Mems Pressure Sensor in Automotive Industry

MEMS: MEMS is a process technology used to create tiny integrated devices or systems that combine mechanical and electrical components. The Micro Electro Mechanical Systems technology (MEMS) allows to integrate on the same silicon substrate both electronic circuits and optical mechanical devices, adopting semiconductor fabrication technologies similar to those used to make integrated circuits. A MEMS device can be thought of a “brain” (an integrated IC) and a set of “arms” and “eyes” through which it monitors and controls the environment. Micro sensors (the “eyes”) gather information from the environment measuring position, movement, temperature, pressure, magnetic field, optical and chemical phenomena, that information can then be processed by the “brain” which acts on the environment by means of the “arms”, that are basically micro actuators.

 

Transducer: A transducer is a device that transforms one form of signal or energy into another form. The term Transducer can therefore be used to include both sensors and actuators and is the most generic and widely used term in MEMS.

 

Sensor: A sensor is a device that measures information from a surrounding environment and provides an electrical output signal in response to the parameter it measured. Over the years, this information (or phenomenon) has been categorized in terms of the type of energy domains but MEMS devices generally overlap several domains or do not even belong in any one category. These energy domains include:

Mechanical - Force, Pressure, Velocity, Acceleration, Position

Thermal - Temperature, Entropy, Heat, Heat flow

Chemical - Concentration, Composition, Reaction rate

Radiant - Electromagnetic wave intensity, Phase, Wavelength, Polarization, Reflectance, Refractive index, Transmittance

Magnetic - field intensity, flux density, magnetic moment, permeability

Electrical - voltage, current, charge, resistance, capacitance, polarization

 

Actuator: An actuator is a device that converts an electrical signal into an action. It can create a force to manipulate itself, other mechanical devices, or the surrounding environment to perform some useful function.

 

What are the materials used to fabricate MEMS? 

 

There are many materials that used in MEMS manufacturing, Silicon‐Compatible Material, such as: Silicon, Silicon Oxide and Nitride, Thin Metal Films, Polymers Other Materials and Substrates: Glass and Fused Quartz Substrates, Silicon Carbide and Diamond Gallium Arsenide and other Group III‐V Compound. Semiconductors, Polymers, Shape‐Memory Alloys. But the most common material used in MEMS is Silicon since it has the features like  the second most abundant element in the Earth's crust coming after oxygen, making up 25.7% of it by  weight. In its crystalline form, it has a dark gray color and a metallic luster and silicon possesses excellent materials properties, which make it an attractive choice for many high‐performance mechanical applications.

 

 

 

How MEMS are fabricated?

 

Most of the MEMS fabrication methods are adopted from standard IC technology. The most common techniques are bulk micromachining and surface micro machining:

1) Bulk Micromachining

In bulk micromachining, a 3D micro‐mechanical structure is built directly on the silicon wafer by selectively removing portions of the substrate. The exposed area on the substrate is subjected to further chemical etching.

 

2) Anisotropic etching

Utilize the crystallographic structure of the silicon lattice.

 

3) Isotropic etching

In this the silicon substrate is attack in all directions with equal rate.

 

4) Surface Micromachining

Surface micromachining is based on the deposition of layers on the substrate, and on the subsequent definition of the micro‐mechanical structure by means of photolithographic techniques. Surface micromachining builds structures on the surface of the silicon by depositing thin films of ‘sacrificial layers’ and ‘structural layers’ and by removing eventually the sacrificial layers to release the mechanical structures. The MPFI (multi point fuel injection) system is used, assuring proper air fuel ratio to the engine by electrically injecting fuel in accordance with various driving conditions. MPFI system injects fuel into individual cylinders, based on commands from the ‘on board engine management system computer’ – popularly known as the Engine Control Unit/ECU. These techniques result not only in better ‘power balance’ amongst the cylinders but also in higher output from each one of them, along with faster throttle response. The electronic fuel injection system supplies the combustion chambers with air/fuel mixture of optimized ratio under widely varying driving conditions.

 

Early airbags required the installation of several bulky accelerometers made of discrete components mounted in the front of the car, with separate electronics near the airbag (at a cost of over $50). Today, because of MEMS, the accelerometer and electronics are integrated on a single chip at a cost of under $10. The small size (about the dimensions of a sugar cube) provides a quicker response to rapid deceleration. And because of the very low cost, size (about the dimensions of a sugar cube) provides a quicker response to rapid deceleration. And because of the very low cost, manufacturers are adding side impact airbags as well. The sensitivity of MEMS devices is also leading to improvements where size and weight of passengers will be calculated so the airbag response will be appropriate for each passenger.

