An object-oriented simulation-based method for emulation development for testing shop control software

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Virginia Tech


An emulator is a computer program that mimics the behavior of a production facility as seen by the control program. Emulation has been used as a tool for dynamic, off-line testing of control software for automated manufacturing systems. However, research efforts in emulation have focused mainly on controllers for equipment, AGVS, and workstations. This research focuses on emulation for testing shop control software.

Though conceptually simple, emulator development efforts tend to be ad hoc in nature and lack a strong conceptual framework. Currently, the effort involved in developing an emulation model may outweigh the potential benefits. The approach used in this research centers around adapting a detailed simulation model, i.e., used for testing control strategies, for emulation, i.e., control software testing. This approach promotes software reuse and thus limits the emulation development task.

Due to the limitations of conventional simulation languages and modeling techniques, a simulation model is not readily adapted for emulation. The main problems lie in turning off the control logic in a simulation and interfacing the model to the actual controller. An object-oriented modeling methodology was developed for systematically transitioning from simulation to emulation. Basically, the method calis for encapsulating manufacturing control logic into controller objects and modeling the system from the perspective of exchange of messages among controllers. The developed method also promotes the rapid development of a driver for verification and validation of the emulation model.

A prototype system was developed to demonstrate the feasibility of the emulation development method. The MODSIM II language for object-oriented simulation was used to implement the object classes.

The second issue addressed is the ability to use emulation to test shop control system in faster-than-real-time (FRT) mode. Currently it is necessary to test the system in real-time, which makes it impractical to observe extended operation of the shop. The mixed-mode emulation method, which switches time-advance between real-time and next-event modes, was developed. Issues in implementing and using the mixed-mode and the delay-scaling technique for FRT emulation were discussed. Experimental results showed that mixed-mode emulation had the potential to reduce run-times by more than 50% over real-time emulation.



CIM, software testing, object-oriented simulation, manufacturing automation