In recent years, various large and small coal enterprises located in Jining City, Shandong Province, have collaborated with relevant universities and research institutions to conduct scientific and technological research on the urgent problems in the application of sensors to the operation and control technology of coal mine belt conveyor systems. They have also actively applied existing sensor technologies and achieved many valuable research results and practical experiences, which have certain reference value for other coal enterprises to ensure the safe operation and improve the efficiency of coal mine belt conveyor systems.
1. Sensor Protection Characteristics of Centralized Control System for Coal Mine Belt Conveyors
The Nantun Coal Mine of Yanzhou Coal Mining (Group) Co., Ltd. has studied and summarized the protection characteristics of sensors in the centralized control system of the mine belt conveyor.
(1) Coal level protection sensor
The installation height should not be lower than 200mm below the level of the conveyor belt at the machine head. This ensures that when objects such as coal or gangue touch the vertical universal guide rod of the coal level sensor and deviate from the center line by 15°, the power supply to the conveyor will be automatically cut off and an alarm will sound after a delay of 1.8 seconds (adjustable), thus protecting the coal level. The coal level protection sensor can be a single universal switch to reduce size, improve sensitivity, and be unaffected by temperature and humidity changes, but it must be waterproof and moisture-proof. Alternatively, a contactless leakage current sensor can be used, as sensor failure is more likely to occur in dry roadways, leading to malfunction.
(2) Speed protection
Also known as anti-slip protection, this can utilize Hall effect magnetic sensors and magnetic induction sensors. These sensors must be small, long-lasting, highly sensitive and reliable, waterproof and moisture-proof, easy to install, and equipped with a test button. During installation, the sensor is placed on the eccentric position of the flange of the unloading roller of the belt conveyor using a magnet, ensuring it does not extend beyond the roller's outer edge. The sensor receiver is fixed to the unloading arm frame of the belt conveyor or to a bracket corresponding to the magnet on the roller shaft, with an installation interval of 35-50mm. Speed protection primarily detects a decrease in friction between the main and auxiliary rollers and belt slippage, automatically stopping the belt conveyor. Failure to stop the conveyor can cause short-term belt friction, potentially leading to a fire in the tunnel. It can also detect situations where the unloading roller fails to rotate due to bearing damage, resulting in increased motor load, equipment damage, and conveyor belt fire. This protection requires a 1.8-second delay before normal detection upon startup. When the driven roller speed of the belt conveyor drops below 30% of its rated speed, it automatically cuts off the power supply to the belt conveyor motor.
(3) Smoke sensor
An ionization smoke sensor with a built-in test button can be used. The smoke sensor is typically installed at a location downwind and free from water spray, within 5 meters above the drive roller. It activates when smoke forms around the belt conveyor or when the flammable gas concentration exceeds the standard (reaching 0.1%), with an activation delay of ≤1.5 seconds, enabling automatic shutdown of the belt conveyor.
(4) Anti-deviation sensor
One universal switch is used. One set of anti-deviation sensors is installed at the middle of the belt storage bin at the head of the belt conveyor and on both sides of the tail. The rolling guide rod is 50-100mm from the outer edge of the idler roller. When the belt deviates, the vertical universal guide rod moves under the push of the belt, and automatically stops after a 5-10 second delay when it deviates 15° from the center line.
(5) Temperature sensor
When a belt conveyor runs for an extended period or when one of its dual motors fails, the load and torque increase, leading to a rise in equipment temperature. Typically, a temperature sensor is installed with its heat-absorbing surface pressed against the motor housing or on the main conveyor drum to continuously monitor the temperature of the motor and drum. The protection temperature is set to +80℃, and the system will shut down when the temperature exceeds 3℃. Metal thermally conductive materials are used for this purpose.
(6) Longitudinal tear protection
When the conveyor belt tears, the sensor transmits an electrical signal to the main unit, immediately enabling the stop protection function.
(7) Tension drop protection device
Its function is to stop the belt conveyor when slippage occurs between the conveyor belt and the drive drum, but since anti-slip protection is already in place, this protection is not necessary.
