Re: Controllable torque electric motor questions

Richard Ferguson wrote:

I was having a discussion with someone about a machine design, and I suggested that a variable torque motor would be a good solution. I further suggested that a DC motor might be a good choice, high torque at low RPM. But neither of us are electrical engineers.

When I did a little internet research I concluded that there are probably multiple solutions, given today's electronics, and that a DC motor might not be the best choice. I noticed, for example, that most universal motors are not reversible. Operating motors at zero rpm can tend to overheat them. Operating universal motors at low load tends to overspeed them.

This application calls for a controllable torque to be output from the motor. This torque would be applied for several seconds at zero RPM, until the load started to move, and then the motor would rotate for a few seconds and stop. In a few seconds, the motor would reverse at minimal load, stop, and the cycle would start again. The machine would need to be able to apply the same torque in both directions. Some kind of position sensor would be used to determine the motor starts, stops, and reversals. A gear reduction would apply the motor torque to the machine, with a total machine rotation of 1/4 or 1/2 turn. 1/2 turn in 3 seconds would be 10 RPM at the final gear reducer output. The amount of power involved is not a great deal, since rapid operation is not important, and a lot of gear reduction makes sense. My guess is that the final torque, after gear reduction, would be in the range of 10 to 100 foot pounds. If we assume 100:1 gear reduction, the motor would need to put out around 1 foot pound of torque at zero to 1000 RPM. A small variable speed electric drill would have more than enough power, but might not survive the tough duty cycle. Without finding the formula, my guess is that the power required would be much less than 1 HP, given appropriate gear reduction, but perhaps good to oversize the motor to prevent overheating it. Kind of an odd application. The RPM is not a major concern, but being able to set the torque is key. Most likely, the torque needs to be controlled by a PLC (showing my age) or computer, but manually setting the torque by turning a dial or punching in a number may be acceptable. It is not clear how precise the torque control needs to be, probably +/- 10% would be more than adequate, given that we are automating something that has been done by hand by a craftsman.

Oh, and the application is similar to twisting square bar for decorative railings, something common for blacksmiths. Pretty easy to do if you get the bar red hot. (Obligatory metal content).

I tried to describe the application thoroughly, but I am sure that I left something out.

Anyone care to propose a motor/controller solution that would meet these requirements?

Does anyone know of a good reference that would help me understand the options?

Or am I asking at the wrong forum? Anyone care to suggest a better place to ask this question?

Richard


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
http://www.wescottdesign.com

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