The Praxis24 Sensor System for Use in the Closed-Loop Control of Standing

Our aim is to develop a safe and reliable closed-loop sensing system for the implanted FES system (‘Praxis24’, Neopraxis Pty. Ltd., Sydney, Australia), which will be light weight and easy to don/doff. Open-loop FES standing has typically been achieved for 10-20 minutes. This system will be used by paraplegic individuals for prolonged standing (20 minutes +), sitstand-sit transfers and stepping. Our present closed-loop control of standing is achieved by monitoring the bilateral knee angles using electro-goniometers (Biometrics Ltd.), resulting in stance for over 60 minutes by subject CS (T10-paraplegic male; ASIA: A) and the stimulation time being reduced to less than 10%. Stance stability is achieved by the Andrews’ Anterior Ankle-Foot Orthosis (AFRO). Closed-Loop control of stand-sit transfer is achieved by monitoring the bilateral knee angles and angular velocity using electro-goniometers (Biometrics Ltd., UK), trunk inclination and vertical acceleration using accelerometers (ADXL05, Analog Devices), resulting in diminished slamming onto the seat. Our next generation sensing system is based on sensor packs (Neopraxis Pty. Ltd., Sydney, Australia) located bilaterally on the thigh and shank. An additional pack is located on the trunk. Each pack is capable of measuring 3-D acceleration (2x ADXL202 2-D accelerometers, Analog Devices), and 1-D angular velocity (ENC-05D rate gyroscope, Murata). Each pack converts the sensor signal into digital values and performs further computation on these values. Preliminary results indicate that the knee angle can be computed from signals generated by the thigh and shank sensor packs. This paper describes the possibility of a simple knee buckle detection strategy using the angular velocity and accelerometer output of the sensor packs. Initial results indicate that the rate gyroscope is more sensitive to motion than the accelerometers, which makes the rate gyroscope give a significant signal the earliest in a knee buckle (200 msec earlier that the goniometer or the accelerometers).