(8) Emergency stop protection along the direction
When an emergency stop switch along the route is activated, this device can immediately provide parking protection.
(9) Comprehensive protection
For individual belt conveyors, tear protection devices and braking devices can be installed to prevent reverse rotation when stopping in inclined aisles for upward or downward transport. Temperature sensors are installed on the main drum of the conveyor to monitor the drum temperature at any time, and an automatic water spraying device is also provided to automatically spray water and cool the main drum when it overheats.
2. Sensors in the centralized control system of ground belt conveyors
The centralized control system for ground belt conveyors developed by Shandong University of Science and Technology and Tianzhuang Coal Mine of Linyi Mining (Group) Co., Ltd. in Yanzhou City uses SLG500 series PLC as its core and is equipped with sensors with various functions. It uses Ethernet to connect the conveyor belt monitoring substation to the coal preparation plant monitoring center, so as to realize the remote real-time monitoring of the conveyor belt electrical control system by the centralized control center. It can achieve relatively complete detection and protection of important parts of the system, and can detect and deal with accidents in a timely manner.
(1) Key issues that need to be addressed
Reliable data transmission between various sensors and the on-site electrical control box during belt conveyor operation; remote data transmission during belt conveyor operation.
The real-time communication between monitoring centers was addressed by setting up a master-slave configuration structure for the server; conflicts between remote control and existing control of the belt conveyor system were resolved by setting up three control modes: local, centralized automatic control, and centralized manual control; and real-time recording of data during the safe production process was ensured.
(2) Centralized control system
The centralized control system for belt conveyors is divided into two types: simple control and protection control. The centralized control system adopts a distributed control structure, mainly composed of a ground control center, vibrating screen monitoring substations and coal bunker monitoring substations, sensors, drive units, and a communication network. The vibrating screen monitoring substation controls the 201 coal feeder, 201 belt conveyor, vibrating screen, 301 belt conveyor, large block belt conveyor, hand-sorting belt conveyor, recycling belt conveyor, crushing belt conveyor, crusher, and recycling screen, etc. The coal bunker monitoring substation controls the clean coal bunker belt conveyor, steep-angle belt conveyor, and trestle belt conveyor. The monitoring substations are responsible for data acquisition from field equipment, belt conveyor start/stop control, and data communication with the control center. The data acquisition device consists of various sensors responsible for monitoring the operating parameters of the conveyor belt.
(3) Sensor selection
The sensors mainly include temperature sensors, speed sensors, belt misalignment sensors, tear sensors, coal level sensors, smoke sensors, and pull-wire switches.
GEJ30 belt misalignment sensors are installed symmetrically in pairs at the head and tail of the belt conveyor; GVD1200 longitudinal tear sensors are installed approximately 10m from the material receiving point along the conveyor belt's running direction; GUJ30 coal pile switches are installed at the coal feeding point; GQQ0.1 smoke sensors are suspended downwind of the drive unit; and KHJ30 emergency stop switches (pull-wire switches) are installed every 50m, fixed to the belt conveyor frame on one side of the walkway in the roadway. All these sensors output switch signals, which are high when the conveyor belt is running normally and low when a conveyor belt fault is detected.
A GW D-type temperature sensor is installed on the main drum of the belt conveyor; a GSC200-type speed sensor is installed near the drive unit. Both temperature and speed sensors output signals with a frequency of 200–1000 Hz. A CHA series ultrasonic level gauge is installed on top of the coal bunker to detect the height of the coal pile. It is powerful, reliable, and has a maximum range of 50 m, outputting a standard 4–20 mA analog signal.
To protect the motor of the belt conveyor, the system employs an integrated WJB-Z100M motor protection device. It detects the motor current using a 4–20mA analog signal.
All the above signals are connected to the PLC after being isolated and shielded, and participate in the emergency shutdown and interlock shutdown of the equipment. Some sensors are also waterproofed.
