For several years, car manufacturers have looked to the smartphone to provide a model for the development of an increasingly sophisticated user experience, particularly for the driver. Cars have followed the smartphone's lead by, for instance, providing a larger Centre Information Display (CID) screen offering higher resolution and fast, responsive capacitive touch sensing.

Now the automotive industry is looking to another feature of the smartphone's user interface, the fingerprint sensor, to enhance and modernise the driver's interface to functions in and even beyond the car. In fact this – and other forms of biometric authentication – appear to show great promise if implemented with sensitivity to user privacy and the limitations of the automotive operating environment. But the use cases of biometric authentication in the car look set to differ, perhaps surprisingly, from those of the smartphone.

__Personalising the user experience __

The obvious assumption about fingerprint sensing in the car is that it should be used as a convenient and secure replacement for the key both for providing access to the cabin and for starting the engine. In the smartphone, of course, fingerprint sensing performs this security function, barring access to any person other than the registered owner. In a car, however, fingerprint sensing is an unsatisfactory form of security in vehicles for two reasons.

The first is because of a difference in the usage model of a car from that of a smartphone. A car may be driven by people other than its registered owner. For instance, users of a valet parking service need to give the valet a means of starting the car. Equally, a driver who finds himself or herself incapacitated might want a suitably insured person to drive the vehicle on their behalf. A fingerprint sensor, then, can never entirely replace a key.

There is a security as well as a convenience reason why a fingerprint sensor cannot be the sole means of securing the car. This is because every fingerprint sensing technology in existence has a 'false acceptance rate'. Occasionally, every fingerprint sensor will wrongly identify a stranger's fingerprint as that of the registered user. Even the smallest risk that a potential car thief could steal this expensive asset simply by pressing a fingerprint sensor is too great for car manufacturers to accept. For security reasons, therefore, fingerprint sensing would always need to be backed by a supplementary form of access control. Conventional RF-operated keys, then, are not about to be superseded by fingerprint sensors on the door handle or Start button.

Fingerprint sensing does, however, enable two far-reaching improvements to the driver's user experience: personalisation, and payment authentication.

In a car with two or more regular drivers, the fingerprint sensor may identify who is driving, allowing the car's operations to be configured to that driver's preferred settings. This affects convenience settings such as the position and height of the driver's seat, mirrors and steering wheel, comfort settings such as the temperature and direction of the cabin air stream and vents, and entertainment settings, such as Bluetooth pairing with the driver's phone, the choice of favourite radio stations, the sound balance in the audio system, and even the appearance and menu configuration of the CID.

Personalisation of the user interface strengthens the emotional bond between owners and their vehicle, making it uniquely theirs. In an important way it also improves the driver's ability to make use of the sophisticated features and functions in a car, many of which in today's cars are hidden behind multi-layered menu structures and complex sets of commands. Research has shown that for every step added to a user interface, 10% of the users drop out. Personalisation via a fingerprint sensor reduces the number of steps to one or even none for many aspects of the user interface, thus making valuable features much more readily accessible to users.

In-car fingerprint sensors can also dramatically improve the user's experience of making payments inside the car, for instance when paying road tolls and parking fees, or to charge a payment card at a drive-through store.

At a parking garage, for example, a number plate recognition system would automatically identify the car, and a payments back end would link the car to a pre-registered payment card. The payment terminal would then send a payment authorisation request wirelessly to the car. The driver would authenticate the use of the payment card to pay the parking fee by pressing the fingerprint sensor (figure 1).

Standards such as the UAF specification published by the FIDO Alliance (fidoalliance.org) define processes for acknowledging biometric indicators such as fingerprints as an alternative to passwords or PINs. Authentication by fingerprint is a far quicker and easier operation than leaning through the driver's door window to insert a card in a reader and then enter a PIN on a keypad.

Fingerprint sensing

Figure 1: Fingerprint sensing implemented in a capacitive touchscreen would provide a quick and convenient way for the driver to authenticate a payment.

