Question

How to set up Proportional-Integral-Derivative (PID) control

Summary

Set up PID control on the sc200 controller

Answer

Please see the below information on how to configure PID control option in the sc200 controller with hints on tuning:

To start, enter the set point, the phase and a value for the Prop Band then watch the process and see how long and how close the sc200 can get the process to the set point. Integral and Derivative should be set to zero. Once you have a good understanding of how the sc200 responds to changes in the process, then you can add an integral value and see how the process responds.

To make the process respond faster you can decrease the Prop band or increase the Integral. It is recommended to make one change at a time and then to be patient and watch how the process responds to each change.

You do not want the process to oscillate, which means the output switches between 4 mA and 20 mA. To stop a process from oscillating you want the process to respond slower, so increase the prop band and/or decrease the integral value.

Example 1: Customer needs to adjust the pH of incoming sample of 10 down to a pH of 6.5

Set mode - Automatic

Phase - direct (process is above the set point, we are trying to drive down the value, and as we value decreases so does the mA output

Set point - 6.5

Prop band - 2 (4 mA= 6.5, 20 mA =8.5)

Integral - 20 min

Derivative = 0

Transit time = 0

Example 2: Customer needs to keep the DO levels in a basin above 2.5

Set Mode - Auto

Phase - reverse (process is normally below the set point and we are driving the concentration up, as the process value increase the mA output will decrease.

Set point - 2.5 ppm

Prop band - 2.5 (20 mA = 0 ppm 4 mA = 2.5 ppm)

Integral - 30 minutes

Derivative - 0

Transit time = 0

**Set Mode**-__Auto__- The sc200 will look at the Process Variable and adjust the 4-20 mA automatically.__Manual__- the operator inputs the value of the mA output

**Phase**- direction in which the signal responds to process change

__Direct__- mA will decrease as the process variable decreases; process normally increases and you want to drive value down to the set point. 20 mA would is above the set point (see example 1 below).

__Reverse__- mA will decrease as the process variable increases; process normally decreases and you want to drive the process up to the set point. 20 mA is below the set point (see example 2 below).

**Set Point**- the desired value

**Prop**

**Band**- is the number of units above or below your set point (depending on the phase) where the output will vary proportionally as the process changes (see examples below). This is similar to linear control where "set low" or 4 mA is the

**Set Point**and "set high" or 20 mA is the proportional band. This value should be provided by the customer, a good starting point would the raw value of the process before any changes.

**Integral**- time period between changes in the output. The output is held during each time period and then the output is recalculated and adjusted based on the process variable. This value should be provided by the customer. A good starting point would be 0 and then increase by 10 minute intervals, if the process begins to oscillate (cycle between 4 and 20 mA)then the Integral should be reduced.

**Derivative**- used to compensate for the rate of change,

__most application this should be set to 0__

**Transit time**- the time it takes for the sample to move from the injection point to the sensor. There should

__not__be a value entered for both Integral and transit time.

**PID Tuning**:

To start, enter the set point, the phase and a value for the Prop Band then watch the process and see how long and how close the sc200 can get the process to the set point. Integral and Derivative should be set to zero. Once you have a good understanding of how the sc200 responds to changes in the process, then you can add an integral value and see how the process responds.

To make the process respond faster you can decrease the Prop band or increase the Integral. It is recommended to make one change at a time and then to be patient and watch how the process responds to each change.

You do not want the process to oscillate, which means the output switches between 4 mA and 20 mA. To stop a process from oscillating you want the process to respond slower, so increase the prop band and/or decrease the integral value.

Example 1: Customer needs to adjust the pH of incoming sample of 10 down to a pH of 6.5

Set mode - Automatic

Phase - direct (process is above the set point, we are trying to drive down the value, and as we value decreases so does the mA output

Set point - 6.5

Prop band - 2 (4 mA= 6.5, 20 mA =8.5)

Integral - 20 min

Derivative = 0

Transit time = 0

Example 2: Customer needs to keep the DO levels in a basin above 2.5

Set Mode - Auto

Phase - reverse (process is normally below the set point and we are driving the concentration up, as the process value increase the mA output will decrease.

Set point - 2.5 ppm

Prop band - 2.5 (20 mA = 0 ppm 4 mA = 2.5 ppm)

Integral - 30 minutes

Derivative - 0

Transit time = 0

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