Due to their high precision, high speed, high efficiency, and safety and reliability, CNC systems are increasingly widely used in the equipment manufacturing industry. A CNC machine tool is an automated machine tool equipped with a program control system, capable of moving and machining parts according to a pre-programmed sequence. It integrates the latest technologies in mechanics, automation, computer science, and measurement, utilizing sensors in the CNC machine tool control system to monitor displacement, position, speed, pressure, temperature, and tool wear.
Application of displacement monitoring sensors in CNC machine tools
The sensors used in CNC machine tools mainly include photoelectric encoders, linear gratings, proximity switches, temperature sensors, Hall sensors, current sensors, voltage sensors, pressure sensors, liquid level sensors, rotary transformers, inductive synchros, speed sensors, etc. , which are mainly used to detect position, linear and angular displacement, speed, pressure, temperature, etc.
CNC machine tool requirements for sensors
Sensors in CNC machine tools typically need to meet the following conditions:
(1) High reliability and strong anti-interference ability;
(2) Meets the requirements for accuracy and speed;
(3) Easy to use and maintain, suitable for machine tool operating environment;
(4) Low cost.
Different types of CNC machine tools have different requirements for sensors. Generally speaking, large machine tools require high speed response, while medium and high precision CNC machine tools mainly require high accuracy.
Displacement detection
Displacement detection sensors mainly include pulse encoders, linear gratings, rotary transformers, and inductive synchros.
1. Application of pulse encoders
A pulse encoder is an angular displacement (speed) sensor that converts mechanical rotation into electrical pulses. Pulse encoders can be divided into three types: photoelectric, contact, and electromagnetic, with photoelectric encoders being the most widely used.
In the figure, photoelectric encoders are equipped at the ends of the X-axis and Z-axis respectively for angular displacement measurement and digital speed measurement. The angular displacement can indirectly reflect the linear displacement of the slide or tool holder through the lead screw pitch.
2. Applications of linear gratings
Linear gratings are made using the phenomena of light transmission and reflection. They are commonly used for displacement measurement, have high resolution, and are more accurate than photoelectric encoders, making them suitable for dynamic measurement.
In the feed drive, the grating ruler is fixed to the machine bed, and the pulse signal it generates directly reflects the actual position of the slide. The servo system that uses the grating to detect the position of the worktable is a fully closed-loop control system.
3. Applications of rotary transformers
A rotary transformer is an induction micro-motor whose output voltage is a continuous function of angular displacement. A rotary transformer consists of a stator and a rotor. Specifically, it comprises an iron core, two stator windings, and two rotor windings. The primary and secondary windings are placed on the stator and rotor, respectively, and the degree of electromagnetic coupling between the primary and secondary windings is related to the rotor's rotation angle.
Rotary transformers can be used to transmit rotation angles or electrical signals in synchronous and digital servo systems; they can also be used to solve functions in solving devices, hence they are also called solvers.
Rotary transformers generally come in two structural forms: two-pole windings and four-pole windings. Two-pole rotary transformers have one pair of magnetic poles each in the stator and rotor, while four-pole windings have two pairs of magnetic poles each, primarily used in high-precision testing systems. In addition, there are multi-pole rotary transformers used in high-precision absolute testing systems.
4. Application of inductive synchros
An inductive synchro is made using the principle that the mutual inductance of two planar windings changes with position. Its function is to convert angular or linear displacement into the phase or amplitude of an induced electromotive force, which can be used to measure linear or angular displacement. Based on its structure, it can be divided into linear and rotary types. A linear inductive synchro consists of a fixed scale and a sliding scale. The fixed scale is mounted on the machine tool bed, and the sliding scale is mounted on the moving part, moving with the worktable. A rotary inductive synchro has a fixed stator and a rotating rotor. Inductive synchros have advantages such as high accuracy and resolution, strong anti-interference ability, long service life, simple maintenance, long-distance displacement measurement, good manufacturability, and low cost. Linear inductive synchros are currently widely used in large-displacement static and dynamic measurements, such as in coordinate measuring machines, CNC machine tools, high-precision heavy machine tools, and machining center measuring devices. Rotary inductive synchros are widely used in the rotary tables of machine tools and instruments, as well as in various rotary servo control systems.