 

Sections with major advancement

 

Fuel Injector Pressure Sensor: The MPFI (multi point fuel injection) system is used, assuring proper air fuel ratio to the engine by electrically injecting fuel in accordance with various driving conditions. MPFI system injects fuel into individual cylinders, based on commands from the ‘on board engine management system computer’ – popularly known as the Engine Control Unit/ECU. These techniques result not only in better ‘power balance’ amongst the cylinders but also in higher output from each one of them, along with faster throttle response. The electronic fuel injection system supplies the combustion chambers with air/fuel mixture of optimized ratio under widely varying driving conditions.

 

Tier Pressure Sensor: The direct tire-pressure monitoring method places a sensor module at each wheel. The sensors measure the pressure in each tire and transmit the data wirelessly to a central receiver in the vehicle, which analyzes the information and displays it to the driver. The information varies from simple warning lights when pressure gets too low to readouts of pressure measurements. Some systems may also include pressure information about the spare tire. The main advantage of the inferred method is that it is relatively inexpensive, since it requires no extra hardware. It has no battery life concerns or remote sensors that can be damaged by tire mounting or road hazards. On the other hand, it won't detect significant under inflation when all four tires are equally soft or when two tires on the same side of the vehicle are under inflated, according to a NHTSA test report. Typically, a tire-pressure sensing module is located inside the rim of the wheel. The MEMS package must stand up to vibration, heat, and corrosive fluids. Tire-pressure sensor contains several components. A MEMS pressure sensor is the key element, but the package may also include a temperature sensor, voltage sensor, accelerometer, microcontroller, radiofrequency circuit, antenna, and battery.

 

Airbag system: In this application, an accelerometer continuously measures the acceleration of the car. When this parameter goes beyond a predetermined threshold, a microcontroller computes the integral of the acceleration (i.e., the area under the curve) to determine if a large net change in velocity has occurred. If it has, the air bag is fired. The decision to fire front air bags has to be made in dozens of milliseconds; the decision to fire side air bags must be made even more quickly because the car door is closer to the occupant than the steering wheel or dashboard.

 

Rollover detection system: Few vehicles have rollover detection systems, but automakers are rapidly adopting this feature. This is particularly true for vans, pickup trucks, and sport utility vehicles, which are more likely to roll over because of their higher center of gravity. These systems read the roll angle and roll rate of the vehicle to determine if it is tipping over. If it is, the system fires the side curtain air bags to protect the occupants. Rollover detection systems use a gyroscope to read the roll rate. The roll rate is integrated to determine the roll angle of the vehicle, but roll rate data alone are not enough to predict if a vehicle is (or will be) rolling over. An accelerometer reading vertical acceleration (Z axis) is also required because large roll angles can be encountered in banked curves with no possibility of rollover.

 

Vehicle dynamic control system: Vehicle dynamic control (VDC) systems help the driver regain control of the automobile when it starts to skid. If the VDC works properly, the driver may not even be aware that the system intervened. A VDC system consists of a gyroscope, a low-g accelerometer, and wheel-speed sensors at each wheel (the wheel-speed sensors may also be used by the ABS). Wheel speed is measured, and the predicted yaw (or turn) rate of the car is compared with that measured by the gyroscope. A low-g accelerometer is also used to determine if the car is sliding laterally. If the measured yaw rate differs from the computed yaw rate, or if lateral sliding is detected, single-wheel braking or torque reduction can be used to make the car get back in line.

 

Throttle Position Sensor: The TPS is a potentiometer attached to the throttle shaft. A voltage signal is supplied to the sensor, and a variable voltage is returned. The voltage increases as the throttle is opened. This signal and the MAP output determines how much air goes into the engine) so the computer can respond quickly to changes, increasing or decreasing the fuel rate as necessary.

 

 

Conclusion:

The potential exists for MEMS to establish a second technological revolution of miniaturization that may create an industry that exceeds the IC industry in both size and impact on society. Micromachining and MEMS technologies are powerful tools for enabling the miniaturization of sensors, actuators and systems. In particular, batch fabrication techniques promise to reduce the cost of MEMS, particularly those produced in high volumes. Reductions in cost and increases in performance of micro sensors, micro actuators and micro systems will enable an unprecedented level of quantification and control of our physical world. Although the development of commercially successful micro sensors is generally far ahead of the development of micro actuators and micro systems, there is an increasing demand for sophisticated and robust micro actuators and micro systems. The miniaturization of a complete micro system represents one of the greatest challenges to the field of MEMS. Reducing the cost and size of high-performance sensors and actuators can improve the cost performance of macroscopic systems, but the miniaturization of entire high-performance systems can result in radically new possibilities and benefits to society. The problem of controlling stable and unstable time delayed processes has been tackled by proposing a new parallel cascade control structure. NEMS stands for Nano-Electro-Mechanical-Systems is the technology that is similar to MEMS, however it involves fabrication on the nanometer scale rather than the micrometer scale.