# Encoders

Hello,

I did some research and came to this conclusion. That you can’t say how accurate a given encoder can be.

And that a good rule of thumb is that you need a resolution of 2 to 5 times more than your desired accuracy.

So if I want an accuracy (=measured movement deviation of real movement) of 0.02 (1/50) mm then I need to select an

encoder that can deliver at least 100 counts per mm (0.01 mm gives me 1 count) and the best (5times) 250 counts per mm (0.004 mm gives me 1 count).

So an encoder that has a nominal resolution of 73 pulses/mm.

But to know the amount of counts per mm I need to chose the counting logic (X1,X2 or X4).

For example if I use an X4 counting logic then I have 73*4 = 292 counts/mm.

And if I use an X2 counting logic then I have 73*2 = 146 counts/mm.

so this encoder would do Is this correct ?

But this is only how much deviation i will have against the real value.

So if i wan tot calculate the smallest movement that the encoder can measure (very important detail). I need to know how many mm or inches 1 revolution is. This will give me the PPR for the encoder that i need to use. I found info about choosing the PPR value saying this :

Try to chose a PPR that is an even multiple of the value you are trying to measure or display. For example, if one revolution of the encoder equates to 12 inches of travel, you might chose a 1200 PPR encoder. This can eliminate or simplify the need for a calibration constant or scaling factor and more importantly, it eliminates the possibility of accumulating a rounding error over many cycles of the encoder. In this example you would be able to measure the travel to a resolution of 1/100 of an inch. You should also consider any 2x or 4x counting logic in your controller. If your controller can "see" pulses on both the A and B channels (2x logic), then it will count 2400 pulses for every 12 inches of travel in our example. If the controller counts both the leading edge and the trailing edge of each of the pulses on both channels (4x logic), then it will count 4800 edges per revolution and your effective resolution would increase to 1/400 of an inch per count.

So the smallest movement you can measure for this encoder is 0.0025 inch/count. (Conversion inch to mm *25,4)

Thank you for your time and help.

Kind regards,

Pieter De Vos

Hi, can you please explain what you want to attempt, before rushing into a mathematical explanation.

I gather you want to measure linear movement with a rotary encoder, to do this you will have to connect the rotary shaft to the linear device with a wheel.
The diameter of the wheel will be one of the parameters that will decide your mm/count and its resolution.

There are encoders that offer up to and more than 5000 counts/revolution, and are programmable by switches inside or by oem programming.
I'm not following about a multiplier. Its as though you expect to be able to multiply the number of pulses out of the encoder and hence get your added resolution.

Sorry but its not that easy to do.

Tom...

TomGeorge:
Hi, can you please explain what you want to attempt, before rushing into a mathematical explanation.

I gather you want to measure linear movement with a rotary encoder, to do this you will have to connect the rotary shaft to the linear device with a wheel.
The diameter of the wheel will be one of the parameters that will decide your mm/count and its resolution.

There are encoders that offer up to and more than 5000 counts/revolution, and are programmable by switches inside or by oem programming.
I'm not following about a multiplier. Its as though you expect to be able to multiply the number of pulses out of the encoder and hence get your added resolution.

Sorry but its not that easy to do.

Tom...

Hi,

Yes, i want to measure a linear movement with a CPT (cable position transducers) also called stringpot that has a rotative encoder.

exactly what you say is that the diameter of the wheel will be one of the parameters that will decide your mm/count and its resolution. So 1 count gives me .... mm. That's what i want to know.

Before you have the 5000 counts/revolution you need to decide an encoding (X1,X2 or X4).

So if i have Ch A en Ch B and Ch A is 90° before Ch B. Now if Ch A is before Ch B then their will be one count up.

If Ch B is before Ch A then their will be a count down. This is an example of X1 encoding.

X2 encoding will count on the upwards and downwards flank off ChA and for X4 encoding every upwards and downwards flank will be used for upwards or downwards counting.

That's what I meant with the multiplier function.

So for example If I only use X1 encoding i will have 100 counts/revolution. If i use X2 encoding i will have 200 counts/revolution and if i use X4 encoding i will have 400 counts/revolution.

The only thing I want to know is what the smallest difference of movement (Let's call it deltaX) can be monitored.

So I know that it is good enough for my application.

Thank you for your time.

Kind regards,

Pieter De Vos

Hi, thank you, a stringpot, that makes sense.
These devices are usually available through distributors that know what they are talking about.
If you have chosen a particular unit or have a manufacturer in mind, then I would contact them and discuss your requirements.

Tom...