Controller Terms and Definitions Flashcards
Controller
a device which operates automatically to regulate a controlled variable
Error
algebraic difference between setpoint and process variable
Direct acting
increase in signal results in an increase in output (for error = b-r)
Reverse acting
increase in signal results in a decrease in output (for error = r-b)
Set Point
The desired quantity, property, or condition of the process.
Output
The signal from the loop controller that manipulates the final control element.
Process Variable
The measured quantity, property, or condition of the process.
Manual Control (open loop)
Operator controls FCE (valve) directly, no feedback.
Automatic Control (closed loop)
Controller controls FCE (valve), based upon the feedback (PV input signal) being compared to the set point, and some combination of ON-OFF, P, I, & D algorithms.
Local Setpoint:
Set point is set at, or on the controller by the operator
Remote setpoint:
Set point is adjusted from a location away from the controller. This could be at a remote auto/man (set point/output) station in the field, or from the primary controller output to the secondary controller set point in a cascade loop.
Gain
the amplification (or de-amplification) of the input signal to result in the output signal.
Proportional Band (%PB):
Similar, but inversely related to gain. Defined as the % of input signal change required, resulting in a 100% change in output signal.
PB = 100%/Kc Kc = 100%/PB
Offset
The remaining (steady state) error after a Proportional control algorithm’s correction. Inherent in P-only control
Final Control Element
a device that receives the output value from a controller to manipulate or regulate material or energy into or out of a process
Load Change
Disturbance in the process that causes the controlled variable to deviate from setpoint
Alarm function
the process of monitoring state of condition of a variable and comparing it to a pre-set condition or value
Function Block
pre-built processing units tied together by software used to build a process controller used to control a process
Integral Action (reset)
A mode of control used after the Proportional correction that eliminates the offset over a period of time.
The main purpose of Integral is to eliminate offset for very precise control. The gain units for Integral mode can be either “minutes per repeat” (MPR) or “repeats per minute” (RPM), depending upon the device manufacturers preference. The “repeats” refer to the time it would take for the particular Integral mode setting to repeat the amount of output change caused by the Proportional mode corrective action.
Proportional Action
A change in controller output that is proportional to the size of the difference between the SP and the PV (error), and based upon a multiplication factor (“gain” or “proportional band” setting). (O/P = error * gain). The correction is brief and results in some remaining error, called “offset”. mc = Kce + mo
Derivative action (rate):
A mode of control that only reacts to changing error.
The derivative action occurs first, to make a very quick, radical correction. It is followed by the Proportional correction and finally the Integral correction (if used). Derivative raises the gain very high (temporarily) to cause extreme valve movement, much like On/Off control. Derivative can be used to increase response in slow processes like temperature and level, but is usually avoided in fast reacting processes like pressure and especially flow. The units of Derivative are in “minutes” (or fractions of minutes).
Control Loop Tuning
The specific settings for Proportional (gain or %PB), Integral (m/r or r/m) and Derivative (minutes) must be adjusted to optimum values for proper control. If the settings are too low, sluggish response results and it takes too long for the PV to be brought back to the SP. If the settings are too high the loop becomes too sensitive and it over-reacts to errors. This can cause excessive cycling (oscillations of the PV around the SP), or even total loss of control.
P-Only Control
where only proportional action is used in the PID block
PI Control
Where Proportional and Integral tuning parameters are used in a control loop
PD Control
Where Proportional and Derivative tuning parameters are used in a control loop
PID Control
Where Proportional, Integral and Derivative tuning parameters are used in a control loop
Reset Windup
In case of a prolonged deviation of the PV from the SP, the integral mode keeps integrating the error until the output signal reaches saturation (0% or 100%). Reset windup occurs commonly during a process Shutdown or an interruption of the feedback signal.
Setpoint tracking
technique of having the process variable become the controller setpoint when controller is in manual control mode to prevent large process bump when returning controller to automatic mode also called bumpless transfer
Feedback:
control design where any change in the process variable is communicated to the controller to be compared with the desired variable to allow the controller to respond to the process change by adjusting the controllers output
