Drum rope. Theory of Constraints: Intrinsic Simplicity and Constraint Management

The new flagship 1C product is distinguished by a more analytical approach to enterprise automation - instead of implementing many individual functions, developers are trying to select the most successful and promising techniques and develop functionality that allows them to be used in the enterprise. The most striking example of this approach is the capabilities of 1C:ERP for production planning and control, built on the basis of the theory of system constraints. To effectively use this functionality, it is necessary, first of all, to understand not the capabilities of the program, but to understand all the principles and premises of the theory of system limitations. This article will provide an overview of all the key points of this theory that have found their application in 1C:ERP.

Theory of Constraints, "Drum-Buffer-Rope"

According to the theory of constraints proposed by E. Goldratt, in each production one can identify a small list of work centers, which are “bottlenecks”, the productivity of which limits the productivity of the entire production as a whole. To achieve maximum production productivity, these bottlenecks must be used as efficiently as possible and, where possible, expanded.

Conceptually, the theory of constraints suggests concentrating specifically on ensuring maximum production throughput and maximum speed of production of finished products. To achieve these goals, it is proposed to abandon a number of familiar and ineffective production traditions.

Traditionally, most companies concentrate on maximum load of all work centers, this leads to the accumulation of large stocks of semi-finished products that do not have time to be processed at production bottlenecks. This has two negative consequences at once. The first is the risk of obsolescence, spoilage, or loss of need for accumulated stocks of semi-finished products, which is a direct loss of money. The second is the need for a larger volume of working capital, which is “frozen” in stocks of semi-finished products. Also, traditionally, companies strive to increase the volume of processed batches of semi-finished products in order to reduce the time required to switch to the production of other products, because in this case, the productive operating time for each work center will be higher.

The theory of system constraints suggests, as far as possible, not accumulating stocks of semi-finished products, but ensuring the fastest possible passage of products through all stages of the production process, including by reducing batches of materials processing. This approach allows for shorter production times from raw materials to final products. With this method, stocks of semi-finished products often may not be created, which also solves the problems of freezing and the risks of writing off these semi-finished products. Next, a description of the production planning methodology according to the theory of system constraints will be given.

"Drum-buffer-rope." Application of the principles of the theory of constraints in production management

To make the most of bottlenecks (“key work centers”), you must adhere to the following rules:

Limited resources should never be idle.
It is necessary to reduce the time overhead of bottlenecks. For example, if changeover is required between the release of different products, the order of production of different batches of products can be determined in such a way as to reduce changeover time.
If it is possible to perform individual production operations on other work centers that are not bottlenecks, it is advisable to try to transfer these operations to other machines.
If a certain percentage of defects occurs in production, it is advisable to carry out quality control operations before processing semi-finished products in bottlenecks, because otherwise, their resources will be wasted on processing obviously defective products.

To implement the first two of these principles (the most important), the “Drum-Buffer-Rope” (DBR) technique is used. The main steps in using the technique are as follows:

Identify work centers that are bottlenecks. The technique calls these bottlenecks drums.
Ensure the most efficient loading of drums. To do this, you should create a detailed schedule for processing different products at key work centers. Downtime of key work centers should be eliminated or reduced to the possible minimum. The schedule should be drawn up in such a way as to reduce the time for changeovers if they are necessary between the processing of different products.
Subordinate the work on other work centers to the work of the drum. This means that the start of production of a product must be planned in such a way that it arrives on the drum no later than the planned start time of processing on the drum. Those. The time to start production of products depends on the time they pass through the drum. The BBV method states that the “drum” pulls the “rope” so that the production of the product begins at the first work center (the so-called “pull” production scheme).

Defining Buffer Sizes

To understand the BBB technique, it is very important to understand the role of the buffer. For various reasons, the production schedule may be disrupted. The buffer allows you to ensure that problems in other areas lead to a disruption in the drum’s operating schedule (and, accordingly, a disruption in the overall production schedule). The buffer size must be selected in such a way that parts always arrive on time for processing on the drum. In the BBB technique, a “buffer” refers to the entire duration of the production cycle in front of the drum, and not just the margin of time added for reliability to the average processing time (which, perhaps, is better consistent with the traditional understanding of the word “buffer”). Those. The execution time of individual production operations in front of the bottleneck is summed up and indicated by one number - the buffer size.

One of the fundamental points of the whole concept is the choice of buffer size. The size of the buffer should be determined not by simply summing up the execution time of all operations included in it, but by adding a significant time margin. The target buffer size must be selected in such a way that even in the event of production disruptions in areas “hidden” in the buffer, the total time to complete all operations does not exceed the buffer time. In practice, this means that the buffer can exceed the pure technological time of execution of the operations included in it by three or more times, because It is the multiple reserve of time that provides the necessary guarantee of timely completion of all operations.

The main goal of choosing the size of the buffer is the timely completion of all operations included in it, so that disruption of production in the buffer does not lead to downtime at the narrow workplace located after the buffer, because idle time of a narrow workplace reduces the total output of the entire production.

It is important to understand that allocating a buffer with a large margin of time does not lead to an increase in processing time as the volume of production batches increases. Production time = buffer time + drum operating time * number of products (batch of products).

Example Let the production stage be carried out on three consecutive DCs with the productivity: DC1 - batch of up to 5 pcs. for 1 hour, RC2 - 1 piece/hour, RC3 - batch of up to 3 pieces. in 4 hours. Thus, RC1 and RC2 have greater productivity and are included in the buffer before RC3. The time of this buffer should be designed to prepare a full batch of launches, allowing simultaneous processing on RC3. Because RC3 processes batches of 3 products simultaneously, then the buffer time must be calculated for 3 products. The net time for performing technological operations in the buffer is 4 hours. For the specified conditions, the production of one batch of 3 products will take 4+4=8 hours, the production of two batches of 6 products will take 4+4*2=12 hours. With an increase in the number of manufactured products, the first term, showing the operations hidden in the buffer (4h), will remain unchanged. An example is illustrated in the figure.

If you increase the buffer to 12 hours, then only one term in the above equations will increase; the time to produce 6 and 8 products will be 16 and 20 hours, respectively. Those. The buffer shows one-time time spent in front of a bottleneck workplace to produce an arbitrary number of products.

