Abstract: This paper lists several commonly used displacement sensors in long-stroke displacement control. Based on the analysis and comparison of the principles, characteristics, and limitations of various displacement sensors, it explains in detail the characteristics of the static magnetic grating long linear displacement sensor. The static magnetic grating displacement sensor has a unique design, direct position detection, and theoretically no lifespan limit. Keywords: Static magnetic grating sensor, long linear displacement control, PLC. The detection and control of object displacement has always been the most commonly used and fundamental positioning method in industry. Currently, products have undergone significant changes in structure and performance, with more complete functions, higher reliability, and development towards automation and intelligence. The performance of displacement detection products is mainly reflected in meeting accuracy, reliability, timeliness, durability, and the degree of automation. ● Introduction to Static Magnetic Grating Displacement Sensor The static magnetic grating displacement sensor is a new type of high-reliability displacement sensor product developed and manufactured by Wuhan Yingjia Technology Development Co., Ltd. based on the absorption and assimilation of the world's most advanced magnetic displacement sensor technology. Since its introduction, it has been widely favored by industries such as metallurgy, machinery, petrochemicals, transportation, and water conservancy. General-purpose displacement sensors, level gauges, gate opening meters, crane stroke detectors, and hydraulic cylinder stroke detectors produced using this technology are widely used in harsh industrial environments. Its non-contact displacement measurement method is suitable for an extremely wide range, overcoming the shortcomings of imported products such as high price, inadequate after-sales service, poor environmental adaptability, limited range, and long order cycles. It can interface with various automatic control systems. This static magnetic grating displacement measurement technology borrows the principle of "vernier caliper differential scale micrometry," relying on the Hall effect of the Hall line array to analyze the digital absolute displacement code, ensuring accuracy and reliability. The commercialization process of this original new technology has been very rapid. I. Principle of Static Magnetic Grating Displacement Sensor 1. The static magnetic grating displacement sensor consists of two parts: a "static magnetic grating source" and a "static magnetic grating scale." 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. 2. When the "static magnetic grating source" moves relative to the "static magnetic grating ruler" axis without contact (relative gap tolerance and relative attitude tolerance up to 50mm), the "static magnetic grating ruler" analyzes the digital displacement information and directly generates a digital signal of displacement higher than millimeters. It fully utilizes the resources of the embedded microprocessor, increasing the data update speed to the millisecond level to adapt to displacement responses at movement speeds below 5m/s. It directly measures object displacement and angle, with a standard resolution of 1mm, and currently the highest resolution reaching 0.2mm, with a maximum measurement length of 2km. The mechanism is similar to a "vernier caliper," where a 1mm graduation can distinguish 0.02mm, resulting in a 50-fold increase in resolution. The spacing between the magnetic grating array and the Hall encoding array differs, much like the different scales of a differential slide bar on a vernier caliper. Through a series of iterative algorithms, the resolution of the static magnetic grating absolute encoder can reach 0.2 mm or less. 3. The "spatial static magnetic grating," as the name suggests, uses a permanent magnet to act on the displacement measuring device without contact through a spatially subdivided grating-like area. The magnetic field's area can be subdivided into grating-like spaces of considerable width and thickness, each 0.2 mm thick. These "spatial gratings" have very fixed numbers, and during the continuous switching between power on/off and power off, or when the permanent magnet leaves and returns to the area of ​​action, they can accurately reflect the detected position or angle, truly achieving "absolute encoding." In simpler terms, the Hall encoding array elements only have two states: on and off. Due to different physical positions, each Hall switch element contains different position information. Through high-speed computer scanning and identification, "absolute encoding of spatial linear position" is achieved. Regardless of the measurement range, as long as the installation accuracy is guaranteed, very small indication errors can be obtained. It is basically unaffected by external factors such as temperature, humidity, stray magnetic fields, and electromagnetic interference. 