PID Settings | Proportional-Only Control
In this article, we’re going to look at the effects of Proportional-only action on a flow control loop.
Proportional-only control is the simplest of the PID controller actions but is seldom used in industrial production processes.
The problem with P-Only control is something called Offset. In this article, we’re going to explain Offset and show you how and when it occurs.
A typical flow control loop
The controller in our loop is typical of most controllers. We can adjust the setpoint and switch between auto and manual. In manual control, we can adjust the controller output to the exact amount we want.
Our flow loop has a pump to move our process fluid. We can turn the pump on or off which will be very helpful for observing PID responses in static and dynamic conditions.
Ok… Let’s kick things off with a look at our flow control loop P&ID.
The flow is manipulated by the pneumatic final actuator FCV200. The controller FRC200 produces an electrical signal that operates the pneumatic final actuator through a Current to Pressure (I/P) converter FY200.
The Differential Pressure Transmitter FT200 sends an electrical signal representing the flow rate to the controller.
PID Settings 1) Proportional-Only
let’s look at a flow loop in operation. OK… Let’s start investigating the effects of adjusting the amount of Proportional action on our flow loop.
The controller output range is 4 to 20mA representing 0 to 100%. When the process is stable, the controller output will be at some value between 4 to 20mA.
The Process Variable is also represented by a 4 to 20mA signal from the Flow transmitter. This signal represents 0 to 100% of the process variable.
Alright, now that we know how the flow loop operates, let’s move on to Proportional Control. So,… what is Proportional Action?
The controller’s job is to ensure that the Process Variable is the same as the Setpoint. The controller is always looking at the difference between the Setpoint and the Process Variable. This difference is often referred to as the Error.
Proportional Action is an Amplification of the Error. This Proportional amplification has no units and often is referred to as Gain. This amplified result is added to the current controller output.
OK. Now we have a simple math formula.
New Controller output = Previous Controller output (CO) + (Gain × Error)
Let’s look at an example of Proportional Action and how that math formula applies.
We’ll start with the pump OFF. With the controller on Manual, we’ll set the output to 50%. We’ll set the Gain to 1.
Now let’s set the controller to Auto. With the pump OFF, the PV or Process Variable will be 0.
Watch what happens when we change the Setpoint from 0% to 10%. The Setpoint change of 10% produced a 10% change in Controller output. Why? The Controller Output (CO) changed by 10% from 50% to 60%.
Earlier we said that Proportional action results in a change in controller output based on the simple math formula: Controller output = CO + (Gain × Error)
Our Setpoint changed from 0 to 10% which produces an error of 10%. The Gain is set to 1.
Because the Gain is 1, the proportional action will only add 10% to the controller output. The Controller output was at 50%, so adding the proportional response will move it to 60%.
Let’s try it again, but this time, we will increase the Gain to 2. This time the CO changed from 50% to 70% which is a 20% change. Why? Because the Gain is now set at 2, therefore the proportional action will add 20% to the controller output.
Keep in mind that the pump has been OFF for both of our experiments. With the Pump OFF, we have no possible change in the process variable therefore our Proportional Action is acting on a static condition.
Once the pump starts, we are in a dynamic environment and variables are changing with time.
Let’s turn the pump ON and watch the Proportional Action dynamic response. We start with the Process Variable at the setpoint value of 75%.
When we introduce a setpoint change to 65%, the Controller initiates Proportional Action and attempts to change the Process Variable to move to 65%. But, the Process Variable only goes to 69% and stays there!
A proportional-only controller does the job expected of it, at least until a disturbance, process variable, or setpoint change happens.
Offset in PID control
In the beginning, the Process Variable equals the Setpoint and the Error is zero. The process variable is stable. A process disturbance will cause the process variable to change and the controller will act.
Unfortunately, the process will recover back to a condition of stability, BUT NOT with the process variable at the set point.
The difference between process variable and setpoint is called Offset and occurs in Proportional-Only Controllers.
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In the next part, PID Settings | Proportional-Integral Control, we’ll show you how adding Integral (I) action will help bring the Process Variable back to the Setpoint thus eliminating the Offset.
Ok… let’s summarize what we’ve discussed…
– The Controller’s job is to ensure that the Process Variable is the same as the Setpoint.
– The controller is always looking at the difference between the Setpoint and the process Variable and this difference is often referred to as the Error.
– Proportional Action is an Amplification of the Error.
– Proportional amplification has no units and often is referred to as Gain.
– A Simple math formula for Controller output after Proportional action is: Controller output = CO + (Gain × Error)
– A Proportional Only Controller will result in an Offset condition after a disturbance or Setpoint change.
If you want to learn more, you might want to review our other articles:
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