1. Overview
With the continuous development of urban construction and the increasing number of high-rise buildings, elevators have a wide range of applications in the national economy and daily life. As a vertical transportation tool in high-rise buildings, elevators are inseparable from people's daily lives. In reality, elevators operate based on external call signals and their own control mechanisms. Since the call is random, an elevator is essentially a human-machine interactive control system. Simple sequential or logical control cannot meet the control requirements; therefore, elevator control systems employ random logic control. Currently, elevator control commonly uses two methods: one uses a microcomputer as the signal control unit to collect elevator signals, set operating status and functions, and realize automatic scheduling and collective operation functions, while drive control is performed by a frequency converter; the second control method uses a programmable logic controller ( PLC ) instead of a microcomputer to achieve signal collective control. From the perspective of control method and performance, there is not much difference between the two methods. Most domestic manufacturers choose the second method because their production scale is smaller, and designing and manufacturing microcomputer control devices themselves is more expensive; while PLCs have high reliability, convenient and flexible programming, strong anti-interference capabilities, and stable and reliable operation. Therefore, current elevator control systems widely use programmable logic controllers.
2. Ideal operating curve of the elevator
Extensive research and experiments have shown that the maximum acceptable acceleration for humans is am ≤ 1.5 m/s², and the rate of change of acceleration ρm ≤ 3 m/s³. The ideal operating curve for an elevator can be classified into triangular, trapezoidal, and sinusoidal waveforms based on acceleration. Since a sinusoidal acceleration curve is difficult to achieve, and the maximum acceleration and the rate of change of acceleration at the transition points during starting and braking are both greater in a triangular curve than in a trapezoidal curve (i.e., the rate of change of acceleration from +ρm to -ρm or from -ρm to +ρm), it is rarely used. The trapezoidal curve, being easier to achieve and having a good rate of change of acceleration index, is widely used. The ideal operating curve for an elevator using a trapezoidal acceleration curve is shown in Figure 1.
Intelligent frequency converters are elevator-specific frequency converters specially designed for the requirements of flexible speed regulation, control, and high-precision leveling in elevators. They can be used with common three-phase asynchronous motors and feature intelligent software, standard interfaces, menu prompts, and the ability to input elevator curves and other key parameters. They are easy and quick to debug, can automatically achieve single and multi-level functions, and have an automatic deceleration curve optimization function. Speed control systems composed of these converters have short crawl times and short leveling distances. They are suitable for both dual-winding and single-winding motors, with a maximum design speed of 4 m/s. Their unique computer monitoring software allows for contactless control of input/output signals via a serial interface.
In an elevator system powered by a frequency converter, when the frequency converter receives a call direction signal from the controller, it starts the motor based on the set speed and acceleration values. After reaching the maximum speed, it runs at a constant speed. Upon reaching the deceleration point of the destination floor, the controller sends a signal to cut off the high speed. The frequency converter then reduces the maximum speed to the crawling speed using a set deceleration rate. During deceleration, the frequency converter automatically calculates the distance between the deceleration point and the leveling point and calculates an optimized curve, allowing it to operate according to the optimized curve. This reduces the low-speed crawling time to 0.3 seconds . During leveling, the frequency converter adjusts the leveling accuracy by adjusting the leveling speed or braking ramp. Specifically, if the elevator stops too early, the frequency converter increases the low speed value or decreases the braking ramp value; conversely, it decreases the low speed value or increases the braking ramp value. When the elevator is 4-10 cm away from the leveling position, a leveling switch automatically disconnects the low-speed signal. The system then achieves high-precision leveling according to the optimized curve, thus ensuring accurate and reliable leveling.
3. Elevator speed curve
The comfort of elevator operation depends on the magnitude of its acceleration *a* and rate of change of acceleration *p* during operation. Excessive acceleration or rate of change of acceleration can cause passenger discomfort. Simultaneously, to ensure elevator efficiency, the values of *a* and *p* should not be too small. The elevator operating curve that guarantees the optimal values of *a* and *p* is called the ideal operating curve. The ideal elevator operating curve should be a parabolic-linear composite velocity curve, meaning that the elevator's acceleration and deceleration processes consist of a parabola and a straight line. Whether the given elevator curve is ideal directly affects the actual operating curve.
