Design and Practice of an Elevator Control System Based on PLC

By Ricardo Hudson,2014-03-03 20:31
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Design and Practice of an Elevator Control System Based on PLC


    2008 Workshop on Power Electronics and Intelligent Transportation System Design and Practice of an Elevator Control System Based on


    1, 211Xiaoling Yang, Qunxiong Zhu, Hong Xu

    1 College of Information Science &Technology,

    Beijing University of Chemical Technology, Beijing 100029, China

    2 Automation College of Beijing Union University,Beijing,100101, China



    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 "minimum waiting time" to run 2 elevators in parallel 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.

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    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 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

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    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 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 Output points

    description address description address

    1-8 floor up hall-call I0.0-I0.7 1-8 floor up hall-call lamp Q0.0-Q0.7

    2-9 floor down hall-call I1.0-I1.7 2-9 floor down hall-call lamp Q1.0-Q1.7

    Q2.0-Q2.7, 1-9 floor car-call I2.0-I2.7, I3.0 1-9 floor car-call lamp Q3.0

    I3.1-I3.7, 1-9 arrival sensor up moving lamp Q3.1 I4.0-I4.1

    door open button I4.2 down moving lamp Q3.2

    door close button I4.3 Seven segment LED display of Q3.3-Q3.7

    door close location switch I4.4 elevator’s position Q4.0-4.1

    door open location switch I4.5 door opening Q4.2

    up leveling sensor I4.6 door closing Q4.3

    down leveling sensor I4.7 up moving Q4.4

    fire switch I5.0 down moving Q4.5

    driver operation switch I5.1 full load lamp Q4.6

    touch panel switch of car door I5.2 high speed operation Q4.7

    overload I5.3 low speed operation Q5.0

    Forced speed changing switch I5.4 acceleration of starting Q5.1

    full load I5.5 deceleration of braking Q5.2-Q5.4

     alarm beeper 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


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    Figure 3 The combination of the calls

    Figure 4 The cancellation of up calls

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    In Figure4, the auxiliary relay M4.0 is the up moving flag of the elevator. When the current direction of the elevator is up, M4.0’s contacts are closed; on the contrary, when

    the current direction of the elevator is down, M4.0’s contacts are opened. M0.1 to M0.7

    denotes the car-calls’ stopping request signal of floor 2 to floor 8 respectively.

    This program has two functions:

    (1) Make the elevator response the normal down hall-calls when it is moving down, and when a down hall-call is served, its registration is canceled.

    (2) When the elevator is moving up, the corresponding floor’s down hall-call it

    passing by is not served and the registration is remained.

    The cancellation of down hall-calls is reversed with up hall-calls. 3.4 Elevator’s direction

    The elevator may be moving up or down, depending on the combination of hall-calls and car-calls. The following ladder diagram in Fig.5 illustrates that the elevator will move up.

    Figure 5 Up moving of the elevator

    Figure5 shows that when the calls corresponding floor is higher than the elevator’s

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    current location, the elevator will go up. Here the auxiliary relay M4.0 is used as the up-moving flag. When the elevator is moving up, the up-moving lamp is illuminated, so the M4.0 is connected on. When the elevator arrives the top floor, the up-moving lamp is off and the timer starts. After 0.2s, the M4.0 is disconnected, the up-moving display is off. Here we used M4.0 to replace Q3.1 which can ensure the cancellation’s reliability.

    3.5 Elevator’s floor-stopping

    Figure6 shows the ladder diagram of the elevator’s floor-stopping function.

    As showed in Figure6, M6.4 is the flag of floor-stopping signal. M6.6 is the floor-stopping signal sent by the driver. M7.0 is the fire signal sent by the fire switch. And M6.7 is the forced speed changing signal. When either of these contacts act, the system should send out the floor-stopping signal.

    Figure 6 The elevator’s floor-stopping

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    4. Minimum waiting time algorithm

    In traffic of elevator systems, there are two types of control task usually. The one is the basic control function to command each elevator to move up or down, to stop or start and to open and close the door. The other is the control of a group of elevators.

    The main requirements of a group control system in serving both, car and hall calls, should be: to provide even service to every floor in a building; to minimize the time spent by passengers waiting for service; to minimize the time spent by passengers to move from

    [1]one floor to another; to serve as many passengers as possible in a given time.

