Absolute Rotary Sensor, where to find one?

[Dizztroy]

I'm pretty new to this board but I've been working with different Phidgets for a couple of years now. However while I do implement phidget software, I'm not a person who does know a lot about the hardware.

At the moment I've got a nice idea I want to work on. But to make it work I do need an absolute rotary sensor. The reason for this is, that I want to attach a rotary sensor to a desk chair which can turn. The turns need to be absolute, so I know that from a start position, this char has turned x angle. I know this could work with an incremental rotary sensor, however this chair isn't going to be in the exact same orientation every single time the software starts.

I've been searching trough the rotary sensors but none of them is absolute (phidgets) unless I overlooked something.
What could I use to make this work and could I use a 3rd party rotary sensor and connect it to a highspeed ecoder phidget to read out the data?

I hope someone could help me on this prototyping idea. =)

[mparadis]

There are a few options:

- Use a continuous rotation potentiometer
A rotary potentiometer (like the 3583)will be able to report absolute location regardless of where it starts up. The downside is that there is a small blind spot where the value resets from 100% to 0 where the sensor value doesn't change.

- Use an encoder with an index channel
The index channel activates at a specific spot on the encoder wheel, so the position will still be unknown until you make one full rotation in the worst case. The only encoder sold here with an index channel is the 3532.

- Use an incremental encoder and some other form of index
You can place some sort of limit switch or magnetic sensor on the rotating part of the chair that gets triggered at a known location. The advantage of this over an indexed encoder is that you can have multiple sensors or limit switches to reduce the angle that needs to be traveled before your position is known.

- Use an absolute encoder
Absolute encoders express the current position as a series of binary outputs. I haven't used one of these, but it may be possible to interface it with a Phidget digital input board.