Design of low-power wireless node module for motor temperature monitoring system

When the motor runs at high speed for a long time, it will generate a lot of heat, causing the temperature of the main components to rise and the motor to burn out. For example, the traction motor of the EMU train may bring serious safety hazards. Due to the special structure of most motors, the traditional infrared shaft temperature monitoring system cannot detect the temperature of the motor. The goal of the design system is to measure the temperature of the motor in real time and prevent the motor from overheating and causing failure. The temperature detection system proposed in this paper uses a low-cost, low-power MCU with an embedded Cortex-M0 core to send the temperature data collected by the temperature sensor to the monitoring center through a highly integrated, low-power RF chip.

1 Module structure and hardware design solution
1.1 Wireless module solution
The network node of this system consists of four main parts: data acquisition, processing, transmission and power supply. The sensor detection unit uses sensors to collect information about the objects to be measured in the monitoring area, and uses the DS18B20 chip; the microcontroller unit realizes data analysis, processing and storage functions, and uses the LPC1114 with an embedded Cortex-M0 core provided by NXP; the wireless transmission unit is responsible for low-power short-distance communication between nodes, and uses the Si4432 chip provided by Silicon Labs; the power supply unit uses a small, high-capacity battery to ensure the long life and miniaturization of the node.
The hardware structure of this node module is shown in Figure 1.

1.2 Module Hardware Implementation
The temperature sensor transmits the detected temperature data through the GPIO port of the MCU via a one-line data interface. The MCU reads and writes the internal registers of Si4432 through the enhanced SPI interface, and can flexibly configure various parameters. In addition, through the four-wire SPI interface, namely MOSI, MISO, SCLK and nSEL. MOSI is used for serial data transmission from LPC1114 to Si4432; MISO is used for serial data transmission from Si4432 to LPC1114; SCLK is used to synchronize serial data transmission between LPC1114 and Si4432 on MOSI and MISO lines; nSEL is used as a chip select signal. Only when the chip select signal is at a low level, the operation on Si4432 is effective. For the specific hardware circuit design, refer to the application manual provided by Silicon Labs. The resistance and capacitance parameters of the receiving low-noise amplifier matching circuit and the transmitting power amplifier matching circuit provided in the circuit can enable Si4432 to achieve better communication effects. The LP filter circuit of the RF front end is simulated by ADS. Its S parameter simulation is shown in Figure 2. It can be seen that it has very small return loss in its 240-960 MHz passband.

[page]

The temperature sensor circuit is directly connected to the GPIO port of LPC1114 through the DQ data line to realize temperature acquisition data transmission. LPC1114 communicates with the PC through the JTAG interface or ISP mode to realize online debugging of the module program. The relevant circuit design refers to the circuit of the core board of LPC1114. It is necessary to pay attention to the setting of the pull-up resistor in the JTAG interface. In this scheme, a 3V16AH battery is used to provide power. In the circuit design, different capacitors of 2.2μF, 100nF, 100pF, and 10pF are used to realize the power filter circuit. At the same time, the choke inductor is used to provide a DC bias voltage for the Si4432 transmitter power amplifier.

2 PCB design considerations
In this digital/analog hybrid circuit, the quality of PCB design will directly affect the overall performance of the module. The following is a brief description of the key issues of PCB design in this scheme:
(1) The digital and analog power supplies in the design should be isolated by choke inductors to prevent the digital high-frequency power supply from interfering with the analog signal. Decoupling capacitors should be added to the power access end and should be as close to the Si4432 chip as possible. The filter capacitor should also be as close to the corresponding pin as possible, so that better filtering performance can be obtained;
(2) In order to eliminate the inductive effect between the traces, as many vias as possible should be arranged in the free space on the PCB. In order to achieve better RF communication effect, the entire PCB should be covered with ground copper. After providing a good RF ground, the ground copper area of ​​the TX/RX area helps to reduce or even avoid radiation interference;
(3) The RF front-end circuit should use 0402 package inductors and capacitors as much as possible to reduce electromagnetic interference effects. The RF inductors are placed perpendicular to each other to reduce coupling. The RF high-frequency part requires a 50 Ω transmission line as a connection.
The module PCB layout and wiring effect is shown in Figure 3.