__Biometric indicators: which will the car industry favour? __

Various forms of biometric sensing are under consideration by the car industry, including facial recognition, iris recognition and heart-rate variability (a parameter which may be measured by a wireless health-monitoring wristband) as well as fingerprint sensing. Looking into the future, there is some interest in the potential to use biometric indicators such as heart rate, blood pressure and sweat to make a reading of the driver's state of health and state of mind. This might allow the car to issue an alert to the driver, for instance to park the car and rest if the indicators suggest that the driver is drowsy. With the advent of advanced driver assistance systems (ADAS), the car may even take over control from the driver and park itself in a safe place.

Today, however, the industry's preferred biometric indicator is the fingerprint. This is because the operation of fingerprint sensors is well understood by users, and because the technology of fingerprint sensing has been proven in the mobile device industry.

There are in fact three fingerprint sensing technologies available to car manufacturers today:

• Capacitive fingerprint sensing is the technology used in the world's best-selling smartphones, in which its ultra-small size – a sensing pad just a few tens of microns thick and a small controller IC – and low power usage are extremely attractive. In a small number of cases, capacitive fingerprint sensing might fail because of 'dry finger' problems, or when the user has severe scarring or calluses. • Optical fingerprint sensing is familiar to travellers who have crossed the US border, where an image of a fingerprint is captured and linked to passport information. An optical sensor is highly reliable and accurate, but the sensors require a backlight to illuminate the finger, and are still comparatively bulky compared to capacitive solutions. • Ultrasonic sensing offers reliable detection of fingerprints in 3D, but has not found its way into mainstream mobile devices. It is today an expensive and relatively unproven option. The benefits of capacitive fingerprint sensing that appeal to smartphone makers appeal equally to the car industry. In particular, the ultra-thin sensing pad gives car designers the total freedom they want to design the styling, shape and configuration of the sensor unit to suit the form and functions of the cabin.

Prototypes and demonstrations developed by Synaptics show how a fingerprint sensor may be integrated smoothly into the normal design of a car's interior without requiring additional surface space, and without cluttering the design of the cabin. Fingerprint sensors may readily be integrated, for example, into a touch-sensing directional pad mounted on the steering wheel (figure 2).

The capacitive fingerprint sensor (circled in red) Figure 2: The capacitive fingerprint sensor (circled in red) is shown on the left spoke of the steering wheel in this working demonstration created by Synaptics. The fingerprint sensor also doubles as a directional pad for navigating the in-car user interface.

This space-saving design allows the fingerprint sensor to be used with both the driver's hands on the steering wheel. In the Synaptics demonstration, the fingerprint sensor is combined with a force sensor also integrated into the sensing pad. This means that an authentication event may be triggered only when the pad is pressed, eliminating the risk that the driver could inadvertently authenticate a payment, for instance, when resting the hands naturally on the steering wheel.

Requirements for a successful implementation There are three elements to a successful fingerprint sensing implementation. The mechanical design of the sensing pad, the sensitivity of the controller IC, and the algorithms running on the IC for accepting or rejecting fingerprints.

The sensing controller IC and the software it runs may be sourced from third-party suppliers such as Synaptics. Synaptics has the advantage that its automotive fingerprint sensor solutions draw on technology shipped in tens of millions of smartphones worldwide. The learning and development baked into this technology ensure that automotive implementations can achieve just as high a performance as implementations in the mobile phone.

The mechanical design – the area and thickness of the sensing pad, the material it is made of, and its positioning in the vehicle – will be decided by the car manufacturer. Here, OEMs may draw on the experience that Synaptics has had in the smartphone market: smartphone manufacturers have experimented with many different configurations of the sensing pad, for instance. Some have worked, some have not: knowledge of previous design iterations will allow automotive manufacturers to complete sensor designs faster and avoid experimenting with design configurations that are known not to work.

As a proven technology, then, the capacitive method is set to be the first technology for fingerprint sensing to be adopted in the car. Time will tell whether it turns out to be preferred by the automotive industry in the longer term, and no doubt optical, ultrasonic and other technologies will be evaluated, and other forms of biometric sensing may be introduced.

Today, however, capacitive fingerprint sensing is both liked and understood by users of smartphones, and it provides a ready-made way for car manufacturers to make their users' experience of their products more convenient, more personal and more secure. __ About the author__ Raja Bose is Senior Staff Scientist and Research Technologist at Synaptics.