I was asked to mount a Vex encoder onto a FP gearbox, so we could measure it's run-speed. It was a fun little challenge.
Nicely done, JV.
Since you had to cut the gearcase anyway, wouldn’t it have been easier to mount a geartooth sensor near the motor pinion and just measure the electrical frequency of its signal? Or was the challenge specifically to do the job with a Vex encoder?
Gear tooth sensors only work on gears made from ferrous metals. Fisher price gears are plastic.
EDIT: Oops, I guess I didn’t read the post carefully enough.
I suggested counting pinion teeth. The motor pinion is metal. A geartooth sensor such as Allegro’s ATS665 will generate one pulse per pinion tooth at rates up to 20 kHz, which should be sufficient for the FP speed range.
But maybe JV was asked to use a Vex encoder for some reason?
This was a “quick and dirty with what we have on hand” project.
I had plenty of Vex encoders on hand. 
My first idea was to attach some retro-reflective tape to the output, and use a banner sensor to count pulses. This would have been the zero-effort approach, and I think they decided it wouldn’t annoy me enough, so they wouldn’t let me do it.
During this project I relearned to hate the FP gearbox (specifically the grease they use inside it).
JV
Ha…that is the truth, and the reason why i was so happy when Joe Johnson found banebots.com and their transmissions.
And you’ve got to love the giant globs of grease piled up on one side of the face of a gear (does wonderful things for balance), and none on the teeth. At least that’s how ours was.
What is the accuracy? how fast can it track without going mad?
Is that a bracket to support the shaft on the other end or is that just a collar visible on the end of the shaft?
The great JVN does it again. Good looking piece of work there John looks promising. Very cool and exciting good luck with that. I am sure it will help FIRST in some way with the motor specs of the Fisher price motor.
-Drew
Accuracy -
It measures 1/528 revolution of the gearbox output. It can handle up to 1700 pulses/sec, but will see 1233 pulses/sec in regular usage.
There is no opposite supporting bracket; that is just a shaft collar shown in the picture.
JV
So, do the VEX gears mesh with that specific stage of the F-P gearbox? I haven’t compared the pitches of each specific stage recently…
However, I can already envision a better use of this, which is to mechanically couple the F-P motor directly onto a existing VEX-bot. (I know the electrical add a couple more components.)
Well done, shall we keep our eyes out for a whitepaper? (j/k John!)
BEN
Ben,
Yes, there is a 24DP gear stage in the gearbox (or a stage that is close enough to 24DP that it meshes nicely.)
It’s a real short whitepaper:
Cut open the gearbox, next to the 24DP gear.
Mesh a vex gear with this gear.
Mount stuff around it.
JV
It just goes to prove, “Simplicity is the mark of Genius.” :yikes:
How do I mount this? I’m so confused with one day left. I need to be able to stop a forklift at 4 positions. Im using a winch system with one of these motors and all I have is the gear tooth sensor (the chip thing). I know how to connect it to the RC i think, but where do I get that red thing from? and how do I connect it do the motor? please help!