How are VAV damper controlled?

[lostdude]

Ya, I know, very basic question...

We got Trane VAV units controlled with Siemen TEC (Terminal Equipment Controller). Looking on the wiring diagram the damper is wired to DO (digital output). When I saw this, I was like, "what?"

I'm guess it's using some type of pulse signal from the digital output to set the damper position. If so, anyone know the detail of the signal form? How hard would it be to change the VAV to take analog signal?

[kontrolphreak]

Most VAV controllers with integrated damper actuators use drive-open drive-close actuators (or sometimes called floating control) that pulse the actuator either open or close for a set amount of time (seconds) to achieve the airflow setpoint. It seems to be the industry standard (ones I have worked with: ALC/Alerton/Delta/Reliable/t.a.c. Xenta/Andover) due to probably cost. But it is a good and stable control method, with an analog output you will probably need a PID that is tuned well and then the actuator is going to be making small (lets say 0.1V) adjustments all the time and the output is going to have alot of on time. I haven't seen an integrated VAV controller with damper actuator that is controlled by an analog signal, there are probably ones out there.

Why are you wanting to change the digital signal to an analog one? Are the actuators part of the board or external?

kontrol out

[lostdude]

Thanks, it makes more sense now. So I guess the position I'm seeing when I use Datamate Base (Siemens) is just an estimate rather than actual feedback.

I post in another thread that I want to go with an open system using PLC. PLC can use digital output and analog output. I would think analog output would be give better control since the same PID are at work anyway. It seems Trane does make analog VAVs according to their website. I will call them up and see if I can learn more.

Well, if I can't do it with PLC then I would also look at an open standard VAV controller instead of proprietary stuff.

[dngrsone]

Analog control is all well and fine, but as kontrolphreak said, it will use a lot more power than a pulse-controlled floating actuator.

[kontrolphreak]

Some actuators have a true feedback, others as you said calculate the position by adding the time it has pulsed open or closed to give a position. Most actuators are around 90 second runtime, so if you have pulsed it on 9 times at 5 second pulses it should be approx 45 seconds = 50%.

For floating/drive-open drive-close control I like software that can leave the DO for ON energized at 100% and the DO for OFF energized a 0%. That way whenever it reaches 100% or 0% you know it is truely opened or closed (sometimes the calculations drift over time and it only pulses for a total of lets say 80 seconds so you are not really at 100% open, then when you drive it closed your calculations are screwed aswell). AutomatedLogics old VAV controllers did this, the new ones I don't know about.

I have never had a problem with this setup and wouldn't change it unless I saw a very good reason to do it.

kontrol out

[dngrsone]

For floating/drive-open drive-close control I like software that can leave the DO for ON energized at 100% and the DO for OFF energized a 0%. That way whenever it reaches 100% or 0% you know it is truely opened or closed (sometimes the calculations drift over time and it only pulses for a total of lets say 80 seconds so you are not really at 100% open, then when you drive it closed your calculations are screwed aswell). AutomatedLogics old VAV controllers did this, the new ones I don't know about.

Alerton VAV-SDs do this as well.

[kontrolphreak]

dngrsone,
Do you work with Alerton? If so can you answer the question I posted here:

http://www.hvac-talk.com/vbb/showthread.php?t=132224

kontrol out

[lostdude]

For floating/drive-open drive-close control I like software that can leave the DO for ON energized at 100% and the DO for OFF energized a 0%. That way whenever it reaches 100% or 0% you know it is truely opened or closed (sometimes the calculations drift over time and it only pulses for a total of lets say 80 seconds so you are not really at 100% open, ..Thanks, that's what I'm looking for.

I can easily duplicate that functionality in a PLC using a pair of DO. This way I can use the same VAV and my home brew controller.

[shifferbrains]

Thanks, it makes more sense now. So I guess the position I'm seeing when I use Datamate Base (Siemens) is just an estimate rather than actual feedback.


It’s true the TEC never really knows where the actuator is in the stroke. But it is very good at knowing where it told it to be. It's also true that over time it gets a little lost. Manually commanding the actuator to the zero position recalibrates the system (no longer lost). Siemens came up with an automatic recalibration procedure that works very well. Once every 12 hours the TEC Manually commands the actuator to zero. It recognizes the zero position then releases it back to normal operation. You have the option of disabling this process or customizing the timing if you wish. The only down side to this recalibration procedure that I have seen is that you get the occasional customer inquiry about his air conditioning going off every day for a few seconds then coming back on.

All in all the Siemens TEC's have proven to be a very good and reliable controller.

[lostdude]

..All in all the Siemens TEC's have proven to be a very good and reliable controller.I'm sure TEC is just as good as any other VAV controller. Our system is showing its age in other places as well at DPU panel, BMC, etc... I like to use this oppertunity to move toward an open system without being tied to Siemens Insight.

I would love to keep the TEC if I can talk to TEC directly via OPC or some standard type of network.

[tlp261]

For what it's worth, the design engineers say that floating actuators will outlast modulating in a VAV application by a factor of 5 or more.

[Balibe]

Not sure of all VAV controllers, but the Siemens TEC, Trane UCM and Carrier ComfortID all use floating point control. Out of these three, only the Carrier ComfortID uses a 0-10vdc feedback signal to know damper position. The Trane damper has open/close end switches to stop the signal to the motor once the damper reaches the fully open or closed position.

At my last company, we serviced a 12 story building with Trane equipment and Seimens 600 controls which had 300+ Trane VAV's with TEC's. They worked great, the biggest issue was motor or gear failures on the dampers.

[comfortmaster1]

Thanks, it makes more sense now. So I guess the position I'm seeing when I use Datamate Base (Siemens) is just an estimate rather than actual feedback.

The VAVs that I see usually have some kind of air flow sensor.

[chesehd]

Quick question that may have significant bearing on your plan: are these the old, conical, 6 min Trane VAV's or the new style? If it's the old style you're going to be putting yourself through a bit of a PITA reinventing the wheel. If you're looking for a feedback signal you're going to have to drop the bottom and come up with a new plan for that worm gear setup.

Just wondering.

Ken

[lostdude]

These VAV are about 14 years old and yes, I think they are all conical shaped. Feedback is nice to have but I think if I can replicate the calibration program it should be just fine. There is dP (differential pressure) feedback to measure the CFM.

I am fully aware this is somewhat akin to reinventing the wheel. This is not a typical building but a factory with hundreds of existing PLCs. There are only about 40-50 VAVs for the office area, not all are on the network and some just haven't worked right for years. We have to pay for support contract for PLCs AND Siemens Apogee so I rather get rid one these system. TEC is made specifically to control VAV so I rather keep it if I can interface it to a standard industrial protocol or PLC. There are interface made for Lon (see http://www.fieldserver.com) but TEC is on a propiertary network...

no, i haven't made my final decision yet but the infomation you guys provided are very helpful. Thanks.