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PID Settings | Proportional-Only Control
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⌚Timestamps:
00:00 - Intro
00:50 - A typical flow control loop
01:53 - Proportional-Only control
03:14 - Examples
05:41 - Offset in PID control
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In this video, 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 video, we’re going to explain Offset and show you how and when it occurs.
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.
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.
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.
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.
We’ll set the controller to Auto. With the pump OFF, the PV or Process Variable will be 0.
The Setpoint change of 10% produced a 10% change in Controller output. Why? The Controller Output (CO) changed by 10% from 50% to 60%.
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%.
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.
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.
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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