Thus, the buffer shows a one-time expenditure of time in front of a bottleneck workplace to produce an arbitrary number of products. In general, allocating a buffer of time may not only not increase, but will most likely even reduce the overall production time. The reason is this: in most manufacturing plants, there is a huge difference between the total net processing time and the total time a product is in production. The first value for most types of products ranges from several minutes to an hour per unit, the second can reach several weeks and even under the best production conditions is measured in several days. This is a consequence of the fact that each unit of production waits much longer for its turn than is directly processed. The buffer time in front of a narrow workplace only “legitimizes” the idleness of the product while waiting for processing. But due to the fact that, thanks to the buffer, the downtime of a bottleneck workplace in the entire production will be eliminated, the actual processing time for a batch of products can be reduced.

Here doubts may arise whether the buffer time will slow down the production of products if small batches are produced. Fundamentally, a buffer can slow down the average production time of a small batch of products. However, the presence of a buffer will ensure that the batch will actually be released within the specified time. The absence of a buffer reserve allows you to plan the release faster, but such an optimistic plan cannot always be fulfilled.
If we accept the concept that the work time in the buffer should be taken into account, then another advantage arises. In the BBB, there is no need for high accuracy in standardizing the execution time of all technological operations in the buffer. The time required to readjust machines and move parts between work centers can be ignored altogether, because the buffer provides sufficient time margin. Thus, the task of planning the production schedule is greatly simplified and is reduced only to planning the drum operating schedule.

It is worth emphasizing that the BBB technique not only allows you not to waste time on operational planning, but directly says that such planning can be harmful. If the work center “hidden” in the buffer has excess capacity, it must perform operations in the order in which the parts will arrive on the drum. Otherwise, its local optimization may lead to disruption of the delivery of parts to the drum. It is advisable to optimize the work order only for those work centers that have only a small power reserve compared to the drum. For such work centers, the number of changeovers and downtime should be reduced as much as possible.

The BBV technique proposes to call a buffer not only the work performed in front of the drum, but also the work performed after the drum, before the release of the finished product. In “1C: Enterprise Management” these buffers are named: buffer before and buffer after. By setting the time reserve in the buffer after, you can, in the same way as for the buffer before, abandon detailed operational planning and guarantee the release of products by the planned time.

Buffer management

A key task of buffer management is to monitor and respond to production delays that could delay the transfer of semi-finished products to the drum.

It is proposed to set the buffer time to a minimum with a triple margin relative to the pure production time, and to assess the state of the buffer it is divided into three zones: green, yellow and red. This division allows you to quickly understand which production tasks are at risk of failure. As long as the buffer is in the green zone, everything is fine. When the buffer is in the yellow zone, it is possible that production will not be completed on time, control is desirable. The red zone buffer must be dealt with urgently to avoid delays in the transfer of the workpiece to the drum.

If the share of each zone is equal to a third of the buffer time (in “1C: Enterprise Management” this is exactly the case) -

production control will be very simple:

In a normal situation, production may already end while the buffer is in the green zone.
If production has not even started while the buffer is in the yellow zone, you can manage to complete it even with a time reserve. But the stock in such a buffer is no longer redundant. Production must begin before the buffer enters the red zone.
Even if the buffer falls into the red zone, you can ensure timely completion of production if you make every effort to ensure that the work included in the buffer is completed as quickly as possible. Production that falls into the red zone requires strict control to ensure its completion as quickly as possible.

Thus, for each of the three buffer zones there is a clearly defined response strategy.

It is important to emphasize that buffer time should not be wasted. Those. There should be no peace of mind that production that is idle while the buffer is in the green zone is a normal phenomenon. It is fundamentally important to protect yourself from the “student syndrome” and complete production tasks at the end of the buffer time. The time reserve in the buffer is not to ensure that the work centers included in the buffer work slowly, but to protect the drum from possible problems, such as a problem on the work center working directly in front of the drum. If work on the DC in front of the drum occurs when the buffer is already in the red zone, this will delay the transfer of production to the drum. Therefore, production must begin immediately and run while the buffer is in the green zone. If the buffer enters the red zone, it means there is a high risk of disruption to the production plan, so frequent entry into the red zone is a reason to study and eliminate the problems that cause this.

As stated above, initially it is suggested to choose the buffer time with a triple margin. With stable production in the green buffer zone, the buffer time can be reduced if necessary to speed up the release of a batch of products.

Simplified technique, UBBV

For a large number of manufacturing enterprises, the constraint of the enterprise as a whole is not production capacity, but market demand. The production capacity of these companies allows them to produce more than what the market requires. In such a situation, when production capabilities exceed production requirements, the “drum-buffer-rope” technique can be simplified. This simplified technique is commonly called “simplified drum-buffer rope”, UBBV.

In the usual BBB method, the limitation is the drum; accordingly, all production capacities before it do not need to be planned in detail, because they will have time to complete the necessary operations with a reserve before transferring production to the drum. In the case where the constraint (market demand) is located outside the scope of production, all production can not be planned in detail, but managed as a general buffer that controls the timely release from production.

Thus, the UBBV methodology proposes not to plan production within a period, because It is known that production facilities can fulfill the production plan with a margin. In UBBV, it is only necessary to ensure that production with excess capacity is completed by the specified date. Therefore, in UBBV, production control is reduced only to monitoring the status of the buffer, similar to its control in BBB. The task of planning in UBBV is only to determine the size of the buffer: large enough to ensure timely production, and not too large so as not to overestimate the overall production time.

As in the case of BBB, in the BBB technique it is necessary to control the frequency of the buffer entering the red zone. If this happens often, the procedure should be as follows:

It is necessary to study the reasons for entering the red zone of the buffer.
If the reason is due to internal problems of the production itself, they should be eliminated.
If the reason is a small buffer time and market demand allows it to be increased (i.e., a longer standard production time will not lead to a decrease in demand), then a buffer time with a large margin should be selected.
If the buffer time cannot be increased and the reason for the delays is a small reserve of production capacity relative to the needs for finished products, two options are possible:

In situations where the productivity of all production areas is approximately equal, it will be necessary to increase production capacity (if it is important to reduce the risk of possible production disruption).
If you have a work center with a throughput that is noticeably less than that of other DCs, you should switch to the BBB method, because it allows for optimal production planning and greater precision in production control.

