| Improving the wheel encoders Kategorie: Pro-Bot128 (von PepeV - 12.02.2010 13:08) | ||
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Hi all, The wheel encoders are a weak spot of the otherwise wonderfull design of the Pro-bot128. They easily pick up noise like interference from lights in the surroundings or electric interference (cross talk) from signals that drive the motors of the bot (as I wrote earlier on this forum). Tuning the position of the phototransistors and the IR-diodes as suggested in the manual is useless. At least I did not notice any improvement. I would like to share here some better tips to improve the reliability of the wheel encoders. Key point is that the slopes of the pulses from the phototransistors should be as steep as possible. The steeper they are, the less time is spent near the thresholt value of the input port of the Mega128 and the less time there is for interference signals to create extra thresholt crossings. There are two ways to improve the steepness of the slopes of the pulses. 1. Increase the difference between the reflection of the dark and the light segments on the encoder gear wheel. I used aluminium foil and printer blackend paper. This is how. Take a sheet of ordinary paper. Glue aluminium foil onto it with the shiny side on top. With a drawing program draw a circle with eight black and white segments like the plastic sticker that came with the bot. Also draw a white inner circle where the axis will come. Make two copies, for each gear wheel one. Print them. Cut out the circles. Cut out the white segments but leave the inner circles intact to keep the black segments attached. Glue the resulting four leaf clovers on the paper/aluminium sheet you prepared earlier. Cut out the circles and cut out the inner circles for the axes to obtain the improved stickers for the encoder gear wheel. 2. Decrease the distance between the encoder gear wheel and the phototransistor and the IR-diode. There is a lot of blacklash (space) between the encoder gear wheel and the phototransistor and the IR-diode. This can be deminished by clamping a washer on the end of the axis of the gear wheel. Take an ordinary steel washer with a hole that is slightly to small and file it out until it fits securely on the end of the axis. Leave just a little space for the gear wheel to run smoothly. A retaining ring will do the same job but might be more difficult to obtain. This will make the wheels run smoother and with less noise too. Regards, Pepe | ||
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