With the availability of a motion driver for 6DoF platforms, I thought it was time to try building a six degree of freedom rig based on the classical Steward Platform.

The idea of my construction is based on the Hexatech simulator from Cruden.

As you can see, the rotation point is high, around shoulder level. This helps to avoid false cues and also keeps the platform height down, to make it suitable for my attic. It also allows for longer actuators.

The heart of this 6DoF is the linear actuator, which I designed based on following criteria:

 
Outside view is a wooden box. Balancing bungee via 10 strings in folded configuration.

Description of the principle:
The drive shaft is fastened to double timing belt via a wooden block. The 10 strings balancing bungee are also fixed to this wooden block to relieve the belts from the platform static load.

The main drive shaft is positioned such that there is no force offset between belt and shaft, which keeps the forces mainly axial, so the Teflon coated slide bearings at the shaft top are not stressed in sideways force too much.

Drive calculations:
The two timing belts are driven via a 72/20 teeth timing belt transmission. It gives an outgoing force of 50N/A motor current.
At 12A motor current (24V supply) I get 600N outgoing shaft force.
With 24V drive, the motor does 11 turns per second. The belt drive does  3 turns per second, which gives 0.5m/sec drive speed.
With total 180kg platform weight, I calculated I need 50kg of balance force per actuator at platform lowest position, which requires 10 strings of 8mm bungee. The actuator length ranges from 1.3m to 1.7m.

 

Google Sketchup file of the Linear Actuator can be downloads here.
I have used layers, so you can view each component separately. 

A video of the Linear Actuator design and some inintial tests can be seen here.

The triangular bottom frame was made from 3mm thick 50mm square metal tubing, the side posts were made of 2.5mm thick 60mm square tubing. top plate is 6mm, and connects to the rod-end bearings.

Preliminary setup with half-finished actuators:

Total platform footprint will be 2.3mx2.3m, and max pitch / roll angles +/-15~17 degrees.
For the software I plan to use Ian's 6DoF driver, and the hardware will be based on hardware servo loop, similar to Platform drive III and IV.

Most materials for the actuator have been custom made in Taiwan. (see Mechanical Components V page)


 
Main Drive assembly and Position Sensor shaft assembly

Update January 2015:

The original actuator has some drawbacks: It is relatively long but travel is limited, and the motor position is such that adjacent actuators will touch each other in the Stuart configuration. The dual pulley-belt system also makes it rather expensive. Markus has successfully build another actuator, which is based on Ian Hopper's original linear actuator design.

My updated linear actuator incorporates ideas from both Markus' and Ian's design:

Some drawings of the new setup:

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