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  1. #1
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    Dec 2004
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    please advise what's the difference between those two control valves. Thanks!

  2. #2
    Join Date
    Dec 2004
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    San Francisco Bay Area
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    While both qualify as modulating control, the floating point control is effected by using a pair of digital outputs to power the valve open on one leg for a certain amount of time (a 60 second actuator will be powered for 30 seconds to move it from fully closed to 50% open), and powering it closed on another leg for the desired time to achieve the desired position. It requires the controller to time its outputs, and requires a precise actuator for repeatable action. Many controllers have logic within them to "pulse" the closed or open outputs for a short time when it expects the valve to be fully closed or open, just to make sure the actuator is synchronized with where the controller thinks it is.
    Proportional control uses an analog output (either 4-20mA, or 0-10VDC (or 0-5VDC, depending on the controller) to more precisely position the actuator. It doesn't require any timing function, as 5V puts the actuator at 50% every time, 7.5V puts it at 75% every time.
    Floating point control is cheaper to buy, but harder to troubleshoot, and requires a good quality actuator to avoid the condition where the controller loses track of exactly where the actuator is actually positioned.

  3. #3
    Join Date
    Oct 2003
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    Minnesota
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    Originally posted by ceng
    please advise what's the difference between those two control valves. Thanks!
    Davem had it right.

    The only thing I'd add is a couple points.

    When using a proportional signal controller that takes a 0-10 VDC, 2-10 VDC, etc signal; make sure you ground the shield for the cable to the same ground plane as that used by the controller. Otherwise you can end up with a floating signal where induced voltage causes the signal to drift, and thus the actuator. In some cases, not often, I've seen it be so severe we had to run a separate ground strap from the equipment the actuator was attached to, back to the controller back plane.

    Problem gets worse with longer runs, and or if cable is running close to a lot of EMF generating equipment. DO use shielded cable. And use a generous gage sized conductor. To minimize voltage drop. We standardize on 18 gage. Haven't had a problem. But I've seen installations where someone decided on 20 or even 22, and a 100 foot one way run ... not good.

    That said, we'll use the proportional signal actuators where we want more accuracy. And save a couple bucks by using floating point actuators for things like reheats, fin tube radiation valves, etc. As long as these items are not located where freezing is a likely risk. (In which case we use proportional signal, spring return actuators.)

    As Davem mentioned, most controllers made for HVAC service will have settings so that you can tell the controller to "bump" up against the stops when going 100% open or closed to "recalibrate" a starting position to make up for slight timing errors. Other controllers go a step further and allow you to choose a set "calibrate" period of every so many hours. ie If set for 8 hours, for a brief time every 8 hours the controller will run the valve or damper fully shut for the lenght of time equal to the full stroke travel time of the actuator ... usually times 2. Then it'll reposition actuator at the required position. Some cheaper floating points need more frequent calibration, others ... like Belimo, I usually set to do it once every 24 hours.

  4. #4
    Join Date
    May 2002
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    9,564
    ... I think I should emphasize there is a big cost difference between the two styles. (floating point is less expensive)

    I see many manufacturers (ie Trane on VAV or CV reheat) routinely put in floating point actuators when the specification states proportional. Just looked at a project with over 70 valves specified proportional where Trane put in floating. I was called in to look at the job a few years later and found that right away... If I was the Spec engineer I'd be aggravated.

    If you are an engineer, make sure that when you specify proportional, that is what you get. Personally, I make sure our VAV reheats are proportional and not floating point.

    You pay for precision, but it's worth it in many instances.

    [Edited by sysint on 04-04-2005 at 12:14 PM]

  5. #5
    Join Date
    Dec 2004
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    41
    Thank you for all of your input. I have been confused by this for long time..

  6. #6
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    Nov 2004
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    226
    Sys,
    Are you saying that floating control is NOT proportional?

  7. #7
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    May 2002
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    I'm saying the actuator doesn't respond proportionally to a given signal. A floating point actuator is time dependant.

    A floating point actuator has to be calibrated regularly because it will "lose position" over time. Where a controller "thinks" the actuator position is may not be where the actuator actually is.

    However, (it's said) a properly set up floating point actuator will generally have a longer life span due to less overall cycling.

    They are cheap. You primarily see them because they are inexpensive.

  8. #8
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    Oct 2003
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    Minnesota
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    Originally posted by sysint
    I'm saying the actuator doesn't respond proportionally to a given signal. A floating point actuator is time dependant.

    A floating point actuator has to be calibrated regularly because it will "lose position" over time. Where a controller "thinks" the actuator position is may not be where the actuator actually is.

    However, (it's said) a properly set up floating point actuator will generally have a longer life span due to less overall cycling.

    They are cheap. You primarily see them because they are inexpensive.
    True enough.

    An actuator which hunts continuously/excessively is not good. And will fail much sooner than otherwise. If one sets up a good control loop, the system will find a balance point and one should see the actuator move at approximately the rate of temperature change occurring in the air being conditioned. Discounting a bit of delay time before it initially responds. Perhaps a minute or so. You want to have a slow initital reaction to change so as to preclude the system over responding to a stray breeze. For instance a door opening momentarily while someone enters or exits.

