Abstract: This paper introduces the composition principle and technical characteristics of an X-ray thickness gauge, and proposes solutions to some problems encountered in its application. Keywords: X-ray thickness gauge , AGC 1. Overview As a precision instrument for online measurement of plate thickness, the thickness gauge plays a crucial role in the AGC control system of the entire rolling mill. It provides real-time thickness deviation signals to the AGC system, and the accuracy and sensitivity of the signal directly affect the thickness quality of the rolled plate. The Radiometrie RM 215 thickness gauge provides accurate, high-speed, non-contact thickness measurement for strip steel. It features a modular design, flexible configuration, easy maintenance, economy, and excellent performance. Users can improve product quality based on the real-time feedback of the absolute value or deviation of the measurement results provided by the RM 215 thickness gauge. Therefore, the RM 215 thickness gauge is widely used in hot and cold rolling lines due to its fast response speed, low noise, and high accuracy. 2. Composition and Technical Characteristics of X-ray Thickness Gauges 2.1. Composition of the Thickness Gauge A typical plan view of the Radiometrie RM 215 thickness gauge is shown in Figure 1. The thickness gauge mainly consists of a main control cabinet, junction box, alarm lights, and a C-frame. 2.1.1. C-Frame The C-frame is located after the finishing mill in the rolling mill line and includes the mechanical parts, probe, radiation source, and receiving source. The X-ray probe and high-voltage source are imported from abroad, offering high resolution and short response time, resulting in more accurate measurement data and ensuring the stability and reliability of product quality. The mechanical parts consist of a C-frame, a drive cylinder, and front and rear limit switches. Inside the lower part of the C-frame, there is a small control and calibration cylinder and its auxiliary mechanism. The cylinder can be driven to perform measurement, calibration, and shielding operations via software control buttons on a CRT touchscreen. The radiation source is located in a black lead box inside the lower arm of the C-frame, with a shutter on it to control the on/off state of the radiation. Located directly above the source box, the cylindrical structure on the upper arm of the C-frame houses the X-ray detector, ionization chamber, and preamplifier circuitry. 2.1.2 Main Control Cabinet: The main control cabinet contains an industrial computer, a touchscreen display, a main cable terminal for the wiring cabinet, analog and digital input/output modules, a high-speed counter, and an Ethernet module. The industrial computer is equipped with a Motorola 680LC40 microprocessor motherboard, 4MB DRAM, 1MB flash memory, a printer port, and floppy and hard disk drives. The computer acquires analog signals such as thickness, length pulses, and probe temperature, as well as digital signals such as cylinder position, through a multi-functional data acquisition board. It performs calculations, outputs, displays thickness, controls machinery, and provides a thickness deviation signal to the rolling mill's AGC system. The program uses the popular object-oriented programming language C++. Model parameters can be flexibly configured according to actual production conditions. Historical data can be used to adaptively optimize model parameters; measured data can be used for offline model debugging; and measured data can be synchronized and scanned for effective control of mid-waves and edge waves. 2.1.3. Alarm Lights: Yellow Light (X-ray Indicator): When the yellow light is off, it indicates no X-rays are emitted; a flashing light indicates X-rays are about to be emitted; and a solid light indicates that X-rays are being emitted and the thickness gauge is working normally. Generally, when the yellow light is on, keep away from the C-frame to prevent radiation damage from X-rays. Red Light (Shutter Open): When the red light is off, it indicates the shutter is closed; a flashing light indicates the shutter is about to open; and a solid light indicates the shutter is open. Green Light (Shutter Closed): When the green light is off, it indicates the shutter is open; and a solid light indicates the shutter is closed. 2.2. Working Principle of the Thickness Gauge: For X-rays, after penetrating the material being measured, the attenuation law of the ray intensity I is as follows: Where I0 — incident ray intensity; μ — absorption coefficient; h — thickness of the material being measured. When μ and I0 are constant, I is only a function of the plate thickness h, so measuring I allows us to know the thickness h. X-ray thickness gauges work by measuring the intensity of X-rays as they penetrate the object being measured, specifically by measuring the amount of X-rays absorbed by the steel plate. The thickness of the object is determined based on the energy value of these X-rays. The X-ray probe converts the received signal into an electrical signal, which is then amplified by a preamplifier and finally converted by the dedicated thickness gauge operating system into a visually apparent actual thickness signal. The intensity of the X-ray source radiation is related to the emission intensity of the X-ray tube and the intensity of the X-rays absorbed by the steel plate. For a given thickness within the system's range, the required X-ray energy value can be determined using an M215 X-ray detector. When measuring any specific thickness, the system sets the X-ray energy value to ensure successful measurement. For a given thickness, the X-ray energy value is constant. When the safety shutter opens, X-rays pass through the steel plate between the X-ray source and the probe. The steel plate absorbs some of the energy, and the remaining X-rays are received by the probe, located directly above the X-ray source. The probe converts the received X-rays into an output voltage of a corresponding magnitude. Changing the thickness of the steel plate being measured will alter the amount of X-rays absorbed, resulting in a change in the amount of X-rays received by the probe and consequently, a change in the detection signal. 