Controller Terms and Definitions

Question 1

Controller

Answer 1

a device which operates automatically to regulate a controlled variable

Question 2

Error

Answer 2

algebraic difference between setpoint and process variable

Question 3

Direct acting

Answer 3

increase in signal results in an increase in output (for error = b-r)

Question 4

Reverse acting

Answer 4

increase in signal results in a decrease in output (for error = r-b)

Question 5

Set Point

Answer 5

The desired quantity, property, or condition of the process.

Question 6

Output

Answer 6

The signal from the loop controller that manipulates the final control element.

Question 7

Process Variable

Answer 7

The measured quantity, property, or condition of the process.

Question 8

Manual Control (open loop)

Answer 8

Operator controls FCE (valve) directly, no feedback.

Question 9

Automatic Control (closed loop)

Answer 9

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.

Question 10

Local Setpoint:

Answer 10

Set point is set at, or on the controller by the operator

Question 10

Remote setpoint:

Answer 10

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.

Question 11

Gain

Answer 11

the amplification (or de-amplification) of the input signal to result in the output signal.

Question 12

Proportional Band (%PB):

Answer 12

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

Question 13

Offset

Answer 13

The remaining (steady state) error after a Proportional control algorithm’s correction. Inherent in P-only control

Question 14

Final Control Element

Answer 14

a device that receives the output value from a controller to manipulate or regulate material or energy into or out of a process

Question 15

Load Change

Answer 15

Disturbance in the process that causes the controlled variable to deviate from setpoint

Question 16

Alarm function

Answer 16

the process of monitoring state of condition of a variable and comparing it to a pre-set condition or value

Question 17

Function Block

Answer 17

pre-built processing units tied together by software used to build a process controller used to control a process

Question 18

Integral Action (reset)

Answer 18

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.

Question 19

Proportional Action

Answer 19

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

Question 20

Derivative action (rate):

Answer 20

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).

Question 21

Control Loop Tuning

Answer 21

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.

Question 22

P-Only Control

Answer 22

where only proportional action is used in the PID block

Question 23

PI Control

Answer 23

Where Proportional and Integral tuning parameters are used in a control loop

Question 24

PD Control

Answer 24

Where Proportional and Derivative tuning parameters are used in a control loop

Question 25

PID Control

Answer 25

Where Proportional, Integral and Derivative tuning parameters are used in a control loop

Question 26

Reset Windup

Answer 26

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.

Question 27

Setpoint tracking

Answer 27

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

Question 28

Feedback:

Answer 28

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