On Thu, 11 Feb 2016 16:33:48 -0600, "Terry Coombs"
wrote:

wrote:
On Thu, 11 Feb 2016 13:41:56 -0600, Tim Wescott
wrote:

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

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.

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.
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

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 . ?
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