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NOV-DEC 2017

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INTECH NOVEMBER/DECEMBER 2017 49 AUTOMATION BASICS Passive loopback process simulation Passive loopback process simulation (PLPS) is the answer. While ideally the PLC simulation will be on a separate PLC or PC that is commu- nicating with the PLC under test, you can run the PLPS on the same PLC, with some restric- tions, which we will discuss. It does not require any extra equipment or software packages. All it requires is a basic understanding of the external equipment and of how that equipment interacts with the PLC. For most process applications, there is minimal need to simulate advanced control schemes. The primary goals are to simulate on/ off for motors and open/close for valves, and cycle through sequences, demonstrate safety shutdowns, and then be able to demonstrate that the program reacts as it should to numer- ous upset scenarios. Most programs resolve to two major sections: device control and process control. Device con- trol logic supervises a specific field device, like a motor, and can have the following attributes: l the output command to turn on a motor l the interlock logic that will automatically shut off the motor in emergencies l the permissive logic that, when in auto mode, will allow the motor to start/stop based on a set of conditions l the timer that monitors the motor's auxiliary contacts to ensure the motor starts within a reasonable time limit l the restart inhibit timer that prevents multiple restarts and gives the windings time to cool l the motor run clock l the auto/manual switching logic l the remote/local switching logic Other forms of device logic include (but are not limited to): l on/off valves l throttling valves l variable frequency drives l stepper motors Process control logic is logic that is not spe- cific to a device, but that controls the on/off, start/stop, speed, percent open/closed, and other values that drive one or more sections of device logic. Some examples of process control logic are: l sequence logic that energizes/deenergizes and otherwise communicates with the device logic fragments to operate the field equip- ment sequentially l proportional, integral, derivative ratio and other higher-level control algorithms l combinational and supervisory logic that monitors multiple sensors and drives device logic in response to stimuli In its most basic form, PLPS is all discrete (dig- ital). The developer creates a simulation logic fragment for each PLC discrete output. Each fragment simply mon- itors the status of the software tag assigned to a physical discrete output and loops the signal back to a soft- ware tag associated with the physical dis- crete input that rep- resents feedback from the field device. Injecting a short wait interval in this "loop back" will more accurately reflect the fact that a motor or valve has mechanical lags and so does not react instantaneously. More advanced PLPS not only loops the dis- crete output back to its associated feedback discrete input, but also ramps any associated analog input signals to their normal operating Process control logic Device control logic PLC CONTACTOR MOTOR A B C C M A DO B C CMD DI Device SIM logic AUX NOT AVAILABLE FOR TESTING Figure 2. Simple motor simulation scheme Process control logic is logic that is not specific to a device, but that controls the on/off, start/ stop, speed, percent open/closed, and other values that drive one or more sections of device logic.

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