Lecture 6 - Control Systems Flashcards
Define in one sentence a control system?
A control system is a network of devices and components that regulates and influences the behavior of a system to achieve or maintain a desired state
What is the role of sensors in a control system?
They act as the eyes and ears of the system since they feel and see what is happening
What is a controller in a control system?
A controller in a control system plays the role of the brain. When the sensor tells it a piece of information, the controller makes a decision for the future steps of the system
In a control system, what would be the role of the actuators?
These play the role of muscles, they follow whatever command is fed to them by the controller
What does PV stand for?
Process variable
What does SP stand for?
Set point
What are the two classes of control systems?
1) Open-loop systems
2) Closed-loop systems
Define an open-loop control system
Control system where the output is determined solely by the input command, dosen’t take any feedback mechanism to monitor or adjust the output based on the actual performance. (Control action is totally independent from the output)
What do open-loop control systems often get used for?
They get used for mainly simple applications since they have well defined input-output behavior
What is another term used to refer to closed-loop control systems?
Feedback control
Define a closed-loop control system
1) Output is continuously compared with the desired result
2) The control output (which is the system’s input) is modified and adjusted to reduce deviation
3) Forces the response to follow the reference
What would be an example of a simple open-loop controller?
ON-OFF controller; switches the output on and off according to a set timer
Name 3 pros to a simple open-loop controller
1) Simple and economical
2) Easy to maintain
3) Stable
Name 3 cons to a simple open-loop controller
1) Inaccurate
2) Unreliable (don’t know if the goal output is ever reached)
3) Change due to disturbance cannot be corrected (lack of feedback)
What is an example of a simple closed-loop controller?
ON-OFF controller; controls temperature while using a temperature sensor and a switch.
ON-OFF controller will switch the output when the output crosses a setpoint
Define what is a Process Variable (PV)
The system parameter that needs to be controlled, such as a temperature, pressure, flow rate, etc
Define what is a sensor
Used to measure the process variable and provide feedback to the control system
Define what is the set point (SP)
The desired or command value for the PV
Define what may be an ‘Error e’
The difference between the SP and the PV; it’s used by the control system to determine the action to get/reach that desired input.
Define the term ‘rise time’
The amount of time the system takes to go from 10-90% of the steady-state or final value (difference in time between those two values)
Define the term ‘percent overshoot’
The amount that the process variable overshoots the final value, expressed as a percentage of the final value
Define the terme ‘settling time’
The time required for the process variable to settle whithin a certain percentage (commonly 5%) of the final value
Define the term ‘steady-state error’
The final difference between the process variable and set point
What is a PID controller?
Controller which calculates an error value as the difference between SP and PV and then applies a correction based on the following three:
1) Proportional
2) Integral
3) Derivative Terms
What does Kp represent?
Kp represents the proportional constant that accounts for the present error value
Whats does Ki represent?
Integral constant that accounts for historical error values
What does Kd represent?
Derivative constant that accounts for future error values
What does SSE stand for?
SSE: Steady-state error
What is SSE?
SSE is the difference between the desired final output and the actual one
How can SSE be mitigated?
1) Adding a compensating term to the setpoint and the outpoint
2) Dynamically: adding an integral term
What does Kp account for?
Kp accounts for present error
What does Kd account for?
Kd accounts for future error
What does Ki account for?
Ki accounts for historical error value
What happens in the case where Kp is high?
Large change in output + reduces rise time
(will reduce but never rid of SSE)
What happens when Kp is too high?
Introduces oscillations; repeatedly goes above and under SP
What happens when Kp is low?
Delays system response
(reduce Kp for sensitive systems that overshoot easily)
What happens when Kd is too low?
Too overshot
What happens when Kd is too high?
Much under the SP line
What does Kd do?
1) Increases system stability
2) Reduces overshoot
3) Improves transient response
What effect does Kd have?
Has a damping effect by reducing oscillations that may have been caused by a large Kp.
What are the 3 categories of loop responses?
1) Under Damped
2) Critically Damped
3) Over Damped
Define ‘under damped’
PID tuned to prevent the oscillation of the graph but results in an overshoot of the PV and somewhat continuous oscillation which eventually settles at the Setpoint.
Define ‘critically damped’
This is the ideal situation where the PV may approximately equal to the SP; overshoot does not occur and the process responds in a non-oscillatory and controlled manner
Define ‘over damped’
The system responds in a very slow rise time manner which causes non ideal and unnecessarily slow
What is tuning in the context of PID controllers?
Tuning is the adjusting of each Kp, Ki and Kd until a desired overall response is obtained
What are the steps to designing a PID controller?
1) Obtain an open-loop response and determine what needs to be improved
2) Add proportional control to improve rise time
3) Add derivative control to improve overshoot
4) Add an integral control to eliminate SSE
5) Adjust/tune all the parameters until one obtains the desired overall response
What is the effect of the proportional control over the 4 main elements?
Rise Time: Decreases
Overshoot: Increases
Settline Time: Small change
SSE: Decreases
What is the effect of derivative control over the 4 main elements?
Rise Time: Small change
Overshoot: Decreases
Settline Time: Decreases
SSE: Small change
What is the effect of integral control over the 4 main elements?
Rise Time: Decreases
Overshoot: Increases
Settline Time: Increases
SSE: Eliminate
