Rotational Position Sensor

[TomFin]

Hi,

Currently I have a shaft that has a limited rotation from +90 to +15 degrees. Clamped to the end of the shaft is a cam that engages 3 different on/off switches which lead to a "pc board". Each location of the 3 switches can be adjusted via screw, which is how the mechanism is callibrated by assemblers. The switches are cumbersome and I'd like to move to a less mechanical method of tracking the shaft's angular position while retaining the ease of adjustment by an assembler. I know this is a general description of my situation,but if someone could point me in the right direction of whats out there I'd appreciate it. Thankyou in advance

 

[Skogsgurra]

Your field of choices is very wide. It depends a lot on how fast the shaft is moving, what accuracy you need, the environment, dirt vibrations, heat etcetera.

One of the simplest solutions is an encoder and a counter. Plus a set of comparators that output a signal when the respective position is arrived at. But such a solution loses its reference everytime power is cycled off/on. And that is usually not acceptable.

Another, rather expensive way of doing it (at least if you need good precision - better than, say, one degree) is to use an absolute encoder and comparators (no counter needed). I will always know where it is. No need for "grid search" after power off.

There are also very simple potentiometric solutions. All you need is a potentiometer to pick up position and then a set of potentiometers to set the different angles where you need an output. Simple potentiometers have a limited life. But there are non-contacting ones that have an extremely long life. The electronics needed is extremely simple (and cheap). Google "non-contacting potentiometer".

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[TomFin]

Well the shaft would be rotating at about 5rev/min and accurate to 1 degree. It would experience vibration but concealed from dirt,sand,etc. Thanks for the suggestions.

 

[ScottyUK]

Shaevitz have a range of top quality rotary position transducers using RVDT technology. These are small and have high survivability in arduous environments. They're arguably easier to interface to than an encoder too, but Skogs might disagree with me on that! Cost should be a little cheaper than a mid-range encoder, but it will definitely be more expensive than anything involving a potentiometer.

http://www.schaevitz.com/

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[nbucska]

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[Skogsgurra]

No Scotty. Don't disagree at all. But a lowly pot is even simpler. And less costly. I think that it will work quite well in this application - even if the numberts are unknown to some extent.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[m777182]

As I understand your problem your shaft does not make 2*PI rotation. In such a case a noncontact device you may construct with the aid of a LED beam and opto transistor or with a magnetic proximity switch.In both cases the position can be mechanicaly adjusted independently of the shafts angular position which in turn you may track with a permanent magnet and a Hall efect sensor. Each time your shaft passes the particular light beam you can perform the Hall sensor calibration at that point.
m777182

 

[ScottyUK]

If you go for the potentiometer, most of the big manufacturers have a conductive polymer precision pot for servo applications in their range. These have a long life and good linearity. A normal pot will become noisy and wear.


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Sometimes I only open my mouth to swap feet...