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Capacitive touch application: Automotive door handle

Posted: 18 Apr 2016 ?? ?Print Version ?Bookmark and Share

Keywords:touchscreens? door handle detection system? capacitive detection? rotary knobs?

There are several technical challenges to overcome to achieve best-in-class capacitive detection in a door handle detection system. The most relevant being that the system must:
???Achieve a proper detection distance to allow sufficient time for the system to wake up and perform its authentication tasks. The typical distance for unlock detection is between 1 and 4 cm.
???Consume as little power as possible. The capacitive detection is the only function that must always be awake, as it is responsible for the initiation of the whole unlock process. Typical average power consumption is in the range of 50-100?A.
???Be reactive enough to provide a transparent user experience. The driver should never encounter a locked door due to the fact that the unlock system was not woken up sufficiently in advance. The typical response time is 20ms.
???Be fully reliable in disturbed conditions. The capacitive detection has to detect the driver hand in disturbed conditions (rain, interference, etc.) while rejecting all the unwanted touch signals potentially induced by these disturbances.

Technical proposal
In addition to proximity capacitive detection, a door handle system must provide communication with the PEPS microcontroller (MCU), including serial communication, power line communication, LIN, CAN, etc., and provide support for additional wireless interface (NFC, etc.). It must also perform application-specific tasks. These various system requirements lead to the need of MCUs with very different packages, memory sizes and communication interfaces.

In answer to all the door handle challenges, Atmel has developed a hardware PTC module coupled with the QTouch software library. Thanks to this combined hardware and software solution, Atmel can deliver a hardware capacitive acquisition system with a small footprint library, fully configurable by software. The solution also requires very limited CPU usage and thus very low power consumption.

In answer to the various system requirements, Atmel has integrated this hardware PTC module and software library in a large number of automotive-qualified microcontrollers. This provides a single-chip solution that answers both the capacitive proximity detection function and the other communication and application tasks.

Figure 2 shows two examples of door handle systems with different system requirements. Both can be fulfilled with a single Atmel MCU embedding the same PTC and QTouch library, but with different processor cores, package sizes, memory footprints and communication interfaces.

Figure 2: Two examples of door handle systems with different system requirements.

Technical introduction to Atmel PTC and QTouch library
Figure 3 shows the main functions inside the PTC hardware module.

Figure 3: The main functions inside the PTC hardware module.

Input control: this module multiplexes the multiple sensor channels inputs. In mutual-capacitance mode, both X (driving signals) and Y (receiving signals) are used. In self-capacitance mode, only Y (drive/receive signals) are used. It should be noted that the PTC is able to manage both self-capacitance and mutual capacitance in the same application.

Compensation circuit: This circuit selects the compensation capacitor to compensate for the sensing capacitor (calibration feature). It also selects the value of the internal serial resistor.

Acquisition module: this module manages the complete acquisition process. Its main tasks include:
???Charge transfer operation
???Signal amplification
???ADC conversion
???Oversampling management
???Output results for post-processing

Focus on low level PTC and QTouch library features
The complete description of the hardware and software features of the PTC and QTouch library are described in the technical documentation given at the end of this article. We will now examine a selection of the low-level features of the PTC, which are relevant in the case of a typical door handle capacitive proximity detection.

???Timings considerations

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