While the auto-tune function of the PXU PID control works well, in some cases it might be necessary to fine tune the PID output manually. There are a variety of ways to tune PID loops. This document describes two different manual tuning procedures.
APPLICABLE PRODUCTS
All PXU series process controls
PRE-REQUISITES
None
IN THIS ARTICLE
Accessing display loop PID values
The PID values can be accessed for manual turning, by pressing the D button.
Scroll through the display loop options until you see.
Prop- Proportional band value
Intt- Integral band value
Dert- Derivative band value
Once the desired parameter has been selected, adjust with the up or down arrow key. Press the D key again to exit back to the main display.
PROCEDURE 1:
Simple Manual Tune Procedure with Very Little or No System Oscillation During the Tune Procedure
• Lower the derivative value to 0. We will not change this value from zero after this first step.
• Lower the integral value to 0. Easy second step.
• Raise the proportional value 100.0.
• Increase the integral value to 100.
• Slowly lower the integral value and observe the system’s response.
• Since the system will be maintained around the setpoint, change setpoint and verify if the system corrects it in an acceptable amount of time. If it's not acceptable or you would like a quicker response, continue lowering the integral value.
• If the system begins to oscillate again, record the integral value and raise the value to 100.
• After raising the integral value to 100, return to the proportional value and raise this value until oscillation ceases.
• Lower the proportional value back to 100.0, then lower the integral value slowly to a value that is 10% to 20% higher than the recorded value when oscillation started. (Recorded value times 1.1 or 1.2)
Change the setpoint and watch as the system tracks quickly and efficiently. If you experience an overshoot that is not desirable, consider using the setpoint ramp parameter. It is most useful at system start-up or when a large setpoint change is introduced during system operation. Reference the PXU product manual for more information on setpoint ramp rate.
PROCEDURE 2:
Utilizing the System Oscillation to Determine Optimum Proportional and Integral Values.
During this procedure, you will locate the ultimate gain value utilizing the proportional value only. Then you will introduce error correction with the integral value. As you can see above, the rate of change, or derivative value, may be more of a nuisance. You will have to move back and forth between the parameters, mostly proportional and integral, with an occasional setpoint change as we manually tune the unit.
Completing a manual tune of a quick responding system may seem like a tedious process, but you can complete the process in a reasonably short time period with acceptably tight control results.
• Lower the derivative value to 0. We will not change this value from zero after this first step.
• Lower the integral value to 0.
• Raise the proportional value to a high value (I often use 150.0).
• Change the setpoint value to develop a difference between actual process value and setpoint value.
• Lower the proportional value slowly. You may see some correction, but there will always be a difference between the process value and the setpoint value you programmed (steady state error).
• As you lower the proportional band slowly, you increase the risk of initiating a system oscillation. If oscillation becomes large and is not acceptable, record the proportional band value and then raise the band value until oscillation ceases.
• Since the proportional band has been raised to stop the system oscillation, this is a good time to raise the integral value to 100. Eventually we will use the integral value for error correction.
• Return to the proportional band and lower the value slowly until a value that is double the earlier recorded value is reached.
• Now lower the integral value slowly and the error between setpoint value and process value will decrease.
• Since the system will be maintained around setpoint, change setpoint and verify the system corrects in an acceptable amount of time. If not acceptable or you would like a quicker response, continue lowering the integral value.
• If the system begins to oscillate again, record the integral value and raise the value to 100.
• After raising the integral value to 100, return to the proportional value and raise this value until oscillation ceases.
• Lower the proportional value to the double value used earlier in the set-up and then lower the integral value slowly to a value that is 10% to 20% higher than the recorded value when oscillation started. (Recorded value times 1.1 or 1.2)
Change the setpoint and watch as the system tracks quickly and efficiently. If you experience an overshoot that is not desirable, consider using the setpoint ramp parameter. It is most useful at system start-up or when a large setpoint change is introduced during system operation.
ADDITIONAL INFO
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