3. Mining Infrared Rotary Shaft Surface Temperature Sensor
By Hefei Coal Science Research Institute and Yanzhou Coal Mining (Group) Co., Ltd. Jining
The HKPW type mine infrared rotating shaft surface temperature sensor developed by No. 2 Coal Mine is an intrinsically safe instrument that can effectively prevent catastrophic accidents of belt conveyors and other electromechanical equipment, providing an ideal tool for their safe operation.
(1) Origin of the topic
Real-time measurement of the surface temperature of rotating shafts in mechanical equipment is a crucial measure to ensure safe operation, especially for large equipment. Previously, shaft surface temperature measurement relied on contact methods, which suffer from mechanical wear and can easily damage the sensing element. The HKP W-type sensor overcomes these drawbacks by utilizing the non-contact nature of infrared thermometry. When a belt conveyor experiences overload, slippage, or malfunction, the infrared surface temperature sensor non-contactly measures the shaft's surface temperature. When the temperature exceeds a set value, it outputs a power-off signal to associated equipment, cutting off the conveyor's power supply and preventing catastrophic accidents. Simultaneously, it sends audible and visual alarm signals to notify personnel to promptly troubleshoot the problem.
(2) Design concept
① Determination of Infrared Radiation Band. The infrared radiation band of the target object is determined according to Wien's displacement law. The measurement temperature range for this project is 0–150℃, corresponding to a band of 10.6–7.0 μm. Considering other factors, the infrared receiving band range is determined to be 8–14 μm. Within this range, a larger radiation power can be obtained, while avoiding the influence of absorption bands such as water vapor and gas in the air on the measurement.
② Infrared Detector Selection. Given the relatively low temperature of the target object in this project, a thin-film thermopile is suitable. Thin-film thermopiles offer high accuracy, excellent repeatability, and high operational stability; however, their disadvantages include lower sensitivity and a longer response time. These drawbacks are secondary to this project. They selected a CRC-1 type thin-film thermopile consisting of 15 pairs of thermocouples connected in series in a radial arrangement, with a response time of less than 20ms (with a window) and a built-in temperature compensation diode.
③ Determination of the optical system. The purpose of setting up the optical system is to focus the received radiation onto the detector's receiving surface, thereby improving system sensitivity and protecting the detector. Since the selected infrared band is 8–14 μm, the transmittance of ordinary optical glass is extremely low in this band. Improving transmittance requires depositing multiple layers of anti-reflective coatings, which are expensive. Therefore, they adopted a reflective optical system consisting of two mirrors with metal coatings, which has no chromatic aberration and a relatively small field of view.
The aperture can be made larger and is easy to assemble and adjust. For the same reason, the protective window is made of optical plastic film, with a transmittance of over 70% in the 8-14μm band.
(3) Basis for determining the main technical indicators
This sensor is used for monitoring the surface temperature of belt conveyor shafts. Its main technical specifications are the alarm temperature setpoint and allowable error. The upper limit temperature depends on the belt's flash point and the bearing's maximum allowable operating temperature, while the lower limit temperature is determined by the shaft surface temperature under rated load. Data indicates that the maximum allowable operating temperature of the bearing is 150℃, and actual measurements show that the shaft surface temperature during normal operation under rated load is approximately 60℃. Within the range of 60-150℃, 120℃ is a suitable alarm temperature. Under strict specifications, this infrared sensor's measurement error is less than ±2%. These specifications include: an infrared source using a blackbody furnace, an emissivity of 1, and a fixed distance. However, these conditions are not always met in actual use. This is because the emissivity of the measured object is related to its material and surface condition. The radiation coefficient is usually determined based on reference data provided by the manufacturer, which introduces a degree of subjectivity, leading to occasional errors. Furthermore, the non-uniformity of the measured target material and surface finish also contributes to measurement errors. Considering these occasional errors, a high sensor precision is meaningless; therefore, the sensor's relative error is specified to be no greater than ±3%. According to value engineering theory, reducing unnecessary precision can significantly reduce costs and increase value.
