The historically formidable challenge of navigation has been greatly simplified and made manageable for many old applications, while enabling innovative new ones, thanks to today’s MEMS-based motion sensors which are often combined with GPS modules.
MEMS-based Motion Sensors
In the past, accurate and affordable electronic sensing of motion and its various factors was difficult. Gyroscopes, magnetic compasses, star finders, and accelerometers were the most common components and subsystems available, and often required significant manual assistance for set-up and calculations. As a result, widespread use of electronic motion sensing was limited to advanced applications such as ships, aircraft, missiles, and spacecraft. The availability of easy motion sensing for low-cost drones, autonomous vehicles, or even the orientation of handheld smartphones was inconceivable.
Now, MEMS-based sensors make determining motion and orientation fairly routine. In doing so, it has changed the way we think about benefits of acceleration measurement, which is a surprisingly revealing and pervasive parameter.
MEMS-based Accelerometers
Note that MEMS-based accelerometers are very different than just micro-sized versions of the spinning wheel gyros sold as toys but also used in precise guidance systems; instead, MEMS units use vibrating tuning-fork structures and measurement of the parameter shifts due to motion. Also keep in mind that some applications only need to sense relative motion and position, since they have a fixed reference base (think of assembly robotic arms in a factory) while other applications do need to know absolute location (drones and unmanned vehicles, for example).
Basics of Motion Sensing
The phrase “motion sensing” actually encompasses a range of specifics and objectives. The fundamental parameters are the hierarchy of three vectors of position, velocity, and acceleration. Basic physics defines velocity as the time derivative (rate of change) of position x (v = dx/dt), and acceleration as the time derivative of velocity v (a = dv/dt).
The complementary operations are true as well, of course—velocity is the time integral of acceleration, while position is the time integral of velocity. In principle, if you know one of the three and measure time intervals, you can integrate and differentiate to find the others. (You still need to know your starting point for true navigation, of course, even as you determine one or more of the other three.)
Precision Accelerometers
Some systems rely solely on precision accelerometers in conjunction with analog or digital integration to determine velocity and position. The challenge is that even tiny errors in the sensed acceleration signal due to transducer imperfections or noise can build up to large, unacceptable errors in the results, whether integrating or differentiating. As a result, many systems choose to use a combination of sensors to independently determine these critical parameters.
Inertial Measurement System
There’s more to motion sensing than just the three factors. Depending on the application, a motion-sensing subsystem may also include single-axis or multi-axis gyroscopes or an electronic compass to determine orientation, and a GPS to determine location (but note that GPS signals are not always available). A gyroscope-based motion system, which includes a trio of orthogonal accelerometers and gyros (for x, y, and z axes), is often called an inertial measurement unit (IMU) because it determines orientation and acceleration without any external reference or received signal (Figure 1).
Figure 1: The classic IMU architecture is completely self-contained and features an accelerometer and gyroscope for each orthogonal axis, so it can completely determine both motion and orientation without the need for any external reference such as GPS, sightings (stars or landmarks), or geolocation beacon (radio signal).
That makes it the appropriate solution for applications such as underwater vehicles, vehicles in tunnels, or spacecraft, where GPS or the Earth’s magnetic field may be unavailable or too inaccurate. Even if they are available, an IMU may be used in conjunction with another motion-sensing subsystem for cross-checking results.
There are many motion-sensing applications that do not need the full capabilities of an IMU or the measurement of distance, velocity, and acceleration. For example, an airbag sensor just needs to measure acceleration, to determine if a sudden change in speed of the vehicle indicates a crash. Where the car is now or how it got there is irrelevant; similarly, vibration sensing for rotating machinery only needs an accelerometer to detect excess vibration and possible bearing failure; and an over-speed lockout on a vehicle needs only to measure that one parameter, and does not need to know position or acceleration.
Magnetic Sensing Devices: Fluxgate
The magnetic compass using a rotating needle is among the oldest navigation tools. It is moderately accurate, requires no power, and is fairly reliable. However, it is not compatible with electronic systems, and is difficult to calibrate or compensate for errors due to nearby metal objects.
