An accelerometer is a device that can be used to measure acceleration in one, two, or three orthogonal axes. It is typically used to measure in-plane velocity and position, inclination, tilt, or orientation in two or three dimensions with respect to the acceleration of gravity, and to measure vibration and shock. It works by using a proof mass to detect motion and gravity. This proof mass can be a mechanical structure or heated gas.
The most popular today are of the micro-machined MEMS variety and largely consist of two basic types: those that work on the capacitive approach and ones that work on the heated-gas principle. These two types constitute the vast majority of MEMS accelerometers on the market.
This accelerometer measures less than 2.0 square mm in size. Here you can see it in reference to a common sewing needle and thread.
There are some other micro-machined types, as well, but these are used in specialized applications and are not generally suited for low-cost mass-market applications. These include piezo-resistive, piezo-electric, and strain-gauge accelerometers.
Capacitance-type accelerometers account for a large number of accelerometer applications, but gas-heated types provided by MEMSIC are beginning to make in-roads in consumer electronics, toys, automotive, industrial, home appliances, portable hand tools, and military and aerospace markets. Depending on the application, there are many advantages to using thermal MEMS acceleration versus capacitance types.
The capacitance approach is more suitable for applications requiring higher bandwidth. However, all capacitive sensors intrinsically have resonance from its mechanical structure resulting in unwanted ringing effect, which disturbs the normal sensing mechanism. Such effects have been a major issue in many sensitive applications such as automotive electronic stability control (ESC). For this reason, the heated-air sensor which has its natural roll off bandwidth from the sensor itself is better suited for ESC and rollover automotive applications, as well as for many consumer electronics applications which require inclination and human motion information.
Heated gas acceleometers are more expensive than capacitive ones. Hence theie lack of populatiry in consumer products. You also don't gat the "tap detect" function because of the lower bandwidth.
Rob, I believe the MEMS accelerometer application had something to do with gauging patients' motion, such as during walking, but I don't recall the exact details.
Auto Crash and Rollover Sensing requires higher bandwidth due to the higher da/dt jerk content of crash signals. ESC requires < 1.0 g because lower amplitude and slower da/dt and less rapidly chaning forces are involved.
The inherent reliability of these thermal accelerometer due to the absence of moving parts makes them ideal for some designs. Not sure how much these devices will be limited as specialty components or if additional research will enable them to penetrate additional applications. Seems to be part of the challenge going forward.
I didn't realize accelerometer were getting deployed into medical applications. Make sense. Do you recall what the applications were? Heart monitoring, perhaps.
Yes, I remember that the Japanese flying machine was a great story. The size of the accelerometer makes for a wide range of applications. Years ago, Deisgn News flew me out to Research Triangle to do a video interview with Motorola when they included an accelerometer in a phone. It was used for games and to shake the song list to the next song.
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