Design and Practice of an Elevator Control System Based on
PLC
Xiaoling Yang1, 2, Qunxiong Zhu1, Hong Xu1 1
College of Information Science &Technology,
Beijing University of Chemical Technology, Beijing 100029, China 2
Automation College of Beijing Union University,Beijing,100101, China
yxl_lmy@ sina.com, zhuqx@mail.buct.edu.cn,
Abstract
This paper describes the development of 2 nine-storey elevators control system for a residential building. The control system adopts PLC as controller, and uses a parallel connection dispatching rule based on \mode. The paper gives the basic structure, control principle and realization method of the PLC control system in detail. It also presents the ladder diagram of the key aspects of the system. The system has simple peripheral circuit and the operation result showed that it enhanced the reliability and performance of the elevators.
1. Introduction
With the development of architecture technology, the building is taller and taller and elevators become important vertical transportation vehicles in high-rise buildings. They are responsible to transport passengers, living, working or visiting in the building, comfortable and efficiently to their destinations. So the elevator control system is essential in the smooth and safe operation of each elevator. It tells the elevator in what order to stop at floors, when to open or close the door and if there is a safety-critical issue.
The traditional electrical control system of elevators is a relay-controlled system. It has the disadvantages such as complicated circuits, high fault ratio and poor dependability; and greatly affects the elevator’s running quality. Therefore, entrusted by an enterprise, we have improved electrical control system of a relay-controlledelevator in a residential building by using PLC. The result showed that the reformed system is reliable in operation and easy for maintenance.
This paper introduces the basic structure, control principle and realization method of the elevator PLC control system in detail.
2. System structure
The purpose of the elevator control system is to manage movement of an elevator in
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response to user’s requests. It is mainly composed of 2 parts:
2.1 Electric power driving system
The electric power driving system includes: the elevator car, the traction motor, door motor, brake mechanism and relevant switch circuits.
Here we adopted a new type of LC series AC contactors to replace the old ones, and used PLC’s contacts to substitute the plenty of intermediate relays. The circuits of traction motor are reserved. Thus the original control cabinet’s disadvantages, such as big volume and high noise are overcome efficiently.
2.2 Signal control system
The elevator’s control signals are mostly realized by PLC. The input signals are: operation modes, operation control signals, car-calls, hall-calls, safety/protect signals, door open/close signal and leveling signal, etc. All control functions of the elevator system are realized by PLC program, such as registration, display and elimination of hall-calls or car-calls, position judgment of elevator car, choose layer and direction selection of the elevator, etc. The PLC signal control system diagram of elevator is showed in Figure 1.
Figure 1 PLC signal control system diagram
2.3 Requirements
The goal of the development of the control system is to control 2 elevators in a 9-storey residential building.
For each elevator, there is a sensor located at every floor. We can use these sensors to locate the current position of the elevator car. The elevator car door can be opened and closed by a door motor. There are 2 sensors on the door that can inform the control system about the door’s position. There is another sensor on the door can detect objects when the
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door is closing. The elevator car’s up or down movement is controlled by a traction motor.
Every floor, except the first and the top floor, has a pair of direction lamps indicating that the elevator is moving up or down.
Every floor, has a seven segment LED to display the current location of the elevator car.
The first step for the development of the elevator control is to define the basic requirements. Informally, the elevators behavior is defined as follows.
(1)Running with a single elevator
Generally, an elevator has three operation states: normal mode, fire-protection mode and maintenance mode. The maintenance mode has the highest priority. Only the maintenance mode is canceled can the other operation modes be implemented. The next is fire-protection mode, the elevator must return to the bottom floor or base station immediately when the fire switch acts. The elevator should turn to normal operation mode when the fire switch is reset. Under normal operation mode, the control system’s basic task is to command each elevator to move up or down, to stop or start and to open and close the door. But is has some constraints as follows:
Each elevator has a set of 9 buttons on the car control panel, one for each floor. These buttons illuminate when they are pressed and cause the elevator to visit the corresponding floor. The illumination is canceled when the corresponding floor is visited by the elevator.
Each floor, except the first and the top floor, has two buttons on the floor control panel, one to request an upelevator, one to request a down-elevator. These buttons illuminate when they are pressed. The illumination is canceled when an elevator visits the floor, then moves in the desired direction.
