VIRTUAL COMMISSIONING FOR PLC SIMULATION.doc

VIRTUAL COMMISSIONING FOR PLC SIMULATIONMinsuk Ko, Dae Soon Chang, and Sang C. Park Ajou University Department of Industrial EngineeringSan 5, Woncheon-dong, Yeongtong-gu, KoreaE-mail: sebastianminsukKEYWORDSPLC Simulation, Virtual device model, Virtual commissioning.ABSTRACTIn this paper, a template-based modeling methodology is proposed for the effective construction of a virtual plant and it can be used for PLC simulation. As the proposed methodology provides high fidelity modeling power, the virtual plant consists of virtual devices which include sensors and actuators. One of the key ideas of the proposed methodology is to provide a virtual device template model and this is separated into two parts, a physical model and a logical behavior model. When both the physical and the logical models are defined, we can simply define a virtual device model by combining the two sub-models. The proposed template model approach provides two major benefits: (1) Significant reduction in the time and efforts for the construction of a virtual plant, and (2) Reduction in the stabilization time of a production system through PLC simulation.INTRODUCTIONNowadays, product life cycles are reduced in the constantly changing marketplace. Therefore, modern manufacturing systems must possess sufficient responsiveness to adapt their behaviors efficiently to a wide range of circumstances. Recently, in order to respond to these demands, including high productivity and production flexibility, the use of the concept of a virtual commissioning (VC) has been widely accepted (Pellicciari 2009). In the past, VC was applied to small size (cell) manufacturing system. However, due to the recent development in computer technology, it is possible to apply VC technology (VCT) to a huge manufacturing system (line, factory). As a part of this revolution, offline programming for robots and verification of control program (Hibnio 2006) along with the virtual device models have emerged in various industries. Figure 1 shows the procedure to build a production system which is based on a concurrent engineering approach. This has two major design activities: mechanical design and electrical design. The mechanical design phase produces a physical model which includes the hardware configurations of a production system whereas the electrical design phase describes the control program of the system. Usually, electrical design involves programmable logical controllers (PLCs), because PLCs are currently the basic and universal tool for the automation of manufacturing processes. Traditionally, the development of the PLC (Programmable Logical Controller) controlled applications, mechanical design and electrical design have been performed sequentially (Hibnio 2006) and partly on-line. So, the control engineer has to wait with the programming, verification and optimization of the control code until the mechanical engineer has completed his or her work (Fray 2000). It is an inefficient manufacturing process and it delays time required for the product to reach market. So, many manufacturing companies adopt a more attractive method to do this in a concurrent engineering approach and totally off-line and here, both the mechanical and the control engineers work simultaneously. Moreover, a simulation based on the VCT has been considered as an essential tool in the design and analysis of the complex system which cannot be easily described by analytical or mathematical models (Hoffman 2010). As the implementation of a manufacturing line requires heavy investment, many companies apply VC simulation to the production system design in order to ensure that a highly automated manufacturing system will successfully achieve the intended benefits. Figure 1. A framework for control level simulationThis demand has resulted in the concept of PLC simulation. PLC simulation can be described as a model that executes digital manufacturing processes within a computer simulation (Hibnio 2006). In order to verify the mechanical and electrical designs of the production system, it provides a realistic effect as a test run for the production system. This is done, by using the 3D graphic model which appears to be the same as a real shop floor, and the logical model that drives a PLC in a real factory.The objective of this paper is to develop an efficient method for the construction of models for PLC simulation in an automotive manufacturing system. The proposed model construct method employs a template model which consists of the physical model and the logical behavior model. The overall structure of the paper is as follows. Section 2 illustrates the architecture of the proposed template-based modeling methodology. Section 3 describes an efficient construction methodology for a template model which can be synchronized with a control program. Finally, concluding remarks are given in Section 4.APPROACH FOR TEMPLATE-BASED MODELING METHODOLOGY FOR PLC SIMULATION As PLC programs contain only the control information, without device models, it is necessary to build a corresponding virtual plant model (a set of virtual device models) to perform simulation. However, construction of a virtual device model for the physical model and the logical model alike requires an excessive amount of time and effort, as we cannot use both the models directly for PLC simulation due to some limitations. Sometimes, the virtual device model construction requires more time compared to PLC programming. This serves as the motivation for exploring the possibility of finding out a template-based modeling methodology for building a virtual device model. In order to apply the virtual device model to the PLC simulation that is connected with the shop floor environment, we have to progress sequentially through the some procedures as follow; After the construction of the simulation environment using virtual device models and layout, users have to progress sequentially to Manual mode simulation and Automatic mode simulation.Figure 2. Various components belonging to a virtual device modelIn this PLC simulation environment in order to achieve control level verification, the virtual device model has to represent behaviors of the device that of actual system. However, users have to invest much time and effort in the construction of a virtual device model that is suitable for the control level environment. This is shown in Figure 2. It is necessary to add virtual sensors and correct motions to achieve the intended control objectives for each task in the physical model. Furthermore, the most time-consuming task is the development of the logical model for analyzing device behaviors based on the process information. This is due to the process design information which contains only the process sequence of the production system and not the control level information, the level of sensor and actuator. Therefore, in order to define the logical model of a device as a DEVS(Discrete Event System Specification) model, users have to analyze the device behavior specifically according to the signals that are in a control program. As the logical model has high modeling DOF (Degree Of Freedom), users have to consider on how to determine the set of DEVS components for the representation of the device behaviors.When both the models are determined suitable for simulation, they