heating, ventilation and air conditioning (hvac) controls variable air volume (vav) systems, vissim...

7/13/2019 Heating, Ventilation and Air Conditioning (HVAC) Controls Variable Air Volume (VAV) Systems, VisSim Tutorial Part 2

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VisSim Tutorial

The VAV controller that matches this application, single-duct, pressure-dependent, cooling-only VAV, isconstructed from four main components:

A PID controller

A pulse width modulator

An electric two-state damper actuator

Sensors and feedbac components

The PID Controller

The Proportional, Integral and Deri!ati!e "PID# controller is designed to monitor the room temperatureand control the VAV damper position$ The PID control algorithm used to de!elop the VAV system is astandard PID controller with anti-windup and lag filter deri!ati!e calculation, as shown in %igure &'$

Figure 10. Implementation of the PID controller

This PID controller can be de!eloped using the already e(isting PID controller found in VisSim)s *ontrolToolbo($

The Pulse Width Modulator

The general definition of a pulse width modulator "P+# is that it is a control component that transformsanalog signals into digital pulses$ In today)s VA* component maret, it costs less to command thedamper position through a pair of pulse width modulated output points than through a single analogoutput point$ This is because digital point data ac.uisition costs less than analog point data ac.uisition$

Therefore, it is more economical to output digital /pen0*lose signals to a two-state VAV actuator than tooutput analog '1 to &''1 commands to an analog VAV actuator$ In addition, because most VAV

building installations contain hundreds of VAV bo(es, building owners can sa!e considerably on theirVAV installations by choosing pulse width modulated output points instead of analog output points$

To transform the PID analog commands into digital pulses, a P+ is needed$ The modulator estimatesthe position of the damper and sends a stream of /pen0*lose pulses until the commanded position isreached$ If the estimated position is lower than the commanded position, the P+ sends a stream of

*opyright &223$ Visual Solutions, Inc$ All rights reser!ed$Author: 4ebil 5en-Aissa, 6ohnson *ontrols, Inc$

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/pen pulses$ Similarly, if the estimated position is greater than the commanded position the P+ sends astream of *lose pulses$

%or e(ample, if the VAV actuator stroe time is & minute "that is, the actuator goes from to '1 to &''1in & minute# and the PID commands a 7'1 position, gi!en an initial damper position of '1, the P+does the following:

Sends 8' consecuti!e & second /pen command pulses, where each pulse causes the VAV damper

actuator to open by &$9931

stimates the damper position and when the estimated position is at 7'1 "within a deadband#, it

stops dri!ing the actuator

The final position of the damper ; Initial position < 8' &$993 ; '1 < 7'1 ; 7'1

If the initial position of the VAV damper is 371, the P+ does the following:

Sends &7 consecuti!e & second *lose command pulses, where each pulse causes the VAV damper

actuator to close by &$9931

stimates the damper position and when the estimated position is at 7'1 "within a deadband#, it

stops dri!ing the actuator

The final position of the damper ; Initial position - &7 &$993 ; 371 - =71 ; 7'1It is ob!ious that when using the two-state digital actuator, the accuracy of the damper position issacrificed$ The precision inconsistency is illustrated when the initial estimated position is not e.ual to the>real? position of the damper$ To accomplish better damper position tracing accuracy, the followingalternati!es are a!ailable:

@se an accurate analog actuator which commands the damper position directly from the PID output$

Install a position feedbac de!ice such as a resisti!e feedbac potentiometer$ 4ote that installing e(tra

de!ices on the VAV bo( can affect its manufacturing and installation costs$

@se a P+ and apply o!erdri!e when the estimated damper position reaches '1 or &''1$

To ensure that the position of the damper is synchronied with the pulse width modulated commands, ano!erdri!e algorithm is added to the P+)s logic$ The o!erdri!e algorithm, although !ery simple, is a

!ery effecti!e approach to sol!ing the synchroniation problem$ The o!erdri!e is applied e!ery time theestimated damper position is either at '1 or &''1$

