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Measuring Current output to Stepper Motor

Posted by rm446 
Measuring Current output to Stepper Motor
July 08, 2014 08:55PM
I want to get a measurement of the current being outputted to my feed stepper motor in real time. I believe I could use this information with the stepper's spec to estimate the torque output of the stepper at any given time (and thus determine the pressure drop across the hot end).

What's the best way to achieve this? From what I've heard there's no convenient software in place that'd make the RAMPs simply display the current signals it outputs.

Are there any spare pins on the RAMPs well suited to the task of measuring current? My Printer is fairly bare-bones (2013 Type A Series 1, no heated bed, no SD Card, 1 print head) so there's a good number of open pins but alas I don't quite understand what's used for what. I know there are 2 other sets of thermistor pins, can they be adapted to measuring current instead of resistance?

I'd be very grateful for any assistance you guys can provide.

Thanks!
Re: Measuring Current output to Stepper Motor
July 08, 2014 11:24PM
You cant read current... Not directly

The mega can read voltage via a ADC , the stepper driver provides a sense resitor just for this purpous. you can then caculate curent via maths. See vref

The thermisters are also reading voltage to determine the resitance. See Thermister theory

Any spare analog input pin can be used for this...

Cant help with the software side...
Re: Measuring Current output to Stepper Motor
July 09, 2014 02:42AM
Quote
rm446
I want to get a measurement of the current being outputted to my feed stepper motor in real time.

Unless your stepper driver is broken, this current should be constant all the time. Stepper driver chips which feature sort of a "standby current" exist, but none of A4983, A4988 or DRV6625 are in this group.


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Re: Measuring Current output to Stepper Motor
July 14, 2014 02:09PM
On my motor drivers, I am able to read and adjust the voltage to the meters with a multimeter. The black probe (gnd) went to the ground wire on my power supply for convienence and a little balancing act (aligator clips would have been nice). The red probe went to the potentiometer (metal) on the driver board. With the meter set to 2V (?) I was able to read the current voltage with the USB plugged in. I don't think plugging in the 5A power supply was necessarily needed.

The balancing act came in when I was using a screw driver to adjust the potentiometer, with the red probe touching the screw driver rather than the potentiometer directly. This took both hands, so I had to lay / wedge the black probe just right to complete the circuit. Not recommended though as you supposedly can short your motor driver.

Using the VREF that Dust linked, I think you can estimate the current. However to measure it directly, you need to wire the multimeter into the circuit between the motor and the motor driver. I have seen it done in a few YouTube videos, but could never find a diagram for trying it myself.
Re: Measuring Current output to Stepper Motor
July 16, 2014 10:51PM
Quote
Kurzaa
On my motor drivers, I am able to read and adjust the voltage to the meters with a multimeter. The black probe (gnd) went to the ground wire on my power supply for convienence and a little balancing act (aligator clips would have been nice). The red probe went to the potentiometer (metal) on the driver board. With the meter set to 2V (?) I was able to read the current voltage with the USB plugged in. I don't think plugging in the 5A power supply was necessarily needed.

The balancing act came in when I was using a screw driver to adjust the potentiometer, with the red probe touching the screw driver rather than the potentiometer directly. This took both hands, so I had to lay / wedge the black probe just right to complete the circuit. Not recommended though as you supposedly can short your motor driver.

Using the VREF that Dust linked, I think you can estimate the current. However to measure it directly, you need to wire the multimeter into the circuit between the motor and the motor driver. I have seen it done in a few YouTube videos, but could never find a diagram for trying it myself.

Get yourself a spare red multimeter probe, they are very cheap.
Make the probe tip into a small screw driver and then when you make the adjustment to the driver board you will also be able to read the voltage, no balancing act needed.
Modify a spare black multimeter lead so that it has a pin end that can be plugged into a GRD socket (spare driver board socket for instance.) and then you have that connection taken care of as well.


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Re: Measuring Current output to Stepper Motor
July 17, 2014 01:15PM
Quote
regpye
Get yourself a spare red multimeter probe, they are very cheap.
Make the probe tip into a small screw driver and then when you make the adjustment to the driver board you will also be able to read the voltage, no balancing act needed.

Modify a spare black multimeter lead so that it has a pin end that can be plugged into a GRD socket (spare driver board socket for instance.) and then you have that connection taken care of as well.

Wow, great idea. Thank you for the suggestion.
Re: Measuring Current output to Stepper Motor
August 24, 2017 07:55PM
I have the same issues.
I have the Arduino R3 and the CNC shield with the 4988 drivers. when I check the voltage the set screw on the driver is maxed out but only reading 1.31
any ideas on what is wrong. no power supply power just from the usb cable. I also have an issue with the stepper motors not moving my X and Y axis, I assume it is because of the low voltage.
I am using this on a DIY Laser Engraver I am building from scratch.


My steppers are:holding torque:450mN.m
2 Phase
3.73 volts
1.5A Amps
resistance: 5.0 ohms


Thanks for any help.
Bruk
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