1. Introduction to Elevator Control
Elevators are vital transportation tools in modern buildings, ensuring the safety of people's lives and property. Improving elevator operating efficiency, reducing energy consumption, minimizing mechanical wear, and extending elevator lifespan are all crucial research topics. An elevator is a complete set of equipment used for fixed-position lifting between floors, characterized by safety, reliability, comfortable ride, accurate stopping, ease of operation, and high transportation efficiency. It consists of a lifting and traction system, a guiding system, safety devices, and an electrical control system.
Currently, elevator control generally employs two 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, while drive control is handled by a frequency converter. The second method uses a programmable logic controller (PLC) instead of a microcomputer to achieve collective signal control. In terms of control method and performance, there is no significant difference between the two methods. Domestic manufacturers mostly choose the second method because their production scale is smaller, and designing and manufacturing their own microcomputer control devices is more costly; while PLCs offer higher reliability and more convenient and flexible programming. This design uses a Mitsubishi FX2 series PLC to control a static magnetic grating displacement sensor to achieve elevator leveling control.
In elevator control systems, static magnetic grating displacement sensors are used for adjusting elevator leveling. The electrical control system is the "central nervous system" of the elevator, and its quality directly affects the overall elevator performance. Passenger elevators and medical elevators prioritize passenger comfort, which is related to travel time. To achieve greater comfort, acceleration and deceleration times must be extended, thus increasing travel time and reducing elevator efficiency. Therefore, to ensure high elevator efficiency, acceleration and deceleration should have appropriate limits and be smooth. This places the following requirements on the electrical control system:
Safe and reliable, easy to troubleshoot, and the simpler the wiring, the better, provided that the usage requirements are met.
The noise and vibration should be low, the components should be selected reasonably, the electromagnetic noise should not be loud, the structural components of the installation parts should have sufficient rigidity, and there should be anti-loosening measures.
It can adapt to the requirements of frequent starts, stops, adjustments, and reversals, has good speed regulation performance, and is easy to switch working modes. Acceleration, deceleration, and constant speed should be smooth, the speed curve should be smooth, and there should be no micro-movements before reaching the station.
It can achieve automatic leveling, and the leveling must be accurate.
It can adapt to varying loads over a wide range and can start under heavy loads.
Based on the characteristics of elevator operation and the above requirements, the elevator's operating speed should conform to the curve shown in Figure 1. The leveling error should conform to the specifications in Table 1.
2. Introduction to Static Magnetic Grating Displacement Sensor
The static magnetic grating displacement sensor consists of two parts: a "static magnetic grating source" and a "static magnetic grating ruler." The "static magnetic grating source" uses an aluminum alloy-sealed passive neodymium iron boron magnetic grating to form a magnetic grating encoding array. The "static magnetic grating ruler" is encapsulated in a specially made high-strength aluminum alloy tube with an embedded microprocessor system, and uses a switch-type Hall sensor to form a Hall encoding array. The aluminum alloy tube is externally treated with an anti-oxidation plastic coating. When the "static magnetic grating source" moves non-contactly (with a relative gap tolerance and relative attitude tolerance of up to 50mm) along the axis of the "static magnetic grating ruler," the "static magnetic grating ruler" analyzes the digital displacement information, directly generating a digital signal of displacement exceeding millimeter-level displacement. By fully utilizing the resources of the embedded microprocessor, the data update speed is increased to the millisecond level to adapt to displacement responses at speeds below 5m/s.
3. Comprehensive Features of the Product
Long service life: Non-contact detection of positions and angles avoids mechanical damage, theoretically with no lifespan limit;
Resistant to harsh environments: Operating temperature range of -40℃ to +100℃; suitable for continuous high dust, mud, underwater, and high impact/vibration environments.
Direct absolute measurement: Directly indicates displacement in millimeters or rotation angle, no conversion required, unaffected by power outages, and allows for arbitrary positioning control;
Extremely long measuring range with moderate resolution: 260 mm to 2000 m length range, resolution 0.2 mm to 1 mm;
Abundant data interfaces: 4-20mA, 1-5V analog outputs, various serial and parallel data interfaces, and various fieldbuses such as PROFIBUS;
Easy to install and maintain: It can be installed and operated without constraints while maintaining an appropriate clearance.
