Re: ATM Stepping Motors - fast step - lower power

[Adrian Pederson <PEDERSON@uwplatt.edu>]

>May I suggest a National LM18245 stepper motor drive IC.  It relaces the 
>resistors with a current limiting switcher supply.  Two of them are capable of 
>driving a bipolar stepper motor in microstep fasion.  Not only do they have 
>the normal step and direction controls, but also have a three bit dac wich is 
>capable of controlling the outputs from zero to max in sixteen steps.  
>National provises some good app notes.  I've been seriously considering using 
>them to  control the current through my peltier cooler of my CB camera from a 
>battery.  All you need for this application is an inductor capable of 3 amps 
>at about 15khz.  The chips will also save your power supplies some power.
>
>Clear skys  MIke Chibnik

I just got a pair of these chips in the mail FREE from National. I was playing
with them this afternoon and they do a good job of microstepping without the
need for the CPU to control the pulse width modulation. The 4-bit DAC is used
to limit the motor current with the direction pin controlling the polarity, in
other words it can run a bipolar motor (mine are actually unipolar motors with
the center tap leads unconnected) from a single supply, 12V in my case, but the
chips are rated at 55V, 3A.

My microstepping DAC values still need to be adjusted to make the movement a
little smoother. When using a strict sin/cos relation, the shaft slows down and
speeds up again, a periodic error due to one coil pulling too strongly. But
this can be fixed by "filling out" the sine pulse, increasing the values near
the zero-crossings. I be playing with these values more once I get the chips
soldered down, they have a somewhat strange pin spacing which forced me to
wire-wrap my current connection which turned out to be rather noisy as the pins
are loose in the socket.

Adrian Pederson
pederson@uwplatt.edu