Theory of restrictions in the functionality of 1c:erp.

To support the theory of constraints and BBB and BBBV techniques, the production management functionality offers the following operating procedure:

At each stage of production, a bottleneck can be identified - a key type of work center , for which information about its specific productivity is indicated. For all jobs performed before and after it, a generalized execution time is specified for which they can be completed - buffers.
The production lead time at each stage is defined as the processing time of all products at the key type of work centers, plus the time of buffers before and after. To calculate the processing time of products at a key type of work centers, various parameters of its functioning are taken into account: specific productivity, work schedule, production frequency, the possibility of simultaneous processing of different products under conditions of synchronous and asynchronous start of processing of various products (examples - high-temperature ovens and drying chambers, respectively ).
At each stage of production, a detailed drum schedule can be created to optimize drum performance (for example, reduce the number of changeovers). Control of buffers for each production job (route sheet)can be performed using a traffic light system, according to the BBB method. Alternatively, production control within a stage can be carried out using the UBBV method.

Situations are possible when the production of different items requires a different ratio of processing time at different work centers, i.e. Some products require more time at one DC, while others require more time at another DC. In such cases, at the stages of production in the program, you can specify several types of work centers and the required operating time for them to produce one batch of products. The program will determine the bottleneck in each planning interval automatically, in accordance with which type of DC will operate at its capacity limit in this interval.

Division of production into stages

The production planning and control system in ERP is built not only to optimize production throughput. It is also aimed at solving other problems: delimiting areas of responsibility of employees, monitoring intermediate production results (including cost accounting), etc. Different tasks have conflicting goals.

Thus, from the point of view of productivity optimization, it is desirable to determine the single bottleneck of the entire production chain.

From the point of view of organizational control of production and other aspects of planning:

It is undesirable to combine operations that take place in different workshops into a common production buffer, because It is unclear who will be held responsible for untimely execution of operations in the buffer.
For a long production process, it may be necessary to establish intermediate points to which additional materials must be transferred into production. The transfer of materials to the very beginning of production may be associated with the freezing of working capital for the sake of too early delivery, such a transfer may delay the start of production due to the need to wait for materials from the supplier. There may be other reasons.
The theory of system constraints involves minimizing the accumulation of large batches of products for transfer to the next stage of production, because Such enlargement of processing batches can be useful, in general, only to speed up the work of the bottleneck. But when different production workshops are geographically separated, it will be too wasteful to move each product blank between them separately. From the point of view of cost savings, it is more rational than planning to prepare a certain batch of products in the first workshop and transport the entire batch. Thus, production planning in the second workshop should be carried out from the time of receipt of the batch of products from the first workshop.

Thus, because Since production management has many additional goals and objectives, to solve them it is necessary to divide production into stages and set control time points at which each stage should begin or end. Each stage of production is treated as an independent production system for which a production plan is created and controlled. To plan production at each stage, the planning logic of the theory of system constraints is used: the maximum volume of production at a narrow workplace of a given stage is estimated. To control the production execution plan, the BBB technique is used, where the drum is identified as the bottleneck of this particular stage.

To summarize the capabilities of ERP for dividing production into stages, we can say the following:

If it is necessary to maximize output at any cost and there are no other limiting conditions for production, you can designate all production as a single stage, find the slowest section on it and plan production according to the maximum theoretical output volume.
For complex production, it is impossible to single out a single bottleneck and subordinate all other processes to its maximum use: no less important are the tasks of planning the supply of materials, reducing costs by combining transportation batches between stages, and increasing controllability by delineating areas of responsibility. To solve all these problems, it is necessary to consider production as a series of separate stages, the planning of which must be carried out independently. Already when planning and controlling a separate stage, you can fully use all the principles of the theory of system constraints. Type of work centers - work centers with the same production capabilities (but possibly different productivity). Because for production planning, it does not matter which of the identical DCs the production will be carried out on - the type of DC is indicated.

5. DRUM-BUFFER-ROPE (DBR) METHOD

The “Drum-Buffer-Rope” method (DBR-Drum-Buffer-Rope) is one of the original versions of the “push-out” logistics system developed in the TOC (Theory of Constraints). It is very similar to the limited FIFO queue system, except that it does not limit the inventory in individual FIFO queues.

Rice. 9.

Instead, an overall limit is set on the inventory located between the single production scheduling point and the resource that limits the productivity of the entire system, the ROP (in the example shown in Figure 9, the ROP is area 3). Each time the ROP completes one unit of work, the planning point can release another unit of work into production. This is called a “rope” in this logistics scheme. “Rope” is a mechanism for controlling the restriction against overload of the ROP. Essentially, it is a materials issue schedule that prevents work from entering the system at a rate faster than it can be processed in the ROP. The rope concept is used to prevent work in process from occurring at most points in the system (except critical points protected by planning buffers).

Since EPR dictates the rhythm of the entire production system, its work schedule is called “Drum”. In the DBR method, special attention is paid to the resource that limits productivity, since it is this resource that determines the maximum possible output of the entire production system as a whole, since the system cannot produce more than its lowest capacity resource. The inventory limit and the time resource of the equipment (the time of its effective use) are distributed so that the ROP can always start new work on time. This method is called “Buffer” in this method. The “buffer” and “rope” create conditions that prevent the ROP from being underloaded or overloaded.

Note that in the “pull” logistics system DBR, the buffers created before the ROP have temporal rather than material in nature.

A time buffer is a reserve of time provided to protect the scheduled “start of processing” time, taking into account the variability in the arrival at the ROP of a particular job. For example, if the EPR schedule requires that a particular job in Area 3 begin on Tuesday, then material for that job must be issued early enough so that all of the steps preceding the EPR processing (Areas 1 and 2) are completed on Monday (i.e., in one full working day before the required deadline). Buffer time serves to “protect” the most valuable resource from downtime, since the loss of time of this resource is equivalent to a permanent loss in the final result of the entire system. The receipt of materials and production tasks can be carried out on the basis of filling the “Supermarket” cells. The transfer of parts to subsequent stages of processing after they have passed through the ROP is no longer a limited FIFO, because the productivity of the corresponding processes is obviously higher.

