Design of wireless thread-controlled micro-heating platform based on STM32W108

1. Introduction

Temperature is a basic thermodynamic parameter, and its measurement and control are widely used and of great significance in production, life and scientific research, such as metallurgy, mining, and refrigeration. In the fields of chemical engineering and life sciences, it is sometimes necessary to make the temperature control platform portable and miniature, or to avoid on-site operation as much as possible.

At the same time, with the development of wireless communication and semiconductor technology, wireless measurement and control technology represented by wireless sensor networks has begun to be applied, such as smart home, environmental monitoring, etc. As part of the wireless measurement and control system, the portable micro-heating platform with wireless remote temperature control can greatly facilitate people's production, life and scientific research.

In response to this demand, this paper designs a digital closed-loop wireless micro-heating platform based on the STM32W108 wireless microcontroller newly launched by STMicroelectronics, which consists of PT100 temperature detection, PWM driven heating, and Zigbee wireless communication. The remote temperature control of the micro-heating platform is implemented through programming, ensuring the mobility and performance stability of the node.

2. System overall design and key technologies

2.1 Overall design and block diagram

The designed wireless thread-controlled micro-heating platform can be divided into three parts in principle: temperature detection circuit based on PT100 and low-power op amp, PWM heating drive circuit based on low-leakage current MOS tube and high-efficiency thin-film ceramic heater, and control and communication unit with STM32W108 as the core; the three form a complete temperature control closed loop and provide an external Zigbee wireless communication interface and a serial port for monitoring. The system principle block diagram is shown in Figure 1.

2.2 Temperature Detection Circuit

Considering the device cost, temperature measurement range, detection circuit complexity, and response time, this design uses thin-film encapsulated PT100 elements, which are lower in cost and faster in response than traditional platinum wire PT100. 0.5 is less than 10s, and the linear temperature measurement range can reach -200℃~800℃. High-temperature thermal conductive adhesive is used to bond PT100 to the heater to ensure mechanical reliability and high thermal conductivity.

Considering that when the temperature range is large, the PT100 resistance changes greatly and the constant voltage bridge method system has large nonlinearity, this design uses constant current source excitation. Literature shows that when the thin film PT100's own current exceeds 1mA, it will generate self-heating, so this design uses the LM334 adjustable constant current source chip from National Instruments, and sets the current to 100A through an external resistor.

After the excitation current enters PT100, the output voltage maintains a strict linear relationship with the temperature. The original voltage is buffered by the post-stage emitter follower and enters the SK second-order amplifier filter circuit, with a cutoff frequency of 40Hz and a quality factor of 0.707. The op amp chip used for follower and amplifier filtering is ADA4501-2, which integrates dual op amps and has a 1.8V low-power supply. After the above conditioning, when the PT100 temperature ranges from -50℃ to 500℃, the analog voltage output by the SK circuit has a nominal range of 0.1~1.1V. The analog voltage can be directly converted to digital by the built-in ADC (1.2V reference voltage) of STM32W108 to achieve temperature feedback.

2.3 PWM heating drive circuit

The essence of PWM is that the transmitted power is modulated by pulse width.

The heater in this design is a square thin-film ceramic resistor heater with a side length of 1 cm. The power passing through is the heating power. The PWM wave frequency is set to 100Hz, and the duty cycle is from 0 to 100%, which is given by the timer module of STM32W108. It is connected to the gate of the low-power, high-power MOS tube CDS16301Q2, and the source-drain is used as the current path of the heater. The leakage current of the MOS tube is only 1mA, and the maximum source leakage current is 5A. It is measured that the open-loop heating steady-state value of the heating circuit at room temperature can reach about 500℃, and the power consumption is 4W.

2.4 Control communication unit circuit

The main control unit uses the 32-bit ultra-low power consumption, harsh environment wireless processor STM32W108 launched by ST in 2009. The chip is based on the ARM Cortex-M3 core, with strong processing power and high cost performance. The chip integrates 8KB RAM and 128KB FLASH, and has rich interface resources, such as the ADC module, timer PWM module, RF communication module, and UART module used in this design.

The power supply system uses a single external 3.3V voltage supply, and the on-chip transformer converts it to 1.8V for storage and analog power supply, and 1.25V for core power supply. The clock system uses an external 24MHz passive crystal and a built-in 10KHz clock generator to generate clock signals for the core, internal bus, RAM, timer, etc. through a built-in frequency division circuit.

To achieve remote portable data transmission, the system uses the RF transceiver module of STM32W108 to provide wireless communication.

The module complies with the IEEE 802.15.4 MAC layer standard and provides the maximum hardware support for Zigbee. The chip also comes with a hard-core protocol stack that complies with Ember Zigbee. In terms of peripheral circuits, a PCB microstrip inverted F antenna design is adopted, and the DBF71A001 RF communication filter launched by SOSHIN is selected, which integrates balun and 2.45GHz bandpass filter functions to ensure maximum effective power transmission.

2.5 Embedded Software Design

The main embedded software program of STM32W108 is shown in Figure 2. After power-on, the processor core, hardware access layer initialization and board-level initialization are first performed, including memory space configuration, AD startup, wireless reception configuration, etc. When an RF reception event occurs, the hardware writes the event into the RF reception flag register and the corresponding cache. Then enter the whlie (1) main loop to query the RF reception status register. If a data packet is received, the target temperature is configured according to the instructions in the data packet; if not, it is configured according to the last temperature control target temperature. Then read the current temperature value detected by the AD module and calibrate the system error, calculate the PWM duty cycle based on this, configure the timer output, and send the actual value of this temperature control to the host computer through RF, and execute the main polling again, and repeat this process.

3. Test results analysis

The room temperature is 18.2℃. The temperature of the micro-heating platform is remotely set to 200℃ by wirelessly sending instructions from the host computer 30 meters away. The actual temperature value is monitored by the TES1307 thermocouple thermometer produced by TES. The temperature response of the system from 0 to 20 minutes is shown in Figure 3.

It can be seen from the test data that 10 minutes after the temperature control program is started, the working surface of the micro-heating platform can achieve an error within ±3°C and remain stable.

4. Conclusion

This paper designs a wireless thread-controlled micro-heating platform based on STM32W108, in which the temperature control uses PWM to drive the high-temperature ceramic heater, the temperature feedback uses the PT100 excited by the constant current source, and the integrated RF module is used to achieve wireless communication and program control. Experiments show that the heating platform can achieve remote temperature control through wireless data transmission, and has high temperature control accuracy, compact design, and mobile flexibility, meeting the special needs of biochemistry, medicine and other fields for portable and wide-range heating.