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The number of sensors used in automobiles has risen dramatically in the last decade. Current vehicles can contain 40-50 of them. Researchers are looking for new sensors and new ways to package them.

Multi-sensor modules

Researchers (D. Sparks, T. Noll, D. Agrotis, T. Betzner, and K. Gschwend) at Delphi Automotive Systems are investigating multi-sensor modules. In addition to the increased number of sensors used in current vehicles, one or more vehicle-wide data-communication networks are being used. These networks link various sensors to actuators and control centers and enable a variety of new automotive functions. Complex antilock braking systems, drive-by-wire, heads-up display, navigation, and traction control can all use vehicle networks.

Increased use of wiring has caused the harness weight to increase from 4 kg (9 lb) fifty years ago to between 37 and 91 kg (81 and 200 lb) in today's vehicles. Multiplexing is an efficient way to reverse this weight increase trend and is being employed by a number of vehicle suppliers. Bus protocols vary with manufacturers and include Class 2, CAN, GMLAN, UART, ABUS, VAN, SCP, BEAN, and ACP.

Individual sensors can be fitted with data-bus communication capability. This can be done by providing a processor, protocol controller, and transceiver chip to the sensor module. A cost penalty goes along with this conversion from a "dumb" analog sensor to a "smart" sensor capable of vehicle-wide communications. This cost can exceed several dollars per sensor.

Examination of sensor function and placement shows that a variety of sensors lend themselves to being placed in localized clusters. These multi-sensor modules can share a single set of communication chips, thereby lowering the overall cost associated with the conversion to a smart sensor network.

Packaging is one of the more expensive pieces of sensor manufacturing. By using a single housing, sensor cost is also reduced by eliminating multiple connectors and cables. Reducing the number of connectors, integrated circuits, and cables not only saves costs but also improves reliability and the assembly process and reduces vehicle weight.

A first-generation prototype module contains angular rate sensors and low-g linear accelerometers oriented along the three axes of the vehicle. Two high-g accelerometers for impact detection are also included as is a barometric pressure, temperature, and carbon dioxide sensor. A second prototype module contains only motion sensors, including smaller second-generation angular rate sensors to reduce the overall module size. Both modules are CAN compatible-for bus communication.

The majority of the sensors used in the prototypes are safety-related or could be used as the core of an automotive black box or data recorder. Accelerometers and angular rate sensors can be used to fire frontal and/or side airbags or raise roll bars as well as increase tension in seatbelts or adjust braking. Such sensors require faster communication rates and high priority. This leads to the potential need for multiple bus interfacing for an individual sensor cluster. Temperature or barometric pressure sensors most likely would be tied to a slower vehicle-wide bus.

Another sensor cluster may be identified for comfort sensing. This cluster may consist of multi-zone, non-contact surface temperature sensing, a humidity sensor, and an air quality sensor. Placing these sensors on one substrate with customized packaging helps eliminate problems with finding multiple locations for separate sensors in an area of the vehicle where appearance design considerations are an overriding concern. The databus interface connects this cluster to the control head of the heating, ventilation, and air conditioning system with a small, fixed number of wires. This makes it possible to add sensing functions as cabin air control shifts from temperature to comfort control.

Sensors usually have location requirements. Low-g accelerometers for ride control need to be near the center of gravity of the vehicle. Adaptive cruise control using radar or laser sensors and smog sensing must be located in the front of the vehicle, perhaps in the grille. Comfort sensors are located near the HVAC control head in front of the dash. However, a sensor cluster with databus interface may be placed in the headliner above the rear view mirror. This provides benefits in minimizing or eliminating difficult trade-offs between cabin interior design objectives and functionality. It also creates opportunities for integrating other sensors such as rain and fog, occupant position, and vehicle intrusion sensing.

There are some sensors that cannot be conveniently clustered. To augment the effectiveness of sensor cluster electronics, analog input from these isolated sensors can be fed into the module, if located in the vicinity of the box. The sensor cable would go directly to an ADMUX input, and then out into the data communication line. Examples of sensors that may be connected in this manner include steering wheel position and dispersed interior temperature sensors.

A study has shown that out of the 40-50 sensors in a vehicle, 15-20 could be grouped into multi-sensor modules. This would cut the number of sensor housings, connectors, and bus electronic devices by 30-40%. Using a bus for applications like angular-rate and acceleration sensing can actually eliminate the need for redundant sensing. As an example, yaw sensing can be required by braking, adaptive cruise, and navigation systems. Going to a central module can eliminate two of the three sensors normally required for these functions.

Future trends include putting several sensors on one silicon chip.

Pressure sensor

Integrated Sensor Solutions, Inc. (Michael L. Dunbar) has developed a small outline pressure sensor for use in various automotive applications including vehicle dynamic control (VDC), gasoline direct fuel injection, common rail diesel fuel injection, and other emerging systems. The sensor uses a stainless steel diaphragm for fluid compatibility and can be built in versions to measure a wide range of pressures from 0-700 kPa (0-100 psi) up to 0-180 MPa (0-26,000 psi). Advanced diagnostic and protection functions are included in the electronics to alert the electronic control module of possible sensor or system failures. All signal conditioning functions are integrated into a single CMOS circuit to reduce cost, size, and complexity.

Digital programmable sensor system architecture by Motorola.

Programmable CMOS integrated pressure sensor

Researchers (W. Czarnocki, X. Ding, J.P.Schuster, Motorola Automotive and Industrial Electronics Group and B. Roeckner, Motorola Corporate Communications Research Lab) have developed an integrated pressure sensor which uses a custom digital signal processor and non-volatile memory to calibrate and temperature-compensate a family of pressure sensor elements for a wide range of automotive applications. Unlike previously introduced analog solutions, this programmable signal conditioning engine operates in the digital domain using a calibration algorithm that accounts for higher order effects beyond the realm of most analog signal conditioning approaches. It also provides enhanced features that typically were implemented off-chip (or not at all) with traditional analog signal conditioning solutions that use laser or electronic trimming. A specially developed digital communication interface permits calibration of the individual sensor module via connector pins after the module has been fully assembled and encapsulated. Post trim processing is eliminated and calibration and module customization can be performed as an integral part of end-of-line testing at the completion of the manufacturing flow. The integrated circuit contains a pressure sensor element that is coprocessed in a submicron mixed-signal CMOS wafer fabrication process, and which can be scaled to a variety of automotive pressure-sensing applications. Both digital and analog sensor outputs are available.