A CNC machine tool is an automated machine tool equipped with a program control system, which can move the machine tool and process parts according to a pre-programmed program. It integrates the latest technologies in mechanics, automation, computers, measurement, and microelectronics, and uses a variety of sensors.
sensor
Sensor (measuring element; transducer): A device or apparatus that can sense a specified measurand and convert it into a usable output signal according to a certain rule.
A sensor generally consists of three parts: a sensing element, a conversion element, and a basic conversion circuit.
A sensor is a physical device or biological organ that can detect and sense external signals, physical conditions (such as light, heat, and humidity), or chemical compositions (such as smoke), and transmit the detected information to other devices or organs. It is a device or apparatus that can sense a specified measurand and convert it into a usable signal according to a certain rule; it typically consists of a sensing element and a conversion element.
A sensor is a detection device that senses the measured information and transforms that information into an electrical signal or other required form of output according to a certain rule, in order to meet the requirements of information transmission, processing, storage, display, recording, and control. It is the primary link in realizing automatic detection and automatic control.
Sensor Function
Sensors are often compared to the five major human senses: light sensors – vision, sound sensors – hearing, gas sensors – smell, chemical sensors – taste, pressure sensors, temperature sensors, and fluid sensors – touch.
Classification of sensitive elements:
① Physics category, based on physical effects such as force, heat, light, electricity, magnetism, and sound.
②Chemistry-related, based on the principles of chemical reactions.
③ Biological categories, based on molecular recognition functions such as enzymes, antibodies, and hormones.
Based on their basic sensing functions, sensitive elements can generally be divided into ten categories: thermal sensitive elements, photosensitive elements, gas sensitive elements, force sensitive elements, magnetic sensitive elements, humidity sensitive elements, sound sensitive elements, radiation sensitive elements, color sensitive elements, and taste sensitive elements (some people have even divided sensitive elements into 46 categories).
Application of sensors in CNC machine tools
Basic requirements for sensors in CNC machine tools
1) The sensor has strong anti-interference capabilities and high reliability;
2) The sensor must meet the standards and requirements for speed and accuracy;
3) The sensors are relatively easy to use and maintain;
4) The sensor has a low cost.
Different types of CNC machine tools have different requirements for sensors. Generally, high-precision and medium-sized CNC machine tools focus on the accuracy of sensors, while large CNC machine tools mainly focus on the speed response of sensors.
1. Applications of displacement detection
Sensors used in CNC machine tools for displacement detection mainly include linear gratings, pulse encoders, and rotary transformers.
1) Some applications of linear gratings
Linear gratings are made by utilizing the phenomena of light reflection and projection. They are commonly used for displacement detection, have high recognition capabilities, and have higher detection accuracy than optical encoders. They are widely used in dynamic measurement.
During the feed drive process, the linear encoder is fixed on the machine bed. The actual displacement of the slide is directly reflected in the pulse signals generated by the linear encoder. The servo system that uses the linear encoder to detect the specific position of the worktable is a fully closed-loop control system.
2) Some applications of pulse encoders
A pulse encoder is a type of speed sensor or angular displacement sensor that converts mechanical rotation into electrical pulses. There are three types of pulse encoders: electromagnetic, contact, and photoelectric. Among these three types, the photoelectric type is the most widely used. It is equipped with a photoelectric encoder at both the x-axis and z-axis ends, and is mainly used for detecting digital and angular displacements. The angular displacement is indirectly reflected by the linear displacement of the tool holder based on the lead screw pitch.
3) Some applications of rotary transformers
A rotary transformer is essentially an induction micro-motor. In this type of rotary transformer, the angular displacement and output voltage have a continuous functional relationship. The rotor and stator together constitute the rotary transformer. Specifically, a rotary transformer consists of two rotor windings, an iron core, and two stator windings. The primary and secondary windings of the rotary transformer are placed on the rotor and stator, respectively. The electromagnetic coupling between the primary and secondary windings is related to the rotation angle of the rotary transformer rotor.
2. Applications of position detection
There are certain differences between position sensors and displacement sensors. Position sensors are mainly divided into proximity sensors and contact sensors.
1) Applications of contact sensors
Contact sensors typically work by the contact and pressure between two objects. Common examples include limit switches and two-dimensional matrix position sensors. Limit switches are relatively simple in structure, inexpensive, and reliable in operation. When an object encounters a limit switch during movement, the internal contacts of the limit switch actuate, thereby controlling the entire process. If a limit switch is installed at each end of the x, y, and z axes, it controls the range of motion. A two-dimensional matrix position sensor is installed and configured on the inside of the robotic arm. This sensor is mainly used to detect the contact position between itself and an object.
