control loops tune a fish. control loops tuning of a control loop involves selecting loop parameters...

Control LoopsControl Loops

Tune a FishTune a Fish


Tuning of a control loop Tuning of a control loop involves selecting loop involves selecting loop parameters to ensureparameters to ensure

stable control under allstable control under alloperating conditions.operating conditions.


Preparation for TuningPreparation for TuningFirst look at the controlled device. First look at the controlled device. Oversized valves or dampers control Oversized valves or dampers control

flow when they are less than 50% flow when they are less than 50% open.open.FlowFlow

PositionPosition

Virtually all of the control of flowVirtually all of the control of flowoccurs between 0% and 50%occurs between 0% and 50%


Preparation for TuningPreparation for TuningWhen dealing with oversized valves or When dealing with oversized valves or

dampers, it is important to tune the dampers, it is important to tune the loop when the position is between loop when the position is between

0% and 30%.0% and 30%.FlowFlow

PositionPosition

Loop must be tunedLoop must be tunedin this regionin this region


Preparation for TuningPreparation for TuningBetween 0% and 30% of the actuator Between 0% and 30% of the actuator

position, the flow changes rapidly. position, the flow changes rapidly. This is the region of high gain. This is the region of high gain.

FlowFlow

PositionPosition



Preparation for TuningPreparation for TuningIf a loop is tuned in a region of high If a loop is tuned in a region of high gain, it will be stable in a region of gain, it will be stable in a region of

low gain. Try to tune loop when gain low gain. Try to tune loop when gain is maximum.is maximum.FlowFlow

PositionPosition



““Optimal” Tuning Optimal” Tuning ConditionsConditions

Tune temperature control loop at Tune temperature control loop at minimum cooling load and when minimum cooling load and when

chilled water is coldest.chilled water is coldest.Tune static pressure control loop Tune static pressure control loop

when all boxes are closed.when all boxes are closed.Tune VAV box flow control loop when Tune VAV box flow control loop when

duct static pressure is highest. duct static pressure is highest.



The gain of the control loop is highest:The gain of the control loop is highest: When the cooling valve is almost When the cooling valve is almost

closedclosed When the VSD is almost at minimumWhen the VSD is almost at minimum When the VAV damper is almost When the VAV damper is almost

closedclosed



Under these conditions, large Under these conditions, large stabilizing proportional bands and stabilizing proportional bands and

integration constants will be integration constants will be required.required.

These parameters will then produce These parameters will then produce very stable control results under very stable control results under

other conditions.other conditions.


Step #1Step #1Adjust integration and deadband value Adjust integration and deadband value

to zero.to zero.The first step is to tune the control The first step is to tune the control

loop using the proportional band loop using the proportional band only.only.

If stability cannot be produced with If stability cannot be produced with proportional only control, it will be proportional only control, it will be impossible to produce stability with impossible to produce stability with

integration.integration.


Step #2Step #2Adjust the proportional band used to Adjust the proportional band used to

control the loop, finding stability.control the loop, finding stability. First, adjust the proportional band to aFirst, adjust the proportional band to a

large value. The larger the value, the large value. The larger the value, the lessless sensitive the loop will be. Start with: sensitive the loop will be. Start with:

- Cooling Valve : 100.0 (Deg C)- Cooling Valve : 100.0 (Deg C)- Static Pressure : -1500.0 (Pascal)- Static Pressure : -1500.0 (Pascal)- VAV Box Flow : 10000.0 (CFM)- VAV Box Flow : 10000.0 (CFM)

If the loop cycles, double the valueIf the loop cycles, double the value


Step #2Step #2Adjust the proportional band used to control Adjust the proportional band used to control

the loop, finding stability.the loop, finding stability. Next, keep halving the proportional bandNext, keep halving the proportional band

until the system becomes unstable. Then until the system becomes unstable. Then slowly increase the proportional band slowly increase the proportional band untiluntil the control loop is stable. Small the control loop is stable. Small continuouscontinuous cycles of constant magnitude (i.e. ±0.5°C) cycles of constant magnitude (i.e. ±0.5°C) is okay. is okay.


