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Address issues in capacitive-touch interface design (Part 2)

Posted: 05 Sep 2011 ?? ?Print Version ?Bookmark and Share

Keywords:capacitive sensors? Parasitic capacitance? tuning?

The performance of capacitive sensors is greatly dependent on physical property/characteristics of the sensor board and environmental/operating conditions. For example, sensor performance is affected by sensor capacitance changes due to PCB manufacturing process variations, whenever overlay material or thickness changes, or the PCB vendor changes.

The challenges do not stop here. Parasitic capacitance also varies with environmental conditions (noise floor) such as temperature and humidity. Thus, a board tuned in the Alps may not work in the hot and humid climate of Hong Kong, resulting in more time and labor to tune the board again. To minimize yield losses due to process variation or vendor change, tuning needs to take into expected differences based on statistical analysis.

There may also be reasons when the board layout needs to be redone such as changing a button size, moving traces on the PCB to incorporate minor changes in the schematic, resizing the PCB to address different EMC/EMI issues, and so on. All of these modifications require that sensors be tuned again. Moreover, the tuning process needs a communication protocol and a host-side application to observe and analyze the raw sensor data. Extra I/O will be needed since tuning must be done after the final board has been made, creating potential issues for systems with pin constraints.

capacitive sensing system

Figure 1: Here's aself-tuning-based capacitive sensing system.

It is clear that tuning is not an easy job to do, requiring significant expertise and experience with the chips involved and an understanding of capacitive sensing effects at very low signal levels. When coupled the time-to-market constraints of the appliance market, tuning can impose significant delays and increases to system cost.

To cost-effectively handling design constraints and market needs, tuning is most efficiently implemented to be handled by the appliance itself. An ideal self-tuning system will perform this task as shown in figure 1.

In systems capable of self-tuning capacitive sensing, there will be numerous algorithms to achieve a workable touch-sense system. At a basic level, self-tuning done by the appliance is no different from manual tuning. Looking at figure 1, it can be seen that some tasks are done once upon power up (one-time compensation) while other must be performed continuously (dynamic compensation).

One-time compensation
Self-tuning capacitive sensing systems must calculate the best parameter settings for the sensors based on the appliance and expected operating environment.

Clock: The capacitive sensing systems referred to in this article are based on switched-capacitor theory. In contrast, a physical sensor capacitor is emulated to form a resistor by charging and discharging the sensor capacitor in consecutive cycles. Emulated resistance is proportional to the sensor capacitor value and is measured using a current source in conjunction with an analog-to-digital conversion stage to compute the actual value of sensor capacitor.

Proper emulation of resistance requires the sensor capacitor to be charged and discharged at a frequency that provides enough time for the capacitor to charge and discharge fully. Therefore, the switching frequency should be adjusted in accordance with the absolute sensor capacitance and should be reduced if the sensor capacitance is higher.

Resolution: Since this system converts the capacitance of the sensor to counts, the smallest change in capacitance which can be measured is dependent upon the resolution of system. The required resolution can be calculated using the parasitic capacitance and required sensitivity.

Scan Time: This is one of the most important turning parameters from a system specification point of view. However, the noise added to the system will increase as the resolution of scanning is increased. To compensate for this increase in noise, the scan time of sensors must be "stretched" to integrate the noise and reduce its effect on capacitance measurements.

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