2) Application of proximity switches
A proximity switch is a switch that emits an "action" signal when an object is within a preset distance of the switch, without requiring direct contact between the switch and the object. Proximity switches mainly include Hall effect, inductive, and capacitive types. In CNC machine tools, proximity switches are primarily used for controlling the table travel, tool selection, and hydraulic cylinders.
In tool selection control of the tool turret, as shown in the diagram, the four cams from left to right correspond to proximity switches SQ4 to SQ1, forming a four-bit binary code. Each code corresponds to a tool position, such as 0110 corresponding to tool position 6. Proximity switch SQ5 is used for parity checking to reduce errors. Each time the tool turret rotates through a tool position, it sends a signal. This signal is compared with the tool position command from the CNC system. When the tool position signal of the tool turret matches the commanded tool position signal, tool selection is complete.
Hall sensors are sensors that utilize the Hall effect. When a semiconductor such as germanium is placed in a magnetic field and a current is passed through it in one direction, a potential difference will appear in the perpendicular direction; this is the Hall effect. A small magnet is fixed to a moving part; when the part approaches the Hall element, the Hall effect is generated, thus determining whether the object is in position.
3. Applications of speed detection
A speed sensor is a sensor that converts speed into an electrical signal. This type of sensor can detect both linear speed and angular velocity. Common examples in CNC machine tools include pulse encoders and tachogenerators.
A pulse encoder generates a pulse for every unit angular displacement, and works in conjunction with a timer to detect angular velocity. Tachogenerators come in DC and AC versions; they have two main characteristics: first, the slope of the ratio of the generator's speed to its output voltage is relatively large; second, the relationship between the generator's speed and output voltage is linear.
Typically, speed sensors are mainly used in CNC machine tools to detect the speed of the servo units in the CNC system.
4. Applications of pressure testing
A pressure sensor is a sensor that converts pressure into an electrical signal. Based on their working principle, pressure sensors can be classified into capacitive, piezoelectric, and piezoresistive sensors.
Pressure sensors are a collective term for sensors that detect the energy interaction between substances such as solids and gases. The distance between the two electrodes and their areas in a capacitive pressure sensor determines the sensor's capacitance. Due to their good stability and high sensitivity, capacitive pressure sensors are widely used in CNC machine tools.
Pressure sensors in CNC machine tools can detect workpiece clamping. If the set value exceeds the clamping force, the clamped workpiece will loosen, and the system will sound an alarm and stop operating. In addition, pressure sensors can also be used to detect changes in the cutting force of lathe tools.
5. Temperature detection and application
A temperature sensor is a device that converts temperature into a signal or resistance value. Common examples include thermistor sensors or resistance temperature detectors (RTDs). In CNC machine tools, temperature sensors detect temperature and then provide appropriate overheat protection and temperature compensation.
During the operation of CNC machine tools, the cutting of moving parts and the operation of motors generate varying degrees of temperature. This heat is not easily dissipated quickly and is unevenly distributed, easily causing temperature differences. This can lead to overheating and deformation of the CNC machine tool, significantly affecting the machining accuracy of parts. To avoid these phenomena, temperature sensors are installed in temperature-sensitive parts of the CNC machine tool. This setup converts the temperature signal into an electrical signal and transmits it to the CNC system for temperature compensation.
In addition, appropriate temperature sensors should be installed in parts of equipment or components such as motors that require overheat protection to provide certain overheat protection.
6. Tool wear monitoring and application
Tool wear can negatively impact the surface roughness and dimensional accuracy of workpieces. Therefore, to prevent this from occurring in CNC machine tools, it is necessary to monitor tool wear. When tool wear occurs, the load on the machine tool spindle motor increases, causing changes in the motor's voltage and current, as well as its power. These power changes are detected by Hall effect sensors. If the power change leads to an alarm in the CNC system or the machine tool stops running, the tool should be adjusted or replaced immediately.
The above-mentioned sensors have been widely used in CNC machine tools. However, with the continuous updating and development of CNC machine tools and sensors, some sensors have been phased out, such as rotary transformers. Similarly, some newer sensors are constantly emerging, which leads to the updating and improvement of CNC machine tools.
