more on sine wave encoder

> Date: Sun, 24 Sep 2000 18:23:06 -0000
> From: "Mariss Freimanis" <geckohall@...>
>Subject: Re: sine wave encoder outputs


>1) Encoders have a cyclic amplitude modulation; a sine/cosine
>amplitude of +/- 1 volt may be +/- 1.2 volts when the encoder is
>turned 180 degrees.

True. The sine wave decoders normalize the output to allow for this. This
is why 4 outputs are provided, although it can be shown that only three are
necessary.

>2) Any resolution enhancing scheme depends on a 90 degrees
>relationship between the two channels. This cannot be gauranteed.

The phase relationship has a maximum error which is specified in the
manufacturers literature.

>3) The waveform more than likely is a "quasi-sinewave". This means it
>actual waveform will be somewhere between a sinewave and a triangle
>wave.

For grating systems not designed to have sine wave output, this is true.
The encoder I have has generates quite accurate sine waves.

>4) Even if the encoder is perfect, the circuitry necessary to extract
>additional resolution is complex and expensive.

This was my question. Who has a simple circuit? I know a a scheme with a
tree of comparators and op amps, but thought maybe there was a simpler
solution. The simpler solution should spend very little time doing
calculations. For example, the arctangent can be calculated with a look up
table (PROM and a latch technique), which still isn't very simple.

>Apart from the "non-dithering" advantage, the expense of
>interpolation is better spent on a higher resolution digital encoder.
>If anyone is interested, I can post a circuit I designed that
>extracts an analog voltage proportional to position between encoder
>counts.

Have you priced arcsecond encoders?





Elliot Burke