3 Module software design
3.1 Software flow
The software system of this module can be roughly divided into the following parts: initialization part, data transmission part, data reception part. In the system software design, modular layered design is still adhered to. The initialization module includes the initialization of LPC1114, the initialization of SPI, and the initialization configuration of Si4432's internal registers such as wireless transceiver frequency, working mode, and transmission rate. The relevant register configuration can be obtained from the Excel calculator provided by Silicon Labs. The software design process of each module above refers to the application manual provided by Silicon Labs, which can greatly shorten the research and development cycle. The interface function implementation program of Si4432 and MCU data communication is as follows:

In addition, in order to give full play to the low power consumption advantage of this solution, a power management part is added to the system software. Its function is to detect the power supply of the system in real time. If the system is detected to be powered off, the power-off information is sent to the center, and the sleep command is sent to the node at the same time. The device node in the sleep mode wakes up every half an hour to query whether the center has been powered on and working. If the center has been powered on and working, the node enters the working state. If the center is not detected to be working, the node continues to sleep. It mainly includes two parts: power-off process and power-on process. The specific implementation process is shown in Figure 4 and Figure 5 respectively.

[page]

3.2 Module system software debugging
This design adopts the LPCXpresso eclipse programming environment provided by NXP for free, and uses the SPI and GPIO control related drivers in the Cortex-M0 core provided by NXP to realize the data transmission between SPI and GPIO and Si4432 and DSl8820 respectively. The PC uses the LPC-Link emulator and Cortex-Debug interface to implement the ISP debugging mode for LPC1114, which greatly improves the development efficiency.

4 Module index test and energy consumption analysis
This module scheme has been applied to the truck axle temperature measurement system and meets the design requirements of this scheme. The RF index of the module was tested by the spectrum analyzer. Figure 6 is the spectrum of the module transmission signal observed by the spectrum analyzer at a center frequency of 410 MHz. The transmission power of Si4432 is up to 10 dBm, and the receiving sensitivity can reach -110 dBm. The communication distance in open areas can reach 2 km. When the transmission rate reaches 100 Kb/s (or above), the bit error rate is less than 0.075%, which can meet most wireless data transmission performance requirements.

Since all nodes in the system are powered by batteries, frequent battery replacement is often not allowed after installation on site, so the system has high requirements for energy saving. From the hardware point of view, the main power consumption is MCU and RF chip, and MCU and RF chip have working mode and sleep mode. In sleep mode, their power consumption is very small, so to save energy, the CPU and RF chip should be operated in sleep mode, combined with the system power management software design, to achieve the maximum energy saving of the module, the specific data is as follows: LPC1114 working mode (3.3 V voltage) current is 220μA, in sleep mode current is 6μA. RF chip Si4432 transmit current is 85 mA, receive current is 18.5 mA, power-saving mode current is 1μA. According to the average working level of the node and calculated according to the minimum cycle, the node working time and sleep time are both 17 s. Thus, in the shortest period of 34 s, the node current consumption is:
8.4×(0.22+18.5+0.002+0.001)+1.7×0.04×85+25.6×(0.006+0.001)=163 mA.
Then the 16 A·h battery in this module can last for 135.3 days, achieving the purpose of reducing the energy consumption of the module.

5 Conclusion
This paper designs and implements a low-power, low-cost, stable and reliable wireless temperature detection system node device solution. In the entire system test, the performance of this module in all aspects has achieved ideal results. It is necessary to further solve the problem of the installation and fixation of the equipment in the motor equipment and the electromagnetic interference of the high-speed rotation of the motor to the wireless module.