Capacitive sensing expands to multi-touch all-point sensing in automotive apps

The images created in Sci-Fi movies tend to push the limits of reality, such as when Tom Cruise uses a multi-touch screen to browse through information in the movie Minority Report. Capacitive sensing technology, however, has changed the way we interact with devices.  We no longer simple push buttons or throw switches.  Rather, we can touch, slide, and pinch the data itself on a touchscreen to interact with it.  The far-off interfaces of the movies are not such science fiction anymore.  In fact, they are already being adapted and designed into a variety of applications including our cars.

Multi-touch all-point sensing is an extension of capacitive sensing which has made it possible for touch technology to become much more intuitive by sensing the presence of multiple fingers simultaneously and being able to recognize gestures.  This article covers the basics of capacitive sensing and the evolution of touch sensing technology in automotive environments. After discussing the construction and internal operation of a multi-touch touchscreen/trackpad, it will also address the paradigm changes brought about by multi-touch all-point sensing in human machine interfaces (HMIs).

Multi-Touch Capacitive Sensing

At the heart any multi-touch all-point system is a capacitive sensor composed of a pair of adjacent electrodes. When a conductive object such as a finger comes in proximity to these electrodes, there is additional capacitance between the electrodes (see Figure 1) that can be measured with the help of a microcontroller. Alternately, capacitive sensing can also be used for proximity sensing where no contact is required between the sensor and the user's body. This can be achieved by increasing the sensitivity of the sensors.

Figure 1. Capacitive Sensing

Capacitive sensing is increasingly being used to replace mechanical buttons, knobs with touch sensitive buttons, and sliders (see Figure 2) for in-car systems like infotainment controls, trunk release, Heating Ventilation & Air Conditioning control (HVAC), and Passive Keyless Entry sensors (PKE). The consequent reduction in the number of mechanical components and grooves (which require more complex molds, trap dust, etc.) enhance reliability and reduce system costs.

Figure 2. Capacitive Sensing Interfaces

Touchscreens and Trackpads

Touchscreens enable users to directly “touch” the application in a device, thus reducing the dependence on external buttons. Similarly in a trackpad, users can interact with the system using instinctive actions and gestures like touching, tapping, pinching, and dragging. Touchscreens come in one of three main forms: Single-Touch, Multi-Touch Gesture, and Multi-Touch All-Point (see Figure 3).

Single-touch touchscreens were predominantly based on resistive touchscreen technology, locating both the screen and buttons within the same area. The limitations of resistive single-touch touchscreens are the ability to detect only one finger per screen at a time, limited gesture recognition capabilities, and poor performance due to wear and tear of the sensors.

These limitations led to the development of projected capacitance technology and multi-touch gesture touchscreens based on it. Multi-touch gesture touchscreens do not depend on pressure to detect user interaction.  They are also able to support  simultaneous multi-touch recognition and tracking along with gesture detection used to manipulate screen viewing size and orientation of web page views.

Multi-touch all-point refers to a touch sensing surface's (trackpad/touchscreen) ability to recognize the presence of two or more points of contact on the surface at the same time. Users have ten fingers across two hands and presence of multiple passengers in a car farther increases that number. Music browsing, map manipulation, and body electronic controls like seat position are a few examples of automotive applications that are prime candidates for this level of rich touchscreen functionality.

Trackpads in cars make it convenient for the driver to operate systems such as the navigation and audio subsystems without having to stretch out to reach the center console. Trackpads also enable character recognition, making alphanumeric keys redundant.

Figure 3. Types of Touchscreens

A Touchscreen typically has a glass/plastic covering overlay along with two  layers of transparent conductors such as Indium Tin Oxide (ITO) that are separated by an insulating material (see Figure 4).  Patterns are etched on the ITO layer to form a grid of capacitors. The ITO layer has a high level of transparency which helps make touchscreens brighter and easier to read. Since there is no pressure required for detecting touch, the screen becomes more durable.

In a trackpad, the capacitive sensor construction is similar except that the system has an opaque covering layer and simple copper layers as sensors.

Figure 4. Touchscreen layers

The touchscreens sensors are scanned in order to detect any change in sensor capacitance to detect an finger touches. The data is analyzed to identify gestures, finger range, and finger direction movement.  The system may also drive output devices like LEDs, or control motors.

Capacitive touchscreens and trackpads with 10-finger support are now increasingly being adopted in cars, serving as the integrated interface for various car systems.  These multi-touch all-point sensing systems also allow multiple users within a car to simultaneously access the touchscreen. In addition, automotive networking protocols like CAN/LIN help integrate distributed electro-mechanical systems into a central console. This enables HMI designers to coordinate operation of all subsystems within the console from a single user interface to create an HMI with a unified style and increased flexibility while offering developers greater control over look and feel. 

Multi-touch all-point sensing is helping create displays with an intuitive and engaging user interface and offers a compelling value proposition. Touchscreens and trackpads will be increasingly more pervasive with larger sizes and more capable processors, until they eventually become the automotive interface of choice.

About the Author

Aditya Kaul currently works with Cypress Semiconductors as a Product Marketing Engineer in the Automotive business unit focusing on proprietary products.  Apart from a BS in Electronics and Telecommunication, he has also completed an MBA.