Many researchers are studying and developing various kinds of man-machine systems. Especially, a wearable robot, such as anexoskeleton power suit, is one of the most remarkable fields. In this field, more accurate and reliable sensing system for detecting human motion intention is strongly required. In most of conventional man-machine systems, torque sensors, tactile pressure sensors and EMG sensors are utilized in a man-machine interface to detect human motion intention. These sensors, however, have some limitations. For example, it is hard to install and secure torque sensors on the joints of a human body. It is not easy to correlate the data from a tactile pressure sensor to the human motion intention. Although the EMG sensor can detect human motion intention, the sensor system is complex and expensive, and suffers from electric noise. We have been developing an innovative sensor suit which, just like a wet suit, can be conveniently put on by an operator to detect his or her motion intention by non-invasively monitoring his or her muscle conditions such as the shape, the stiffness and the density. This sensor suit is made of soft and elastic fabrics embedded with arrays of MEMS sensors such as strain gauges, ultrasonic sensors and optical fiber sensors, to measure different kinds of human muscle conditions. In the previous paper, the muscle stiffness sensor for detecting muscular force was developed according to the fact that the muscle gains its stiffness as it is activated. Its superior performance was reported through experiments in which the sensor was applied for the assisting device for the disable. In this paper, the ultrasonic sensor disk is proposed as one of the sensor disks embedding the sensor suit. This sensor is based on an original principle and non-invasively detects activity of specific muscle. It is clear that the square of ultrasonic transmission speed is in proportion to the elasticity of the object and in inverse proportion to the density. It is estimated that the elasticity and density of the muscle increase or decrease as the muscle is energized. Then, it is hereby expected that the muscular activity is measured by the ultrasonic sensor. In this study, the feasibility of an ultrasonic sensor for detecting muscular force is shown thr- ough experiments.

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