Steppers and max torque

I am using an UNO, GRBL v.8 and a CNC v3 shield with DRV8825 drivers to control 3 stepper motors using G-Code Sender. The power supply is a 24v 20A 480w.

I am not running a cnc machine but using this setup for a completely different application. The 3 steppers are attached to 3 components in an HF antenna tuner. The application is really irrelevant as my question will be about how to achieve the most torque from my steppers given the setup.

I would like to learn how to achieve the most torque from the steppers for both homing and x,y,z stepping. After I home the steppers I will send a simple code like this to send the steppers to their preset spots: g90 g01 f200 x -.697 y-0 z -11.406

The stepper is a NEMA 23 size 2.5A 1.3nm 184oz in.

I have the DRV8825 drivers set at 1/4 step.

Here are my G Code Sender parameters:

Machine control step size - .1mm/step (under machine control tab)

$0=175.000 (x, step/mm)
$1=200.000 (y, step/mm)
$2=175.000 (z, step/mm)
$3=5 (step pulse, usec)
$4=150.000 (default feed, mm/min)
$5=150.000 (default seek, mm/min)
$6=28 (step port invert mask, int:00011100)
$7=25 (step idle delay, msec)
$8=10.000 (acceleration, mm/sec^2)
$9=0.050 (junction deviation, mm)
$10=0.100 (arc, mm/segment)
$11=25 (n-arc correction, int)
$12=3 (n-decimals, int)
$13=0 (report inches, bool)
$14=1 (auto start, bool)
$15=0 (invert step enable, bool)
$16=0 (hard limits, bool)
$17=1 (homing cycle, bool)
$18=0 (homing dir invert mask, int:00000000)
$19=75.000 (homing feed, mm/min)
$20=100.000 (homing seek, mm/min)
$21=100 (homing debounce, msec)
$22=1.000 (homing pull-off, mm)

Thanks for your help!

kevinga:
I am using an UNO, GRBL v.8 and a CNC v3 shield with DRV8825 drivers to control 3 stepper motors using G-Code Sender. The power supply is a 24v 20A 480w.

......

The stepper is a NEMA 23 size 2.5A 1.3nm 184oz in.

I have the DRV8825 drivers set at 1/4 step.

First, the max output of a DRV8825 is a lot less than 2.5 amps - don't expect it to run close to full output without some very serious forced cooling. 1.5amps to 1.7amps would be more realistic.

For that motor you really should have a drive capable of 3.5amps or more to give a bit of headroom - unfotunately they are a lot more expensive.

Second, using fractional steps reduces the torque because the fractional steps are achieved by using the coils to pull against each other.

Third, in case it is relevant, the torque will fall significantly as the motor operates at higher speeds.

...R
Stepper Motor Basics

Thanks for the reply but given what I currently HAVE, what settings will give me the most torque.

Thanks.

Set the current limit for the DRV8825 as high as you dare, use single steps and move slowly.

AFAIK if the DRV8825 overheats it will just shut down and you will lose position. A heat sink and a fan will help.

In other words you are going to have to do some experiments.

…R

Robin2:
Second, using fractional steps reduces the torque because the fractional steps are achieved by using the coils to pull against each other.

Not using microsteps often leads to miss-steps via resonance, a much more important issue
in practical situations. And the coils aren’t fighting each other, they act in quadrature, so that
the torque is fairly constant.

The highest torque is achieved by only moving slowly - dynamic torque
drops off rapidly with speed in a stepper. Higher supply voltages extend the torque to
higher speeds better.

So: Use microstepping, higher supply voltage (if within part ratings of course), move slower.

If possible stay below the first resonance point too…

I can only enforce what @Robin2 said. There is an ongoing dispute about torque being reduced or not by using microstepping - I gave up as too many experts pretend to know better, but the discussion is still controversely. My experience is: in some cases torque is reduced, in some not (and cases means to take into account EVERYTHING, from mechanical devices fixed to the steppers till steppers and drivers themselves. For sure: the resonance point is a critical issue. That's why I only decide wether to use microstepping or not, when the (mechanical) prototype is ready to be tested and as close as possible to the device which will be the one to be used in real operations. And my decision is based on test, test, test.

One addition:
If your application allows (depending on the load situation of each axis) you can cool the DRV8825 down by setting the driver in sleep mode. But this will really drive current and thus torque to ZERO for the respected axis.

If your application doesn't allow that torque in idle periods goes down to zero, you will need drivers, which automatically reduce torque in stand still periods automatically to a set level (e.g. 1/2, 1/3).

But that is not possible with DRV8825 drivers (unfortunately).