Rice. 10. An example of organizing buffers in the DBR method
depending on the position of the ROP

It should be noted that only critical points in the production chain are protected by buffers (see Figure 10). These critical points are:

the resource itself with limited productivity (section 3),
any subsequent process step where the part processed by the limiting resource is assembled with other parts;
shipment of finished products containing parts processed with a limiting resource.

Because the DBR method focuses on the most critical points of the production chain and eliminates it elsewhere, production cycle times can be reduced, sometimes by 50 percent or more, without compromising reliability in meeting customer shipment deadlines.

Rice. 11. Example of supervisory control
passing orders through the ROP using the DBR method

The DBR algorithm is a generalization of the well-known OPT method, which many experts call the electronic embodiment of the Japanese “Kanban” method, although in fact, between the logistics schemes for replenishing the “Supermarket” cells and the “Drum-Buffer-Rope” method, as we have already seen, there is a significant difference.

The disadvantage of the “Drum-Buffer-Rope” (DBR) method is the requirement for the existence of a ROP localized at a given planning horizon (at the interval of calculating the schedule for the work being performed), which is only possible in the conditions of serial and large-scale production. However, for small-scale and individual production, it is generally not possible to localize EPR over a sufficiently long period of time, which significantly limits the applicability of the considered logistics scheme for this case.

6. LIMIT OF WORK IN PRODUCTION (WIP)

A pull logistics system with a work in process (WIP) limit is similar to the DBR method. The difference is that temporary buffers are not created here, but a certain fixed limit of material inventories is set, which is distributed to all processes of the system, and does not end only at the ROP. The diagram is shown in Figure 12.

Rice. 12.

This approach to building a “pull” management system is much simpler than the logistics schemes discussed above, is easier to implement, and in a number of cases is more effective. As in the “pull” logistics systems discussed above, there is a single planning point here - this is section 1 in Figure 12.

The logistics system with WIP limit has some advantages compared to the DBR method and the FIFO limited queue system:

malfunctions, fluctuations in the rhythm of production and other problems of processes with a margin of productivity will not lead to a shutdown of production due to lack of work for the EPR, and will not reduce the overall throughput of the system;
only one process must obey scheduling rules;
there is no need to fix (localize) the position of the ROP;
It is easy to locate the current EPR site. In addition, such a system gives fewer “false signals” compared to limited FIFO queues.

The considered system works well for rhythmic production with a stable range of products, streamlined and unchangeable technological processes, which corresponds to mass, large-scale and batch production. In single-piece and small-scale production, where new orders with original manufacturing technology are constantly being put into production, where product release times are dictated by the consumer and can, generally speaking, change directly during the manufacturing process of products, then many organizational problems arise at the level of production management. Relying only on the FIFO rule in the transfer of semi-finished products from site to site, the logistics system with a work in progress limit in such cases loses its effectiveness.

An important feature of the “push” logistics systems 1-4 discussed above is the ability to calculate the production time (processing cycle) of products using the well-known Little formula:

Release time = WIP/Rhythm,

where WIP is the volume of work in progress, Rhythm is the number of products produced per unit of time.

However, for small-scale and individual production, the concept of production rhythm becomes very vague, since this type of production cannot be called rhythmic. Moreover, statistics show that, on average, the entire machine system in such industries remains half underutilized, which occurs due to the constant overload of one equipment and the simultaneous downtime of another in anticipation of work related to products lying in line at previous stages of processing. Moreover, downtime and overloading of machines constantly migrate from site to site, which does not allow them to be localized and to apply any of the above logistics pull schemes. Another feature of small-scale and individual production is the need to fulfill orders in the form of a whole set of parts and assembly units by a fixed deadline. This greatly complicates the task of production management, because The parts included in this set (order) can be technologically subjected to different processing processes, and each of the areas can represent an ROP for some orders without causing problems when processing other orders. Thus, in the industries under consideration, the effect of the so-called “virtual bottleneck” arises: the entire machine system on average remains underloaded, and its throughput is low. For such cases, the most effective “pull” logistics system is the Calculated Priority Method.

7. COMPUTABLE PRIORITIES METHOD

The method of calculated priorities is a kind of generalization of the two “push” logistics systems discussed above: the “Supermarket” replenishment system and the FIFO system with limited queues. The difference is that in this system, not all empty cells in the “Supermarket” are replenished without fail, and production tasks, once in a limited queue, are moved from site to site not according to the FIFO rules (i.e. mandatory discipline is not observed “ in the order received"), and according to other calculated priorities. The rules for calculating these priorities are assigned at a single production planning point - in the example shown in Figure 13, this is the second production site, immediately following the first “Supermarket”. Each subsequent production site has its own executive production system (MES - Manufacturing Execution System), the task of which is to ensure timely processing of incoming tasks taking into account their current priority, optimize internal material flow and timely show emerging problems associated with this process ,. A significant deviation in the processing of a particular job in one of the sites can affect the calculated value of its priority.

Rice. 13.

The “pull” procedure is carried out due to the fact that each subsequent section can begin to perform only those tasks that have the highest possible priority, which is expressed in the priority filling at the “Supermarket” level not of all available cells, but only those that correspond to priority tasks. Subsequent section 2, although it is the only planning point that determines the work of all other production units, is itself forced to carry out only these highest priority tasks. Numerical values of task priorities are obtained by calculating the values of the criterion common to all in each section. The type of this criterion is set by the main planning unit (section 2), and each production section independently calculates its values for its tasks, either queued for processing, or located in the filled cells of the “Supermarket” at the previous stage.

For the first time, this method of replenishing “Supermarket” cells began to be used at Japanese enterprises of the Toyota company and was called “Production Leveling Procedures” or “Heijunka”. Nowadays, the process of filling the “Heijunka Box” is one of the key elements of the “pull” planning system used in the TPS (Toyota Production System), when the priorities of incoming tasks are assigned or calculated outside the production areas executing them against the backdrop of the existing “pull” replenishment system of the “Supermarket”. (Kanban). An example of assigning one of the directive priorities to an executing order (emergency, urgent, planned, moving, etc.) is shown in Figure 14.

