The devices that measure and act on the process variables are of fundamental importance in implementing a control system. Their values are used by controllers, together with the reference values that are inputted, to generate the control signals. This is outlined in the following diagram, where we have the following:

• Set point (SP): This is the reference and the desired value for a specific process variable
• Controlled value (CV): This is the calculated value for the control variable to fit the reference value
• Process value (PV): This is the measurement of the process value that comes from the process:

Let's think about a very simple example. When you turn on your oven, the SP is the temperature that you want to set. The CV is the value of the electric current that is feeding the heating elements of the oven, and the PV is the real temperature of the oven that is measured by the sensor.