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  1. #1

    Floating point control questions

    I just want to know more about floating point control.

    1) Why does it called floating point control?
    2) Please confirm my understanding of floating point control is correct.

    My understanding:

    Floating point control is also called 3-position control (clockwise, counter-clockwise, off). Basically, there are two DO's or BO's signals (24VAC signal) to control a device (i.e. valve actuator). One DO drives the actuator in one direction (i.e. clockwise) and the other DO drives the actuator in another direction (i.e. counter-clockwise). If there is no input to the actuator, the actuator stays in that current position (non spring return; fail in place). And floating point control depends on time. For example, for a 90-second-run-time valve actuator, the DDC controller needs to send a DO signal for 90 seconds in order to completely open or close the valve. If the DDC controller send a DO signal to the valve actuator for 45 seconds when the valve is completely close, the valve will open 50%. The valve actuator will do self-calibration regularly (how often?).

    3) What is floating point control is cheaper than modulating control? Is it the circuit board design or the manufacturer process of the parts (i.e. valve actuator)?

    Thanks!

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    Your understanding is correct.
    The self-calibration frequency is usually adjustable to options like 1 hour after the controller schedules off, or every 12 hours etc.

    I don't know whether the price difference is something that needs to be taken into account these days or not (altho I'm sure someone on here will), but I've always thought that DO/BO's are able to be used for either floating modulation or for switching other 'stuff'.

    Altho that said I used to work for Reliable controls, and their controllers don't have any BO's in the sense of relay outputs (all 'Universal'), if you want a BO you wire an output to a 10VDC relay, which suprisingly isn't as annoying as it sounds, it's quite good.
    The DDC system... guilty until proven innocent

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    Quote Originally Posted by Knife Switch View Post
    Your understanding is correct.
    The self-calibration frequency is usually adjustable to options like 1 hour after the controller schedules off, or every 12 hours etc.

    I don't know whether the price difference is something that needs to be taken into account these days or not (altho I'm sure someone on here will), but I've always thought that DO/BO's are able to be used for either floating modulation or for switching other 'stuff'.

    Altho that said I used to work for Reliable controls, and their controllers don't have any BO's in the sense of relay outputs (all 'Universal'), if you want a BO you wire an output to a 10VDC relay, which suprisingly isn't as annoying as it sounds, it's quite good.
    Though, at that point (having to add the extra relay) I wonder if the cost is worth it over just using an analog input valve.

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    Quote Originally Posted by Knife Switch View Post
    I don't know whether the price difference is something that needs to be taken into account these days or not (altho I'm sure someone on here will), but I've always thought that DO/BO's are able to be used for either floating modulation or for switching other 'stuff'.
    Pricing almost always gets taken into account. If one is talking one or two actuators. No big deal. But if you're doing a project that'll involve hundreds of VAV controllers with HW reheats, for instance, and the price differential between a floating point actuator and an equivalent 2-10 VDC actuator is $5 to $10. Then the possible savings can be one of those 4 digit numbers (>= $1000) that a salesman or project manager pays attention to.

    Heck, I've had arguments with them over the question of whether to buy said actuators with or without electrical connection pigtails already factory installed. i.e. One job involved 300 and some odd VAV reheat valves. Salesman decided to order them without pre-installed pigtails. I asked him why in the world did he do that? His answer was that he saved $4 apiece that way. Right .... I told him it was costing the company a heck of a lot more than $4 apiece to have the installers work without em.

    Especially given the fact that the valves with actuators were ordered and delivered directly to the mechanical types to put into the system. So controls installer, in order to wire em, was working on ladders or lifts, in high and often difficult to work places. Having to remove actuator cover, try to peer in at tiny terminals, get the correct wire attached to correct spot, get cover back on, etc. In many cases it went slow. And a lot of mistakes were made.

    I told the guy that next time, either cough up the bucks for pre-installed pigtails. Or if ordering actuators without them, have them delivered to our installers FIRST. So an apprentice or someone could be set to the task of making up and attaching pigtails for them, while having the luxury of having them some place where he could easily see and work. Then the installers could pass em on to the mechanical types.

    As concerns whether or not a BO/DO can be used only for floating point motor control or for switching other things. Depends. On the controller, the controller manufacturer's ideas, etc.

    I've seen controllers where a pair of BO's were dedicated to floating point motor control, in firmware. Others are more flexible. One can configure device so that a pair can be used for floating point motor control, or each can be assigned for separate usage.

    In some programmable controllers, one is not limited by any firmware restrictions, you use BO's however you wish.

    Quote Originally Posted by Knife Switch View Post
    Altho that said I used to work for Reliable controls, and their controllers don't have any BO's in the sense of relay outputs (all 'Universal'), if you want a BO you wire an output to a 10VDC relay, which suprisingly isn't as annoying as it sounds, it's quite good.
    I've never worked with Reliable controls. But have seen the same thing with other controllers. I'm not real fond of the idea. It has its pros and cons. I just find it annoying to HAVE to always put in that extra relay.