    There are many dispatching algorithms for elevator’s group control. Such as

    [2]Nearest-neighbor Algorithm, which the elevator always serve the closet request next;

    [3]Zoning Algorithm which by analyzing the traffic of elevator system with unequal floor and population demand to dispatch the elevator; and Odd-even rule, which an elevator only serves the odd floor and the other only serves the even floor.

    The Nearest-neighbor Algorithm minimizes the length of the elevator’s empty move

    to the next request. It usually has very small average waiting times, but individual waiting

    [2]times can become quite large. The Zoning Algorithm usually used in buildings which has

    heavy traffic situations, such as the office building at lunch time.

    Compared to the office building and shopping mall, the traffic flow of residential buil dings is relatively low and even in every floor. Secondly, people usually think of elevators as purely functional objects and the experience of riding an elevator is time waited for most of them. Furthermore, there exist immense problems when attempting to satisfy all requirements.

    Considering all of the reasons above, we adopted the “minimum waiting time”

    [4]algorithm to realize the 2 elevators’ parallel running.

    4.1 Evaluation function

    The goal of the “minimum waiting time” algorithm is to predict the each elevator’s

    response time according to all calls, and select the elevator which has the shortest response time to serve.

    When there is a call, the system calculates out the function values of each elevator according the evaluation function showed in (1) and (2):

    J(*)=Min[J(1),J(2),…,J(n)] (1)

    J(i)=Tr(i)+KTd(i)+KTo(i) i=1,2,...,n (2)

    J(i) is the evaluation index of each elevator; Tr(i) denotes the time of the elevator

    directly moving to the destination corresponding the latest call from its current floor; To(i)

    denotes the additional acceleration and deceleration time of a floor-stop of the elevator; Td(i) denotes the average time of the passenger boarding and alighting the elevator; and K

    is the sum of hall-calls and car-calls. But when a hall-call and a car call corresponds the

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    same floor, the K is only calculated one time.

    4.2 Calculation of minimum waiting time

    In equation (2), K is a certain value, To and Td can be obtained by means of statistics. Tr = T*L, where T denotes the average time of the elevator passing by one floor; L denotes

    the desired floors of the elevator from current floor to the hall-call floor.

    In order to calculate the L value, we defined the 2 elevators are A and B respectively;

    YA,YB denotes the current floor of elevator A and B respectively. H is the corresponding

    key value when a hall-call button is pressed, and H=floor number of the hall-call.

    We defined 4 tables for the PLC realization: up hall-call registration table, down hall-call registration table, car-call registration table of A and car-call registration table of B.

    When a certain call button is pressed, its floor value is recorded in corresponding table.

    Here we take elevator A as an example. First, define the variable MA, MB and MW.

    Where MA, MB denotes the extreme value of car-calls with same direction of A or B’s

    movement respectively.

    When elevator A is up-moving, set MA is equal to the maximum value in car-call registration table A; when elevator A is down-moving, set MA is equal to the minimum

    value in car-call registration table A.

    MW denotes the extreme value of hall-calls with same direction of A’s movement.

    When elevator A is up-moving and up-hall-call value?YA, set MW=0; otherwise, set

    MW is equal to the minimum value in up-hall-call registration table A. When elevator A is

    down-moving and up-hall-call value?YA, set MW=0; otherwise, set MW is equal to the

    maximum value in down-hall-call registration table A .

    Thus, we can determine the L value according to YA, H, MA and MW. There are 3


    (1) When the hall-call’s direction is opposite to elevator A’s movement:

    L=|YA-MA|+|MA-H| (3)

    (2) When the hall-call’s direction is same as elevator A’s movement and it is in the

    front of elevator A:

    L=|YA-H| (4)

    (3) When the hall-call’s direction is same as the elevator A’s movement and it is in the

    back of elevator A:

    L=|YA-MA |+|MA-MW|+|H-MW| (5)

    So the i-th floor’s minimum waiting time can be calculated by (6) as follows:

    Time(i)=TL(i)+KTd(i)+KTo(i) i=1,2,...,n (6)

    When the calls change during the operation of elevators, the system calculates the minimum waiting time of each elevator. Then it allocates the current call to the elevator

    which has small value. When the each elevator has the same value, then the current call is

    prior to elevator A.

    When an elevator is wrong or not in service, the system can exit the dispatching algorithm and turns to a single elevator running mode.

    4.3 Algorithm realization

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