Control and regulate
In engineering, quantities can be both controlled and regulated.
When controlling, a quantity is manipulated without being able to compensate for outside influences. When regulating, a quantity is maintained at a specific value in order to compensate for outside influences.
In the following example, controlling means that the person can set the heat output at a fixed value. The heater cannot compensate for the drop in room temperature when a window is opened.
In the example below, regulating means that the person can increase the heat output if the room temperature drops to below 20 °C. If the room temperature rises above 20 °C, the heat output is reduced.
Basic concepts of regulating
In the example, the current for the electric heating is the manipulated variable. The changeable resistance is the actuator. The hand that operates the actuator is the control. The actual room temperature is the controlled variable or the process value. The desired room temperature is the command variable or the setpoint value. The electric heating is the control process. The thermometer is the sensor. The temperature loss from opening the window is the disturbance variable.
So this means that the person measures the process value (room temperature) with the sensor (thermometer), compares the process value (room temperature) with the command variable (desired room temperature) and uses the actuator (changeable resistance) to manually regulate the manipulated variable (heating current), in order to compensate for the disturbance variable (temperature drop from opening the window). The person is therefore the controller.
The control device is formed from the actuator and the control.
The control and controller together form the regulating device.
The following picture gives an abstract portrayal of the situation described above.
The comparing element uses the sensor to compare the command variable with the process value. If the command variables and process value deviate from one another, this results in a positive or negative loop error that in turn changes the process value.
Control loop
The process value x influences the manipulated variable M by means of the regulating device. This creates a closed circuit that is also known as a control loop.
If, in the example above, the window is opened, the temperature in the room drops. The person must increase the heat output of the heater. If the heat output is increased too much, it will get too hot. The person must then reduce the heat output.
If the heat output is increased or reduced too quickly, then the control loop starts to sway. The room temperature fluctuates. It is either too hot or too cold. To prevent this, the person must carefully and slowly reduce or increase the heat output.
Loop error
The loop error is the difference between the command variable and the process value. In other words: the deviation of a process value from a set value.
e = SP – PV
The loop error e brings about a change to the manipulated variable M.
The example above illustrates this very well: if, with a desired temperature of 20 °C (= command value w), the room temperature is 22 °C (= process value PV), this results in the loop error:
e = SP – PV = 20 °C - 22 °C = -2 °C
In this case, the negative sign indicates a reversing action: the heat output is reduced.
In a control loopstate of equilibrium, the loop error is zero or very small. If the command variable changes or there is a disturbance, a loop error arises. The loop error is corrected by means of the manipulated variable M.
Description of the individual parameters