+hen the estimated damper position reaches '1, the actual damper position might be at '1 or at apercentage that is near '1, maybe 71 or &'1$ This percentage uncertainty might cause unwanteddamper leaage$ In addition to damper leaage, as the position uncertainties accumulate during the daylong VAV operation, the control system might lose trac of the damper position and be way off the actual!alue$ This causes bad control especially when these tracing errors reach magnitudes greater than 7'1$

The o!erdri!e algorithm is describe below$

Begin Algorithm A-2

If " The stimated Damper position B The @pper o!erdri!e threshold# Then

%or "time ; ' to time C; Stroe time#

*ommand damper /P4 for Stroe Time

If " The stimated Damper position C The ower o!erdri!e threshold# Then

%or "time ; ' to time C; Stroe time#

*ommand damper */SD for Stroe Time

End Algorithm A-2


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Heating, Ventilation and Air Conditioning (HVAC) Controls: Variable Air Volume (VAV) Systems

%igure && shows the VisSim implementation of the o!erdri!e algorithm described by algorithm A-=$

Figure 11. Overdrive algorithm implementation in VisSim

The VisSim implementation of the P+ with o!erdri!e is shown in %igure &=$

Figure 12. VisSim implementation of the PW !ith overdrive

The Electric Two-State Damper Actuator

The damper actuator used in this VAV system is a two-state digital input "/pen0*lose# electric actuator$The actuator recei!es streams of /pen or *lose pulse width modulated signals, then rotates the dampera(le based on the width and direction of the signals$

The two-state electric damper actuator model is implemented with an anti-windup integrator described bythe following e.uation:

Present Position " Old Position#&

Stro$e%ime &Present 'ommand %ime duration( Direction"2#

where

Direction;


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Direction; -& if the command is *lose

The anti-windup integrator algorithm, based on the pre!ious e.uation is described by the followingalgorithm:

Begin Algorithm A-3

If "Present Position B; &''1# A4D "/P4 */A4D#

Present Position ; &''1

If "Present Position C; '1# A4D "*/S */A4D#

Present Position ; '1

Present Position ; /ld Position < "&0stroeTime# "Present*ommandTimeDuration# Direction$

End Algorithm A-3

Since VisSim already offers a built in anti-windup integrator bloc, called limitedIntegrator, the VisSimimplementation of the actuator is !ery simple and is shown in %igure &8$

Figure 1). *lectric t!o+digital input actuator

Sensors and Feedback Components

/nly the room sensor is discussed in this section because it is the only sensing component in the VAVapplication$

The Room Sensor

The room sensor is typically a nicel- or platinum-resisti!e element and is highly sensiti!e to temperature

changes$ As the temperature of the room changes, the resistance of the metal fluctuates due to thetemperature dependent dilatation properties of the metal$

The room temperature sensor senses the temperature of the room, using its resisti!e element, thengenerates a !oltage which is linearied and fed bac to the VAV controller$ The !oltage signal generated

by the sensor is obtained by mounting a current source across the resisti!e element$ The !oltagemagnitude, based on /hm)s aw, is e.ual to the product of the magnitudes of the current source and thesensing element)s resistance$

The linearied resistance response to temperature !ersus time is appro(imated by a linear first ordersystem with a aplace transform of the following structure:

,oom %emperature Sensed&S( " -ctual ,oom %emperature&S( ./

S

p

$ +&"2#

where/pis the gain of the linearied sensor, for most linearied sensors /p; &$

"Tau# is the time constant of the sensor$ +hen adding the dynamics of the plastic enclosure of

the sensor, typical !alue of are about & to 7 minutes$


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The VisSim representation of the first order appro(imation of the sensor is shown in %igure &E$

Figure 1. First order room temperature sensor

System Simulation and Oser!ationsIn this section, all the VAV models de!eloped earlier are put together and simulated$ Then, the results areobser!ed and discussed$

To test the control algorithm and understand the dynamics of the VAV system, the room model isdisturbed using disturbance loads that simulate daily en!ironmental room conditions$ As mentionedearlier in e.uation 9, the room model has three load components:

Interior loads generated by constant and changing load disturbances

(terior loads generated by the outdoor climate

4egati!e loads generated by the control system to maintain the temperature setpoint

As shown in %igure &7, an aggressi!e set of simulated loads is applied to the room model$ The load profileapplied in this simulation is constructed from the following load components:

