Howvabout an H-bridge motor driver driving a pully motor back (left and right).
One pulley on left side of target, one on right.
Single cable/rope loop going around one pulley on the top and coming back on the underside of
the pulley across the target under the other
pully and connected to the starting point creating
a closed loop around the pulleys. Targets are
hung from lower part of loop. Code runs random speed up and slow down loop. Target move left to right. You can vary delay between speed changes
to get longer periods of consistent speed.
Detecting hits is much more complicated because
projectile is so small, but if I had to do it I
would make the target out of material that makes
the most noise on impact and simply detect
impact sound using parabolic dish microphone
panning left and right (another h-bridge driver)
controlled by same arduino so it knows EXACTLY
when the speed will change and by how much
and it knows (by geometry) what spoed to drive the panning motor to track the targets. If you
record the impact sound and view it on an iphone spectrum analyzer app (I have one) and find put
the impact frequency you can built a notch detect
circuit (Narrow Band-Pass Filter) and then you can add a comparator
level detect circuit set to trigger for sound.
When you move closer to target sound increases
and you adjust trigger level down. Whe you move
away from target sound level goes down so you have to readjust trigger level lower.
H-bridge mosfet bds are cheap on ebay. I just
bought two at about $5/each.