November 1, 2014
We think we made something of a breakthrough today regarding which kind of actuation use (pneumatics, electric, or hydraulics). We started the day by discussing the relative advantages and disadvantages of each of these options.
Earlier in the week Tony talked to two product specialists, one familiar with pneumatics and hydraulics, and the other familiar with electric linear actuators.
Hydraulics are extremely heavy and overkill for the kind of speed and force we need. We need about 400lbs delivered at the back of the elbow to lift 200lbs at the hands, and we need for the cylinder to travel about 6 inches in around a second to get the range of motion and speed we’re after. The smallest cylinder we were able to find (1.5″ diameter) delivers almost 10 times as much force as we need. We estimated that the hydraulic components alone would weight about 200lbs. Having all that additional weight would make the suit very cumbersome to operate.
Pneumatics on the other hand, can deliver the speed and force we need, are lightweight, and are the least expensive option. The biggest problem with pneumatics is that for the suit to be self-contained, we would need either an on-board compressor, or a pressurized tank. A compressor that could keep up to the pressure we need (around 100psi) would be bulky and require a lot of power. A pressurized diving tank looked like an attractive option since a standard tank pressurized to 3000psi would be enough to power the suit for an hour or so, and can be refilled at a dive shop for about $5. The biggest problem with this solution is safety — we don’t feel comfortable having a high pressure tank. If the wearer were to fall and break the valve we’re not sure what would happen, but it wouldn’t be good.
We were initially leaning toward electric linear actuators because they are simple, light, safe, and relatively easy to control, both in speed and direction. Unfortunately the ones we found were either fast, or could provide the force we need, but not both. The electric specialist was able to point us to the ETH series of actuators from Parker. They are plenty fast and plenty strong, but they’re expensive, about $1500 per actuator. We are now thinking that if we use the strong, slow, and less expensive actuators for the upper torso (they’re about $400 each), and the Parker actuators on the lower torso where we need the speed to avoid falling over, we’ll still be within our $8K budget. So at this point, that’s our plan.
To get an accurate understanding of both the speed and force we’ll need to walk successfully, we decided that next we will build a tethered lower torso using pneumatics. We can experiment with different speeds and forces quickly and inexpensively, and armed with that knowledge we should be able to order exactly the electric actuators we need.
After the team meeting, Thelonius and Ed started working on a rolling support frame that will hold the suit up, and that we can attach to when we’re learning to walk without fear of falling over.
Thelonius sketching a design for the support frame.
Ed started repurposing some casters we had lying around from The Viper. The support frame will roll on these.
Cole finished building the control board for the prototype electric elbow. We’ll test it next week.
Joseph continued to work on the Inventor model for the final suit.
We were even treated to a guest – Dr. Richard Piccioni (Dr. P), a science teacher at Bay stopped by to see what we were up to. Thelonius gave him a quick refresher on welding techniques.
Goals for next week: continue work on the support frame and Inventor model, test the control board for the electric elbow, and start construction of the pneumatic lower torso.