A constant internal load component of magnitude 8''' 5tu0hr, is used to simulate constant loads

inside the room, such as lights, computers, and coffee machines$

A changing internal load component simulated by a full wa!e rectified sinusoidal signal with a

magnitude of =7'' 5tu0hr and a period of &$37 hours$ This load component simulates loadsgenerated by groups of humans entering a conference room, generating heat, then lea!ing$

A changing e(ternal load component simulated by a sinusoidal wa!e of magnitude =''' 5tu0hr and a

period of E$9 hours$ This load component simulates the outside temperature drop during earlymorning and later afternoon, and outside temperature increase around &' am to = pm$


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Figure 1. oad profile

As loads change in the room, the VAV control system generates >anti-loads? or >negati!e? loads to cancelthe heat generated by the internal and e(ternal temperature disturbances $ The VAV system controls the

position of the damper and regulates the amount of cold supply air needed to eliminate all the loads in the

room$ To properly control the temperature of the room, the VAV system has to first reach the temperaturesetpoint, then match the disturbance loads$ If the disturbance loads are matched and the temperature ofthe room is at setpoint, the temperature setpoint is maintained$ %igure &9 shows how the VAV system isable to eliminate the internal and e(ternal disturbance loads by generating a negati!e load, from supplyair, that matches the magnitude of the room loads$

Figure 13. oad control

/nce the temperature setpoint is reached and the loads are matched, the VAV controller easily maintainsthe setpoint because all loads are canceled and no temperature change is generated$


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Figure 14. V-V s5stem temperature control

%igure &3 shows the performance of the PID controller, which maintained a star!ed? because it is commanded at ma(imumwithout it being able to satisfy the room loads$ VAV bo( star!ation is common for o!er-sied and under-

pressuried VAV installations, it is also more common in hot climates$


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Figure 17. Starved V-V damper response

%igure =' shows the load matching response of the star!ed VAV system$ 4ote that when the loads aregreater than the system)s capacity, the VAV damper is at ma(imum, as shown in %igure &2, and thetemperature is abo!e the temperature setpoint, as shown in %igure =&$

Figure 20. Starved V-V s5stem load matching

%rom %igure =&, it is obser!ed that the VAV system does not ha!e enough capacity to maintain thetemperature setpoint$ In other words the system)s capacity cannot eliminate the disturbance loads$ As aresult, the room temperature de!iates from the re.uired setpoint while the VAV damper is at its ma(imum

position$


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Figure 21. Starved V-V s5stem temperature control

The same star!ed beha!ior is also obser!ed when increasing the temperature of the supply air$ In fact, ifthe temperature of the duct system supply air is increased, the VAV system is not able to generate enough>negati!e? loads to eliminate the disturbances in the room$

%igures ==, =8 and =E show the response of the VAV damper, load matching and temperature controlwhen the supply air temperature is increased from 77 % to 9' %$

Figure 22. V-V damper response !hen suppl5 air temperature is at 30 F


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Figure 2). V-V load matching !hen suppl5 air temperature is at 30 F

Figure 2. V-V temperature control !hen suppl5 air temperature is at 30 F

Re"erences aines, Hoger +$ and *$ ewis +ilson$ &22E$8V-' S5stems Design 8and9oo$, =nded$ craw

ill, 4ew Jor$

Sauer, arry 6$ and Honald $ owell$ &22E$Principles of 8eating Ventilating and -ir 'onditioning$

ASHA, Atlanta$

Sun, Tseng-Jao$ &22E$-ir 8andling S5stem Design$ craw ill, 4ew Jor$

&22E ASHA andboo$ &22E$8V-' Fundamentals, ASHA, Atlanta$

&227 ASHA andboo$ &227$8V-' -pplications, ASHA, Atlanta$

(cerpted with permission from:>odeling and Visual Simulation in Industry,? by A$ ulpur and P$ DarnellPublished by International Thomson *omputer Press, 5oston, A: &223$

*opyright &223$ Visual Solutions, Inc$ All rights reser!ed$Author: 4ebil 5en-Aissa 6ohnson *ontrols Inc

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