Speed Sensor for Ball Shooter

[David Brinza]

Originally Posted by 6600gt

Dosen't the photo sensor have only a 500us response time?

Note: This post really deals with measuring the ball speed, not the shooter...

Per the attached spec sheet from Rockwell Automation, the photo receiver 42SMR-7100 has the following response times:

1ms ON/1.5ms OFF

So, if you are going to measure ball speed based on the time the ball is blocking a single light beam, the sensor will indicate OFF ~1.5ms after the beam is blocked and ON ~1ms after the beam becomes unblocked. So the sensor will indicate the ball has passed through the sensor ~0.5ms faster than actual.

So, if you're blocking the beam across the full ball diameter (approx 18 cm) and the elapsed time according to the optical sensor is 14.5ms, the actual crossing time is 15ms. That would put the ball speed at 12 m/s - just barely legal !

In terms of assessing measurement error, the actual response times for the optical sensor are needed (plus any jitter, which should be small). Error in the assumed ball diameter (i.e. is the sensor measuring the full ball diameter or is the ball off-center or out-of-round?) also needs to be taken into account (this is probably the dominant factor, 1% error would require better than 2 mm knowledge). If you use the FRC controller timer interrupts as eugenebrooks has described in:

http://www.chiefdelphi.com/forums/sh...99&postcount=8

the timing measurement error is insignificant (better than 100 ppm).

I think we're going to try this...stay tuned!