# Homework Help: Incremental rotary encoder motor help

1. Jan 14, 2016

### AC130

1. The problem statement, all variables and given/known data
Hi please see the image that I have attached. The text colored in red is the data I need to calculate.

2. Relevant equations

3. The attempt at a solution
The encoder has 128 counts per revolution
So to calculate corresponding angle change this I what I did
I divided encoder count in decimal number by 128 which gave the number of revolutions, I then multiplied it by 360 to find the angle change

So for example I did
6350/128 = 49.61 revolutions
49.61 * 360 = 17859.34 degrees

Is this method right?

To calculate no load speed RPM, I divided the revolutions by (time*60) to get it to RPM

So for the 6350 encoder count which gave 49.61 revolutions

I did

49.64/(5.25*60) = 0.1575 RPM

Is this RPM calculation right?

For me it looks as if the RPM is well below and so I must have made a mistake somewhere but I don't know specifically where

#### Attached Files:

• ###### encoder motor.JPG
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2. Jan 14, 2016

### Hesch

That's correct.

To calculate the speed, you must know the sample frequency. Say it is 1Hz. Then you will have 6350 counts per second.
Convert to RPM. ( 128 counts per rev, 60 sec/min ).

The result is close to 3000 RPM.

If the sample frequency were 2Hz, the motor speed would be the double ( ≈ 6000 RPM ).

Last edited: Jan 14, 2016
3. Jan 15, 2016

### AC130

Hi can you please tell me where RPM came from?
I know that it took 5.25 sec for the angle change and so frequency would be 1/T = 1/5.25 = 0.1905Hz

But I don't really know how to find RPM

4. Jan 15, 2016

### Hesch

So 49.61 rev. within 5.25 sec, which gives 9.449 rev./sec ≈ 567 rev/min. = 567 RPM. ( 1 min = 60 sec ).

The RPM's in #2 is just an example, not knowing your mentioned 5.25 sec.

5. Jan 16, 2016

### AC130

Ok thanks I understand it now