Workshop control system model based on double-layer control loop

Abstract

The invention provides a workshop control system model based on a double-layer control loop. According to the model, control variables are constructed in a flow rate form, and the order input rate and the productivity are selected as the control variables of the system, that is, the release of orders and the production capacity of the system are continuously adjustable. For a certain manufacturing unit, the constraint of the manufacturing unit is mainly from two aspects. On one hand, the manufacturing units are influenced by internal factors of the manufacturing units and need to be adjusted according to real-time states of the manufacturing units; and on the other hand, the manufacturing units are globally controlled by the workshop system. When the two are combined to see the problem, a local-to-overall framework is formed. According to the workshop control system model based on the double-layer control loop, a double-feedback control structure is designed from the angle of a control theory. And the production capacity adjustment response rate and the stability of the inventory level of products in process are improved.

Description

Technical Field

[0001] The invention relates to a workshop control system model based on a double-layer control loop. Background Art

[0002] Production control system modeling is the prerequisite for realizing adaptive control of workshop manufacturing systems. It must meet two requirements: (1) The production control system model should be able to reflect the specified performance in the production control task, such as rise time, adjustment time, overshoot or corresponding frequency domain characteristics; (2) The established production control model is convenient for designing controllers and can dynamically adjust system parameters to gradually achieve tracking convergence.

[0003] Traditional production control system modeling methods neglect analysis of the system's dynamic behavior. Modeling methods based on control theory offer the advantage of applying feedback mechanisms from control theory to establish a connection between the system's design and operational layers. This control-theoretic modeling approach can be applied to systems ranging from a single workstation to an entire workshop. Different production control objectives and control variables can be set for different production scales.

[0004] However, existing production control systems cannot fully map the production control block diagram of discrete manufacturing workshops. They cannot autonomously adopt corresponding control strategies when disturbances cause changes in workshop system parameters, thereby achieving adaptive control of the system. Summary of the Invention

[0005] The purpose of this invention is to provide a workshop control system model based on a double-layer control loop to solve the problems raised in the above background technology.

[0006] (1) How to establish a production control system model for a discrete manufacturing workshop, so that when disturbances cause changes in workshop system parameters, corresponding control strategies can be adopted autonomously to achieve adaptive control of the system.

[0007] To achieve the above objectives, the present invention provides the following technical solutions.

[0008] A workshop control system model based on a double-layer control loop;

[0009] The workshop has a manufacturing unit, which includes a flow rate input module, a work-in-process control module, and a flow rate output module. The flow rate input module obtains a planned or ideal work-in-process inventory level indicator by combining an order input rate indicator with a planned or ideal production lead time indicator. The work-in-process control module obtains a planned productivity indicator by combining a planned or ideal work-in-process inventory level indicator, an actual work-in-process inventory level indicator, and a work-in-process inventory control gain parameter. The flow rate output module obtains an actual productivity indicator by combining the planned productivity indicator with the delays incurred during the actual production process.

[0010] Assumption: G c is the system production capacity adjustment gain, T d is the system production capacity adjustment delay time, Tlt is the production lead time, PR plan is the planned productivity, PR real is the actual productivity;

[0011] The transfer function expression of the delay in the production process is:

[0012]

[0013] A production capacity regulation loop is introduced in the workshop manufacturing unit. The transfer function of the production capacity regulation loop is:

[0014]

[0015] Planned productivity PR plan and actual productivity PR real The difference accumulates over a period of time to form work-in-progress inventory.

[0016] On the basis of the above technical solution, the present invention can also be improved as follows.

[0017] Furthermore, it also includes a backlog task controller, which obtains the backlog task control output index by comparing the system planned output parameters, the system actual output parameters and the backlog tasks with the backlog task control gain, and obtains the system production capacity adjustment gain by comparing the backlog task control output index with the actual productivity.

