Here’s a little more insight into how the Accident Waiting to Happen table worked:
6 Ultrasonic sensors were arranged around the perimeter of the table. When observers stood in the range of these sensors, the PICAXE perimeter detection system picked up where the nearest person was standing and a Festo PLC moved the object away from them. The table mechanism consisted of a linear actuator, attached centrally to a 180 degree rotary actuator. This allowed enough freedom of movement to position the object anywhere on the circular table.
Four inputs told the PLC which position to move to as shown below.
The plan view of the table shows how this translated to real life. The linear actuator runs the full diameter of the table, so only needed to rotate by 180 to allow the object to be positioned anywere on the table. The numbered lines represent the sensor locations, the dashed lines represent intermediate zones:
The ‘encoder’ consisted of a semi-circular array of bars fixed to the linear actuator. The PLC counted the pulses from the capacitive sensor in order to keep track of the position of the rotary actuator.
This proved to be a very effective way of controlling the position, which didn’t need to be too accurate. Variations in the stopping time of the rotary cylinder added to the natural, fluid motion of the object on the table.