(4) Sensor calibration and adjustment
The experiment was conducted in a constant temperature chamber using a blackbody radiation source. The sensor and the radiation source were on the same central axis, and the window was 20 mm away from the radiation surface. The room temperature was set to 20°C; the emissivity was adjusted to the "1" position, the temperature compensation potentiometer was adjusted to the middle position, and the blackbody radiation source was set to 60°C. The main amplifier amplification factor was adjusted so that the display showed 60.0; the emissivity potentiometer was adjusted so that the display showed 100.0, and the emissivity was calibrated to 0.6; the constant temperature chamber temperature was adjusted to 30°C, and the displayed value was recorded as 100.0 ± Δt; the temperature compensation potentiometer was adjusted so that the display showed 100 ± (1 - 0.618) Δt; the radiation source was adjusted so that the display showed 120°C; the comparator setting potentiometer was adjusted to activate the audible and visual alarm and output an alarm signal.
(5) Sensor installation and debugging
① Installation. The sensor is installed radially on the shaft of the target object, with the window 20mm from the shaft surface.
② Emissivity Correction. When the device being measured is not working, fine-tune the emissivity potentiometer to make the displayed value equal to the ambient temperature.
4. Improvements to the sensors in multi-functional automatic spraying devices
The PJD-7 multi-functional automatic spraying device is a nationally patented piece of equipment used for automatic water spraying in underground coal mines. It has a wide range of applications and is relatively advanced and practical. Yanzhou Coal Mining (Group) Co., Ltd.'s Nantun Coal Mine encountered some problems during the application of its automatic overheat protection function on belt conveyors. Through research with the manufacturer's technical personnel, improvements were made to the sensors, significantly reducing the failure rate and improving sensitivity and reliability, making it more suitable for on-site applications on belt conveyors.
(1) Existing problems
① Sensors. The temperature and smoke sensors in the PJD-7 multi-functional automatic spray device were initially designed with analog outputs. Interference would occur when two units were connected in parallel or over slightly longer distances, resulting in poor sensitivity and frequent malfunctions. However, in underground coal mines, whether using fixed or mobile belt conveyors, the increased transport distance and volume lead to 3-4 or even more drive points. Each drive roller must be equipped with an automatic overheating sprinkler system, requiring one temperature sensor, one smoke sensor, one automatic sprinkler column, and one control box. To reduce consumption and the number of on-site devices, 3-4 identical sensors need to be grouped together, further exacerbating the interference problem. Therefore, ensuring stable sensor output signals becomes crucial.
② Automatic overheating sprinkler system. The automatic overheating sprinkler protection for belt conveyors should have a simple testing method in field use. However, the wiring of the PJD-7 multi-functional automatic sprayer is complex, making maintenance extremely inconvenient.
③ Electric ball valve. As a key component of automatic sprinkler systems, the electric ball valve has a high failure rate. After a short period of use, water can enter the motor wiring chamber along the valve stem, causing grounding damage to the motor. Furthermore, a more serious problem is that if the temperature sensor reaches the set temperature and activates, but the power is cut off before the electric ball valve opens, the valve will not open, the water flow will be blocked, and not only will the automatic sprinkler function fail, but this can also easily lead to serious consequences.
(2) Technological improvements
① Sensors. The system changes the analog signal output of the temperature sensor and smoke sensor to digital output. The fault signal judgment is moved from the main control box circuit to the sensor's own circuit, and the sensor transmits the digital signal to the main control box. This not only achieves the goal of multiple sensors working together without interference, but also greatly improves the reliability of the PJD-7 multi-functional automatic spray device.
② Automatic Sprinkler System for Overheating. To simplify the on-site testing of the automatic sprinkler system for overheating, a set of normally open test buttons was installed on the temperature sensor, connected in parallel across the thermistor Rt, while changing the output signal format of the temperature sensor. During normal operation, IC1 acts as a voltage regulator, and RW provides a reference voltage to IC2 (sensitivity can be adjusted by fine-tuning the reference voltage). The thermistor Rt has a relatively high resistance, so the voltage at the inverting input of IC2 is higher than the voltage at the non-inverting input, resulting in a low output potential and the 24V relay not activating. When the temperature near the drive drum where the temperature sensor is located reaches the set value, the resistance of the thermistor Rt decreases, the voltage at the inverting input of IC2 decreases, the potential at the non-inverting input is higher than the potential at the inverting input, resulting in a high output potential, transistor T operates, and the 24V relay is energized, transmitting a closed or open switching signal. During the test, pressing the test button short-circuits the thermistor Rt, which is equivalent to reducing the resistance of Rt, causing the 24V relay to be energized and energized. The switch signal output by the relay enters the control box of the PJD-7 multi-functional automatic spraying device, and the control box issues a command to open the electric ball valve and immediately stop the belt conveyor.