Rice. 14. Example of assigning a directive
priority to fulfilled orders

Another option for transferring tasks from one site to another in this “pull” logistics system is the so-called “calculated rule” of priorities.

Rice. 15. Sequence of executed orders
in the calculated priority method

The queue of production tasks transferred from section 2 to section 3 (Figure 13) is limited (limited), but unlike the case shown in Figure 4, the tasks themselves can change places in this queue, i.e. change the sequence of their arrival depending on their current (calculated) priority. In fact, this means that the performer himself cannot choose which task to start working on, but if the priority of tasks changes, he may have to, having not completed the current task (turning it into the current WIP), switch to completing the highest priority one. Of course, in such a situation, with a significant number of tasks and a large number of machines on the production site, it is necessary to use MES, i.e. carry out local optimization of material flows passing through the site (optimize the execution of tasks already being processed). As a result, for the equipment of each site that is not the only planning point, a local operational production schedule is drawn up, which is subject to correction every time the priority of the tasks being executed changes. To solve internal optimization problems, we use our own criteria, called “Equipment Loading Criteria”. Jobs awaiting processing between sites not connected by the “Supermarket” are ordered according to “Queue Selection Rules” (Figure 15), which, in turn, can also change over time.

If the Rules for calculating priorities for tasks are assigned “externally” in relation to each production site (Process), then the Site Equipment Loading Criteria determine the nature of the internal material flows. These criteria are associated with the use of optimization MES procedures on the site, intended exclusively for “internal” use. They are selected directly by the site manager in real time, Figure 15.

Rules for selection from the queue are assigned based on the priority values of the tasks being executed, as well as taking into account the actual speed of their execution at a specific production site (section 3, Figure 15).

The site manager can, taking into account the current state of production, independently change the priorities of individual technological operations and, using the MES system, adjust the internal production schedule. An example of a dialog for changing the current priority of an operation is shown in Fig. 16.

Rice. 16.

To calculate the priority value of a specific job being processed or awaiting processing at a specific site, a preliminary grouping of jobs (parts included in a specific order) is carried out according to a number of criteria:

Number of the assembly drawing of the product (order);
Part designation according to the drawing;
Order number;
The complexity of processing the part on site equipment;
The duration of passage of parts of a given order through the machine system of the site (the difference between the start time of processing of the first part and the end of processing of the last part of this order).
The total complexity of operations performed on parts included in this order.
Equipment changeover time;
A sign that the processed parts are provided with technological equipment.
Percentage of part readiness (number of completed technological operations);
The number of parts from a given order that have already been processed at this site;
The total number of parts included in the order.

Based on the given characteristics and calculating a number of specific indicators such as tension (the ratio of indicator 6 to indicator 5), comparing the values of 7 and 4, analyzing the ratios of indicators 9, 10 and 11, the local MES system calculates the current priority for all parts found in one group.

Note that parts from the same order, but located in different areas, may have different calculated priority values.

The logistics scheme of the Calculated Priority Method is used mainly in multi-item production of small-scale and single types. Featuring a "pull" scheduling system and using local MES to ensure high-speed orders flow through individual production areas, this logistics design uses decentralized computing resources to maintain process efficiency in the face of changing job priorities.

Rice. 17. Example of a detailed production schedule
for workplace in MES

A distinctive feature of this method is that the MES system allows you to draw up detailed schedules of work performed within the production area. Despite some complexity in implementation, the method of calculated priorities has significant advantages:

current deviations that arise during production are compensated by local MES based on the changing priorities of the tasks being performed, which significantly increases the throughput of the entire system as a whole.
there is no need to fix (localize) the position of the ROP and limit the work in progress;
it is possible to quickly monitor serious failures (for example, equipment breakdown) at each site and recalculate the optimal sequence of processing parts included in various orders.
The presence of local production schedules in certain areas allows for operational functional and cost analysis of production.

In conclusion, we note that the types of “pull” logistics systems discussed in this article have common characteristic features, these are:

Preservation in the entire system as a whole of a limited volume of stable reserves (current reserves) with regulation of their volume at each stage of production, regardless of current factors.
An order processing plan drawn up for one site (a single planning point) determines (automatically “pulls out”) the work plans of other production departments of the enterprise.
Promotion of orders (production tasks) occurs both from the next section in the technological chain to the previous one using the material resources consumed in the production process (“Supermarket”), and from the previous section to the next one according to FIFO rules or calculated priorities.

LITERATURE

Jonson J., Wood D., Murphy P. Contemporary Logistics. Prentice Hall, 2001.
Gavrilov D.A. Production management based on the MRP II standard. - St. Petersburg: Peter, 2003. - 352 p.
Womack D, Jones D. Lean production. How to get rid of losses and achieve prosperity for your company. — M.: Alpina Business Books, 2008, 474 p.
Hallett D. (translation by Kazarin V.) Pull Scheduling Systems Overview. Pull Scheduling, New York, 2009. pp.1-25.
Goldratt E. Purpose. Goal-2. - M.: Balance Business Books, 2005, p. 776.
Dettmer, H.W. Breaking the Constraints to World-Class Performance. Milwaukee, WI: ASQ Quality Press, 1998.
Goldratt, E.. Critical Chain. Great Barrington, MA: The North River Press, 1997.
Frolov E.B., Zagidullin R.R. . // General Director, No. 4, 2008, p. 84-91.
Frolov E.B., Zagidullin R.R. . // General Director, No. 5, 2008, p. 88-91.
Zagidullin R., Frolov E. Control of manufacturing production by means of MES systems. // Russian Engineering Research, 2008, Vol. 28, No. 2, pp. 166-168. Allerton Press, Inc., 2008.
Frolov E.B., Zagidullin R.R. Operational scheduling and dispatching in MES systems. // Machine park, No. 11, 2008, p. 22-27.
Frolov E.B., . // General Director, No. 8, 2008, p. 76-79.
Mazurin A. FOBOS: Effective production management at the workshop level. // CAD and graphics, No. 3, March 2001, p. 73-78. — Computer Press.

