Application of CAN bus in elevator remote monitoring system

Preface

As people's quality of life improves, smart buildings have become a trend. In smart buildings, the safe and stable operation of elevators is crucial. However, this is difficult to guarantee due to the limitations of funds and technology. Therefore, it is quite necessary to detect elevator failures in a timely manner and repair them quickly. At present, the domestic elevator service level is mostly still limited to the problem of the on-site elevator, notifying the maintenance center, and the maintenance center sends a special person to the site to investigate and troubleshoot. The disadvantage of this situation is that the response speed is slow and a special person needs to be sent to the site to monitor. The elevator remote monitoring system provides a powerful tool for improving elevator maintenance and responding in a timely manner.

At present, large foreign elevator companies have mature elevator remote monitoring systems, but the high price is a high barrier, and their monitoring systems are only developed for their own elevators, with poor compatibility. Based on the above situation, we have developed a remote monitoring system that can be suitable for different types of elevators. For multiple elevators installed in a certain area (a building, a group of buildings, a community, a city, a country, etc.), these elevators can be centrally remotely monitored, managed, data maintained, counted, analyzed, fault diagnosed and rescued. Its purpose is to perform remote data maintenance, remote fault diagnosis and processing, early diagnosis and early elimination of faults, statistics and analysis of elevator operating performance and fault conditions, and select a reasonable elevator dispatching plan based on the analysis.

System composition

The elevator remote monitoring system consists of three parts: data acquisition card, video monitoring, and monitoring workstation. Its overall scheme structure is shown in Figure 1. The data acquisition card (CAN-232 data conversion card in Figure 1) is connected to the elevator communication card in the elevator controller responsible for data exchange with the monitoring system through the CAN bus, and the wiring method adopts the bus type. If the bus length exceeds 100 meters, a 120-ohm terminal resistor should be connected at the end of the bus to suppress signal reflection. These two 120Ω resistors play a very important role in matching the bus impedance. Ignoring them will greatly reduce the anti-interference and reliability of data communication, or even make it impossible to communicate, which has been verified on site. The data acquisition card connects to the COM1 port of the workstation through the RS232 bus to communicate data with the workstation.

Figure 1 System composition

The camera is installed inside the elevator car to collect images inside the car. Its video signal is transmitted to the video switcher via a video cable. The switching of the video switcher is realized by the MCU on the data acquisition card controlling its analog switch. The MCU receives the control command from the computer from the COM1 port of the workstation computer, and then controls the analog switch to select and output from multiple video signals according to the command. At the same time, the video switcher has an amplification function to extend the transmission distance of the video signal, so that the workstation computer can obtain high-quality images. The switcher transmits the selected video signal to the video capture card via a video cable. The video capture card is installed on the PCI interface of the workstation computer. Its main function is to convert the analog video signal of the camera into a digital video signal and transmit it to the workstation computer for further processing and storage.

The workstation receives data sent by the data acquisition card and the video acquisition card for processing and displays the elevator operation status and the image in the car, and provides multiple alarm methods when the elevator fails, and stores various useful information in the database. In this way, users can grasp the elevator status information and the image information in the elevator car through the workstation, and can query the elevator's archive information, the historical event library of the elevator operation, the elevator's previous fault information, query the performance analysis results of the elevator in a certain period of time in history, print reports, etc.

CAN bus introduction

Data is transmitted through the CAN bus, which is a local area network that supports serial communication in distributed real-time control systems. Due to its high performance, high reliability, good real-time performance and unique design, it has been widely used in data communication between various detection and actuators in control systems. Its main features are: (1) Multi-master bus, each node can actively send information to other nodes on the network at any time; (2) Using unique non-destructive bus arbitration technology, nodes with high priority are given priority to transmit data, which can meet real-time requirements; (3) It has the function of point-to-point, point-to-multipoint and global broadcast data transmission; (4) The maximum number of valid bytes per frame on the CAN bus is 8, and there are CRC and other verification measures. The data error rate is extremely low. In the event of a serious error in a node, it can automatically leave the bus, and other operations on the bus will not be affected; (5) The communication distance is as far as 10km (5kb/s), the communication rate can reach up to 1MB/s (40m), the actual number of nodes can reach 110, and the communication medium uses twisted pair cable, and optical fiber can also be used; (6) The CAN bus has only two wires. When the system is expanded, the new node can be directly connected to the bus. The system is easy to expand and flexible to modify. Therefore, the CAN bus has become an ideal bus for distributed computer control systems.