    Of course, in some instances one does want a faster rate of response but one should keep such to the minimum necessary occassions. For ordinary room air conditioning, in most instances, a fast response is not desireable. Human senses detect fast changes far more easily than gradual ones. One of the reasons that trying too hard to hold a tight temperature control in an occupied room can generate complaints, as well as cause components to fail sooner.

    As a general rule of thumb, that's far older than I am, the average person notices, becomes conscously aware of, a temperature change of about 2 degrees. My observation has been that they'll definitely notice if this change occurs quickly and repeatedly. But will tend to not immediately notice if the change is slow, ie over 15 minutes or longer. At least, not immediately. After some time, usually half hour or longer if a slow change, person might start becoming aware of feeling a bit cooler or warmer. But many (most) will become almost instantly aware, and annoyed in an office seeting for instance if the changes are rapid. Even if only slight. This holds not only for temperature, but also for air flow. Their ears and skin will tend to notice more rapid changes and fluctuations in air flow more readily than with slower changes. Even more than they'll become "aware" of large changes in velocity that are more significant, if those changes occur more slowly.

    I've had to do follow up on a project or more than a couple occassions where complaints came in from customer and occupants. Where in fact rooms were well within specified parameters of temp and air flow. Original tech, or programmer back in the office had set up control loops that were in fact too fast and tight. And all I did was slow things down. In some cases even allowing a larger temp swing (tho more gradual)... BUT ... complaints stopped and occupants were more happy.

    This is not to mention things like one site were original system programmer set up tight control loops for room temperature control (VAV system), with the result that damper motors hunted almost constantly. Net result was that within a year we were replacing damper motors. By a year and a half, a lot of damper motors. One of those situations where things fell thru the cracks. Repairs being done by our service department and over on my side of the house we weren't even aware of the issue. Once someone FINALLY mentioned it at a meeting, I sent a good man to check. He readjusted loops, and problem went away.


  9. #9
    Thanks. I got the difference between floating controller and proportional controller.

  10. #10
    Join Date
    Feb 2009
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    Northeast
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    Now we are mixing in another device. A valve actuator (or any actuator) and a controller are different things in my head. This thread has covered the differences in a floating and a proportional valve actuator. Though the idea a proportional actuator is better than a floating actuator is a fallacy. Just different devices and each can be a better solution to specific applications (Maybe I don't want an electronic board in the location where the actuator is). A controller is the part of the control system where an input is evaluated and an output is generated. The controller can be as simple at a toggle switch or as complex as a computer.

    My definition of a proportional controller would be a controller where the output changes over a range as the input changes over a range. This could have a timed based floating output or a proportional output as both are a proportional control signal.

    My definition of a floating controller would be a controller with 2 outputs where: 1 output activates above a limit and is deactivated below the same limit; and the second output activates below a limit and is deactivated above the same limit. Now additional stuff could be added like time limits, differentials at the switching points, and deadband between the limits, but I am aiming for the bare bones description. A floating controller could control a floating valve, but it would have trouble with a proportional valve without some more stuff to change the output.

  11. #11
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    Nov 2006
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    Wow, 9 year old zombie post. Must be some kind of internet record.
    If you can't fix it with JB Weld, Duct Tape, and Ty Wire it has to be replaced.
    No good deed goes unpunished.
    If you want to take off friday to go fishing then make sure you train your helper right.

  12. #12
    Join Date
    Feb 2009
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    I decided to expand on the misconception that a proportional actuator is better than a floating actuator. Some users think that a % on the computer screen means that is how much the device is outputting. Most devices (like valves) do not vary their effect in a linear proportion to the actuator signal. For instance a simple radiator; First the signal has to vary to the actual start point of flow through a valve. Rarely at the first change of the signal off of Zero. Could be as high as 25% on a typical ball valve. Then the heat output of a radiator is not linear to water flow. It is not even linear to the water temperature in the pipe. Bottom line; at zero % there is zero and at 100% there is all it can do. The % change in effect for any 1 % change between the ends of the range is just a guess.

    So now we apply a PI controller on a closed loop and the controller takes care of finding the % value that exactly balances the load to maintain the setpoint we are trying to control. As long as the balance point can be achieved the % value to the actuator doesn't really matter. A Floating or a Proportional valve can be equally good (or bad) for finding the physical position where the input energy it is modulating matches the load required to maintain a balanced condition at setpoint.

    There are applications where a proportional actuator makes our life easier. For instance setting a minimum OA damper position. (Note that is not the same as minimum outdoor air Flow!). Another is where two actuators need to be sequenced, one before the other. It is also easy for the service man to use a meter at the actuator and see the position signal from the controller. It may require a computer connected to the controller to know the actuator position signal in floating applications. Either actuator should be easy to test by a trained service person. When a proportional actuator is restored to service, the proportional actuator will return to the position the controller is outputting. The floating actuator's controller will need some input to restore operation. Usually the setpoint can be kicked or the controller reset with no problems.

    So floating actuators tend to be applied in positions where simple control is needed. Especially where there are multiple units like terminal units where the cost advantages add up. So floating actuators have the image of being low quality because they cost less and are used on simple stuff. Actual quality depends on how they are built and applied.

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