Electronic PID Controllers

A controller is a comparative device that receives an input signal from a measured process variable, compares this value with that of a predetermined control point value (set point), and determines the appropriate amount of output signal required by the final control element to provide corrective action within a control loop. (T-length) is the measurement between the instrument connection and process connection of the thermowell.

An electronic sensor (thermocouple, RTD or transmitter) installed at the measurement location continuously sends an input signal to the controller. At set intervals, the controller compares this signal to a predefined set point. If the input signal deviates from the set point, the controller sends a corrective electric output signal to the control element. This electric signal must be converted to a pneumatic signal when used with an air operated valve, such as a Watson McDaniel HB Series Control Valve. The conversion can be made using a Watson McDaniel TA901 I/P Transducer, which converts a 4 to 20 mA electric signal to a 3 to 15 PSI air signal. As an option, a Valve Positioner such as the Watson McDaniel CA2000 may be used to send an air signal to the Control Valve. These Positioners can be controlled with a 3-15 psi air signal from a Pneumatic Controller or a 4-20 mA signal from a PID Controller.

I&M (pdf)

Description

Electronic PID Controllers are designed for use on applications where large load changes are expected. Or where extreme accuracy and fast response times are needed.

PID Control is a feature of Watson McDaniel TR890 Electronic Controllers. PID combines the proportional, integral and derivative functions into a single unit.

Proportional (P) — Proportional controls react to the size of the deviation from set point when sending a corrective signal. The size of the corrective signal can be adjusted in relation to the size of the error. Just change the width of the proportional band. A narrow proportional band will cause a large corrective action in relation to a given amount of error, while a wider proportional band will a cause smaller corrective action in relation to the same amount of error.
Integral (I) — Integral control reacts to the length of time that the deviation from set point exists when sending a corrective signal. The longer the error exists, the greater the corrective signal.
Derivative (D) — Derivative control reacts to the speed in which the deviation is changing. The corrective signal will be proportional to the rate of change within the process.
Auto-Tuning will automatically select the optimum values for P, I and D, thus eliminating the need for the user to calculate and program these values at system startup. This feature can be overridden when so desired. On some models, the control element can be manually operated.