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 (QTouch^TM) and mutual-capacitance sensors (QMatrix^TM). 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.

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.

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