Controlled variable
the variable that we want to control / keep constant. it has a set point
Manipulated variable
a variable that we change to maintain the controlled variable at the desired set point
Disturbance variable
a variable that affects the output but is outside the system’s control
(basically everything not controlled by a valve or similar)
Define regulator control
adjusting a manipulated variable to MAINTAIN a controlled variable at a desired value
ie. aims to minimise the effect of a disturbance on a specified process
Define servo control
adjusting a manipulated variable to CHANGE a controlled variable to a NEW desired value
ie. aims to impose a change on the system
Define feedback control
a control strategy where a measured output is compared to a desired set point, and an action is taken to reduce the error.
define error in feedback control
epsilon e = T(sp) - T(m)
the difference between the set point and the measured value
here, using T for temperature
describe two setbacks of feedback control
an error must occur in order for any control action to be made
feedback loops can introduce instability to an otherwise stable process
name the elements of a simple feedback control loop
a sensor
a controller
a final element (often a control valve)
role of a sensor
measure a controlled variable
role of a controller
measures the deviation of an output variable from its set value, and adjusts an input variable to make a correction
define feedforward control
a proactive control method that measures the DISTURBANCE (rather than the output) and uses a model to calculate the necessary control input
name / describe the three control objectives
Safety - often layers in addition to control systems
Operability - make the plant do what the operator wants
Profitability - strategic control level - achieving desired production rate, quality and efficiency.
if there are n controlled variables, how meany manipulated variables are required to control the system?
at least n manipulated variables
