Force-sensing: A third dimension in automotive touch controls

Force-sensing: A third dimension in automotive touch controls

Technology News |
By Christoph Hammerschmidt

In consumer electronics devices such as smartphones, tablets and laptop computers, this problem is solved by a combination of a graphical user interface and a user input device, which enables rapid navigation through highly complex and multi-layered software menu structures which present numerous choices to the user.

The appropriate user input technology is different for different device types: in the desktop PC, the mouse continues to be the most common user input device more than 30 years since its introduction in mainstream personal computing. In laptop computers, the touchpad has come to dominate. In the recent past, the favoured user input device in small form-factor computers such as smartphones and tablets has become the finger.

Fig. 1: touchscreen user interfaces optimised for the automotive environment,
such as the BMW iDrive, emphasise ease of viewing and use.

Indeed, the touchscreen offers a remarkable set of advantages, since it is intuitive and responsive, occupies almost no space, and eliminates the need for a second, peripheral device such as a mouse. In the automotive use case, however, the touchscreen has a serious drawback: it requires focussed attention on the screen to place the finger in the correct place. In a car, this necessarily draws the driver’s attention away from the road.

True, display and touchscreen system manufacturers, including Synaptics, have developed touchscreen designs adapted to the car, providing large icons in an uncluttered user interface design to make it easier for the driver to operate at arm’s length (see Figure 1). But the best solution from the point of view of car manufacturers, which are governed by extremely tough safety regulations, is one that that can be operated without requiring the driver to take his or her eyes off the road or hands off the steering wheel.

This has led automotive suppliers to experiment with methods for implementing thumb-operated touchpads mounted on the front spokes of the steering wheel. The driver may operate such touchpads safely without moving his or her hands from their natural position on the steering wheel. When combined with a heads-up display, this input method allows the driver’s eyes to remain on the road at all times.

TouchPad technologies: a question of size, cost and performance

There appears, of course, to be a ready-made model for the automotive touchpad: touchpads in laptops are based on proven capacitive sensing technology which has been developed, over a period of many years, to support large touch surfaces and multi-finger inputs, and to recognise gestures. Capacitive sensing touchpads are extremely light, thin and robust. Laptop manufacturers can readily calibrate the sensitivity of the touchpad so that it responds quickly to user inputs without suffering from interference caused by stray capacitance.

Automotive suppliers, however, encountered difficulties in early attempts to implement capacitive touchpads in cars. Briefly, car makers experimented with a touchpad in the centre console, before touchscreens were commonly adopted. A touch input device that required the driver to take a hand off the steering wheel was never likely to find favour, however.

The requirement, then, was clearly for steering wheel-mounted touchpads. But an attribute of conventional capacitive sensing touchpads that provides an advantage in laptop computers – their high sensitivity – becomes a drawback in the car: because the touchpad is sensitive to the slightest touch, there is a risk that it will register inadvertent touch events as drivers shift the position of their hands on the steering wheel in the normal course of driving.

A Mercedes Benz response to this problem has been to take a different technological route altogether, demonstrating at the 2016 Consumer Electronics Show an optical finger navigation (OFN) touchpad mounted on the steering wheel. OFN technology performs well, but suffers from various drawbacks from the point of view of automotive applications. It is:

  • vulnerable to optical interference from ambient light sources including sunlight;
  • apt to cause irritation to a proportion of users who are sensitive to the wavelength of the touchpad’s laser backlight; and
  • most important of all, expensive.

Nevertheless, this type of steering wheel-mounted, thumb-operated touchpad appeals strongly to users: it is intuitive, safe, sleek in appearance, and robust, since unlike a conventional mechanical button, it contains no moving parts.

And the implementation of a touchpad allows the car maker to perform a thorough decluttering of the driver controls and user inputs. In luxury and premium vehicles, which are loaded with many user-selectable features and functions for comfort, driving style, infotainment and so on, the use of single-function mechanical switches and buttons can lead to a proliferation of user input devices on and around the steering wheel.

