# How to calculate encoder motor ratio?

Discussion in 'General Electronics Chat' started by Sagan820, Dec 30, 2017.

1. ### Sagan820 Thread Starter New Member

Jul 10, 2016
19
0
Hello all,

I was wondering if there is a method of calculating the encoder motor ratio for a given system? I have a 12800 step/rev stepper motor and a 360 pulse/rev, 5000 rpm, quad encoder. Google search suggested this ratio was for accuracy but I couldn't find any info on the lower and upper limit of the ratio. For my project, I have 4 evenly spaced locations of interest in 360 degrees.

2. ### WBahn Moderator

Mar 31, 2012
24,560
7,700
I don't think you are giving enough information. Is this encoder mounted to the stepper motor, or are they two separate components?

If I understand you correctly, you have something that you want to position at four locations that are 90° apart. Is that correct?

How is the motor connected to whatever is being positioned?

Most encoders are not absolute encoding, only relative; if this is the case then you will need some means of establishing a reference location.

How accurate do you need the position that it stops in to be? And, please, do not say something like, "Exact," or, "As close as possible." Give a tolerance that is barely good enough, be it ±10° or ±0.1° or whatever.

3. ### Sagan820 Thread Starter New Member

Jul 10, 2016
19
0
Hi WBahn,

Sorry I didn't provide enough info. I learnt of the topic few hours ago and the data I listed were the only things I saw from the threads.

So the encoder and the motor are connected by a coupling. The other end of the motor is attached to a gearbox and together they rotate a metal disk. On the metal disk, there are 4 circular holes that are 90 degrees apart. The home position will be one of the holes and the system will index relative to the home position.

I don't have the dimension of the disk and the holes with me atm. Say the metal disk's diameter is 5 inches and the holes' are 1 inch, what ratio would give me +/- 2 degree tolerance?

Thank you.

Jul 18, 2013
18,843
5,885
So are you using the 360 pulse encoder or a metal disk with 1" holes?
Max.

5. ### Sagan820 Thread Starter New Member

Jul 10, 2016
19
0
Hi Max,

I was trying to provide as much detail as possible but that seemed to be more confusing.

The encoder/motor ratio is what I'm trying to find. The encoder, motor, and a gearbox will rotate the metal disk. Metal disk makes one revolution for every 3 encoder revs. I'm not sure if that is relevant to estimate the ratio.

I hope that makes some sense. I'm fairly new to stepper motor and encoder, and I'm learning as I go.

Cheers.

6. ### LesJones Well-Known Member

Jan 8, 2017
2,304
693
So you have 1080 pulses from the encoder per rev of the metal disk. So you will have 0.3333 degrees between pulses which is well within your accuracy specification. (Doing quadrature decoding normally give 4 transistion per line of the encoder so you would probably only have 0.083 degrees between counts.) As the output of you stepper motor is geared down with a ratio of 1:3 one revolution of the disk would be 38400 steps. so one step is 0.0094 degrees. Do you plan to control the rotation by counting stepper motor pulses or by counting transistions from the encoder ?
As there ar 4 positions that you want the disk to stop in you need to count either 270 pulses from the encoder or provide 9600 steps to the stepper motor.
Les.

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7. ### Bernard Expert

Aug 7, 2008
5,226
600
If all holes are the same, it would seem that an auxillary hole or slot would be necessary to mark home. A hole might detect up to 98 encoder pulses ?