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 A Novel, Media-Compatible Pressure Sensor for A small-outline pressure sensor has been developed for use in various automotive applications including vehicle dynamic control, gasoline direct fuel injection, Michael L. Dunbar and Today's advanced vehicle control systems require an increasing number of intelligent sensor inputs in order to meet system-level performance and reliability demands. These sensors must also achieve a higher degree of accuracy, and incorporate self-diagnostic and protection functions.
A small-outline pressure sensor (SOPS) has been developed to meet the requirements of a variety of automotive applications. This advanced device (see Photo 1) combines a stainless steel sensing element with ASICs that perform signal conditioning, fault detection, and protection functions, all contained in a compact housing with an integral connector. All sensor signal conditioning, including amplification, error correction, temperature calibration, and filtering are performed by a single CMOS ASIC [1]. Diagnostic, overpressure clamping, and protection functions are performed by a second ASIC. The use of integrated electronics eliminates the need for ceramic hybrids, thick or thin films, and laser trimming. The sensor package and manufacturing process were designed to facilitate inline calibration and automated assembly. An automated sensor manufacturing line has been installed, implementing a zero-defect philosophy through integrated product tracking and line control software. The sensor can be tailored to the requirements of a particular application by changing its calibration values through software modifications. And various sensing elements can be used in order to support a range of operating pressures. Stainless Steel Sensing Element The sensing element consists of a stainless steel diaphragm with polysilicon resistors configured as a Wheatstone bridge. The polysilicon resistors are vapor deposited onto a SiO2 insulating layer on the surface of the steel. Photolithographic processes are used to form the resistors in the optimal locations on the diaphragm. Gold metalization is then deposited to make the electrical connections to the resistors. Finally, a protective stratum of silicon nitride is added as a passivation layer. Figure 1 illustrates the manufacturing flow that produces the sensing element. The resistive bridge configuration is driven by the input supply voltage, Vs. As pressure, Pin, is exerted on the steel diaphragm by the pressure medium, the resulting strain causes a change in the electrical resistance of the polysilicon elements such that the bridge becomes unbalanced. As a result, the sensing element produces a differential output voltage (Vd) that is proportional to both the input pressure and the supply voltage: Vd = K1 • Pin • Vs (1)
where: K = generic constant that varies from one unit to another The completed sensing element is e-beam welded to the pressure port per system specifications. This welding process creates a strong bond that can withstand the extreme pressure and temperature cycles characteristic of automotive applications. Sensor qualification and durability tests have demonstrated that this process is suitable to meet demanding system requirements over the vehicle's lifetime. This construction technique produces a highly stable element that withstands harsh media, and the elements can be modified to satisfy specific application requirements. To handle various pressure ranges, for example, elements with differing diaphragm thicknesses can be added. Similarly, the machined HEX and thread can be produced to meet specific customer or application needs. Signal Conditioning Circuitry A key function of the signal conditioning electronics is to correct errors inherent in the sensing element. These errors vary from unit to unit, necessitating a manufacturing process in which each sensor is individually measured over the application's operating conditions. Based on test data, the electronics for each sensor are adjusted by means of electronic trimming. Digitally programming the ASICs minimizes sensor, interface, and electronics errors. Digital calibration coefficients, stored in nonvolatile memory on each ASIC, make external components or laser trimming unnecessary. (The conventional approach of laser trimming thick film passive components in a hybrid circuit based on sensor characterization data obtained earlier in the manufacturing process has many drawbacks, including limited product accuracy, large physical product size, high component count, long-term performance drifts, and high manufacturing costs.) Resistor compensation circuits typically create interdependencies in the compensation resistors; the trimming algorithm must therefore precisely take into account the effect one resistor adjustment will have on the entire circuit. With electronic trimming, each parameter can be separately adjusted and the process can be repeated multiple times if desired. Conversely, thick film laser trimming allows only one attempt due to its irreversible nature. The signal conditioning ASIC used in the pressure sensor is based on the block diagram shown in Figure 2. The sensing element output is processed through a number of gain stages, with digital correction to the analog signal path accomplished by means of coefficients programmed into the chip during the manufacturing process.
