Control loops are fundamental components in the field of instrumentation, playing a crucial role in maintaining and regulating processes. In instrumentation, a control loop consists of sensors, controllers, and actuators working together to achieve a desired output or setpoint.
Primary Element
The primary component of a control loop is the sensor. This is the primary element. The sensor may measure level, temperature, pressure, or many other variables. What’s important is that the value it reads is indicative of the process being controlled. We call this value the primary variable..
For example, let’s take a look at the cruise control on your vehicle. A speed sensor is what detects the speed at which your vehicle is traveling. The vehicle speed measured in KM/Hr would be the primary variable and the speed sensor would be the primary element.
Let’s say that we have our cruise control set to travel at 100 KM/Hr but our vehicle is currently driving up a hill, causing us to travel at about 85 KM/Hr.
How will our vehicle know that it needs to speed up?
Control Element
The primary element would transmit a signal to the control element. The control element expects the signal to match the value we had set our cruise control to. We refer to this value as the setpoint. The main goal of the control element is to have the setpoint be as close to the primary variable as possible. When there is a difference between the setpoint and the primary variable, the control element will create a control signal based on the difference.
Now our vehicle is aware that it is traveling below the setpoint, but how does it increase the speed?
Final Element
The control signal travels to the final element. The final element is usually an actuator that is manipulated to adjust the primary variable. In this example, the final element would be the throttle actuator. Since we are traveling below our setpoint then the control signal would tell the throttle actuator to increase the speed.
Now let’s say that our vehicle sped up and is now going 105 KM/Hr. We are now traveling faster than our setpoint which will cause our control element to send a signal that would cause the throttle to decrease.
These three elements (Primary, Control, and Final) make up what is called a closed-loop circuit. A closed loop circuit gathers an Input(Primary Variable) and sends it to a controller(Control Element) that compares it to a setpoint and develops an error signal(Control Signal).
An actuator (Final Element) receives the error signal, and it changes in proportion to the signal, subsequently adjusting it accordingly.
-Image of our closed-loop cruise control circuit.
The details on how the control element determines the control signal to send to the final element is part of a more advanced topic called PID tuning which will be covered in a later post.
Additionally please check out our other intro article on Analog Signals. Finally, I’d like to recommend Chapter 2 of the Instrumentation and Process Control Textbook.