③ Electric ball valve. The leakage problem of the electric ball valve was analyzed and found to be caused by two main factors: the sealing quality of the ball valve itself and poor alignment between the valve stem and the motor gear. By strictly controlling the quality of the ball valves during procurement and improving manufacturing precision, the leakage was largely eliminated. To ensure that the water circuit could still be manually connected even after the electric ball valve lost power, a manual valve was designed to be connected in parallel with the electric ball valve. This manual valve can be opened manually when necessary.
(3) Improvement effect
This device has been operating for many years on 10 main conveyor belts in the mine, with good results.
① It solves the problem of multiple temperature sensors malfunctioning when connected in parallel, allowing one control box to connect multiple sensors. For belt conveyors with relatively close drive points, only one control box is needed to achieve the protection function, saving production costs.
② The test method is simpler, and the simulation test can be completed on the sensor itself. The self-test part is brought forward, which is more conducive to detecting the state of the sensor's own circuit.
③ It eliminates the safety hazards caused by the electric ball valve not being able to open, and further improves the applicability of the PJD-7 multi-functional automatic spraying device on site.
5. Sensor Installation for Belt Conveyor Belt Protection Device
The Xuchang Coal Mine of Zibo Mining (Group) Co., Ltd., located in Rencheng District, Jining City, has summarized its experience in installing sensors for the ZJZ-S(A1) belt protection device of the belt conveyor.
The ZJZ-S(A1) conveyor belt protection device consists of an explosion-proof and intrinsically safe integrated protection control host and various intrinsically safe sensors. The integrated protection control host consists of four main parts: a power supply board, a wiring board, a display board, and a control board. The intrinsically safe sensors consist of six main parts: coal pile protection, temperature protection, smoke protection, belt misalignment device, anti-slip device, and solenoid valve.
(1) Sensor wiring and installation
The temperature sensor, smoke sensor, and speed sensor are active sensors. They are all powered by DC 12V from the main unit and have identical wiring: the sensor's "+" terminal connects to the main unit's +12V terminal, and the sensor's "-" terminal connects to the main unit's ground terminal. The sensor outputs are uniformly designated with "K," and they connect to the "Speed," "Over-temperature," and "Smoke" terminals on the main unit, respectively. The belt misalignment sensor and coal pile sensor connect to the main unit's "Coal Pile" and "Belt Misalignment" terminals, respectively, and the main unit's ground terminal connects to the sensor's ground terminal. The solenoid valve connects to terminals K1 and K2 on the main unit's power board. When the temperature at the monitoring point exceeds the specified value (+80℃) or is tested using the test temperature button, it provides temperature fault protection.
After installation, a thorough inspection is necessary to ensure everything is correct before powering on and testing the functionality of all protective sensors. Damaged protective sensors should be repaired or replaced promptly. However, sometimes after replacing a sensor, the tape protection device may fail to activate or malfunction, primarily due to incorrect wiring. For example, if the "+12V" and "-" terminals of the smoke sensor are reversed, the smoke sensor indicator light will not illuminate. Furthermore, since its power supply is DC "12V," prolonged power supply will burn out the main control board. If "-" and "K" are reversed, pressing the test button will cause the red light to flash, but the voice alarm will not sound, and the corresponding red light on the display panel will not illuminate. If "K" and "12V" are reversed, the green light will not illuminate.
