Mechanical components III
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Here is an overview of the mechanical components that I used for the platform:


Rack-shelf metal is great for this type of experiment. You can bolt it together easily, make lots of adjustments, and with some cross-beams, a very strong structure can be build. The even spaced holes let you easily make accurate symmetrical constructions.
I used 30x50mm, 2mm thickness material. It was quite cheap in Taiwan, about US$ 1 per meter. You can get it up to 2.5m long.
In Europe, check for "dexion slotted angle"

 
For the cables I used 2.5mm steel cable, connected to the V-belt and bungee cords. Cable fasteners are standard hardware stuff. Bigger type cable fasteners were used to clamp the cable to the V-belt.

 


The actuator is made from normal car V-belt and pulleys. I used "A" type belt, 2" pulley on the motor that drives a 6" pulley mounted on the drive axis via #22 belt. Then a 2" pulley on the same drive axis moves a 2nd 68" outer diameter V belt. The mechanical drawings of the pulleys are shown on the side. I had these made in Taiwan, hence the Chinese comments.

The actuators in this motion platform use bungee cords to counter the weight of the platform.
The bungee cord is 8mm type, found in camping shops. To determine the length and amount of bungee cords needed, I did some measurements:


A length of bungee cord, mounted to the ceiling. The other end has a basket, that can be filled with lead weights. The stretching of the cord can be measured as a function of weight added. I did this for several cord lengths. The results are shown in the graphs.

As can be seen, bungee cord has a non-linear behavior in the beginning and when fully stretched. Also there is a slight hysteresis between stretching and un-stretching. (middle graphs) The shorter the cord, the smaller the linear range. The force needed to reach the linear range is the same. Longer cord lengths seem better, for a more constant force over the displacement in the linear range. But a compromise must be chosen to avoid excessive lengths.

Above graph shows some detailed characteristics of one particular cord length (1 meter). From this length, total stretched length will become 1.75m, with 28cm of travel in both directions  from 1.48m center value. Center force is 48N, so one cord is good for 4.9kg platform lifting weight.  I used 1 meter lengths with 8 in parallel. Totally about 9 meters per actuator.


 


The motors are real industry DC servo motors. I was lucky enough to find them on the junkyard one day. They are specified as:
Typical power 230W, current 4.4A, 2100rpm@ 52V, torque 1.05Nm. I used this motor with 36V drive voltage. For quick heave movement, you really need some power. All three motors running in full heave surge consume about 450W peak each.
DC servo motors can be expensive. Check this place for cheap second-hand stuff.



The roll-bearing wheels came from old production line conveyer belt rollers. They are aluminum with V-groove, and inside have double ball bearings with 15mm inner shaft.


For the end-stop switches I used micro-switches, with extended lever, that is pulled down by a rope and a spring.

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