Validation of electrolytic-liquid tilt sensors for human motion measurement

The development of inexpensive, low-weight, no-invasive sensors that can be used to accurately measure human motion is important for the evaluation of biomechanical risk during a variety of industrial work activities. Electrolytic liquid tilt sensors that measure the angular position of an object relative to the gravitational force vector may provide a valuable means for assessing the postural demand of work tasks. For example, the biomechanical cost to the low back could be assessed during manual material handling activities, or the demand to the shoulders could be evaluated during overhead construction work. Tilt sensors were tested during static and dynamic activities, using isovelocity and isoacceleration dynamometers to move the sensor. The output voltage of the sensors was found to be linearly proportional to the angular deviation of the dynamometer within an 150 degree range (r > 0.99). During constant angular velocity and constant angular acceleration movements, the correlation between the output of the dynamometer and sensor was high and linearly dependent on angular position. Hence, the sensors are capable of accurate motion measurement during static and controlled dynamic movements. Because of the inertia of the liquid within the sensor, its output during sudden acceleration/deceleration may cause some artifact which requires more extensive investigation.