Elevator control system based on Siemens PLC

1 Introduction
In conventional automatic control systems, sensors and actuators are wired independently, and a system consisting of multiple sensors and actuators requires a large number of wires. The application of communication buses to measurement and control systems can not only save a large number of wires, but also improve the reliability of the system. There are generally two types of industrial buses that have been widely used. One is the master-slave structure, such as RS-485 communication. This communication bus has been widely used in industrial control, and its communication method is command-response. The host sends query signals to each sub-controller at regular intervals, and then each sub-controller reports its own status. This communication method is less difficult to develop, but the communication actually consumes a considerable part of the resources of the main controller. Therefore, this method does not fully exert the powerful computing function of the main controller. The other type is the autonomous communication method of each node, such as the CAN bus of Omron and Mitsubishi, and the LONWORKS bus of NEWLIFT. The reliability and communication rate of this type of bus are substantially improved compared with the previous one, but the cost is relatively expensive.
2 Siemens Actuator-Sensor Interface Bus AS-Interface
In view of the advantages and disadvantages of the two popular serial bus control methods, Siemens has absorbed the advantages of the two control methods and launched the AS-Interface (remote I/O) bus technology. AS-Interface is an actuator-sensor interface bus system, which collects scattered I/O signals through slave stations and transmits them to the AS-I master station with only two signal lines. The AS-I master station calls in sequence, and the longest cycle time is 5ms. In the event of an error, the AS-I slave node has the function of automatically shutting down the bus, cutting off its connection with the bus, so that other slave stations are not affected, and its fault problem can be reflected on the AS-I master station in time. Each frame of AS-I information has CRC check and other error detection measures to ensure the high reliability of the AS-I bus. The direct communication distance of the AS-I bus can reach up to 100m, and the maximum distance extended through the relay station is 300m. The AS-I bus can install up to 248 sensors and actuators.
The connection between the Siemens PLC host and the actuator-sensor-interface slave station is through the AS-I master station, and there is no need to edit the communication program additionally. For engineers, the remote I/O corresponds to the corresponding bit in the image area, which conforms to their programming habits and is very convenient. Due to the application of two-wire communication, the system connection line adopts a card-line piercing structure, which greatly reduces the wiring volume, and the unique AS-I ladder cable eliminates the possibility of wiring errors. Compared with the previous PLC control system, it can save a lot of cables and greatly reduce the installation workload.
3 Elevator control system
The elevator control system has experienced a considerable technological leap from relay control to PLC plus speed regulator control. The existing products are also formed and the performance is quite stable. The basic structure of the existing elevator control system is shown in Figure 1. The control center is in the rooftop machine room, and all signals in the hoistway and car are transmitted to the control center in a point-to-point form through a large number of cables.

Figure 1 Traditional elevator control system
Traditional elevator control systems have the disadvantages of being difficult to maintain and inefficient due to excessive wiring, complex installation, and difficulty in modification and expansion. Elevator users' requirements for elevators are no longer limited to the pursuit of basic functions such as system safety and reliability. They have put forward higher requirements for elevator comfort, efficiency, self-fault diagnosis, remote monitoring and other intelligence, as well as elevator debugging and maintenance simplicity. Therefore, there is an urgent need for a high-efficiency and high-reliability fieldbus technology to meet user requirements, and AS-Interface bus technology is the best choice. The physical implementation of the AS-I bus is two-wire communication, and the wiring adopts a card-wire piercing structure. The AS-I slave station can be easily connected to the bus, and the unique AS-I ladder cable eliminates the possibility of wiring errors. The following is
a discussion on the AS-Interface bus system of Siemens S7-200CN PLC to realize elevator control.

