You will do doubt have seen pictures from ‘RoboCup’ showing robots booting footballs from one end of the pitch to the other. Building an electromechanical robot like this is entirely within the capability of the hobbyist with the help of a few cheap everyday items.
In order to give the ball a good kick the robot’s feet are powered by linear sdenaids. Accelera-tion is more impor-tant than force, however, and so we eschew readily available solenoids which generally operate cn 12 V or 24 V and which, although power-ful, are much too slow for our purposes. The integral of force over time (or impulse) produced by a coil with an iron armature depends, disregarding constant factors such as turns count, coil geometry and permeability, on the change in the coil current. The faster we wish to change the current, the higher the voltage we will have to use. And so we need a high voltage supply. We can generate a high voltage using the flash from a disposable camera of the sort that can sometimes be had for free from photography shops.
The camera elec-tronics includes a high-voltage cas-cade circuit with a storage capacitor for the flash These components are ideal for pressing into service as part of a robotic footballer. Open the camera carefully. First remove the battery making sure not to burn your finger by touching the capacitor contacts. For safe-ty’s sake discharge the capacitor using a resistor of a few kilo-ohms I before removing the printed circuit 1 I board. Because we will later want the capacitor to be charged con-tinuously, bridge the power supply switch connections. The circuit in the camera tested by the author (made by Kodak) charges a 120 pF high voltage voltage capacitor to 330 V in 16 s from a 1.5 V battery. Next we turn to the sewing box for inspira-tion. We need two cotton reels from which we will fashion inductors using enamelled copper wire. On the one hand it is advan-tageous to use wire that is very thin so that we can have as many turns as possible and hence a high inductance, while on the other hand the high ohmic resistance of this arrangement limits the maximum current that can be achieved; we need to find a good compromise. To simplify making the windings with very fine wire, first wrap the coil former with a layer of thin double-sided adhesive tape.
This will hold the wire in place as you wind the first layer. Use adhesive tape again after each successive layer of wire. Finally, wrap the finished coil in insulating tape so that just the two connection wires (with extra insu-lation) protrude. The two iron cores can with a little luck be found in the clearance bin at an electron-ics shop. If not, you can resort to do-it-yourself: the cores can be ordered from any metal warehouse that can supply steel rounds. Ensure that you do not buy vanadium steel or a non-ferrous metal. The size should be chosen so that the lengths of metal pass through the cotton reels without too much play. In each drill a hole in one end and fit a small washer to prevent the light compression spring from sliding down. The spring ensures that after each kick (Figure la) the foot will return — smartly to its initial position (Figure 113). The cores are fitted into the coils and a plastic cylinder, which will be the part that actually makes contact with the ball, is attached to the free ends. Figure 2 shows how simple the drive circuit can be. A type TIC126D thyristor wired between the high-voltage generator and the coil triggers the kick. The thyristor is, in turn, triggered optically via an LDR, which ensures isolation between the high voltage electronics and the control circuit.
