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
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
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
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
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
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.