Aplicaciones PID

Advanced PID controller features

1MC.60.F1.02 - VLT is a registered Danfoss trademark

Co

ntents

■ IntroductionIntroductionIntroductionIntroductionIntroduction ................................................................................................................................................................................................................................. pagepagepagepagepage 2 2 2 2 2

■ Single Feedback and SetpointSingle Feedback and SetpointSingle Feedback and SetpointSingle Feedback and SetpointSingle Feedback and Setpoint ................................................................................ page 2page 2page 2page 2page 2

■ TTTTTransmitter connections to the driveransmitter connections to the driveransmitter connections to the driveransmitter connections to the driveransmitter connections to the drive ........................................ pagepagepagepagepage 3 3 3 3 3

■ One Feedback and One Setpoint withOne Feedback and One Setpoint withOne Feedback and One Setpoint withOne Feedback and One Setpoint withOne Feedback and One Setpoint withFeedback ConversionFeedback ConversionFeedback ConversionFeedback ConversionFeedback Conversion ...................................................................................................................................................... pagepagepagepagepage 3 3 3 3 3

■ ■ ■ ■ ■ TTTTTwo Feedback signals with One Setpoint page 4wo Feedback signals with One Setpoint page 4wo Feedback signals with One Setpoint page 4wo Feedback signals with One Setpoint page 4wo Feedback signals with One Setpoint page 4

■ TTTTTransmitter connections to the VLransmitter connections to the VLransmitter connections to the VLransmitter connections to the VLransmitter connections to the VLTTTTT ............................................. pagepagepagepagepage 6 6 6 6 6

■ TTTTTwo Feedback signals and Two Feedback signals and Two Feedback signals and Two Feedback signals and Two Feedback signals and Two Setpointswo Setpointswo Setpointswo Setpointswo Setpoints ..... pagepagepagepagepage 7 7 7 7 7

■ TTTTTransmitter connection to the VLransmitter connection to the VLransmitter connection to the VLransmitter connection to the VLransmitter connection to the VLTTTTT ....................................................... pagepagepagepagepage 8 8 8 8 8

■ Serial communicationSerial communicationSerial communicationSerial communicationSerial communication ...................................................................................................................................................... pagepagepagepagepage 8 8 8 8 8


MC.60.F1.02 - VLT is a registered Danfoss trademark2

■■■■■ Introduction:A PID (Proportional Integral Derivative) controller isusually used to control variable speed drives (VSD) inHVAC systems. This controller can be part of theBuilding Management System, part of an OEMdevice such as an Air Handling Unit or PumpController, or part of the frequency converter (VSD).The PID controller monitors the system byexamining signals, called Feedback signals, that aretransmitted from sensors placed in the system andcomparing them with their expected value orSetpoint. The PID controller then adjusts the speedof the variable speed drive until the Feedback signalequals the Setpoint.

While many VSD’s contain a PI or PID controller,they are often too complicated to set up and do nothave the capability needed for HVAC applications.As a result, an external PID controller may beneeded. This increases the cost and complexity ofthe installation. The PID controller in the VLT® 6000has been specifically designed for HVAC applicationsto provide full functionality and simple programming.The terminology used in connection to the PIDcontroller is well known within the HVAC industry.Feedback and setpoint settings can directly be donein the most common units such as Pa, PSI, m³/h,CFM, °C, °F etc.

Two Zone control, for example, is an importantfeature for HVAC applications which is not requiredfor most industrial applications. In HVAC systems, itis frequently necessary to monitor two feedbacksignals. The VLT® 6000 is capable of handling thetraditional single feedback with a single setpoint aswell as two feedbacks with a single setpoint, andtwo feedbacks with two setpoints.

Fig.1:Fig.1:Fig.1:Fig.1:Fig.1: Constant pressure control with one feedback

Fan

Filter

Diffuser

PressureTransmitter

Since the system does not contain VAV boxes orhas dampers that modulate, the sensor type andlocation is not critical to achieve the proper energysavings. A simple velocity sensor or pressure sensorcan be used. In this example, a pressure sensor isplaced downstream of the air filter. The setpoint isthe pressure that corresponds to the required flow.As the resistance of the filter varies, the pressuredownstream of the filter also varies. The driveincreases or decreases the speed of the fan tomaintain a constant pressure and therefore thedesired air flow.

