Next-generation automotive pressure sensors using advanced MEMS technology
13 March 2013
MEMS technology will constitute a growing proportion of the automotive pressure sensor market over the coming years. The performance benchmarks and the higher level of integration it offers will allow it to see greater and greater uptake.
Pressure sensors are widely used throughout the industry, with all manner of electronics design benefiting from their use. In the last couple of decades, automotive has been one of the main sectors to drive demand for these devices. The development of new systems in conventional internal combustion engine vehicles, as well as hybrid electric vehicles, is demanding complex control systems. Tasks such as air intake and exhaust gas management, regenerative braking, fuel delivery, ESP and anti-lock braking systems all rely on pressure sensing in their control electronics. The automotive industry is predicted by all observers to continue to contribute heavily to the pressure sensor market growth.
Many different sensing technologies have been used so far for measuring pressure in an automotive environment. Examples include strain gauges on metal substrates, plus capacitive and piezo-resistive sensors using ceramic materials. As new application scenarios begin to emerge, the shortfall in performance of these devices is starting to become evident however. Usually sensors made of ceramic or metal materials are used to sense pressures of several Bars, but for pressure levels below the 5 Bar mark their sensing elements become relatively big and expensive. As a result more advanced sensor technologies are being introduced.
The need for reduced fuel consumption, as well as tighter exhaust gas emission legislation, will mean that in the future an increasing number of automotive applications will require the rapid and accurate acquisition of detailed pressure data. There are several factors leading to greater use of pressure sensors to enable more fuel efficient automobile design. Firstly, there are the improvements that these allow to engine air intake management. By connecting a pressure sensor to the air intake manifold it is possible to measure the air flow rate. Advancements in engine design place higher demands on existing pressure sensor devices. For instance more exhaust gas re-circulated in the engine intake makes for a harsher operating environment for the sensor specified. Then there is the engine fuel delivery system where the need for a more accurate monitoring of the fuel line pressure is pushing the operational limits of conventional sensors like for instance the ones measuring very high pressures in common rails of direct injection engines.
In addition to these, increasingly stringent European legislation on the emission of pollutant gases like NOx(nitrogen oxides) as well as particulate matters need to be taken into account. To get rid of NOx and particles, special systems are created that will again require the use of some form of sensor technology – often being pressure based. The main types of engines that generate particulates are diesel based. The most effective way to get rid of particulates is to work with the so called diesel particle filter. The filter is connected in the exhaust pipe and traps the particulates. Pressure sensors are utilised to monitor the pressure drop over the filter so that the engine control unit (ECU)can change the combustion conditions in order to automatically regenerate the filter. Reductions in NOx are being enabled through deployment of exhaust gas recirculation (EGR) systems - which mix exhaust gasses with fresh air at the engine intake. Elsewhere, selective catalytic reduction (SCR) system sallow urea to be mixed with the exhaust gas to initiate a chemical reaction which converts NOx into non harmful chemicals. Pressure sensors take care of controlling the amount of urea mixed and can also be used to monitor the EGR flow. Finally the development of hybrid vehicles using start-stop systems is decreasing or even removing completely the vacuum level at the intake of the engine during the engine stop condition. The engine vacuum is used in the brake booster to assist the driver with braking. It becomes important to have an accurate monitoring of the vacuum level in the brake booster in order to actuate an auxiliary pump when the natural engine vacuum is no longer enough for the booster.
Migration to MEMS technology
As new applications areas open up from pressure sensors within automobile design, the drawbacks of conventional sensor technology are more and more apparent. Their bulkiness and relatively high unit cost puts limitations on where they can be specified. For a large proportion of applications they are simply not suitable. As a few new automotive pressure sensor applications work below 5 bar they are much better served by miniaturised pressure sensors. Micro-electro-mechanical system (MEMS) based devices are seeing greater uptake here as a result.
