IMU raw and processed data for computing human joint angles

The goal of this study was to compute the relative angle of human joints such as the knee flex/extension angle using two IMUs. To do so, we utilized two 6-axis (accelerometer, gyroscope) low-cost IMUs (MPU6050, TDK-Invensense, CA, USA) that were mounted on a custom developed test apparatus that replicated the human knee motion. The custom test apparatus contained a single motor that repeatedly rotated one of the IMUs from 0 ~ 180 degrees at a one of three predefined speeds (slow - max speed of 25 deg/s, medium - max speed of 100 deg/s, fast = max speed of 200 deg/s) about one of the three rotation axes (yaw - rotation about gravity vector, pitch - rotation orthogonal to yaw and roll, roll - rotation orthogonal to yaw and pitch) for 25 minutes. One IMU was stationary and mounted securely on the side of the test apparatus while the other IMU was mounted on the motor axis and rotated about the motor-axis. The motor speed could be programmed via a microcontroller. The rotation axis could be changed by re-configuring the orientation of the IMUs. An optical encoder, which was placed on the motor shaft, was used to measure the true rotated angle. As the test apparatus rotated the IMU, raw accelerometer and gyroscopic data from the two IMUs as well as the encoder data were collected using a microcontroller. Five computational algorithms (i.e., accelerometer inclination angle, gyroscopic integration, Complementary Filter, Kalman Filter, Digital Motion Processing) IMU data were used (see https://github.com/ssong47/get_joint_angles_using_imus) to calculated the relative angle between the two IMUs. These computed angles were compared to the gold standard (i.e., encoder angle). A total of 9 trials were collected = three rotation axes (yaw, pitch, roll) x three speeds (slow, medium, fast). Each trial contained two phases: 1) a short calibration phase where the IMUs were stationary and placed such that the origins were parallel to each other, and 2) test phase when the one of the IMUs started to move at the pre-defined speed and rotation axis.