3.1 Method for generating velocity curves
The system utilizes an FX2-64MR PLC, and to meet input/output point requirements, three expansion modules (FX-8EYT, FX-16EYR, and FX-8EYR) and an FX2-40AW twisted-pair communication adapter were added. The FX2-40AW is used for system serial communication. The ideal speed curve is output using the PLC's D/A module. The digitized ideal speed curve is pre-stored in the PLC register. During program execution, the data is written to the D/A module via a lookup table, converted into an analog signal, and then output as the ideal speed curve.
3.2 Accelerating the generation of a given curve
The 8-bit D/A converter outputs 0-5V/0-10V, corresponding to hexadecimal values from 00 to FF, with a total of 255 levels. If the elevator acceleration time is between 2.5 and 3 seconds, conservatively, the time interval between each table lookup during elevator acceleration should not exceed 10ms.
Because the elevator logic control program is the largest, and the PLC operates using a periodic scanning mechanism, the usual lookup table method results in excessively long intervals between lookup instructions, failing to meet the accuracy requirements of the given curve. During PLC operation, the information exchange between the CPU and various devices, the execution of the user program, signal acquisition, and the output of control quantities are all performed in a fixed sequence using a cyclic scanning method. Each cycle requires querying, judging, and operating all functions. This sequence and format cannot be manually changed. Typically, one scan cycle completes six steps: operation monitoring, information exchange with the programmer, information exchange with the digital processor, information exchange with the communication processor, execution of the user program, and input/output interface services. Within one cycle, the CPU executes the entire user program only once. This mechanism has its advantages, but its real-time performance is poor. Excessive scan time directly affects the system's signal response. Minimizing the CPU's cycle scan time while ensuring control functionality is a complex problem. Generally, the only solution is to minimize the user program execution time. The program scan time of the elevator logic control section has exceeded 10ms. Although some methods have been adopted to reduce the program scan time, it is still impossible to reduce the scan time to below 10ms. Meanwhile, the braking section curve adopts a distance-based principle, and the response time for each distance segment should not exceed 10ms. To meet the real-time requirements of the system, an interrupt method is used in the speed curve generation, thus effectively overcoming the limitations of the PLC scanning mechanism.
The acceleration startup is handled by a periodic interrupt service routine. This type of interrupt cannot be toggled by the program; once set, it continuously interrupts at the set time intervals. Therefore, the startup conditions must be placed in the interrupt service routine, and the interrupt will return if the conditions are not met.
3.3 Generation of deceleration and braking curves
To ensure the completion of the braking process, braking condition judgment and deceleration point determination need to be performed in the main program. Before the deceleration point is determined, the elevator is in either acceleration or steady-speed operation. The acceleration process is completed by a fixed-period interrupt. After accelerating to the maximum value of the corresponding mode, the acceleration program's running conditions are no longer met. After each interrupt, the acceleration program is not executed again, and the program returns directly from the interrupt. The elevator runs at the maximum value of the corresponding mode. After reaching the deceleration point of that mode, a high-speed counting interrupt is generated, and the deceleration service program is executed. In this interrupt service program, the conditions for modifying the counter setting value are changed to ensure execution in the next interrupt.
In the PLC's internal registers, the values in the deceleration curve table are arranged from largest to smallest. Each interruption increments the table pointer by 1, ensuring that the lookup value for the next interrupt will be less than the current lookup value. Gate and level zone determinations are provided by external signals to guarantee the reliability of the deceleration process.
4. Elevator control system
4.1 Characteristics of the elevator control system
The starting segment of the elevator operating curve is a crucial stage affecting elevator comfort, which is directly related to acceleration. According to control theory, to make a quantity change according to a predetermined law, it must be directly controlled. For the elevator control system, to make acceleration change according to an ideal curve, acceleration feedback must be used. Based on the motor torque equation: M—MZ=ΔM=J(dn/dt), it can be seen that the rate of change of acceleration reflects the change in the system's dynamic torque. Controlling acceleration controls the system's dynamic torque ΔM=M—MZ. Therefore, a time-based acceleration control principle is adopted in this segment. When the speed in the starting ascending segment reaches 90% of the steady-state value, the system switches from acceleration control to speed control. This is because in the steady-speed segment, the speed is a constant value with minimal fluctuations, and the acceleration change is small. Furthermore, using closed-loop speed control can maintain a certain accuracy in the steady-state speed, creating conditions for precise leveling in the braking segment. Although PI controllers are used in both the speed ascending and steady-speed segments, the PI parameters are different in the two segments to improve the system's dynamic response.
