Servo - November 2009

today. We see present-day humanoid robots such as Honda’s Asimo with a huge backpack full of state-of-the-art batteries and are a bit disappointed that it can operate for less than an hour. It makes you wonder just what will be required in the way of a battery or other type of system to provide power for an exoskeleton that must deliver much more force than small experimental robots.

Electrically-Powered Joints

Most robotic exoskeletons have utilized electrical systems and electric motors to power the joints. Torque requirements at the knee, elbow, and shoulder joints are quite high and many designers have used worm drive gearing at these points. The placement of large gearmotors at these locations is difficult and some designers have positioned the large motors at different locations and have used flexible shafts to deliver the rotational power to gear assemblies at the joints.

All in all, the sum total of the motors, gear systems, structural members, and batteries adds up to a lot of mass and weight that must be moved in addition to any prospective payloads.

Pneumatic and Hydraulic Systems

As previously mentioned, Monty Reed and others have used compressed air systems for their suits. This is due to the large amount of force that a pneumatic cylinder can exert, but even Monty realizes that his future suits must contain a more compact power system.

Pneumatic cylinders act just like our muscles to pull or push an appendage. In a human, the ulna bone of the lower arm is pulled upwards to the humerus bone of the upper arm by the bicep muscle which is attached by tendons. Think about the tremendous force that muscle is exerting on the bones when a person is ‘curling’ a 100 pound barbell, for example. At 50 pounds per arm of barbell weight, and an assumed ratio

of 10:1 (arm length, tendon attachment point radius), each bicep must pull at a force of 500 pounds each. A pneumatic exoskeleton cylinder must also be able to exert the same force.

Another design issue with pneumatic and hydraulic exoskeletons is the difficulty of varying the cylinder’s pressure to accommodate differences in load conditions. As you can imagine, a barbell will exert the greatest load on the arm’s cylinder when it is extended at 90 degrees; proportionally less pressure is required when the weight is at a lower or higher position.

It is hard to rapidly change pneumatic or hydraulic pressure using a simple open/close solenoid valve. A motorized valve will deliver the same system high pressure when barely cracked open; the flow rate and movement will just be slower, not the force.

Issues with

Exoskeletons

Large robots have large cavities for motors, actuators, and battery systems. An exoskeleton must contain a full-sized human being, as well as those motors, actuators, power sources, and control systems. The human being must be able to enter the suit easily, be comfortable, and not claustrophobic, and have padding and physical protection from injuries. Control systems must be easily reached and manipulated by the operator.

These caveats sometimes doom an otherwise excellent design. If the human takes up most of the inside space, where do the batteries, joints, motors, and controls go?

Another issue that I mentioned earlier is the balancing problem. Bipedal robots have sophisticated algorithms to coordinate pace and length of steps all the while controlling momentum, acceleration and deceleration, turning, balance, and the positions of many joints. Inverted pendulum and accelerometer feedback systems can stabilize a robot that normally walks at a specific pace but a human wearing an exoskeleton walking at various paces presents a

more critical balance problem.

Full robotic exoskeletons, versus a system that augments only the lower legs pose a mass/momentum problem. Abrupt acceleration or deceleration mass can cause the suit to tip forward or backward.

Moving a large mass out in front of the suit can also cause a tipping problem. Designers compensate for this unbalanced condition by the use of pressure strain gauges at the front and back of the feet to determine a neutral balance point, and then shifting the leg joints to compensate.

Hardiman Leads the Way

In the mid ‘60s, General Electric engineers in their Schenectady, NY facility came up with Hardiman, shown in Figure 2. ‘Hardi’ is short for Human Augmentation Research and Development Investigation. This massive, strength-amplifying exoskeleton weighed 1,500 pounds and touted a 750 pound lifting capacity. Bomb loading, nuclear plant operations, underwater construction, and even space uses were envisioned by GE. Hardi had 30 joints and two massive arms interconnected by hydraulic and electrical lines.