Design and application of threadless micro-heating platform

Temperature is the basic parameter of thermodynamics, and its measurement and control are widely used and important in production, life and scientific research, such as metallurgy, mining, and refrigeration. Among them, in the chemical industry, life sciences and other fields, sometimes the temperature control platform needs to be portable and miniature, or try to avoid the operation of personnel on the spot.
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 homes and environmental monitoring. As a 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, based on the STM32W108 wireless microcontroller launched by STMicroelectronics, a digital closed-loop wireless micro-heating platform with PT100 temperature detection, PWM-driven heating, and Zigbee wireless communication is designed, and the remote temperature control of the micro-heating platform is realized by programming. , To ensure the mobility and performance stability of the node.
2. System overall design and key technology
2.1 Overall design and principle block diagram
The designed threadless control micro-heating platform can be divided into three parts based on the principle: temperature detection circuit based on PT100 and low-power operational amplifier, PWM heating drive circuit based on low leakage current MOS tube and high-efficiency thin-film ceramic heater, with STM32W108 as The control and communication unit; 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 as in Fig. 1.



2.2 Temperature detection circuit
Taking into account the cost of the device, the temperature measurement range, the complexity of the detection circuit, and the response time, this design uses a thin-film package PT100 element. Compared with the traditional platinum wire PT100, the cost is lower and the response is faster. 0.5 is less than 10s, and the linear temperature measurement range can be Up to -200℃~800℃. Use high temperature thermal conductive glue 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 change range is large, and the constant piezoelectric bridge method system is non-linear. This design uses a constant current source for excitation. The literature shows that when the self-current of the film PT100 exceeds 1mA, it will generate self-heating. Therefore, the LM334 adjustable constant current source chip from National Instruments is used in this design, and the current is set to 100A through an external resistor.
After the excitation current enters the PT100, the output voltage maintains a strict linear relationship with temperature. The original voltage is buffered by the post-emitter follower and enters the SK second-order amplifying filter circuit with a cut-off frequency of 40Hz and a quality factor of 0.707. The op amp chip used to set the easy-going amplifier filter selects ADA4501-2, integrated dual op amp, 1.8 V low power supply. After the above conditioning, when the PT100 temperature range is -50℃~500℃, the nominal range of the analog voltage output by the SK circuit is 0.1~1.1V. This analog voltage can be directly used by the built-in ADC (1.2V reference voltage) of the STM32W108 ) Perform analog-to-digital conversion to achieve temperature feedback.
2.3 PWM heating drive circuit
The essence of PWM is that the transmission power is modulated by pulse width.
The heater in this design is a square sheet ceramic resistance heater with a side length of 1cm, and the power passed is the heating power. The PWM wave frequency is set to 100Hz, and the duty cycle is from 0 to 100%. It 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 heater current path. The leakage current of the MOS tube is only 1mA, and the source leakage current is 5A. It is measured that the steady-state value of the heating circuit can reach about 500℃ at room temperature and the power consumption is 4W.
2.4 Control communication unit circuit
The main control unit adopts the 32-bit ultra-low power consumption and harsh environment wireless processor STM32W108 introduced 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 abundant interface resources, such as ADC module, timer PWM module, RF communication module, UART module used in this design.
The power supply system uses a single external 3.3V voltage supply, and the on-chip transformer is converted to 1.8V for storage and analog power supply, and 1.25V for core power supply. The clock system is generated by an external 24MHz passive crystal and a built-in 10KHz clock generator, and the built-in frequency divider circuit provides clock signals for the core, internal bus, RAM, timers, etc.
In order to realize remote portable data transmission, the system uses the RF transceiver module that comes with STM32W108 to provide wireless communication.
The module complies with the IEEE 802.15.4 MAC layer standard and provides hardware support for Zigbee. The chip also comes with a hard-core protocol stack that complies with Ember Zigbee. In terms of peripheral circuits, the PCB microstrip inverted F antenna design is adopted, and the DBF71A001 radio frequency communication filter introduced by SOSHIN is selected, which integrates the functions of balun and 2.45GHz bandpass filter to ensure effective power transmission.
2.5 Embedded software design
The embedded software main program of STM32W108 is shown as in Fig. 2. After power-on, first perform processor core, hardware access layer initialization and board-level initialization, including memory space configuration, starting AD, wireless receiving configuration, etc. When an RF receiving event occurs, the hardware writes the event to the RF receiving flag register and the corresponding buffer. Then enter the whlie(1) main loop, query the RF receiving status register, if there is a received data packet, configure the target temperature according to the instructions in the data packet; if not, configure the target temperature according to the last temperature control. 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 true value of the temperature control to the host computer through RF, and perform the main polling again , So repeatedly.
3. Test result analysis
The room temperature is 18.2℃, the instruction is sent wirelessly through the host computer 30 meters away, and the temperature of the micro heating platform is set to 200℃ remotely, and the actual temperature value is monitored by the TES1307 thermocouple thermometer. The system is at 0~20min The temperature response is shown in Figure 3. 

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