Re: stepper vs. servo

Harvey,

Have a look at:

http://www.allegromicro.com/techpub2/stp/stp01-2.pdf

The waveforms I mentioned are on pages 4,5,and 6...

Yes, current in the coils may be adjusted using PWM for different
values. Though not the only way, it is the most typical method used.

>In CCED,Harvey White <madyn@i...> wrote:

<snip>Perhaps stepped square waves means something different to me.

>I'm thinking of the steps that are 1/4 1/2 3/4 and 1 in making a
>square wave, which waveform then approximates a triangular wave.

The steps are in the sin/cos relationship, which is not 1/4, 1/2,
3/4, and 1.

> I'm thinking of arriving at these current levels by PWM, which over
> time, averages out to the particular current you need, but the
> driver sees, and delivers, a square wave.

Yes, but the COIL of the motor is energised in the sin/cos
relationship BY that square wave (PWM) FROM the driver.

We appear to be lost among the different points in the circuit,
limited by the words we use... In a driver there is a "square" wave
at more than one point. It's not the sma one, and each may be used
for a different purpose IN the drive, but they all are cortrectly
described as square waves. (if we ignore that a REAL SQUARE wave is
50% duty cycle; these are more correctly called pulse trains.)

> >Look at ANY stepper driver waveforms and you'll see the steps I'm
> >talking about. If the drive is more than halfstep, you'll see the
> >multiple levels which make the sine wave approximation.
>
> Then this is a different approach than the one that I'd consider.
> You're running the output stage in something other than class D,
> switching. I'd think that this would run the output dissipation of
> the drivers up a bit.

Let me change one word in my paragraph above:

>Look at ANY stepper COIL waveforms and you'll see the steps...

Hope that helps,

Ballendo