EIT week35-2 Capacitive Touch Sensing Circuit and Signal Processing

Capacitive Touch Sensing Circuit and Signal Processing

Overview

• Course: 6.810 Engineering Interaction Technologies
• Professor: Michael Wessely, Interactive Matter Lab

Build Capacitive Touch Sensing Circuit & Measure Touch Signals

Circuit Configuration

• Capacitor Components: Consists of two conductive plates, representing the touch button and the user’s finger.

时间轴
|-----充电阶段-----|----放电阶段----|-----充电阶段-----|----放电阶段----|
    (电容充电)         (电容放电)          (电容充电)         (电容放电)

Basic Working Principle

• Voltage Discharge Process: \(\begin{equation} \text{"send" pin outputs a positive } 3.3V \text{ pulse, "receive" pin measures the discharge time.} \label{eq:voltage_discharge} \end{equation}\)

• Discharge Time Analysis:

  • The closer the user’s finger and the more overlap with the touch sensor, the slower the overall discharge time due to increased capacitance.
  • Discharge time (Tau, ( \tau )) is affected by resistance (R) and capacitance (C): \(\begin{equation} \tau = RC \label{eq:tau} \end{equation}\)

Processing Touch Signals

Converting Discharge Time into Touch Signals

• Thresholding: \(\begin{equation} if (capacitance > 100) touch = true; \label{eq:touch_condition} \end{equation}\)
  • Adaptive Thresholding: Adjusts threshold based on running mean or median to reduce noise and false positives.

Signal Noise and Solutions

• Problem: Simple thresholding may interpret noise as touch.
• Adaptive Thresholding Approach:
  • Uses a running filter (mean or median) to dynamically adjust the threshold: \(\begin{equation} \text{if (running_filter_new > running_filter_old + threshold) \{ touch = true; \}} \label{eq:adaptive_threshold} \end{equation}\)

Filters and Window Size

• Running Mean and Median Filters:
  • Running Mean Filter: Suitable for data without massive spikes, using a window of size n.
  • Running Median Filter: Effective when single values show massive spikes.

Example Configurations

• Examples of Adaptive Thresholding:
  • Demonstrations show how mean and median filters handle fluctuations and touch events.

Additional Sensors Enabled by Capacitive Sensing

• Types of Sensors:
  • Movement: Buttons, sliders, dials, multi-touch pads, pressure.
  • Other Motions: Proximity, folding, rotation, linear elongation, shear, shape.