Agilent's Optical Encoders


Thread Starter

Joe Lindo

Is there a direct relationship between the A output and rotor shaft position? Can I use this output to drive a TTL binary counter and get good motor position information? Are there pitfalls, not using a quadrature decoder, driving the binary
counter directly?
If your motor never runs in reverse, stalls, or drifts backwards, this will work. Quadrature lets your counter know what direction the motor is turning. In a 1400 ppr incremental encoder A channel will pulse 1400 times per rev if your go backwards it will still pulse. B channel will also pulse 1400 times per rev, but it is out of phase with A channel. Thus in forward you get A pulse prior to B. In reverse you get B pulse prior to A. The Z or marker pulse gives you one pulse per rev and is relvent to shaft position. This can be used just as a prox, posibly for a reset. Your only other option is an absolute encoder that will output a gray code value that relates to shaft postion. This option is more expensive and requires more inputs. Hope this helps, good luck.

Robert Scott

By not using a true quadrature decoder circuit you sacrifice four things:

1. Ability to read position in both directions
2. Noise immunity
3. Higher resolution
4. Auto-zero on index pulse

If your application only involves rotation in one direction, then 1. may not matter. The noise immunity is provided by quadrature circuits through the use of a digital filter based on a shift register. (A transition is recognized only after it has persisited for a given length of time.) If your application has a short run between the encoder and the counter, and if the counter has some form of filtering and hysteresis, then you may get reliable counting by simply counting the A-phase pulses. It all depends. Also, a true quadrature circuit will give a count for each edge of both the A and B phases. That's four counts per "line" on the encoder. A typical simple counter will just give one count on the rising edge of the A phase. That's 4 times poorer resolution with the counter method. But if your encoder has excess resolution, then that may be acceptable too. One final feature of true quadrature circuits that you may find useful is the ability to load a preset count automatically upon detection of the index pulse (once per rev). If you use a counter, you will have to devise your own zeroing scheme.