4 Application of AS-Interface Bus in Elevator Control System
4.1 Hardware Implementation
Siemens S7-200CN PLC with AS-Interface bus function
has good performance and powerful functions, supporting trigonometric functions, square root, logarithmic operations and other functions; it can be edited and monitored online; it supports remote monitoring through a modem; it can diagnose faults, perform single scans, and force outputs; it can edit variable status tables, and use multiple windows that can be opened at the same time to display signal status and status tables at the same time. Therefore,
the elevator control system based on S7-200CN PLC is a networked, intelligent, and highly cost-effective control system.
In terms of the hardware implementation of the system, after careful investigation and demonstration, it was found that the sensors and actuators of the elevator control system are basically concentrated in the shaft and the car, and the machine room has only one actuator, namely the speed regulator, but no sensors. Therefore, it is not reasonable to use the machine room as the control center. In order to optimize the hardware layout of the system, the project made the following adjustments to the traditional elevator control system: the control system and the traction system of the elevator are physically separated, which changes the state of the traditional elevator system where control and traction are inseparable. The advantage of this is that it truly realizes the separation of strong and weak electricity, greatly improves the anti-interference of the system, and further ensures the safety and reliability of the elevator system; since most of the elevator signals are in the car and the shaft, if the control center is placed in the machine room, even if the AS-Interface bus technology is applied, the AS-I slave stations it needs are very considerable (taking a 10-story 10-station elevator as an example, there are about 100 signals in the car and the shaft, and the I/O number of an AS-I slave station is at most 8, that is, 13 slave stations are required to meet the requirements). This system, even if it is advanced but not economical, is difficult to be accepted by the project. The project's approach is to transfer the control center to the top of the car. This transfer greatly reduces the cost of the control system without reducing its advancement (also taking a 10-story 10-station elevator as an example, there are about 48 signals in the shaft and the machine room, and only 6 slave stations are required).
Figure 2 shows an elevator control system that uses AS-I bus technology based on the above ideas. The control center is located on the top of the car and consists of three parts: CPU226CN (PLC), EM223 (PLC expansion), and AS-I master station. The signals on the car are directly connected to the I/O of the PLC, and the signals from the hoistway and the machine room are transmitted to the AS-I master station through the AS-I slave station. The on-site installation is very simple.

Figure 2 AS-I elevator control system
4.2 Introduction to related Siemens control components
The following is a brief introduction to the performance and function of CPU226CN, AS-I master station CP243-2, expansion EM223 and AS-I slave station.
(1) S7-200CN master controller (CPU226cn)
● Composition
This machine integrates 14 inputs/10 outputs, a total of 24 digital I/O points. It can be connected to 7 expansion modules, with a maximum expansion of 168 digital I/O points or 35 analog I/O points. 13K bytes of program and data storage space. 6 independent 30kHz high-speed counters, two independent 20kHz high-speed pulse outputs, with PID controller, 1 RS-485 communication/programming port. It is a controller with strong control capabilities. As shown in Figure 3:

Figure 3 CPU226CN
●Function
Installed on the top of the car, responsible for controlling the position of the car, driving the car door, receiving various electrical signals from the car, processing various signals between the AS-I master station and the modem communication.
(2) AS-I master station
●Performance
The AS-I cycle time is no more than 5ms. The maximum current allowed by the AS-I connection cable is 3A. It can be directly connected to an external 24V power supply. Its address range: one 8DI/8DO digital module and one 8AI/8AO analog module. It can be seen that the response time and load capacity of the AS-I master station are extraordinary. As shown in Figure 4:

Figure 4 CP243-2
●Function
Installed on the top of the car, responsible for communicating with the main controller and controlling the AS-I slave station.
(3) Expansion
●Performance
The EM223 expansion unit has 8I/8O, a total of 16 digital signal input and output ports, with photoelectric isolation, low power consumption and other functions.
●Function
An important reason for placing the control center on the top of the car is that most of the elevator signals are concentrated on the car, and these signals can be sent directly to the control center in parallel, which is a very economical and feasible method. Therefore, it is obvious that when the number of I/O points of the CPU224 itself cannot meet the requirements, it must be supplemented by expansion (EM223).
(4) AS-I slave station
The signals from the shaft and the machine room are connected to the elevator control system through the AS-I slave station. Therefore, the AS-I slave station is installed in the shaft and the machine room in a dispersed manner, responsible for processing the signals in the call box and controlling the speed regulator.
4.3 Software Implementation
Siemens S7-200CN series PLC automatically maps the AS-I slave to 8 analog input words (AIW0~AIW7) and 8 analog output words (AQW0~AQW7). For engineers, programming the AS-I slave is no different from programming ordinary I/O. You only need to add a small program to achieve the mapping of the slave I/O to the PLC.
The list of starting AS-I and mapping conversion programs is as follows:
LD SM0.1
SI Q3.7, 1
RI Q3.0, 4
LD SM0.7
BMW AIW0, VW1000, 8
BMW VW2000, AQW0, 8