[0018] Further, assume that: D is the number of user orders within the sampling time, d is the delivery time, and ST is the daily working hours;

[0019] The order input rate of the system is equal to the planned or ideal production rate. The order input rate can be defined as the daily average of user demand during a sampling period:

[0020]

[0021] After adopting such a structure, this workshop control system model based on a double-layer control loop constructs control variables in the form of flow rate, and selects order input rate and productivity as the control variables of the system, that is, the release of orders and the system production capacity are continuously adjustable.

[0022] A manufacturing cell faces two primary constraints. On the one hand, it is influenced by its own internal factors and needs to adjust based on its real-time status. On the other hand, it is subject to the overall control of the shop floor system. Combining these two aspects creates a local-to-global architecture. This shop floor control system model, based on a two-layer control loop, employs a dual-feedback control structure designed from a control theory perspective. This improves the responsiveness of production capacity adjustments and the stability of work-in-process inventory levels. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a principle block diagram of the three basic unit modules of the internal framework of a discrete manufacturing workshop adopted in this workshop control system model embodiment based on a double-layer control loop.

[0024] Figure 2 This is the production control block diagram of the manufacturing unit in the workshop control system model embodiment based on the double-layer control loop.

[0025] Figure 3 This is the block diagram of the workshop production control system in this workshop control system model embodiment based on a double-layer control loop. DETAILED DESCRIPTION

[0026] To make the above-mentioned objects, features, and advantages of the present invention more readily apparent, specific embodiments of the present invention are described in detail below with reference to the accompanying drawings. The following description sets forth numerous specific details to facilitate a full understanding of the present invention. However, the present invention can be implemented in many other ways than those described herein, and those skilled in the art may make similar modifications without departing from the scope of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0027] The terms "vertical," "horizontal," "left," "right," and the like as used herein are for illustrative purposes only and do not represent the only implementations.

[0028] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which this invention pertains. The terms used in this specification of the present invention are for the purpose of describing specific embodiments only and are not intended to limit the present invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0029] The unit modules of the internal framework of the discrete manufacturing workshop are mainly composed of Figure 1The three basic unit modules are serial operation, split operation and assembly operation, and parallel operation is equivalent to the "parallel" form of multiple serial operations, so parallel operation is not considered separately. From the perspective of material flow, each module only contains two links: input and output. Since this application uses the flow rate model idea to construct control variables, the input and output of the system are viewed from the perspective of continuous flow. Therefore, this application is based on Figure 1 (a) The form of control system modeling for the basic unit ignores the branching or merging of material flows. The simplified model will inevitably have a certain impact on the control effect. However, this application analyzes the problem from a system perspective, so a certain compromise is made between model simplification and control effect. Figure 1 (b) and (c) involve the branching or merging problem of material flows. In this method, the order flow matrix is ​​defined and described using the state space method.

[0030] For manufacturing cells within a shop floor, random factors during the production process, such as equipment failures, delayed workpiece arrival, or worker absences, can prevent scheduled production tasks from being completed on time, leading to a backlog within the cell. However, from a shop floor system perspective, the backlog within the cell represents only a portion of the work-in-progress within the entire production or manufacturing process. Therefore, from a systemic perspective, the backlog within a manufacturing cell is the work-in-progress within the system.

[0031] In the following, the backlog tasks within the unit are collectively referred to as work in progress.

[0032] If the production process is represented by a first-order lag link, the following can be obtained based on the basic idea of ​​the flow rate control model: Figure 2 Considering the manufacturing system under the distributed control architecture, the production capacity of the system can be adjusted according to production needs. The production capacity adjustment loop is introduced in the manufacturing unit model, and its transfer function is

[0033] Figure 2 The meaning of each parameter is as follows:

[0034] I ordermi : input rate of manufacturing unit i;

[0035] G cmi : Production capacity adjustment gain of manufacturing unit i;

[0036] T dmi : Delay time for production capacity adjustment of manufacturing unit i;

[0037] T ltmi : Production lead time of manufacturing unit i;

[0038] PR pmi: planned production rate of manufacturing unit i;

[0039] PR rmi : actual production rate of manufacturing unit i;

[0040] T ltmi *: planned production lead time of manufacturing unit i;

[0041] WIP mi : Actual work-in-process inventory level of manufacturing unit i;

[0042] WIP mi *: planned work-in-process inventory level for manufacturing unit i;

[0043] G wmi : WIP inventory control gain of manufacturing unit i;

[0044] Cap plani : Planned output of manufacturing unit i

[0045] S: Laplace transform.

