I am thinking about a making a game where people throw a ball and speed of the ball is measured. My main problem is that people do not throw a ball at exactly the same place every time. I expect that I need to cover at least a 1 meter (3ft) square area in which the ball can pass. So what is a good approach for making a sensor to cover this area? My ideas so far:
Doppler is too complex for me, probably also too expensive too and difficult to "aim" at the ball.
Two "gates" with laser light or IR detection and measuring time difference seems relatively simple to me. But how to assure that the ball is detected when it passes a gate? Is it is possible to cover the area by using several mirrors to create a "mesh" of laser light on the surface? Maybe 2 lasers per gate would be OK to create a sufficiently fine grained mesh?
Does my plan make sense? Are small lasers powerfull enough to travel several meters incl. mirror bounces?
Any tips or sharing of experience is very welcome!
use one laser and let it make a zig zag pattern to the detector by placing the laser under a small angle and you will get a dozen or more "trip wires"
LASER)/////////(very fast pulse detector)
The pulse detector must be able to switch in the period the beam is broken.
Light travels 300.000KM/sec =300.000.000.000 mm
in one nanosecond it travels 30 centimeter.
if your ball is 10 cm in diameter and it flies at 72KM/hour that is 72000 meter /3600 seconds = 2000 cm per second.
The maximum disruption of the laser beam will be 10 cm == 1/200 second
so the signal pulse disruption will be in the order of 1..5 milliseconds.
should be doable with Arduino but you need a fast (on/off) light sensor
You can use two laser pointers and two ldr resistors. When the ball interrupts the first laser, store the time, and when the ball interrupts the second laser, substract the time stored from the last time readed.
laser 1-------------------------LDR
| | |
| | |
O Ball
Laser2------------------------LDR
PD. Sorry for my bad english, it's not my native language
Thanks for your replies. Looks like I am on the right track, but indeed need to assure that the sensors have fast enough reaction time. And maybe the distance between the 2 gates should not be too short to allow the arduino get both readings.
On to the experimenting phase...
How accurate do you need the speed? I build a TDR to measure and find breaks in cables and I had considered altering it's input method to use laser detectors so I could measure distance via time of flight. The timing chip I use can measure in the 45-90 pico second range so quite accurate and I could measure to about 2cm on average. Laser would maybe be slightly less accurate. http://forum.arduino.cc/index.php?topic=183770.0
vier:
You can use two laser pointers and two ldr resistors.
Don't use LDRs (light dependent resistors) for this; they don't react quickly enough for something fast and small like a ball. A phototransistor/photodiode of some type is definitely preferred.