(2) Installation of belt protection sensor
The coal pile protection sensor is installed 300mm directly above the lower end of the conveyor belt or the center point of the coal bunker's sloping section. It is secured using a dedicated lifting tool or support; a counterweight-type coal pile protector is preferred if available. The belt misalignment protection device is installed on the last set of belt storage bins at the head of the belt conveyor. It is fixed perpendicular to the conveyor belt using a dedicated support and bolts, ensuring that the conveyor belt within 50mm of the edge of the upper idler can reach the anti-misalignment switch. The speed protection (also called anti-slip protection) device is installed at the belt storage guide roller. The sensor is mounted on a dedicated bracket on the guide roller, and the magnet is mounted on the hub of the guide roller. The gap between the two should not exceed 30mm. The connection and installation should be secure, level, and aligned to ensure smooth operation. The smoke protection sensor is suspended above the two main drive rollers in the downward direction of the airflow, at a height as close as possible to the material feed on the upper conveyor belt, but not touching it. A height of 300mm is generally recommended. A dedicated hanger is used for suspension. The sensors undergo a ground test once a month to ensure sensitivity and reliability. The temperature protection device is located above the drive roller, and those with insertion holes extend into the device.
For machines without insertion holes, place them parallel to the top of the transmission frame, no more than 50mm above the rollers. Use machines with test buttons and secure them with a dedicated hanger. The automatic sprinkler system is positioned 200mm directly above the center of the two main drive rollers, with the two nozzles pointing downwards. Secure it with a dedicated bracket and bolts. The water gate should open normally.
The driver on duty should test each of the above six safety features once and keep a record. If any issues arise, they should be dealt with immediately. If the issues cannot be resolved, they should be reported promptly and repaired by a dedicated maintenance personnel to ensure safe production.
6. Experimental Device for Sensor and Centralized Control System of Coal Mine Belt Conveyor
Shandong University of Science and Technology and Lunan Fertilizer Plant of Yanzhou Coal Mining (Group) Co., Ltd. have developed an experimental device for a belt conveyor centralized control system, based on the design and application of a coal mine belt conveyor centralized control system. The device uses a microcontroller, temperature sensor, smoke detector, limit switch, and proximity switch to achieve real-time monitoring of belt conveyor faults such as belt misalignment, overspeed, temperature rise, and tearing. It reproduces the actual working environment and process flow of the coal mine belt conveyor transportation system in the laboratory. The device can be used for actual production and can also cultivate students' innovative awareness and ability through secondary development.
(1) Experimental setup
The experimental setup consists of two belt conveyors, a control box, a host computer, sensors, limit switches, and proximity switches. Temperature sensors measure the temperature of the conveyor shafts; smoke detectors sense the smoke and its concentration generated by friction between the rubber belt and coal dust, or other causes; limit switches detect belt misalignment, coal level on the belt (excessive coal level indicates coal accumulation), and belt tearing; proximity switches measure the belt speed. The microcontroller continuously uploads the collected temperature, speed, and smoke data to the host computer for real-time display. In case of a fault, the microcontroller activates an audible and visual alarm and transmits the fault information to the host computer for display on the screen, allowing for immediate action to eliminate the fault or implement protection measures.
(2) Hardware composition
The analog signals measured by the temperature sensor and smoke detector are filtered and amplified, then converted into a 0-+5V voltage signal. This signal is then converted into a digital signal by an A/D converter and input to the microcontroller. The microcontroller compares this digital signal with a set upper limit. If the signal exceeds the set range, the corresponding indicator light on the control panel flashes and an alarm sounds until the fault is resolved by relevant personnel. In case of conveyor belt misalignment, coal accumulation, tearing, or overspeeding, the normally open contacts of the limit switches and proximity switches will close. This switch signal is filtered by an RC low-pass filter circuit before being input to the microcontroller. The microcontroller then drives the audible and visual alarm and uploads the signal to the host computer.
① Microcontroller. The low-power, high-performance 8-bit CMOS microcontroller AT89S52 is selected, which is fully compatible with the instructions and pins of industrial 80C51 products.
