# Rotational Position Sensor

Discussion in 'The Projects Forum' started by 360modina, Dec 11, 2008.

1. ### 360modina Thread Starter New Member

Oct 22, 2008
9
0
At the moment I am mentoring a high school robotics building team and I came across a problem that I need a little help with. For a potential sensor we are considering using a digital semicircular sensor capable of measuring angular position.

The sensor must have be able to measure the angular position to a precision of 0.3 degrees.

My questions are:

What is the number of concentric rings and on-off sensors are needed to achieve this precision?

Is their a simple formula that I'm missing here, it appears like this could be easily solved with some knowledge of this type of sensor.

I apologize that I cannot give too much detail on the subject, It just so happens that I am actually a chemistry teacher and don't have much experience behind the formulas that govern digital rotation sensors.

Regards,
Mr. Evans

Apr 5, 2008
15,647
2,346
Hello,

It depends on the type you look for.
With an absolute you need th number of rings dependend on the resolution.
Witn an incremental sensor you are counting the number of marks after the indexmark.
You say that you want to detect changes of 0.3 degrees.
A full circle has 360 degrees. so the number of positions you need is 360 / 0.3 = 1200 positions.

Binairy coding looks like this.
1,2,4,8,16,32 ... 1024 for 2^0 , 2^1 , 2^3 , 2^4 , 2^5 ... 2 ^ 10 so you need 11 rings to encode 1200.

http://mechatronics.mech.northwestern.edu/design_ref/sensors/encoders.html

http://www.educypedia.be/electronics/sensorsencoders.htm

Greetings,
Bertus

3. ### Søren Senior Member

Sep 2, 2006
472
28
Hi,

If you are referring to something like this (a 1°/step for the steering in an autonomous robot), there is indeed a formula.

ceil(log(n)/log(2))

Where
n = number of segments
and ceil means "if not integer, round up to nearest (higher) integer"

log(1200)/log(2) = 10.229 (to 3 decimal places)
Rounding up gives you the number of rings and sensors needed = 11