 |
||||
|
||||
Measuring RPM via Photo reflector
Computer
Interface
|
Measuring RPM using a photoreflector This is a follow-up to the Handy method of measuring RPM article. After obtaining a few photoreflectors I used one to measure the gearhead motor's RPM, both with and without the gearbox attached. I removed the gearbox from the motor, then disassembled it and counted the gear's interlocking teeth in order to calculate the gearbox ratio. Starting with the small gear on the motor's shaft and ending with the large gear on the gearbox's shaft, the teeth count was as follows: 10-25, 12-25, 12-25, 12-25, 12-23. The resulting, simplified gearbox ratio is therefore 43:1. To obtain as accurate a measurement as possible I had to experiment with various encoding wheels. I tried using smaller wheels to help reduce 'wobble' i.e. the slight up and down motion of a wheel as it rotates on top of the motor's shaft but is not exactly perpendicular to the shaft - I'm sure there must be a technical term for this phenomenon but I don't know it :) I found that 'wobble' had a more negative impact on the measurements taken with the photoreflector than it had had with a CdS photoresistor. Photoreflectors measured in cm's I also printed the encoding wheels on types of paper and card that had differing reflective properties. Various encoding wheels Finally the best results when measuring fast RPMs were obtained when I cut out the black segments from a 38mm diameter, 12 segment encoding wheel which had been printed on highly reflective, thick photo paper (the bottom right wheel in the photo).
It appears that uncut encoding wheels are most commonly used in robot encoders. From my own limited experiments, it would seem that the precise alignment of a photoreflector(s) with an encoding wheel is essential. I used the same circuit to measure RPM as before, except that I changed resistors to suit the photoreflector. Actually, I had two means of checking whether my photoreflector measurement of the motor's RPM was correct or at least close! Apart from the manufacturer's motor specifications, I had also previously taken measurements using a phototransistor and an infrared LED that I'd salvaged from an old 1.2MB floppy disk drive. Based on 9V, the final RPM measurements were, after correcting for slight differences in the battery's voltage:
According to the manufacturer's spec, at 9V the motor's RPM should be within a 10% +/- tolerance of 4,650. The calculation of the gearbox's efficiency is:
Although I had found that the gearbox had turned smoothly and easily by hand after I had detached it from the motor, I was still pleasantly surprised by it's measured high efficiency. Cheers, KD P.S. I'm curious as to how well the photoreflectors will work when used in a robot encoder circuit. It'll be sometime before I reach the stage of building a robot with an encoder circuit myself but I 'might' be able to give away three or four to other robotics hobbyists in the near future. I'll post an update here and possibly on the comp.robotics.misc newsgroup in a week or two after I've corresponded with the manufacturer ;-) P.P.S. I have to follow-up again with the manufacturer but they are presently observing the Chinese New Year holidays so I mightn't have any further news until mid February. Update: Unfortunately I can't give away a few of my
photoreflectors (I only have a few samples). I had considered placing
an order and reselling them (the retail price would have been very competitive)
but the manufacturer wouldn't provide a warranty! |
|||
