There are currently many solutions to obtain information on the location, direction, and motion of pointing, driving, and guiding devices. In fact, it is becoming increasingly common for many applications to rely on the Global Positioning System (GPS). However, when facing indoor navigation and dealing with more complex and environment related challenges, relying solely on GPS is not enough. For such applications, different types of sensors can be used to improve the system's ability to judge actual motion from abnormal actions. The ability of a specific sensor used to handle special navigation problems depends not only on the performance of the sensor, but also on the unique dynamic characteristics of the application.
Most applications involve different detection techniques, none of which can independently meet the application requirements. For GPS, obstacles can block satellite reception, making it prone to errors. Another common navigation aid device is a magnetometer, which requires clear contact with the Earth's magnetic field. However, in industrial environments, there can be many magnetic field interferences, which can result in the reliability of the magnetometer not always being at its optimal state. Optical sensors are affected by line of sight obstruction, while inertial sensors are generally not affected by these interferences, but they also have some limitations, such as the lack of an absolute reference point (where is the north?).
Sensor selection
The 20-year application history of the automotive industry has proven that MEMS inertial sensors have high reliability, as well as the advantages of low power consumption, small size, and low cost. Their successful applications in mobile phones and video games also demonstrate their commercial appeal. However, there are significant differences in current performance levels, and devices suitable for gaming cannot handle high-performance navigation issues. For example, the performance level required for precision industry and medical navigation is typically one order of magnitude higher than that of MEMS sensors used in consumer electronics devices.
In most cases, the motion of the device is relatively complex (multi axis motion), thus requiring a complete inertial measurement unit (IMU) that can integrate up to six degrees of freedom of inertial movement (three linear and three rotational).
For example, ADI's ADIS16334 iSensor IMU can adapt to many industrial instruments and automotive applications. In many cases, four or more additional degrees of freedom can be integrated, including three-axis magnetic detection and uniaxial pressure (altitude) detection.
Inertial measurement units can output highly stable linear and rotational sensor values, which must be compensated for the following influencing factors: temperature and voltage drift, bias, sensitivity and nonlinearity, vibration X, Y, Z-axis alignment errors. Inertial sensors have different drift degrees due to their quality, and designers can use GPS or magnetometers to correct this drift. In addition to good sensor design, the main challenge for navigation applications is determining which sensors to use at different times. Inertial MEMS accelerometers and gyroscopes have been proven to have good auxiliary effects for designers to complete a complete functional detection system design.
In indoor industrial or medical environments where GPS signals are interrupted and mechanical and electronic devices generate magnetic interference, designers must achieve mechanical guidance through non-traditional solutions. Many emerging applications, such as surgical tool navigation, require significantly higher accuracy than car navigation. In all of these cases, inertial sensors are an option that can provide the necessary dead reckoning guidance to maintain accuracy when the line of sight is obstructed or other sources of interference can have adverse effects on non inertial sensors.
A universal inertial navigation system (INS) can be used to guide anything from surgical tools to cars, airplanes, etc. The INS model includes a Kalman filter, which was first used for the Apollo moon landing mission and is now widely used in phase-locked loops for mobile communication to provide a mechanism for combining multiple good but imperfect sensors to obtain the best estimation results regarding position, direction, and overall dynamic characteristics.
In the field of surgical applications, INS can play a role in assisting navigation, helping to align artificial joints such as knee or hip joints based on the individual characteristics of the patient. In addition to achieving better alignment (to improve comfort) and faster, less invasive surgeries, using the right sensors can also help eliminate hand tremors and fatigue issues.
In recent years, pure mechanical alignment has been supplemented by optical alignment, but just like the GPS signal blockage problem that hinders car navigation, potential line of sight obstruction in the operating room can limit the accuracy of optical sensors. Inertial guided surgical alignment tools can assist in supplementing (or even replacing) optical guidance without any visual issues, while providing potential advantages in size, cost, and automation.
While consumer applications strive for small-sized, low-power, and multi axis inertial sensors, some sensor developers also attach great importance to developing compact, high-precision, low-power, and high-performance sensors. These sensors with good environmental adaptability are sparking a wave of adoption of MEMS inertial sensors in the industrial, instrumentation, and medical markets
