Signal transmission between the elevator car and machine room is achieved via a traveling cable. The elevator IoT system needs to collect elevator data, such as vibration data, door operator current signals, and car audio/video signals. This requires utilizing existing traveling cable channels or adding new data channels for signal transmission. To ensure reliable and stable data interaction between the elevator IoT platform and the elevator car signal acquisition device, a high-quality data transmission channel needs to be established between the elevator car and machine room. This paper, combined with practical engineering cases, studies Wi-Fi point-to-point wireless transmission and power line carrier wired transmission. After comparative research, it is concluded that the elevator IoT shaft transmission scheme based on power line carrier technology has the best performance. This scheme has been practically applied in several projects with good results and has significant potential for wider application.
Keywords: elevator IoT, shaft transmission, point-to-point Wi-Fi, power line carrier
1. Research Background
The Internet of Things (IoT) provides the technical means to realize intelligent elevators. One of the basic functions of elevator IoT is to collect elevator operating status data and transmit remote information. This requires establishing an interactive channel between the elevator IoT platform and various monitoring points in the elevator control system . As shown in the structure diagram of the elevator control system (Figure 1), if it is necessary to collect car data (such as elevator car vibration data, alarm signals, etc.), the existing elevator control system communication protocols generally do not include these signals. Therefore, a separate testing device needs to be installed on the car top or inside the car. Similarly, if elevator IoT voice and video intercom is required, voice and video intercom devices, such as LCD displays and one-button alarm devices, also need to be installed inside the car. These additional devices need to interact with the elevator IoT platform through data transmission devices. Considering that it is generally difficult to guarantee complete coverage of operator signals within elevator shafts, especially in residential areas or areas on the edge of operator base station coverage, the wireless signal attenuation is severe due to the metal obstacles of the shaft and car. Therefore, the feasibility of directly installing a data transmission unit on the elevator car top or inside the car is limited. In addition, considering that elevator IoT generally requires data collection from the elevator control system through a data collector and access to the operator network through the data collector or a data transmission unit near the collector, adding a separate data transmission unit on the car top or inside the car also poses a cost challenge for elevator IoT.
Based on the aforementioned application background, a data transmission channel between the machine room and the elevator car needs to be established. Existing elevator shaft traveling cables typically contain minimal communication cables. A standard 28-core or 18-core cable generally includes: 4 cores for AC220V signals (2 cores for lighting, 2 cores for door operator power); 4 cores for AC110V signals (door lock, brake); 4 cores for communication signals (communication power, communication twisted pair); 3 cores for intercom signals; 2 cores for door zone signals; 2 cores for the car door lock behind the through door; and 2 cores for the asynchronous main unit's auxiliary car door lock. Without altering the control system software and electrical design, it is difficult to carry data such as audio/video intercom device data, alarm signals, and car vibration data. Ideally, the best solution is to achieve data transmission between the car and the machine room without increasing the number of traveling cable cores, changing the electrical circuit, or modifying the software. Based on practical engineering application research, this paper proposes several solutions to achieve the integrity and reliability of the elevator IoT data link.
Figure 1 Schematic diagram of elevator control system
2. Scheme Design
2.1 Wi-Fi peer-to-peer
(1) System structure
Data transmission between the hoistway and machine room is achieved by installing a Wi-Fi board on the top guide rail of the hoistway and another on the top guardrail of the car. The system structure is shown in Figure 2. Considering the convenience of obtaining power for the Wi-Fi boards during actual installation, they are directly powered by AC220V, using the same power supply as the hoistway lighting. The hoistway-top Wi-Fi communicates with the data acquisition unit (or data transmission unit) in the elevator control cabinet via communication methods such as RS232, CANBUS, and RS485. The car-top Wi-Fi is responsible for input data acquisition and elevator status transmission. Data from the elevator IoT platform, such as elevator advertisements, emergency rescue information, car fault alarm signals, and vibration data, can be exchanged bidirectionally via the communication link between the car-top Wi-Fi and the hoistway-top Wi-Fi.
