*2.1. Sensor System*

This paper describes the method to acquire and analyze the IMU data so that an abnormal gait from a combination of hip and knee motion can be detected. The main system configuration is a pair of IMU sensors to create a network of joints and links of the human leg (Figure 1). The study uses Bluetooth technology for wireless transmission, and an Android device or a PC acts as a host receiver for the data transmitted using these sensors. They have a host application installed to allow the collection of data in the receiver. A MATLAB program involving mathematical calculations provides the analysis of the sensor data.

The specifications of the wireless IMU system are as follows:


• The NRF24L01+ is a 2.4 GHz Radio-Frequency (RF) transceiver integrates an RF transceiver, a full-speed USB 2.0 compliant device controller, an 8-bit microcontroller, and flash memory.

**Figure 1.** A custom designed IMU network to record leg movements: the sensor system diagram and the manufactured wireless IMU sensor circuit.

The motion sensor module consists of a 9-axis sensor made by 'Invensense', MPU9150. The motion processor acquires the data collected by the accelerometer, gyroscope, and magnetometer, to generate the data in the format of quaternions. The MPU-9150 features three 16-bit analog-to-digital converters (ADCs) for digitizing the gyroscope with a full-scale range of ± 250, ± 500, ± 1000, and ± 2000◦/s (dps). The noise rate is 0.005 (◦/s/ <sup>√</sup> Hz). However, our test showed about 5◦/h stability of the sensor assembly at the 20 Hz sampling rate.
