Pnuematic Controllers Flashcards
What are the two types of controllers
Pneumatic Controller 1:On-off Controller
Pneumatic Controller 2: Proportional Controller
Define a controller
A comparative device that receives, an input signal from a measurement device about a measured process variable and compares this value to a to a set point (predetermined control point value) and then determines the appropriate amount of input signal to send the FCE to provide corrective action within the loop
What is a pneumatic controller?
Mechanical device designed to measure temperature or pressure, then mechanically compare this info aka the input signal against the setpoint to determine the appropriate correctove air signal and transmits the corrective air signal to the final control element which is pneumatic control valve
What does the pneumatic controller operate through?
Coordination of it or pressure sentence system, and it’s air signal release system
What are the main components of an electrical transmitter
MEA
Mechanical Sensing device
Electrical transducer
Amplifier With feedback
Draw the electrical transmitter block diagram
….
Explain the electrical transmitter diagram
The input causes a change in the mechanical sense device, which generates a mechanical signal that is sent to the electrical transducer to be converted to an electrical signal.
Which is then applied to the amplifier, which increases the signal, aka output. There is also feedback between the amplifier and electrical transducer
Give examples of the mechanical sensing device in an electrical transmitter
Bourdon tube
Bellows
Diaphragm with linkages
What happens in the mechanical sensing device of the electrical transmitter
A force or potential is created when the process variable changes in value
What can the electrical transducer be
It can be a variable resistance, inductance or capacitance
When does the electrical property change?
In response to variations in the mechanical input
What is the purpose of the amplifier with feedback
To amplify the electrical change produced by the electrical transducer and to give feed back its output to electrical transducer to create an opposing force to the input
What is an On/Off control
Control that only recognizes that deviation exists and has only 2 extremes, i.e., 100% on or 100% off
What will the deviation between the setpoint and the measured pv cause in I/0 control
It will cause a full corrective signal to be produced
What are examples of an on off controller
A home hot water heater
Thermostat controlled air furnace
Compressed air to a furnace tank
Describe the operation of the On off controller
Using a home water heater. When the temperature of the water in the tank falls below the set point, the burner turns on until the water temp meets the setpoint, and then it turns off again It is only a matter of time before the water begins to cool again and the cycle repeats itself
What does the discrete controller hold the variable
It does not hold the variable at the set but keeps the variable within the proximity of the setpoint in the dead zone
What do the tuning parameters essentially determine the
The magnitude of the correction
The duration of the adjustment
The speed at which the adjustment is made
How much correction should be made, and what is it determined by?
The magnitude of the correction( the changed in the controller output) determined by the proportional mode
How long should the correction should be applied
The duration of the error determined by integral
How fast Should the correction be applied
The Speed at which the correction is made is determined by the derivative mode
What does proportional control react to when sending a corrective signal
The size of the deviation from the set point
How is the corrective signal adjusted the relation to the size of the error
The size of the corrective signal is adjusted to the size of the Error by changing the width of the proportional band
A narrow proportional band will cause
A large Corrective action in relation to the given amount of error
A wider proportional band will cause
A smaller corrective action in relation to the same amount of error
What is the proportional band express in terms of? Give its formula
The percentage change in error that will cause 100% change in the controller output
PB= (Δ input% divided by Δ output % ) x 100=100/gain
What is proportional gain expressed as?
The percentage change in output divided by the percentage change in input
What is the gain formula?
Gain = Δ input% /Δ output %
Gain = 100/PB
What is proportional control?
This type of action is where the controller produces an output signal, which is proportional to the error
What is the proportional control action represented by?
K e
Where k is a constant and E is the error
When is integral mode added?
If offset cannot be tolerated in the proportional only control
What does PI stand for?
Proportional plus integral control
What does the integral action integrate and what does it cause?
It integrates any difference between the measurements and it’s set point and causes the controller to change until the difference between the measurement and set point is zero
What is the integral action represented by?
K/R ∫ edt
What is the purpose of the integral mode and how does it achieve this?
To return the process variable to the set point and it does despite repeating the action of proportional mode as long as the error exist
What is the advantage and disadvantage of integral mood?
The advantages it eliminates errors
Reset wind up and possible overshoot
What is another name for integral action?
Reset action
What is another name for proportional action?
Offset action
How is the PID controller accomplished
I added derivative adjustment to the PI controller
What does the derivative action give?
An output which is proportionate to the derivative or rate of change of error
What is another name for the derivative action?
Rate control
What’s mathematical expression is the derivative action represented by?
KD de/dt
What kind of function is the rate action
Rate of change of the error
What action is the of rate action?
This action is to apply immediate response that is equal to the proportional plus reset action that would have occur number of minutes in the future
What is the rate action unit?
Minutes
What is the advantage and disadvantage of the derivative action?
Rapid outputs reduces the time that is required to return the PV to the SV in a slow process
Dramatically amplifies, noisy signals
What is a major limitation of the flopper amplifier
It has a limited a handling capacity
What is another name for the pneumatic amplifier?
Air relay
