ENHANCEMENT OF INFRARED BIOTELEMETRY FOR MONITORING AMBULATORY PATIENTS OVER WIDE AREAS

Ante Santic, Faculty of Electrical Engineering and Computing, Zagreb, Croatia
Michael R. Neuman, Case Western Reserve University, Cleveland, USA
Vedran Bilas, Faculty of Electrical Engineering and Computing, Zagreb, Croatia

INTRODUCTION A limitation of infrared (IR) telemetry is that its range is usually confined to a small space such as a room, and shadows and non-reflecting surfaces require that multiple receivers be used for reliable operation. The goal of this work was to optimise infrared diffuse telemetry by determining the most efficient pulse transmission and extending the area over which the patient can freely move. This system is applied in the rehabilitation of patients with lower extremity injury.

METHODS A biotelemetry system for measuring forces on the feet and crutches or cane as a patient walks has been developed. Capacitive transducers sense up to six different forces on the legs while crutches or cane have a transducer based on IR radiation. Information was transmitted using time-division multiplexing and pulse position modulation. Pulse durations were 5m s, and the eight channels were sampled at 8x200=1600Hz. This keeps transmitter input power at 160mW, so that a small 9V rechargeable battery ( Varta, 110mAh) can be used for 4 hours of continuous operation. The whole transmitting system including the sensors requires twice as much energy and uses two additional 9V batteries. A personal computer with A/D performs signal processing and displays the 8 channels of data. A unique aspect of the system design is the use of multiple receiving units to include larger areas. This means that several rooms as well as halls or staircases can be covered. This is possible because transmitted pulses received simultaneously by receivers that are within range can be summed, and the noise from the other receivers can be muted. This improves transmission reliability by increasing the signal-to-noise ratio when the transmitter is not close to any receiver. Automatic gain control circuits at each receiver help to compress signal dynamic range especially when the transmitter is close to a receiver.

RESULTS The system has performed reliably in interior areas as large as 10 x 10 m and in up to 3 different rooms simultaneously. As many as 10 receivers have been used with the system at one time.

DISCUSSION The advantages of IR telemetry of simplicity and reliability over short ranges have been extended using multiple receivers with intelligent signal summing. There is no theoretical limit to the number of receivers that can be used in such a system, so it should be possible to extend the range to be even greater than reported. The utility of the system in the rehabilitation clinic has been demonstrated.