Capacitive proximity detection spurs automotive convenience features
There has been a steady rise in demand for proximity detection sensors in automotive applications because they can reliably detect the presence of objects near the sensor surface without physical contact.
The number of possible proximity detection applications is numerous:
Door entry control (keyless entry): Detecting a hand approaching the door handle to initiate unlocking Illuminating and waking up the touch screen when a hand approaches the screen surface Switching interior car lights on/off when the hand is near the sensor Detection of simple spatial gestures to switch devices on/off Sensing the presence of large objects around the car during parking
Many different proximity detection methods exist, for example, capacitive, infrared, ultrasonic, optical, etc. For the 5 to 300 mm proximity detection range, capacitive sensing has many advantages compared to other methods: Excellent reliability, simple mechanical design, low power consumption, and low cost.
The Atmel® capacitive proximity detection technology sensors are based on charge-transfer technology—a method pioneered by the company where voltage is generated on the sampling capacitor during the repetition of a specific control sequence applied over the I/O pins. Atmel holds multiple patents in the area of charge-transfer technology for self-capacitance sensors (QTouchTM) and mutual-capacitance sensors (QMatrixTM). This charge-transfer technology offers advantages compared to other capacitive measuring methods including: Increased flexibility, high sensitivity, improved moisture resistance, and noise immunity.
Technology basics
Capacitive proximity sensors measure the capacitance change between the single electrode and ground (self-capacitance sensors) or between two electrodes (mutual capacitance sensors) as objects approach the electrodes. While constant capacitance is between 10 to 300pF, the capacitance changes are typically extremely small, ranging from a few fF to several pF. Because the electrical field lines around the self-capacitance sensors spread far away from the sensing electrode, self capacitance is the preferred proximity detection method over mutual capacitance where field lines are largely concentrated in the area between the transmitting and receiving electrodes.
Characteristics of capacitive proximity sensors for automotive applications include:
High sensitivity: Detecting small changes in the measured capacitance requires increased and stable sensitivity. In many cases the oversampling technique enables increasing the measuring resolution of existing methods at the cost of slower performance, extra memory, and additional calculations.
Special measures should be taken to reduce negative effects on sensitivity caused by capacitive loading, especially if the sensing electrode is placed on a conductive surface (metal plane, car body, etc.). An active shield layer is used to reduce the negative effect of capacitive loading between the electrode and the conductive surface as shown in the figure below. A further advantage of active shields is their neutralizing effect on water films.
Fig. 1: Active shielding of a capacitive proximity sensor
(For information on using active shields, please refer to page 7 in this Atmel application note.)
Moisture resistivity: Moisture-induced changes in the measured signals can be more significant than changes from approaching objects. Water film on the surface is one of the biggest problems for capacitive solutions. Water films are more or less conductive and create a change of the measured signals that is similar to normal touch events. There are mainly two ways to handle effects caused by water films:
a) Use of active shields (described above)
b) Shorter charge transfer time—the water film could be utilized as a distributed RC circuit (as shown below). Reduced charge transfer pulses will prevent full charging of the distributed capacitors C and hence reduce the impact of the water films. Best results can be obtained if the charge transfer time is in the range of 100 to 250 ns.
Fig. 2: A water film can act as a distributed RC circuit
A proper mechanical design of the sense area and the use of the appropriate materials prevent the emergence of thick water films on the sensing area.
Temperature stability: In automotive applications extreme and rapid temperature changes may occur at any time. Special care should be taken with regards to a stable mechanical design—even the smallest gap changes near the conductive surfaces may cause false detection.
Noise immunity: Due to the high sensitivity, noise interference could compromise normal operation of the proximity sensor. The electrical and mechanical design of the PCB should be carried out to avoid noise interference caused by adjacent cables or conductive surfaces.
Fast response time: The expected response time is usually between 10 and 100ms The following sections provide more detailed scenarios of automotive capacitive proximity detection.
Door entry system
One example of capacitive proximity detection is in car door entry systems.
The proximity sensor that detects hand approaches is located within the car door handle (1). Once object proximity has been detected, the main unit (2) sends a wake-up signal via the LF antenna (3) which activates the car key transmitter (4). The transmitter then exchanges information with the RFID receiver (5) and—if the code matches the main control unit (2)—the door is unlocked. The entire process of proximity detection and ID recognition takes a fraction of a second. This means when the hand pulls the door handle, the door is already unlocked.
The advantage of using proximity detection rather than touch detection in door entry systems is the extended time to identify a person. As a result, the door lock state will always be resolved before the door handle is pulled.
Spatial gestures to switch devices on/off
The simultaneous use of two or more capacitive proximity sensors enables using simple spatial gestures such as hand waving in front of the device to be detected. The figure below shows a simple example of such a system to switch lights on/off inside the car—a wave of the hand in front of the light in one direction causes the light to switch on, a wave of the hand in the opposite direction switches it off. The system is able to analyze the signals from the proximity sensors and to decide whether to switch the lamp on or off.
Fig. 4. Spatial gestures can be used to switch devices on/off inside the car.
There are many different options available for designing sensing electrodes inside a light—from using thin copper wires to conductive polymers that can be applied directly over the plastic.
Conclusions
Implementation of capacitive proximity sensors in automotive applications paves the way to a broad range of comfort applications Moisture and rapid temperature changes are the main challenges for capacitive proximity sensors used in automotive systems.
Atmel’s QTouch and QMatrix technologies have been implemented in multiple touch controllers supporting touch buttons, sliders, and wheels, as well as touchscreens. Proximity detection support is also available with some of the standard products. The company is now developing and manufacturing new proximity methods and algorithms to increase sensitivity to support finger or hand detection ranges of up to 200 mm and more.
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