Control of a four-level elevator system using a programmable logic controller

International Journal of Electrical Engineering Education, Apr 2002 by Cheded, L, Al-Mulla, Ma'an

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.