Question for the CD community… We want to put encoders on our shooter wheels, however we have a small problem - we’re using 1/2 inch shafts, and none of the encoders from US Digital support shafts that big!
We’re direct-driving the shafts, so there’s no real chance for us to stick the encoder elsewhere, short of adding belting over to an idler sprocket just for the encoder (which we don’t really want to do).
So, any suggestions out there for a reasonably priced encoder that works on a 1/2 inch shaft that would mount similar to the US Digital ones?
Does the shaft stick out the end? If so, can you find someone with a lathe to turn the end of the shaft down to the correct size?
We have a few latching Hall Effect sensors on order, for our shooter encoder. Then we need to figure out how to put a magnet on the shaft, or the wheel, or something.
Unfortunately, no. The design we’re going with will have 2 550’s powering each wheel, and they’ll be mounted directly onto both ends of the shaft (through an appropriate gearbox). As you can imagine, this design does present some interesting challenges… like how to get an encoder on there!
Something to think about from our experience in Aim High is static electricity. In 2006 our ball shooter was a terrific generator of high voltage charges. Sparks could jump half an inch! That same year we used those larger US digital encoders and went through about half a dozen of them…destroyed by static.
If you have an all metal shooter it may not be an issue because the charge will have somewhere to go. Otherwise think about where the electrons will go to roost.
Interesting you should mention that…we were playing with the VanDeGraff generator in our teacher sponsor’s room…and I completely missed the fact that we’re building our own on the robot!
We’re trying to keep things simple and avoid having an idler like that just to power the encoder (although we have done similar idlers before).
bear24rw, thanks for pointing those out… I swear, I looked all over but must have missed those somehow. It sucks that they come out to be about $70 each though!
We had a similar issue in finding a decent encoder. I found and gutted an old Ball Mouse, using the optical components to sense a slotted wheel.
A ball mouse has four IR LEDs (clear case) that shine onto four Photodiodes (red case). Starting with a 7805 voltage regulator (to convert robot’s 12V), I fed a pair of LEDs (wired in series) the 5V through a 100 Ohm resistor.
Each photodiode has a surface-mount 10k resistor associated with it. +5 goes to one side of the photodiode, I pick off the signal from the other side of the photodiode (which also goes to the resistor), and the other side of the resistor is ground.
The Photodiode output is fed through a 74LS14 inverting Schmitt Trigger ('cause that’s what I had) but I suppose any TTL device would buffer it OK. (A Schmitt Trigger is used exactly for this kind op application, ‘debouncing’ any inputs)
I think the cRio can handle the data rate, if I measure the time between pulses to check speed. If not, we might use an Arduino board to handle the counting via Interrupts and also do the PID calculations, feeding only motor speed control data back to the cRio.
Testing it tomorrow.
(Are you old enough to know what a Ball Mouse is?)
That’s interesting. I’ve never taken apart a ball mouse before, but now I want to. The only question is where to find one. I suppose I could go ask at the local ancient history museum…
(and yes, I’m old enough… I started programming about 18 years ago :p)
Has anyone tried using last years Rockwell Automation light sensor for the kit of parts? They spec it as having a 1Ms response time. Unknown to me is how focused it is and how close it can be to the axle.
I pick them up at garage sales all the time, have a box full. Just to take them apart and use for things like this. Just have to keep your eyes open. The local thrift stores probably have some, too.
