Motors drive exoskeleton movement

A little more than a decade ago, Richard Little and Robert Irving set out to design a robotic exoskeleton – now known as Rex – after Irving was diagnosed with multiple sclerosis. They wanted to develop a device that would enable wheelchair-bound individuals to once again stand, walk, climb stairs, and become upright and mobile.

Adding to their interest in developing Rex was the fact that both founders had direct experience with the day-to-day challenges and side effects of prolonged wheelchair use via immediate family members – both of the developers’ mothers used wheelchairs. So, the plan was to develop a method for those once confined to wheelchairs to be able to stand and walk.

Today, the exoskeleton comes in two versions: Rex Personal, released in 2007, and Rex Rehab. Both alleviate many of the health issues wheelchair users suffer because of the lack of mobility.
 

Design and components

The benefits of Rex include:

• No crutches or a walking frame required to provide stability, leaving the user hands free
• Usable by people with a complete spinal cord injury up to the C4/C5 level
• Protects the shoulders from injury
• Enables navigation of stairs and slopes (REX Personal only)
• Addresses the medical complications of wheelchair use

With four double tethered leg straps, an upper harness, and an abdominal support, security is simple yet ample. The specially designed cuffs hold legs firmly, but without creating pressure points, and are constructed from hospital grade materials, including hypoallergenic foam padding that prevents pressure areas from forming.

Powered by a rechargeable, interchangeable battery, Rex can be used continuously for more than two hours on one charge, and because Rex is designed to be inherently stable, it uses no power when standing.
 

Strong motors, secure stance

The original design and prototype for Rex required close work with potential users and, throughout time, the machine evolved joint by joint. First, the knee was built, then it was actuated, and then it was controlled. Next the process was repeated for the ankle. In addition, each joint uses complex geometry and precisely-orchestrated actuators while the exoskeleton’s bones swivel around virtual pivots.

Rex is a highly complex electromechanical device. Each exoskeleton has thousands of precision parts, including limbs that are controlled by a network of 29 microcontrollers for the personal version, (27 microcontrollers are in the Rex Rehab version). The special arrangement of the microcontrollers makes it possible for users to move and react within seconds.

The maxon motors control the limbs, which move in the same way as a human leg. Ten DC RE 40 maxon motors are used in each exoskeleton. The RE 40 is equipped with powerful 150W motors delivering efficiency of more than 90%. The mechanically commutated DC motors are characterized by good torque behavior, high dynamics, a very large speed range, and a long service life. The heart of the motor is the ironless rotor that ensures detent-free running of the drive.

Although the exoskeleton weighs 84 lb, because of its design, the user does not carry any of this weight. The system is controlled with a joystick and control pad, a control setup that the designers say required no movement or nerve functions to use the exoskeleton. Functioning at approximately 3m/min, the Rex is not very fast, but the user can move forward safely and will not lose balance. Whether it is switched on or off, the exoskeleton always remains stable.

Rex Bionics chose to use the maxon motors for a good reason: Rex is a highly sensitive medical product and the safety of the users is of utmost importance. The largest challenge, where the motors are concerned, was uniting quality, size, and power in a single product. The decision seems to be working.

 

maxon motors
www.maxonmotor.com

About the author: Elizabeth Engler Modic is the editor of TMD and can be reached at 330.523.5344 or emodic@gie.net.