During the braking phase of the system, it is necessary to control the deceleration to ensure comfort, while also strictly controlling it according to the relationship between the elevator's speed and distance to ensure leveling accuracy. Before the system speed drops to 120 r/min, in order to balance both aspects, a method is adopted that primarily controls the time based on acceleration, while simultaneously correcting the acceleration setpoint curve based on the deviation between the actual speed and the theoretical speed at each braking distance. For example, at a certain distance L from the leveling point, the speed should be reduced to Vm/s, but the actual speed is higher than V′m/s. This indicates that the applied braking torque is insufficient. Therefore, after calculating the given deceleration value -ag at this point, a negative deviation ε is added to it, so that the given deceleration value at this point is corrected to -(ag+ε), which increases the negative deviation between the given deceleration and the actual speed, thereby increasing the braking torque and making the speed drop to the standard value quickly. When the motor speed drops to 120r/min, the car is only a few centimeters away from the leveling point, and the elevator's running speed is very low. To prevent the elevator from stopping before reaching the leveling area and to ensure that the elevator can enter the leveling area quickly, proportional regulation and time optimization control are used in this section to ensure that the elevator enters the leveling area accurately and in a timely manner, so as to achieve accurate and reliable leveling.
4.2 Elevator Control Structure
Since elevator operation is controlled based on floor and car call signals and travel signals, and these calls are random, the system employs stochastic logic control. This means that while sequential logic control fulfills the basic elevator control requirements, the system controls the elevator's operation in real-time based on random input signals and the elevator's corresponding state. Furthermore, the car's position is determined by the pulse count from a pulse encoder, which is then sent to the PLC's counter for control. Additionally, a proximity switch is installed on each floor to detect the system's floor signal.
To facilitate observation, the elevator's direction of travel and the floor it is on are displayed using LEDs and LED lights, while the call signals for the floor and car are displayed using indicator lights (the switches have indicator lights).
To improve elevator operating efficiency and leveling accuracy, the system requires the PLC to effectively control the car's acceleration, deceleration, and braking. This is achieved based on the car's actual position and the control algorithm of the AC speed control system. For elevator safety, the system should be equipped with reliable fault protection and corresponding displays. The PLC-based elevator control system consists of the following main components.
4.2.1 PLC Control Circuit: The PLC receives call signals from the control panel and each floor's call button, function signals from the car and 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 also sends signals to the frequency converter for running direction, start, acceleration/deceleration, and braking/stopping.
4.2.2 Current and speed dual closed - loop circuit: The frequency converter itself is equipped with a current detection device, thus forming a current closed loop; through the rotary encoder connected coaxially with the motor, two-phase pulses a and b are generated and enter the frequency converter. While confirming the direction, the speed closed loop is formed by pulse counting.
4.2.3 Displacement Control Circuit: As a means of transporting people, elevators, under potential load conditions, require not only safety and reliability but also smooth operation, comfortable ride, and accurate stopping. A variable frequency speed control dual-loop control can basically meet these requirements. While utilizing the existing rotary encoder to form the speed loop, the PG card of the frequency converter outputs a pulse count proportional to the motor speed and elevator displacement. This pulse count is then introduced into the high-speed counting input port of the PLC. By accumulating the pulse count, the pulse equivalent is calculated using equation (1), thereby determining the elevator position. Elevator Displacement
h=SI
In the formula, I—cumulative pulse count; S—pulse equivalent;
S = plc / (pr) (1)
l—reduction ratio; D—traction wheel diameter; P—number of pulses per revolution of the rotary encoder; r—PG card frequency division ratio.
4.2.4 Terminal Station Protection: When the elevator is moving upwards, the upward direction relay, fast-moving auxiliary contactor, fast - moving contactor, door lock relay, and upward contactor are all energized and engaged, the brake is released, and the elevator moves upwards. When the car encounters the upper forced speed change switch, the PLC's internal latching relay is energized and engaged, and timers Tim10 and Tim11 begin timing. The timing duration can be set according to the terminal station floor spacing and elevator speed. After the upper forced speed change switch is activated, the elevator switches from fast-moving to slow-moving operation. Under normal circumstances, the elevator should stop when leveling at a floor. If the car continues to move upwards without stopping, when the Tim10 set value decreases to zero, its normally closed contact opens, the slow-moving contactor and the upward contactor are de-energized, and the elevator stops running. When the elevator car encounters the forced speed change switch, and for some reason fails to switch to slow speed operation, and the fast speed contactor fails to release, when the Tim11 setpoint decreases to zero, its normally closed contact opens, de-energizing both the fast speed contactor and the upward contactor, causing the elevator to stop. Therefore, regardless of whether the elevator is running at slow or fast speed, as long as the forced speed change switch issues a signal, the elevator can be stopped using Tim10 and Tim11, regardless of whether other protection switches at the terminal station are activated, thus making the elevator terminal station protection more reliable.