4PLC-controlled static magnetic grating displacement sensor to achieve elevator leveling control
To ensure that the elevator automatically levels itself upon reaching a floor, an automatic control system, or elevator automatic control device, is required. The control component of this device is a static magnetic grating displacement sensor. Taking a 30-story elevator as an example, the installation diagram is shown below.
The car shown in the diagram is located on the first floor above the basement. The static magnetic grid source is installed parallel to the outdoor floor in the elevator shaft, with one source on each floor. The static magnetic grid ruler is installed on the car and is 1.2 meters long . Two static magnetic grid sources are installed on the basement floor to detect whether the car has reached the bottom and the direction of movement.
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 adjusts elevator operation in real-time based on random input signals and the elevator's corresponding state. Furthermore, the car's position is determined by a static magnetic grating displacement sensor and fed into the PLC's counter for control. Additionally, a static magnetic grating source is installed on each floor to detect the system's floor signal.
a . When the elevator is moving upwards, the static magnetic grating ruler detects the static magnetic grating source in the upward direction, the brake opens, and the elevator moves upwards. When the car encounters the upper forced speed change switch, the PLC's internal latching relay is energized, and timers Tim10 and Tim11 begin timing. The timing duration can be set according to the terminal floor distance and elevator speed. After the upper forced speed change switch is activated, the elevator switches from fast to slow operation. Under normal circumstances, the elevator should stop when leveling at the 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 and upward contactors are de-energized, and the elevator stops. If, after the car encounters the upper forced speed change switch, the elevator fails to switch to slow operation for some reason, and the fast operation contactor fails to release, when the Tim11 set value decreases to zero, its normally closed contact opens, both the fast operation and upward contactors are de-energized, and the elevator stops. Therefore, regardless of whether the elevator is running at slow speed or fast speed, as long as the forced speed change switch sends a signal, the elevator can be stopped by Tim10 and Tim11, regardless of whether other protection switches at the terminal station are activated, thus making the elevator terminal station protection more reliable.
b . When the elevator needs to descend, as long as 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 for the elevator to descend normally. The protection principle of the lower station is similar to that of the upper station and will not be repeated.
c . Floor counting uses a relative counting method. Before operation, the number of pulses corresponding to each floor height is measured through self-learning, and the data is stored in 30 memory units DM06~DM21 for each of the 30 elevator floors. The floor counter (CNT46) is a bidirectional counter. When reaching the floor counting point, it increments or decrements by 1 according to the direction of travel. During operation, the accumulated value of the high-speed counter is compared with the number of pulses corresponding to the floor counting point in real time. 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 of the counting pulse, the floor counting is triggered by the rising edge of the floor counting signal.
d . When the high-speed counter value equals the number of pulses corresponding to the fast speed change point, if the elevator is running at high speed and there is a floor selection signal for this floor, a fast 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.
e . Gate signal; when the value of the high-speed counter CNT47 is within the range of the number of pulses corresponding to the gate, a gate signal is sent.
5. Software Design Features
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 consists of an array of registers containing the static magnetic source information for floors above the elevator's current location that send upward call signals. The upward secondary priority queue consists of registers containing the static magnetic source information for floors below the elevator's current location that send upward call signals. This real-time arrangement of the four priority queues by the control system provides the foundation for implementing stochastic logic control.
The system employs a first-in-first-out (FIFO) queue. Based on the elevator's direction of travel, non-zero cells (cell 70 when there is a call, and cell 0 when there is no call) in the priority queue in the same direction are sent to the register queue (FIFO). The first-in-first-out read instruction (SFRDP) is then used to send the data in the first cell of the FIFO to the compare register.
Random logic control is employed. 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 so, the pulse count of the corresponding register is compared with a 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 stops at that floor. After this process is completed, the saved data is sent back to the comparison register to achieve random logic control.
6. Conclusion
Elevator leveling control is achieved by using a Mitsubishi FX2 series PLC to control a static magnetic grating displacement sensor. This enables intelligent elevator control, resulting in a comfortable elevator ride. The comfort during starting, deceleration, and leveling remains consistent regardless of changes in car load, achieving satisfactory results.