Figure 2. Structure diagram of elevator IoT shaft data transmission system based on Wi-Fi solution
(2) Working principle
The Wi-Fi protocol is managed by the IEEE 802.11 working group and has undergone development through standards such as IEEE 802.11, IEEE 802.11a, IEEE 802.11b, IEEE 802.11n, IEEE 802.11g, IEEE 802.11ac, and IEEE 802.11ad. It supports ISM bands from the traditional 2.4GHz and 5GHz to 60GHz, with peak transmission rates ranging from 2Mbps to 7Gbps. Considering the actual working conditions in elevator shafts, and taking into account comprehensive requirements such as wireless interference, communication distance, and communication speed, the Wi-Fi board in this project operates in the 2.4GHz band and supports IEEE 802.11b, IEEE 802.11n, and IEEE 802.11g.
The car roof Wi-Fi and hoistway roof Wi-Fi communicate point-to-point, each using an external planar directional antenna for signal amplification. The Wi-Fi board's network role (AP (Access Point) or STATION) is selected via a mode selection switch on the board. In AP mode, the Wi-Fi board acts as the Wi-Fi access point, the creation point of the Wi-Fi network, and the central node of the network, allowing other devices to connect and providing data access services. In STATION mode, the Wi-Fi board functions like a wireless terminal; it does not have access point functionality but can connect to an AP. Matching between Wi-Fi networks is achieved through SSIDs. Each wireless AP needs a unique network name, and the SSID distinguishes different Wi-Fi networks. This solution allows selection of SSIDs via a setting switch, supporting a total of 16 SSIDs. Considering that the number of adjacent or same hoistway elevators will not exceed 16, 16 SSIDs meet practical application requirements. The set SSIDs are displayed on the Wi-Fi board's built-in display module for easy differentiation during on-site commissioning. This solution is designed to determine the current working status using LED indicators. When the network connection is successfully established, the network indicator light stays on; otherwise, it remains off. The data indicator light flashes during wireless data transmission and reception and remains off when there is no data transmission or reception.
(3) Case Application
Figure 3. Case study of elevator IoT shaft data transmission based on Wi-Fi solution.
This case study requires transmitting real-time elevator status data, such as floor, direction, special status (maintenance, fire protection, malfunction, overload, full load, ARD), trapped passengers, and rescue personnel location, to the multimedia display screen inside the elevator car. A data communication link is established between the in-car multimedia display screen and the machine room gateway via Wi-Fi. Since the requirements for the amount of interactive data and the interaction rate are not high, RS232 communication is used between the Wi-Fi board, the in-car multimedia display screen, and the nodes or gateway, as shown in Figure 3.
2.2 Power line carrier
(1) System structure
Power line carrier (PLC) is a wired transmission method. By installing a pair of PLC modules inside the control cabinet and on the elevator car top, a communication link between the control cabinet and the car can be established by reusing the hoistway lighting cable in the traveling cable. The PLC module provides RS232 and Ethernet ports for communication with peripherals, as shown in Figure 4. When the elevator has high floors or the power line quality is severely polluted, reusing the lighting cable can easily lead to data loss and unstable communication. This problem can be solved by using the two spare cores of the traveling cable. The PLC module is easy to install and has a transmission rate of up to 94 Mbps, making it suitable for voice and video transmission applications.
Figure 4. Structure diagram of elevator IoT shaft data transmission system based on power line carrier scheme
(2) Working principle
The basic principle of power line carrier communication is to modulate the signal from the processing unit of the communication master station A, couple it to the power line via the power coupling interface, receive the signal from the power coupling interface of station B, demodulate it, process the demodulated signal to restore the original signal, and send it to the communication equipment through the communication interface, as shown in Figure 5. The principle of sending data from station B to master station A is similar. The coupling interface consists of a coupling capacitor and a filter forming a bandpass filter, which allows high-frequency carrier signals to pass while preventing power frequency signals on the power line from entering the carrier equipment.
Figure 5. Schematic diagram of the working principle of power line carrier technology
Power line carrier technology can be divided into narrowband power line carrier and broadband power line carrier from a bandwidth perspective. Narrowband power line carrier typically has a bandwidth of tens of kHz. The legal frequency band for power line carrier as stipulated by my country's telecommunications authorities is 40-500 kHz, with a basic carrier bandwidth of 4 kHz. However, the bandwidth occupied by a single-directional carrier channel in actual power line carrier equipment is an integer multiple of the basic frequency band width. Broadband power line carrier generally operates in the 2-30 MHz range. Compared to broadband power line carrier, narrowband power line carrier has advantages such as reduced signal attenuation, longer communication distance, and lower cost, but the 40-500 kHz band experiences significant interference. Currently, commonly used spread spectrum techniques for power line carrier communication include direct sequence spread spectrum, chirp, and orthogonal frequency division multiplexing (OFDM). In addition, frequency hopping (FH), time hopping (TH), and combinations of these techniques are also commonly used.
