Newbie question: rotary encoders

[Jonathan]

Hi,
I've been researching rotary encoders for a project. A rotary encoder would
be excellent for the UI I have in mind.

Simple question: Do all rotary encoders, by nature, turn infinitely in
either direction? I'm reading the descriptions of some in my Digikey
catalog and they say "32 position," which makes it sound like there's a stop
after the last position.

Please help me understand what this really means, and what the deal is with
travel stops. The encoders in mice and my car stereo do not have stops.

thanks,
Jonathan

 

[Roger Hamlett]

Hi,
I've been researching rotary encoders for a project. A rotary encoder would
be excellent for the UI I have in mind.

Simple question: Do all rotary encoders, by nature, turn infinitely in
either direction? I'm reading the descriptions of some in my Digikey
catalog and they say "32 position," which makes it sound like there's a stop
after the last position.

Please help me understand what this really means, and what the deal is with
travel stops. The encoders in mice and my car stereo do not have stops.

thanks,
Jonathan

First, there are two fundamentally very different 'families' of encoders.
The first are 'absolute' encoders, which return a quite long 'position'
value, with the length dependant on the accuracy involved. Normally a 'grey
code' is used, so that if one bit is in error (hasn't yet transitioned), the
worst case error on position is still only one bit - unlike normal binary,
where if - for instance - the high bit of an eight bit value had not yet
changed, the result could be half a turn in error!... The second type are
'incremental' encoders, which just return two 'pulse trains', that are
technically 'in quadrature' (one leads the other by 90degrees). For these
you can decode just one 'edge', two edges (rising and falling on one of the
trains, or all four edges (rising and falling, on both trains). The
relationship of the two waveforms, gives direction, while counting the
edges, then allows position to be determined. This is the approach used on
mice, with normally 90 'teeth', giving one degree resolution (if all four
edges are used).
Both types, can have end stops, and in the case of the units with relatively
low counts, 'detents'. Often end stops are optional, just involving
inserting a pin into the casing.
Absolute encoders, need more wires, and are more expensive.
If you need a type without an endstop, look for the keyphrase 'continuous me
chanical rotation'.
If you look at (for example), the Panasonic EVQ series from Digikey, you
will see at the start of the 'mechanical' description, it says "360 degree
(endless)", for the rotation angle. Similarly the CTS encoders, say
'continuous rotation'.

Best Wishes

 

[Jonathan]

Hi,
thanks for such a quick and detailed response!

-Jonathan