Capacitive sensing—Integrating multiple interfaces

Capacitive touch sensing is a popular alternative to mechanical switches and knobs in consumer electronics, white goods, and automotive applications. In traditional designs, capacitive touch sensing is implemented using a dedicated IC while advanced user interface features such as proximity sensing, LED dimming, haptics and liquid level detection are implemented with one or more separate IC’s. Using multiple IC’s increases board size, BOM cost, and time-to-market. In this article we discuss how to design advanced user interfaces using a single SoC.

Part I of this article gives a brief overview of capacitive touch sensing and talks about different feedback mechanisms. In this section We will demonstrate how to integrate multiple user interface features using a single SoC and how to overcome some of the problems caused by this integration. In Part II, we will discuss how capacitive sensing used for application beyond touch sensing. Here we will discuss the following

• Proximity Sensing: This technology is being widely adopted across many applications. We will briefly discuss the technology and the challenges faced during implementation.
• Liquid Level Sensing: This feature is a step beyond the user interface, but it too can be implemented using capacitive sensing technology. We will explain the benefits of using the capacitive sensing method as well as how to overcome common issues with this method.

The basics

Capacitive touch sensors consist of copper pads connected to capacitive sensing controller input pins with traces. Figure 1 shows a typical capacitive sensor.

When a finger is not touching the overlay, the capacitive sensing controller measures some parasitic capacitance (C_P) as shown in Figure 2. C_P is the sum of the distributed capacitance on the sensor.

When a finger comes in contact with the overlay it forms a simple parallel plate capacitor called finger capacitance (C_F). In the presence of a finger the total sensor capacitance (C_X) is defined by Equation 1.

The capacitive sensing controller monitors the sensor capacitance by converting the measured capacitance into a digital value called Raw Counts. The Raw Counts value is used to detect the presence of a finger on, or near, the sensor. Figure 3 shows the block diagram of a capacitive touch sensing pre-processing circuit.

Feedback Mechanisms

Adding feedback mechanisms to your user interface differentiates your end product and makes the user’s interaction with your product complete. Haptic (tactile), visual (LED, LCD), and audio (buzzer) are the most common types of feedback. Multiple types of feedback can be used in a single user interface. In the past, adding feedback to a user interface meant using multiple IC’s in your design, adding BOM cost and board size. A SoC can integrate all of these features into a single chip.

Haptic Feedback

When you touch a button on your cell phone screen, you feel a vibration. This is an example of haptic feedback. The vibrations are created by an amplifier connected to an actuator (DC motor) with Eccentric Rotating Mass (ERM). Haptic feedback enables capacitive touch sense buttons to feel like they press and release and improves the usability of sliders, scrolling lists, and list end stops. Also, haptic feedback gives capacitive touch sense buttons an edge over mechanical buttons by allowing you to select different tactile feedback for each button. Figure 4 shows a user interface designed with two controllers, one for capacitive sensing and one for haptics drive control.

This design includes:

User Interface – An overlay surface that senses a finger touch.

Capacitive Sensing Controller – A microcontroller that interprets the touch input signals from the user interface. It captures analog signal information and converts it into digital information. It then sends this information to the host.

Host or Embedded Application – The host or embedded application determines what the user is touching and commands the haptics drive controller to send the appropriate signal to the actuator hardware.

Haptics Drive Controller – A microprocessor that enables the amplifier and drives a PWM signal based on the host device’s input.

Amplifier – The amplifier drives the haptics actuator with a differential output.