Evgeniy Borisovich Frolov

Performance indicators have taken the corporate world by storm. Company employees concentrate on achieving target indicators, the fulfillment of which determines salary increases and career growth. Few employees think about how their individual successes affect the results of the company as a whole.

It seems to us that local improvements certainly lead to global improvements. Practice proves that this is not so. We simply accepted this assumption as an axiom and organized all the work on its basis. Creator of the Theory of Constraints Eliyahu Goldratt in the book "Target. Continuous Improvement Process" reveals the danger of this approach: by following old, incorrect assumptions, we are leading our company into a trap.

This article will be useful to those who are tired of improving
everything in a row. We'll show you how to improve your company's performance by influencing several key factors.

Goals and indicators

The main purpose of a business is to make money. This is obvious. But the more the company grows, the more the main goal moves away from each employee. Management personnel will not escape this fate either - managers are increasingly mired in the abyss of statistical data and key performance indicators.

An Internet marketer strives to increase website conversion and reduce the cost per click.

The factory floor manager focuses on increasing productivity and reducing equipment downtime.

The sales manager prioritizes revenue and sales volume.

At first glance, everything is logical. But Goldratt warns that the race to maximize local performance does not lead to greater system efficiency. Moreover, local optimization can cause a deep crisis in the company.

To avoid equipment downtime in the absence of real orders, the machines are loaded with work for future use. This is good for productivity, but ruins the business: more and more money is “frozen” in the form of work in progress or surplus products that sell poorly and require storage costs.

Sense of purpose

Nice numbers look good on paper, but they drag your company into the abyss.

To cope with a crisis, Goldratt calls for focusing on the most important goal. Once the goal is determined, we need to introduce a simple system of indicators that show as accurately as possible whether we are moving closer to the goal or moving away from it.

Goldratt suggests using three basic metrics: revenue generation rate, tied up capital, and operating expenses.

Revenue generation rate- The rate at which a company makes money through sales.

Bound Capital- money invested by the company in materials and equipment that can be sold.

Operating expenses- money that a company spends to convert tied capital into generating income.

The main advantage of such a system of indicators is that it gives an extremely accurate and clear view of the real situation.

If tied capital increases, for example in the case of an increase in work in progress, then the company's efficiency decreases.

If operating costs fall, efficiency increases.

If we observe high indicators of labor and equipment productivity, but the products are poorly purchased, there is nothing to rejoice at.

System limitation

The production process is built on the basis of a strict sequence of operations: manufacturing of parts, processing, assembly, quality control. In the same way, you can decompose any business process of your company, be it sales, marketing or accounting.

This is important to do for one reason - the strength of the chain is equal to the strength of its weakest link. Once you break down the workflow into sequential elements, you can find the link that limits the productivity of the entire company.

The accountant does not have time to issue invoices for payment on time, as a result, the receipt of money from customers is delayed.

The designer does not have time to layout booklets according to requests from the marketing department.

The online store manager does not have time to call all clients on the day the application is received.

In production, a special term was introduced for this element - "bottleneck". But in fact, a bottleneck can appear in any business function.

Goldratt names the company's bottlenecks system limitations.

Constraints may be related to company policies, rules and procedures, lack of resources and materials, lack of orders, or too slow response to customer needs.

Once your company's main constraint has been identified, there are two things you need to do.

First, the capacity of the bottleneck should be expanded whenever possible.

If the department staff cannot cope with the work, it is worth organizing the timing of working hours. Based on its results, you need to determine how to use your working time more effectively. For example, you can replace some manual labor with automated processes. If this is not possible, the staff will have to be expanded.

When you have increased the limiting power to the limit or are convinced that this is impossible, you need to move on to the second step. In this step, you tailor the entire workflow to accommodate the bottleneck capacity.

There is no point in putting every employee and every piece of equipment at 100% capacity if their product ends up bottlenecked and unable to reach the end customer on time.

According to the theory of constraints, resources that are not a bottleneck should be idle for a certain part of the time. These resources have excess capacity in contrast to the bottleneck, which has insufficient capacity.

Drum - buffer - rope

In order to radically change the business management process, it is necessary to abandon the premises that were previously considered unshakable: the principle of 100% employment of employees or equipment, conditions for the supply of materials, lunch schedules. Instead, Goldratt proposes to build the entire technological process around the company's bottleneck using the method "drum - buffer - rope."

Drum is a limitation that sets the rhythm for the entire work process. Instead of maximizing productivity at each stage, we work to the rhythm of the “drum”, that is, we adapt the workflow to the constraint.

Contextual advertising generates 100 leads per day, but sales managers can efficiently process only 50 applications. Operating costs are rising, while overall service levels are declining. Solution - we set up contextual advertising in such a way as to receive a predictable number of leads, which our managers are able to process efficiently and in a timely manner.

Buffer- reserve before the bottleneck. Since the main constraint determines the efficiency of the entire system, it is very important to use it to the maximum and avoid downtime. The buffer ensures that the bottleneck link works even if one of the previous elements in the chain temporarily fails.

If the main flow of applications comes from the website, you need a backup plan in case of technical problems. For example, managers can call old clients who have not purchased anything for a long time. To avoid downtime, you need to have a ready-made base.

Rope- a mechanism connecting the buffer and the drum. We only introduce new materials into production when the bottleneck buffer has dropped below a certain minimum. If this condition is neglected, we will again return to workflow overload.

Process continuity

It is impossible to optimize the workflow once and for all. Solved problems are replaced by new difficulties. Goldratt emphasizes that the improvement process must be continuous. The continuous improvement model consists of five steps:

1. Find the system limitation. Find out what is limiting the efficiency of the entire company. A bottleneck is something that prevents your business from making more money.

2. Decide how to use the constraint effectively. Decide how to make the most of your bottleneck. It is in this area that there should be no downtime or loss of time.

