Electromagnetic flowmeter analysis capacitive touch sensor promotion application design

Suddenly, capacitive sensors are everywhere. They’re installed in car seats to control airbag deployment and seatbelt pretensioners, in dishwashers and dryers to correct the state of rotating drums, and even in refrigerators to control automatic defrosting. But until now, their biggest potential application has been in touch switches, which are increasingly appearing in consumer electronics.

Because mixed-signal IC processes are widely adopted, this technology allows chip designers to optimize the analog and digital subsystems of the chip to build capacitive sensors with unprecedented sensitivity and durability, and at a cost that is unmatched by mechanical switches.

how to work

Capacitive sensors can basically be divided into three categories: electric field sensors, relaxation oscillator-based sensors, and charge transfer (QT) devices. Electric field sensors usually generate a sine wave of several hundred kHz, then add this signal to the conductive disk of one plate of the capacitor and detect the signal level on the other conductive disk. When the user's mobile phone or another conductive object touches the two plates, the signal level at the receiver will change. By demodulating and filtering the signal on the plates, it is possible to obtain a DC voltage that varies with the change in capacitance; applying this voltage to the threshold detector can generate a touch/no touch signal.

The relaxation oscillator uses an electrode disk, on which the electrode capacitance forms the variable timing unit in the sawtooth oscillator. By feeding a constant current into the electrode line, the voltage on the electrode increases linearly with time. This voltage is provided to an input of a comparator, and the output of the comparator is connected to a ground switch connected in parallel with the electrode capacitance. When the electrode capacitance is charged to a predetermined threshold voltage, the comparator changes state and realizes the switching action - discharging the timing capacitor and opening the switch, and this action will be repeated periodically. As a result, the output of the comparator is a pulse train whose frequency depends on the value of the total timing capacitance. The electromagnetic flow meter sensor reports the touch/no touch status according to different frequency changes.

QT devices use a physical principle called charge retention. For example, a switch applies a voltage to the sensing electrode for a short period of time to charge it, then the switch is turned off, and a second switch releases the charge on the electrode to a larger sampling capacitor. The touch of a human finger increases the capacitance of the electrode, resulting in an increase in the charge transferred to the sampling capacitor, which changes the sampling capacitor and thus allows the detection result to be obtained.

QT devices perform digital signal processing after burst mode sampling. This approach provides higher dynamic range and lower power consumption than competing solutions, and the automatic calibration routine can compensate for drift caused by changing environmental conditions. More importantly, this method is sensitive enough that it does not require a reference ground connection when current passes through thick panels, making it suitable for battery-powered devices. Generally, multiple input channels can be used to implement sliding buttons or rotary buttons, while the dedicated QT series chips can achieve high-resolution linear sliding or rotary interfaces with only three channels with 7-bit (128-point) resolution.

Other possibilities

Many designers are using QT chips to replace resistive touch screens. Because this method only requires a single transparent layer to be laid on the screen for sensing, the absorption of light is greatly reduced compared to multi-layer resistive technology. OEMs also use multi-channel sensors to implement programmable opaque touch surfaces, and the configuration of the panel is adjusted by software, which can help reduce material costs. The same approach also provides users with the possibility to configure the touch screen according to their personal preferences. Users can download specifications from a network server or run a configuration program themselves.

This technology opens the door to many applications, such as electromagnetic flow meters using plane position detection QT devices, and end users can enter Chinese or other complex characters by moving on the numeric keypad of a mobile phone. However, the largest application of this technology is the plane coordinate touch screen, in which the lower surface sensing film can replace the resistive screen, and the single-layer film obtains high transparency and low cost, without the need to open holes in the front panel.