Re: [CAD_CAM_EDM_DRO] Re: voltage based servo controller ?

At 12:16 PM 11/12/2007, you wrote:

>This will almost work, but it will be unidirectional. You'd need
>additional circuitry (pretty simple, though) to flip the aritmetic
>sign of the analog output for the reverse direction.

Actually not. You can either do it as sign and magnitude (as you are
discussing) or as an offset voltage. You get the same resolution for
the same number of bits since sign is a bit.

If we let 0000 (binary) be zero volts, one count will be 0001, minus
one count will be 1111, just the same as integer arithmetic is done
in computers. With 4 bits, there are 16 states.

For sign and magnitude, we see sign => +, 000 => zero AND sign =>
minus, 000 => zero for zero volts out, note that TWO states are zero
valued. Full scale is sign => plus, 111 => max and sign => minus 111
=> negative full scale. For 4 bits, there are 16 states but only 15
*different* states.

At 8 bits it is a crapshoot, 1 in 256 is not that important.

Let's assume the PWM is set for 50% duty cycle = zero and 100% duty
cycle is plus full scale, 0% duty cycle becomes minus full
scale. The problem here is that you need a PID controller to change
the instaneous voltage to something the servo will do well with -- it
can be in software or hardware. If the PWM is at a fixed rate, a
fast filter is pretty easy, if not it needs a slow filter.


David G. LeVine
Nashua, NH 03060