How much PPR do you use on your encoders?

[techhelpbb]

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Originally Posted by Ether

I did not say that. Neither does US Digital.

I said CPR (=Cycles Per Rev per US Digital) is never greater than PPR.

See attachment.

Okay then what exactly is 'cycles per revolution' in that context?

I can see a 100 cycles per revolution encoder yielding 400 pulses per revolution.

I can see a 360 cycles per revolution encoder yielding 1440 pulses per revolution.

Those are all multiples of 4.
I have taken a ton of PCB out of these encoders (they fail a lot and not just in FIRST robots but on my MaxNC CNC tools) and there is no decoding in them.

Are they just being cute and declaring the pulses you could get in the decoder circuit with a 4x decode? Effectively exchanging the meaning of PPR and CPS as other companies use it?

[Ether]

A cycle is one repeating pattern. It's a good way to define an encoder because it's independent of how you plan to use (decode) the signal.

The space between the red lines is one cycle.

One cycle can be 1 count (if you are counting only rising edges on one channel); or 2 counts (if you are counting only rising edges on 2 channels); or 4 counts (if you are counting rising and falling edges on 2 channels).

[techhelpbb]

Originally Posted by Ether

A cycle is one repeating pattern. It's a good way to define an encoder because it's independent of how you plan to use (decode) the signal.

The space between the red lines is one cycle.

One cycle can be 1 count (if you are counting only rising edges on one channel); or 2 counts (if you are counting only rising edges on 2 channels); or 4 counts (if you are counting rising and falling edges on 2 channels).

At best that is a matter of 2 differring terms for the same thing.

Yes using cycle in that context seemingly eliminates the idea you can get 4 pulses from the decoder circuit, but it ignores that not all decoders are 4x decoders.

What is the difference between a cycle and a pulse in a 1x decoder? Nothing. Even over time, a bunch of pulses and a bunch of cycles, still nothing.

Granted: to the point of the OP, U.S. Digital's use of the term is what matters so I will accept the cycles per revolution in their case is 4x lower than the potential PPR as they use the terms. I prefer to think of them the other way and simply assume the quadrature decoder circuit is 1x unless provided as part of the encoder assembly.

Still a lot of good information in here on the point, minus the ambiguity of the terms. The ambiguity of the term is itself notable.

Originally Posted by asid61

If your encoders fail a lot, you may want to consider just switching to magnetic encoders. 115 has killed many, many US Digital encoders in its time.

Also: the failure rate for that style encoder Ether linked has to do with the end play of the shaft with the encoder disc. My MaxNC servos have eventually had encoder failures at the back of the steppers because the axis coupled to the shaft of the stepper exerts at times enough vibration (screw lash) and force to displace the disc and either scratch it or put it out of range of the detector. MaxNC has historically used a similar encoder from other companies and they suffered the same fate. I solved this problem using sealed encoders, a timing belt and a mount for the encoder. With 12 axis between these machines I'd have spent $1,400 on fixing those encoders or more by now without making those changes. Point being - I am noting a limitation of that style encoder not taking a shot at U.S. Digital.

[asid61]

Originally Posted by Ether

I think you have that backwards.

Originally Posted by techhelpbb

At best that is a matter of 2 differring terms for the same thing.

Yes using cycle in that context seemingly eliminates the idea you can get 4 pulses from the decoder circuit, but it ignores that not all decoders are 4x decoders.

What is the difference between a cycle and a pulse in a 1x decoder? Nothing. Even over time, a bunch of pulses and a bunch of cycles, still nothing.

Granted: to the point of the OP, U.S. Digital's use of the term is what matters so I will accept the cycles per revolution in their case is 4x lower than the potential PPR as they use the terms. I prefer to think of them the other way and simply assume the quadrature encoder circuit is 1x unless provided as part of the encoder assembly.

Still a lot of good information in here on the point, minus the ambiguity of the terms.

I like that US Digital doesn't take the decoder into account. Their encoders don't come with decoders, so why should it be their job to decide things for you? They do provide both CPR and PPR if you want to calculate your counts per revolution given your own decoder. All US Digital encoders (to my knowledge) are 4 pulses per cycle, that is, A high, B high, A low, B low, so for them it makes sense to count it that way.
Your point about the 1x decoder seeing CPR and PPR as the same is valid, but then the designer would have to determine that themselves. Are we defining pulses as the actual counts the decoder spits out, or just the step changes coming in from the encoder?
AFAIK FRC encoder decoders (TalonSRX and RoboRio) count all the step changes/pulses anyway, so for the purposes of this discussion I think using PPR = 4 x CPR is the best way to define it.

EDIT: On topic: The results of the poll are pretty interesting. It looks like most people are using 256 or 512 CPR encoders, with a few people using 64 CPR (or just picked the wrong option in the poll... I should have used CPR instead).

[techhelpbb]

Originally Posted by asid61

I like that US Digital doesn't take the decoder into account. Their encoders don't come with decoders, so why should it be their job to decide things for you? They do provide both CPR and PPR if you want to calculate your counts per revolution given your own decoder.

Yes but factually the other guys are often doing no less. They are sometimes using different terms to do it. Course this means you may have to double check.

FRC11 has been using the Grayhill potentiometer style encoders recently. Too many issues with the end of shaft style. Using the terms used as criteria for this topic we get 128 cycles per revolution.

[Munchskull]

Depends where in the gearbox/ system. You don't want to flood the Roborio with data.

[adciv]

Originally Posted by asid61

EDIT: On topic: The results of the poll are pretty interesting. It looks like most people are using 256 or 512 CPR encoders, with a few people using 64 CPR (or just picked the wrong option in the poll... I should have used CPR instead).

The two common encoders are 250 & 360 from US Digital. Not sure about the (much higher) others.