Capacitive touch button solution in induction cooker application

1 Introduction

　　Compared with mechanical buttons and resistive touch buttons, capacitive touch buttons are not only durable, low-cost, simple and easy to install, waterproof and anti-fouling, but also provide functions such as rollers and sliders. However, capacitive touch buttons also have many problems. Because there is no mechanical structure, all detections are small changes in electricity, so they are much more sensitive to various interferences. ST has launched a capacitive touch sensing solution based on the STM8 series 8-bit general-purpose microcontroller platform for home appliance applications, especially induction cooker applications. There is no need to add a dedicated touch chip. Capacitive touch sensing functions can be realized with only a simple peripheral circuit, which is convenient for customers to develop secondary development.

　　2 Solution Introduction

　　ST's capacitive touch button solution detects the capacitance change caused by human touch through the charging/discharging time of a resistor-capacitor network composed of a resistor and the capacitor CX of the sensing electrode. As shown in Figure 1, when a human hand presses, it is equivalent to connecting a capacitor CT in parallel to the sensing electrode, increasing the capacitance on the sensing electrode, and the charging and discharging time of the sensing electrode will increase, thereby detecting the state of the button. The sensing electrode can be directly drawn on the PCB board as a button, roller or slider application style, or it can be made into a spring component inserted on the PCB board, and its detection performance will not be affected even if it is separated by an insulating layer (glass, resin).

　　Figure 1 Working principle of STM8S capacitive touch button

　　The induction cooker heats food using the heating principle of magnetic field induction current. When heating, a strong magnetic field is generated by the coil under the panel. When the magnetic lines of force pass through the bottom of the pot made of a magnetic conductor, the pot cuts the alternating magnetic lines of force and generates eddy currents at the bottom of the pot, causing the bottom of the pot to heat up quickly, thus achieving the purpose of heating food. In this solution, the 44-pin STM8S105S4 is used as the main control chip of the key display board, controlling the scanning of 13 keys, the display of 24 LEDs and a 4-digit digital tube, the communication between I2C and the mainboard, and leaving a SWIM interface for engineers to debug (as shown in Figure 2).

　　The STM8S105S4 uses ST's advanced STM8 core, with a 3-stage pipeline Harvard structure, 3.0~5.5V operating voltage, an internal 16MHz RC that can provide the MCU with a 16MHz operating frequency, low power mode and peripheral clock shutdown function, and a total of 34 I/Os available. The STM8S105S4 has 2KB of RAM and 16KB of FLASH, as well as a 1KB EEPROM data memory with up to 300,000 erase and write times.

　　Figure 2 Principle of induction cooker keypad

　　3 Interference in the working environment of the induction cooker

　　1. Electromagnetic interference

　　When the induction cooker heats the pot, it also induces forward or reverse current on the induction electrodes on the circuit board, which will shorten or increase the charging and discharging time of the keys. This will have a great impact on the detection of the keys and may even cause false operations. Common methods use hardware shielding and zero-crossing detection to eliminate the impact of electromagnetic radiation on the keys.

　　Hardware Shield

　　In the STM8S solution, ST provides design specifications for sensing electrodes and routing and the Driven Shield function as shown in Figure 3 (providing the same driving signal as the button pin on the Shield line, the parasitic capacitance between the electrode and the Shield will not be charged or discharged), which can effectively reduce the impact of the parasitic capacitance of the sensing electrode routing on the button sensitivity.

　　图3 Driven Shield

　　Zero crossing detection

　　1) Hardware zero-crossing detection

　　Zero-crossing point detection can be implemented by hardware. In the hardware design, a zero-crossing point hardware detection circuit as shown in FIG. 4 or FIG. 5 can be added. By judging the key state when the output at the B terminal is a high level, the touch key can be detected when the electromagnetic radiation is minimum.

　　Figure 4 Hardware zero-crossing detection circuit 1

　　Figure 5 Hardware zero-crossing detection circuit 2

2) Software zero-crossing detection

　　Hardware zero-crossing detection increases the complexity of hardware circuit design and the cost of the solution. In our solution, we use software to perform zero-crossing detection for the working environment of the induction cooker, thereby reducing costs and effectively solving the interference of the main power circuit of the induction cooker on the touch buttons.

　　2. Grid interference

　　Because the quality of the domestic power grid varies, in some areas with poor quality, it is easy to affect the normal operation of the induction cooker touch buttons. If the power supply cannot be isolated, you will see the difference in Figure 6 (blue means no key, red means the key is pressed), and these figures are only when the induction cooker is not turned on. When the induction cooker is working, the electromagnetic radiation generated will make the signal seen more chaotic. In the experiment, it was found that the key effect can be significantly improved by isolating from the external power grid or using software filtering.

　　Figure 6: Good quality grid Poor quality grid 1 Poor quality grid 2

　　3. Impact of splashing water and oil

　　When using an induction cooker, water or oil often splashes onto the touch panel, which may cause false triggering of buttons. This solution uses a special software algorithm to reliably distinguish between water or oil splashes and finger pressing.

　　4. Environmental Adaptability

　　When the induction cooker is working, it will generate a lot of heat and moisture. The panel temperature/humidity and circuit board temperature/humidity will fluctuate in a wide range. As time goes by, the glass panel and PCB board will age to different degrees, thus affecting the accuracy of key detection. In ST's solution, an automatic calibration function is implemented, providing real-time environmental detection and realizing an environmental self-adaptation mechanism.

　　4 Conclusion

　　The solution provided by ST includes touch panel self-calibration, software filtering, software zero-crossing detection and environmental adaptation functions, and uses software algorithms to shield interference from various complex environments as much as possible, with low cost and reliable operation. Of course, in the application of other products, there will be some requirements that are different from the induction cooker environment. Here we only introduce some representative interferences, but as long as you understand the working principle of capacitive touch, there are still many ways to deal with various application situations.