It is worth noting that distribution transformers can obstruct power line carrier signals. Since the carrier signal is essentially a high-frequency signal compared to the 50Hz power frequency, the inductive reactance of the transformer prevents the high-frequency carrier signal from passing through smoothly. Furthermore, when the load on the power line is heavy, the actual transmission distance of the power line carrier is significantly shortened. These relative disadvantages, to some extent, limit the further promotion of power line carrier technology.
(3) Case Application
Elevator operating environments are generally harsh, with nonlinear loads in the power supply lines causing high-order harmonics. High-order harmonics are also generated during inverter operation. Actual testing revealed that narrowband power line carrier technology resulted in unstable communication, failing to meet the voice and video transmission rate requirements of the elevator car. This case study employs broadband power line carrier technology with orthogonal frequency division multiplexing modulation. Within the operating frequency band, multiple mutually orthogonal subcarriers are used at certain frequency intervals. Encoded data is modulated onto these subcarriers for transmission, making it suitable for data communication in the harsh elevator environment. This case study utilizes power line carrier technology to achieve remote image and video transmission from the elevator via the elevator's IoT network. Data is transmitted through the multi-functional gateway port in the machine room, via the elevator's traveling cable lighting line, to the power line carrier module on the car top. The network output communicates with the audio/video intercom host on the car top, establishing a data link between the elevator car and the machine room, achieving a real-time rate of 94Mbps. Since the lighting line or spare traveling cable between the hoistway and the machine room is within the same transformer range, there is no need to solve the problem of power line carrier transmission across transformers using relays or other methods. This power line carrier module complies with IEEE 802.3 and IEEE 1901 standards and employs the CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) mechanism. This means that regardless of whether the current signal is busy, the transmitter waits for an inter-frame interval. When the transmitter detects that the current channel is empty, it waits for another inter-frame interval and then continues listening. If the current channel is detected to be empty again, a random backoff process is executed. After the random backoff process, the channel is listened to again. If it is still empty, the transmitter begins transmitting data. During this process, if a busy signal is detected at any stage, the transmitter exits the waiting process and continues to check if the current channel is empty. This proactive collision avoidance method, rather than passive detection, significantly improves the success rate of data transmission.
3. Comparison of Schemes
Elevator IoT shaft transmission needs to consider requirements such as safety, stability, and ease of installation. Regardless of the technology used (wired or wireless), it must not affect the normal operation of the elevator, nor the electrical circuits of the elevator's safety components, and must comply with the electromagnetic compatibility requirements of relevant elevator standards. Establishing a communication link between the machine room and the elevator car is currently mainly used for collecting elevator data related to safety hazards and enabling audio and video intercom in the elevator car during passenger entrapment; therefore, the stability requirements for the transmission scheme are high. In addition, the transmission scheme must also meet the requirement of easy installation. Simple installation reduces interference with the existing elevator system and facilitates the promotion of the solution.
Table 1 Comparison of Point-to-Point Wi-Fi and Broadband Power Line Carrier Shaft Transmission Schemes
A comparison of various parameters (as shown in Table 1) reveals that power line carrier technology outperforms Wi-Fi transmission solutions in terms of stability, transmission distance, transmission rate, ease of installation, ease of use, and compatibility.
4. Conclusion
This paper, starting from the practical application requirements of elevator IoT shaft transmission (security, high stability, and ease of installation, etc.), comprehensively compares point-to-point Wi-Fi technology and broadband power line carrier technology. It studies the application of these two technologies in shaft transmission projects from the perspectives of system structure, working principle, and application cases, concluding that power line carrier technology is more suitable for elevator IoT shaft transmission projects. This solution has been successfully applied in our company's Elevator Star IoT audio-visual intercom system and Elevator Star IoT multimedia system.
Disclaimer: This article is provided by the company. If it involves copyright or confidentiality issues, please contact us promptly for deletion (QQ: 2737591964). We apologize for any inconvenience.
