Sense capacitance
The capacitive sensor consists of a conductor piece, a ground plane, and a controller. In most applications, a piece of copper is used for the conductor piece and the ground is filled with a fill. There is a native (parasitic) capacitance (CP) between the two. When other conductive objects such as fingers approach the sensor, as the capacitance value (CF) of the object increases, the capacitance value of the system also increases. (Figure 1)
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There are several ways to detect an increase in the capacitance caused by CF. In the Field Effect measurement method, an AC voltage divider is used between the sensing capacitor and the system reference capacitor. A change in the capacitance value produced by a finger touch can be sensed by monitoring the change in current on the voltage divider. Charge Transfer uses a switched capacitor circuit and a reference bus capacitance value to repeat the charge transfer step 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, so the voltage value is measured after a fixed number of steps, or the number of steps required to reach a certain voltage threshold is determined. Capacitance value. In addition, the relaxation oscillator is a method of measuring the charging time, wherein the charging rate is usually determined by the value of the fixed current source and the value of the sensor capacitance. Larger sensor capacitors require longer charging times, which are typically measured using pulse width modulators (PWMs) and timers. As for the Continuous Approximation, it is also a method of measuring the charging time of a capacitor, except that the starting voltage is determined by a continuous approximation.
The continuous approximation performed by the PSoC component (the patented Cypress application) uses a set of capacitor-to-voltage converters and a single-slope analog-to-digital converter (ADC). The capacitance value is measured by first converting the capacitance value to a voltage value, then storing the voltage value in the capacitor, and then measuring the stored voltage value by using the adjustable current source. Among them, the capacitance value versus voltage value converter utilizes a switched capacitor technology, which allows the sensor capacitor to reflect the corresponding voltage value according to its capacitance value. The frequency used by the switched capacitor is generated by the oscillator inside the PSoC itself.
The sensor capacitor is connected to the analog multitasking bus and is charged using a programmable current output digital-to-analog converter (iDAC) that also connects to the bus. The charge on each bus is q=CV. When SW2 is open and SW1 is closed, the potential across the CX is zero and the amount of power on the bus is reduced. The reduced value is proportional to the capacitance of the sensor. This charging and discharging action will be repeated all the time, and the sensor capacitor will also become the current load on the bus. (Figure 2)
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With the circuit operation of the switched capacitor, the iDAC determines the amount of constant voltage on the bus in a binary search. This voltage value affects the switching frequency of the switched capacitor, the sensor capacitance value, and the current value of the iDAC. The bus is also equivalent to a bypass capacitor that stabilizes the final voltage. Additional capacitors can be added to the bus to adjust the behavior and timing of the circuit.
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The calculated iDAC value is then used again to charge the bus and measure the charging time required for the bus from the initial voltage to the comparator's threshold voltage. The initial voltage is in the absence of a finger touch, so the charging time can be determined in advance. When the finger touches the sensor, it increases the value of CX and lowers the initial voltage, thus prolonging the charging time measurement. (as above formula and Figure 3)
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Construct sensor
Capacitive sensors come in a variety of styles and functions, and can be used in a wide variety of media, from simple to complex. It is the application's own needs that determine the sensor construction and construction details. The most common sensor styles are buttons and sliders. The button is actually a large conductor piece connected to the controller. The measured capacitance value is compared with a series of critical values, and the measurement result can also be obtained by digital output, or other analog characteristics can be used to further sense the touch.
Dynamic pressure or finger area. As for the slider, many conductor pieces are arranged in a straight line or a radial arrangement. The position of the contact can be determined using an algorithm that calculates the centroid, and the resolution is much larger than the number of stitches used for sensing. Most simple capacitive sensors like buttons or sliders use copper to deposit onto printed circuit boards. However, other substrate materials and deposition media can be used to make circuits, such as highly conductive silver ink. (Figure 4)
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The buttons or touch areas of the dynamic user interface can be configured with any display style. This type of display has a smoother and more intuitive interaction that creates a better user experience. It is more complicated to construct such a system than a simple button or slider. The projected capacitive touch screen adds a transparent conductive material to the display. The conductive surface is deposited by deposition onto a substrate such as a glass or PET film and attached to a control circuit which is then adhered between the touch surface layer and the display. The trigger area is measured in the same way as the slider. The longitudinal and lateral sets of sliders are interlaced to cover the entire display area, and the sliders in these two directions detect the touch position and output x-axis and y-axis data. Since the projected capacitive touch screen has a cover layer above it, it also protects the screen from direct impact, bending, environmental factors and the like that are common to conventional resistive touch screens.
A proximity sensor is basically a large button. The purpose of the proximity sensor is not to detect the exact location of the conductive object, but whether the object is nearby. Since there is no need to know the exact location of the object, the reaction time can be slightly slower (3-4ms vs. 250us). The proximity sensor is much more sensitive; it is designed to even reach a distance of 30 cm. Also, since the proximity sensor does not need to incorporate any display pattern, there is more flexibility in the placement position in the device. Whether it's a copper coil outside the control board or a wire behind the cover, a very basic and cost-effective proximity sensor can be built. (Figure 5)
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Using capacitive sensors
Capacitive sensors are increasingly used. The flexibility, durability, and simplicity of these sensors have created new opportunities for many designers. The basic menu browsing and clicking functions still use the button mode, but with the analog potentiometers, the buttons with analog features can create more simple, cost-effective, reliable and safe functions.
The LG LA-N131DR Air Cleaner uses five capacitive sensors on the buttons on the panel display menu. These buttons allow the designer to design a smooth body with a user interface. The capacitive button detects the presence or absence of a finger touch through the four-mm glass.
The control circuit is built on the side of the double-layer printed circuit board without the sensor. LG uses a PSoC mixed-signal array to control the sensor and output the status to the main device processor. (Figure 6)
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The proximity sensor has a reactive backlight function, which is mainly for night operation or safety considerations. Most of these situations require larger triggering components, such as adult hands or metal cans, to achieve a manageable range. Proximity sensors, buttons, sliders, and even touch screens can all be controlled with a single PSoC processor. The firmware routine can make changes to the state according to user input or host commands.
Create a capacitive sensing application for you
The PSoC mixed-signal array contains an array of configurable digital and analog resources, flash memory, RAM, 8-bit microcontrollers and many other functions. These features allow PSoC to implement innovative capacitive sensing technology in its CapSense family of products. The device can be configured and reconfigured using PSoC's intuitive development environment to meet design specifications or any specification changes. The emergence of new sensing technologies has improved sensing sensitivity and noise immunity, and reduced power consumption and increased upgrade rates, allowing designers to create better applications.
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