On Thu, 11 Feb 2016 18:17:51 -0600, Tim Wescott
wrote:

On Thu, 11 Feb 2016 13:59:30 -0800, etpm 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

Oooh. If you want to sound trendy, you're building a system with haptic
feedback. That'll get you a _lot_ more funding than "a servo motor that
you can feel when it sticks".

You can get motors with low or no cogging torque. I'm not sure if you
can get them _cheaply_, but they're out there. Coreless motors are most
definitely of this variety.

Gear a DC motor down, not too far that the drag of the gear train or the
inertia of the motor is noticeable. Then run a current through the DC
motor that's proportional to the current draw of your servo motor.

The 1940 way to do this would be to use DC motors for servo drive and
haptic feedback, select them carefully, and just connect them in series.
The torque on the feedback motor will, automagically, be proportional to
the torque on the drive motor, without you wasting too many vacuum tubes
or relays (since this in 1940) on making it so.

You'll probably have all sorts of bandwidth issues that'll be easier to
settle if you use fancy electronics and whatnot.
Greetings Tim,
I am indeed designing a haptic system. The mechanical parts I
understand pretty well but the electrotonical and automagical stuff I
need a little help with. Terry suggested the gear train DC motor idea
but I don't think it will work for what I am trying to do. Please
correct me if I am wrong but it seems to me that with the system you
describe above the servo connected to the handwheel will have the same
current passing through it as the servo that is doing all the work. Is
this the case? The handwheel torque resisting device will need to
provide, at most, 40 inch pounds of resistive torque, and this will
require little current, or more accurately, few watts. Less than15
watts on a continuous basis.The servo motor that the handwheel is
controlling, the one that is actually doing the work will oftentimes
be consuming 1800 watts. Would this work with your scheme? As
mentioned in my original post each handwheel will be connected to an
encoder. These encoders will be providing the information needed to
control the position and speed of the main servo motors. I am not
considering something like a SloSyn system. If I turn the handwheel
such that an axis should move .0001" inch I expect that it will. I
can, and have, done this in the past. Used a hand wheel to turn an
encoder to provide a signal to a servo amp connected to a servo motor
connected to a ballscrew connected to a slide in order to move it in
..00005" increments. What I haven't done is to add drag to the
handwheel that is proportional to the load on the servo. Even if the
drag on the handwheel isn't exactly proportional the positioning will
and must always be correct, the encoder to circuitry to servo motor
cannot be affected by whatever is used to add drag to the handwheel.
Cheers,
Eric