Control of a four-level elevator system using a programmable logic controller
Cheded, L
Abstract This project reports on the design and implementation of a PLC-based controller for a fourlevel elevator. The PLC used is an Omron Sysmac C20K with 12 inputs and 8 outputs. The design incorporates an intelligent controller that services all the requests in an energy-saving way, rather than on a first-come, first-served basis. Some suggestions on how to extend this system to the control of more than four floors are also included.
Keywords elevator control; intelligent control; Omron Sysmac; programmable logic control; state diagram
Many engineering systems, including elevator systems,”2 are nowadays controlled by programmable logic controllers (PLCs). The reliance on these PLCs is due to their high reliability and efficiency. In this paper, the design and implementation of a four-level elevator system controlled by a small PLC is discussed.
The PLC may be considered as a special-purpose computer with a basic architecture similar to that of any other known computer such as a central processing unit (CPU).3,4 It is based on a memory and a number of input and output terminals. The software used for PLC programming is based on a special language known as the ladder diagram. The ladder diagram is an easy programming language since it is based on Boolean logic functions. This makes the task of modifying any system much easier and more cost-effective. The size of the PLC is one of the factors considered when it is selected to control a process. PLCs come in various sizes and different capabilities; the sizes range from small controllers with limited inputs and outputs used for controlling small processes, to very large ones with more inputs and outputs provided which are used to control much larger processes and operations. Determining the appropriate PLC to be used is based on analysing the process to be controlled and accordingly identifying the number of inputs and outputs required.
The PLC has many advantages over other control systems. It is known for its flexibility, lower cost, operational speed, reliability, ease of programming, security, and it is easy in implementing changes and correcting errors.3 One of the applications using PLCs is the control of elevator systems. A simulation of such control was successfully tested in a senior-project course in the Systems Engineering Department at KFUPM, where an Omron 12-input and 8-output PLC was used. This paper presents both the hardware and software aspects of the successful design and operation of this PLC-based controller.
Objectives of the project
The objective of the project is the design and implementation of a four-level elevator system controlled by a Programmable Logic Controller (PLC). The design was limited to four levels due to the limited number of inputs provided by the Omron PLC available. Some suggestions are given later as to how to extend the elevator system to more than four levels. Moreover, the design was not based on a first-come first-served basis, since this approach was not found to be practical. As a practical compromise between energy consumption and speed of response, we decided to combine all requests going in one direction (up or down), and then process them in sequential order. This was achieved in the following manner: if the elevator is going up, all ‘up’ requests are given higher priority than the ‘down’ requests until the elevator reaches the last destination upwards. Then the elevator goes down but now gives priority to all ‘down’ requests over the ‘up’ ones. More details on this will be given in the section `Software design’.
Hardware design
The objective of the hardware design is to develop the interface circuit between the PLC and the elevator system and the elevator control panel, with both external and internal requests. These requests are produced by push buttons that send continuous signals to the PLC when activated. Each push button is connected to an LED to identify the request placed. In addition, the four floors are represented by four LEDs, one for each level. Furthermore, an alarm switch is installed to produce a flashing signal whenever activated. This facility was introduced to simulate the desire for a sudden stoppage of the elevator either for reasons of safety or for requests for a repair job to be carried out on the elevator.
In order to obtain the desired setup, we needed to find a way to capture the pulse generated by a depressed push button. We also needed to make sure that the PLC is recognising these signals in order for it to correctly perform the required action. As explained below, both issues were resolved by using set/reset flip flops and relays respectively.
The block diagram of the system’s layout is shown in Fig. 1, where both the interface between the PLC and the elevator system with the control panel are drawn.
Description of the interface circuit
The hardware components used in the project are listed below: Omron Sysmac C20K PLC.
74LS04.74LS279.74LS156, which are inverters, SR flip flops and buffers, respectively.
Voltage Supply. Push buttons, LEN, resistors, relays, a switch, and connecting wires.
Since the number of required inputs and outputs, i.e. 12 and 8 respectively, matches the maximum input/output capability of the PLC used, there is no need for any multiplexing or demultiplexing operations. Thus all inputs and outputs used can be directly controlled by the PLC.
As shown in Fig. 2, the push buttons were connected to the SR flip flops, since the PLC needs continuous signals to process, and so do the lights that indicate the requests placed. The flip flop holds the signal until the reset is activated. The reset of the flip flop is the level position for levels L1 and L4. So when the elevator reaches one of these two levels and a request is placed the output will reset the requested signal. However levels L2 and L3 are reset by software. The reason for that is because L2 and L3 are intermediate levels. So when the elevator is travelling upwards or downwards, it has to either flash at the level it passes to show the current elevator position or service this level if its request has the appropriate direction by setting its request. In this case, it will also reset all requests associated with the serviced levels.
Description of the control panel
The 12 inputs and 8 outputs used in this project are listed and defined in Table 1.
As shown in Fig. 3, the elevator system consists of three sections: internal requests, external requests, and the elevator position. The internal requests are represented by the push buttons inside the elevator which consists of four push buttons (1-4) and a door open (DO) push button. A door close push button could not have been included in the design because of the limited number of available inputs. The external requests are represented by the six push buttons located outside the elevator and distributed according to their corresponding floors. It consists of six push buttons distributed according to the position of the level. The elevator position is displayed by the four LEDs, one for each level, which are directly controlled by the PLC according to the location of the elevator.
