The control system needs the knee and hip angles to properly predict the muscle forces.
This can be performed very simply for the knee joint, since the orthosis limits the
motion of the knee joint to the sagittal plane around a single fixed axis. This allows
the application of a Hall sensor which is accurate and reliable.
As shown in figure 6.6, the magnet is attached to the thigh of the orthosis and the
Hall sensor is placed above the magnet on the circuit board which is fixed to the shank.
During joint rotation the orientation of the magnetic field below the sensor is changed,
which is measured by the sensor.
The Hall sensor is a Philips7 KMZ41, connected to a Philips UZZ9001 signal conditioning
IC [DW00], which readily provides a digital output to the bus.
Due to the three degrees of freedom in the hip joint, a complex mechanical construction
or other sensors, like goniometers, would have to be applied, making the application
of the exoskeleton more uncomfortable. Thus, the hip angle is not measured,
but determined in two different ways: During calibration it is set to 90, because it is
assumed that the operator is sitting upright on a chair. During walking and climbing
stairs the hip angle is set equal to the negative knee angle, which is an unconventional
but sufficient approximation. Besides, the hip angle only affects the prediction of the
two-joint muscles, which are the rectus femoris and the flexor muscles, but the large
forces in the considered movements are contributed by the vastus medialis and vastus
lateralis