[0046] The workshop production control system modeling assumes that: (1) the production capacity of each manufacturing unit can be adjusted synchronously; the production capacity of each manufacturing unit can be adjusted synchronously, mainly considering that the manufacturing units, as the various components of the workshop system, have synchronization when executing the instructions or control strategies of the upper-level production control system; secondly, it is convenient for modeling and simplifies the complexity of the control system.

[0047] Assumptions: (2) The production lead time of each manufacturing unit is equal. For discrete workshops, the production lead time includes the waiting time for workpieces, tooling time, processing time, and workpiece turnover and transportation time. The same production cycle does not mean the production lead time is consistent; the consistent production lead time is used to represent the production synchronization of each unit and facilitate the overall system modeling.

[0048] Assumption: (3) Logistics and transportation time is not considered. Logistics and transportation time is not considered. The control at the workshop level focuses on internal control of the system. The internal logistics and transportation link is part of the production lead time and is therefore not considered separately.

[0049] The production control system at the shop floor falls within the purview of upper-level planning and decision-making, therefore emphasis should be placed on the system's integrity and timeliness. Ideally, after the production control system issues a command, each manufacturing unit within the system should execute it instantly and synchronously. Therefore, assumptions are made to ensure the synchronization of production capacity adjustments and the consistency of production lead times. During the manufacturing unit modeling process, the classification of backlogs and work-in-progress at different levels is a challenge. When modeling the shop floor system, managers are concerned with the completion or compliance of tasks across the entire system; if this falls short of expected capacity, a backlog of tasks will result.

[0050] Based on the control theory, a block diagram of the discrete workshop production control system is established in the continuous time domain, such as Figure 3 This two-layer control loop-based workshop control system model fully maps the production control block diagram of a discrete manufacturing workshop. It can also autonomously adopt corresponding control strategies to achieve adaptive control of the system while realizing changes in workshop system parameters caused by disturbances.

[0051] The disturbances in the workshop system can be roughly divided into two categories: internal disturbances and external disturbances. Internal disturbances, such as equipment failures, delayed arrival of workpieces, etc.; external disturbances, such as changes in orders or demand, urgent orders, etc. Internal disturbances cause workshop system parameters such as production lead time T to fluctuate. lt Changes, external disturbances will cause the probability of delivery to decrease. Therefore, the workshop production control system dynamically adjusts the key parameters of the workshop according to the changes in the key parameters of the workshop.

[0052] This two-layer control loop-based shop floor control system model incorporates three key parameters of the manufacturing system: work-in-process (WIP), backlog, and production rate (PR). It is suitable for describing the dynamic characteristics of the system using continuous flow. Therefore, this two-layer control loop-based shop floor control system model defines system parameters in the continuous time domain.

[0053] The meaning of each parameter is as follows:

[0054] I order : Order entry rate;

[0055] G c : System production capacity adjustment gain;

[0056] T d : System production capacity adjustment delay time;

[0057] T lt : Production lead time;

[0058] PR plan : planned productivity;

[0059] PR real : actual productivity;

[0060] T lt *: Planned or ideal production lead time;

[0061] WIP: actual work-in-process inventory level;

[0062] WIP*: Planned or desired work-in-process inventory level

[0063] G w : Work-in-process inventory control gain;

[0064] G b : Backlog task control gain;

[0065] Cap plan : System plan output;

[0066] Cap real : Actual output of the system;

[0067] The production lead time T in the workshop control system model based on the double-layer control loop is lt * and planned capacity Cap plan Prepared by the production planning department.

