EIT week36-1 Multi-Touch Pad Technology

Multi-Touch Pad Technology

Overview

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

Multi-Touch Pad Design

• Components: Consists of multiple layers, typically two, which are insulated from each other to prevent direct electrical contact.

Capacitance Sensing Methods

Self Capacitance

• Function: Measures the capacitance of a single electrode with respect to the ground (user).
  • Detection Method: Senses touch by individually scanning each row and column, determining touch locations by intersections.

Coupled Capacitance

• Configuration: Measures the capacitance between pairs of electrodes, sensing touch between a transmitter and receiver pair.
• Operating Voltage: Uses a ±3.3V AC signal to charge one plate, with the facing plate showing a phase-shifted signal.
• Finger Interaction: The presence of a finger increases capacitance by acting as a capacitive plate between the electrodes.

Challenges and Solutions

Ghosting Issue

• Problem: Occurs when multiple touches lead to incorrect or ghost touch detections.
• Solution: Time-multiplexing is employed to prevent ghosting by altering the active sending and receiving times of signals to accurately detect multiple touches.
• Sender:
  • This set of electrodes sends (injects) signals into the sensing area, e.g., periodically applying high or low levels (or AC signals, etc.) to “excite” the sensing matrix.
  • Different rows (or columns) are switched to send high or hold low in sequence to scan the entire sensing area row by row (or column by column).
• Receiver:
  • This group of electrodes is responsible for “listening” or measuring the strength of the signal coupled from the transmitter.
  • When a finger touches or approaches the sensing layer, it changes the capacitance or impedance between the transmitter and receiver, and the Receiver detects these changes to determine the location of the touch.
• MCU:
  • On the circuit board, there will be a special touch control chip or MCU (microcontroller) is responsible for the Sender side of the electrodes in order to apply the scanning signal (can be interpreted as a row to send a high level or AC signal, the other rows to maintain a low level or high-resistance state).
  • At the same time, the chip or MCU measures the sense signal on the electrode at the Receiver side.

HP-2

ESP32 微控制器

• 负责控制扫描流程、生成或控制 PWM 信号，以及通过 SPI 接口 读取外部 ADC 芯片 输出的触摸采样数据。
• 同时用于处理、识别触摸位置或手势等逻辑。

Standard Multiplexer（多路复用器）

• 从微控制器（或专用 PWM 引脚）获取发送信号（PWM 信号），根据需要将该信号切换到不同的 TX 引脚。
• 帮助实现行列扫描：一行或几行被激励，其他行保持低电平或高阻，进而完成对触摸阵列的逐行/逐列检测。
• 
  • 4 位进入, 二进制16种情况

Touch Surface（触摸阵列）

• 由交叉排列的 Sender 端（TX） 与 Receiver 端（RX） 电极组成。
• 触摸时，手指会改变 TX 与 RX 之间的电容/耦合强度，进而在接收端引起电压或电流的变化。

ADC 芯片

• 将接收端（RX 引脚）感应到的模拟信号转换为数字信号（经 SPI 向 ESP32 输出），微控制器据此判断是否有触摸发生以及触摸位置。

Applications of Multi-Touch Technology

• Visualization: Using processing software to visualize touch interactions.
• Functional Uses:
  • Light control
  • Music interfaces
  • Messaging systems
  • Busy-hands interactions
  • Gesture and identification on wearable devices (e.g., zippers).

Sensory Capabilities Enabled by Capacitive Sensing

• Motion Types: Proximity, folding, rotation, linear elongation, shear, and shape.
• Touch Interactions: Buttons, sliders, dials, multi-touch pads, and pressure sensors.