2.3 Technical Features: Utilizing the most advanced and stable X-ray source internationally, it is small in size, has a long service life, and is easy to maintain; Modular design ensures a simple equipment structure, quick and easy component replacement, and convenient maintenance; Rapid multi-point calibration technology guarantees long-term system accuracy; Fast, continuous, non-contact measurement with real-time display of high-precision measurement values; Employing dual amplification technology and a unique large ionization chamber to enhance the measurement range and accuracy; Temperature and alloy compensation functions: Since the X-ray thickness gauge is an online measuring instrument, the thickness detected is the thickness of the steel plate in its hot state, which will deviate from the thickness in its cold state. Therefore, a temperature compensation coefficient is added to ensure consistency between the online measurement data and the cold state data. Similarly, changes in the rolled material will also introduce measurement errors. In this case, simply testing samples of different materials and calculating the alloy compensation coefficient can also ensure consistency between the measured data and the actual material thickness. Due to the complex structure of the thickness gauge and the harsh environment it operates in, a series of isolation and anti-interference measures, such as optocoupler, spike filtering, and transformer coupling, were adopted in the design and manufacturing process of the hardware and software to ensure detection accuracy, enabling it to adapt to harsh working environments. This greatly improves the reliability of the electrical control system. The operating software based on a touch screen and WINDOWS operating platform is easy to operate and use. The desired thickness can be set in the main interface . When measuring different alloys, the stored alloy grade can be selected through the alloy grade selection dialog box, and the compensation coefficient can be entered to compensate for the actual product thickness value. The main monitoring screen is shown in Figure 2: Figure 2: Main monitoring screen 3. Application 3.1 AGC control In strip steel production, AGC (Automatic Thickness Control) requires the thickness gauge to provide a thickness deviation signal. The thickness deviation signal is transmitted to the rolling mill control system (PLC) as shown in Figure 3: Figure 3: Thickness AGC Adjustment Principle Diagram. Based on the thickness deviation signal, the rolling mill operator selects the appropriate AGC control mode. The AGC control system uses the PID regulation principle to calculate the tension AGC, speed AGC, and pressure AGC. The calculated control signal is then transmitted to the PLC control system to achieve real-time adjustment of tension, speed, and pressure to roll out strip products of qualified thickness. 3.2 Common Faults and Handling The "C" frame automatically retracts, and even manual operation is not possible, causing the AGC to malfunction, and in severe cases, causing scrap and production stoppage. When the thickness gauge "C" frame automatically retracts, the constant temperature cooling system should be checked for faults, such as undervoltage or low water level faults; whether the thickness gauge voltage is normal; and whether the network connection is normal. If the "C" frame cannot be operated manually, the forward and backward limit switches of the "C" frame should be checked to see if they can reset. To ensure the accuracy of the rangefinder measurement, the lens, air sweeping system, and constant temperature cooling system should be checked regularly. 3.3 Installation Precautions During installation, the following precautions should be taken: the guide rail should be parallel to the rolling line so that the X-rays pass perpendicularly through the rolled material; a dedicated instrument power supply or a regulated power supply should be used to prevent interference from the field; the frame must be grounded, and the power cord and signal cord should be run through separate iron conduits, with the shielding wire connected to the ground wire to prevent interference. The X-ray inspection window should be kept clean to ensure measurement accuracy. Because X-rays are harmful to the human body, to ensure the safety of personnel, an X-ray status alarm light, a current cut-off relay, a temperature relay, and a manual safety lock are provided. These ensure that when the thickness gauge malfunctions, such as when the temperature is too high or the cooling water is shut off, an alarm will sound in time, and the X-rays will be automatically cut off to prevent equipment damage. Furthermore, when the manual lock is engaged, the X-rays cannot be connected, ensuring that personnel are not accidentally injured during maintenance and inspection. 4. Conclusion The Radiometrie RM 215 thickness gauge is designed specifically for hot-rolled and cold-rolled steel plates, aluminum or aluminum foil, cold-rolled steel or steel alloys (such as stainless steel), copper or copper alloys, and other metal and non-metal industries. It enables online, dynamic, real-time, accurate, and non-contact thickness measurement, allowing for real-time production line monitoring and ensuring raw material savings while meeting high-quality product performance requirements. About the author: Ma Jing (1977-), female, engineer, mainly engaged in the development and research of industrial automation.