[0068] Figure 3 In the production control system model, the production control system model mainly consists of three parts, namely

[0069] (1) In the production process, the transfer function is: The production process is simplified into a pipeline flow model, using first-order inertia or lag links. The number of tasks completed per unit time represents the actual productivity PR. real Planned productivity PR plan and actual productivity PR real The difference, after a period of accumulation (through an integral link ) constitutes work-in-progress inventory.

[0070] (2) Two-layer control loop, Gw represents the work-in-process inventory process control gain of the secondary control loop; Gb represents the backlog task control gain of the primary control loop, focusing on production capacity or production capacity planning at the planning level.

[0071] (3) Production capacity regulation loop, the transfer function is: Where Gc represents the adjustment gain; Represents the production capacity adjustment delay link. Its physical meaning is that in a manufacturing system under a distributed control architecture, the system's production capacity can be increased by configuring or reconfiguring the manufacturing units within the system; or by extending operating hours or increasing the startup rate.

[0072] In discrete manufacturing, the processing time of a workpiece is significantly shorter than the time spent in material transportation and transit within the production process. The actual processing time often accounts for less than 10% of the entire production lead time. Therefore, it can be roughly assumed that materials (parts, raw materials, etc.) within the manufacturing system are constantly in motion during a production lead time. The production process is simplified into a pipeline (or flow) model in this model. The difference in time between material flowing into and out of the pipeline represents production delay—the production lead time.

[0073] Production delays are a key factor in the dynamic nature of manufacturing systems. Delays create inertia in the system and can also lead to oscillations. Therefore, when modeling a shop floor production control system, the following two factors must be considered:

[0074] (1) Average value of production delay;

[0075] (2) Distribution of material output flow around the mean of production delay.

[0076] Due to the different production capacities of various production equipment within the workshop manufacturing system and the different transportation times of the logistics system, there are differences in the input and output of materials between the various sections of the workshop, making it difficult to accurately describe the production delay link. In order to facilitate statistical and control analysis, this workshop control system model based on a double-layer control loop uses T lt The expression of the production delay link is shown in formula (1.1).

[0077]

[0078] Where n represents the production delay order of the workshop system.

[0079] According to Little's law, when the input flow and output flow of the system are equal, that is, the system reaches equilibrium, the average delay time can be calculated from the internal logistics inventory (ideal work in progress) WIP* and the planned production rate PR plan To express, that is It is a first-order linear link, so in the above model, n=1 is taken, and its transfer function expression is

[0080] In this workshop control system model based on a double-layer control loop, the backlog controller and the WIP controller form a double-layer control loop. The backlog task controller is mainly responsible for the planning of upper-level production capacity. By cooperating with the production capacity adjustment loop, it realizes the reconstruction of the system's production capacity. Strictly speaking, the backlog control link of this workshop control system model based on a double-layer control loop should be connected in series with a first-order lag link. Because there is a certain delay in the planning and decision-making of production capacity in reality, and considering that the functions of backlog control and subsequent production capacity adjustment are consistent, the parameter Td is uniformly used to represent the delay time constant. Secondly, if a first-order delay link is introduced, the entire production control system will become a third-order system, which is not convenient for subsequent system analysis.

[0081] The work-in-process control loop serves as a correction link in the system's production process and is an effective means of suppressing fluctuations in the production process. Furthermore, when the system's production capacity changes after reconfiguration or configuration, the system's work-in-process threshold (or ideal value), WIP*, will also change. In this case, the WIP controller dynamically adjusts the system's WIP level to maintain stable operation and prevent excessive fluctuations.

