The “basic concept,” of course, ignores the challenges of actually manufacturing the device, cross-coupling vibrations from one axis to the other, calibration, thermal issues, and so forth. And there’s no need to make the vibrating element look like a piano tuner’s tool. Imagine what you could do with a circular structure that vibrates like the mouth of a bell or a wine glass (Fig. 2). There are lots of patents on MEMS structures and many clever ways of adapting semiconductor process flows to manufacturing these devices.
BREADTH OF APPLICATIONS People tend to think of accelerometers in terms of automobile airbag deployment. But really, since movement and position changes are accompanied by acceleration, it’s common to use MEMS accelerometers to detect events that are less dramatic than cars running into each other.
Whenever a device is picked up and put down, the accompanying change of acceleration can be detected and used to generate an interrupt that powers certain device functions on and off, keeping some active while putting others into power-saving sleep states. Think of a handheld device that turns off its backlight until it’s picked up. (Of course, the movement sensor had better consume a lot less power than the backlight!)
More dramatically, a year or so ago, I wrote about portable radios for first responders that signal automatically when the person carrying them stops moving for a certain length of time— for instance, a lone firefighter disabled in a burning building (see “P25 Handhelds Incorporate High-Velocity Human Factors Design”). Or on a battlefield, you wouldn’t want the enemy picking up a radio from a dead or wounded soldier and using it to obtain tactical intelligence, so the radio can be programmed to require re-authentication before permitting user access.
Wisniowski described new defibrillators for use in public places. These devices are designed to help relatively inexperienced rescuers deliver CPR when electric heart stimulation fails. When that happens, Wisniowski said, “A less experienced rescuer might not compress the patient’s chest enough for effective CPR. Accelerometers embedded in the AED’s chest pads can be used to give the rescuer feedback on the proper amount of compression by measuring the distance the pad is moved.”
When Electronic Design publishes a story about energy harvesting, the most common application is vibration monitoring to assess the condition of mechanical systems like industrial pump motors, railcar wheel bearings, and highway bridges. Generally, one reads about the means of using the energy of the vibrations being monitored to power the microcontroller and the mesh-radio node, but I rarely consider the source of the raw data.
Yet that’s a key part of the system. Very small MEMS accelerometers with very wide bandwidth are required to capture an accurate enough profile of the normal vibration baseline and the aberrations. It would ultimately provide enough diagnostic information to allow intelligent analysis of potential time to failure.
So far, we’ve considered displacement and vibration. Shock impulse events are another source of accelerations. Probably the widest use of such sensing is in notebook disk drives. Interestingly, with disk drives, it isn’t the shock itself we want to detect. At that point, it would be too late to do something about it.
As Wisniowski’s “SOS” demonstration suggests, even before the shock itself, it’s possible to detect the changes in g-forces that indicate the notebook has been dropped, which are precursors to damaging shock associated with hitting the floor. During the milliseconds that elapse between the onset of zero-g conditions and the notebook’s impact, the system can order the disk-drive head to be parked.
The Apple iPhone and Nintendo Wii have accustomed us to the use of accelerometers and gyros for gesture recognition—taps, double-taps, and shakes that activate features and adjust operating modes. In addition to adding coolness to the product, providing gesture input has other benefits, Wisniowski observed.
Button-free designs have other advantages in lower system cost and higher ruggedness in products such as underwater cameras. Tap interfaces also are appropriate in wearable and implantable medical devices.
The Wii game control has also introduced a wide audience to tilt sensing. Beyond gaming, tilt sensing offers interesting potential in industrial applications. In these cases, operating a device one-handedly can leave the other hand free for hanging on to a vehicle transiting uneven ground or for control of a bucket or platform. Here, you would use a three-axis accelerometer to detect slow changes in inclination in the presence of gravity, interpreting that as a twist or tilt in one direction or another.
Looking at more prosaic applications than Wii-like control of industrial equipment, lots of jobs would involve tilt-sensing capability. Examples include adjusting industrial weigh scales and pressure for proper orientation.
At the other end of the complexity spectrum, the latest IMUs combine a multi-axis accelerometer, a multi-axis gyroscope, and a multiaxis magnetometer. ADI’s six-degrees-of-freedom IMU provides fine resolution in medical imaging and surgical instrumentation.
EARLY BREAKTHROUGHS In mid-2007, ADI broke new ground with the ADIS16355 IMU. It combines three axes of angular rate sensing and three axes of acceleration sensing, bringing 50 times greater accuracy than other off-the-shelf inertial sensors. It also came pre-calibrated, meaning that data out is accurate regardless of operating temperature. The product designer needn’t incorporate a table of correction values in system code.
At introduction, in 1000-unit lots, the full-range temperaturecalibrated version cost $359, and the room-temperature calibrated version cost $275. The device comes in a cube measuring just shy of an inch per side, with a little extra space required for mounting feet and a flex circuit with a connector on the end.
Sorry, the gyroscopic effect is not the same as the Corrolis effect. I suggest you look this up. (Try Wikipedia) The Rosby for such MEMS sytems would be huge (probably > million) and therefore the Corrolis effect completetly negligible. Gyroscopes typically operate due to torques created by the interaction of the suspension force and its angular momentum .
Jason Pressesky -July 09, 2009
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