■■■■■ Single Feedback and SetpointIn simple HVAC applications, such as controlling asupply fan in a constant air volume (CAV) AirHandling Unit or a Cooling Tower fan, a singlefeedback signal may be sufficient for proper systemcontrol. Normal or inverse operation should beavailable as selections, and the ability to modify thesetpoint for seasonal changes or night timeoperation are important.

The diagram below shows how to control a simplesupply fan in a CAV system. The drive is used toautomatically eliminate the extra energyconsumption caused by oversizing and tocompensate for the changing system as the air filterbecomes clogged (blocked) over time.

Other typical applications for 1 Feedback / 1setpoint:· Supply fans for Variable Air Volume (VAV)

systems (constant static) pressure control· Water supply pumps and pressure booster

systems (constant pressure)· Cooling Tower fans (constant cooling water

temperature)· Condenser water pumps (constant cooling

water temperature or pressure)· Condenser fans (constant pressure in the

refrigeration process)



Fig. 2:Fig. 2:Fig. 2:Fig. 2:Fig. 2: 2-wire transmitter with current signal suchas 4...20 mA DC

Fig. 3:Fig. 3:Fig. 3:Fig. 3:Fig. 3: 3-wire transmitter with voltage signal, suchas 0...10 Volt

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16 17 18 19 20 27 29 32 33 61 68 69

Terminal Strips

Analog Input

Common

Common

24 VDC

■■■■■ One Feedback and One Setpoint with Feedback Conversion

Some HVAC applications require a preciselycontrolled constant flow, such as ventilation systemsin hospitals with highly efficient Heppa filters orcleanroom environments in industrial productionareas.To control the flow, flow meters may be installed inthe air duct. Often these transmitters are nothingmore than pressure transmitters that contain anelectronic circuit that calculates the flow from thepressure measurement.

This calculation is based on the centrifugal pumpand fan laws. These laws state that, if otherparameters in the system are constant, flow isproportional to static pressure squared. Therefore,flow can be determined by taking the square root ofpressure and applying a proportionality constant.

Fig. 4:Fig. 4:Fig. 4:Fig. 4:Fig. 4: Feedback conversion diagram

■■■■■ Transmitter connections to the drive:Terminal 12 and 13 of the VLT 6000 provide accessto a 24 VDC, 200 mA power supply. This can beused to power remote transmitters, so an externalpower supply is generally not needed. The diagramsbelow show how to wire two- and three-wiretransmitters.

Common

Power InSignal Out

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Terminal Strips

Analog Input

24 VDC

Common

Common

The function is activated by simply setting parameter416 to Square Root. The proportionality constant isentered by setting in parameter 413 and 414 theminimum and maximum value of the convertedfeedback signal. Parameter 415 is used to set theunit for this feedback signal.

Programming example:Programming example:Programming example:Programming example:Programming example:Assume that a 4…20 mA pressure transmitter isused and is properly installed downstream of thefilter (shown in picture 1):· Set parameter 314 to Feedback, parameter 315

(AI scale low) to 4 mA and parameter 316 (AIscale high) to 20 mA.

· Run the fan at 100% speed (nominal speed) andused the VLT’s built-in meter to measure thecurrent signal at the analog input terminal 60 (AI60).

· Take a flow measurement at the same locationas the pressure transmitter.

· Set parameter 415, Reference and FeedbackUnit, to the flow unit used

· Use the following equation to calculate the valueof parameter 414 (Maximum Feedback)

Two feedback functions can not be utilized inconnection with the feedback conversion(linearization).

Parameter 414 = flow at 100% speed x

2

100% at 60 Al

mA20



■ ■ ■ ■ ■ Two Feedback signals with One SetpointMany HVAC systems require two feedback signalsfor proper control. Because traditional PIDcontrollers in drives cannot handle this situation,external controllers are frequently purchased. Thisadds unnecessary cost to the installation.Depending on the application, differentmathematical operations will be applied to thesefeedback signals. These two signals may be:

Averaged, as is the case with CO or CO2 sensors inparking garages or temperature sensors in largerooms with CAV heating or cooling systems.

