Re: Nema 42 3100 oz/in motors will not move

I'll take a crack at the "paradox".

Digitally generated frequencies are like a piano keyboard while the
analog generated frequencies of your pulse generator are like a slide-
whistle.

A piano has 88 (?) keys and you have only 88 different frequencies
available. "Acceleration" consists of hitting the keys in ascending
order, making acceleration a series of increasing frequency steps.

This stepping of frequencies is called staircasing. The fewer
the "notes", the coarser the staircasing and the larger the
individual steps are.

The problem occurs on the "note" changes. This requires the motor to
accelerate on each "note" change; the larger the change, the greater
this acceleration must be. Required is a series of sharp acceleration
torque pulses; no acceleration occurs while each "tone" is held. Make
the change in frequency too large and the motor stalls.

Unlike a piano, the slide-whistle can generate any frequency. For it,
acceleration is a continuously ascending tone. The motor accelerates
continuously and smoothly as well.

Both acceleration methods require the same average torque and there
is the rub: The staircased acceleration requires much higher peak
torque pulses from the motor in order to get its average value equal
to the analog method. Of course, the motor stalls on the pulsed
torque peak value.

The digital method can closely approach the analog method by adding
many, many more keys to the "piano keyboard" and playing each much
more rapidly. This decreases staircasing by having many more
frequency steps, each being held for a much shorter period of time.

Experimental lab data I have taken shows the digital method is within
1% of the analog method performance when 1,000 frequency steps
or "notes" are issued per second. This of course requires a lot
of "piano keys", 65,536 in my case.

You can tell by ear if your staircasing is coarse (bad) or fine
(good). Accelerate the motor very slowly, perhaps a couple of minutes
to reach full speed. It doesn't matter if the motor stalls; you are
only interested in the sound.

You should not hear a "keyboard" effect where there are distinct
changes in pitch. Rather it should sound like a continuous ascending
tone with no discernable stepping in the tone pitch. The closer it is
to that, the better your motor performance will be.

Hope that helps.

Mariss








--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Keith Clark" <mail@t...>
wrote:

> I now have Nema 42 3100 oz/in motors in my Bridgeport retrofit. I
> have tired everything I can figure out to get them to work with the
> current software from CamSoft. I am being told that the Gecko
> drives and the stepper motors are proper by the motor manufacturer
> Anaheim Automation. This is because I can make the motors move the
> x axis at 120 ipm using their step pulse generator. All these
> comparisons are being done with the same Gecko drives, powersupply
> etc. (However, the step pulse generator has a potentiometer that
> gradually increases the step pulse rate rather that ramp it up by
> accellerating the step pulse rate.)
>
> The motion control board/software company, CamSoft, says it is not
> their board or software that is the problem and that the motors are
> underpowered. They feel the torque falls off and causes them to
> stall at 20 ipm when using their stepper board and software. I

know

> the motors are capable of much higher rpms as described in the

above

> paragraph.
>
> Can anyone explain this paradox? Got any ideas?
>
> My next option is to send the motors back and get some in the 1000
> oz/in range to see if they work better. Just trying to get some

idea

> if you guys agree.
>
> thanks
> Keith Clark