Controllable torque electric motor questions

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
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"Richard Ferguson" wrote: (clip) 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). (clip) ^^^^^^^^^^^^^^^^^^ Well, I don;'t know much about the electric motor part, but I am very experienced at twisting square bar for handrails, which, as you say, is the metal content of your post. It's normally done cold, for two very good reasons: 1.) 1/2" square stock, which is the most commonly used size, is not hard to twist cold. 2.) If you try to do it hot, any unevenness in the temperature will cause unevenness in the twist. If this is the type of thing you are planning to do, I wanted to be sure you knew this. BTW, the twisting should be done inside a pipe, to keep the bar from looking like a corkscrew.
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I wish I had paid more attention years ago when visiting a fellow in Mass. He had just built a device for twisting bar stock using a pneumatic cylinder to do it. He hit the air valve and instantly 4' of bar stock was uniformly twisted and perfectly straight. Any ideas??? Respectfully, Ron Moore

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Leo Lichtman wrote:

I am not experienced with twisting square bars, but have seen what a local " Wrought Iron " shop used. He used a ordinary induction motor connected to a worm gear reducer. The output of the gear reducer was connected to an automobile transmission. He did not do the bending inside a pipe and they still came out straight. I can't remember exactly the way the square rod connected to the machine, but as I remember it just dropped in a three sided square socket. The distance between the sockets was fixed so when the twisting was being done, the square rod would be getting slightly shorter and would be under some tension. Which is probably why the twisted rods came out straight. The speed was pretty slow, I would guess about 1 rpm. And as I remember he did not use any sophisticated limit switch. Just shut the power off after the rod was twisted the correct number of turns.
So no fancy motor, no fancy turns counter, and the gear reducer was not too expensive as the auto transmission supplied the final drive. I think he may have had a 1/2 hp motor. This is recalled from seeing it a good many years ago.
Dan
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Richard Ferguson wrote:

<snip>
I think you could do this with a VFD and an AC induction motor with a gear reducer. I'm not sure I see why controlled torque is in any way necessary. In fact, it seems like a big mistake, as different bars will take different amounts of torque to start to bend. Why not just turn the motor slowly, and stop when you get the required number of turns. Why do you need to reverse? Just to get the two bar clamps aligned so the straight bar can be inserted? No need to reverse, just go to the nearest "index" position. So, it could all be done going in one direction, it seems. When you have twisted the bar and stopped, it will be under torsional load, and may be hard to remove from the clamps. Not a problem on a manual twister, but there may need to be some mechanism to unload that tension so the finished bar can be removed.
As for motor overheating at low speed, just add a fan to force air through the motor.
Jon
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On Mon, 14 Aug 2006 04:17:09 GMT, Richard Ferguson

The torque of a DCPM (permanent magnet) motor is linearly proportional to its current almost regardless of speed. Some if not most servomotors are designed to deliver a rated torque down to and including zero speed, indefinitely.
If your accuracy requirement is only 10%, then straight current control would probably work fine for you. If you want more accuracy, then you'd have to measure torque somewhere in the system, perhaps with a loadcell, and use that as a feedback signal in a closed-loop control.
One possible error source is the stuff between the motor and the load, e.g. gears and other power transmission components. Their drag torque may not be constant with speed, temperature or over time as things wear and/or the state of lubrication possibly changes. The most accurate and reliable approach would be to control motor current using a feedback signal derived from a torque sensor that meauses actual torque delivered to the load.
One ft-lb of torque at 1000 RPM is about 142 watts or about 0.2 HP. That'll give you a rough idea of what size motor you need -- but you'll want to select a servmotor that can deliver 1 ft-lb continuously at stall. Just one example: Baldor MT-3363-BLYCN can deliver continuous stall torque 11.25 in-lb (almost a ft-lb) when drawing 4.75 amperes. It's rated speed at 100 volts is 2400 RPM, so with slightly more gear reduction than you contemplate this motor would easily do your job. There are many other servomotors available from a number of manufacturers. Google is your friend.
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Sounds like a job for a large scale RC servo. All the gearing and feedback mechanics are built right in. 6 volts dc. available at any hobby shop for 25 bux
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Thanks to everyone who responded. An interesting and educational discussion.
I think that I understand the issues of friction and stiction. ;-) I will side with those who think that friction and stiction will reduce the delivered torque, even at zero speed. As long as the friction and stiction was fairly consistent, it should not be a big concern. However, a prudent designer might plan ahead about how a torque sensor could be installed if more precision was required.
I like the idea of a DC permanent magnet motor rated for stall use. Apparently these types of motors are commonly used as servomotors. Regulating the current to control the torque makes sense, and sounds fairly straightforward, with an appropriate programmable power supply. Looking at the curve on the Baldor website, it appears that torque is proportional to current, while RPM is a function of current (torque) and voltage. I noticed that someone said that temperature was also a factor, although I did not see it on the Baldor site, but perhaps that effect could be minimized by an oversized motor or other conservative design.
I can understand that twisting square stock by machine is usually done cold, but this application is a little different, and does not involve square stock. The process already exists, albeit in a manual form, the goal is just to automate it.
The more I think about this project, I realize that a good controls designer is key. I could probably handle the mechanical design and overall system design, working with the guy who developed the process, but obviously I am not up on servomotors, programmable power supplies, PLCs, etc. The project gets bigger the more I look at it. ;-) I may just offer a bit of free advice and bow out.
Thanks again, I learned some stuff.
Richard
Don Foreman wrote:

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Richard Ferguson writes:

Seems like DC motor is exactly what you want (and I *am* an electrical engineer). A variable- or fixed-current voltage-limited power supply will deliver any torque you care to specify from stall to some very slow top speed.

