New Trends in Automotive Capacitive Touch Sensing Technology

Capacitive touch sensing has always been a design differentiator for the design of highly aesthetic automotive infotainment systems. When devices were adequate for integrating automotive-specific functions, engineers began to use capacitive sensing to develop newer applications beyond infotainment applications to simplify and improve the various user interfaces in the vehicle. Capacitive touch sensing has become a mainstream technology in automotive design when used in conjunction with proximity sensing and automotive-specific functions such as standard network protocols such as LIN and CAN.

Over the past century, cars have evolved from a hodgepodge of mechanical parts to efficient, highly comfortable, and increasingly safe machines. This revolution has been driven primarily by the dramatic increase in the amount of electronic devices used in cars. One thing that remains constant, however, is that user satisfaction has always been based on how convenient and effective the interaction between people and their cars is. Whether it’s the driver interacting with the controls or the passengers interacting with the entertainment system, the joy of owning a car still comes from this interaction.

Systems that measure and track user interactions and provide feedback to the host control system are called automotive human-machine interface (HMI) systems. From the user's perspective, these interactions can be conscious (such as when a passenger actively provides input to the system) or subconscious (such as when the system measures the driver's intentions without the driver's awareness). In addition, as the various systems in the car become more complex, there is a need for the systems to be more responsive and aware of human interactions.

Capacitive sensing - revolutionizing automotive HMI

Engineers are constantly looking to improve HMI systems, making them more intuitive, "cooler" looking, and more accurate. Capacitive sensing technology is at the heart of this ongoing revolution. It is revolutionizing the design and implementation of HMI applications.

In simple terms, a capacitive sensor is made up of a pair of adjacent electrodes. When a person (or other conductor) approaches the electrodes, the capacitance between the electrodes and the conductor increases. By measuring this capacitance, the presence of the conductor can be sensed. The ability to sense the presence of an object is the basis for creating a wide variety of touch sensors. These sensors can be used as buttons, sliders, trackpads, and other common interface components.

Capacitive sensing technology can also be used for proximity sensing, where there is no contact between the sensor and the user's body. This can be achieved by increasing the sensitivity of the sensor. In addition, since this sensor is not limited by the field of view, a single sensor can sense the approach direction of an object in three-dimensional space.

This technology is even more powerful when used with a programmable mixed-signal controller. Programmable devices can intelligently measure capacitance to detect multiple indicators of human proximity: distance, approach direction, gesture recognition, etc. The programmable approach can also integrate other functions such as motor control and LED drive to provide touch/proximity feedback to the user (see below). Programmable devices also allow developers to significantly improve performance by reducing current consumption and improving noise immunity by defining optimal sensitivity thresholds and variable scan speeds.

Capacitive sensing – a proven technology

Over the past five years or so, capacitive touch sensing technology has been widely used in automotive infotainment systems, replacing mechanical buttons with touch-sensitive buttons. Many users are already familiar with capacitive touch controls in navigation systems, such as "fixed" buttons that change function depending on the menu activation state, sliders to zoom images, and controllers that appear on the main display when needed . These systems control navigation, audio, HVAC, and overall vehicle management functions in the vehicle.

Capacitive touch sensing technology can also be used in conjunction with mechanical buttons to create hybrid buttons (see Figure 1) that respond to both proximity/touch (such as function preview) and actual button presses (such as function activation), as shown in the figure below.

Figure 1: Hybrid touch technology implementation

These applications can be further enhanced by using capacitive proximity sensors, for example, to activate a panel with backlight control (see Figure 2) based on the user's proximity.

Figure 2: Proximity-based car audio backlight control

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Factors Driving Capacitive Sensing into Automotive Applications

As capacitive sensing technology matures, electronic design engineers are finding innovative new uses for it in cars. Two factors are driving the technology into the automotive space: first, support for automotive-specific communication protocols; second, the availability of robust IC technology.

Touch sensing controllers have begun to support automotive-specific communication protocols such as LIN and CAN. With the increasing demand for distributed architectures between touch panels and system ECUs (see Figure 3), support for CAN and LIN in touch sensing controllers allows system designers to minimize the number of external components required while improving reliability and reducing BOM costs. In addition, the trend towards integration has also made it easy to migrate functions that were previously implemented on these network buses to touch sensing controllers.

The growing number of mature and robust ICs combined with capacitive sensing technology has enabled semiconductor companies to create new devices with faster scanning speeds, better noise immunity, and lower power consumption. In addition, the increasingly experienced application design team will greatly promote the expansion of capacitive sensing technology into newer application areas. Some of these innovative applications include:

Capacitive Sensing Passive Keyless Entry: Passive keyless entry systems have been around for quite some time. More recently, capacitive sensing has been used to detect the approaching hand of the driver. This automatically initiates encrypted communication between the remote control in the driver’s pocket and the vehicle’s anti-theft control system. Once authenticated, the driver is allowed to enter the vehicle and drive away, without having to pull the key from their pocket to start the vehicle (see Figure 4).

Figure 4: Capacitive touch sensor used in passive keyless entry system

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Proximity detection of approach to the center console: A capacitive proximity sensor in the center console of a car can be customized to provide multiple functions based on the direction and distance of a person’s hand approaching. This allows the center console to adjust or change its response based on whether the driver or passenger is approaching the center console (see Figure 5).

Figure 5: Controlling center console functions based on approach direction

Interior lighting control: Capacitive proximity sensing is also an effective way to implement distance-sensitive lighting control for various interior and exterior automotive systems. Examples of such systems include proximity-based cabin lighting; proximity-based door handle backlighting where the lighting intensity varies with the proximity of the hand; and touch/proximity ceiling lighting via a LIN interface. An IC that combines capacitive touch sensing, LIN communication support, and LED dimming capabilities enables all of these applications to be implemented on a single chip .

Capacitive Touch Sensor Type Switches : Touch buttons are becoming increasingly popular and replacing mechanical switches and buttons due to their superior aesthetics, flexibility and reliability. Emerging applications use capacitive touch buttons to control various mechanical functions in the car, such as operating the sunroof, windows, door locks, mirrors and remote controls. Capacitive touch technology also works effectively and waterproof for external functions such as opening the trunk. Many of these applications are being further promoted due to the emergence of LIN/CAN communication protocols.

Liquid level sensing: In addition to sensing the human body, capacitive sensing technology can also be used to measure liquid conditions, such as detecting fuel, brake fluid and coolant levels.

From enabling "cooler" infotainment systems to providing a reliable method for measuring fluid levels, capacitive sensing is considered a popular and versatile sensing technology in automotive applications. By combining the ability to manage the LCD and transmit control data to system modules via LIN and CAN interfaces, developers can create a single-chip implementation that simplifies design and reduces costs.

The potential of capacitive sensing technology is now being unlocked by the advent of next-generation mixed-signal controllers designed specifically for the automotive industry. As the demands on these systems increase, designers will be able to use capacitive sensing to provide an effective interface technology for a wide variety of applications.