Rolling bearings are generally composed of four parts: inner ring, outer ring, rolling element and cage. The function of the inner ring is to match and rotate with the shaft; The function of the outer ring is to cooperate with the bearing seat and play a supporting role; The rolling element is evenly distributed between the inner ring and the outer ring with the help of the cage. Its shape, size and quantity directly affect the service performance and service life of the rolling bearing; The cage can make the rolling elements evenly distributed, prevent the rolling elements from falling off, guide the rolling elements to rotate and lubricate. The rolling bearing has the advantages of convenient use and maintenance, reliable operation, good starting performance and high bearing capacity at medium speed. Compared with sliding bearing, rolling bearing has larger radial size, poor damping capacity, low service life and large sound at high speed.
At present, there are three feasible methods to detect and diagnose the running state of bearings: temperature measurement, noise and vibration parameter testing technology. Because the temperature measurement technology is simple and easy, it can comprehensively reflect various factors including bearing lubrication state, and all kinds of bearing failures are reflected as the significant temperature rise of the bearing. Therefore, the temperature measurement method can effectively monitor the operation of the bearing and avoid or reduce the loss caused by the failure.
1. ZigBee wireless temperature measurement technology
ZigBee wireless temperature measurement technology is a short-range, low-power wireless communication technology. This name comes from the eight character dance of bees, because bees rely on the “dance” of flying and “Zig” shaking their wings to convey the location information of pollen with their companions, that is, bees form the communication network in the group in this way. It is characterized by short distance, low complexity, self-organization, low power consumption, low data rate and low cost. It is mainly suitable for automatic control and remote control, and can be embedded with various devices.
As the bearing belongs to a large device that is frequently replaced, the rolling mill has a bad site environment and there are many oil stains in the air. If the wired method is used to detect the temperature, it is not only inconvenient to replace, but also frequent plugging and dialing will cause the joint to be polluted by oil stains and affect the data transmission. As an emerging technology, ZigBee technology is a wireless network protocol specially designed for low-speed sensor and control networks. It is very suitable for the field of industrial automatic control and remote control.
ZigBee standard includes physical layer, media access layer, network layer and application layer. It has the following characteristics:
(1) Low power consumption. Due to the short working cycle and low power consumption of sending and receiving information, two No. 5 dry batteries can support one node for 6 ~ 24 months in standby mode. In sleep mode, the battery life can be as long as several years, and its transmission power is about 1 MW
(2) Low cost. The module is cheap and the ZigBee protocol is royalty free.
(3) Short delay. ZigBee’s response speed is fast. It takes only 15 ms to activate from sleep and enter the network after activation
(4) Large capacity. In cluster and mesh structures, a master node can manage up to 254 child nodes. If cascade connection is adopted, the number of nodes can reach 65000.
(5) Safe. It provides three-level security mode and advanced encryption standard.
2. Bearing temperature detection system
2.1 system composition
The system structure block diagram is shown in Figure 1.
The temperature sensor installed in the bearing pedestal inputs the detected temperature signal to the ZigBee RFD module. The sleep timer and energy-saving equipment built in the module can greatly reduce the power consumption of the system and collect temperature and battery voltage data. After receiving the data, the ZigBee network coordinator returns the confirmation information to the corresponding ZigBee RFD module, On the other hand, the received data is transmitted to the main console by wired connection. The LED is used to display the current temperature value detected by each sensor in real time, equipped with alarm, alarm light, etc., and provides a friendly man-machine interface to display, store and record the received data, so as to realize the alarm of abnormal conditions and output control signals to protect the system. The key of the detection system is the design of the measurement node. Its hardware implementation and software implementation are introduced below.
2.2 hardware implementation of measurement node
Jn5139-z01-m00 / M01 module of jennic company is used as the core of ZigBee network node. Figure 2 shows the interface circuit between temperature acquisition hardware circuit and JN5139.
In Figure 2, DS18B20 digital temperature sensor is selected, which has small volume and the measurement temperature range is – 55 ℃ ~ + 125 ℃. Within the range of – 10 ℃ ~ + 85 ℃, the accuracy is ± 0.5 ℃.
The node temperature collected by DS18B20 is input to jn5139-z01-m00 module after passing through the signal conditioning circuit. Jn5139-z01-m00 is a transmitting module based on wireless microcontroller JN5139. It has the characteristics of low power and low cost. It integrates 32-bit RISC MCU core, high-performance IEEE802.15.4 transceiver, 192 KB ROM and 96 KB ram. It can design and implement ZigBee wireless network system at low cost in a very short time. The system node works in the 2.4 GHz frequency band, the transmission rate is 250 KB / s, the transmission distance is more than 100 m, the operation is stable and the reliability is high.