Step #2Step #2Adjust the proportional band used to control Adjust the proportional band used to control

the loop, finding stability.the loop, finding stability. Finally, test the loop for stability under aFinally, test the loop for stability under a

setpoint change. Change the setpoint to setpoint change. Change the setpoint to create an upset and then change the create an upset and then change the setpoint back to the original value within: setpoint back to the original value within:

- Cooling Valve : 45 seconds- Cooling Valve : 45 seconds- Static Pressure : 90 seconds- Static Pressure : 90 seconds- VAV Box Flow : 30 seconds- VAV Box Flow : 30 seconds


Step #3Step #3Add integration to increase response Add integration to increase response time of loop and eliminate offset error.time of loop and eliminate offset error.

First, double the proportional band First, double the proportional band fromfrom whatever value was required to whatever value was required to achieveachieve stable control using proportional only stable control using proportional only control control


Step #3Step #3Add integration to increase response Add integration to increase response

time of loop and eliminate offset time of loop and eliminate offset error.error.

Next, adjust the integration value Next, adjust the integration value to:to:

- Cooling valve : 200- Cooling valve : 200- Static Pressure : 15- Static Pressure : 15- VAV Box Flow : 60- VAV Box Flow : 60


Step #3Step #3Add integration to increase response time Add integration to increase response time

of loop and eliminate offset error.of loop and eliminate offset error. Then, conduct another upset / recoveryThen, conduct another upset / recovery

procedure by changing the setpoint procedure by changing the setpoint andand returning it to the original value. If the returning it to the original value. If the settling time is too long, reduce the settling time is too long, reduce the integration value by 10%. If the loop integration value by 10%. If the loop recovers too quickly or begins to cycle, recovers too quickly or begins to cycle, increase the integration value by 10%. increase the integration value by 10%.


Step #3Step #3Add integration to increase response Add integration to increase response

time of loop and eliminate offset time of loop and eliminate offset error.error.

Finally, continue the upset / Finally, continue the upset / recoveryrecovery procedure and keep adjusting the procedure and keep adjusting the integration value by 10% on each integration value by 10% on each iterationiteration until the loop is tuned. until the loop is tuned.


DeadbandDeadbandThe output of the control loop will not The output of the control loop will not change when the controlled variable is change when the controlled variable is

“close enough” to the setpoint.“close enough” to the setpoint.““Close enough” means within the Close enough” means within the

deadband.deadband.Deadband should be entered when trying to Deadband should be entered when trying to

tune a “noisy” signal such as static tune a “noisy” signal such as static pressure or VAV Box Flow.pressure or VAV Box Flow.

Use a deadband of about 2% of nominal Use a deadband of about 2% of nominal value.value.


Input FilteringInput FilteringUsed to even out a very “noisy” Used to even out a very “noisy”

reading such as static pressure (use reading such as static pressure (use value of 2).value of 2).

Filter Output Last FilteredValueAnalog In Last FilteredValue

Filter Value


Input FilteringInput Filtering

TimeTime

Unfiltered InputUnfiltered Input

Filtered InputFiltered Input


Fail-softFail-softIf an input sensor is unreliable (open / If an input sensor is unreliable (open /

short), the DDC Controller uses a short), the DDC Controller uses a default value instead and flags the default value instead and flags the

output of the control loop as output of the control loop as unreliable.unreliable.


Loop TuningLoop TuningCurrently, most DDC Controllers require Currently, most DDC Controllers require manual tuning by an operator. Enabling manual tuning by an operator. Enabling

technologies will soon permit:technologies will soon permit: Self-tuning (DDC Controller is put in “tuning Self-tuning (DDC Controller is put in “tuning

mode”, calculates optimal tuning parameters mode”, calculates optimal tuning parameters and then is switched back into “control mode”)and then is switched back into “control mode”)

Adaptive tuning (DDC Controller continuously Adaptive tuning (DDC Controller continuously optimizes tuning parameters while controlling)optimizes tuning parameters while controlling)

Pattern recognition adaptive control (Adaptive Pattern recognition adaptive control (Adaptive tuning + response to events such as start-up)tuning + response to events such as start-up)


Troubleshooting Control LoopsTroubleshooting Control Loops


When troubleshooting, When troubleshooting, look at the output of the look at the output of the control loop - not at the control loop - not at the

input.input.A cycling or sluggish control loop can A cycling or sluggish control loop can

be easily spotted from the output be easily spotted from the output signal but is very difficult to spot signal but is very difficult to spot

looking at the sensed input looking at the sensed input (temperature, pressure, etc.).(temperature, pressure, etc.).


