One issue I’ve mentioned in the past here that I’ve been having is that the hip brace I’ve been using that the hip motor is mounted to can twist with the torquing force of the motor lifting up the leg. Here is a photo of what I mean:
So I decided to try out using an external frame backpack and mounting the motor to that to add needed rigidity to prevent against the twisting. Unfortunately, it didn’t work out as well as I had hoped.
I bought an ALICE backpack frame, which is a standard backpack frame that was used by the military. Here you can buy one at Amazon for $28:
Here’s what it looks like:
In order to attach the motor mount to the frame, I used 80/20 aluminum extrusion attached to the rear of the frame and then continued at a right angle to mount the motor parallel with the leg. Here’s what that looked like on (please ignore the messy room in the background :
Here are a couple more photos of the configuration not being worn:
The big issue with using this particular frame (as well as most off-the-self ones I imagine), is that the frame and the related belly strap and shoulder straps are designed to handle the downward forces of the pack weight. So the assumption is that these downward forces will inherently do a good job at keeping the backpack frame rigid against the back. The forces that I’m looking to counter are the twisting forces coming from below the pack. So what happens is that, much like the hip brace, these forces move the pack outward away from the back and in turn the efficiently of the motor actuation is lost. I had originally thought that having the belly strap secured would counteract this but it didn’t turn out to be true. Another problem is that the frame is designed to jut out on the lower part of the back so, as you can see in the photos, a good amount of tubing needed to be added to extend the motor mount to the right place and the downsides of this are a) added weight b) added complexity and c) another component that caused loss of rigidity as the tubing would itself twist.
So now I’m working on what I’ve been thinking about for a some time now which is using casts of the back along with Kydex (ABS) to make a mold particular to the user’s body. Although it adds complexity to the build and also brings in new challenges (e.g. attaching components to a one-off and curved piece) I feel that the results could definitely make up for it. I have already finished up casting of my back (with the help of my lovely wife Christina!) and I have the Kydex molded and ready to add components to it which I’ll post about soon.
Oh also, Klaus has a very cool torso setup for his exo that you can see in his video here:
He combines a backpack that is cabled to a plywood half circle that fits around his waist and is then cinched down with a belly strap. The motor is then mounted on the plywood.