4.4 Workflow
The core of elevator control is to analyze various signals and control the speed regulator, door machine, etc. to drag the car. In the Siemens S7-200CNPLC serial system, various control and data signals of the shaft and machine room are transmitted to the AS-I master station through the AS-I slave station, and then transmitted to the CPU226CN through the AS-I master station. Similarly, if CPU226CN wants to issue instructions to a certain slave station, the AS-I master station must complete it. All signals on the car are directly sent to the CPU226CN through parallel I/O points. The following takes the process of an elevator processing a call signal as an example to briefly introduce its workflow.
When the system is powered on, the CPU226CN performs a power-on self-test. This includes I/O inspection, communication inspection with the master station, and the correctness of the parameters of the current state of the elevator (door state, automatic, maintenance or driver, elevator position, etc.). Once an error is found, it enters the fault state and blocks the express train until all faults are eliminated before entering the normal operation state. Once the slave detects a call signal, it will immediately transmit it to the AS-I master station through the AS-I signal cable, and then send an interrupt signal to the CPU226CN through the master station, and finally transmit the call signal to the CPU226CN for processing. The transmission time of a signal is less than 5ms.
After receiving the signal, the CPU226CN determines the running direction and parking position of the elevator according to the current state of the elevator, and sends instructions to the slave station through the AS-I master station to control the speed regulator and traction machine.
5 Elevator Remote Monitoring System Based on Siemens PLC
5.1 Introduction to Communication Ports
The internally integrated PPI interface provides powerful communication functions for users of S7-200CN. The physical characteristics of the PPI interface are RS-485, and it can work in three modes:
(1) PPI mode
The PPI communication protocol is a communication protocol developed by Siemens specifically for the S7-200CN series PLC. It can be connected to the network through an ordinary two-core shielded twisted pair cable. The baud rate is 9.6kbps, 19.2kbps and 187.5kbps. The programming port integrated on the S7-200CN series CPU is also the PPI communication protocol. It is very simple and convenient to communicate. Only two statements, NETR and NETW, are needed to transmit data signals without additional configuration of modules or software. The PPI communication network is a token passing network. Without adding a repeater, up to 31 S7-200CN series PLCs, TD200, OP/TP panels or host computers (MPI card inserted) can be used as stations to form a PPI network.
(2) MPI mode
S7-200CN can be connected to the MPI network through the built-in interface with a baud rate of 19.2/187.5kbps. It can communicate with S7-300/S7-400CPU.
(3) Free port mode
Free port mode is a very distinctive function of S7-200CN PLC. It enables S7-200CNPLC to communicate with other devices and controllers with any open communication protocol.
5.2 Hardware Implementation
We use the free port method to connect the 485 port of CPU226CN to the modem through a cable, and then connect it to the telephone line, and connect the modem to the computer in the monitoring room. After the connection is completed, the elevator on site can be monitored through dial-up Internet access. Among them, the modem uses Star Online Star 5600db+, and the hardware block diagram is shown in Figure 5:

Figure 5 Remote monitoring hardware block diagram
5.3 Software settings
Since the Siemens STEP-7MicroWIN programming software itself has the relevant settings for remote monitoring, it is not necessary for engineers and technicians to redevelop the communication, saving a lot of costs. After entering the interface of the STEP-7MicroWIN programming software, remote monitoring can be achieved with simple settings.
In Communication, set the Local Modem and Remote Modem to the same model (otherwise the Local
Modem cannot be burned). If the selected hardware Modem cannot be found in the option column, it must be customized. Customize the Modem configure, as shown in Table 1.
Table 1 Customize Modem configure

6 Comprehensive Index Analysis
The application of bus technology in elevators (also known as serial communication elevators) has been adopted in some domestic elevators. Large elevator manufacturers such as Shanghai Mitsubishi, Guangzhou Hitachi, and Tianjin Otis have begun to adopt this technology in large quantities, but for the vast number of small and medium-sized elevator companies in China, the introduction and development of this system will undoubtedly consume a lot of manpower and material resources. Sichuan Jianning Elevator Factory introduced a complete set of Taiwan TS868 elevator serial communication system in 2000. Compared with the current independently developed serial communication system based on Siemens AS-I bus technology, a comparison table is listed, as shown in Table 2.
Table 2 Comparison table of TS868 and SIEMENS (S7-200CN)

It is not difficult to see from the comparison table that the development of the serial communication system based on Siemens AS-I bus technology is excellent in both cost and technology, and is particularly suitable for product upgrades of small and medium-sized elevator companies.
7 Conclusion
The serial communication system based on Siemens AS-I bus technology is very suitable for independent development by small and medium-sized enterprises. Without the need for engineers and technicians to give up familiar PLC control, or large investments, the elevator products can be upgraded and keep up with international trends, allowing the company to gain a good market position in the fierce market competition. The application of this technology in elevators has been successfully implemented in two elevators in the main teaching building of Sichuan Leshan Normal University in October 2001.