Design and Implementation of Elevator Control System Based on PLC and Frequency Converter Liu Yang 12 (Department of Electrical Engineering, Wuhan Electric Power Vocational College, Wuhan, Hubei 430079) Flowchart shows the design of an elevator control system using PLC and frequency converter.
Elevators, as transportation tools, carry a large volume of people and goods, playing a crucial role in the transport of passengers and goods. Currently, elevators generally employ two control methods: the first uses a microcomputer as the signal control unit to collect elevator signals, set operating status and functions, and achieve automatic scheduling and collective operation, with drive control handled by a frequency converter; the second uses a programmable logic controller (PLC) instead of a microcomputer to achieve signal collective control. Most elevators in China use the second control method, primarily because PLC control offers advantages such as high reliability, simple programming, small size, and compact structure. Furthermore, PLCs typically have self-diagnostic capabilities, fault type display, and fault alarm functions. Using PLC control for elevator control not only simplifies external wiring but also improves elevator safety and reliability, and facilitates future functional expansion.
The main task of an elevator is to respond to external inputs, and after calculation by the PLC, determine the elevator's operating mode. A complete elevator operation involves the traction motor starting, running at a constant speed, and then decelerating to a stop. The schematic diagram of its operation is shown below.
Schematic diagram of elevator operation process: Start - Input command - Up/Down - Reach floor - Stop. II. Hardware Design of Elevator Control System: This control system is designed based on a three-story elevator. The hardware consists of an Omron CP1H PLC and a frequency converter (Omron 3G3MZ). Programming and debugging can be performed on a computer using corresponding programming software. During elevator operation, the elevator's operating status can be monitored and displayed through the PLC and computer programs.
In the hardware design, I/O addresses are first allocated according to the input and output points of the PLC, so that each input signal corresponds to the input relay inside the PLC and each output signal corresponds to the output relay inside the PLC.
Input signals: The control buttons of the control system should be located on the elevator's operation panel. For each floor, there should be a floor selection button, a limit switch input to control the elevator's arrival at a specific floor (6 inputs in total), a down call button for the top floor, an up call button for the bottom floor, and one up and one down call button for the second floor (4 inputs in total), one manual door open button input, one manual door close button input, one door open limit switch, one door close limit switch, one elevator up limit switch, and one elevator down limit switch (16 input points in total). Output signals: When a passenger calls a floor, a down call button display should be provided for the top floor, an up call button display for the bottom floor, and one up and one down call button display for the second floor (4 outputs in total); one floor signal display and one car floor button display are required for each floor (3 floors in total, 6 outputs); in addition, one up indicator output, one down indicator output, one motor forward rotation output, one reverse rotation output, one door open output, and one door close output are also required (16 outputs in total).
The elevator control system mainly includes a PLC, frequency converter, photoelectric rotary encoder, car control panel, hall call panel, door operator, and other electrical components, as shown in the block diagram. The intelligent elevator control system mainly consists of two parts: a signal control system and a drive control system. The main hardware includes the PLC host and its expansion, car control panel, mechanical system, hall call panel, door operator, speed control device, floor indicator lights, and main drive system. The core of the system control is the PLC host. The signal control system is implemented through PLC software, including in-car commands, door zone, hall call, operation control, forward stop, leveling signal, and door opening/closing operation program control. In the drive control system, the feedback signal of the elevator's current operating status is directly sent to the PLC, which then sends leveling, speed switching, and start control signals to the drive system. [mPLC elevator control system block diagram] III. Software Design of the Elevator Control System The software design of the elevator control system is a crucial aspect of the control system design. Elevator control systems are essentially personal-computer interactive systems. During elevator operation, various input signals occur randomly, introducing numerous uncertainties and complex self-locking and interlocking relationships. Therefore, elevator control systems employ stochastic logic control. The PLC receives call signals from the control panel and each floor, functional signals from the car and landing door systems, and status signals from the hoistway and frequency converter. Through program judgment and calculation, it achieves collective control of the elevator. While outputting display and monitoring signals, the PLC, based on the requirements of stochastic logic control, sends signals to the frequency converter for starting, decelerating, and stopping, completing the entire elevator operation control process.
The elevator's operation cycle is as follows: detecting call and floor selection signals, determining the elevator's direction of travel, starting operation, and performing operations such as stopping the elevator in the correct direction, calling the elevator, and memorizing in-car selections during operation. Upon reaching a floor, the door opening/closing procedure is executed. The PLC control function flowchart for the elevator in actual operation is shown in the figure. The elevator's operation can be divided into two states: stationary and moving.
(1) When the elevator is stationary and stops at a certain floor, the elevator receives two types of signals: call signals from other floors. The PLC makes a decision to go up or down based on the response process of the call signal, so that the traction motor is energized to go up or down; if there is a call signal at the floor where the elevator is stopped, the door opening and closing program is executed, and then the elevator goes up or down according to the floor selection signal.
(2) When the elevator is in operation, if it is going down, it only responds to the up or down status indication. This allows passengers inside and outside the car to clearly know the elevator's operating status when calling for a lower floor. During the upward movement, it only responds to the call signal from passengers on the upper floor. The elevator has floor indication during operation, and the elevator control system model has been successfully manufactured. Based on the flowchart, the elevator's up and down programs, door opening and closing programs, and inverter parameters were written. After combining these programs, the system ran well after debugging and achieved the control objective. Using a PLC for elevator operation control greatly simplifies the hardware structure of the control system. Furthermore, due to the use of a "soft wiring" method for program control, the system's reliability and flexibility are greatly improved. Of course, the elevator control system has a large number of signal input points, making the initial investment more expensive than a relay system. However, this higher cost can be addressed by reducing the number of input points.
