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15
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Electronic Instrumentation and Measurements
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Using the ADXL202 Accelerometer as a Multifunction Sensor (Tilt, Vibration and Shock) in Car Alarms
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By using an intelligent algorithm, the ADXL202 ( 2 g dual axis accelerometer) can serve as a low cost, multifunction sensor for vehicle security systems, capable of acting simultaneously as a shock/vibration detector as well as a tilt sensor (to detect towing or jacking up of the car) The accelerometer s output is passed through two parallel lters A bandpass lter to extract shock/vibration information, and a low pass lter to extract tilt information This application note describes the basics of such an implementation Introduction The ADXL202 is a low cost, low power, complete dual axis accelerometer with a measurement range of 2 g The ADXL202 outputs analog and digital signals proportional to acceleration in each of the sensitive axes (see Figure 1563)
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Figure 1563 ADXL202 block diagram
Currently automotive security systems use shock/vibration sensors to detect collision or forced intrusion into the car Typically, these sensors are based on magneto-inductive sensing Sensors of this type generally have adequate sensitivity, but fall short in other areas Often a fair amount of signal conditioning and trimming is required between the shock sensor and microcontroller due to variations in magnetic material and Hall effect sensor sensitivity and their frequency response is fairly unpredictable due to inconsistency in mounting In addition such sensors have no response to gravity-induced acceleration, so they are incapable of sensing inclination (a static acceleration) Tilt sensing is the most direct way of detecting if a vehicle is being jacked up, about to be towed, or being loaded onto a atbed truck These are some of the most common methods of car theft today
Part II
Electronics
The ADXL202 is a true accelerometer, easily capable of shock/vibration sensing with virtually no external signal conditioning circuitry Since the ADXL202 is also sensitive to static (gravitational) acceleration, tilt sensing is also possible Tilt sensing requires a very low noise oor which usually necessitates restricting the bandwidth of the accelerometer, while shock/vibration sensing requires wide bandwidth These con icting requirements may be met using clever design techniques Principle of Operation The ADXL202 is set up to acquire acceleration from 0 to 200 Hz (the maximum frequency of interest) Figure 1564 shows a block diagram of the system The accelerometer s output is fed into two lters; a low pass lter with a corner frequency at 125 Hz used to lower the noise oor suf ciently for accurate tilt sensing, and a band pass lter to minimize the noise in the shock/vibration pass band of interest The low pass ltered (tilt) output then goes to a differentiator (described in the Tilt Sensing section) where the determination is made as to whether the accelerometer actually sensed tilt or some other event such as noise or temperature drift Then an auto-zero block performs further signal processing to reject temperature drift The band pass ltered output goes to an integrator (described in the Shock Sensing section) that measures vibrational energy over a small period of time (40 ms) A decision as to whether or not to set off the alarm may then be made by the microcontroller Most of these tasks are most easily implemented in the digital domain and require very little computational power
200 Hz low pass filtered digital output
125 Hz low pass filter (samples averaging) 10 Hz high pass analog filter
Differentiator
Auto-zero recalibration
Tilt alarm Low cost microcontroller
Analog output
Analog to digital converter
Integrator
Shock alarm
Figure 1564 Shock and tilt sensing using the ADXL202
Since the two measurements (shock/vibration and tilt) are basically exclusive and only share a common sensor, their respective signal processing tasks will be described separately Tilt Sensing Fundamentals The alarm system must detect a change in tilt slow enough to be the result of the vehicle being towed or jacked up, but must be immune to temperature changes and movement due to passing vehicles or wind Note that the ADXL202 is most sensitive to tilt when its sensitive axes are perpendicular
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