The buttons on the car control panel or the floor control panel are used to control the elevator’s motion.
The elevator cannot pass a floor if a passenger wants to get off there. The elevator cannot stop at a floor unless someone wants to get off there.
The elevator cannot change direction until it has served all onboard passengers traveling in the current direction, and a hall call cannot be served by a car going in the reverse direction.
If an elevator has no requests, it remains at its current floor with its doors closed. (2)Parallel running with two elevators
In this situation, there are two elevators to serve the building simultaneously. It runs at 7am to 9am and 5pm to 7pm every day.
When an elevator reaches a level, it will test if the stop is required or not. It will stop at this level when the stop is required.
At the same time, to balance the number of stops, the operation of two elevators will follow a certain dispatching principle.
An elevator doesn’t stop at a floor if another car is already stopping, or has been stopped there.
The normal operation of elevators is implemented by cooperation of its electric power driving system and logic control system.
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3. Software design
Due to the random nature of call time, call locations and the destination of passengers, the elevator control system is a typical real-time, random logic control system. Here we adopted collective selective control method with siemens PLC S7-200 CPU226 and its extension modules. There are 46 input points and 46 output points in the system. The I/O points are showed in Table1.
Table 1 I/O address distribute Input points description 1-8 floor up hall-call 2-9 floor down hall-call 1-9 floor car-call 1-9 arrival sensor door open button door close button door close location switch door open location switch up leveling sensor down leveling sensor fire switch driver operation switch touch panel switch of car door overload Forced speed changing switch full load Output points address I0.0-I0.7 I1.0-I1.7 I2.0-I2.7, I3.0 I3.1-I3.7, I4.0-I4.1 I4.2 I4.3 I4.4 I4.5 I4.6 I4.7 I5.0 I5.1 I5.2 I5.3 I5.4 I5.5 description 1-8 floor up hall-call lamp 2-9 floor down hall-call lamp 1-9 floor car-call lamp up moving lamp down moving lamp Seven segment LED display of elevator’s position door opening door closing up moving down moving full load lamp high speed operation low speed operation acceleration of starting deceleration of braking alarm beeper address Q0.0-Q0.7 Q1.0-Q1.7 Q2.0-Q2.7, Q3.0 Q3.1 Q3.2 Q3.3-Q3.7 Q4.0-4.1 Q4.2 Q4.3 Q4.4 Q4.5 Q4.6 Q4.7 Q5.0 Q5.1 Q5.2-Q5.4 Q5.5 About software designing, we adopt the modularized method to write ladder diagram programs. The information transmission between modules is achieved by intermediate register bit of PLC.
The whole program is mainly composed of 10 modules: hall-call registration and display module, car-call registration and display module, the signal combination module, the hall-call cancel module, the elevator-location display module, the floor selection module, the moving direction control module, the door open/close module, the maintenance operation module and the dispatching module under parallel running mode.
The design of the typical modules is described as follows:
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3.1 Hall-call registration and display
There are two kinds of calls in an elevator: hall-call and car-call. When someone presses a button on the floor control panel, the signal will be registered and the corresponding lamp will illuminate. This is called hall-call registration.
When a passenger presses a button in the elevator car, the signal will be registered and with the corresponding lamp illuminated. This is called car-call registration.
Figure2 shows the ladder diagram of up hall-calls registration and display. The self-lock principle is used to guarantee the calls’ continuous display.
Figure 2 up hall-call registration and display
3.2 The collective selection of the calls
Here the collective selection control rules are used. As showed in Figure3, M5.1-M5.7, M6.0 and M6.1 are auxiliary relays in PLC. They denote the stopping request signal of 1st to 9th floor respectively. The auxiliary relay M6.2 denotes the elevator driver’s operation signal. When there is a call in a certain floor, the stopping signal of corresponding floor will output. When the elevator is operated by the driver, the hall-calls will not be served. And the elevator cannot pass a floor at which a passenger wishes to alight. 3.3 The cancellation of the calls
The program of this module can make the elevator response the hall-calls which have the same direction as the car’s current direction, and when a hall-call is served, its registration will be canceled. The ladder diagram of up hall-calls’ cancellation is showed in Figure4.
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