Application of the hottest capacitive touch sensin

2022-08-11
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The application of capacitive touch sensing technology in home appliances

recently, the application of various exciting capacitive sensing technologies in portable media players, notebook computers and the market makes people almost forget that this kind of interface technology has been widely used in the design of home appliances for many years. Significant advances in sensing algorithms and control circuits have made this technology applicable to more application fields. Designers have seen the value of capacitive sensing technology, which can not only replace mechanical keys and diaphragm switches; It can also be applied to various novel applications, such as touch screen and short-range sensor

sensing capacitance

capacitive sensor is composed of conductor, ground and controller. In most applications, the conductor sheet will be made of a copper circuit board, and the grounding will be filled with pouring. There is a native (parasitic) capacitance (CP) between the two. When other conductive objects such as fingers approach the sensor, as the capacitance (CF) of the object increases, the capacitance of the system also increases. (as shown in Figure 1)

to detect the Timken experimental machine method is one of the important methods to measure the extreme voltage function. There are several methods to increase the capacitance value caused by CF. In the field effect measurement method, an AC voltage divider is used between the sensing capacitor and the system reference capacitor. By monitoring the change of current on the voltage divider, the capacitance change caused by finger touch can be sensed. Charge transfer uses the switching capacitor circuit and the reference bus capacitance value to repeat the charge transfer steps from the smaller sensor capacitor to the larger bus capacitor. There is a proportional relationship between the voltage value on the bus capacitor and the sensor capacitance value. Therefore, the voltage value is measured after a fixed number of steps, or the capacitance value is determined by calculating the number of steps required to reach a certain voltage critical value. In addition, the relaxation oscillator is a method to measure the charging time. The charging rate is usually determined by the value of the fixed current source and the capacitance of the sensor. Larger sensor capacitors require longer charging time, which can usually be measured by pulse width modulator (PWM) and timer. As for the successful approximation method, it is also a method to measure the charging time of the capacitor. The difference is that the starting voltage is determined by the continuous approximation method

the continuous approximation method (patented by cypress) performed with PSoC components uses a set of capacitor to voltage converters and a single slope analog-to-digital converter (ADC). The capacitance measurement method is to convert the capacitance value to the voltage value, then store the voltage value in the capacitor, and then use an adjustable current source to measure the stored voltage value. The capacitor to voltage converter uses the switching capacitor technology. This circuit system allows the sensor capacitor to reflect the corresponding voltage value according to its capacitance value. The frequency used by the switching capacitor is generated by the oscillator inside the PSoC itself

the sensor capacitor is connected to the analog multitask bus and charged by using the programmable current output digital analog converter (IDAC) which is also connected to the bus. The charging power on each bus is q=cv. When SW2 is open circuit and SW1 is closed circuit, the potential across both ends of Cx is zero, and the power on the bus will be reduced, and the reduced value is proportional to the capacitance value of the sensor. This charge and discharge action will be repeated all the time, and at this time, the sensor capacitor will also become the current load on the bus. (as shown in Figure 2)

by the circuit operation of the switching capacitor, IDAC will determine the constant voltage value on the bus in the way of binary search method. This voltage value will affect the switching frequency of the switching capacitor, the capacitance value of the sensor, and the current value of the IDAC. The bus is also equivalent to a bypass capacitor, which can stabilize the final voltage. Additional capacitors can also be added to the bus to adjust the behavior and timing of the circuit

the calculated IDAC value is then used to charge the bus again, and the charging time required for the bus from the initial voltage to the critical voltage of the comparator is measured. The initial voltage is without finger touch, so the charging time can be measured in advance. When the finger touches the sensor, it will increase the value of Cx and reduce the initial voltage, so it will prolong the charging time measurement. (the above formula and figure 3)

construction sensor

capacitive sensors have a variety of types and functions, which can use a variety of media, and the implementation styles range from simple to complex. It is the requirements of the application itself that determine the details of sensor construction and construction. The most common sensor styles are keys and sliders. The key is actually a large conductor connected to the controller, in which the measured capacitance value will be compared with a series of critical values, and the measurement results can also be obtained by digital output, or other analog characteristics, to further sense the pressure of touch

or the current finger area. As for the slide bar, it is composed of many conductor sheets arranged in a straight line or radial shape. Using the algorithm of calculating the centroid, the contact position can be determined, and the resolution is much larger than the number of pins used for sensing. Most simple capacitive sensors, such as buttons or slide bars, are deposited onto printed circuit boards using copper. However, other substrate materials and deposition media can also be used to make circuits, such as high conductivity silver ink. (as shown in Figure 4)

the key or touch area of the dynamic user interface can configure its display style arbitrarily. This kind of display has smoother and intuitive interactive operation, creating a better user experience. It is more complicated to construct such a system than the simple keys or sliding bars. The projection capacitive touch screen adds transparent conductive substances to the display. This conductive surface is attached to substrates such as glass or PET film by deposition and connected to the control circuit, and then the substrate is adhered between the touch surface and the display. The trigger area is measured in the same way as the slide bar. The vertical and horizontal sliding bars are staggered to cover the whole display area, and the sliding bars in these two directions will detect the touch position and output x-axis and y-axis data. Because there is another coverage above the projected capacitive touch screen, which is equivalent to the reduction of 70million vehicle layers on the highway in one year, it also protects the screen from direct impact, bending, environmental factors and other common damage to the traditional resistive touch screen

proximity sensors are basically large keys. The purpose of the proximity sensor is not to detect the exact position of the conductive object, but whether the object is nearby. Since the exact position of the object does not need to be known, the reaction time can be slightly slower (ms vs. 250us). The sensitivity of short-range sensor is much higher; Properly designed, it can even reach a distance of 30cm. Also, because the proximity sensor does not need to be combined with any display graphics, the placement position in the device has more flexibility. Whether the copper coil outside the control circuit board or the wire behind the covering layer, a very basic and cost-effective short-range sensor can be built. (as shown in Figure 5)

using capacitive sensors

capacitive sensors are increasingly used. The elasticity, durability and simplicity of the above sensors have created new opportunities for many designers. The basic menu browsing and clicking functions still use the key mode, but using the affordable potentiometer, which has analog characteristics, can build more simple, cost-effective, reliable and safe functions

lg la-n131 and the employment mechanism of private enterprises is more eager for quick success. Dr air purifier uses five capacitive sensors on the buttons for menu browsing of panel display. These buttons allow designers to design a smooth body with a user interface. Capacitive buttons detect finger touch through 4mm glass

the control circuit is built on the side of the double-layer printed circuit board without sensors. LG uses PSoC mixed signal array to control the sensor, and outputs the status to the main device processor. (as shown in Figure 6)

the proximity sensor has a reactive backlight function, which is mainly for night operation or safety considerations. Most of these situations require larger trigger components, such as Cheng? Hands or metal cans, there is a way to reach the controllable range. Close range sensors, buttons, sliders, and even touch screens can be controlled by PSoC's single processor. Firmware routines can change the state according to user input or host commands

create capacitive sensing applications for you

psoc mixed signal array contains an array of Configurable Digital and analog resources, flash memory, ram, 8-bit microcontroller and other functions. These features enable PSoC to realize innovative capacitive sensing technology in its CapSense series products. Using the intuitive development environment of PSoC, the device can be configured and reconfigured to meet the design specifications or any specification changes. The emergence of new sensing technology improves the sensing sensitivity and anti noise ability, reduces power consumption and increases the upgrade rate, allowing designers to create better application products

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