controller terminology

Question 1

Error

Answer 1

algebraic difference between setpoint and process variable

Question 2

Direct acting

Answer 2

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

Question 3

Reverse acting

Answer 3

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

Question 4

Set Point (SP)(r)

Answer 4

The desired quantity, property, or condition of the process

Question 5

Process Variable (PV)(b)

Answer 5

The measured quantity, property, or condition of the process

Question 6

Output (O/P) (CO)(Mc)

Answer 6

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

Question 7

Manual Control (open loop)

Answer 7

Operator controls FCE (valve) directly, no feedback

Question 8

Automatic Control (closed loop)

Answer 8

Controller controls FCE (valve), based upon the feedback (PV input signal) being compared to the set point, and some combination of P, I, & D algorithms

Question 9

Local Setpoint

Answer 9

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. 
  Change in output/change in input

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

Proportional Action

Answer 13

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 x gain). The correction is brief and results in some remaining error, called “offset”. Mc = Kce = Mo

Question 14

Offset

Answer 14

The remaining error after a Proportional control correction. Inherent in P-only control

Question 15

Integral Action (reset)

Answer 15

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 units for Integral 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 setting to repeat the amount of output change caused by the Proportional correction.

Question 16

Derivative action (rate)

Answer 16

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 17

Control Loop Tuning

Answer 17

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 18

P-Only Control

Answer 18

where only proportional action is used in the PID block

Question 19

PI Control

Answer 19

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

Question 20

PD Control

Answer 20

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

Question 21

PID Control

Answer 21

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

Question 22

Reset Windup

Answer 22

If the error doesn’t correct the output will continue driving Up or down to saturation value(0or 100 %). Reset windup occurs commonly during a process Shutdown.

Question 23

Setpoint tracking

Answer 23

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 24

Feedback

Answer 24

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

Question 25

Final Control Element

Answer 25

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

Question 26

Load Change

Answer 26

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

Question 27

Alarm function

Answer 27

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

Question 28

Function Block

Answer 28

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