What are Intertial Measurement Units
An IMU is a Micro-Electro-Mechanical System (MEMS) typically comprised of 3 accelerometers and 3 gyroscopes. The more advanced systems also include 3 magnetometers. These tri-axial sensors are typically referred to as 3 degrees of freedom (DOF) IMU sensors while the more advanced ones are capable of 6 DoF. 3 DoF means the sensors give only orientation (or tilt) data in 3 axes. 6 DoF systems also provide translation or movement data indicating where the sensor is located in free space. Many IMU manufacturers misuse these terms and count the number of sensors as “Degrees of Freedom,” which is a misrepresentation of the inherent characteristics of IMUs. One example of this is when a sensor that includes 3 Gyroscopes, 3 Accelerometers and 3 Magnetometers is touted as a 9 DoF system.
When are IMUs the right choice for motion capture?
→ REAL TIME VISUALIZATION: Inertial motion tracking systems offer real-time motion tracking visualization and requires little to no post-processing.
→ USE IN ANY ENVIRONMENT: Inertial sensors can be used anywhere, inside or outside and with no restrictions on capture volume.
→ GUARANTEED DATA CAPTURE: Inertial sensors do not suffer from any line-of-sight occlusions or data dropouts.
→ HIGH SAMPLING RATES: IMU sensors have very high internal sampling rates, typically at least 800 Hz.
→ DIRECT MEASUREMENT: Inertial sensors directly measure linear accelerations and angular rate changes without the need for differentiating positional data. This allows for the direct estimation of force data.
What are some of the drawbacks of IMUs?
→ POSITIONAL DATA: Positional data is typically not as accurate as either optical or magnetic tracking systems due to the mathmetical estimation process required.
→ DRIFT: Inertial sensors, due to the nature of the technology, usually have some level of positional drift over time.
→ INHIBITED MOVEMENT: IMU systems with multiple sensors can be restrictive and inhibit the movements of the subject for certain activities. With the recent development of small wireless inertial measurement systems, inhibited movement is becoming less of a problem.
→ GLOBAL POSITIONING: Global position data is not provided and must be calculated using either additional GPS sensors or with more advance systems that have a very accurate magnetometer combined with advanced Kalman filtering routines.