...products, machinery monitoring, air and ground transportation, environmental monitoring, HVAC/building automation, home appliances, medical devices and equipment, aerospace, security, and wireless monitoring systems. Sensors are key enabling technology that continues to find opportunities to allow for achieving new and promising, potentially high-volume applications; for example, automotive stability control, rollover detection, cabin air quality monitoring, occupancy detection, tire pressure monitoring; handheld devices (including cell phones, PDAs, tablet computers, GPS-equipped devices); and expanded sensor networks in industrial, commercial or homeland security applications.

However, the sensor's signal must be conditioned and processed to be rendered suitable for use in the product, equipment, or system in which the sensor resides. For example, an uncompensated, raw silicon micromachined pressure sensor is subject to large variations in its output offset and sensitivity, and such a sensor can exhibit offset and sensitivity variations as a function of temperature.

An initial driver of the need for smart sensors has been the proliferation of microprocessor technology, which propelled the need for sensors with an electrical output that could be readily interfaced to facilitate automated, unattended sensing and control. Such measurement and control schemes also require the sensor's analog signal to be amplified and converted to a digital signal that is sent to the system/process controller.

As sensors are finding opportunities to be used in greater numbers in key applications to undertake a multiple number of measurements, there is an increasing need for sensors that can be readily implemented in smart sensor networks that, in turn, allow for more comprehensive and more widely accessible sensor data at a reduced cost as a result of simplifying the wiring and interconnection of multiple sensors. Moreover, smart sensors with signal conditioning/processing capability that are distributed within the environment to be sensed can render the overall control system more efficient by assuming some of the processing and data storage tasks previously handled by the host controller.

APPROACHES TO SMART SENSING

Smart sensors-which can have such capabilities as signal amplification and conditioning (including compensation for temperature and non-linearity), analog-to-digital conversion, self-diagnostics, control logic, memory, and bus communication compatibility-can help streamline and accelerate the design, development, and integration efforts of the OEM who will incorporate the sensor in their product or system. A sensor equipped with intelligent electronics can result in decreased assembly and component costs for the OEM, a reduction in the overall size of the user's system, enhanced reliability (e.g., elimination of any loss in signal quality between the sensor and the system's electronics, the ability to have a longer distance between the sensor and host controller), compared to using a raw sensor and separate control circuitry. Moreover, a sensor with smart electronics and the appropriate bus communications compatibility can more readily be implemented in a sensor network, compared to a raw or "dumb" sensor.

Basic smart sensor functionality tends to include calibration (e.g., offset and full-scale), temperature compensation, and linearity correction. Basic calibration (e.g., offset and gain (span) compensation) of the sensor can be achieved by using trimmable potentiometers or laser trimmed resistors.

There are various approaches to configuring sensors and sensor systems with smart signal conditioning electronics. For example, smart electronics can be integrated with the sensing element, implemented as a separate sensor interface that is part of the sensor package, or configured on a PC board or ceramic substrate. The various elements of a smart sensing system (such as the sensor, sensor front-end ASIC that performs pre-amplification/processing and analog-to-digital conversion, digital circuits with filters and algorithms for processing the sensor's signal, and interfaces for sensor data transmission, control, or direct actuator drive) can also be arranged in various combinations based on the user's requirements.

While there is keen interest in using low-cost, fully integrated smart sensors in varied applications, such as automotive, it is important to provide a sensor signal conditioning and calibration solution that addresses the accuracy, space, and cost requirements of a particular application and provides design flexibility with respect to the system designer's or customer's requirements. In lower-volume applications, it is not often economically or technically feasible to integrate the intelligent signal conditioning electronics with the sensor element or in the sensor package, due to the higher cost of such sensors.

The fabrication of the sensor element may not be compatible with the fabrication methods for the smart electronics. Integrating the signal processing and interfacing circuitry into the sensor chip may, in certain cases, engender sensor packaging, performance, and reliability issues. Moreover, the materials used in the sensor element may not be readily compatible with the integrated smart electronic circuitry, driving up the costs involved in fabricating and manufacturing the sensor with integrated electronics and leading to decreased yields.

Surface micromachining technology, which involves depositing and etching...

NOTE: All illustrations and photos have been removed from this article.



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