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.
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.
... 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.
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.
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.