Accelerometers are used in automobiles to detect rapid collisions and have the ability to sustain a wide range of shock necessary for air bag deployment systems. The choice of accelerometers is founded on a number of factors including size, resistance to noise and temperature variations, and the response mechanism. Airbags is a safety device mounted in vehicles and consists of a flexible envelope that guards occupants from striking internal objects. Various components are necessary for an airbag system. Accelerometers, gyroscopes, impact sensors among others are just a few used to detect an impact from a collision. The signal in the sensing system should reach a threshold for air bag deployment in a matter of few milliseconds. Therefore, rapid detection and sensitivity system should be built to ensure efficiency.
In order to reduce the severity of the accidents, accelerometers senses the sudden increase in the negative acceleration of the moving vehicle and initiate air bag deployment systems. The aim of this paper is to probe the different kinds of MEMS sensors used in this field and determine their responsiveness with changes in the input acceleration. The literature will consider publications from past research and analyze the testing in the 100g acceleration range. This is the real time acceleration at the accident instance. It will also probe the material in use for the manufacture of these sensors as well as their performance analysis. The conclusion derived from the research affirms that when the capacitance, resistance, or voltage of a sensor reaches a threshold value, it amplifies the electric signal and air bag deployment is initiated. The choice of the threshold parameters is dependent on the type of material used for the sensor. For instance, capacitive, piezoelectric, and piezoresistive are measured using different parameters.
Introduction
Advances in microelectronics and silicon micromachining have accorded designers the opportunity to fabricate compact structures conveniently providing a new front for sensors and equipments. The development of compact sensors has, however, been a constraint by a variety of the sensor sensitivity limits. For instance, the development of miniature sensors for most of the important applications often needs drastic improvements in the displacement transducer detection sensitivity. Recent developments based on novel signal detection have led to the production of ultra-high sensitivity microprocessors and micro instruments. Included among the applications designed are micro-accelerometer, micro-magnetometer and infrared detectors. Accelerometers design have high sensitivity and can detect movements in the order of 10-9 to 10-8 and therefore can be used in the Hz to kHz bands.
Fig. an airbag system
Miniature accelerometers have been a field of study for many authors. Some miniature accelerometers are built for large forces such as those in automobile applications for air bag deployment. Others are used in applications with extreme sensitivity for small signals and in this regime, has described capacitive accelerometers with micro-g resolution at 1 1KHz.
Accelerometers functions by proof masses whose motion is determined by accelerations. Thus, the motion of the proof mass is measured with respect to the accelerometer is measured and converted to the motion of the case. In this scenario, better resolution is required for a better approach.
Miniature sensors for automobile deployment fall in the group of micro-sensors. These micro-sensors are referred to as MEMS and are described as any system that performs electrical and mechanical functions and has component dimensions of micrometers. A single micrometer is equivalent to a tenth of the human hair. An example of a micro-sensor is the inertia sensors used for air bag deployment in automobiles and micro-cars. This paper has its focus directed towards miniature sensors for air bag deployment in automobiles.
MEMS is a pioneering technology for miniaturization. It is regarded as the leading technology for the 21st century and is quite inevitable in industrial products and system development trends. An example is the computer miniaturization mechanism which decreased the size of computer exponentially from an equivalent size of a room to a palm-top computer in a period of 50 years. With the surge in market demands, miniaturization is viewed as the only viable option for the production of intelligent, robust, multi-functional and low cost industrial products.
MEMS accelerometers for air bag deployment have made their way into our daily lives due to their characteristics. They are small, lighter, and more reliable and are produced for a fraction of the cost of the conventional accelerometers. MEM accelerometers are used to determine the acceleration experienced by a system and specifically the negative acceleration of the moving vehicle. A processor will determine the magnitude of the acceleration, analyze it and decide whether to deploy the airbags or not.
Several innovations in micromachining have been put in practice to design a commercially viable accelerometer suitable for low gravity acceleration applications. Examples of such applications are the straight beam comb accelerometer and 3D capacitive accelerometer for automotive applications.
MATERIALS
The material choice for an accelerometer is based on the threshold parameter desired. Piezoelectric accelerometer uses piezoceramics such as lead zirconate titanate or single crystals. The upper frequency, low packaged weight and temperature range, are unmatched. On the other hand, piezoresistive accelerometers are mostly used for high shock applications. Various accelerometers in the market are either piezoelectric or piezoresistive. An example is the Tri axial PZT.
Capacitive accelerometers are preferred because they are less prone to noise and temperatures in contrast with piezoelectric and piezoresistive accelerometers. They have excellent power utilization, scale factor stability and excellent bias. In addition, the possibility of using self testing and force balancing techniques are realized using electrostatic force. Most micromachining technologies require minimal or no testing and additional processes. The ability for capacitors to operate as both sensors and actuators gives it an upper hand in comparison with other materials. Their excellent sensitivity and transduction mechanism makes them intrinsically insensitive to temperature. Likewise, capacitive sensing is not dependent on the base material and purely relies on the variation of the capacitance under changing geometry of the capacitor.
Airbags plays an important role in the safety of the driver and the occupants in a vehicle. Airbags research started way back in the 1950s and has become mandatory for every car to act as supplementary safety devices apart from the seat belts. The efficiency of airbags is dependent on the timing of deployment. Usually an effective airbag should be deployed within a few milliseconds. Another fundamental mechanism is the ability of the system to detect the between a crash and a rough driving. Also, the system should clearly note the difference between the severe crash and a minor crash.