4. The matching display controller can realize conventional displacement control functions, integrate on-site display and adjustment functions and data communication conversion functions. It can not only adjust parameters on-site and directly implement position closed-loop control, but also provide multiple hardware forms such as I2C, RS485, RS422, SSI, CAN, PROFIBUS to facilitate direct data acquisition by the host computer. If connected to a fieldbus, it can be directly connected to a frequency converter. For short-distance measurement and control (e.g., less than 2 meters), it provides 4-20mA or 1-5V standard signals. II. Detailed Description of Static Magnetic Grating Displacement Sensor 1. Comprehensive Product Features: 1.1 Long Service Life: Non-contact detection of position and angle avoids mechanical damage, and theoretically has no life limit. 1.2 Harsh Environment Resistance: Operating temperature range of -40℃ to +100℃; withstands continuous high dust, mud, underwater (tested by the Central South National Metrology and Testing Center, this product can operate underwater at 1000m, test certificate number: Force No. 04411486), and high impact and strong vibration environments. 1.3 Direct Absolute Measurement: Directly indicates displacement in millimeters or rotation angles, no conversion required, unaffected by power outages, and allows for arbitrary positioning control. 1.4 Extremely Long Measuring Range and Moderate Resolution: 260mm-2000m length measuring range, resolution 0.2mm; omnidirectional 360-degree angle measuring range, resolution 0.1 degree. 1.5 Abundant Data Interfaces: Offers NBK mode, N mode is built-in, providing standard analog signals such as 1-5V and 4-20mA, and three digital signals: I2C, RS485, and RS422. Type B is the type with a dedicated transmitter, which can provide standard analog signals and richer digital signals such as I2C, RS485, RS422, SSI, high-speed parallel output, Profibus-DP, DeviceNet, EtherNet, CANopen, InterBus, ControlNet, ASI, PF, Modbus, etc. Type K is the type with a dedicated display controller. In addition to all the interfaces of Type B, it also has multiple functions such as display position, closed-loop control, and communication conversion, and can form an independent display control system. 1.6 Easy installation and maintenance: Unrestricted installation and operation under the condition of maintaining a moderate clearance. 2. Product appearance description 2.1 Static magnetic grating displacement sensor: Basic product parameters: ① The static magnetic grating scale is a specially made aluminum alloy "A" tube type (cross-section size 52 mm * 35 mm, 80 mm mounting lip), with a maximum length of 4200 mm and an effective detection length of 250 mm to 4000 mm. ② The static magnetic source uses an aluminum alloy-sealed passive neodymium iron boron magnetic grid with a cross-sectional dimension of 120*25*20mm. ③ Cable length: 2000mm. ④ Resolution: 1mm (0.2mm, 0.5mm, 1mm, 2mm, 5mm) ⑤ Scale-base separation type working gap: 0～10mm (maximum 0～50mm) ⑥ Output interface: RS485 digital output (analog output 1-5V, 4-20mA, bus-type digital output) ⑦ Ambient temperature: -10℃～85℃ ⑧ Encapsulation type: ordinary waterproof type (deep water pressure resistant type) ⑨ Operating conditions: Environment: IP69 (underwater), -10℃～＋85℃, Vibration: omnidirectional amplitude ±5mm, 5～200Hz, Acceleration: omnidirectional 50g, 1 time/minute 2.2 Display controller: Basic parameters of the display controller: ① Display controller dimensions: 130 mm * 100 mm * 55 mm, 140 mm * 100 mm * 65 mm ② The basic function is to display real-time position signals. Extended functions include communication mode conversion and positioning (synchronous) control of single, double, and four-foot scales. ③ The wiring terminals are WAGO. ④ It can independently achieve closed-loop position control and set and store the central position value. 3. Static Magnetic Grating Displacement Sensor: The static magnetic grating displacement sensor is a non-contact, direct measurement type displacement sensor. The "static magnetic grating source" is embedded with multiple sets of coded neodymium iron boron magnetic gratings, and the "static magnetic grating scale" is encapsulated in a 5mm thick special aluminum alloy tube, providing a high level of shock resistance and waterproofing (IP69). The "static magnetic grating source" and "static magnetic grating scale" can be freely separated during product transportation and storage. When the installed sensor moves along the axis of the detection surface of the "static magnetic grating scale," the Hall magnetic grating array in the "static magnetic grating scale" senses a digital displacement signal with a resolution of 0.2–5mm. Of