Subtracted from each other. For example the returnfan in a VAV system being controlled to maintain aconstant difference between the supply and returnairflow (like shown in fig. 5).I applications where a differential pressure sensor isimpractical, the signals from two pressure sensorscan be subtracted (like shown in fig. 6)

Fig.5:Fig.5:Fig.5:Fig.5:Fig.5: A return fan being controlled to maintain a constant difference between supply and return airflow.

Added to one another. This is usually used as anoffset to the feedback signal, in place of modifyingthe Setpoint.

Compared to one another, with control based onthe minimum or maximum signal. In systems thathave two loads of equal size in different locations, itis difficult to decide where to place the sensor. Inthese cases, using two sensors can ensure thatboth loads are satisfied. In some systems, theminimum signal should be used for control forexample when monitoring differential pressureacross two heating or cooling loads. In other casesthe maximum signal should be used. An example forthis is when monitoring temperature while coolingtwo separate critical zones such as a conferenceroom and the presidents office.

The diagram below shows the above mentioned tocontrol the return fan in a VAV system.

FlowTransmitter

FlowTransmitter

Return Fan

Supply Fan



The application shown in fig.5:As the VAV boxes in the system modulate the flowof supply air varies. If the return fan is not alsocontrolled, a negative pressure will be generated inthe building if more air is removed from the zonethan is supplied to it, or the zone will become overpressurized if too little air is removed. Because thesupply and return fan system curves and fan curvesdiffer, controlling both fans from the same drive orthe same control signal often results in pressureregulation problems. To avoid these problems,velocity sensors can be placed in the supply andreturn ductwork and a fixed differential flow can bemaintained. The return fan’s speed is controlledproperly as the VAV boxes modulate.

Another simple example for using the 2 feedbackfunction to create a differential pressure transmitterby using two absolute pressure transmitters:

Fig.6:Fig.6:Fig.6:Fig.6:Fig.6: Subtracting the two pressure transmitters to measure differential pressure

Instead of hydraulic pipes and an expensivedifferential pressure transmitter, two simple absolutepressure transmitters of equal type can be installed,saving installation costs and providing great flexibilityin locating the sensors. The two transmitterlocations do not have to be close to each other,since the electrical signal can be sent over longerdistances.



Transmitter connections to the VLT:When two feedback signals are used with the VLT6000, both signals must be voltage signals. Bothtransmitters also must have the same range. Whenthe PID controller of the VLT 6000 is set up tosubtract the two signals, it is essential to note thatthe feedback signal at terminal 54 is alwayssubtracted for the signal at terminal 53. Thereforethe larger signal must be wired to terminal 53. If thefeedback signals are added, averaged or simplycompared in size, the order of connection does notmatter.

Fig. 7:Fig. 7:Fig. 7:Fig. 7:Fig. 7: Wiring of two 3-wire voltage signaltransmitters

When two feedback signals are used, the VLT 6000requires that they both be connected to its analogvoltage inputs, terminal 53 and 54. Transmitters thatproduce a current signal (such as 4…20 mA), caneasily be used by simply adding two resistors. Theseresistors are connected in parallel with the analogvoltage input (between terminal 53 and 54 andterminal 55) and provide a low resistance path forthe control current signal. The voltage drop acrossthe resistor is the anlaog voltage input for the VLT6000.The value of the resistor is easy to calculate. If thetransmitter is capable of producing 10V at 20 mA,the value of the resistor to use is:

The input scaling of the VLT 6000 analog inputs inthis case would have to be 2…10 Volt.

Fig. 8:Fig. 8:Fig. 8:Fig. 8:Fig. 8: Two 4…20 mA transmitters wired to the VLT6000, powered by its own power supply and actingas feedback signals.

