[sdiy] sin/cos pot

[rsdio at audiobanshee.com]

On Jul 10, 2017, at 4:58 AM, Ove Ridé <nitro2k01 at gmail.com> wrote:
> On 10 July 2017 at 13:12,  <rsdio at audiobanshee.com> wrote:
>> Does anyone know why these pots are cosine and sine instead of two linear ramps that are offset by 90 degrees or 180 degrees?
> 
> It doesn't have to be a sinusoidal shape per se. You could also use
> two triangular outputs in quadrature. (Triangular in terms of the
> sweep waveform.) The unit circle would then become a unit square
> rotated 45 degrees so its corners are on the axis lines, but otherwise
> it would have the same useful mathematical properties in terms of
> indexing. Having two linear ramps proper would be a bad idea, since
> you would have a discontinuity at the max/min transition point.

I see your point, but I don't think that there is always a difference in a practical implementation - at least not for embedded systems.

Even with the triangular waveform pot response, as soon as the wiper gets close to an extreme it becomes more difficult to determine whether it is right before or right after the peak, since both product the same voltage. At that point, you might as well focus on the other dimension because it has a clear indication of the position as long as you're aware of the polarity of the dimension near an extreme.

Two linear ramps would be practically the same, because when one is in the region of discontinuity, you can just focus on the other dimension for an accurate reading. There are certainly endless pots designed this way, and the associated firmware can handle it.


One advantage I see of the triangle as opposed to the ramp is that the voltage changes twice as fast for the same amount of rotation. If you have limited resolution on your A/D, it might be an advantage to have the voltage change be larger, matching your larger quantization steps.

Another advantage is that pure analog calculations might be easier without the discontinuity, since the voltages can just be part of the equation at all times. If your circuit is all op-amps, the triangle setup could feed an analog computer.

Brian