3. Align other actions with this decision. Tailor your entire workflow to suit the power of your bottleneck. Ensure that all other business functions allow the bottleneck to run smoothly

4. Increase Bandwidth Capping Buy additional equipment, hire staff, implement automation, or change work procedures.

5. Proceed to step one. Having eliminated the problem in one bottleneck, we return to the beginning of the algorithm and again find restrictions that can be used to improve the efficiency of the entire system.

Text: Zhanna Omelyanenko

Illustrations: Konstantin Amelin

Illustration: Shutterstock

Voice of the project “Big plans” - Dimitry Chumak, announcer, coach on public speaking, public speaking and voice development. He will be happy to tell you the details personally. Write [email protected]

Afterword:

Sergey Kozlov, General Director of Megaplan

I became acquainted with the theory of constraints, as, in general, it should have been, when I worked at a factory. My then supervisor was very interested in this book. And on New Year’s Eve 2008, when we celebrated the successful defense of the budget with whiskey in his office, he gave me Goldratt’s book “The Purpose.” This was a really good New Year's gift. At least in my mind, many ideas were turned upside down. Even though I now work in IT companies, the book still occupies an important place on my shelf. Of course, there are echoes of the 1980s with their industrialization, but even today they are suitable for design work and the creation of software products. It’s just that the main production forces are different in IT, the “necks of the bottles” and the interaction of workshops are different. From designers to front-end developers, from testing to release.

Friends, what do you think about the futility of improving everything?

5. DRUM-BUFFER-ROPE (DBR) METHOD

Rice. 9.

Note that in the “pull” logistics system DBR, the buffers created before the ROP have temporal rather than material in nature.

Rice. 10. An example of organizing buffers in the DBR method
depending on the position of the ROP

It should be noted that only critical points in the production chain are protected by buffers (see Figure 10). These critical points are:

the resource itself with limited productivity (section 3),

any subsequent process step where the part processed by the limiting resource is assembled with other parts;

shipment of finished products containing parts processed with a limiting resource.

Rice. 11. Example of supervisory control
passing orders through the ROP using the DBR method

6. LIMIT OF WORK IN PRODUCTION (WIP)

Rice. 12.

The logistics system with WIP limit has some advantages compared to the DBR method and the FIFO limited queue system:

malfunctions, fluctuations in the rhythm of production and other problems of processes with a margin of productivity will not lead to a shutdown of production due to lack of work for the EPR, and will not reduce the overall throughput of the system;

only one process must obey scheduling rules;

there is no need to fix (localize) the position of the ROP;

It is easy to locate the current EPR site. In addition, such a system gives fewer “false signals” compared to limited FIFO queues.

An important feature of the “push” logistics systems 1-4 discussed above is the ability to calculate the production time (processing cycle) of products using the well-known Little formula:

Release time = WIP/Rhythm,

where WIP is the volume of work in progress, Rhythm is the number of products produced per unit of time.

7. COMPUTABLE PRIORITIES METHOD

Rice. 13.

Rice. 14. Example of assigning a directive
priority to fulfilled orders

Another option for transferring tasks from one site to another in this “pull” logistics system is the so-called “calculated rule” of priorities.

Rice. 15. Sequence of executed orders
in the calculated priority method

Rice. 16.

Number of the assembly drawing of the product (order);

Part designation according to the drawing;

Order number;

The complexity of processing the part on site equipment;

The duration of passage of parts of a given order through the machine system of the site (the difference between the start time of processing of the first part and the end of processing of the last part of this order).

The total complexity of operations performed on parts included in this order.

Equipment changeover time;

A sign that the processed parts are provided with technological equipment.

Percentage of part readiness (number of completed technological operations);

The number of parts from a given order that have already been processed at this site;

The total number of parts included in the order.

Note that parts from the same order, but located in different areas, may have different calculated priority values.

Rice. 17. Example of a detailed production schedule
for workplace in MES

current deviations that arise during production are compensated by local MES based on the changing priorities of the tasks being performed, which significantly increases the throughput of the entire system as a whole.

there is no need to fix (localize) the position of the ROP and limit the work in progress;

it is possible to quickly monitor serious failures (for example, equipment breakdown) at each site and recalculate the optimal sequence of processing parts included in various orders.

The presence of local production schedules in certain areas allows for operational functional and cost analysis of production.

In conclusion, we note that the types of “pull” logistics systems discussed in this article have common characteristic features, these are:

Preservation in the entire system as a whole of a limited volume of stable reserves (current reserves) with regulation of their volume at each stage of production, regardless of current factors.

An order processing plan drawn up for one site (a single planning point) determines (automatically “pulls out”) the work plans of other production departments of the enterprise.

Promotion of orders (production tasks) occurs both from the next section in the technological chain to the previous one using the material resources consumed in the production process (“Supermarket”), and from the previous section to the next one according to FIFO rules or calculated priorities.

E.B. Frolov, Moscow State Technological University "STANKIN"

The classic Drum-Buffer-Rope mechanism cannot always be applied correctly in practice. Most often it is difficult to maintain the correct sequence, where the Rope comes first, followed by the Buffer, and the Drum is used only in special cases. In addition, a significant obstacle to the successful use of the mechanism is the difficulty in synchronizing sales and the production process. In this regard, it is of rather high interest simplified systemDrum-Buffer-Rope.

Classical system Drum-Buffer-Rope is a mechanism for managing production processes aimed at “expanding” the limitation of the system, subordinating all production to the most efficient use of the limitation. In practice, the construction of such a system includes the development of a detailed work schedule for the constraint (drum), the creation of a protective buffer that prevents the constraint from being idle (buffer), and also the organization of a mechanism for the timely release of work into production (rope).

However, when introducing the BBK mechanism, there is also a hidden premise: sales and production are processes that occur in two self-sufficient departments, and the sales department can sometimes send new orders to production departments, even when the latter are difficult (or completely unable) to fulfill them . However, in this case, it is obvious that this premise must be destroyed. This is a situation in which the market is the constraint, and production must subordinate its work to this constraint.

So what to do in the case where the BBK mechanism has been introduced into production, but it has become obvious that production is no longer a constraint?

In this case, it is inevitable to come across the conclusion that some provisions of the LBC methodology are no longer necessary.