Melexis has developed an innovative proprietary MEMS technology which consists of miniaturised membranes only a few µm thick and a few hundred µm long that can be reliably etched onto a silicon wafer as a post CMOS processing step. During the semiconductor manufacturing process, in addition to the active circuitry, piezo resistors are implanted at the point of maximum sensitivity on the membrane to form a Wheatstone bridge. When this micro-machined membrane senses pressure, it deflects. The piezo resistors convert the stress on the membrane into an electrical signal that is then processed by signal conditioning circuitry fabricated on the same IC.
By working with MEMS technology that is fully compatible with CMOS processes, it has been possible to create a monolithic sensor, so that the sensing element can be placed very close to the signal conditioning electronics. Conventional sensor technologies based on ceramic or metal substrate cannot process the signal conditioning electronics on the same substrate, so there will always be a few mm between the sensing element and the signal conditioning. This makes it more difficult to design a sensor that is robust enough to deal with strong levels of electro-magnetic interference (EMI) and impinges on the overall signal integrity. The development of a MEMS process that is fully compatible with EEPROM memory also means that support for different configurations is possible, such as independent setting of diagnostic functions for over-voltage or under-voltage supply conditions and over-pressure or under-pressure conditions, or fully programmable clamping levels as well as selectable digital filter settings to either further reduce the output noise or increase the sensor response time.
Through its piezo-resistive MEMS based technology, Melexis has been able to introduce a robust pressure sensing product. The MLX90809 has the capacity to address a broad spectrum of pressure sensing tasks within demanding automotive environments requiring a high performance, rugged construction and reliable operation. It is optimised for measurement within the 1 Bar range, achieving a very high accuracy over an extended automotive temperature range. High degrees of performance have been achieved by integrating the MEMS sensing element with a low noise analogue front end and 16-bit sigma delta analogue-to-digital converter (ADC). This analogue chain performs the amplification and offset compensation of the sensing element. The MLX90809 also includes a 16-bit microcontroller to take care of temperature compensation of the sensing element as well as implementing the diagnostic functions needed in safety critical applications. 32 Bytes of fully programmable EEPROM memory can be used to store sensor compensation and configuration settings. It can deliver pressure data via an analogue output voltage ratiometric to the supply voltage or using the SENT digital protocol.
Fully AEC Q100 qualified, the MLX90809is assembled in a compact pre-molded plastic housing. A special cavity molding technology has been developed to create a cost-effective robust IC package. A gel coating covers the front side of the device to safeguard it from harm during manufacturing processes, as well as bringing extra protection to the atmospheric venting connection in gauge mode or extra level of front side protection in a fully differential mode. Extensive media compatibility tests have been performed on the IC to guarantee that it can handle contact with typical automotive contaminants (fuel, oil, etc.).
One of the most important applications for the MLX90809 is within advanced braking systems now being fitted into cars. As already indicated in vehicle start-stop conditions, when the engine is shut off, it is important to be able to monitor the vacuum level within the vacuum brake booster as the natural vacuum source of the engine intake manifold is now in many cases being replaced by an auxiliary pump. This pump has to be actuated to generate the necessary vacuum. The brake force applied by the booster is directly proportional to the vacuum level in the booster relative to the atmospheric pressure. By monitoring the vacuum level in the booster with a relative pressure sensor the ECU knows when to actuate or shut off the auxiliary pump. As the MLX90809 is a highly sensitive and accurate MEMS-based relative pressure sensor optimised for the 1 Bar vacuum range, it is highly suited to this kind of application. Furthermore, its strong signal integrity serves well applications where the reliability of the information is of vital importance to prevent safety critical situations.
In conclusion, it is clear that MEMS technology will constitute a growing proportion of the automotive pressure sensor market over the coming years. The performance benchmarks that it can set and the higher level of integration it offers compared to conventional pressure sensing mechanisms will allow it to see greater and greater uptake as new, highly demanding applications are incorporated into all types of vehicles, both high end and lower end.
Figure 1: The MelexisMLX90809
Figure 2: MLX90809 functional block diagram
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