When the elevator needs to descend, once a selection command is received, the descending direction relay is energized, its normally open contact closes, the latching relay is reset, and both Tim10 and Tim11 are de-energized, their normally closed contacts close, preparing the elevator for normal descent. The protection principle of the lower station is similar to that of the upper station and will not be repeated.
4.2.5 Floor Counting: Floor counting uses a relative counting method. Before operation , the number of pulses corresponding to the floor height is measured through self-learning, and the data for each of the 17 floors is stored in 16 memory units DM06～DM21. The floor counter (CNT46) is a bidirectional counter. When the floor counting point is reached, the counter increments or decrements by 1 according to the direction of travel.
During operation, the accumulated value of the high-speed counter is compared in real time with the pulse count corresponding to each floor. When they are equal, a floor counting signal is issued, incrementing by 1 for upward movement and decrementing by 1 for downward movement. To prevent the counter from counting repeatedly during the high-level period of the counting pulse, the floor counter is triggered by the rising edge of the floor counting signal.
4.2.6 Rapid Speed Change: When the high-speed counter value equals the number of pulses corresponding to the rapid speed change point, if the elevator is running at high speed and there is a floor selection signal for this floor, a rapid speed change signal is issued. If the elevator is running at medium speed or running at high speed but there is no floor selection signal for this floor, no speed change signal is issued.
4.2.7 Gate signal: When the value of the high-speed counter CNT47 is within the range of the pulse number corresponding to the gate, a gate signal is sent.
4.2.8 Pulse Signal Fault Detection: Accurate acquisition and transmission of pulse signals are crucial in the system. To detect faults in the rotary encoder and pulse transmission circuit, a pulse signal presence/absence and pulse error detection circuit was designed to ensure normal system operation through real-time detection. To eliminate cumulative pulse counting errors, a reset switch is installed at the base station and connected to the reset terminal of the PLC high-speed counter CNT47.
5. Software Design Features
5.1 Use a priority queue
Based on the elevator's location and direction of travel, four priority queues were used in the programming: an upward priority queue, an upward secondary priority queue, a downward priority queue, and a downward secondary priority queue. The upward priority queue is an array of registers containing the pulse counts of upward call signals from floors above the elevator's current location. The upward secondary priority queue is a queue of registers containing the pulse counts of upward call signals from floors below the elevator's current location. This real-time arrangement of the four priority queues in the control system provides the foundation for implementing stochastic logic control.
5.2 Adopting a First-In-First-Out (FIFO) queue
Based on the elevator's direction of travel, non-zero cells in the priority queue (cell 70 when there is a call, and cell 0 when there is no call) are sent to the register queue (FIFO). The FIFO read instruction SFRDP is then used to send the data in the first cell of the FIFO to the compare register.
5.3 Random logic control is adopted.
When the elevator approaches a floor's deceleration position in a certain direction, it checks whether there is a call signal in the same direction (upward call flag register, downward call flag register; the corresponding register is 1 when there is a call request, otherwise 0). If there is, the pulse count of the corresponding register is compared with the comparison register. If they are the same, the elevator decelerates and stops at that floor. If they are different, the data in the former register is sent to the comparison register, the original data in the latter register is saved, and the elevator decelerates and stops at that floor. After this process is completed, the saved data is sent back to the comparison register to achieve random logic control.
5.4 Software display is used.
The system uses travel distance to determine the floor and converts it into BCD code for output, which is then displayed on LEDs via a hardware interface circuit.
5.5 Control of the frequency converter
Based on the requirements of random logic control, the PLC can send forward operation, reverse operation, deceleration, and braking signals to the frequency converter, which then controls the motor according to certain control laws and algorithms. Simultaneously, when a system fault occurs, the PLC sends a signal to the frequency converter.
6. Conclusion
The elevator control system using the MIC340 elevator-specific frequency converter can realize intelligent elevator control. However, since the number of people waiting in the elevator and the number of people arriving at each floor are uncertain, even with the AITP artificial intelligence system, the transmitted traffic flow information is still ambiguous. To solve these two major unknown factors in the elevator vertical transportation control system, we need to continue to research and explore in our future work.