have to be connected with one another. The output of this step is usually the virtual device model and this becomes a practical guideline for simulation. Obviously, manufacturer can greatly benefit from PLC simulation by using the virtual device model, but there are still various difficulties that complicate the full utilization of the virtual device model. One of the main obstacles to build a logical model comes from understanding the device behaviors which are set of tasks that are assigned to the device. As a logical model interacts with a virtual factory which may consist of hundreds of machines and products, it is difficult to find out a modeling error of the designed logical model, and physical model during simulation. Sometimes, finding out modeling errors of the virtual device model becomes a bottleneck in the simulation time delay. Therefore before simulation, it is necessary to verify both the logical model and the physical model. In order to cope up with this problem, we apply the template-based modeling approach to build a virtual device model of the PLC simulation. As mentioned earlier, the separation of the mechanical and electrical parts of a virtual device enables concurrent engineering of the mechanical and electrical designs of a virtual device model. Figure 3 shows the detailed construction procedure of a virtual device by using template model. The template model consists of two steps, adjustment step and application step. In the adjustment step, it is necessary to prepare a solid model with motions that are obtained from the mechanical design and a task which is assigned to the device. During this, as we can select a proper template model based on the identified task and the geometric model, efficient adjustment of both the models is possible. Once the template model is identified, a virtual device model can be simply obtained by combining the physical model and the logical model in the application step. Figure 3. Proposed procedure to construct a virtual device model for PLC.TEMPLATE MODEL OF THE VIRTUAL DEVICE The objective of this paper is to propose the template model which is able to provide a practical guideline for the construction of a virtual device model for PLC simulation. As mentioned above, the logical model of the proposed template model is based on the Zeiglers DEVS formalism. In DEVS formalism, one must specify two types of sub-models, atomic model and coupled model (Kim 1994). In the original DEVS formalism, the atomic model is similar to the Timed-FSA (Finite State Automata). It is supposed to represent the core logics of the target device. Formally, an atomic model M is specified by a 7-tuple:X: input events set; S: sequential states set; Y: output events set;: SàS: internal transition function;: Q* XàS: external transition functionQ = (s,e)| s S, 0 e(s): total state of M;: SàY: output function; : SàReal: time advance function.In order to construct a template model of the virtual device, we extracted five major control components from the PLC program and this is shown in Figure 4. PLC program starts with an external precondition which has to be satisfied by other components. Moreover to progress this condition, it is necessary to satisfy the error conditions that are caused by other devices, process sequence, HMI signals, and product code. After checking the external precondition, the PLC program checks the self-condition of the device, the internal precondition. These preconditions role is to check whether the device is located at the proper state for the execution of the task or not. Once these conditions are satisfied then, the device executes the target task. It is possible to separate device output types whether a device is controlled by PLC or own controller. As mentioned before, an automated device in the production system usually executes an output signal for the task through the PLC. On the other hand, if the device is like a robot then, is controlled by its own controller and a task can be executed by calling a program. The output of the task is usually verified with a sensor, timer, or a signal which indicates that the task is complete. As the proposed template model contains these components in the logical model, and physical model, the modeling errors can be avoided elegantly.Figure 4. Components of PLC programFigure 5. Template model of the virtual device model of an AGVBy using the proposed template model, the virtual device model can be constructed as follows.1) Identify a set of tasks in the device, and confirm the PLC output symbols to trigger those tasks. After this, identify a set of input signals whose role is to indicate the state of the device that is based on internal precondition. By using this information, determine a set of states of the device including an initial state.2) Identify int, ext to connect the device states sequentially.3) Identify the required virtual sensors to monitor device states, and to make pairs between a sensor and X (or Y). Then, determine the output function () at the int, and connect it with the required virtual sensor.4) Connect a motion in the physical model with a state, and by considering motion time, determine ta.5) Connect signals in the external precondition with HMI, and test a virtual device model with the PLC program.Figure 5 shows a simple template model by using a sample of an AGV (Automatic Guided Vehicle) with two tasks, M1 (movement from p1 to p2) and M2 (movement from p2 to p1). The two tasks should be triggered by external events, O_X1 and O_X2. When the user selects this template model, it is possible to automatically instantiate the logical model. Moreover, the physical model is instantiated from the predefined physical model which has two motions M1, M2, and also two position sensors S1, S2. The functional relationships will be automatically defined between DEVS components of the logical model and the physical activities in the physical model. This is shown in Figure 3. By using this template model, users can intuitively understand both the logical and the physical aspects of the device. The logical model of the virtual device corresponds to the AGV and it can be described as follows:M = < X, S, Y, int, ext, , ta>X: X1, X2; S: S_P1*, M_P1_P1, S_P2, M_P2_P1 Y: Y1, Y2; int (M_P1_P2 ) = S_P2; int (M_P2_P1) = S_P1 ext (S_P1, X1 ) = M_P1_P2; ext (S_P2, X2) = M_P2_P1 (M_P1_P2) = Y1; (M_P2_P1) = Y2 ta(M_P1_P2)=T1 ; ta(M_P2_P1)= T2As mentioned above, this simple template model contains the major components of the PLC program. As S1, S2 have a role for sensing the AGV position, the preconditions of the AGV can be verified. Moreover, the time to travel between P1 and P2 can be used for the verification of the AGV motion verification. If the time can not satisfied with the purpose of the motion then, the position of the sensors or motion properties has to be readjusted. Finally, the external functions have to be verified with the sensors before sending an external signal. I_P1, verifies to find out whether a task is complete or not. In order to export I_P1, it is necessary to satisfy S1 and Y1 simultaneously. When the simulation has to be initialized by the process reset signal, the virtual model set up with the initial position (P1) and the initial state (S_P1).ACKNOWLEDGMENTS This work was supported by the Defense Acquisition Program Administration under the Contrac