The above POU body represents the standard control loop. The control input is determined by the user (e.g. desired room temperature of 22°C). After the A/D conversion the set point value (SP) is entered as one input value for the PID processing instruction. The measured process value (PV) (e.g. current room temperature) is normally transmitted via a sensor and entered as another input value for the PID processor. F355_PID calculates the standard tolerance e (error value) from the set point value and the process value (e = set value - measured process value). With the parameters given (proportional gain Kp, integral time Ti, ...) a new output value (manipulated value MV) is calculated in increments set by the sampling time Ts. This result is then applied to the actuator (e.g. a fan that regulates room temperature) after the D/A conversion. The analog section represents the system’s actuator, e.g. heater and temperature regulation of a room.
Proportional part (P part)
A proportional part generates an output that is proportional to the error value. The proportional gain Kp determines by how much the manipulated value is increased or decreased. A proportional part can be a simple electric resistor or a linear amplifier, for example.
The P part displays a relatively large maximum overshot, a long settling time and a constant standard tolerance.
Integral part (I part)
An integral part produces an output quantity that corresponds to the time integral and input quantity (area of the input quantity). The integral time thus evaluates the output quantity MVi.The integral part can be a quantity scale of a tank that is filled by a volume flow, for example. Because of the slow reaction time of the integral part, it has a larger maximum overshot than the P component, but no remaining constant error value.
Example:
Input quantity e and the output quantity MVi produced.
Derivative part (D part)
The derivative part produces an output quantity that corresponds to the time derivation of the error value. The derivative time corresponds to the weighting of the derived input quantity. A derivative component can be an RC-bleeder (capacitor hooked up in series and resistance in parallel), for example.
Example:
Input quantity e and the output quantity MVd produced.
PID controller
A PID controller is a combination of a P component, an I component and a D component. When the parameters Kp, Ti and Td are optimally adjusted, a PID controller can quickly control and maintain a quantity at a predetermined set value.
The following equations are used to calculate the controller output MV under the following conditions:
In general:The output value at time period n is calculated from the previous output value (n-1) and the change in the output value in this time interval.
Reverse operation PI-D Control = 16#X000
Forward operation PI-D Control = 16#X001
Reverse operation I-PD Control = 16#X002
Forward operation I-PD Control = 16#X003
