Help please w/ electromagnetic slip clutch

Everybody, I'm working on a project that needs a slip clutch or brake or something. I'm turning handwheels that need some sort of instantly
variable resistance to turning . Anywhere from 0 to 40 inch pounds. I have been looking at magnetic particle clutches and brakes, eddy current brakes, and disc type clutches and brakes. The magnetic particle brakes would seem to be a good solution except that the ones I have seen that can provide enough drag have too much drag when not energized and have too large a diameter. About 3 inches diameter by about 3.5 inches long is the space the clutch or brake must fit in. I am considering rolling my own devices but am not sure how I would do it. Maybe some sort of generator feeding a resistive load. By varying the field strength the resistance to rotation would increase. Whatever the solution is the resistance to rotation must be linear to the current that actuates the device. That's one of the reasons I like the magnetic particle barkes. Thanks, Eric
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On 11/02/16 19:31, snipped-for-privacy@whidbey.com wrote:

Have you looked at automotive aircon clutches. I don't know if they're suitable and normally used for on/off operation but likely cheap to get one from a breaker to test or play with.
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wrote:

Greetings David, Those clutches are way too large. But thanks anyway. Eric
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wrote:

A/C clutches aren't made to slip at all, David. This won't work. Band clutch/brakes have been used in auto tranny setups and on old emergency brakes for early vehicles.
Eric, band brake setups might work for you, with solenoid actuation. Variable voltage to the solenoid could facilitate varied braking.
https://www.youtube.com/watch?v=fSRWf8PY_4Y
Something like this. But done a teeny bit more professionally? <g>
https://www.youtube.com/watch?v=RwVeIbRx7wE

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On Thu, 11 Feb 2016 11:31:49 -0800, etpm wrote:

Not enough information.
These handwheels are to be used to control something, for exercise, what?
I'm pretty sure that when you say "turning ... that need ... turning" you mean you're making things on a lathe that need resistance to someone twisting them.
Does the resistance need to be viscous, tapering off to zero as the speed tapers off? Does it need to be constant down to zero speed? Something else?
The excruciatingly high-tech way to do this is with a direct-drive motor, position sensor, and a fancy control system. That's overkill for a whole lot of applications, but I've done it for something that really needed it.
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On Thu, 11 Feb 2016 13:41:56 -0600, Tim Wescott

The handwheels turn an encoder that makes a servo motor turn a leadscrew. I want to monitor the servo current draw and use it to put a drag on the handwheel. So more work for the servo makes the handwheels harder to turn. I want to be able to feel the machine working. All the way down to zero speed. I thought a hysteresis brake would work until I found out about the cogging effect. I though about using a BLDC motor kit I have to make a skewed rotor to avoid cogging but I'm not sure if it would work, and if it did would it be fairly linear. Eric
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snipped-for-privacy@whidbey.com wrote:

Hysteresis ... How about a small PM DC motor hooked to a load and geared to turn faster than the hand wheel , might reduce or eliminate the cogging . ?
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On Thursday, February 11, 2016 at 2:33:50 PM UTC-8, Terry Coombs wrote:

I'd call it a 'generator' because that's what it is when it dumps power into a load resistor, but yeah, this is a sensible way to go. One could also consider using an AC motor with variable DC exciting current to act as a kind of adjustable brake.
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wrote:

I dunno Terry, You idea sounded good until I tried it a little bit ago. with a couple BLDC motors I had laying around. I just couldn't get enough drag, even with fairly high speeds and high performance motors. Your idea has merit but not with my constraints. Cheers, Eric
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snipped-for-privacy@whidbey.com wrote:

What you want is a small servo motor controlled by a servo amplifier. The torque is proportional to motor current. Just take the current monitor signal from the servo amp, amplify and apply to the handwheel motor.
Trying to do this with clutches or brakes will lead to great frustration. Only the most exotic clutches and brakes are very smooth, and otherwise would put a lot of stick-slip friction into a systemw here you really DON'T want that.
If the main motor is a DC brush servo, it is possible that a resistor can be put in series with it, and the handwheel motor connected across that resistor. Then you would not require a separate small servo amplifier.
Jon
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wrote:

Greetings Jon, The brakes and clutches I have seriously considered are non-contact devices so the stick-slip problem is one I wouldn't need to deal with. I thought about using a small motor as the brake but n oit the way you describe. The main servos are DC brush servos and will be using 20 amps max for about 4 minutes max, with maybe a 7 second period of a 4 amp draw and then 20 amps again and so on. There will be peaks of much higher current draws, maybe 80 amps, but these peaks will be quite short and the servo amps can handle the loads. Can you explain a little about how the resistor in series with the main servo and the handwheel servo, which supplies the drag, connected in parallel with the resistor, would work? Thanks, Eric
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snipped-for-privacy@whidbey.com wrote:
Can you explain a

Well, a very small resistance would be put in series with the main motor. This resistor would be of much lower value than the DC resistance of the handwheel motor. The handwheel motor would then be put across the resistor. Some portion of the main motor current would flow through the handwheel motor, producing a torque. Possibly, you'd also place an adjustable resistor in series with the handwheel motor to set the level of torque feel.
I really don't know if this will work, as when turning the handwheel fast that motor will generate a voltage that will be dumped in the resistor. But, since the servo system with the encoders will make the main motor synch to the handwheel, these back EMF's should be able to be equalized.
This would be a cheat that would allow you to do this without a bunch of electronics, like an extra, small servo amp. As I say, I'm not sure it would work.
Jon
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wrote:

Greetings Jon, If it works it is not a cheat. The handwheel motor would need to be wired backward with respect to the rotation of the main servo so that it will oppose rotaton of the handwheel. I will investigate your idea. Simple and easy to implement. If it works. Cheers, Eric
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snipped-for-privacy@whidbey.com wrote:

How about using DC to brake an AC servo? Higher the voltage the more resistance to motion.
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wrote:

How would that work? How are AC servos made? I'll look online. Eric
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On Friday, February 12, 2016 at 3:45:34 PM UTC-8, snipped-for-privacy@whidbey.com wrote:

Most AC motors (the ones without brushes) will brake if you feed 'em some DC. Steppers (but they cog), shaded-pole, or hysteresis types, all have a rotor without driven windings, and DC on the stator makes them into brakes.
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On Fri, 12 Feb 2016 16:39:36 -0800, whit3rd wrote:

This gets you back to something that brakes proportionally to speed, though.
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wrote:

I thoughty they only braked while spinning and once stopped there is no braking force. I ordered some MR fluid and will try making my own magnetorheological brake. Eric
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    That is true for AC motors. For steppers, since they have a permanent magnet rotor, the braking continues, but things are a bit rough until it gets to a stop. Perhaps sense the speed, and when it comes close enough to a stop, switch from AC motor to stepper. (Make things biggern than you want. :-)

    Have fun with that -- and let us know how it goes.
    Good Luck,         DoN.
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