Capacitive Touch Broadens Switching Options for Equipment Apps

Today’s homes and workplaces are increasingly pervaded by smart appliances, which are designed to be attractive and reliable, and ideally should support interactions that are as easy and rewarding as those delivered by the smartphone sitting in the user’s pocket.

As new smart appliances like televisions, set-top boxes, audio equipment, white goods, PC peripherals, security panels, and industrial controls enter the market, the design of the user interface and the quality of the user experience delivered have a critical influence on market appeal and, ultimately, sales and revenue. Creating a good impression through the user interface is vital, and designers can choose from a range of technologies, such as conventional pushbutton switches or various types of touch sensors.

Switch or Touch—Pros and Cons

机械开关（例如膜或触觉开关）是一种成熟的技术，它具有低成本，易于可用性和直接集成，需要最小的电子设计或软件工程。这些类型的开关还通过给出触觉反馈来确认已检测到纽扣拼接，从而为用户提供了放心。另一方面，机械开关的某些缺点包括响应时间相对较慢，由于依赖运动部件而可靠性差，并且可能需要额外的噪声抑制或弹跳电路。

On the other hand, a touch sensor can create a more modern impression, and it simplifies construction and assembly. It does away with the panel cut-outs typically needed for installing mechanical switches, and helps simplify the mechanical design of the control panel. The panel also can be sealed more easily against ingress of liquids, such as water. This is a valuable feature for many types of equipment, such as kitchen appliances like coffee machines, and simultaneously enhances reliability as well as facilitating cleaning.

Among touch-sensing technologies in use today, which include capacitive, resistive, or piezoelectric sensing, capacitive sensing is widely used among products in the market today. Capacitive touch sensors can be deposited easily on glass or plastic using a process such as screen printing, making it possible to create a variety of shapes such as buttons, sliders, or rotary selectors. Backlighting is also easy to arrange.

在没有任何机械响应的情况下，正如触觉开关提供的确认致动的，设计人员可以使用光来确认触摸的检测。可以利用这来创造时尚的效果，例如将面板深色保持黑暗直至触摸并使用色彩变化效果或高科技图标。还可以引入谨慎的声音蜂鸣声或叮当声来补充触摸控件。所有这些效果都与触摸敏感的用户界面相吻合。

Sensing Principles

一般,电容式感应检测ing a change in the basic capacitance of the sensor pad upon the approach of the user’s fingertip. In the self-sensing type of detector, the approaching finger effectively adds to the sensor’s capacitance. The change increases the time constant of the touch-pad circuit, which is detected by charge-time measurement circuitry implemented in the touch-sensor controller.

An alternative technique is mutual differential sensing. It detects a reduction in the capacitance formed between the sensor pad and a separate excitation pad as the user’s fingertip comes into proximity with the touch panel. The change in capacitance (ΔC) is detected and converted into a voltage (V_OUT).Figure 1说明用于生成V的差分放大器电路_OUT. This has a linear relationship with ΔC, and is compared with a threshold voltage (V_T). Touch detection is indicated when V_OUTexceeds V_T.

Mutual differential sensing delivers an advantage through improved sensitivity for a large dynamic range. The practical benefits of this include extended flexibility in terms of sensor-pattern design, and tolerance of long sensor trace lengths due to superior cancellation of parasitic capacitances (C_P) between pad and ground. In practice, trace lengths up to 500 mm have been demonstrated successfully.

Touch-Controller Implementation

The touch sensor must have an electronic controller with enough channels to support the required number of touch buttons. Historically, the controller has been implemented either as a standalone application-specific IC, or integrated in a microcontroller. A microcontroller-based solution may use a combination of analog peripherals and software, while others provide dedicated on-chip touch-control functionality such as a charge-time measurement unit (CTMU) for use with self-sensing detectors. The microcontroller vendor may provide free capacitive-touch software IP, which can help simplify code development.

Implementing a standalone controller brings a number of advantages. An IC such as在半导体上LC717A00doesn’t require a host microcontroller, and has several built-in features including noise cancellation and water detection, to prevent spurious responses. In addition, patented automated noise and environmental-change compensation technology is also built in to prevent noise-related malfunctions and compensate effects that can alter sensitivity, such as changes in humidity or accumulation of dirt on the sensor area.

The controller’s internal circuitry features a unique C/V-conversion amplifier, which converts the monitored differential capacitance to an output voltage. No need for programming helps shorten development time, and because no additional external components are required, it minimizes bill-of-materials costs.

Figure 2shows the LC717A00 used in standalone mode for controlling up to eight capacitive touch sensors. The device features a communication port, either I²C or SPI, allowing for convenient connection to a host microcontroller if required.

The controller uses the differential signaling method, which delivers high sensitivity and extends freedom for designers to create control panels in a wide variety of shapes with long connection lengths.Figure 3shows a sample touch-panel design with extremely long trace lengths. The data rate is configurable to over 200 Hz, thus enabling its use in equipment that requires rapid touch response.

The controller’s high signal-to-noise ratio (SNR) increases touch-sensing reliability, even while wearing gloves. Oftentimes, this isn’t possible with self-sensing type detectors. Moreover, the design rules governing the shape and size of senor electrodes, and the effects of air gaps as well as overlay thickness and material, can be relaxed.

Figure 4shows an example of signal and noise levels using real sensor data. In this example, the signal is a 17,000-count difference and the noise is a 160-count difference; therefore, the SNR is 106:1.

Conclusion

电容式触摸user-inte有许多优势rface technology for modern smart appliances. Mutual differential sensing ensures reliable touch detection and allows for greater design freedom when laying out the control panel and selecting materials. A standalone application-specific touch-control IC supports convenient and fast implementation, based on this mutual-differential-sensing technique, and can deliver advantages such as elimination of software design, saving on external components, and acceleration toward project completion.

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