Capacitive Touch Sensor System (Second-Generation Capacitive Touch Technology)

Renesas' Solution for Capacitive Touch Development:
All-In-One Microcontroller and Easy-to-Use Evaluation Kit Maximizes Development Efficiency

The usability and quality of a human machine interface (HMI)—the medium through which a human interacts with a machine—has a large influence on product value. The adoption of touch keys in HMI of embedded devices is rapidly expanding. Capacitive touch technology can provide an intuitive user interface, allowing an overlay to be manipulated with the touch of a finger or quantity to be selected by a swipe motion, and so on. This page presents solutions from Renesas that help lower hurdles in touch system development.

Capacitive Touch Sensor System (Second-Generation Capacitive Touch Technology)

This is the "touch" era!
Capacitive touch sensing technology is being rapidly adopted in embedded devices

The adoption of capacitive touch sensing technology is expanding rapidly. Elegant, smooth, and intuitive interface designs with no uneven surfaces have become the natural choice for end users. Touch sensor interfaces are also easy to wipe clean, so they have become popular in home appliances/white goods and healthcare devices.

Furthermore, they have been the subject of increasing attention for their high resistance to environmental factors because the electrodes are situated underneath a resin overlay and not exposed, thus preventing moisture and dust from entering the system, and for their long usage lifetime because of lack of mechanical weardown of moving parts. Thus, adoption of capacitive touch sensing technology in industrial equipment used in buildings, factories, and so on is also rapidly increasing. The growth of this market is expected to accelerate (Figure 1).

Figure 1. Use of touch-key technology in embedded devices is accelerating

Features of Renesas' second-generation capacitive touch sensing technology
High-touch sensitivity and high noise immunity! Supports both self and mutual-capacitance methods

In order to support the vast variety of design, material, and overlay shape demands, changes in capacitance need to be detected with high sensitivity. Renesas has succeeded in vastly improving sensitivity and noise immunity by developing a circuit for converting electrostatic capacitance into current and amplifying and digitizing this value. This solution has almost zero effect from noise.

The conventional recommended thickness for an acrylic overlay is 2 millimeters (mm) or less. However, the Renesas second-generation capacitive touch technology can sense the proximity of a fingertip even when using a 10 mm thick acrylic panel, and passes the strict electromagnetic noise testing stipulated in the IEC 61000-4-3/4-6 level 3 standards. The technology also allows touch panels to be operated even when the user is wearing gloves or proximity sensing where there is no actual touch. Touch can be detected even if there is a small layer of air between the overlay and the electrode.

The technology also supports the mutual-capacitance method, which prevents erroneous detection when the surface is wet. By supporting both self and mutual-capacitance touch sensing methods, the Renesas capacitive touch sensor solution allows the developer to build a highly quality user interface that provides a better end-user experience.

Capacitive touch sensing methods

(1)Self capacitance

A touch button detects the change in electrostatic capacitance produced between an electrode and a human body (fingertip). Conventional touch sensor systems use the self-capacitance detection method. In this method, the electrostatic capacitance between a single electrode and a human body (fingertip) is detected and used to determine whether a touch event is recognized. There is only a single electrode, so the structure is simple (Figure 2).

The circuit containing the electrode has a fixed electrostatic capacitance (parasitic capacitance) between the electrode and the ground. When a fingertip is in close proximity to the electrode, new electrostatic capacitance is produced. In the self-capacitance method, this additional electrostatic capacitance is detected and is used to determine whether a touch has occurred.

In this method, the electric capacitance also increases if liquid comes in contact with the operating surface. Thus, accurate detection is difficult if the surface is wet.

(2)Mutual capacitance

In the mutual-capacitance method, a transmission node and a reception node are used to generate an electromagnetic field, and changes in the electromagnetic field between these nodes are detected (Figure 3). With this method, liquid that comes into contact with the operating surface has almost no effect on the electromagnetic field. Thus, this method can be used even in environments where the operating surface is likely to get wet. In addition, in a self-capacitive touch sensor system where electrodes are arranged in a matrix (grid), a false ghost detection occurs if two or more points are touched at the same time. A mutual-capacitive touch sensor system does not have this issue. Thus, a mutual-capacitive system uses a small number of pins to configure many electrodes, and supports multitouch behavior and more sophisticated operations than simple on (touch) and off (not touching).

In the mutual-capacitance method, a pulse is applied to the transmission node to generate an electromagnetic field between it and the reception node. When a fingertip comes into close proximity, part of the electromagnetic field moves to the fingertip, and the strength of the electromagnetic field detected by the reception node decreases. The electrostatic capacitance also decreases. This drop in capacitance is detected and captured, and recognized as a fingertip being in close proximity.

The mutual-capacitance method requires two electrodes to generate an electromagnetic field, so designing using this method is more complex than designing using the self-capacitance method.

Figure 2. Self-capacitive method of detection

Figure 3. Mutual capacitive method of detection

Tool for automatically adjusting touch sensitivity

Touch keys have a large influence on the physical design of a product, and there are several issues that need to be overcome during development.

For example, if the overlay of the end product will not be flat, there will be a layer of air between the overlay and the electrode, which will reduce touch sensitivity. Touch sensitivity is also an issue if materials other than glass or acrylic are used for the overlay, or if a curved surface is planned.

Parasitic capacitance, which influences the sensitivity of the touch keys, is affected by not only the electrode but also the shape of the circuit board. Therefore sensitivity parameters need to be adjusted by actually setting up a board with the electrode and wiring and measuring the parasitic capacitance.

If the level of sensitivity is found to be unacceptable at this stage, the design needs to go back and rethink the design and shape of the touch interface of the product. Furthermore, whenever a change in the design occurs the sensitivity parameters need to be readjusted. Without an easy way to adjust parameters, the amount of time required for development will inflate.

(1)Workbench6 tool for automatically adjusting touch sensitivity

Workbench is a tool for automatically adjusting touch sensitivity that makes it easy to adjust the touch sensitivity of implementations that use Renesas' second-generation capacitive touch sensing technology.

Workbench6 is GUI-based tool. It automatically measures the parasitic capacitance, etc. of an electrode on a capacitive touch evaluation board to set optimum parameters. Also, the developer can set the touch detection threshold by actually touching the electrode. Changes in capacitance as a finger approaches an electrode are displayed in a graph, so the developer can easily set the threshold value. The tool also allows the developer to visualize the position on a slider or wheel where touch is being sensed,so you can follow along with where your finger is on the panel.

The biggest advantage of using Workbench6 is the ability to try out capacitive touch operation and apply the results to software immediately. In contrast, in the traditional development process the developer has to perform measurements, calculate parameters, adjust the parameters in the source program, and then build. Workbench6 automates a significant portion of this process, so the time required for development can be reduced dramatically (Figure 4). It enables users without specialized knowledge or experience with development to implement a capacitive touch sensing system.

Please see this introduction to the features of Workbench6.

(2)Capacitive touch evaluation kit enables you to start developing immediately

This evaluation kit contains a CPU board, capacitive touch evaluation boards with electrodes for several self and mutual-capacitive touch sensing patterns, Workbench6, and evaluation software.

Since electrode shape differs depending on whether self capacitance or mutual capacitance is used, developers need to build boards depending on which method they are using. This evaluation kit contains four types of boards for evaluating various capacitive touch implementation scenarios. You can also evaluate water resistance, proximity, and so on using this kit. This evaluation kit helps reduce the time needed to design electrode shape, enabling users to start trying out touch interfaces right away. Development time can be greatly reduced.

Technical documents and detailed information on development tools and so on for capacitive touch sensing system (second-generation) are available.