Purpose
The PID block maintains balance in a closed loop by changing the controlled variable (an analog output) in response to deviations from a user-defined set point. The difference between the actual value (an analog input) and the set point value is the error, or deviation.
In response to errors, the PID block calculates an appropriate control output signal, which attempts to reduce the error to zero. The adjustment that the PID block makes is a function of the difference between the set point and the measurement, in addition to the values of the proportional band, the reset, and the rate.
Features
The PID block:
- Is an optional control block.
- Is a secondary block.
- Ultimately receives its value from an upstream Analog Input block even though secondary blocks can intervene between the AI and the PID block in the chain.
- Bases its control period on the scan time selected for the upstream Analog Input block.
- Can send output to another control block or to an Analog Output block.
- Can have its set point value changed by:
- The output of a master PID block (cascaded set point).
- Operator entry into a Data link that references a Target Value field.
- A SETTARG Program block command (in the Batch option) or a script.
- A change in the value of an Analog Input block that is specified as the PID block's set point.
- A change in the value of an Analog Input block connected to a Fanout block. The Fanout block would have the PID block's name listed in its Destination field.
NOTE: When using a Fanout block in this manner, you must append the field F_TV1 to the PID block's name.
- Maintains the balance in the process loop when operating in Automatic mode.
- Suspends updates to the PID algorithm when operating in Manual mode. You can change block parameters, such as the Set Point, Tuning Parameters, and Engineering Units Output from the keyboard in Manual mode.
