As vehicles become more sophisticated and incorporate advanced driver assistance systems (ADAS) to allow them to help the driver and, ultimately, become self-driving, they need to have the data to make the best decision in every case. In many situations, this involves translating a physical property (such as pressure or temperature) or a mechanical movement (travel or rotation) into an electrical signal that can be processed and acted upon.
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Modern vehicles incorporate many sensing elements and the number is growing rapidly as automakers see this as fundamental to road safety and a means of differentiation against their competition. Governments and regulatory bodies agree, and are now starting to mandate certain types of sensing and monitoring before vehicles are allowed to be sold.
Sensors vary hugely, from simple resistive type devices through to modern vision sensors that offer high performance at high resolution – often with in-built processing and error detection and correction.
As vehicles become more complex, engineers have found it useful to segment the vehicle into several sectors as the sensor needs and environment in which they will operate are similar in each sector.
Figure 1: Modern vehicles contain a huge number of sensors
The three primary sectors are:
As automakers are constantly seeking to improve safety and operating efficiency of vehicles, most of the uses of sensor technology on vehicles is geared towards one or both of these goals. Within the powertrain, pressure and temperature are the two most common forms of measurement to provide the engine control unit (ECU) with the data needed to re-tune the engine for maximum efficiency.
Pressure sensing takes place in many areas of the engine including the exhaust manifold, the cylinder, fuel injection system and exhaust gas recycling. Temperature sensors are most often used in the manifold and exhaust as well as to monitor the temperature of the coolant.
Additionally, gas sensing is common in ICE to check the combustion process, including in the cylinder and to confirm the air-fuel ratio. Mechanical sensing in the form of rotary position sensing is used to confirm the position of the crankshaft and camshaft as well as the throttle body and accelerator pedal.
Within the chassis, the focus is much more on safety and movement of the various components. The wheel rotational speed is essential for antilock braking systems (ABS) and general stability. For more advanced active suspension, parameters such as the displacement of the suspension struts, chassis height and body acceleration in all three axes are important to know.
As more vehicles move to drive-by-wire, the angle and torque applied to the steering wheel by the driver will become essential inputs to the system. Simpler, but essential, measurements include tyre pressure and the level of the brake fluid reservoir.
While there continue to be developments in the technology used in the chassis and powertrain, the most rapid and significant changes are taking place in the vehicle body sensing. Here, there are five principal areas of activity including safety, intelligence, navigation, comfort / convenience and security – although many systems encompass more than one of these areas.
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In terms of safety, crash detection is one of the most important functions, using accelerometers to detect rapid deceleration (or acceleration for a rear-end shunt) as well as sensing whether the vehicle has rolled over. Outputs from these sensors will drive the e-call systems that are now mandatory in some jurisdictions, and turn off systems that could present a danger. The tilt sensor has a dual use as it also forms part of the security system, detecting unwanted vehicle movement - when parked and locked the vehicle should not move!
Vision sensing in its many forms (image sensors, radar, LIDAR, ultrasonics etc.) is one of the most significant advances in vehicle technology. This facilitates object detection for ADAS systems as well as many other features such as 360-degree vision, speed limit detection, blind spot surveillance, lane change assistance, reversing cameras (which are now mandatory in some countries) and more. While, in the early days, vision systems were exclusively external, they are now being used in the cabin to monitor driver alertness and the same camera can be used for other applications, such as determining the number and size of passengers – a useful input to the airbag system.
In fact, almost anything the driver currently does is being augmented or replaced by an automated system that involves some form of sensing. Headlight operation is often automated and sophisticated, shaping the beam to give maximum illumination while not adversely affecting other road users.
With the vast amounts of sensors available and the variety of applications they can be used in, the specifics of sensor selection is also very broad. However, there are a number of considerations that apply quite widely:
Sensors to be used in the chassis will see similar conditions as many will be outside the confines of the vehicle. Here, shocks are another hazard, and sensors will be exposed to low temperatures and environmental dampness and dirt.
Inside the cabin is the most forgiving environment, as the temperature is regulated, but even here there can be challenges. Sensors mounted behind the dash will be subject to elevated temperatures due to the confined space as well as the solar gain from sunlight through the windscreen.
Some safety-critical systems (including sensors) use automotive safety integration levels (ASIL) as a method of ensuring that they are designed in such a way as to be safe at all times. Some sensors, especially complex sub-systems such as vision sensors and digital output devices, include this ability internally which is a useful feature, saving design time and effort.
‘Sensor fusion’ contributes significantly to overall system reliability as it allows the data from multiple sensors to be combined, making a system that is greater than the sum of its parts. While simple redundancy uses multiple sensors of the same type so that if one fails, another can take over, sensor fusion combines data from different types of sensor.
For example, an image sensor is able to detect colour and determine the shape of objects in the path of the vehicle. Radar, however, can ‘see’ further but with less ability to identify the object – but it operates much better than a vision sensor in poor weather. Together, the two types of sensor provide a comprehensive system. Should the image sensor fail (for example) then the vehicle may be able to operate using just the radar, albeit with reduced performance (such as restricted speed), thereby giving the vehicle a ‘limp home’ mode to get it to a place of safety.
Sensors are becoming one of the most critical elements of modern vehicles as automakers seek to augment and eventually replace the driver. As one of the fastest growing sectors of the automotive industry, the number of available sensors and the potential applications are growing dramatically.
In selecting a sensor for a given application, the most important thing is for the designer to fully understand the needs of the application in terms of sensor performance and the environment the sensor will operate in. Only then will they be able to select the most appropriate device
Below we’ve highlighted our leading suppliers for components suited to automotive sensors.
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