There is an electronic equivalent to the magnetic compass: the fluxgate. It uses a magnetic core wrapped with a wire coil, and this coil is stimulated with an alternating current that induces known changes in the core’s magnetic field and output signal. Any external magnetic field will cause deviations from these expected changes; these deviations can be detected and amplified to sense the field’s strength with high precision.
While the fluxgate arrangement is obviously more complicated in concept than sensing a compass needle or its equivalent, it is has no moving parts, is small yet sensitive, and can be arranged to minimize effects of stray externals fields due to nearby objects. It does require a somewhat unusual electric interface, and the DRV421 from Texas Instruments is designed for this class of application (Figure 2). It combines a fluxgate sensor, signal conditioning, and a compensation-coil driver in a single IC, yielding sensing accuracy of better than 0.1 percent. To maintain linearity (another critical performance parameter), the IC simplifies the task of minimizing cross-coupling between the fluxgate sensor itself and the external compensation coil, as well as unwanted emissions from fluxgate’s own excitation.
Figure 2: The DRV421 IC from Texas Instruments provides accurate, non-contact magnetic-field sensing in a 4 × 4-mm package, via its combination of fluxgate sensor, signal conditioning, and compensation-coil driver. (Source: Texas Instruments)
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Accelerometer Sensors in Cars
Today’s cars use accelerometers for much more than airbag deployment; they are used as part of the roll-over detection/prevention system, theft alarms (when the car is being jostled or lifted); as well as ride stabilization and smoothing. Single-axis units such as the Freescale MMA2201KEG (Figure 3) are part of a series of MEMS accelerometers that include signal conditioning, a 4-pole low-pass filter, and temperature compensation for enhanced performance. Zero-g offset, full-scale span, and filter cut-off frequency are factory set and thus require no external active or passive devices, while a compete self-test capability verifies system functionality. The IC operates from a 5-V supply and has a nominal 50 mV/g sensitivity rating.
Figure 3: The Freescale MMA2201KEG is a basic single-axis accelerometer with analog output of 50 mV/g over a range of ±40 g, and is a good fit for many non-navigation acceleration and motion applications. (Source: Freescale Semiconductor)
Applications of MEMs Accelerometers
The applications of accelerometers such as this tiny device with ±40-g range, housed in a 16-lead 7.5 × 10 mm SOIC package, go beyond the obvious of airbags or navigational situations. It can be used for vibration monitoring and recording, appliance control, computer hard-drive protection (when acceleration suddenly drops to zero, the drive is falling and its heads need to be retracted immediately), computer mice and joysticks, virtual-reality input devices, and even sport diagnostic products, to cite just a few of the ways that single-axis acceleration can quantify what was both obvious and hard to measure.
A representative gyroscope IC shows how radically MEMS technology has transformed the “spinning rotor” gyro approach. The ADIS16250 low-power gyroscope from Analog Devices (Figure 4) is a complete angular rate-measurement system providing factory-calibrated 14-bit digital sensor data to a processor via simple SPI serial interface port. The device’s range can be digitally set to three different values: ±80°/sec, ±160°/sec, and ±320°/sec. It needs a single 5-V supply, is offered in an 11 mm × 11 mm × 5.5 mm LGA package, and offers 2000-g shock survivability even when powered.
Figure 4: The ADIS16250 low-power gyroscope from Analog Devices is a MEMS device with internal A/D converter offering 14-bit resolution and an SPI port to simplify interface to the system processor, along with other features that enhance its functionality. (Source: Analog Devices)
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It is targeted at a wide range of motion-related applications such as instrumentation control, platform control and stabilization, navigation, and robotics. Like many motion-rated MEMS ICs, it is available with extended temperature-range performance and meets stringent automotive specifications for robustness, which cover temperature, vibration, thermal shock, and voltage transients.
Sensing of motion-related attributes—speed, direction, position, and acceleration—has always been a challenge, requiring large, power-hungry, and complex sensors. This situation has changed radically with the availability of MEMS-based sensors such as accelerometers and gyroscopes, as well as high-performance interface circuitry that can compensate for sensor shortcomings. As a result, system designers can now address a wide range of new application insights and design opportunities.