Author Topic: Linear actuators with potentiometer feedback  (Read 1721 times)

dp

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Linear actuators with potentiometer feedback
« on: June 27, 2013, 03:51:29 PM »
I'm looking to build a heliostat for greenhouse (and maybe regular house) heating, and am considering using something like these: http://www.aliexpress.com/store/group/Miniature-linear-actuator-with-Potentiometer/108393_211205913.html

Does anyone have any experience with them? The advantages in my mind are that they are already available in weatherproof IP ratings, don't need additional endstops or positional feedback, and would be relatively easy to control. Various brackets are available for mounting them which ought to simplify the mechanics.

Any downsides? They're more expensive than steppers (but I think they're still very reasonable), and I am a little worried that the potentiometer feedback won't be precise enough.

Thanks!

David


Gabriel

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Re: Linear actuators with potentiometer feedback
« Reply #1 on: June 27, 2013, 05:32:08 PM »
I personally have never used these, but I've always been curious to how accurate they are. Like you said they should be relatively easy to control. Granted, the stepper based program is already up and running, so you would no doubt spend more time coding if you go this route, but probably less time building the machine because the actuators are finished and ready to mount.

It would be nice if there was a datasheet somewhere that gave some info on this type of actuator's accuracy. I've also wondered if their positioning varies some depending on both the temperature and on wear on the potentiometer over time.

How do the potentiometers even work in these. Do they just give you the measured resistance so that you can count the revolutions or what? If that's the case then they aren't really saving any work. I think there are types of linear actuators that do give you the length when you measure the resistance, but it's not obvious to me that these do. Like I said though, I have no experience with these things.

A greenhouse is generally a pretty big target, so accuracy might not be a huge deal in this case anyway, especially if the heliostat isn't set up very far from it.

The biggest disadvantage with linear actuators in general is that they restrict the range of motion for the machine. Whether or not this really matters depends on both your latitude and the time of the year you are most interested in getting energy from the sun. Where the target is in relation to the heliostat can make a difference too.



dp

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Re: Linear actuators with potentiometer feedback
« Reply #2 on: June 28, 2013, 12:56:00 AM »
I might ask the supplier about the accuracy. I'm pretty sure it uses a multiturn pot connected to one of the final stages of the gearbox, and so the wiper resistance is proportional to how far the actuator is extended -- very simple to interpret. It's almost like a big RC servo (and you could control it in much the same way). The pot will only have a limited number of cycles, but given the average duty cycle of a heliostat I doubt that would be a problem. Temperature variation might be an issue. With all these issues, I suspect the only way to be sure would be to try -- hopefully worth $150 or so to find out...

I agree it would be nice to use a worm gear for the azimuth drive, but I can't find a simple way to do the mechanics with off the shelf parts, hence linear actuators for now. I did build a telescope azimuth drive once that was built by moulding a threaded rod into some epoxy putty to make the wheel, and then using nylon threaded rod for the worm gear itself...

dp

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Re: Linear actuators with potentiometer feedback
« Reply #3 on: June 28, 2013, 06:57:53 PM »
The supplier says 0.3mm accuracy and 20,000 cycles. I'm a little skeptical about both, but great if true (plenty good enough for a heliostat, and ought to last decades).

Gabriel

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Re: Linear actuators with potentiometer feedback
« Reply #4 on: June 29, 2013, 05:58:49 AM »
The accuracy is actually a lot better than I expected. It will be interesting to see how well they work out.

I've been making my worm gears on my drill press. Basically, I cut out a circle, mount it on the drill press using a jig that hold it the correct distance from the center of the chuck, and cut the grooves for the threads using a tap. Once you're setup, it actually goes pretty fast. That's just a simple DIY method, but it's really not all that different from how they are normally made.