At present, there are two types of CAN bus devices that are widely popular: one is an independent CAN controller, such as SJA1000 and Intel82526/82527, etc.; the other is a microcontroller with CAN, such as P8xC592 and 16-bit microcontroller 87C196CA/CB, etc. SJA1000 is an independent controller used for regional network control CAN in mobile targets and general industrial environments. It is a replacement for PHILIPS Semiconductor PCA82C200 CAN controller BasicCAN and it adds a new working mode PeliCAN, which supports CAN2.0B protocol with many new features. The main new features of SJA1000

(1) Reception and transmission of standard and extended structure information
(2) Receive FIFO 64 bytes
(3) Single/dual acceptance filter with mask and code registers in standard and extended formats
(4) Error counter for read/write access
(5) Programmable error limit alarm
(6) Most recent error register
(7) Error interrupt for each CAN bus error
(8) Arbitration loss interrupt defined by function bits
(9) One-time transmission without retransmission when error or arbitration lost
(10) Listen-only mode, CAN bus monitoring without response and error flag
(11) Supports hot plugging, non-interference software-driven bit rate detection
(12) Hardware disables CLKOUT output

Data transmission and protocol conversion module

CAN-232 conversion card

This part completes the functions of data acquisition, protocol conversion, fault judgment, communication with the workstation, and control of video switching. Among them, the CAN controller selected is PHILIPS's SJA1000, and the transceiver selected is PAC82C250. The signal isolation uses a high-speed optocoupler 6N137. It receives the elevator status data frame in the CAN protocol format sent by the elevator communication card, and then converts it into a standard RS232 format data stream to transmit to the workstation. Since the communication protocols of various elevator controllers are different, it is necessary to perform protocol conversion in the data transmission module to convert the format of the elevator status signal into a data format that meets the requirements of the workstation software protocol, so that the monitoring software can be compatible with different types of elevators.

The following is a detailed introduction to the communication between the acquisition card, the elevator communication card and the workstation.

Communication process between conversion card and communication card

The CAN bus works in a multi-master mode, and more than 110 nodes can be connected to the bus. Therefore, one acquisition card can connect to the communication cards of up to more than 110 elevators. However, in actual applications, considering the real-time nature of the monitoring software, one workstation monitors 16 elevators and assigns a station number as an identity to each elevator (the DIP switch on the elevator communication card of each elevator is set to 1-16). The host computer monitoring software collects data from an elevator every 40ms. It first sends the station number of the elevator to the acquisition card, and then waits to receive data. When the acquisition card receives the station number sent by the host computer, it fills the station number into the first byte of the CAN frame, and sends this CAN frame with only one data byte to the bus, and then waits to receive the data sent by the elevator.

16 elevator communication cards are connected to the acquisition card. During initialization, the address receiving code and the mask code are set to only receive the data sent by the acquisition card. When the communication card receives the station number sent by the acquisition card, it compares it with its own station number. If they are different, it will ignore them. If they are the same, the elevator status data will be sent to the bus. During initialization, the acquisition card is not set to receive data from all communication cards. When it receives the data on the bus, it confirms whether it is the data sent by the collected elevator. If it is correct, it will perform protocol conversion and convert the data received from the elevator communication card into a format that meets the requirements of the host computer. Then, based on these status information, the elevator fault diagnosis is performed to determine whether the elevator is operating normally. If not, it is determined what fault has occurred or what fault may occur, and a fault alarm or pre-alarm is performed. After the fault judgment and other processing, the fault code and other data are sent to the host computer together, otherwise the operation is abandoned.

In CAN bus communication, the initialization module is more important, a key point, and also a difficult point. During initialization, first enter the reset mode, and then configure the registers of the CAN controller. However, in practice, it is found that hard reset is more reliable. As long as the time is sufficient, the CAN controller can enter the reset state, but the values ​​of some registers of the CAN controller are uncertain at this time. Soft reset is just the opposite. It may not necessarily make the CAN controller enter the reset state, but once it enters the reset state, the register value of the CAN controller is a certain reset value. In practical applications, these two reset methods are used together for good results. Therefore, a Watchdog circuit is also designed in the hardware circuit, which can also prevent the microcontroller from freezing or the program from "running away". The initialization program flow chart is shown in Figure 2, and the overall program flow chart of the acquisition card is shown in Figure 3.

Figure 2 CAN bus initialization module

Figure 3 Capture card flow chart

Communication process between conversion card and workstation

The acquisition card and the workstation communicate through the RS-232 bus. The baud rate of the MCU serial port is 19200bs. The workstation monitoring software uses VisualBasic and SQL, and the Settings property of its MSComm control is set to "19200, E, 8, 1". Using the serial port to connect to the workstation can meet the requirements of the remote monitoring system in terms of speed and reduce costs. If a CAN bus adapter card based on the PCI bus is used, although the communication rate can be increased, it also increases a lot of costs. Moreover, the tasks of protocol conversion and fault judgment must be transferred to the host computer, which increases the burden of the host computer and affects the real-time performance of the entire system. Comprehensive comparison shows that CAN-232 has a higher cost-effectiveness and is suitable for use in this system. The monitoring software can obtain the rotation angle = ∠AOC and the pitch angle = ∠COB as shown in Figure 4 every 40ms. In order to coordinate the angle obtained with the angle obtained by COMPASS, the angle modulation is performed with due north as zero degrees, and the angle range is from 0 to 359.9.

Figure 4

in conclusion

The system has a compact and reasonable structure and can be adjusted to any position in the hemisphere as needed. In this system, the power of the prototype is 1.5W, the weight is 2.5kg, it can load a 10kg antenna (0.7kg in the antenna of this system), and the rotation speed is 4 degrees per second. The tracking effect of the prototype is relatively good in the engineering test, and the reliability of operation is guaranteed by software. The results show that the response characteristics meet the requirements and the accuracy fully meets the actual needs.