■

�500mA20

Volt10R ==

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Common

Power In

Transmitter 1

Signal Out

Terminal Strips

Analog Input

Common

Common

24 VDCTransmitter 2

Common

Power InSignal Out

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16 17 18 19 20 27 29 32 3361 68 69

Transmitter 1

Terminal Strips

AnalogInput

Common

Common

24 VDCTransmitter 2

500 Ohm



■ ■ ■ ■ ■ Two Feedback signals and Two SetpointsJust as with two feedback signals with one setpoint,the need for two feedback signals with twosetpoints is frequently encountered. Whenever twocritical zones exist that do not have the samesetpoint value, two feedback, two setpoint control isrequired. Many pumping systems require themonitoring and control of two loads that differ in sizeand pressure drop. For these cases, 2 Zone controlis selected, Feedback 1 is associated with Setpoint1 and Feedback 2 is associated with Setpoint 2.The two independent groupings are continuouslymonitored to satisfy both zones requirements. If 2Zone Minimum is programmed, the drive willregulate the system so the feedbacks are equal to

Fig. 9:Fig. 9:Fig. 9:Fig. 9:Fig. 9: Two zone control in a secondary chilled water system

or greater than their associated setpoints. If 2 zonemaximum is programmed, the Feedbacks will beequal to or less than their associated Setpoints. Thisis used when controlling different temperatures in acooling situation.

The diagram below shows how to control twodifferent loads in a secondary pumping system. Asthe temperature varies in each room, the drive willadjust the speed the secondary chilled water pump.

Other typical applications for 2 Feedback /2 Setpointcontrol include:· VAV systems with two major duct systems

supplying VAV boxes· CAV systems with two different setpoint

requirements for 2 building zones



■■■■■ Transmitter connection to the VLT:The transmitters are connected to the VLT in thesame way as shown above with 1 setpoint and 2feedbacks. The feedback connected to terminal 53is assigned to setpoint 1 and the one connected toterminal 54 to setpoint 2. Both transmitter musthave the same output signal and range.

Programming example for 2 feedback/2 setpointProgramming example for 2 feedback/2 setpointProgramming example for 2 feedback/2 setpointProgramming example for 2 feedback/2 setpointProgramming example for 2 feedback/2 setpointcontrol:control:control:control:control:Transmitter ratings:Supply: 15-30 VDCOutput: 0…10 VRange: 0…10 Pa

Parameter Parameter Parameter

number description value

100 CONFIG. MODE CLOSED LOOP

415 Ref./FDBK UNIT Pa

413 MIN. FEEDBACK 0

414 MAX. FEEDBACK 100

308 AI [V] 53 FUNCT. FEEDBACK

309 AI 53 SCALE LOW 0

310 AI 53 SCALE HIGH 100 *

311 AI [54] 54 FUNCT. FEEDBACK

312 AI 54 SCALE LOW 0

313 AI 54 SCALE HIGH 100 *

314 AI [mA] 60 FUNCT. NO OPERATION

* For maximum control accuracy, it is suggested toused the VLT 6000’s meter display (to be read in atthe Local Control Panel, LCP) to measure the actualvoltage at the terminals of the feedback signals.SCALE HIGH is set to the values accordingly.

The reference range is generally set to equal therange of the transmitter. In this way, the setpoint(reference) can be set to any value that thetransmitter can produce. If required, the referencerange can be set to a range that is narrower thanthe transmitter’s range, but it can not be set to awider range.

Parameter Parameter Parameternumber description value

204 MIN. REFERENCE 0

205 MAX. REFERENCE 100

Parameter Parameter Parameter

number description value

417 2 FEEDBACK CALC. 2 ZONE MIN

418 SETPOINT 1 30

419 SETPOINT 2 60

■■■■■ Serial communication:In large HVAC systems, variable speed drives areoften connected to the Building ManagementSystem (BMS) via serial communication utilizingproprietary protocols such as Johnson Controls’sMetasys N2 and Siemens FLN or open protocolssuch as LonWorks.The BMS typically is the overall control system,monitoring and controlling each individualcomponent of the system. Nevertheless the actualcontrol of single applications often can bedecentralized. The VLT 6000HVAC is fully capable ofoperating as a stand alone closed loop applicationutilizing the built-in PID controller. In this case, thePID controller would be set up as described in thisnote, while the drive receives the start/stop signaland the one or two feedback signals directlythrough the serial communication bus (par. 537Busfeedback 1 and par. 538 Busfeedback 2).

The VLT 6000 is programmed to match its input tofollow the specification of the transmitter:

In this example, load 1 requires a pressure of atleast 30 Pa and load 2 at least 60 Pa.

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Aplicaciones PID