# potentiometer like rotation angle sensor

I have a set of flight simulator gauges which use a motor to move various indicators. The rotary position of the indicator is measured with a potentiometer like device, but it has 5 terminals instead of three. 1 is the wiper, and the other 4 are taps at 90 degrees on the resistive element. The resistive element is a full 360 degrees with no gaps. Measuring between any 2 of the taps gives about 2.8 Kohms. The rotation sensor was made by a NEI which has been acquired by Honeywell, which has no data for the part. These are old devices, wondering if similar devices in use today and how best to read them for determining the rotation angle. Thanks!

• Try the search term "rotary encoder". – Hearth Mar 29 at 22:45
• ... and you'll get links for rotary encoders which are not similar. – Transistor Mar 29 at 23:05

First of all, no, they're not commonly used any more. Digital shaft encoders are much less fussy and more reliable.

Probably the original way to read it was to apply balanced sinewaves to opposite terminals, with one set shifted by 90° relative to the other. Then as the shaft rotates, the wiper picks up a more-or-less constant-amplitude sinewave whose phase varies directly with the shaft position.

simulate this circuit – Schematic created using CircuitLab

Another way to read it with a microcontroller would be to drive the four taps with GPIO outputs, and read the wiper with an ADC input pin. Drive one tap high at a time, while driving the other three low. Depending on which quadrant the wiper is in, only two of the combinations will give you a signal on the wiper, and that lets you figure out which quadrant, with the ADC telling you where it is in that quadrant.

• So used like a resolver? Interesting! – Spehro Pefhany Mar 30 at 0:19
• I suspect the out-of-phase sine waves was how they were originally read, definitely old tech that I suspect predates most micros.I don't have any of the original circuits, just a set of gauges got off eBay, no info what they came out of. Would be difficult to modify the hardware as they are very custom parts. I will probably use the second method since is easy to do with modern micros. Thanks! – dmsdms2000 Mar 31 at 20:27

If they're available they'll be difficult to find. I suggest that you do some reverse engineering on a working unit. Record the voltages on each tab at 45° increments of the shaft.

Angle | Tab 1 | Tab 2 | Tab 3 | Tab 4
------+-------+-------+-------+------
0  |       |       |       |
45  |       |       |       |
90  |       |       |       |


You may be able to use the results to work out how to simulate the pot with a microcontroller's analog outputs driven by rotary encoder.

Alternatively you may be able to use something like the AlpsLine RDC80 which manages the 360° rotation with two overlapping 340° tapers.