Location detection
Position sensors are used to detect position and reflect a certain state of a switch, unlike displacement sensors. There are two types of position sensors: contact and proximity.
1. Applications of contact sensors
Contact sensors operate by the contact and pressure of two objects. Common examples include limit switches and two-dimensional matrix position sensors. Limit switches are simple in structure, reliable in operation, and inexpensive. When an object moves and encounters a limit switch, its internal contacts activate, thus controlling the movement. For example, installing limit switches at both ends of the X, Y, and Z axes of a machining center allows for control of the movement range. Two-dimensional matrix position sensors are installed inside the palm of a robotic arm to detect the contact position between the arm and an object.
2. Applications of proximity switches
A proximity switch is a switch that emits an "action" signal when an object approaches it to a set distance, without requiring direct contact with the object. There are many types of proximity switches, mainly including self-inductive, differential transformer, eddy current, capacitive, reed switch, and Hall effect switches.
The main applications of proximity switches in CNC machine tools are tool selection control, table stroke control, and piston stroke control of hydraulic cylinders and pneumatic cylinders.
In the tool selection control of the tool turret, as shown in the figure, 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, it indicates that 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.
Speed detection
A speed sensor is a sensor that converts speed into an electrical signal. It can detect both linear speed and angular velocity. Common types include tachogenerators and pulse encoders.
The characteristics of a tachogenerator are:
(1) The output voltage and rotational speed are strictly linearly related;
(2) The slope of the output voltage to speed ratio is large. It can be divided into two categories: AC and DC.
A pulse encoder generates a pulse when it undergoes a unit angular displacement, and with the help of a timer, the angular velocity can be detected.
In CNC machine tools, speed sensors are generally used for speed detection of servo units in CNC systems.
Pressure detection
A pressure sensor is a sensor that converts pressure into an electrical signal. Based on its working principle, it can be divided into piezoelectric sensors, piezoresistive sensors, and capacitive sensors.
It is a general term for detecting the energy of the forces between all substances, including gases, liquids, and solids. It also includes pressure gauges that measure pressures above atmospheric pressure and vacuum gauges that measure pressures below atmospheric pressure.
The capacitance of a capacitive pressure sensor is determined by the electrode area and the distance between the two electrodes. Due to its high sensitivity, good temperature stability, and large pressure range, it has seen rapid development recently. In CNC machine tools, it can be used to detect workpiece clamping force. When the clamping force is less than the set value, it causes the workpiece to loosen, triggering an alarm and stopping the tool feed. Additionally, pressure sensors can be used to detect changes in the cutting force of a lathe tool. Furthermore, it is also used in lubrication, hydraulic, and pneumatic systems to detect pressure in oil or air circuits. When the pressure in the oil or air circuit falls below a set value, its contacts will activate, sending a fault signal to the CNC system.
Temperature detection
A temperature sensor is a device that converts temperature into resistance or other electrical signals. Common types include platinum and copper-based resistance temperature detectors (RTDs), semiconductor-based thermistors, and thermocouples. In CNC machine tools, temperature sensors are used to detect temperature for temperature compensation or overheat protection.
During the machining process, the rotation of the electric motor, the movement of moving parts, and cutting all generate heat, and the uneven temperature distribution causes temperature differences, which can lead to thermal deformation of the CNC machine tool and affect the machining accuracy of the parts. To avoid the influence of temperature, temperature sensors can be installed in certain parts of the CNC machine tool to sense the temperature signal and convert it into an electrical signal to be sent to the CNC system for temperature compensation.
In addition, temperature sensors should be installed in places such as motors that require overheat protection, and an overheat alarm should be triggered by the CNC system when overheating occurs.
Tool wear monitoring
Tool wear, when it reaches a certain level, will affect the dimensional accuracy and surface roughness of the workpiece; therefore, tool wear must be monitored. When the tool wears, the load on the machine tool spindle motor increases, and the motor's current and voltage will also change, resulting in a change in power. This power change can be detected by a Hall sensor. When the power change reaches a certain level, the CNC system will issue an alarm signal, and the machine will stop operating. At this point, the tool should be adjusted or replaced promptly.
With the development of sensors and CNC machine tools, some sensors will be phased out, such as rotary transformers, while new sensors will continue to emerge, making CNC machine tools more perfect and more adaptive.
