The open loop gain dictates the response of a negative feedback loop.
The magnitude of the open loop gain is the product of the gains of each of the elements in the loop and should be a dimensionless number.
Thus, the value of G dictates the response.
The subsequent pages of this Work Aid have a summary of all the responses possible due to a step-load change or a step set point change.
NOTE: The only acceptable responses are C1, C2, C3 and CQAD.
Constant amplitude cycling C4 or increasing oscillation C5 are not acceptable.
C1 OVERDAMPED RESPONSE
C2 CRITICALLY DAMPED RESPONSE ½Æ
GL ½ < 0.5
C3 UNDERDAMPED RESPONSE
CQAD QUARTER AMPLITUDE DECAY
*C UNIFORM OSCILLATION
Æ
OVERDAMPED
STEP - LOAD UPSETS
qI
r1
c1
c2
c3
c4
c5
r2 r1 r2 r1
r1 r2
r2 r1 r2 r1 r2
STEP SET POINT CHANGES
GLOSSARY
cascade control Control in which the output of one controller is the set point for another controller.
closed loop Several automatic control units and the process connected so as to provide a signal path that includes a forward path, a feedback path, and a summing point. The controlled variable is consistently measured, and if it deviates from that which has been prescribed, corrective action is applied to the final element in such direction as to return the controlled variable to the desired value.
control loop Starts at the process in the form of a measurement or variable, is monitored, and returns to the process in the form of a manipulated variable or "valve position" being controlled by some means.
controlled variable In a control loop, the variable the value of which is sensed to originate a feedback signal.
controller A device which operates automatically to regulate a controlled variable.
controller algorithm (PID)
A mathematical representation of the control action to be performed.
DCS Acronym for Distributed Control System.
DDC Acronym for Direct Digital Control wherein a computer performs all the functions of error detection and controller action.
error In process instrumentation, the algebraic difference between the indication and the ideal value of the measured signal. It is the quantity which algebraically subtracted from the indication gives the ideal value.
feedback control Control in which a measured variable is compared to its desired value to produce an actuating error signal which is acted upon in such a way as to reduce the magnitude of the error.
feedforward control Control in which information concerning one or more conditions that can disturb the controlled variable is converted, outside of any feedback loop, into corrective action to minimize deviations of the controlled variable.
final actuator (or final controlling element)
The forward controlling element which directly changes the value of the manipulated variable.
gain (magnitude ratio) The ratio of change in output divided by the change in input that caused it. Both output and input must be expressed in the same units, making gain a pure (dimensionless) number.
input to process Mass or energy applied to the process.
instrument In process measurement and control, this term is used broadly to describe any device that performs a measuring or controlling function.
instrumentation The application of instruments to an industrial process for the purpose of measuring or controlling its activity. The term is also applied to the instrument themselves.
intrinsically safe equipment and wiring
Equipment and wiring which are incapable of releasing sufficient electrical or thermal energy under normal or abnormal conditions to cause ignition of a specific hazardous atmospheric mixture in its most easily ignited concentration.
loop gain The combined output/input magnitude ratios of all the individual loop components multiplied to obtain the overall gain.
manipulated variable A quantity or condition which is varied as a function of the actuating error signal so as to change the value of the directly controlled variable.
margin gain The sinusoidal frequency at which the output/input magnitude ratio equals unity and feedback achieves a phase angle of -180 degrees.
open loop Control without feedback; for example, an automatic sprinkler or an automatic washing machine.
output of process A product related signal delivered by the process.
overdamped Damped so that overshoot cannot occur.
overshoot The persistent effort of the control system to reach the desired level, which frequently results in going beyond (overshooting) the mark.
phase The condition of a periodic function with respect to a reference time.
phase difference The time, usually expressed in degrees, by which one wave leads to or lags another.
primary element The system element that quantitatively converts the measured variable energy into a from suitable for measurement.
process Is defined as a physical or chemical change of matter or conversion to energy; e.g., change in pressure, temperature, speed, electrical potential, etc. A second definition of a process from a Chemical Engineering point
process control The regulation or manipulation of variables influencing the conduct of a process in such a way as to obtain a product of desired quality and quantity in an efficient manner.
reaction curve In process control, a reaction curve is obtained by applying a step change (either in load or set point) and plotting the response of the controlled variable with respect to time.
response Reaction to a forcing function applied to the input; the variation in measured variables that occurs as the result of step sinusoidal, ramp, or other kind of input.
set point An input variable which sets the desired value of the controlled variable.
stability That desirable condition in which input and output are in balance and will remain so unless subjected to an external stimulus.
static gain (zero-frequency gain)
The output/input amplitude ratio of a component or system as frequency approaches zero.
steady state A state in which static conditions prevail and all dynamic changes may be assumed completed.
step change A change from one level to another (ideally in zero time.) supply Mass or energy input to process.
transmitter A transducer which responds to a measured variable by means of a sensing element, and converts it to a standardized transmission signal which is a function only of the measured variable.