Improved position pick-up
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This page describes an improved construction for position pick-up. It makes use of a spring-loaded gear construction. (The idea to use a spring tape measure came from Douwe, who uses these for the position pick-up of his 6doF platform. http://www.xs4all.nl/~jippes/ ).

I used a normal size 2m spring tape measure. When you open it up, there is a plastic pulley inside, with the tape measure on the outside, and a spring coil in  the inside of the pulley. Remove the tape measure, we only need the plastic pulley and the inside spring coil.
Two 13mm diameter ball-bearings need to be glued to the sides of the pulley. I increased the side holes to make a good fit for the bearings. The pulley now runs smoothly on a 5mm metal shaft.

Both ends of the 5mm shaft need to be threaded. A slot needs to be cut in the center of the shaft.

Now the shaft can be inserted in the pulley, and the spring-coil end can be inserted in the slot of the shaft. Finally a gear wheel needs to be glued to one side of the pulley. Now the coil spring can be rolled up, creating a relatively constant force to the pulley.

A wire is fastened to the pulley, and goes around a couple of turns. The other end is fastened to the platform leg where vertical position needs to be measured. The shaft is turned until good spring force is obtained, and then bolted tight to the side bars. The position pick-up 10-turn potmeter is gear-coupled to the pulley.

The construction is slip-free, and the potmeter accurately follows the platform leg height. The amount of potmeter turns depends on actuator travel, pulley diameter and gear ratio. A short video shows the movement of this type of position pick-up.

 

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