Timing is, therefore, essential for a lifesaving experience. It must be able to detect an impact and deploy in few seconds while also preventing deployment in the lack of collision. Sensor is the component that detects all this conditions and acts appropriately. The most common design of a crush sensor is the steel ball. Steel ball resides inside a smooth bore and is held by a stiff spring or a permanent magnet. Thus, the smooth ball is inhibited from moving when the car drives past rough terrain and over bumps. However, the motion of the ball is set when the cars acceleration is rapid in the negative direction. It suddenly moves forward and subsequently triggers on an electrical circuit that initiates the inflating of the airbags.
MANUFACTURING TECHNIQUE SELECTION
The three 3D capacitance accelerometer has sensing capability of ±100g at 10kHz bandwidth. It can also survive a shock of up to ±150 g that is seen in extreme conditions. It has considerable stability in extreme temperatures that provides maximum efficiency no matter the conditions.
The main structural components include spring supported mass linked with dampers to provide the sufficient damping effect. Springs and dampers are connected to a shell, and the mass produces a displacement when there is acceleration. In the case of an inertial accelerometer, a mechanical sensing element converts the acceleration into force which is shifted as a displacement and causes a variation in the capacitance. The capacitance is detected and converted into an electrical signal. The inertia of the proof mass prevents the motion of this element when there is an external force acting as a reference frame to which the proof mass is held by the spring. The surrounding gas ambient or the internal dissipation of the spring damps the proof mass. The basic diagram of an accelerometer is as shown.
A 3D capacitive accelerometer has the following features; high density central mass, fixed fingers, movable electrode, glass substrate, and sensing electrodes. The central mass has a maximum size for maximum displacement while the movable electrodes are located between the fixed fingers. Glass substrate is used for support. Sensing electrodes are better placed to sense the displacement of the central mass.
Fig.2 A simplified design of a 3D capacitive accelerometer with horizontal springs.
The central movable fingers are connected to the proof mass and other several fingers. Fixed electrode plates are connected to the fixed substrate made of glass. The sense capacitor is placed between the fixed fingers and the movable fingers in a parallel manner. The sensing organ is the bilateral comb operating in the x-y direction of the proof mass. When the acceleration input is zero, the central proof mass is in a state of balance and capacitors C1 and C2 have equal values. The voltage across them is also zero. Under acceleration, the movable fingers records a displacement through the action of the inertial force and the space between them and fixed plate is altered thus C1 ceases to be equal to C2. The moving and the fixed fingers results in a parallel plate capacitor with capacitance given by the expression C=€ A/d where € is the permittivity and area is the A of overlap between all fingers in the sensing region. When a DC voltage is applied to the capacitor, electrostatic force is achieved, and the amount of force is a function of capacitance and the voltage. Therefore as, the mass is displaced a distance x through acceleration a, the capacitance is changed.
The accelerometer represents a second order damper system and springs are used to attain the displacement of the proof mass. The spring constant is given using the expression
The mass is the total masses of the central proof mass and the fixed and movable fingers and is arrived at using the equation where density of silicon is given by 2230Kg/m3.
and Therefore capacitance can be derived from .
MEMS comb accelerometer function in the same way. The moving parts are comprises of the four folded beams a proof mass and movable fingers. Two anchors are located on the left, and the right side of the fixed mass and the central movable mass is connected to both anchors. In the absence of acceleration, the movable fingers are located in the middle of the right and left fixed fingers. In this state, the left and right capacitance pairs C1 and C2 are the same. Acceleration along the horizontal position parallel to the plan of the device imparts an inertial force –Ms.a along the opposite direction. The resultant beams deflect and the movable mass and movable fingers are displaced a distance x along the direction of the inertial force. Consequently, the left and the right capacitances are changed altering the differential capacitances C1 and C2. A measure of the change in the differential capacitances gives the value and the direction of the acceleration.
The comb accelerometer integrates both sensing and self-test capacitances. The self-test capacitors are used for self testing during in-field usage through electrostatic force. In the test mode where no acceleration is recorded, a driving voltage is supplied to the left and right driving fingers. The electrostatic force will pull the movable fingers towards the right and left and left directions. The measurement of displacement and the device response is critical information for the effectiveness of the device and is specifically important for airbags deployment before they are fixed in automobiles.
Device simulation is performed using more accurate analysis tools such as COSMOL multi physics simulation and ANSYS FEM to derive the device sensitivity and other device parameters. The results are crucial for engineers in design optimization.
CONCLUSION
Thos report has primarily investigated miniature accelerometers for automotive airbag deployment in terms of features, materials and design concepts. The materials utilized are in Nano dimensions because they are anticipated to exhibit higher tensile strength and Young’s modulus. This feature enables them to sustain real time accident scenarios. The change in acceleration applied in terms of the body load results in the change in capacitance. The change in capacitance is given in electrical signals, which are inputted, to the air bag deployment system and amplified. Capacitance reaches a threshold value determined by the application of a 100 g force and initiates air bag system using electronic circuitry.
Future inventions are geared towards designing and simulating accelerometers using carbon nanotubes. Carbon nanotubes are known to withstand considerable amounts of stress. The challenge is, however, attributed to the fact that the fabrication of the device is in nanometer dimension.
Works Cited
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Frauenfelder, Mark. Make: Technology on Your Time. O'Reilly Media, Inc., 2011.
Reed, Graham T. Silicon Photonics:The State of the Art. John Wiley & Sons, 2008.