② Temperature Sensor. The economical and cost-effective IRT P300L infrared temperature sensor manufactured by Exergen Corporation (USA) was selected. Infrared sensors utilize the physical properties of infrared light for measurement. They include an optical system, a detection element, and a conversion circuit. Measurement is performed without direct contact with the object being measured, offering advantages such as high sensitivity and fast response. The IRT P300L infrared temperature sensor has a temperature measurement range of 0–150℃ and outputs a 4–20mA/0–5V electrical signal. Its measurement accuracy is unaffected by pollution, humidity, or electromagnetic interference.
③ Smoke Detector. The SS-168P photoelectric smoke detector is selected to detect the presence and concentration of smoke in the environment. Its features include: photoelectric detection; ceiling mounting; optical labyrinth; low-power CMOS microprocessor; special moisture-proof design; manual testing and reset functions; stable and reliable operation; operating voltage of 9VDC/12VDC; and extremely high smoke sensitivity.
④ Limit switch. Select LXW5-11G1 type. The limit switch can be installed on a relatively stationary object or a relatively moving animal. When the animal and the stationary object move relative to each other, it can sense the displacement between them and drive the switch contacts to close or open via a linkage, thereby controlling the on and off of the circuit.
⑤ Proximity Switch. An LJ12A-3-4-Z/BY type inductive proximity switch is selected to detect the conveyor belt's running speed. The inductive proximity switch consists of a high-frequency oscillator, a detection circuit, an amplifier circuit, a trigger circuit, and an output circuit. The high-frequency oscillator generates an alternating electromagnetic field at the proximity switch's detection surface. When a metal object approaches the detection surface, the eddy currents generated in the metal absorb the energy of the high-frequency oscillator, weakening or even stopping the oscillation. The oscillation and cessation states of the high-frequency oscillator are converted into electrical signals, which are then shaped, amplified, and converted into binary switching signals, and finally output after power amplification.
7. ControlNet monitors the status of sensors and protection systems on downhole belt conveyors.
Yanzhou Coal Mining (Group) Co., Ltd.’s Jining No. 3 Coal Mine uses ControlNet as the backbone network of the underground belt conveyor centralized control system to realize centralized control of the belt conveyor. Operators can directly monitor the various sensors and protection status of the belt conveyor in the ground dispatch room. They can also predict equipment faults based on the collected data, troubleshoot in a timely manner, and ensure equipment operation.
(1) System Composition
① Belt protection devices. These mainly include belt misalignment switches, pull-wire switches, longitudinal tear sensors, coal pile sensors, smoke sensors, slippage sensors, water sprinkler systems, and signaling devices along the belt line.
② Electrical control device. With a programmable logic controller (PLC) as the core, the PLC measures and collects process quantities and field data in real time. After comprehensive processing by modules, the data is transmitted to the controlled equipment in the field to complete the automatic start and stop of the equipment. The data is also sent to the downhole gateway and the surface workstation by the communication module. The control panel is connected to the main control substation. The field equipment information obtained from the PLC is displayed on the control panel, and the field equipment is controlled through the control panel.
③ CST Controllable Drive Unit. Mainly composed of a controllable start-up transmission section, cooling system, PLC control system, and hydraulic control system. It features excellent controllable start-up performance, a feedback system, and real-time control of start-up acceleration. It also enables controllable braking, allowing the belt conveyor to stop at a predetermined deceleration within a specified time. It offers high speed regulation accuracy, strong stability, and high reliability. Using CST can reduce basic belt tension, improve transmission efficiency, and achieve high safety with a relatively small safety factor.
④ Tensioning device. The YZL-300 hydraulic tensioning device is used, which can automatically tension the conveyor belt when it starts or stops.
(2) Implementation of ControlNet network centralized control
The control substations of each belt conveyor are connected to the underground ControlNet network via the communication interface module on the PLC. ControlNet then transmits the monitoring information from each substation to the ground dispatch and control center for centralized monitoring. The mine currently has five main coal flow belts: East, West, North, North Expansion, and Mining Area 16. Each substation transmits its monitoring information via ControlNet in three directions (East, West, and North) to the underground gateway (which aggregates and transmits information from all sources upwards). The data is then transmitted to the ground dispatch and control center for centralized monitoring.