Not true of many DC motors. They are typically designed to deliver lots of torque at stall or low speed, and to dissipate heat by radiation/convection rather than by fanning themselves. Think of elevators, or industrial conveyors.
Look for a continuous stall torque rating in the specifications. Multiply by your gear ratio. (Note: gearing efficiency doesn't matter at zero rpm.) Find one that delivers the oomph you need.
Your arms only put out fractional horsepower. So a motor shouldn't present much problem replacing them to twist stuff.
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On Mon, 14 Aug 2006 01:04:00 -0500, Richard J Kinch

His example cited 1000 RPM motor speed with 100:1 reduction.

His specified motor torque (1 lbf-ft) already assumed multiplication by a 100:1 gear ratio.

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Richard J Kinch wrote:

Gearing efficiency _does_ matter at zero RPM. A gearbox's stated efficiency loss is almost entirely due to frictional losses, which you need to take into account going from the torque at the motor to the torque at the shaft. With a 100:1 ratio it'll be hard to effectively control the torque at the output by controlling the torque on the input, unless you pay a lot of attention to the effects of the gearbox on torque.

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Tim Wescott writes:

If nothing's moving then frictional loss is by definition zero.
Maybe you mean, "stiction"?
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Richard J Kinch wrote:

True, but if your mechanism is designed to not move you need neither motor nor gearbox. As soon as things start moving the gearing efficiency _does_ matter a _lot_. Why? because Colombic friction is not a viscous drag that is somehow proportional to velocity, it is a constant torque that must be overcome.
I suppose that while your statement that "gearing efficiency doesn't matter at zero rpm" is technically correct, you left out the "but it matters for _any_ non-zero speed"

No, I wanted to avoid that question entirely if I could. But if you have to study up on it this may help: http://www.wescottdesign.com/articles/Friction/friction.html .
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Tim Wescott writes:

The point is, the *stall* torque developed at slow speeds isn't affected by friction. As a linear instead of rotational example, air cylinders develop force = area x pressure, without regard to friction.
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On Mon, 14 Aug 2006 17:20:00 -0500, Richard J Kinch

Perhaps a more relevant point is that if nothing is moving, nothing useful gets done in the context of the OP's inquiry. He cited 0 to 10 RPM at the load as the range of interest. For zero speed he could skip the motor and gears, just hang a weight on a lever. If he wanted to apply and release that torque, then the weight might hang on a rope and be lifted by a pneumatic or hydraulic cylinder.
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Don Foreman writes:

Yes, as Xeno taught us.
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Richard J Kinch wrote:

No. Not unless you're looking at the torque developed by the armature of the motor, which is a singularly useless place to be looking when what you care about is the output of a 100:1 gear box.
At the output of the gearbox, the torque will be
torque = (motor torque) * (gear ratio) - (gearbox friction)

No. Not unless you're looking at the force developed at the face of the piston, which is a singularly useless place to be looking when what you care about is the force developed by the cylinder assembly.
At the output of your hypothetical air cylinder the force will be
force = area x pressure - friction.
If friction is much bigger than area x pressure then you have to regard it a great deal, or resign yourself to poorly working machinery.
To the OP:
Sorry for the confusion. This is one of those special USENET moments when you have to read the posts, think a bit, and figure out who's right. Have fun.
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Tim Wescott writes:

And the friction vanishes as the system slows. The ultimate torque as you approach a stall is not affected by friction.

Friction is a monotonically increasing function of velocity, and zero at zero velocity. So a moving air cylinder pushing against a monotonically increasing force will develop an ultimate force = area x pressure when it comes to a stall from equilibrated forces, regardless of friction. The friction only affects the *rate of approach* to the final force, not the magnitude of final force. Similarly for torque in the rotational case.

I would say friction bigger than area x pressure is going to amount to locked brakes.
To the extent you have stiction, that matters.
It may help to think of it intuitively in terms of a more perfectly frictional system, such as a magnetic induction brake, rather than gears that have a combination of friction and stiction combining through gear ratios.
You can also think of it as a restriction in an air line. It slows the delivery of downstream air volume, and it lowers the downstream pressure under flow, but in the no-flow case you still have full pressure at the terminal.
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wrote:

Greetings Tim, I use motors a lot in the shop. I just took a quick look at the link you posted. Thanks a bunch. Eric R Snow
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On Mon, 14 Aug 2006 04:17:09 GMT, Richard Ferguson

I did something very like this a long time ago, except it was one way and more continuous. I used a DC motor input to the reducer. Nowadays, I'd use a small AC induction motor and VFD. The torque part was controlled by a magnetic particle slip clutch, which are very linear and repeatable over 0-24 V on the actuating coil. The application was web tension, and the sensor was a roller mounted on load cells. Worked extremely well.
Pete Keillor
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