Software design
The elevator system may run in two different control modes: dumb control and intelligent control. In order to achieve the complete design, all possible transitions and stages the elevator system has to go through were considered and a complete flowchart was drawn for this.5
Control modes
Of the two different control modes: dumb control, or intelligent control, the dumb control is not popular these days because it is not practical. It is usually used for transporting material and equipment in buildings, where all the floors have to be visited sequentially and continuously. On the other hand, the intelligent control responds to requests placed by users by ordering and processing them in an intelligent manner. This type of control is used in most applications requiring modern elevators. Hence only the intelligent mode was designed and implemented as discussed below.
Intelligent control
The intelligent control is based on taking all requests and ordering them in an intelligent manner such that the earlier-mentioned compromise between energy consumption and speed of response is met. All possible transitions were included as shown in the state diagram of Fig. 4.
Description of the operation of the elevator under intelligent control
The elevator starts at level 1. It opens the door for 5 s, then checks for requests in upper levels. The movement from one level to another is represented by a timer. The transition between two successive levels takes 8 s. As soon as a request is serviced, the door opens for 5 s to take passengers in, and then proceeds to the next request to be serviced. Whenever a level is passed by, its light flashes for 1 s to indicate the current position of the elevator on its way to its required destination. The requests whose direction (up or down) is similar to the current direction of the elevator are always serviced before those made in the opposite direction, regardless of which requests were made first. The system continues to service all the remaining requests in a similar way. Whenever no more requests are left to service, the elevator will simply remain at the level it was last at, keeping the door open for 5 s and then closing it until a fresh request is made. However the door is programmed to never open in between levels, and whenever the alarm switch is activated, the alarm signal starts flashing and the elevator stops at the next immediate destination, opens the door and freezes all requests until the alarm is set off again.
An illustrative example on the intelligent control of the elevator is explained below: Assume the following requests: 21), 3U, 4D, were made and the elevator is currently at level 1. The PLC will then perform the following sequence: First, all up requests are serviced, i.e. in this case only 3U will be serviced. Next the elevator reaches the fourth floor to service 41), and finally it services the remaining down requests, which in our case is 2D.
The complete ladder diagram and program for the intelligent control mode can be found in ref. [5].
Suggestions for extending the current system to a higher-than-4-level
elevator system
The elevator system designed and implemented in this project was restricted to four levels. This restriction was due only to the limited number of inputs and outputs provided by the PLC used. However, to extend the system to more than four levels, some suggestions are made below:
Suggestion 1
Use a more powerful PLC. PLCs come in different sizes and with various capabilities. When increasing the number of levels in the elevator system, the designer must identify the number of inputs and outputs required to select the most suitable PLC.
Suggestion 2
Use two Omron PLCs working together. One PLC can be dedicated to the control of the lower four floors while the other controls the upper three floors. Information about the switch from the set of four floors to the set of three floors and vice-versa can be transmitted from one PLC to the other via two communication lines, C 12 and C21, as shown in Fig. 5. The line C12 will transmit to PLC2 the information that PLCI has sensed the requests for the upper three floors (FS-F7) and once all requests associated with the lower four floors (FI-F4) have been serviced, the control of the upper three floors will be automatically transferred to PLC2. A similar function is carried out by C21. In this case, instead of having 12 inputs and 8 outputs only, 22 inputs and 14 outputs would be required as shown in Fig. 5.
Suggestion 3
The use of a more elaborate input/output interface board together with a single Omron PLC is worthwhile investigating. It should however be borne in mind that, in this case, the overall system cost should be kept lower than that in the previous two suggestions.
Conclusion
In this paper, the successful design and implementation of the intelligent control of a 4-level elevator system using only a small educational PLC was discussed. The design includes a simple scheme that aims at a good compromise between energy consumption and speed of response without requiring any extra circuitry. Some suggestions as to how to extend the design to handle a larger number of floors were also given. Finally, it is hoped that this work has demonstrated that, despite their limited control capabilities, small educational PLCs, when fully exploited, can indeed tackle industrial control jobs of modest size in a cost-effective way.
References
I G. C. Barney and S. M. Dos Santos, Lift Traffic Analysis: Design and Control (Peter Peregrinus, 1977).
2 G. R. Srtakosh, Vertical Transportation: Elevators and Escalators (John Wiley, New York, 1967).
3 I. Warnock, Programmable Controllers (Prentice Hall, Englewood Cliffs, 1988).
4 J. W. Webb, Programmable Logic Controllers: Principles and Applications (Macmillan, New York, 1988).
5 M. Al Mulla, Control of a 4-level Elevator System Using a Programmable Logic Controller, Senior Project, Systems Engineering Department, KFUPM, September 1988.
L. Cheded’ and Ma’an Al-Mulla2
1Systems Engineering Department, KFUPM, Saudi Arabia
2Saudi Aramco, Dhahran, Saudi Arabia
E-mail: cheded@kfupm.edu.sa; maanmm@yahoo.com
Copyright Manchester University Press Apr 2002
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