The stepper is turning and I try to hold it (with my fingers, or something stronger).
How can I sense and measure the resistance I'm applying to the stepper? The resistance in not strong enough for stepper to miss any steps.
I'm new to electronics.. need some links and phrases on this topic.
-- Google "torque sensor".
Orient the motor horizontally, wrap a string around the shaft, and see how much weight it can lift. If it's a husky motor, you may need to increase the shaft diameter somehow. The radius of the shaft multiplied by the force from the weight is torque, measured in foot-pounds or newton-meters or some such units. The stepper should be rated in such units, too.
You could also use the string to pull on a spring scale and see how much force you can get before the motor starts missing steps.
You can also un-mount the motor, apply a load, and measure the reaction torque on the motor body.
John
Or clamp a stick to the shaft, and turn the motor to lift the stick. Then use the known mass and center of gravity of the stick, plus some trigonometry, to figure out the motor torque at the point of stall.
-- www.wescottdesign.com
Sorry, I wasn't clear..
The method you are proposing is mechanical, but I'm searching for a pure electrical method.
I don't want to measure how much the motor CAN lift, but how much the motor IS lifting.
A stepper has its driver, transistors etc, and when I apply a resistance to the shaft, something in the circuit probably happens that can be electrically measured, right?
Maybe "torque sensor", as John Fields said, I haven't searched yet.
John Lark> >
In one word, a dynamometer.
Yes, something. Steppers are complex and waste essentially 100% power at standstill or when stepping slowly, so there's no simple relationship between power consumed and power delivered like you'd see from a brush-type DC motor. It depends on the driver, too... L/R drivers behave different from choppers.
I'd vote to measure reaction torque on the can. Or do the string-torque thing and calibrate something observable in the drive electronics against torque at some given speed, and then use that.
Try measuring power supply current as you load the shaft, and see if that's a useful indicator of torque. It might be at some speeds.
John
A stepper isn't the proper type of motor to use if you're trying to sense torque/weight of objects/stall resistance. For simplicity, you can use a DC motor with a incremental quadrature encoder attached to the shaft to detect forward or reverse movements. This detected signal would then drive the motor to maintain position. From there, you can measure the amount of current the DC motor is using which can then translate to torque or foot pounds of the object pulling on it.
You can make your own encoder from a disc with holes in it and use 2 U-type Optical Rx and Tx units, offset from each another to generate a quadrature signal. Or, I suppose a plastic gear could be used with 2 U-type sensors.
In any case, you need to be able to detect shaft movement to lock it into position so that you can then take a current reading on the supply that is driving the motor.
One could use another DC motor attached to the same shaft which would generate DC voltage that is polarized, give you the direction and speed of the motor how ever, you wouldn't be able to bring it to a complete stand still this way.
-- This is USENET, not email, so please don\'t top post. Instead, bottom post or in-line post when it\'s appropriate. Thanks, JF
-- Dumbass, whether you think it\'s "proper" or not, what he wants to do is sense and measure the torque being generated by a _stepping_ motor when its rotation is being resisted.
Measuring change in the electronics sounds complicated... (holding current vs torquing current.)
I can imagine some spring loaded 'gizmo' that couples the shafts together and flexes under different torques. But now you have to get the signal out of a spinning thing... doable but...
How about measuring the force where the motor mounts to whatever is holding it in place? A little strain gauge or something? (Not sure exactly how this would be done.)
George H.
Oh excuse me..
Since I've done this sort of thing many times in various applications, why don't you step up to the plate and show us how's it done with a stepper motor? God of all Electronics and what ever else you hold yourself so high on the pedestal.
Since I offered no viable solution with a stepper motor and suggested a DC PM/SHUNT type, I must of had a reason?
Varying the current to the coils of the stepper until the slip takes place, to me, isn't a very accurate method. But don't let me stand in the way of progress. I'm sure you have a solution.
A stepper motor can perform that task how ever, you still need a feed back/angle sensor and special control over the current in the coils to allow movement so the weight of the object can be accounted for.
No matter how you look at it, it's still not a good option, so I'll wait for your solution with that superior intellect of yours!.
-- Oh, my, you certainly get testy when you\'re corrected, don\'t you?
Yes
Hi John, I hate the slip rings part. (but I've never used slip rings maybe they are better than I guess.) I did think about mirrors and lasers... It still sounds to complicated. I've got to measure time delays between the arrival of pulses. (OK maybe not that hard, but someone has to do the optical alignment.... what happens if things move under load? OK you can make it beefier.)
What's wrong with putting a strain gauge under the motor mount. I can make the other half of the motor mount a swivel joint and almost get a calibrated torque readout.
George H.
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