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1. Read this entire procedure prior to attempting to tune your process loop.

2. The first step is setting up your Integral (Reset) and Derivative (Rate) where they are not a factor and will not affect the controller’s output. If the controller has a Cycle Time function, set it to 5 seconds or faster. Note that Cycle Time is only used when the controller has a relay or solid state relay output. The Cycle Time tuning parameter tells the controller’s output the speed at which the output is to be cycled, usually set in seconds. To increase the Cycle Time speed, decrease the Cycle Time value.

3. Set the Proportional Band or Gain to a safe value. In most processes, a safe value would be 50% of the controller’s input range. Make sure you read the controller’s operation manual to understand how the control manufacturer’s tuning parameters numbering system works. A Proportional Band of 10, depending on the controller manufacturer, could represent 10 degrees, 10% of the controller’s input range, or 10% of the controller’s setpoint.

4. Start the process and observe the control reaction. If the process responds too fast, and there is a fast oscillation, the Proportional Band is too small (see illustration below). Increase your Proportional Band, upset the process by changing the controller’s setpoint as described earlier, and observe the process again. Continue this step as many times as necessary to achieve the best possible control response. If you have a slow response, proceed to step 5.

5. If you have a slow response, your Proportional Band is too big and you will get a large overshoot before the output begins to respond (see illustration below). Decrease your Proportional Band, upset the process as outlined in the beginning of this article, and observe the process again. Continue this step as many times as necessary to achieve the best possible controller response.

If the process is maintaining the controller’s setpoint after step 5, proceed to step 8.

6. You will probably observe that there is a Proportional offset from the setpoint. If not, a "P" only controller may be all that is required for your particular process. If you do have a Proportional offset, it is time to add Integral (Reset). Introduce Integral gradually until your process returns to setpoint. Remember, after making each tuning parameter change, disrupt the process by changing the controller’s setpoint by at least 10% of the controller’s input range and then monitor the process’s new controlled response. If the reaction time is a little too fast or too slow, trim your Proportional Band (see steps 4 and 5). Also, make sure you understand the numbering system for the Integral. Integral can be set as either repeats per minute, or minutes per repeat. Review your controller’s operations manual for the proper setting.

If the process is maintaining the controller’s setpoint after step 6, proceed to step 8.

7. If you observed that on start-ups and during process disruptions that the controller’s output response was lacking and/or that the Proportional and Integral functions would not provide the process with satisfactory control, Derivative (Rate) can be used. Introduce Derivative gradually until your process returns to setpoint. Remember, after making each tuning parameter change, disrupt the process by changing the controller’s setpoint by at least 10% of the controller’s input range and then monitor the process’s new controlled response.

A note about Derivative: Derivative in most processes is only required when the process is prone to sudden upsets. Sudden process upsets might include a door opening on an oven or chamber, a cold wind that might cool down a machine when a door is opened, or a process load change (removing or adding product to your process).

8. If the controller uses a solid state relay driver output with a solid state relay as the final output device, and your process is maintaining the controller’s setpoint, no further tuning adjustments are required.

9. If the process is maintaining the controller’s setpoint, and the controller's output is a mechanical relay, decrease the controller’s Cycle Time by increasing the Cycle Time value until the process stability begins to deteriorate. At this point, increase the Cycle Time just enough to regain control. The purpose of this step is to eliminate any excess output cycling which could cause premature failure of the controller’s and/or the final controller’s life span. Typical outputs where this step would be important are mechanical relays, solenoids, and other mechanical switching devices.

This tuning procedure may only be necessary if your process controller does not have auto-tune capability. Always read your operations manual to verify if auto-tune is available with your controller. Auto-tune in some instances will require additional tuning adjustments. I have seen on many controllers that the Cycle Time function is not adjusted by the auto-tune feature. Make sure you start with a cycle time of approximately 5 seconds, then proceed to step 7 and 8 of this procedure.