The world of wearables is expanding, as accessories and clothing incorporate sensors and electronics connecting us to the IoT. Wearables, which can be considered data collectors worn on or close to the body, have requirements beyond just functioning well.
As the ads touting sporty watches point out, no one wants to strap a large box of electronics on their wrist, ankle or head or interrupt their day recharging their devices. Comfort and design are as important as an extended battery life, not only to look good, but to prevent interfering with the main purpose of the system. If the wearable is a glove being used to analyze a golf swing or a shoe insert used to capture a running gait, the sensor package can't add extra weight or bulk, which could potentially change the subject's natural motion. That means compact, lightweight devices need to operate on low-power, offer longer battery life and be small enough to not be noticeable.
Wearables targeting Sports, Health/Wellness and Virtual Reality systems depend on motion and position sensing systems to provide analysis, control, feedback and guidance. These systems typically include three-axis accelerometers to detect tilt and motion; some add three-axis magnetometers to detect north for compass orientation and position data to work in conjunction with GPS systems.
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Gyroscopes
Gyroscopes (gyros), which detect angular velocity and react to changes in direction, were late coming to the wearable market. Gyros can add precision orientation readings and respond quicker to rotational changes but are more complex in both mechanical design and computational rigor. Until recently, they had larger footprints and higher power requirements than the other sensors. Most gyro designs require a driving voltage even while at rest which results in a noticeable power drain on the system.
So, while useful for capturing rotational movement about a joint or axis system, designers were reluctant to include gyros. Some systems would simulate the function of a gyro using data from the accelerometers and magnetometers in what's known as a software gyro. Over time, companies have been successful at shrinking the size and lowering the power draw of gyros.
MEMS
As the Micro-Electro-Mechanical Systems (MEMS) processes that produced low-cost, high-volume accelerometers were adapted to provide gyroscopic functionality, MEMS gyros were gradually incorporated into multi-sensor systems. Initially only available for one-axis, most gyros now measure the three axes of x (roll), y (pitch) and z (yaw). Increasingly, accelerometers and gyros are packaged together as six-axis combination products and are likely to become the standard inertial sensor.
Providing multi-sensor packaging not only keeps physical footprints compact, it enables the various sensors to synchronize output to the same position in time. Incorporated into a wearable band or article of clothing, signals representing changes in linear and angular motion as well as gravity and magnetic field orientation can be provided for processing. Combining data from multiple sensors gives more accurate and precise measurements.
Gyros are not typically used alone; they accumulate positional errors which eventually compromise positional data. They are also subject to drift and can be temperature sensitive; you'll see almost all units include a temperature sensor, allowing systems to compensate gyro readings for temperature differences.
Accelerometer data can be used to reset the gyros, correcting errors in real time. It isn't only gyros that benefit; overall more precise information about motion and position can be obtained when gyro outputs are combined with accelerometer outputs in a process known as sensor fusion. Sensor fusion takes the data from each sensor and combines it to address weaknesses or shortcomings of any one sensor and provide a much more precise result.
MEMS gyro sensors, as the manufacturing technique implies, are electrical and mechanical in nature, fabricated at dimensions used in integrated circuit fabrication technology. Suspended or supported structures are sculpted within the substrate layers. Being mechanical, they are subject to the laws of kinetics and Newtonian physics.
While MEMS gyros operate differently from the spinning wheel gyro toys used to illustrate the gyroscope in motion, the same forces which cause the toy gyroscope's precession to seemingly defy gravity are at work in the MEMS gyros. And the same equations of angular velocity and laws for conservation of angular momentum apply. The 'mass' in the MEMS gyros are carved from portions of the silicon substrate and the forces acting on the units can be measured by sensing the deflection of the physical material. MEMS gyros utilize vibrating or oscillating masses rather than a spinning wheel, where the Coriolis Effect is responsible for the measurable deflections.
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May The Force Be With You
The Coriolis force, named for Gustave Gaspard de Coriolis (1792- 1843) a French physicist who studied dynamics of rotating bodies and who is credited with first describing it, is directed 90 degrees from the rotation of the angular velocity. The Coriolis force is in effect whenever the gyro experiences angular rotation. Most MEMS gyros are designed so when the Coriolis force deflects the vibrating material, capacitive circuits detect any movement resulting from angular motion. Transducing and conditioning electronics (usually provided by ASICs) output a voltage or digital signal proportional to the change detected.
MEMS gyros have traditionally required more power than other sensors because the mass used to detect angular momentum has to be kept in constant motion. The vibrating mass has to oscillate within constraints for the gyroscopic motion to be detected; that requires control circuity for reliable measurements. In addition to the driving circuitry, the sensing electronics to detect the motion also require power.
MEMS gyros are usually specified by power supply current, which is the typical consumption when operating (mA); bandwidth, the frequencies at which the outputs are valid (Hz); sensitivity, the amount of change in the output signal per degrees/second (dps); the drive voltage required; temperature sensitivity; drift; zero-rate offset, the output level when the gyro is not rotating and noise level. For digital sensors, there is also a sampling time between measurements.
Conclusions
Some MEMS manufacturers are rising to the wearable challenge by going beyond just providing low-power sensors. Units like Kionix's new family of accel-gyros not only include low power gyros and accelerometers but were developed specifically to lower overall system power requirements. The KXG03 incorporates auxiliary bus control to manage and buffer external sensors, provides power management and has synchronization features. By taking on the management of auxiliary sensors and providing wake/sleep modes, host microcontrollers can receive data more efficiently and remain in sleep mode longer.
Breakthrough six-Axis accel-gyro combo sensors like Kionix's KXG07 and KXG08 represent a significant technological leap forward in reducing the power consumption of motion sensor systems. Their patent-pending technology enables full high-speed operation of the onboard accelerometer, gyro and temperature sensor at power consumption levels as low as 0.2 mA. This puts motion control functionality at power levels demanded by always-on wearables. System designers will be able to incorporate gyro functionality without burdening their systems or their users, so expect more motion based features in the wearable world.
About the Author
Dr. John M. Chong, PhD is Vice President of Product and Business Development at Kionix. He is responsible for productizing and supporting Kionix's growing portfolio of sensors, and for developing new business opportunities. Previously he was Director of Product Engineering and was also responsible for developing the manufacturing test capabilities for Kionix's accelerometer products. Before joining Kionix in 2006, Dr. Chong worked for Calient Networks, a company focused on using Optical MEMS to support increased bandwidth through the automated management of fiber optic networks. He completed his B.S. and his Ph.D. in Electrical Engineering at Cornell University, where he worked on novel techniques for the design and manufacturing of Microfludic MEMS. John holds a number of patents, has spoken at numerous conferences about sensors and their role in the Internet of Things and currently serves on the Governing Council of the MEMS Industry Group.
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