Open AccessExisting shock sensors for car protection using the sensitive elements (SE) of piezoelectric, microphone and electromagnetic types and the analogue circuitry of signal processing, have a number of essential shortcomings:
- piezoelectric sensitive elements have no characteristics repeatability that complicates their use in mass production;
- microphone sensors are structurally complicated and demand difficult information signal processing;
- sensitive elements of electromagnetic sensors demand individual control (a specified clearance to be set between a magnet and the coil).
Use of analogue elements (resistors, capacitors) in the amplifier and filter circuits reduces temporary and temperature stability of characteristics. An adjustment of the sensor operating zones via variable resistors on a printed circuit is extremely inconvenient and doesn't allow to change quickly the sensor sensitivity depending on an external situation (for example, to increase quickly an operating zone of the sensor with an alarm system of a key fob when securing a car in the country or in the woods, or to reduce it in the street with heavy traffic streams).
An analogue circuit–based sensor design disables its automatic adaptation to such external impacts as a rain, a stream passing by cars, etc.
The article considers how to solve some of above problems while designing the two-zone digital shock sensors with a SE of electromagnetic type. It shows the SE design developed by the authors as a module containing the coil and a magnet, secured on the coil axis in a silicone extension. The circuitry solution and algorithms of signals processing allowed authors to realize a remote control of the prevention and alarm zones (with 16 gradation of sensitivity). The algorithm of self-adaptation to the repeating external impacts is proposed. The developed method to form the basic levels of digital comparators for each gradation of sensitivity enables the sensor to have the straight-line adjusting characteristic.
The developed shock sensor is implemented as stock-produced items included in the car alarm systems of the trademark “Excellent” (made by JSC Magic Ring Ltd.). Operating experience (about 40 thousand pieces realized now) allows a proper adjustment of sensors to be possible on cars with any body type. A sufficient level of the sensor sensitivity proves it. The reliability of sensors is confirmed by failures statistics in use (0,08%). Failures are, usually, caused by the SE coil wire break in a soldering point of the coil leads to the printed circuit. However a number of shortcomings are also revealed.
Experimental data processing of a test batch of sensors has shown that existing SE of electromagnetic type have the frequency of free vibrations f 805Hz and a decay constant of transient under shock 0,2s. At these parameters a SE pass-band makes only f 1,5Hz, and the sensor responses only to the shock front, rather than to the secondary vibrations of a body. It is an essential drawback of the narrow-band sensors.
The experiments conducted on test batch of sensors (10 samples) have shown essentially different sensitivities of sensors under shock in the direction of the sensitivity axis and perpendicular to it. This difference reaches 4-6 gradations of sensitivity. It means that when mounting a sensor on the particular car the orientation of sensitivity axis of the sensor will be of crucial importance that is also a shortcoming.
It is shown that overcoming the specified drawbacks is hardly because of the improvement of sensitive elements suspension of electromagnetic sensors. Three-axial accelerometers implemented using MEMS technologies seem to be the most perspective application here.