310202cA PID Control Part A Flashcards
What is the ISA definition for a control algorithm?
A mathematical representation of the control action to be performed
What is a transient response?
The behavior of the controlled variable during the transition between 2 steady states.
List 3 types of damped transient responses
- Underdamped - system oscillates around its final steady state value before settling at that value
- Overdamped - system resets without overshooting the final steady state value
- Critically damped - system sits between an underdamped and overdamped condition
What is an underdamped system?
An underdamped system oscillates around its final steady state value before settling at that value
What is an overdamped system?
An overdamped system resets without overshooting the final steady state value.
What is a critically damped system?
A critically damp system sits between an underdamped and overdamped condition
It is the fastest response to the final state state value without an overshoot
There is only one critically damped response for a given control system
What is a marginally stable response? And what makes it marginally stable?
A marginally stable response is also called sustained oscillations.
The oscillations have the same amplitude and period
It is stable because it is not growing in amplitude, but unstable. Because it never settles to a finally steady state value.
Describe a critically damped response and explain what is required to differentiate it from an overdamped response
A critically damp response is the fastest response possible without an overshoot of the final steady state value.
Extensive testing is necessary to determine the critical damped response from an overdamped response
What are 2 parameters used to differentiate one underdamped response from another?
- How long it takes to reach a steady state
- Decay ratio; the ratio of the amplitude of the first 2 successive overshoots.
Proportional action changes the controller output (CO) by the ______ multiplied by the _______________
Proportional action changes the controller output (CO) by the error multiplied by the proportional gain
What is the relationship between proportional gain and proportional band?
The smaller the proportional band setting, the higher the gain
The higher the proportional band setting, the smaller the gain
Define offset
Offset is the deviation of the controlled variable (CV) from the set point (SP) after a load change, it allows the controller output (CO) to change
The gain setting of a proportional only controller affects how much offset occurs after a load change
The higher the gain setting, the smaller the offset is after a load change for a proportional only controller
In a proportional only controller, what setting affects how much offset occurs after a load change?
The gain setting
The higher the gain setting, the smaller the offset is after a load change
How do you solve for error in a direct acting controller?
e = PV - SP
How do you solve for error in a reverse acting controller?
e = SP - PV
A proportional controller (P) responds to what aspects of an error signal?
- The presence of an error signal (the difference between the PV and the SP)
- The direction of the error signal (positive or negative)
- The magnitude of the error signal
A proportional integral controller (PI) responds to what four aspects of the error signal
- The presence of an error signal (the difference between the PV and the SP)
- The direction of the error signal (Positive or negative)
- The magnitude of the error signal
- The duration of the error signal
How do you adjust the integral response time of a proportional integral controller (PI)
By changing the controller reset (integral gain)
How do you change a proportional integral pneumatic controller into a proportional only controller?
- Put the controller in manual. The manually adjusted output pressure is now connected to the integral gain valve
- Adjust the controller output to 50%
- Fully open the integral gain valve
- Wait one minute and then fully close the integral gain valve
When the integral gain valve is closed, reset action stops because the bias value of the controller can no longer be changed. This turns the PI controller into a proportional only controller. The pressure trapprd in the bias bellows becomes the bias setting
How do you calculate proportional action?
Proportional action = (Gain)(error)
Since the algorithm for proportional action uses gain (Kc) instead of proportional band (PB), the PB setting needs to be converted to a Kc setting, how do you do this?
How do you convert a reset or integral time setting into reset or integral action?
How do you adjust the controller bias setting?
Adjust the thumb wheel labeled COo
If the system is at a steady state (SP = PV) how does the right side react if the SP is stepped up?
The lever moves away from the nozzle. The relay output pressure decreases and therefore the pressure in the feedback bellows decreases until a new force balance is reached at a lower controller output.
For a systen to remain at steady state, what forced must be balanced?
The force generated by the feedback bellows (FB) must equal to force generated by the controller bias (COo) adjustment
Which direction should the gain pivot be moved to increase the controller proportional gain?
To the right towards the feedback bellows
What is required for the output of a proportional only controller and automatic to change from its bias setting?
There must be an error between PV and SP
If a proportional only controller has an offset normal conditions, what action is necessary to eliminate the offset?
You must adjust the bias setting until there is no more offset
