Vitalik gave me the idea to use a stiff stripe: cinch one end of it up to the motor and put an obstacle in front of it. The motor will bend the stripe forcing it to accumulate the energy. Eventually the stripe kicks the target.
As a motor I took standard one from Lego NXT kit. As a stripe I chose a blade. Other objects are just bend and not resilient enough.
To increase the power of the stroke I piled 3 blades one upon another. But actually I could not elicit the sufficient power from this mechanism. The plastic ball is lifted 10-30 cm above the platform but I expected at least couple of meters.
The process is cyclic. At each cycle we get one stroke. There is no need to devise the triggering mechanism.
It is tricky to choose right position of a stripe’s barrier, motor power and the number of blades.
The energy of the blade is too weak. The ball was hit up to 30 cm in vertical direction.
An alternative may be to use compressed air and a pump, but I hardly know which accessory to chose.
Somehow use the rubber. The downside is a construction of triggering component.
Choose more resilient stripe. In this case more powerful motor should be selected too. Or choose another configuration of the resilient metal. In this case the problem is to connect it somehow to the motor.