particular interest is that the Hall magnetic grating array is only sensitive to the movement of the "static magnetic grating source" along the axis of the "static magnetic grating ruler" detection surface, and is insensitive to lateral displacement, vertical movement, tilting in any three-dimensional direction, ambient temperature, operating voltage, and other factors. This brings a series of advantages. First, this new displacement sensor is not only easy to install, but also offers tolerant conditions for installation and operation: taking a sensor with a resolution of 1mm as an example, it can reliably operate with a gap between 1mm and 15mm between the "static magnetic grating source" and the "static magnetic grating ruler" detection surface; lateral displacement or tilting of the "static magnetic grating source" during installation is not a problem; and during equipment operation, deviation of the "static magnetic grating source" from any direction other than the axial direction is also unaffected. Second, changes in many external conditions such as temperature and humidity have almost no impact on the normal operation of the sensor. These are valuable characteristics in industrial settings. Due to the use of the spatial static magnetic grating induction principle and permanent magnets, this sensor is not suitable for use in extremely strong magnetic fields or in environments with a large number of particles or powdery ferromagnetic materials. If such applications are necessary, protective measures must be taken. With a display controller, the static magnetic grating displacement sensor becomes an embedded computer device integrating position sensing, position control, and position display. It can independently meet the position display and control requirements of most industrial sites, and can also coordinate with industrial control computers, frequency converters, and PLCs. The display controller is also used in situations where position control is not required but only position display is needed. The display controller is for indoor installation. In the entire position control process, position setting, positioning command assignment, and drive control are the three main stages. For example, a moving feeder needs to load material at point A, wait at point B, and unload at point C. These three points A, B, and C are the positioning points (senter) we are interested in, and we can store these positioning points in the controller long-term. In actual industrial control, a small interval of a certain width, called "positioning accuracy" (calibrate), should be set for points A, B, and C. Since moving objects have inertia, choosing a suitable stopping (deceleration) point (little) is also more reasonable. When an object approaches a target, we might want to use a "pulse" method to make it more accurately positioned. The control functions described above can all be selectively debugged and stored on the keypad. 4. Two Measurement Methods for Static Magnetic Grating Displacement Sensors 4.1 Linear Method: The above diagram shows the working scenario of a static magnetic grating displacement sensor. In the diagram, 1 is a passive "static magnetic grating source" made of neodymium iron boron permanent magnet steel, and 2 is an active "static magnetic grating ruler." H represents the gap between the ruler and the source. The linear method uses one static magnetic grating ruler and one static magnetic grating source. The static magnetic grating source moves within the length range of the static magnetic grating ruler. The range = length of the static magnetic grating ruler - 100mm. It should be noted that the passive "static magnetic grating source" of the linear displacement sensor is mounted on a moving object. The active "static magnetic grating ruler" is fixedly installed, requiring a gap of 1-12mm between the "static magnetic grating source" and the "static magnetic grating ruler." The length of this measurement method is limited by the static magnetic grating ruler, with a maximum range of 12 meters and an accuracy of 1 millimeter. Multiple "static magnetic grating sources" can be used on one "static magnetic grating ruler" to achieve multi-point detection. 