Controllable torque electric motor questions

snipped-for-privacy@omitthisatt.net says...


10% torque control should be doable with a DC motor, either PM or brushless. A servo amp capable of operating in torque mode, a motion controller, and an encoder would be required to drive the motor.
Unless you need very fast accelerations (which doesn't seem likely), and size the motor conservatively, a standard PM DC gearmotor fitted with an encoder would be adequate. The torque/current ratio will vary with temperature, so you don't want to limit the temp rise of the motor.
Boston Gear can supply a motor/gearhead assembly, or you can buy a C- face motor and gearhead separately. You do need to figure out a way to mount the encoder if you can't get a motor with auxiliary shaft.
I'm partial to Galil controllers and Copley amps. The Galil stuff is expensive, though I bet you can find something on ebay. For example...
http://offer.ebay.com/ws/eBayISAPI.dll?ViewBids&item 0015861978
There are plenty of other good controller mfrs, Galil is just my personal preference. You could use a PLC as a motion controller, but it'd be much clumsier and not nearly as flexible.
I built an automated station for a video cartidge assembly machine that did almost precisely what you've described, but on a much smaller scale. It wound the leader (these were unstuffed cartridges - leader only) all the way onto one hub; applied a specified torque to test the leader attachment to the other hub; reversed and tested the attachment to the other hub; reversed again and positioned the leader for splicing the tape into the cartridge.
Ned Simmons
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
snipped-for-privacy@nedsim.com says...

Ned Simmons
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Richard Ferguson wrote:

You are correct that a universal-wound motor would be a bad idea. You probably want a DC servo motor that's sized for the service you intend. As already pointed out there are DC motors that are designed for continuous high torque slow duty.
I don't think that you'll be happy with trying to control torque through a 100:1 gearbox. I could be wrong, and the first thing you may want to do when you get hardware is to give it a whirl, but I think you'll find that the gearbox will introduce too much uncertainty in your torque, even if the motor is doing exactly what you want.
Were I doing this I would go with the suggestion for a load cell. Better yet, you might want to consider a torsion spring on the output of your gearbox. If you could stand the low speed, winding up the spring through the gearbox should give your controller time to react appropriately, and putting rotation sensors on each side of the spring should give you a good indication of torque.
sci.engr.control would be a good place to post this question. There's lots of guys there doing this sort of practical control, so the answers will be based on solid ground.
--

Tim Wescott
Wescott Design Services
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

Good idea!
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
From your description, I'd look at air cylinders. Air pressure controls torque. have the cylinder drive a rack, rotation on a pinion. KISS
--
<()> An apple a day keeps the doctor away.
Two apples a day gets the doctor's OK.
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Karl Townsend wrote:

If you can do that mechanically I'll second it. If you only need to rotate through 90 degrees or so a mechanical linkage rather than a rack may be a better idea -- but only if it's simpler than a rack.
--

Tim Wescott
Wescott Design Services
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Richard Ferguson wrote:

The application is not clear to me, but I can give you a way to do it. Years ago, I worked for a company that made aircraft simulators. The rudder pedals had to apply a force to the pilot reflecting the conditions of flight. The torque sensor was a rod of 1/4" spring steel about 4' long, with a servo pot at each end. One end of the rod had a worm gear/screw, driven by a Bosch weed whacker PM motor. The other end of the rod had a pulley around which the cables to the rudder pedals were wound. As torque was applied via the rudder pedals, the torque bar would twist. The twist puts the pots at different settings. The difference in resistance is proportional to torque. That torque value, along with the aircraft simulation inputs, was used to control the motor. The motor control was cheap and elegant. The motor was controlled by a triac, running off the AC line. The motor controller determined which half of the AC wave would be gated to the motor, giving motor direction, and the percentage of that half wave, to control drive. In operation, "on the ground" you could work the rudder pedals back and forth with little force as the motor servoed to put no torque on the rod. As "airspeed" increased, more force would be required to push the rudder. In a fault condition (one of the situations had the instructor sneaking over to pull the fuel cutoff for one of the engines)the pedal would come at you with (IIRC) 150 ft/lbs of force. It was inexpensive, simple, and worked very well. The system was designed by Jim Patmore, who was a genius.
Kevin Gallimore
-
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

Might you be a consultant? <G>

Mr Patmore may well have been a genius, but this is a straightforward (classic even) closed loop control with torque feedback. That said, it is elegant in its simplicity and economy.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Don Foreman wrote:

That was one of his claims to fame. Anyone could design a textbook circuit. He could design something that worked using a bent paper clip and a marble. I had known him for years before I knew he held a patent. I had had a chassis bent up in the sheet metal shop and had set it up in the lab when he walked by. In passing, he said "I have a patent on that". I said "you can't have a patent on a three sided box". He crooked his finger. I followed him back to his cubicle. He opened his file drawer and pulled out a stack of patents about an inch and a half thick. He shuffled through them and pulled out a patent on a three sided box. I picked up the stack and started going through them. The first electronic digital voltmeter (Non-Linear System's was electromechanical with a stepping relay, Jim's used triode flip-flops). The complementary symmetry output stage ("using a n-type transistor and a p-type transistor" says the patent). The shift register keyboard debounce circuit. Jim could do anything. He was a pilot. Scuba diver. Dirt bike racer. Sang in a barbershop quartet. Kept race horses. Had a lathe and made model airplane engines. I was a kid in the lab and he took care of me. I could learn more working for him than I could in any school.
Kevin Gallimore
-
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

<PEDANT MODE> Friction (Newtonian) is independent of velocity and is non-zero at zero velocity.
Viscous drag is a monotonically increasing function of velocity and is zero at zero velocity.
</PEDANT MODE>
If it were otherwise, nuts and bolts would not be of much use :-)
Mark Rand RTFM
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Mark Rand wrote:

Love that HTML. :-) ...lew...
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.