Arduino & CNC shield current consumption problem

Using Arduino Motors, Mechanics, Power and CNC
1

Hello, I am using a cnc shield V.3 with four A4988 drive tuned to provide maximum 1.8 A (each drive) and a lab power supply (30V-10A set to operate at 12 V). The configuration hase four motors: 1X axis, 1 Z axis and Y axis is cloned (2 motor for the same axis). I have noticed that no matter which axis I am moving (or if I am moving XY together) the same current is drawn from the power supply (1.8A). I am commanding the system with UGS.

I would expect instead that:
-When I move X axis (1 motor alone) I should see 1.8 A consumption
-When I move Y axis( two motors) I should see 3.6 A consumption
-When I move XY together (3 motors) I should see 5.4 A consumption

If I see always 1.8A this should mean that when more axis are moving togheter (i.e. XY diagonal motion) each motor is drawing less current, so a lower holding torque is available which means lower feed force.

Does anyone know why this is happening and what I need too do to fix this?
Thanks!

2

It depends on your meter. A DMM may not be able to measure PWM current properly. Also the motor current consumption depends on the actual torque.

3

Stepper motors pull current all the time, not just when they are stepping.

4

Oops, you're right. Which stepper motors are used? Current driven motors are controlled by PWM in the driver.

5

You are assuming steppers and stepper drivers are like DC motors - they are utterly different and this behaviour is exactly what I'd expect - constant load all the time.

6

Why?
A higher voltage means a lower supply current.

1.8A is too high for the A4988.
The DRV8825 would have been a better choice.
Leo..

7

Higher voltage means higher current on voltage driven steppers, and faster stepping. Driver boards may come with current limiting for current driven steppers and reduced power idle state.

1.8A for 3 motors (0.6A each) should be within the driver specs. The motor specs are still secret, preventing any specific help :frowning:

8

But OP is using constant current A4988 stepper drivers.
Like switching buck converters, supply current will go down when supply voltage goes up.
Leo..

9

thanks. I agree, stepper current should be constant, BUT for each motor. So if I have 3 motors then I should see 3 times the current. Maybe I can try with the $1=255 command enabled (which means that all the motors, after any movement are kept in hold) and then I should see the current increasing.

Increasing voltage should provide better motor beahviour (let me say, more "square" current waveshape), actually i noticed that increasing from 12 to 15 V , the current absorbed goes down from 1.8 to 1.5A (without any change on the potentiometer on the A4988 drive). I tought that increasing voltage supply shouldn't change current absorbtion, or at least should increase it (if I were not operating at the current limit of the A4988). This is in line with Wawa comment.

In the end it seems that the grbl is sending command in series and not in parallel (maybe this is the way it works) so that the motor are always powerd one at time, and never together, therefore XYmovement are actually rboken up in to small X-Y-X-Y-X...sequences.

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