mne601 tech presentation 1

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MnE 602, Fall 2004

Technical Presentation

By David Basile

PID Control - An Overview



What is PID Control?

• PID Control is a method of automaticallycontrolling a manufacturing or process parameter to a desired setpoint (SP).

• PID Control is a Closed Loop control method.Meaning that feedback from the process is utilizedin order to gauge how close you are to where youwant to be.



Components of PID Control

• Three things are needed in order to apply PID controlto an application:• Sensor / Feedback - Provides the controller with the

current value of what you are trying to measure. This

measurement is typically referred to as feedback or processvariable (PV).

• Controller (PLC) - Looks at where you are currently at andsends a signal to the output in order to move the processcloser to the setpoint.

• Output - Changes the magnitude of the variable we aremeasuring. Sometimes referred to as Control Variable (CV)



Closed Loop PID Control

Output or Control Variable (CV)

Examples:

Encoder (Position)

Tachometer (Velocity)

Thermometer (Temperature)

PLC

Motor

Valve

Ou

tp

uts

Inp

utsPID Algorithm

Sensor Input or Process Variable (PV)

Affects Value Of



Error and Disturbances

• As you drive, disturbances affect the speed of the car.(Wind Resistance, Friction, Inclines etc.)

• This might slow you down which creates an error inyour driving speed. Now you are moving 50mph.

• Error (e) is the difference between the setpoint(55mph) and and the process variable (PV,speedometer reading of 50 mph) that we are attemptingto hold.

e = SP-PV = 5 mph



PID - What does it stand for?

• PID Stands for the following:

• P - Proportional Control

• I - Integral Control

•D - Derivative Control

• These Three Components combine to provide control at

the desired setpoint.



Proportional Control

• The Output (CV) is proportional to theerror.

CV(t) = k · e(t)

k is a constant of proportionality referred to as Proportional Gain. It isessentially the slope of the output function.

• This is very intuitive. To correct a large

error in your speed, you would press downthe pedal a large percentage of the way to

the floor.



Integral Control

•Proportional Control is effective, but because the load / disturbances are always changing our

proportional equation alone cannot compensate and so a steady state error or offset will remain.(We must stay on the defined line according to P only control)

CV(t) = k · e(t)

Offset

k =1

e(t)

CV(t)

Setpoint



Integral Control

• To eliminate this offset, the integral control doesexactly what is says. It integrates the error over time. It is basically like moving the ProportionalLine up and down the axis (changing the y-intercept of the line)

CV(t) = Kpe(t) + Ki ∫ e(t) dt where Ki is the reset constant

• This would be like trying to catch up to the car ahead of you. Over time if you did not see the car getting closer you would start to increase

the pressure on the pedal. The longer you waited without gaining onhim the harder you would press down. So the integral looks at howlong the error has existed and contributes based on that factor.

Proportional Term Integral Term



Derivative Control• One additional tool might speed up the time it takes to get

to the desired setpoint. In the case of Derivative action,how quickly the error is changing allows us to react moreeffectively.

• Derivative Action adds another contributing term to the

output that is proportional to the rate of change of theerror.

• Might look at this as if one of the tires blew, your speedwould drop rapidly. In order to get back to 55 mph youwould need to press down even further on the gas pedal.

CV(t) = Kpe(t) + Ki ∫ e(t) dt + Kd de/dt where Kd is the rate constant

Proportional Term Integral Term Derivative Term



PID In Summary

• PID is the workhorse of industry for closed

loop control of manufacturing processes.

• It has proven itself effective in many

applications ranging from motion control to

refinery operations for over 50 years.

mne601 tech presentation 1

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