    OTOH, I've seen plenty of instances where I wished a controller had more analog outputs available. So having at least a few universal outputs available onboard the device would've been real handy. Or more analogs, and then one could just pop in a 10 volt relay if you needed 1 or 2 more digital outputs than what the controller came with.

    Chuckle, of course it doesn't matter. You're always gonna come up with the instance were you're short just 1 lousy input, or analog output, or digital output. So you either upscale to the next controller model in the line, at more cost, which then has more I/O than you actually need. (My personal preference)

    Or you have to fart around with I/O expansion modules. Which always seem to carry a premium price to them. Not to mention you then need extra cabinet space to put em in, extra wires to connect em, etc.

    Or you resort to playing games with the network, add to the traffic, add complexity, etc and use an I/O point on another controller as if its actually located on the first. I'm not to fond of that solution. Do it often enough, but don't like it. I keep that down to an absolute minimum whenever I can.
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    Quote Originally Posted by osiyo View Post
    Pricing almost always gets taken into account. If one is talking one or two actuators. No big deal. But if you're doing a project that'll involve hundreds of VAV controllers with HW reheats, for instance, and the price differential between a floating point actuator and an equivalent 2-10 VDC actuator is $5 to $10. Then the possible savings can be one of those 4 digit numbers (>= $1000) that a salesman or project manager pays attention to.
    For sure, I agree with you. What I meant to say was that these days is an AO more expensive than a DO? I'd guess it would be..

    As for Reliable controllers I initially scoffed a bit at them, they seem so 'underdeveloped', and very simple. Text based programming, similar but a bit better than PPCL, all the engineering interfaces look like an excel spreadsheet etc... But then those very things make them surprisingly stress free, theres hardly any 'personality' glitches with them.

    2-10v? Methinks we may work for the same company...
    The DDC system... guilty until proven innocent

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    Quote Originally Posted by CraziFuzzy View Post
    Though, at that point (having to add the extra relay) I wonder if the cost is worth it over just using an analog input valve.
    Well that's what ok about it, you can if you want. You never run out of AO's or DO's, if you have an output, you've got whichever you want. Altho of course you can kinda do it with any brand controller, Reliables kind of streamline it, if you command an output to 0-10v, it'll go there, but if you just say 'on' or 'start' then it automatically knows to go to 10v till you say 'off' etc. A small thing, but a good thing.
    The DDC system... guilty until proven innocent

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    Quote Originally Posted by supra33202 View Post
    ...(non spring return; fail in place)...
    Usually, but there are spring return floating point actuators.

    Quote Originally Posted by supra33202 View Post
    ...The valve actuator will do self-calibration regularly (how often?)...
    Generally, the calibration will be done by the controller, not the actuator. Calibration methods vary from manufacturer to manufacturer.

    Quote Originally Posted by supra33202 View Post
    ...is floating point control is cheaper than modulating control?...
    Yes. Not so much at the controller level, but the cost of the actuators. Check pricing on Belimo NMB24-3 (floating) and NMB24-SR (proportional) and note the relative difference in cost.
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    So to use a FP actuator on a controller with only UOs like the Reliable you would need to add two relays to switch 24VAC? I guess I would use Modulating actuators in this case.

    Besides cost FP control should be done by a switching triac not a relay. Triacs typically can cycle a lot more than relays before you get to specified life span. Floating control switches the output a lot more than on off and can wear out relays. But a relay will last the term of most construction warranties Especially for something like the odd fintube radiation you need to cram onto a spare outputs of a nearby controller.

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    Keep in mind that FP control also requires 2 outputs, as opposed to a single for analog control. Since most VAV controllers I've seen have 3 Outs, it'll usually work fine - but if you need a humidifier on there as well - that'll cause problems.

    Technically, you could program the UO's as a tri-state output (0V Close, 5V neutral, 10V Open), and then you'd need some sort of voltage threshold controlled relays - all to 'save' the cost of a servo actuator.

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    Actually come to think of it, reliable do a triac out option controller.
    The DDC system... guilty until proven innocent

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    To clarify a spring return actuator that is 'floating point/tri-state' will also have 24V (or whatever the actuator power is) fed to the actuator constantly. This is the 'holding voltage' to hold the actuator in whatever place was last sent to it by the DO/BO/Triac. Lose the holding voltage the actuator goes to its' fail safe position no matter what the DO/BO/Triac is telling it to do.

    *Note that modulating actuators are much easier to troubleshoot. For long term and being tech support on the other end of the phone I would rather have a modulating actuator.
    Last edited by crab master; 11-21-2010 at 01:00 PM. Reason: *Note
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