This physical characteristic is cleverly utilized in capacitive touch sensing technology. Taking a common touchscreen as an example, one or more sensing electrodes are arranged on the screen. These electrodes are connected to a capacitive sensing circuit and form an electric field. When a user touches the screen, the human body, as a conductor, enters the sensing range of the sensing electrodes, changing the electric field distribution around the touch location, thus causing a change in capacitance at that location. Specifically, when a finger approaches or touches the touchscreen, a new capacitive coupling path is formed, connecting in parallel with the existing capacitive system, increasing the total capacitance. The capacitive sensing circuit continuously monitors the capacitance change between the sensing electrodes and ground. Once a touch operation occurs, the capacitance change at the touch location causes a change in the voltage or charge in the capacitive sensing circuit. This change is captured and converted into an electrical signal, which is then analyzed by a signal processor to determine the touch location and related parameters.
Capacitive touch sensors are mainly divided into two categories: mutual capacitance configuration and self-capacitance configuration. Mutual capacitance configuration consists of two terminals: a transmitting electrode and a receiving electrode, and is the preferred solution for touch-sensitive displays. In mutual capacitance sensing technology, the TX pin provides a digital voltage, measuring the charge received on the RX pin. When a finger is placed between the TX and RX electrodes, the mutual capacitance decreases, and the charge received on the RX electrode also decreases accordingly. The touch state is determined by detecting the change in charge on the RX electrode. Self-capacitance configuration, on the other hand, grounds one terminal of the sensing capacitor, suitable for simple applications such as touch-sensitive buttons, sliders, or scroll wheels. Self-capacitance uses one pin to measure the capacitance between this pin and the power ground. When a finger is placed on the sensor, the system capacitance increases, and the presence of a finger touch is detected by measuring the change in voltage.
Applications of capacitive sensing in automobiles
1. Hands-off detection (HOD) in intelligent driving assistance systems
In autonomous or semi-autonomous driving modes, ensuring the driver's constant attention to road conditions and hands-free operation is crucial. Capacitive sensing technology plays a key role here. Taking the three-zone HOD (Hardware-Oriented Driving) technology used in the Hongqi EHS9 as an example, when the driver holds the steering wheel, a capacitive circuit is formed with the sensing layer wrapped around the wheel; when the hand leaves the wheel, the change in capacitance immediately triggers a signal, alerting the driver to safety. Furthermore, this technology has completed the calibration and verification of 26 driving gestures, effectively eliminating the impact of accidental touches.
There are several solutions for hands-off detection, including measuring steering wheel torque, monitoring with image sensors, and capacitive sensing. Among these, capacitive sensing has become the mainstream due to its high accuracy and controllable cost. However, the high EMC (electromagnetic compatibility) environment of automobiles can affect the performance of capacitive sensing chips, thus affecting detection accuracy. For example, ams has developed a capacitive sensing chip specifically designed for hands-off detection (HOD), which can measure not only the capacitance value of the hand gripping the steering wheel but also the resistance value of the human body to ground, correcting the results to avoid bias.
2. Capacitive sensor steering wheel
A capacitive sensing steering wheel uses the principle of capacitive sensing to detect the steering wheel's rotation angle and direction. Inside the steering wheel are a reference plate and a moving plate that rotates with the wheel. When the steering wheel is turned, the distance between these two plates changes, and the capacitance changes accordingly. This change is converted into an electrical signal by a sensor, then into a digital signal, and transmitted to the vehicle's electronic control unit (ECU) to determine the steering wheel's rotation angle and direction, thereby controlling the vehicle's movement.
The Mercedes-Benz EQS's capacitive touch-sensitive steering wheel includes a two-zone sensor pad that detects whether the driver's hands are on the wheel. The touch control buttons in the spokes also have capacitive functionality, allowing for intuitive operation via swipe gestures and button presses, minimizing the need for a mechanical interface. The Avita 11, Volkswagen ID. series, and certain Cadillac models also utilize capacitive touch-sensitive steering wheel technology. These steering wheels offer significant advantages: providing more precise tactile feedback for more responsive and accurate operation; reducing hand fatigue and improving driving comfort; accurately recognizing hand movements to effectively prevent misoperation; and greatly enhancing driving safety at high speeds or in emergency situations.
3. Touch control in automotive human-machine interaction systems
Capacitive sensing technology has revolutionized the way cars interact with their human-machine interface (HMI). More and more cars are using touch buttons and sliders to replace traditional mechanical buttons and knobs, and these are being applied to systems such as in-vehicle entertainment, trunk opening and closing, heating, ventilation and air conditioning (HVAC) control, and passive keyless entry sensors (PKE).
In automotive touchscreens and trackpads, capacitive sensor structures detect finger touches by scanning changes in sensor capacitance, and analyze the data to recognize gestures, finger range, and direction of movement. Capacitive touchscreens and trackpads supporting up to ten fingers are increasingly becoming the choice for integrated automotive interfaces. Automotive network protocols such as CAN/LIN can integrate distributed electromechanical systems into the central console, allowing HMI designers to create a unified user interface style and increasing design flexibility. For example, touchscreens provide convenient operation methods for functions such as music browsing, map manipulation, and seat position adjustment.
4. Other in-vehicle applications
Capacitive sensing technology has numerous applications in automotive interiors. These include door opening and closing, ambient lighting adjustment, audio system control, and climate control. Non-contact capacitive sensing ensures high sensitivity even with wet hands or while wearing gloves. amsram's capacitive sensing technology, employing orthogonal decoding, integrates seamlessly with ambient lighting, enabling individual brightness and color adjustment for each LED.
Occupant presence detection in the cabin can also be achieved through capacitive sensing, regulating passenger and driver posture and ensuring cabin safety. A foot switch uses changes in sensing circuitry to open the trunk, and liquid level detection can be used to detect the ethanol content in blended fuels, among other things.
