Trying to measure current draw on a robot and am getting confusing information...

Thread Starter

Chris D

Joined Dec 31, 2016
4
Hi all,

I am building a robot, it is powered with two 12 volt batteries in series. The "brains" of the robot are a Raspberry Pi and an Arduino. The drive system is two stepper motors (Nema 34 size) that should be drawing 3.5 amps each.

Now that the robot is able to roam around and things are getting closer to putting this thing into action, I am trying to pay close attention to power draw, battery drain etc.

So, I connected my multimeter's RED probe up to the postive terminal of one battery (remember I have two in series), and connect the black lead up to the cable that would normally plug into that terminal on the battery. Set the meter to read AMPs, powered it up with just the logic and sensors active. It showed that it was drawing 0.97 Amps which sounds about right, no surprise there.

Then I repeated the test but also powered up the stepper motors/drives (Gecko). Oddly enough, the system is showing that it is only using 1 additional Amp with both motors powered. Knowing that the Gecko drive does go into a "Low power" mode, I then ran the steppers up to speed which showed it drawing yet another Amps (for 3 total).

I was expecting to see a current draw of 7 or 8 Amps.

Am I missing something in what I am doing?

For further information, there are inrush current limitting thermistors between the power source and the stepper drives which are 4 Amps Thermisters.

Chris D.
 

ericgibbs

Joined Jan 29, 2010
10,449
hi Chris,
I guess your motors drives are PWM or stepping pulses.?? If so, a meter on DC current. will give a misleading reading.?
E
 

Thread Starter

Chris D

Joined Dec 31, 2016
4
The meter is between the battery and the drives, not the drives and the motors. As such, the effect of PWM or Step/Dir would (or should) have no effect on the current reading.

Chris D
 

ericgibbs

Joined Jan 29, 2010
10,449
hi,
That assumption is incorrect, the power drawn from the power supply [ battery] will be pulsing.
Use an oscilloscope and a low value series resistor between the 0V of the battery and the project to measure the current.
Do you have a scope.?
E
 

Thread Starter

Chris D

Joined Dec 31, 2016
4
hi,
That assumption is incorrect, the power drawn from the power supply [ battery] will be pulsing.
Use an oscilloscope and a low value series resistor between the 0V of the battery and the project to measure the current.
Do you have a scope.?
E
I only have a "Prop Scope" which is not a real scope. I will have to check the voltage limits of it, I believe 24 volts exceeds it capabilities.

Chris D
 

Thread Starter

Chris D

Joined Dec 31, 2016
4
Stepper motors are funny things.

24 volts is maybe kinda low, on my mill I'm running nema 23 540 oz/in motors with a 48v power supply. The thing is with steppers you actually can use steppers that are too big for the application and they will be inefficient. But I guess you'll know this ;)

http://www.geckodrive.com/support/step-motor-basics.html
Yes, 24 volts is low, but when running on batteries, you have to make compromises. I doubt that the motors are too big for the application, assuming everything is correct with the drives, current supply, thermisters, etc.

The Robot is about 4 feet tall, weighs about 35 pounds, wheels are mounted to the motor shafts (direct drive), and they require a very slow acceleration so as to not skip steps or stall out. However, if the motors/drives are only getting 1 Amp each of current instead of the 3.5 Amps, this could explain the need for very slow acceleration.

Chris D
 

ClassOfZero

Joined Dec 28, 2016
114
Personally from a mechanical point of view I wouldn't mount the wheels directly to the wheels. Unless you have done the math there is no real way of knowing whether the motors are the correct size.
Another thing that needs looking at are the torque curves of a motor, you may find that a smaller motor with reduction might be a better option. All in all 35 pounds isn't all that much.
I've seen quite a few threads on cnczone regarding rapid speeds (yanks seem to be obsessed with this) and different sized motors, bigger does not always mean better.
What eric said regarding current measurement is correct.
I think of stepper motors a bit like 2 strokes, there's a sweet spot for power.
 
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