It’s a nightmare. You’re a firefighter or a cop. You’re someplace dark and dangerous. Maybe it’s a fire scene or an earthquake-shattered building. Maybe you’re a soldier doing reconnaissance, or a coal miner, or a caver. Something very bad or very stupid happens and you’re trapped.
Thankfully, your multiband personal radio still works. It broadcasts an alarm at Incident Command if you don’t change position for several minutes, but what is your position? GPS, GLONASS, Galileo—they’re all iffy down inside this mess. An extremely sensitive personal inertial navigation system would be handy in that radio.
In June, Analog Devices released the ADIS16407 iSensor inertial measurement unit (IMU), which integrates a tri-axis gyroscope, tri-axis accelerometer, tri-axis magnetometer, and pressure sensor, with associated signal conditioning, in a single package (Fig. 1). That’s 10 degrees of freedom. Every IMU is factory-calibrated for sensitivity, bias, alignment, and temperature. Each sensor has its own dynamic compensation formulas, maximizing accuracy of sensor measurements.
The pressure sensor and the tri-axis magnetometer, which adds the ability to sense the Earth’s magnetic field relative to the device, is the major advance over the company’s ADIS16334, increasing the degrees of freedom from six to 10.
“Navigation technology used in first responder or unmanned vehicles not only requires multiple axes of sensing, precisely aligned, but also cross-integrated to discern tracking/location in dynamic environments,” says Bob Scannell, business development manager at Analog Devices. “Where no one single sensor provides the required precision, the solution involves merging multiple sensor types with a deep knowledge of the dynamics of the application environment.”
This is the latest entry in Analog Devices’ family of microelectromechanical-systems (MEMS) devices for inertial navigation. The earlier ADIS16365 is already used in TRX Systems’ Sentrix Tracking System, which enables first responders to navigate unfamiliar terrain and track personnel (Fig. 2).
The existing Sentrix system comprises a tracking unit and a command station. The tracking unit houses the MEMS device, which computes and communicates 3D location, movement, non-movement, status, and posture via a body-area network to a personal communications system.
The command station translates the information from the IC into a 2D/3D, real-time location. Its motion classification algorithm identifies specific human gaits such as walking and crawling and supports both manual and automatic alarming. Systems can work a full shift of more than eight hours between recharges. (For a video demonstration of the system in action, go to www.youtube.com/watch?feature=player_embedded&v=Z571TFAml4g.)
The gyro is built on the same process that Analog Devices uses for its automotive airbag accelerometers, i.e., the electronics and mechanical structures are integrated on the same substrate. Beyond that, it’s much more complex.
The gyroscope comprises a mechanical sensor structure and two electronic subsystems. One subsystem establishes a vibration in a resonator structure that creates the necessary velocity, or momentum, that, with imposed rotation, is the source of the Coriolis force. The other subsystem detects this Coriolis force-induced movement with an accelerometer using capacitive detection.
Electronically, when a gyro structure experiences a displacement, the Coriolis movement is at right angles to it and the inter-digitated electrodes function as capacitor plates, reminiscent of the variable capacitors in old tube radios (Fig. 3).
To obtain the motion necessary to produce the Coriolis effect, the company uses an electromechanical oscillator that drives the mechanical structure at resonance—about 15 kHz. The oscillation is started from the noise of the system with the loop in a high-gain startup configuration. When full oscillation is detected, the gain is reduced to maintain signal fidelity and enable accurate zero-crossing detection in the comparator.
Displacement of the vibrating structure is about 10 µm, but the displacement caused by the Coriolis force in the capacitor is on the order of 1 Angstrom—about the dimension of a hydrogen atom, which results in a capacitance change of about 90 aF (attofarads). But dealing with noise knocks this down to the point where the electronics are dealing with a capacitive change of about 12 zF (zeptofarads) based on about 16 Fermis (16 × 10–15 m) of movement.
If you find the physical and electronic scales here incredible, check out “Single-Chip Surface-Micromachined Integrated Gyroscope with 50°/Hour Root Allan Variance,” from ISSCC 2002.
That’s just one gyro. In all, there are separate gyroscopes, accelerometers, and magnetometers for x, y, and z axes, plus the barometric pressure sensor.
Three-axis Magnetometers and an altimeter
The principal difference between the ADIS16407 and the last generation of the company’s iSensors is the addition of tri-axis magnetometers and a pressure sensor—essentially, an altimeter.
Given the unreliability of satellite positioning in underground and in-building situations, the combination of magnetometers and inertial (gyro) sensors represents a “belt and suspenders” approach. While inertial sensors excel at short-term dynamic response to movement, they tend to be subject to some long-term drift mechanisms.
On the other hand, magnetometers have excellent long-term stability as a reference, but suffer somewhat on short-term response and hard-iron and soft-iron effects. (Hard-iron distortion is produced by materials that exhibit a constant, additive field to the earth’s magnetic field. Soft-iron distortion is caused by materials that distort rather than add to the planet’s magnetic field.)
Combining inertial sensors with magnetometers enhances coverage under varying and dynamic conditions. The merging is accomplished digitally in a Kalman filter. As for the pressure sensor, that final degree of freedom, it adds a more precise determination of relative altitude.
This kind of technology is also applicable to robotics, navigation, and medicine. In September, Analog Devices described an application for an inertial measurement sensor with six degrees of freedom employed by OrthAlign Inc. in its KneeAlign 2 portable surgical navigation system.
The KneeAlign 2 guides alignment of the femur and tibia bones during in TKA (total knee arthroplasty), a surgical procedure in which pieces of the knee are replaced with artificial parts. It enables orthopedic surgeons to quickly determine the center of rotation of a patient’s femur and calculate the precise angles to cut the bone. It replaces more expensive, camera-based systems. (For a video of the procedure, see http://orth-align.com/products_technology/surgical_video.asp.)
Earlier Work At Honeywell
The Analog Devices ADIS16407 iSensor is the first “10-degrees-of freedom” inertial measurement unit this sophisticated and compactly integrated, but it does have an ancestor in Honeywell’s 2- by 2-in., DRM4000 Dead-Reckoning Module. Like the TRX Systems Sentrix, the DRM4000 was developed for military and first-responder personnel. It was created by Tom Judd of Point Research Corp., which Honeywell has acquired.
Judd described the DRM in “A Personal Dead Reckoning Module,” a paper he presented at an Institute of Navigation conference in 1997. Honeywell has released preliminary information on a newer DRM module, the DRM-5, which measures 3.4 by 2 in.
Both Honeywell modules can correlate data from external GPS receivers using Kalman filtering and interface via RS-232. Published accuracy specifications for both DRMs are 2% of distance travelled horizontal, 1.5 M vertical, and 1° compass accuracy.