Logix5000 example programs

You slowly start to increase the force until the vehicle is going at the target speed. Within an industrial environment, the same principle is applied to an array of different systems. A heating element will be controlled in order to maintain the temperature setpoint defined by the system.

Similarly, a pump will be controlled in order to maintain a certain flow of a liquid. The implementation of a PID control loop on a Programmable Logic Controller is simplified in many instances through dedicated instructions. For a heated tank, this would be the current temperature. Next, we have the Control Variable. This value will be tied to the piece of instrumentation which has an impact on the system and allows it to be controlled by the system.

In a heated tank, this would be the contact which enables the heating element. The instruction will require you to set a setpoint which is the target for the PID control loop. This variable is critical and can be changed by the PLC program in order to bring the process to the proper value. For example, the setpoint may be varied based on the recipe of the product as well as the stage the tank is in. Based on these values, the control loop will be more or less effective at controlling the setpoint of the process.

Simulating a PID loop without an active system is tricky and requires a timer which periodically updates the response of the virtual values directly on the PLC. Building the logic for simulating the PID can be done as follows. Add a TON Instruction which is tied to a local timer. Update the PID as well as the process variable based on the completion of the timer.

Here are the rungs which make this happen. As you adjust the three gains discussed above, you will see different responses. The PID Instruction can solve many inefficiencies of a system.

A PID system relies on the fact that the control mechanism is able to adjust the system into a steady condition. In other words, a heater which is heating a tank must be able to provide enough energy in order to raise the temperature to the desired level. The engineering team will need to go back to the drawing board and either increase the heating capacity or increase the insulation.

For example, a flow-based system which is controlled by a valve over 30 feet away from the flow meter, may experience problems. These problems are due to the fact that the feedback loop is too slow and as the liquid propagates, the feedback is sent 30 seconds later.

In a steady state condition, the loop can be tuned for a continuous flow, but at the beginning, it will struggle as it needs to be much more aggressive. Each one needs to be tuned independently unless they are completely identical. Having the same starting point is fine, but relying on the same values will most likely result in frustrations. For additional tips, see Search Frequently Asked Questions. Be sure to post complete, working code and avoid snippets.

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