C. Payload and Potentiometer

A close-up of the motion plant – that is, the solenoids, payload, and potentiometer – is shown in Figure III.C.1. The apparatus is constructed on a 0.5" thick aluminum plate that measures 12" by 20". The aluminum payload is sized at 2.4" x 1.5" x 1.5", weighs 13 oz. (0.368 kg), and is secured directly to the solenoid plungers. The 13 oz. include the mass of the solenoid plungers that weigh 3 oz. each. The solenoids are positioned so that they face one another on opposite ends, making a perfectly straight axis. They are secured to the plate so that the payload is slightly elevated from the plate, decreasing the friction between the payload and plate. The spacing between the solenoids allows a 0.8" (0.02032 m) payload travel.

The payload displacement is measured by a Markite type 2986 plastic film element potentiometer. The payload itself is attached to the plunger of the potentiometer. Its electrical travel is 1.0" and mechanical travel 1.3". The friction or actuation force for this model potentiometer is specified at 6 oz. (1.67 N). The actual measurements done during original construction showed that the friction force for the installed potentiometer is approximately 4.5 oz. (1.25 N). A stiff compression spring is placed around the plunger's shaft to ensure that the payload remains aligned with the potentiometer. Removal of this compression spring will cause the payload to be misaligned so that it won’t be able to move the potentiometer’s plunger.