[0082] The workflow of the double-layer control loop:

[0083] (1) When the shop floor system is operating normally, the WIP controller is mainly used to stabilize the work-in-progress level (below the WIP* threshold);

[0084] (2) If the production lead time continues to increase due to internal disturbance factors, the WIP controller will improve the production lead time by reducing the order input rate;

[0085] (3) If urgent orders are inserted and the delivery date is tight, the order input rate cannot be reduced and the WIP has reached the threshold. In this case, production capacity adjustment is needed to expand actual production;

[0086] (4) When the system production capacity is expanded or adjusted, the system becomes stable again. At this time, the WIP threshold needs to be raised and the order input rate needs to be increased to improve the utilization of the system. If the WIP threshold is not adjusted, some system resources will not be fully utilized, resulting in a decrease in utilization.

[0087] Adjusting production capacity has a profound impact on the stable operation of shop floor manufacturing systems. A common misconception in industrial production is that when setting production targets, faster and larger adjustments to production capacity will rapidly increase production capacity. However, due to the high rate of order flow input, a large amount of work-in-progress (WIP) is generated within the shop floor manufacturing system, causing production fluctuations, disrupting the production cycle, and ultimately extending the production cycle. Furthermore, considering the distributed control architecture of discrete shops, production capacity can be reconfigured within the system's internal organizational structure. Therefore, actual production capacity is flexible and adjustable. Introducing a production capacity adjustment loop facilitates simulation analysis of system characteristics under different system organizational models.

[0088] Ideally, capacity adjustment can be considered instantaneous, i.e., Td = 0; however, in practice, the capacity adjustment time Td of the system depends on the magnitude of the capacity adjustment. The value of parameter Td, to a certain extent, indicates the flexibility of the system. The system design uses a first-order delay link to represent the capacity adjustment delay, and its transfer function is

[0089] When the shop floor manufacturing system is in steady state, ideally, the system's order input rate Iorder is equal to the planned (or ideal) production rate PR plan The order input rate can be defined as the daily average of user demand within a sampling period:

[0090]

[0091] Where:

[0092] D: the number of user orders within the sampling time (standard working hours or pieces);

[0093] d: delivery time (days);

[0094] ST: Daily working hours (hours / day).

[0095] The above is only one embodiment of the present invention. It should be pointed out that for ordinary technicians in this field, several modifications and improvements can be made without departing from the principles of the present invention, and these should also be regarded as falling within the scope of protection of the present invention.

Claims

1. A shop control system model based on a double-layer control loop, characterized by: The workshop has a manufacturing unit, which includes a flow rate input module, a work-in-process control module, and a flow rate output module. The flow rate input module obtains a planned or ideal work-in-process inventory level indicator by combining an order input rate indicator with a planned or ideal production lead time indicator. The work-in-process control module obtains a planned productivity indicator by combining a planned or ideal work-in-process inventory level indicator, an actual work-in-process inventory level indicator, and a work-in-process inventory control gain parameter. The flow rate output module obtains an actual productivity indicator by combining the planned productivity indicator with the delays incurred during the actual production process. Assumption: G c Adjust the gain for system production capacity, T d is the system production capacity adjustment delay time, Tlt is the production lead time, PR plan is the planned productivity, PR real is the actual productivity; The transfer function expression of the delay in the production process is: A production capacity regulation loop is introduced in the workshop manufacturing unit. The transfer function of the production capacity regulation loop is: Planned productivity PR plan and actual productivity PR real The difference accumulates over a period of time to form work-in-progress inventory.

2. The workshop control system model based on a double-layer control loop according to claim 1 is characterized by: It also includes a backlog task controller, which obtains the backlog task control output index by comparing the system planned output parameters, the system actual output parameters and the backlog tasks with the backlog task control gain, and obtains the system production capacity adjustment gain by comparing the backlog task control output index with the actual productivity.

3. The workshop control system model based on a double-layer control loop according to claim 1 is characterized by: Assumptions: D is the number of user orders within the sampling time, d is the delivery time, and ST is the daily working hours; The order input rate of the system is equal to the planned or ideal production rate. The order input rate can be defined as the daily average of user demand during a sampling period:

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