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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?

Many parameters have to be considered to define the timings of a door handle proximity detection system. These include:
???The sensor capacitor value
???The serial resistor value (internal and external)
???The clock of the hardware PTC module for signal acquisition (PTC_CLK)
???The Charge Share Delay parameter of the PTC (CSD)
???The clock of the CPU for signal processing (CPU_CLK)

The sensor capacitor value is dependent on the sensor size, the layout parasitic capacitance, and the optional shield layer. This capacitor is automatically compensated by the internal compensation circuit of the PTC and has to fit within its compensation range.

The serial resistor can be internal and, optionally, external.

The main considerations are:
???Large sensor capacitor generally leads to better proximity detection
???Large serial resistor leads to a better low-frequency noise filtering
???Large capacitor and resistor lead to a larger charge time constant, which will increase the response time and power consumption.

The flexible clocks options are useful for finding the best trade-off between detection performance and power consumption.

The PTC_clock is used to perform the charge transfer sequence and the ADC conversion. It has to be slow enough to ensure a full charge transfer operation, and fast enough to minimise ADC conversion time. To find the best balance in between, the Charge Share Delay (CSD) setting can also be used. This CSD adds additional cycles delay during the charge transfer sequence, but not during the ADC sequence. It allows the selection of a faster PTC_CLK to speed up the ADC conversion, while slowing down the charge transfer process according to the charge time constant of the system.

The CPU_CLK is used to post process the data output by the PTC. It is a digital process managed by the CPU of the micro. The selection of the CPU_CLK may depend on the maximum peak current specification and maximum average current specification. A high CPU_CLK decreases the average current consumption but increases the peak current consumption.

Gain and oversampling considerations
The PTC module embeds a gain stage and an oversampling/filtering feature. Increasing the gain stage leads to an increase of not only the signal amplitude but also the noise level. Increasing the number of samples allows the design to average the signal and increase the SNR, but also to increase the acquisition time.

The table shows the resulting signal values depending on the filter_level (i.e., the number of acquired samples, from 1 to 64) and the gain values (i.e., the amplification, from 1 to 32). The resulting signal values are a combination of averaging and amplification of the signal in order to give the best sensitivity and SNR trade off.

Table: The resulting signal values depending on the filter_level.

In a door handle system, it may be of interest to increase the gain settings to improve the capacitive detection distance, and to increase the filter level to improve the stability and reliability of the signal under all conditions. On top of these considerations, the filter level also has to be minimised to keep the power consumption as low as possible.

Capacitive detection in a door handle system is a demanding design challenge. To overcome this challenge, Atmel has introduced a new family of automotive MCUs that embed a Peripheral Touch Controller hardware module and QTouch software library.

About the author
Gilles Robert is Senior Field Application Engineer at Atmel. He specialises in Automotive Touch Solutions.

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