By replacing multiple conventional buttons with a single touchpad for selecting choices in a software menu, the car maker can improve the steering wheel’s appearance, eliminate the risk of mechanical failure inherent in a conventional button, and make the user interface far simpler and easier for the driver to use.

Now car manufacturers are evaluating methods for achieving this reliably, with a compact solution which has a lower cost than OFN.

Combining capacitive sensing with force and haptics

In collaboration with automotive system suppliers, Synaptics has been developing system designs which will enable the implementation of proven, compact and responsive capacitive sensing touchpads without suffering from the problem of inadvertent touch events. Extensive user testing of prototypes shows that capacitive-based force sensing allied with capacitive touch sensing, and backed by haptic feedback, provides exactly the right combination of a responsive feel with total control over the selection of items in a software menu.

The addition of force sensing means that the user is required to actively press the touchpad to activate a function in the menu system. When the system registers a press, it confirms this back to the driver with haptic feedback – typically a short vibration of the touchpad. It is the combination of a press confirmed by a vibration, and potentially a sound as well, which gives the driver sufficient confidence to grip the steering wheel in ordinary use without worrying about inadvertently triggering an unwanted action in the software user interface.

Fig. 2: ForcePad technology from Synaptics senses the change
in capacitance as the touch-sensing surface flexes inwards towards
a capacitive force-sensing layer.

Capacitive force sensing is already a proven technology: Synaptics’ invention and implementation of force sensing is in use in the portable computing market today including ForcePad technology for the notebook PCs touchpad, and ClearForce for smartphone touchscreens. Apple later helped to commercialise force sensing with its 3D Touch. Force sensing depends on precise mechanical specification of the touchpad itself and a lower layer in close proximity to the touch surface. The touch surface flexes as the user presses it, bringing the two layers closer together, and changing the capacitance of the two surfaces. By configuring the threshold at which the change in capacitance is recognised as a press, the automobile OEM or tier 1 system supplier can calibrate the force with which the user must press the touchpad in order to activate a function.

This technology is proven, robust and simple in its conception. The most difficult aspect of its implementation is the precise mechanical assembly of the force sensing layers, to tolerances specified in microns. Extremely accurate and repeatable mechanical assembly, however, is a core competence of the automotive sector, and so this technology is particularly suitable for use in vehicles.

Fig. 3: a Synaptics demonstration of a steering wheel touchpad
with force sensing and haptics.

A demonstration of a touchpad with force sensing and haptic feedback was presented by Synaptics at the Consumer Electronics Show in January 2016 (see Figure 3). It draws on knowledge acquired in the development of the ForcePad product. But the ForcePad portable computing product is itself not suitable for direct implementation in cars. Why is this?

First, the lifetime expectation for vehicle components is higher than that in portable computing products, and the temperature rating of materials is also more demanding: vehicle components have to withstand more extreme temperatures than a laptop computer does. Second, the standards which limit electro-magnetic interference and susceptibility in vehicles are far more stringent than those in consumer products, and the ForcePad product requires modification in order to comply with automotive EMC standards.

Finally, in a laptop computer the user’s requirement is for a large surface area to support a wide range of gestures and touch types. In a steering wheel implementation, the user’s movement is far more restricted, and the range of gestures and touch actions to be supported is far smaller.

For this reason, Synaptics is in the late stage of developing with industry partners an automotive-specific implementation of touchpad technology with force sensing capability and haptic feedback. Using materials rated for use in vehicles, this EMC-compliant technology will provide the temperature and lifetime ratings required by the automotive industry, and it will carry the quality assurance offered by Synaptics, which is the world’s largest supplier of touchscreen and touchpad controller ICs to the mobile phone and computer industries.

Alongside other user interface innovations being introduced by the automotive industry, such as heads-up displays and large form-factor touchscreen displays, the force-sensing touchpad promises to dramatically re-model the array of user input devices presented to the driver, providing a cleaner and more attractive set of controls which is easier to use, and which better ensures that the driver’s attention stays where it is meant to be – on the road.

About the author:

Sunil Thomas is Senior Director, Automotive at Synaptics

All images: Synaptics


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