Axis vs. Actuator-Driven Design

When we started this project, we wanted to build a full 6-DOF motion simulator, allowing for rotation and translation in every direction.  In the interest of keeping costs low and simplifying the initial prototype, we started with a 2-DOF version, which only allows for pitch and roll rotation.  We used linear actuators in the design so that we could use the same actuator design in the full 6-DOF version down the road.

Linear actuator design

As I design the second-revision 2-DOF frame, it is becoming increasingly clear that linear actuators may not be the best option.  Here are the problems:

  1. As these actuators now stand, they cost > $1k each to build.
  2. The actuators are designed to both lift and rotate a load, so they are significantly overpowered for this 2-DOF design, which only needs rotation.
  3. The actuators only allow for +/-45 degrees of rotation.  An axis-driven design would easily allow for at least +/-90 degrees of rotation, even without slip-rings.  That would be a wild ride!
  4. The linear actuators are fairly complex assemblies, time-consuming to build.
  5. The overall assembly is more bulky because of how the actuators must be mounted.

Don’t get me wrong, I’m glad that we have the actuator design!  We’ll definitely use it on more capable motion platforms down the road!

But I’m fairly confident that an axis-driven design would help solve all of these issues.  By spinning each rotation axis directly, instead of lifting the edge of the axis with a linear actuator, we increase the allowable rotation angles significantly (although we may have to increase the frame size to allow for the necessary space).  Without linear rails and idler pulley assembles, the transmission becomes more simple.  We can size the motors appropriately without having to lift a load.  Driving the rotation axis directly will require less space.  And finally, my initial estimates suggest that I can shave between $400 and $600 off the cost of each axis with the more simple design.


A potential downside is that we’ve never attempted a design of this type, so there will probably be some hiccups along the way.  Backlash might be a big problem with the cheap gearboxes we are using.  Without some kind of shaft coupler, high peak torques could damage the gearbox.  And so on.  We can solve problems as they arrive, I’m sure.

The verdict?  An axis-driven design is at *least worth exploring seriously.  I’m hammering out details of the design now.

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