Strange stepper behavior. A real puzzle.
Alphawookie
Posts: 14
Ok,
I know this is NOT the right way to hook up steppers, but I was curious, so I did it.
I began when I needed to add some resistance to a big stepper to keep the current down for testing. I just put another stepper in series to make the resistances add up.
Then I began to wonder "what if", so I took two eight-wire steppers and hooked them up with the ( - ) wires from motor A to the ( + ) wires on motor B. The motor B ( - ) wires were taken together back to the Little Step. This is what happened.
Both were identical NEMA 23's, about 2.2 ohm per phase, driven by a 5V supply.
At low step rates, both motors performed at roughly half strength. That's what I expected. Forward, bachward, ramps up and down, nice and smooth.
The bizzare started when the step rates got to the mid 400's/sec. One motor would turn, the other would not. To test what kind of torque I was getting, I stopped the spinning shaft with my fingers, and THE OTHER MOTOR STARTED RUNNING!
I was puzzled in the extreme. Then I gave the stopped shaft a spin, and the one that was running stopped.
I played with it over and over, and the same results. Some sort of electrical "differential" thing going on.
Any of you have anything to add on this? (other than "why in the H#!! would you want to do this?) It's got me interested to say the least. If I can learn what's going on, I'm sure to have a better understanding of NORMAL stepper behavior.
Thanks,
Tom
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Engineering is the art of doing·something well with one dollar, which any bungler can do with two after a fashion.
I know this is NOT the right way to hook up steppers, but I was curious, so I did it.
I began when I needed to add some resistance to a big stepper to keep the current down for testing. I just put another stepper in series to make the resistances add up.
Then I began to wonder "what if", so I took two eight-wire steppers and hooked them up with the ( - ) wires from motor A to the ( + ) wires on motor B. The motor B ( - ) wires were taken together back to the Little Step. This is what happened.
Both were identical NEMA 23's, about 2.2 ohm per phase, driven by a 5V supply.
At low step rates, both motors performed at roughly half strength. That's what I expected. Forward, bachward, ramps up and down, nice and smooth.
The bizzare started when the step rates got to the mid 400's/sec. One motor would turn, the other would not. To test what kind of torque I was getting, I stopped the spinning shaft with my fingers, and THE OTHER MOTOR STARTED RUNNING!
I was puzzled in the extreme. Then I gave the stopped shaft a spin, and the one that was running stopped.
I played with it over and over, and the same results. Some sort of electrical "differential" thing going on.
Any of you have anything to add on this? (other than "why in the H#!! would you want to do this?) It's got me interested to say the least. If I can learn what's going on, I'm sure to have a better understanding of NORMAL stepper behavior.
Thanks,
Tom
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Engineering is the art of doing·something well with one dollar, which any bungler can do with two after a fashion.
Comments
·
This effect is similar to the electronic differential·hands on experiment that I had mentioned in the link below.
http://forums.parallax.com/showthread.php?p=609161
·
Another thing you can do with two steppers that's cool... without any power applied, match the phases and
appropriately connect the·two stepper motors·together.· Don't forget to connect the common(s).·· Now hand
spin one stepper motor and watch what happens to the other stepper motor. [noparse]:o[/noparse])
·
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Beau Schwabe
IC Layout Engineer
Parallax, Inc.
Post Edited (Beau Schwabe (Parallax)) : 11/8/2006 5:02:22 PM GMT
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(Frequently heard from other's)
Tommy, I know it wasn't designed to·x, but can you make it·do x·anyway?
·
What are you referring to? (Tr/Tx) doesn't mean anything to me. If it's cool I'd like to know!
I was wondering if you could use a small stepper in lieu of an encoder, sort of like a Hall-effect sensor with 200 counts/rev. If so, it could help me out on my pin shortage problem. (I just need one more....)
Tom
I remember reading this can damage the magnetic flux of the stepper, I can't lay my hands on where or what I read just that it stuck in my mind perhaps it is not correct.
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Think outside the BOX!
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(Frequently heard from other's)
Tommy, I know it wasn't designed to·x, but can you make it·do x·anyway?
·
·
·· I think a lot of people are going to be more used to seeing RX/TX for receiver and transmitter.
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Chris Savage
Parallax Tech Support
·
"spin one stepper motor...I remember reading this can damage the magnetic flux of the stepper."
·
Please let me know if you find that information,· I have never heard that before.
·
Basically a stepper is a mini multi-phase generator
·
Most (small) steppers that I have seen consist of a single rare earth magnet with specialized
washers creating an interdigitated ...N-S-N-S... pole around the circumference·of the armature.
The coils usually in pairs have matching "teeth" that align to the ...N-S-N-S... pitch.
Depending on the number of phases the stepper has, depends on how each coil group is aligned
to the ...N-S-N-S...armature.
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Beau Schwabe
IC Layout Engineer
Parallax, Inc.
Thanks again Chris.
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(Frequently heard from other's)
Tommy, I know it wasn't designed to·x, but can you make it·do x·anyway?
·
http://www.servorepair.com/servo_talk_sample.htm
This is not exactly what I read before but similar, I remember what I read said just spinning the motor without driving it can reduce the magnetic field. The clip below talks about back EMF. Like I say, I am by no means even qualified to speak on magnetism it's one of those very deep physics subjects. I can't remember short term stuff but sometimes what I read sticks. I will keep looking.
Clip below from [noparse]:http:[/noparse]//www.shinano.com/xampp/faq_stepper.php#11
"
How does demagnetization occur?
The best way to demagnetize something is to expose it to a high frequency-high amplitude magnetic field. This can be accomplished by running the control system to spin the rotor at high speed when the rotor is actually stalled, or spinning the rotor at high speed against a control system trying to hold the rotor in a fixed position. These will both expose the rotor to a high amplitude high-frequency field. If such operating conditions are common, particularly if the motor is run near the curie temperature of the permanent magnets, demagnetization is a serious risk and the field strengths (and expected torques) should be reduced accordingly! (top)
How does the counter EMF reduce the motor torque?
In a permanent magnet or hybrid stepping motor, the magnetic field of the motor rotor changes with changes in shaft angle. The result of this is that turning the motor rotor induces an AC voltage in each motor winding. This is referred to as the counter EMF because the voltage induced in each motor winding is always in phase with and counter to the ideal waveform required to turn the motor in the same direction. Both the frequency and amplitude of the counter EMF increase with rotor speed, and therefore, counter EMF contributes to the decline in torque with increased stepping rate. (top)
"
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Think outside the BOX!
For reasons that you normally don't see a frequent demand generator with permanent magnets in it, I will agree, that over time and thermal abuse a magnet will eventually loose its ability to remain magnetized.
Another point is that the configuration that I mentioned is not connected to any drive circuitry that "could" be damaged by voltages generated by an externally spun stepper motor nor is the drive circuitry trying to move a stepper that is stalled or trying to turn in the opposite direction.
The back-to-back hands on configuration that was mentioned is simply using the stepper as a multi phase generator to power another stepper in the same phase sequence. I would be impressed if a hand spin could get a sustainable RPM over a significant period of time that would generate enough heat to warrant any damage to the magnet in the stepper.
!!! Warning - No user serviceable parts inside !!!
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Beau Schwabe
IC Layout Engineer
Parallax, Inc.
Just a random bit of information
Brad