Re: Sherline CNC stepper motors getting "stuck"

Hi,

Reagrding Question # 2 below: Why more current in microstepping isn't
necessarily better

Look at the following table for percent of current for each phase
with a given microstep (1/8th step mode):

uStep A Cur B Cur
1 0.00 100.0 (this could be considered at FULL STEP)
2 19.51 98.08
3 38.27 92.39
4 55.56 83.15
5 70.71 70.71
6 83.15 55.56
7 92.39 38.27
8 98.08 19.51
9 100.0 0 (this could be considered at FULL STEP)

In the preceeding table you can see how the percentage of current
varies for each coil as it steps through the microstepping sequence.
The current value roughly follows a sine wave through the cycle. If
we used an example of Vref set for a 1 Amp motor, then at step
5, .7071 amps would be going to both A and B phases. They would have
equal pull. As we continue (up or down), the rotor will follow the
higher current. Now imagine if you set the Vref for 2 amps per phase
(but the motor is still only a 1 amp per phase motor

Multiply everything by 2 and we get:
Step A Cur B Cur
1 0.00 200.0
2 39.02 196.16
3 76.54 184.78
4 111.12 166.3
5 141.41 141.42
6 166.3 111.12
7 184.78 76.54
8 196.16 39.02
9 200.0 0
The B coil is fully saturated until step 7, while the A coil starts
being fully saturated at step 4 (and continues to be so in step
9..14). Microstepping has been totally smeared out for microsteps 4,
and 6, and badly distorted for 2, 3, 7, and 8. At step 1 and step 5
you would be OK position-wise (and similarly at step 9). This would
be fine for half-stepping, but not for 1/8th stepping (and the motor
would overheat pretty badly too probably).

This is why you don't want to have too much current when you are
microstepping. You actually end up loosing resolution, and you no
longer get a nice smooth sine wave current. The smooth sine wave
current is one of they key features than makes microstepping
advantageous. In the above senario you kind of bounce from half-step
to half step (you probably notice a lot of vibration in the motor
when you apply too much current).

Hope this helps some.

Jeff


--- In CAD_CAM_EDM_DRO@yahoogroups.com, "builder4wd
<builder4wd@y...>" <builder4wd@y...> wrote:

> Hi everyone,
>
> I've been working on my Sherline CNC machine with Xylotex driver
> board (set to 1/8th step) and 24 V power supply. My motors are 6-
> wire, 2 amp/phase. I have two questions:
>
> Problem #1
>
> When I try to machine a test part, I get problems with the motors
> getting "stuck"... I think desynchronized with the signals coming
> from the driver board. The motor will accelerate, get

desynchronized

> and stop moving, then start moving again during the deceleration
> phase at the end of the move.
>
> The machining is done at only 300 mm/m or about 12 inch/minute. I

get

> this problem even when I just have the mill move in air (with no
> material being cut). At first, I thought that my motors were set to
> go too fast, but I can set my motors to jog at 25 ipm, yet I still
> get the problem when I run the G-code. Any ideas on what is the
> cause/remedy?
>
>
> Question #2
>
> Since I have 6 wire motors, I set my Xylotex board to output 1/2

the

> rated current, but my motors get stuck at even the 8000 Hz step

rate.

> To fix this, I tried reducing the current from the drive to less...

I

> found that at about 0.73 amps, I can increase my step rate to 18000
> Hz when I jog the motors. My question is, if I reduce the current
> output on the driver, what do I gain/lose? What are the tradeoffs?
>
>
> I would really appreciate any help on this. Thanks.