If the output is 0% or If the output is 0% or 100%, then the loop is 100%, then the loop is saturated. A saturated saturated. A saturated loop is loop is notnot in control. in control.If the loop is saturated, then If the loop is saturated, then

increasing the difference between increasing the difference between the setpoint and input will have no the setpoint and input will have no

effect.effect.


Output = 0% or 100%Output = 0% or 100% A loop becomes saturated when the A loop becomes saturated when the mechanical system does not allow the mechanical system does not allow the

input to achieve setpoint.input to achieve setpoint.

For example, the if chilled water For example, the if chilled water temperature is 12°C and the off-coil air temperature is 12°C and the off-coil air

temperature setpoint is set at 10°C, temperature setpoint is set at 10°C, the chilled water valve will be 100% the chilled water valve will be 100% open but the off-coil air temperature open but the off-coil air temperature will never reach the desired setpoint.will never reach the desired setpoint.


ExampleExampleTemperature Setpoint = 12.0° CTemperature Setpoint = 12.0° C

Temperature Input = 18.0° CTemperature Input = 18.0° CValve Output = 100%Valve Output = 100%

Proportional Band = 4.0° CProportional Band = 4.0° CDeadband = 0.2° CDeadband = 0.2° C

AnalysisAnalysis This loop is saturated, decreasing This loop is saturated, decreasing

the setpoint will have no effect!the setpoint will have no effect!


ExampleExampleTemperature Setpoint = 12.0° CTemperature Setpoint = 12.0° C

Temperature Input = 13.0° CTemperature Input = 13.0° CValve Output = 20%Valve Output = 20%

Proportional Band = 4.0° CProportional Band = 4.0° CDeadband = 0.2° CDeadband = 0.2° C

AnalysisAnalysis This loop is in control, changing the This loop is in control, changing the

setpoint will have an effect!setpoint will have an effect!


Using data from last Using data from last example:example:

Error = 13.0°C - 12.0°C = 1.0°CError = 13.0°C - 12.0°C = 1.0°CError* = 1.0°C - 0.2°C = 0.8°CError* = 1.0°C - 0.2°C = 0.8°C

Output = 100% x (0.8/4.0) = 20%Output = 100% x (0.8/4.0) = 20%Usually, Proportional plus Integral control Usually, Proportional plus Integral control

is used. In this case, it is not possible to is used. In this case, it is not possible to calculate the output because the calculate the output because the

Integral Term cannot be calculated.Integral Term cannot be calculated.


If the output repeatedly If the output repeatedly swings between 0% and swings between 0% and 100%, then the loop is 100%, then the loop is

cycling. A cycling loop is cycling. A cycling loop is notnot stable.stable.

If a loop is cycling, the loop is too If a loop is cycling, the loop is too sensitive for the mechanical system. sensitive for the mechanical system. Reduce the sensitivity of the loop by:Reduce the sensitivity of the loop by:

Increasing the Integral TimeIncreasing the Integral Time Increasing the Proportional BandIncreasing the Proportional Band


If the output is between 0% If the output is between 0% and 100% and reacts very and 100% and reacts very slowly to setpoint changes slowly to setpoint changes or system disturbances, the or system disturbances, the

loop is overdamped.loop is overdamped.If a loop is overdamped, it executes If a loop is overdamped, it executes sluggish response. Increase the loop sluggish response. Increase the loop

sensitivity by:sensitivity by: Decreasing the Integral TimeDecreasing the Integral Time

Decreasing the Proportional BandDecreasing the Proportional Band


After changing the After changing the setpoint, the output shifts setpoint, the output shifts smoothly from one value smoothly from one value between 0% and 100% to between 0% and 100% to another value between another value between

0% and 100%0% and 100% Control loop is working fine.Control loop is working fine.

Don’t mess with it!Don’t mess with it!

Control LoopsControl LoopsThis loop will never stabilize. The proportionalThis loop will never stabilize. The proportional band and/or integration value is too small. band and/or integration value is too small.

TimeTime

Control LoopsControl LoopsBetter -- integration value still a little low.Better -- integration value still a little low.

TimeTime


TimeTime

Ideal for a temperature control loop. DecreaseIdeal for a temperature control loop. Decreaseintegration value a bit for static pressure loop.integration value a bit for static pressure loop.


TimeTime

Ideal for a static pressure control loop.Ideal for a static pressure control loop.

control loops tune a fish. control loops tuning of a control loop involves selecting loop parameters...

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