Capacitive sensing touchscreens are given for the "iPhone generation." But there are four key technical issues automotive manufacturers should consider when implementing them.
Capacitive sensing touchscreens are now being adopted by the automotive industry for use in center information displays (CIDs). The adoption of capacitive touch in CIDs is a direct response to the demands of the ‘iPhone generation’ of car owners, no longer willing to accept that the touchscreen in their $30,000 car may be worse than a smartphone costing $500 or less.
While capacitive touch sensing is a preferable technology for the automotive user interface (UI), the CID brings automotive manufacturers a new set of challenges for ensuring these implementations match the ease of use of smartphones, as well as comply with rigorous automotive requirements for safety and reliability.
These challenges include: noise and interference, an enhanced user experience, style differentiation, and product stability.
Noise and interference
The array of electrodes that powers capacitive touchscreens has an unwanted secondary function. It acts as an antenna, picking up interference from electro-magnetic noise in a car. A larger display creates a larger ‘antenna,’ and thus, more noise with the potential to disrupt touch operations.
Previous touch solutions often trade off noise immunity against performance, but this can be mitigated by implementing high-grade noise filtering schemes, such as self-capacitance and mutual capacitance. Self-capacitance offers a more responsive feel and enables other features like proximity sensing, while mutual capacitance supports multi-touch functions.
By combining self- and mutual capacitance, the controller IC can implement noise filtering in both hardware and firmware, and ensure that noise is eliminated when converting raw measurements of capacitance into recognized touch events.
An enhanced user experience
While mobile phone operation requires a user’s constant attention, the user’s attention in the vehicle must be focused on the road. Larger icons in the vehicle do make them easier to locate with a quick glance, but two recent enhancements to the touchscreen can help the user operate the touchscreen without looking at it.
Haptics can signal to the user that a button is being actuated without the user needing to look at the screen, providing a pulse or vibration in response to a button press. Tactile feedback can be supplemented by simultaneous sounds that signal the recognition of specific touch events.
Force sensing prevents the screen from registering inadvertent touches. An additional benefit is that the on-screen controls can be incrementally adjusted in response to the force of the user’s touch, such as increasing the volume faster or more gradually.
A car’s cabin provides far greater scope for differentiated styling of a display than a smartphone form factor. The vehicle console is certainly one defining design feature, but the touchscreen can also be customized. The choice of cover lens materials – glass or polycarbonate – makes a difference both to the look and the performance of the system. In addition, if the console is slightly curved inward towards the driver, the surface of the cover lens can have a matching curve.
While standard touchscreen controllers are configured to operate with a cover lens of a uniform thickness, this particular use case requires varied thickness and the response of the touch controller IC to be optimized for the thickness of the cover lens. With a successful curved implementation, automotive manufacturers can surpass what’s been achieved by smartphone and tablet manufacturers.
As these systems enter mass production, it’s essential that every manufactured unit performs as intended — but elements of the assembly are subject to inherent manufacturing variations. In particular, variations in thickness between one cover lens unit and another are commonplace. So too is variation in the resistance of production units of the ITO electrode layer. Both these variations have the potential to affect the touchscreen’s sensitivity and accuracy.
Through statistical simulations and tools that optimize the controller's configuration across the variability in cover lens thickness, ITO layer resistance and other properties of the touchscreen assembly, automotive OEMs are able to better prepare for these variations.
The adoption of capacitive sensing touchscreen technology enhances the functionality of the UI and the user experience. Paying due attention to these four technical design issues will help automotive manufacturers achieve a successful, high-performance implementation of of capacitive touch technology with the fewest possible design iterations.
—John Brady is senior director (Automotive) at Synaptics