Automotive sensors

Automotive sensors

Steven J. Prosser

Keywords: Automotive, Sensors, Harsh environments

The trend towards ever increasing use of electronically controlled electrically actuated systems on vehicles is creating new challenges and opportunities for automotive sensor designers. The development of advanced systems such as brake-by-wire and steer-by- wire are well underway in various companies around the world. In the early 1980's, the value of electronics in the vehicle was just a few percent, but by the year 2000 it had dramatically grown with figures of around 17 per cent with some vehicles reaching 30 per cent being quoted in various market studies. The automotive sensor market has similarly expanded in response to the increased requirements for sophisticated electronic control. As an example, the early 1990's saw the first silicon based accelerometer for airbag crash sensor applications. These devices have now achieved extensive market penetration.

Over the past ten years, we have seen a range of sensors manufactured at high volume, including those for speed/position, oxygen, air mass flow, acceleration, pressure, and temperature. Anyone who has listened to a technical presentation on automotive sensors has very likely received the message; “there are three important factors to becoming a successful automotive sensor supplier; low cost, low cost and low cost”. Sensor cost continues to be a significant factor in the selection criteria of automotive system designers. The total cost of a sensor not only includes the sensing technology but also the signal conditioning electronics, packaging, connector/cable harness and testing.

The harsh automotive environment continues to provide challenges to the sensor designer. It has frequently been quoted that automotive suppliers must deliver the robustness of military devices and follow the pricing of consumer products. Over recent years the challenging environment has been under-hood, but in the future, there is an increasing drive to mount electronic control modules, and their associated sensors, on actuators (e.g. steer-by-wire electric motor or brake-by-wire caliper-mounted motor) or directly on the engine. This poses major issues for the design of reliable, low cost sensors that will withstand the extremes of temperature, vibration, and electromagnetic interference. The availability of 42 volts power will result in the enablement of many advanced systems (e.g. electro-magnetic valve actuation, electric cabin heating) which in turn will demand many new sensors.

Electronically controlled chassis systems have enhanced safety enormously by optimising the interface between tyre and road surface, either in the longitudinal, lateral or vertical directions, e.g. anti-lock braking, traction control, active roll and vehicle stability control systems. In the future, hydraulic braking systems will be replaced by fully electrical systems, the brake-by-wire systems. There are still many challenges to be overcome in brake-by-wire systems before they emerge on the commercial market, but the need for advanced sensors is clear. In addition to the existing wheel speed sensors, lateral accelerometers, yaw rate sensors, steering wheel angle, pedal position sensors, tyre pressure monitoring and brake function monitoring are becoming available.

We have already seen the advent of radar distance sensors for adaptive cruise control (ACC). The sensor feeds data on the distance, relative speed/acceleration of the preceding vehicle to the following car's electronic control unit, which then automatically adjusts the following vehicle's speed to maintain a safe distance between them. The vehicle returns to the set cruise control speed when the traffic clears. ACC systems with advanced brake control technology is the leading edge of what is expected to become a completely autonomous driving system which may allow future vehicles to safely navigate along pre- routed traffic systems. New traffic infrastructures are already being demonstrated that enable traffic to move with fewer jammed highways. Video-based sensing systems are also being developed to support obstacle detection, lane departure warning and driver alertness monitoring.

Finally, sensors are also helping to improve the interior environment for the driver and passengers. Airbag safety systems have continued to develop with more sophisticated deployment strategies involving seated weight sensing and vision-based occupant detection. Humidity, air quality and sunlight sensors are being developed for cabin comfort control. Occupant sensing will enable the optimisation of the heat/cooling distribution inside the vehicle. Today, rain sensors are being used to automatically activate the windscreen wipers and seat/pedal/steering wheel position sensors being used to support customised driver preferences.