Actuator Designs – 2016 to Now

Like I mentioned in a previous post, we’ve been at this project for more than a year now.  Early-on, we explored different motion control options and decided that we needed a good actuator design to enable the kind of motion needed for complete immersion.  To-date, most of our development efforts have been focused on that need.

Early exploration of kinematics for the platform. Helped us decide on the required strength of the servos.

First, the Final Version

Here’s the “final” version, below.  We’ll make modifications to improve manufacturability and improve the general appearance, but this is close to what we’ll be using for the simulator designs.

In the video, the position target is being changed randomly every half second.  A PC app of our design is setting the position, and a PLC is handling the actual motion control.

Version One – Ballscrew

Our first attempt (completed just before Christmas last year!) used a ballscrew and a smaller servo motor, compared to the final version.  We ultimately decided that the ballscrew was too noisy and that it had too much trouble moving as quickly as we wanted it to move, so we switched to a belt-drive system for the next version.

Version 1 actuator used a ballscrew and a smaller servo motor. We decided it was too noisy, and had too much trouble moving as quickly as we wanted.

Version Two – Timing Belt

This version used a timing belt and a linear shaft / bearing for motion.  It had no trouble moving quickly, but it wasn’t nearly as compact as the version 1 design.

Version 2, U-Channel belt assembly.
Version 2 Actuator – fast and quiet, but had idler pulley alignment issues.

This design had belt tracking issues when the belt was under load, because the idler pulley (at the top of the picture) was only supported on one end, and by the aluminum frame at that.

Attempting to fix belt tracking issues with a new tensioner assembly.

We built a tensioner assembly to support the other end, and it worked.  But it was unwieldy and imprecise – a bad design overall.  So we decided to move to rectangular aluminum stock for the main body, instead of U-channel.

Version Three – Rectangular Body

In an attempt to fix belt and pulley alignment issues, we switched to rectangular aluminum stock.  This made it easy to support the idler on two sides, as shown below:

Revision 3 actuator. Allowed idler to be supported from two sides.

The idler was attached to steel plates on each side, and the belt was tensioned by pulling the idler upward using a CAM assembly (the circular plate shown in the image was rotated about a slightly off-center point, pulling it tight).

This design worked, but the it was more bulky than we liked.  And the linear bearing and shaft design itself didn’t do much to constrain shaft rotation under load, which again caused belt tracking issues.  It isn’t shown, but when fully extended, this actuator was *very tall.  That made it hard to use.

Version Four – Linear Guide, Modified Tensioner

So, we switched from the linear bearing to a linear rail and guide system (as shown in the first video).  This keeps the entire assembly nice and compact.  The idler is supported on both sides, but the tensioning actually takes place on the drive pulley instead of the idler.  This made the tensioning process and the idler assembly itself much more simple.

We also added limit switches to prevent crashes in case of bad control inputs.

This is the version shown in the video at the top of the post, and the one used on the 2-DOF prototype.

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