First, there is a strict schedule that determines the operation of the constraint: its presence determines that the constraint is always involved in the work. This also implies unconditional adherence to the planned work schedule for the limitation. And while all of these are certainly useful things to constrain the factory's production process, they make the factory itself "inflexible" to changes in market demand: demand increases, the customer demands orders to be completed in a shorter time, order volumes become too large, etc. .p.

Secondly, the classic mechanism drum-buffer-rope requires the creation of three types of buffer:

Shipping buffer – to ensure delivery of orders on time;
Restriction buffer – to ensure the operation of the restriction in case of disruptions in the work schedule;
Assembly buffer - for the timely receipt by the assembly shop (located in the production system after limitation) of all the resources necessary for the assembly.

However, in practice, many businesses do not use the build buffer, and in fact work to provide additional protection to the shipping buffer. This is a signal that in fact the BBK system does not provide any mechanism for prioritizing signals arriving from buffers. First of all, the problem is: am I shipping the product, or am I just keeping the constraint running?

Thirdly, the BBK mechanism is extremely complex! There are a lot of things that the BBK system software cannot fully take into account. For example:

the interdependence of some production stages and other technological limitations that require additional streamlining;
For some production stages (for example, drying ovens), which simultaneously process several orders (or process orders in parts), it is too difficult to create a work schedule;
if the constraint is a set of similar but not identical machines, the scheduling problem is also very significant;
the need to repeatedly pass orders through a stage that is a constraint (or through several constraints)

Finally, in practice there is often a need to reorganize the schedule. In fact, given the strict work schedule restrictions, this could lead to fundamental changes in all processes, which could result in shifting order deadlines. And this does not at all contribute to maximizing the efficiency of using the market constraint.

Since there is no software that could take into account all these difficulties when implementing a LBC system, it is always necessary to have additional programs that will try to take into account all these unaccounted aspects. At a minimum, this complicates the use of the BBK mechanism. At most, this causes a loss of confidence in this mechanism.

This can't make you happy. It turns out that despite the simplicity of the idea embedded in the BBK mechanism, its practical application becomes much more complex than necessary. The classic LBC system calls this paradox the “successful decision conflict”: “do simple things to get good work” versus “make it complicated to get maximum results.” This conflict can be resolved by examining the extent to which simple decisions and complex decisions affect the final results. In some situations, simple solutions can contribute to problems, for example:

1. We may find ourselves underutilizing the constraint in production. If the real constraint is the market, this means that production capacity must be greater than market capacity (ie, the enterprise must be able to respond to increased demand). That is, the plant must operate in such a way that its capacity is underutilized - in other words, the limitation should not be used 100%.

2. The order in which work is submitted to the constraint leads to significant loss of time, which results in underutilization of the capacity of the production constraint. This situation occurs when the constraint is affected by a number of interdependent production steps. This is a fairly rare occurrence, but in such cases it is necessary to introduce a more efficient drum-buffer-rope mechanism.

3. The constraint stage does a lot of work that is essentially unnecessary. This should not happen if at this moment Kanat “pulls” orders into the workshop that the market really requires.

Basic principles of the simplified BBK system

What are the key principles of the classical LBC system that remain the same for the simplified system? There are three main aspects:

1. Subordination in relation to the market (we must know whether we provide the established delivery times for the order);

2. Rope (do not send too many materials into the production process so that the constraint receives only the necessary materials on time and does not create unnecessary work in progress);

3. Reduce the workload of the entire production.

To ensure point 1, the sales department has a tool at its disposal that helps quickly answer the question “What will be the delivery date for this order?” The standard answer to this question would be standard lead time. However, this tool will calculate its lead time option by submitting that order to the constraint at the next possible time, to which 1/2 of the shipment buffer will be added.

If the result obtained exceeds the previously valid standard lead time, then the new (longer) time will be used, since it is most likely that it is more adequate. If the result obtained is less, then the previously valid standard time is used and a longer order buffer is introduced, which will ensure that the order arrives at the constraint link at the right time. (Yes, exactly to everyone order a separate “order buffer” is assigned, which will help control its implementation).

To ensure points 2 and 3, simple rules must be established for production workshops. Each order has its own buffer, which is set separately for each order. Work enters the system taking into account the time allotted for its completion - i.e. according to the same principles that apply to Kanat in the classical BBK system. The priority is set depending on the color assigned to the order buffer. Work centers will receive this information daily and prioritize their activities, and instead of different signals coming from the build, constraint, and dispatch buffers, there will be only one kind of signal.

But it just looks too simple.

Would such a system create too much work in progress?

Will there be a threat of a reboot (or, conversely, idleness) of the restriction if there is no detailed work schedule for its work? What if there are emergency orders or other changes in order volume? The first two points are discussed above.

What about emergency orders? If your business environment allows for this to happen, some planned orders will be delayed so that emergency orders can be processed. At the same time, it will be necessary to ensure the delivery of these goods in a shorter time frame if required by the sales department. If, when orders enter the system, their priority changes, then it is necessary to change the order fulfillment time.

For example, if an order needs to be delivered a week earlier, the time allotted for its completion will be reduced and a new buffer priority will be calculated. There is no need to make complex changes to the constraint link's schedule.

However, there are several points where you should be especially attentive. If a plant operates under both a make-to-order and a work-to-stock system, then store-to-stock orders are prioritized based on consumption from buffer stocks. If the system does not have ways to protect against such changes, then we can influence the timing of the “to-order” work.

In addition, you should be careful with interdependent processes, the execution time of which can significantly increase/decrease under the influence of the work that takes place in the work center. Here, first of all, it is necessary to check whether these stages can be optimized so that significant problems do not arise when shifting work priorities. Another solution might be to identify the preferred work sequence and have the senior foreman make decisions on the fly based on that sequence and whether the buffer is long enough to accommodate all the production changes made.

A rollback to the classical LBC system should be a last resort when all the above methods within the simplified LBC mechanism will not be able to effectively eliminate the problem of inefficient waste of time at the limiting stage.

To summarize the above, the essence of the simplified BBK system is that that only one type of buffer determines both the priority and release schedule of work, and the activities of the sales department in drawing up plans for the passage of orders through the entire system. Simple and effective.

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