Motor shield & current limiting?

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lithium
 
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Motor shield & current limiting?

Post by lithium »

Does this motor shield provide current limiting for stepper motors? We're planning on using this motor with an external power supply.

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adafruit_support_bill
 
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Re: Motor shield & current limiting?

Post by adafruit_support_bill »

If you power the motor with the rated voltage, the coil resistance provides the current limiting. 216-259 mA is within the limits of the shield capabilities. This motor should work fine with the shield.

Ranger1
 
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Re: Motor shield & current limiting?

Post by Ranger1 »

Im confused? How can you power this with 12VDC????? I tried this same motor and it heats up the chips in like 10 seconds. The motor just vibrates with the continuos code sketch.

Do you run 12VDC in the +M/GND terminals, or the arduino ext. power socket with the jumper in place?

Why is there no documentation on how to do this?

Or, do you upload the code, disconnect the USB, plug in 12VDC in the +M/GND and then run? Can you put 12VDC on the +M/GND term's and have the USB connected? And if so, is the jumper in place? Why is this so confussing to me? I feel so stupid on such an easy issue.

Sorry if I interrupted, or butted in, but I really want to know how to get the same motor you got working.

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adafruit_support_bill
 
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Re: Motor shield & current limiting?

Post by adafruit_support_bill »

Do you run 12VDC in the +M/GND terminals, or the arduino ext. power socket with the jumper in place?
You use the external power terminals with the jumper NOT in place. When using external power, you have to remove the jumper to isolate the motor power from the Arduino power.
Why is there no documentation on how to do this?
It is right here:
If you would like to have the Arduino powered off of USB and the motors powered off of a DC power supply, plug in the USB cable. Then connect the motor supply to the PWR_EXT block on the shield. Do not place the jumper on the shield. This is a suggested method of powering your motor project

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lithium
 
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Re: Motor shield & current limiting?

Post by lithium »

When we measure the resistance of the coil with a multimeter, it's actually 5 ohms for each coil. Wouldn't this cause a very high current when the motor is holding position? Also, wouldn't this cause a much higher current than what is acceptable by the motor? We're still using the same motor linked to in the first post.

Edit: I should note that we just got the shield a few days ago, and haven't had a chance to start toying with it.

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Re: Motor shield & current limiting?

Post by adafruit_support_bill »

it's actually 5 ohms for each coil. Wouldn't this cause a very high current when the motor is holding position?
The spec sheet says it should be 50 ohms. Double check your measurement & contact the supplier if it doesn't meet the spec.

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lithium
 
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Re: Motor shield & current limiting?

Post by lithium »

We double checked the resistance, and it is in fact 5 ohms. There's also a sticker on the motor stating it has 5 ohms.

We've read on the internet that some higher torque steppers have lower resistances and require current limitation, and it appears as though this might be the case here. Would it be possible to limit the current on the DC power supply before it's connected to the shield? Would there be any negative side effects to the shield by doing this?

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Re: Motor shield & current limiting?

Post by adafruit_support_bill »

We double checked the resistance, and it is in fact 5 ohms. There's also a sticker on the motor stating it has 5 ohms
Then it is not the same motor in the link.
Would it be possible to limit the current on the DC power supply before it's connected to the shield?
That would only limit the total current available. It would not control the current to each phase when multiple phases are energized (which is pretty much all the time).
Would there be any negative side effects to the shield by doing this?
Limiting the current will limit the torque. If your coil resistance is only 10% of what is in the spec sheet, you can expect a similar reduction in torque if you limit the current to match.

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studen
 
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Re: Motor shield & current limiting?

Post by studen »

Hi,
I am also trying to limit the current that my motor draws because I am facing a problem of overheating. One solution is to be done with resistors. But is it possible to be done from the software e.g. libraries?

Thanks

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Re: Motor shield & current limiting?

Post by adafruit_support_bill »

You could implement a crude form of current reduction (not really current limiting) by reducing the maximum duty cycle for the PWM in the library. However, as with a resistor-based solution, it is 'open loop' since it does not have any feedback to confirm the actual current level.

A better solution is to use a true current-limiting driver such as the Synthetos boards we have in the store.

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studen
 
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Re: Motor shield & current limiting?

Post by studen »

Thanks a lot for the reply. One more question. If one moves the stepper motor with ''SINGLE'' type of movement, what type of signals does the controller send to the motor? Square signals? Please let me know in case of the ''Adafruit motor shield'' library and the ''Accel Stepper'' library.

Thanks.

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Re: Motor shield & current limiting?

Post by adafruit_support_bill »

Square signals. There is no PWM for SINGLE, DOUBLE or INTERLEAVE modes.
Since AccelStepper calls the Adafruit library for all steps, this is true for AccelStepper as well.

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