Re: [CAD_CAM_EDM_DRO] Re: stepper vs. servo

On Sat, 14 Feb 2004 06:29:16 -0000, you wrote:

>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.

Yep, I was thinking of and describing the steps, not the actual
current relationships.

>
>> 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.

Yep, only question was how... <grin>

>
>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.)

Works for me.

>
>> >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...

I'll go check out the waveforms. I was thinking of this in terms of
the drivers, which, for greatest efficiency, need to be saturating.
I'll look at their output stage and see what they've done.

With PWM, you can (probably, since I'm not an expert in this kind of
design) use a single H bridge for the driver. With a circuit that
attempts to control current by a stepped (read... triangular wave
approximation) drive, I think the driver circuit becomes more
complicated.

Harvey

>
>Hope that helps,

I think we're arguing counterclockwise and anticlockwise....

H.

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