4.2 Long Linear Type: The long linear type uses a single magnetic scale and multiple magnetic sources arranged in a line along the relative motion axis. The magnetic scale senses position signals using a relay method, with a range equal to the product of the scale's length and the number of sources, up to a maximum of 2000 meters. The magnetic sources are arranged in a line, with each source defined as a "segment." These segments are numbered from left to right as 1, 2, 3, 4, 5..., representing the unit of measurement in meters. The magnetic scale has a resolution of millimeters. Combining these two features, displacement over long distances can be represented in millimeter resolution. The embedded computer system within the magnetic scale has a state-of-the-art high-speed memory chip that stores the segment numbers and direction of movement. After power-off, as long as the inertial slippage does not exceed the length of the magnetic scale, the intelligent analysis program can accurately reconstruct the actual displacement. It is important to note that the active "static magnetic grating ruler" of the static magnetic grating long linear displacement sensor is mounted on a moving object. The passive "static magnetic grating source" is fixedly installed, requiring a gap of 1-12mm between the "static magnetic grating source" and the "static magnetic grating ruler". 5. Some application examples of static magnetic grating displacement sensors: <1> Shanghai Zhenhua Port Machinery (Group) Co., Ltd. – Container crane spreader length control system <2> Kunming Iron and Steel Group Co., Ltd. – Rolling mill trolley positioning system <3> Xiamen Haideco Hydraulic Machinery Equipment Co., Ltd. – Trestle pitch angle measurement system <4> Wuhan Iron and Steel (Group) Corporation – Stacker-reclaimer long stroke positioning system <5> Wuhan Iron and Steel (Group) Corporation Coking Plant – Coke oven three-car interlocking system <6> Wuhan Iron and Steel (Group) Corporation – Floating crane trolley travel positioning system <7> Wuhan Iron and Steel Ironmaking Plant No. 6 blast furnace Bischoff circumferential valve position gauge <8> Wuhan Iron and Steel (Group) Corporation Industrial Port – —- Batching trolley positioning control system<9>, Wuhan Iron and Steel (Group) Corporation coking plant —- Coal tower scale positioning system<10>, long stroke cluster positioning system for quay cranes<11>, logistics trolley displacement detection system<12>, large workshop overhead crane cluster control system<13>, flame cutting machine control system for ironmaking plant<14>, ore conveying hopper car position detection system for ironmaking plant<15>, ore powder stacking and leveling machine pitch detection system<16>, yard crane and rail crane trolley positioning system<17>, dual-scale synchronous measurement and control system<18>, coke oven four-car interlocking control system<19>, rolling mill longitudinal and transverse uncoiling line saddle car walking positioning system<20>, stacker-reclaimer unmanned operating system III. Comparison of displacement sensors 1. 1. Incremental Rotary Encoder 2. Absolute Rotary Encoder 3. Magnetostrictive Displacement Sensor 4. Grating Incremental Grating Ruler 5. Potentiometer Displacement Sensor 6. Contact Magnetic Grating Displacement Sensor 7. Inductive Displacement Sensor 8. Encoded Cable Displacement Sensor 9. Laser Displacement Sensor 10. Ultrasonic Sensor 11. Eddy Current Displacement Sensor 12. Magnetic Linear Displacement Sensor Brief Comparison: For 500-meter travel control: 1. Rotary Encoder Position Detection Technology: This technology uses a rotary encoder mounted on the wheels to detect the locomotive's position, a relative positioning method. Advantages include low cost (not exceeding 20,000 RMB) and easy installation. Disadvantages include wheel slippage and high maintenance workload. 2. Magnetostrictive Displacement Sensor: Constrained installation, high accuracy, but also high price. Currently, it cannot reach 500 meters. 3. Encoded Cable Displacement Sensor: Cumbersome installation, relatively heavy, limited lifespan (approximately 3-5 years), not fireproof, high-temperature resistant, and low accuracy. The price will not be less than 100,000 RMB. 4. Other Sensors: This discussion focuses on a 500-meter travel range, so I will not elaborate further. IV. Static Magnetic Grating Long Linear Displacement Control Scheme (500-meter travel positioning control for the main trolley) A 3.2-meter static magnetic grating ruler (actual measurement length is 3 meters) and 167 static magnetic grating sources (500 meters/3 meters) are adopted. There are two installation methods for the static magnetic grating sources: (1) The static magnetic grating sources can be buried at equal intervals next to the track and flush with the ground; (2) The static magnetic grating sources can be installed at equal intervals next to the electric track and flush with the electric track. A 3.2-meter magnetic scale is installed on the vehicle, parallel to the magnetic source, with a 10mm gap between the scale and the source, as shown in the diagram below. Explanation: A1, A2, A3, A4, A5...A(n-3), A(n-2), A(n-1), An are all magnetic sources, n=167. L1 is the distance from point A1 to point A2 (3 meters), L2 is the distance from point A2 to point A3 (3 meters), LN is the required positioning control stroke (500 meters), JCL is the length of the 3.2-meter magnetic scale fixed to the moving vehicle, and CL is the length of the moving vehicle (i.e., the main carriage).