Help please w/ electromagnetic slip clutch

As an alternative to a current-proportional clutch, could you adopt a PWM system, to run your device more like the anti-lock brakes on a vehicle?

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I never heard of magnetic particle brakes. (I did google it) The problem with a generator or an eddy current thing, is that the damping will be proportional to the velocity, and I don't think you want that. You want variable friction..(I think) just something that rubs?

George H.

This sounds like a job for hydraulics.

A simple DC motor will do it. Not a generator, but a motor. It must rotate though, cannot stay long in the same position. Would wear out the brushes.

w.

Greetings Helmut, I posted the same message on rcm that I posted here. Jon Elson suggested using a motor too. His idea is to put a small resistance in series with the main servo and connect the small handwheel motor in parallel with the resistor. He's not sure it will work. Is his thinking sound? If so then I will try it out and report back here and on rcm. Cheers, Eric

Hydraulics would work but I won't be using any hydraulics.

Cross-posting would have been appropriate in this case.

That's basically a really crappy way of doing what I suggested. Using a current sense resistor and a current amplifier to drive your handwheel motor is a good way of doing my suggested "echo the current" thing.

The "motor in parallel with a resistor" will tend to have a torque that's proportional both to how hard the servo motor is pushing and to how fast you're rotating the handwheel -- I don't think that's what you want.

Greetings Tim, I don't crosspost because if some ass decides to reply to my post with something off topic and insulkting I don't want it to pollute other newsgroups too. I see enough of it on rcm. Regarding the resisting torque changing with handwheel speed you are right-I don't want the torque to change except by the changing current draw of the main servo. I tried to find a small amount of MR fluid so I could try making my own brake that way but no luck so far. I did order some ferrofluid even though the MR fluid is better. I may just end up with a brake that uses friction disc(s). Since the brake must be adjustable it could certainly be adjusted for wear of the friction surfaces. As I was writing this I realized that I cannot use a motor to apply torque, drag can only be used. This is because I don't want the handwheel spinning when the servo gets a movement command from a CNC control. So when the machine is being operated as a CNC machine the handwheels won't move. I'm a little frustrated right now because I have all the components for the machine conversion except the handwheel brakes. I have the machine, the servos, ballscrews, servo amps, encoders, power supplies, control, wire, VFD, contactors, switches, etc. And I have done a conversion before. Eric Eric

Is there any indication out of the CNC machine that lets you know when it's CNC-ing and when it's following the handwheels? If so, just use that to turn the haptic feedback on and off.

I know the feeling.

I ordered some magnetorheological fluid to make my own brakes with. It looks like the best option. Apparently its response to current is pretty linear which makes things easier. And it is easy to seal against leaking. All that is needed is a ring magnet around the shaft that comes out of the area containing the MR fluid. The fluid will form its own seal around the shaft. Eric

If magnetism causes the fluid to stiffen, do the ring magnets make it stiff? How do you apply magnetic field when you want it to brake? What happens if the shaft gets magnetized? Getting the fluid might only solve a small part of the problem.

The fluid is magnetized between two surfaces that move past each other, such as discs or a cylinder within a cylinder. The surface area is important. The small area where the ring magnet is provides little area for friction. Also, the magnetic field is kept low. So the friction is quite low and can be lower than a typical lip seal or o-ring seal. In the area where the braking effect is wanted the magnetic field can make the MR fluid as think as peanut butter. Ferrofluid can also be used as the seal and I have some of it on the way too. Ferrofluid does not form the fibrils that MR fluid does because of the much smaller particle size, so this means that it doesn't thicken the way MR fluid does. Eric