Diagnostic and Protection Circuitry A second IC serves as a protection ASIC for I/O and diagnostic functions by guaranteeing that the output voltage will be in a predefined diagnostic region to warn the electronic control unit of a problem. The ASIC clamps the output voltage at 92% of Vs to distinguish overpressure conditions from system faults. The following lists some of the IC's functions as well as some of the fault conditions included in the diagnostic feature set:
Predefined diagnostic regions:
Mechanical Design The pressure sensor assembly was designed for compatibility with an automated, inline manufacturing flow. The unique design concept permits sensor calibration and assembly operations to be automated with a relatively low capital investment. Automated manufacture improves the sensor's consistency, quality, and reliability while reducing production cost. By mechanizing the mechanical assembly processes, tighter dimensional controls can be achieved on mechanical tolerances, improving the sensor's performance in high-vibration environments by preventing undesirable mechanical motion. This approach also facilitates product traceability and inventory control by placing all operations under direct computer control.
As shown in Figure 3, the electrical signals from the sensing element are connected to the electronics PCB via wire bonds to an insert-molded plastic component that is in turn soldered to the PC board by an automated soldering apparatus. An insert-molded contact bridge, soldered to the PC board, contains the pins that provide electrical connections to the mating connector (see Photo 2,). This assembly is subjected to appropriate pressure and temperature points in an automated, inline calibration and test system. The test system computer uses these characterization data to compute the digital codes necessary to adjust the signal conditioning ASIC parameters to match its mated sensing element. The test system then programs these codes into nonvolatile memory on the ASIC before executing a final electrical test.
The sensor construction is completed by crimping a molded plastic housing onto the assembly prior to laser marking and packing. The sensor housing, along with the contact bridge pins, is designed to meet the requirements for the customer's desired electrical connector. Product Performance The SOPS can be custom-manufactured for a variety of automotive pressure sensing applications. Electronic trimming and flexibility in the production process allow the device to be customized to suit various applications. Alternative pressure ranges, accuracy profiles, mechanical configurations, and so forth can be satisfied by modifying calibration software or the geometry of the sensing element. Table 1 lists some key performance specifications for a typical SOPS. Durability Testing Automotive pressure sensors must withstand very harsh environments without a failure or degradation of performance. To verify that a design can meet these requirements, a durability test plan is developed and a series of tests is performed. Table 2 is a partial list of the reliability and validation tests that have been performed for the SOPS. In all cases, no failures were observed. Conclusion
A small-outline pressure sensor has been developed for use in a variety of automotive pressure sensing applications. The sensor's small size allows its use in systems with tight real estate. The stainless steel diaphragm's welded construction and vapor-deposited resistors make it more reliable, able to withstand harsh environmental conditions, and capable of operating at very high pressures without leaking. Advanced diagnostic functions in the electronics alert the electronic control unit of possible failures within the sensor or the wiring harness. All signal processing functions are integrated into a single CMOS IC to reduce cost, size, and complexity. The sensors are produced on an automated, zero-defect manufacturing line that incorporates an advanced inline calibration and test system that improves sensor accuracy and quality. Reference 1. M.D. Naik and Michael L. Dunbar. May 1997. "CMOS-Based Smart Sensors," Sensors, Vol. 14, No. 5:22-26. Acknowledgments The authors wish to thank the staff at Integrated Sensor Solutions, Inc., San Jose, California, and ISS-Nagano GmbH, Dresden, Germany, for their work in developing the high-pressure sensor. Invaluable assistance was provided also by our partner Nagano Keiki Co., Nagano, Japan, in developing and producing the stainless steel sensing element. * Michael L. Dunbar is Manager, Product Marketing and Sales, and Dr. Karsten Sager is Manager, Product Development, Texas Instruments Automotive Sensors and Controls (formerly Integrated Sensor Solutions, Inc.), 625 River Oaks Pkwy., San Jose, CA 95134; 408-324-1044, x-116, 408-324-1054, mdun bar@ti.com For more information about the SOPS, contact John Pechonis, Texas Instruments Automotive Sensors and Controls, 34 Forest St., MC 23-10, Attleboro, MA 02703; 508-236-1399, fax 508-236-2349, jpechonis@ti.com Neil Coleman is President and Robert E. Coleman is Senior Application Specialist, Signalysis, Inc., 431 Ohio Pike, Ste. 182 South, Cincinnati, OH 45255; 513-528-6164, fax 513-528-6181, neilc@signalysis.com
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