DIY Rotary Encoders?

[birchy]

I will be using LinuxCNC which IS capable of handling closed loop feedback. From what I've read, it's just a case of using a servo profile and editing the parameters to match the motor and encoder.

I've never used a stepper driven CNC, but have worked in engineering for 22 years. The idea of controlling a precision drive system without any feedback seems to go against basic engineering principles as you never know the exact state or position of the machine. I guess that's the trade off from doing it "on the cheap"?

[m_c]

One thing that stands out about those encoders, is the +/- 0.5deg accuracy. On a stepper system, that is less than a full step, and provided you're not expecting huge accuracy then they may be OK. One thing I do wonder is just how uniform the output would be. You may find that if you turn the motor at a constant slow speed, the output pulses may not be uniform due to the inaccuracy.
And having just checked the datasheet for the RMB28, they only go upto 64ppr/256cpr in 5V incremental version, which is only good to 1.4deg.

That's why most accurate positioning uses optical sensing like those encoders from Zapp. With 1024ppr/4096cpr, they're guaranteed to give you an accurate step every 0.09deg.

One thing to be wary of, is you don't exceed the sampling rate of whatever you're connecting the encoders to, however that's more a servo issue when spinning at higher speeds. Using a 1024cpr you hit 25khz input at just above 1400rpm, which you really don't want to be spinning steppers at, however if you were to do something daft like put 4096cpr encoders on, then you're looking at hitting the limit around 350rpm.

I did look at moving to linuxcnc last year, however the lack of information on running closed loop (I found a few people who'd done it, but very little instructions on how to acheive it), and the need to buy additional input cards to gain high enough sampling rates, made me opt for a KFlop instead.