The four pins cts0, rts0, txd0 and rxd0 of JN5139 are connected with MAX232. The current state can be determined through S2 (PROM), that is, the state selection key of programming / operation. When in the running state, the serial port is used to send data, and when in the programming state, the serial port is used to download programs.
2.3 software implementation of measurement node
Use the jennic CodeBlocks development environment of jennic company. The MAC layer software compiled by the company is used to complete the development of the application layer through C language programming.
In Figure 3, when the measured battery voltage is less than 2.8 V, an alarm is given to prompt the staff to replace it in time. If there is no network receiving data, it will be sent every 5 minutes. If there is network receiving, it will be sent every 15 s. only when the temperature data is detected and needs to be sent, the transmitting module will start working and send this group of data, and the whole system will be put into sleep in the rest of the time. The software flow of the data receiving end is shown in Figure 4.
3. Low power strategy and test analysis
3.1 low power strategy
In the system design, according to the actual needs, make full use of various low-power resource modes to shorten the working time of nodes as far as possible, so as to reduce power consumption. The actual capacity of the commonly used battery is related to the discharge current. When the discharge current is small, the total power discharged by the battery is significantly greater than the nominal capacity, or even significantly less than the nominal capacity. Therefore, the following strategies are adopted to reduce the system working current and prolong the node life.
In terms of hardware, low-power and low-voltage devices are selected; For the power supply cycle of dsb20, the power consumption of MAX232 pin is small, such as the power consumption of MAX232 pin; In addition, the unused MCU pins are set according to the data manual, which has a significant effect on reducing the current in the system sleep mode.
In terms of software, use the support provided by hardware to turn off the power of temporarily unused components, such as DS18B20; According to the protocol work cycle, the software controls the working mode of JN5139 for periodic detection and sleep. The method of long-term and short-term selective sleep is selected to detect data. Compared with the single 15 s sleep, this method has 20 times less power consumption when sending data without network reception.
3.2 test analysis
After completing the system design, under the conditions of battery voltage of 3.4 V, transmission power of + 2.5 DBM and reception sensitivity of – 96.5 DBM, combined with the data in the data manual, the working current of the sensor node is actually tested, ignoring the wake-up sleep time, etc. The results are shown in Table 1.
According to the working mode and working cycle in Table 1, the average working current of the node is calculated as:
The battery with a capacity of 1800 MAH is used under the high temperature working environment of 150 ℃, and the discharge depth is 50%. That is, due to the self discharge factor of the battery during the long-term operation of the system, the available power of the battery is 50% of the total capacity. According to the results of formula (1) and table 1, when the system detects the bearing temperature in a cycle of 15 s, the node can work for more than half a year, meeting the design requirements, The calculation formula is shown in formula (2).
3.3 voltage detection
Due to the use of battery power supply, its voltage needs to be detected to prevent the accuracy of measured temperature value from being affected by insufficient voltage.
As shown in Figure 5, there are six 12 bit analog-to-digital converters in JN5139. The successive approximation design is adopted to improve the conversion accuracy, of which four can be used for external data conversion, and the other two are used to connect the internal temperature sensor and internal power monitoring circuit.
The internal power monitoring circuit is used to measure the 24 pin of the chip, that is, the analog power pin VDD. The voltage of the VDD pin is reduced to 0.666 times through a resistance voltage divider in the chip, and then input to the analog-to-digital converter for detection, which can monitor the power supply status in real time.
4. Test results
At present, the development of industrial small sample machine has been completed. Because the working environment temperature of the detection system is between 50 ℃ and 80 ℃, when the bearing fails, it will reach a high temperature of about 100 ℃ instantly, and there are a lot of oil stains in the working environment, it is necessary to test the short-term ultra-high temperature resistance and sealing of the acquisition module. The specific experimental data are shown in Table 2.
The bearing needs to be replaced frequently. Therefore, in addition to the high-temperature working environment, the working capacity under normal temperature and low-temperature environment in winter also needs to be tested. Figure 6 shows the detection experiments of different modules placed in different environments. When placed in low temperature environment, the power supply capacity of the battery decreases significantly and the voltage decreases greatly. At this time, timely replacement of the battery can continuously and accurately monitor the running state of the bearing.
Through experiments and field debugging, the low-power bearing temperature detection system based on ZigBee technology can accurately detect the temperature and its changes during bearing operation, monitor the bearing operation state, and take timely measures in the early stage of bearing failure, so as to avoid major accidents such as damage to rolls and steel plates and explosion caused by oil temperature rise. It is believed that through efforts, a bearing temperature detection system with perfect functions and based on ZigBee technology will be popularized and applied.
Responsible editor: GT