# shaft encoder equation

Discussion in 'General Electronics Chat' started by minoo atef, Dec 25, 2013.

1. ### minoo atef Thread Starter New Member

Dec 25, 2013
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Dear all,
Kindly be informed that I've a shaft encoder with 2500 revolution/min.
I need an equation that converts it to pulses/mm.
Regards,
Minoo

2. ### poofjunior New Member

May 21, 2013
9
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Can you tell us a bit more about the encoder?

"2500 revolution/min." This sounds like the speed of a rotating shaft, not a characteristic of the encoder. Could this be the maximum speed at which the encoder can keep track of rotations?

An encoder (er, at least a quadrature optical rotary encoder), outputs two square waves in response to a rotation. The number of pulses is predetermined by the disc that spins inside of the encoder. By knowing the number of slits inside of that disc, we know how many pulses that the encoder will output in a single revolution.

By chance, do you have a datasheet?

3. ### atferrari AAC Fanatic!

Jan 6, 2004
2,612
738
Is measuring them an option?

That would be real data thus valid enough.

4. ### tubeguy Well-Known Member

Nov 3, 2012
1,157
197
I just caught this,(in Red)
Is that a typo?
Or, are you wanting to convert encoder pulses into linear distance?

Regardless, as mentioned, the pulses per/revolution output from the encoder will be needed.

5. ### MaxHeadRoom Expert

Jul 18, 2013
10,272
2,269
A quadrature encoder is normally stated in pulses/rev, in this case 2500p/r.
With subsequent electronics you have the option of using X1, X2 or X4 the natuaral count.
This is done by using either the single edge of one pulse the two leading edges of two pulses or use all four edges for x4.
Resulting in 2500, 5000 or 10,000 counts/rev.
Max.

6. ### MaxHeadRoom Expert

Jul 18, 2013
10,272
2,269
Just noticed the pulses to mm, when the method used in my previous post is selected, the the subsequent electronics is responsible for scaling it proportionately, depending on what one revolution (or single pulse) represents in the resulting mechanical distance.
This varies from one application to another.
Max.

7. ### THE_RB AAC Fanatic!

Feb 11, 2008
5,435
1,305
To get mm from revs you need to know the gearing, ie; all the mechanical stuff between the rotating encoder shaft and the final movement.

8. ### IC-Man New Member

Jan 3, 2012
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The description of revolution per minute seams to be mixed-up. It could refer either to the maximum rotation speed/min or resolution per rotation. In case of resoltion usually it's pulses or edges per rotation of 360 degree. Some encoder manufacturer have different definition. The best is to look into the encoder data sheet what is ment. If the output is ABZ the definition is often edges per resolution(see: http://ichaus.biz/wp2_simple_measurement ). For sine/cosine outputs it could be cycles per 360 degree. Most likely what is ment is 2500 rotation/min(RPM) as the maximum rotaion speed. The conversion to a linear motion requires knowledge of the gear, as already mentioned above.

9. ### MaxHeadRoom Expert

Jul 18, 2013
10,272
2,269
All the Encoder manufacturers I have encountered state the resolution in the basic counts/rev, as it is up to the consequent user as to how the encoder pulses are used, it would be very confusing to state anything but the basic count.
The multiplication is never done in the encoder.
In the case of the sin/cos type, the subsequent electronics often use the Arc/tangent method to extract a resolution much greater than the detected quadrature sine waves.
Max.

10. ### eliorossi New Member

Dec 30, 2013
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The equation is very simple
X linear displacement mm/rev
Y encoder out pulses pulses/rev
Y/X = pulses/mm
If X Y are nor known you, running the motor for an adequate time and measure the displacement D by micrometer and the number of output pulses N by an electronic counter, the ratio N/D pulses/mm is what you need.
Repeat n times the measurements for different displacement and calculate the mean value and the statistical error in order to evaluate the precision of the system.
I hope I was helpful.