Brushless DC motors keeping muscles in shape.

For a paraplegic person rehabilitation is often a long process. This is why procedures and equipment that can optimise these processes for the patient are critical. maxon brushless DC motors have been used by researchers who developed a recumbent bike at the Bern University of Applied Sciences after many years of research.

Recumbent style bikes in particular are effective for the restoration of lost motor functions of patients with spinal cord lesions. They also help patients with cardiopulmonary and muscle training. Leg muscles can be reactivated with functional electric stimulation (FES) alongside controlled training.

At the Bern University, students and scientists carry out research on rehabilitation methods and their further improvement. Competencies include cardiopulmonary and neurologic rehabilitation following stroke or spinal cord injuries. To supplement this there are feedback systems, automation and the control of modern training devices. Procedures and systems are developed in an interdisciplinary team in cooperation with Swiss industrial companies and rehabilitation clinics. Kenneth Hunt, Professor for Rehabilitation Technology and director of the Institute for Mechatronic Systems at the Bern University of Applied Sciences has founded the Institute for Rehabilitation and Performance Technology. The Scottish-born expert in the field of rehabilitation was co-founder of the “Scottish Centre for Innovation in Spinal Cord Injury” in Glasgow and left his mark on the centre as inaugural Director of Research. He gained experience in the industry during his five years of working at the Department of Research and Technology of Daimler-Benz AG in Berlin.

“Recumbent trikes are suitable for persons who suffer from paraplegia, a stroke or childhood-onset cerebral palsy. “We use methods and technology from the field of competitive sport to improve the rehabilitation process of people who suffered accidents or diseases,” explains Kenneth Hunt. FES was already used on the first generation of recumbent trikes, but at this stage no motors were used yet. The new generation is equipped with an electric motor that provides additional power to supplement the FES. FES alone can only generate a low power of 30 to 40 W, which means that the maximum power and duration of use is very limited.”

DC motor and gear components for support of the leg.
A DC motor is required to support the initial stimulation. The motor keeps the legs of the patient in motion. The motor also gives the controlled training and switches back and forth between brake and generator mode depending on the ability of the patient. The maxon brushless DC motors are fitted in the pedal bearings at the front of the trikes. Demands from the DC electric motors are high. The motor and gearheads have to fit between the pedals to keep the system as compact as possible. According to Kenneth Hunt, the maxon motors and the matching gearheads meet these requirements. The brushless maxon EC flat motors are the perfect solution for many applications because of their flat design. The 90 Watt flat DC motor used in the recumbent trike also must supply ample power for the application.

Motor and brake in one
The motor initially moves the legs of the patient before the muscles are stimulated. The more active the patient is, or the more muscle power is generated by means of the stimulation, the less support is required from the motor. As soon as the muscles generate enough force to move the legs autonomously or to work against a load, the motor acts as brake. For this reason a four quadrant motor controller is necessary to switch between motor and brake operation. An electronic brake chopper is used during braking operation to dissipate the generated energy. The interaction between motor/brake and FES is controlled with custom software that has been developed at the Institute and can run on a PC or micro. The pedals and wheel of the trike are not interconnected by a drive chain. Each component is mechanically decoupled from the other and the chain is not required. The pedals and the wheel are only “coupled” with the control software. This gives a high level of flexibility because the dynamics of the motion can be adjusted with the software and changed in real time. Two prototypes of the recumbent trike have already been manufactured for research. (One system for adults and another for children.) Large clinical studies conducted with the FES system (without motor) in the past have shown that the fitness of the patients is improved, the bone density in the large leg bones is increased and the muscles are built up again. After clinical testing has been completed, the plan is to commercialise the new generation of recumbent trikes as well, with the assistance of an industrial partner. Additional modifications and optimisations are planned for the recumbent trikes.

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