Application of inductive linear displacement sensors in machine tool industry

As the requirements for precision control in the production process in industrial applications continue to increase, more and more customers choose to use analog position control products. Users can therefore optimize production processes more easily, ensure production quality, and reduce production costs and failure rates. This is why linear displacement sensors have made great progress, from potentiometer sensors to magnetostrictive principles, and now to the new inductive detection principle, linear displacement sensors have been continuously developed.

Introduction to Inductive Linear Displacement Sensors

The linear displacement sensor developed based on the working principle of the inductive coil is unique. The technology used by Turck has reached a leading position worldwide. Many excellent characteristics of the standard measuring system have been integrated and further developed systematically. Position detection is no longer done through a magnetic slider but through an inductive oscillation circuit. Therefore, the sensor is completely shielded from magnetic field interference, such as the strong magnetic field interference generated by large motors, and has excellent EMC characteristics.

The measuring principle of the new linear displacement sensor is based on an RLC coupling circuit, which is an innovative technology of the inductive principle. Unlike the detection principle of potentiometer or magnetostrictive sensors, this measurement method has considerable advantages. The sensor integrates a signal transmitter and a receiver coil system, which are accurately printed on the circuit board in the form of printed coils. The transmitting signal coil is activated by a high-frequency AC magnetic field and generates an RLC induction circuit with the position block (resonator). Therefore, the resonator and the receiving coil form an inductive coupling. At the location where the receiving signal coil is arranged, the voltage change is caused by the induction of the resonator and the coil. These voltages are the measurement signals of the sensor. In order to make the measurement more flexible and fast, the sensor contains a rough and a fine measuring coil system. The former is responsible for the rough positioning of the resonator, while the latter is responsible for the fine positioning. The two-pronged approach ensures its accurate measurement. The new detection principle not only guarantees the accuracy of the sensor, but also enables the sensor to work in a non-contact manner. Within the allowable range, even if the position block is offset or jittered, there will be no deviation in the sensor output.

This sensor also features a double-layer circuit board. The first layer of the circuit board is responsible for sending and receiving sensing signals and is located on the sensor sensing surface. Below it is the second layer of the circuit board, which is printed with the receiving signal processor part, which is responsible for converting the signal into digital-to-analog and then forming the output. The two-layer circuit board design makes the blind area of ​​the sensor extremely small, making it the linear displacement product with the smallest blind area on the market.

Given that this linear displacement sensor has many features, it can be used in more application environments, such as injection molding machines, plate rolling machines, rubber machinery, packaging machines, hydropower station gate control and other linear control fields.

Application of Linear Displacement Sensors in Injection Molding Machines

Injection molding machine is called injection molding machine for short. Its working principle is as follows: it utilizes the thermoplasticity of plastic to heat the material through the barrel heating ring to melt the material, and then quickly flows it into the mold cavity at high speed and high pressure. After a period of pressure holding, cooling, and solidification, the mold is opened under the action of the mold clamping system, and the shaped product is ejected from the mold through the ejection device.

Injection molding is a cyclic process, and each cycle mainly includes: quantitative feeding - melting and plasticization - locking the mold - moving the injection table forward - pressure injection - mold filling and cooling - injection table retreat - mold opening - ejection of the product, taking out the plastic part and then closing the mold again for the next cycle.

In this cycle process, there are several stations that need to be precisely controlled, namely mold locking, injection table movement, injection molding and product ejection. The injection molding and mold locking stroke control of the injection molding machine plays a decisive role in the performance of an injection molding machine and the accuracy of the product.

In recent years, the trend of the injection molding market has made more and more manufacturers and users pay attention to the importance of the position system. The old mechanical manual system has been gradually replaced by a modern digital automatic system. Digital measurement not only improves accuracy, but also can use fieldbus technology for communication to improve processing speed. This is already the industry standard in the industry. In earlier applications, potentiometers were used in large quantities on injection molding machines, but since this product is far inferior to magnetostrictive displacement sensors in terms of accuracy, reliability, stroke and life, because it is contact-type, the carbon brushes will inevitably wear out after long-term repeated use, eventually leading to failure. The accuracy of the magnetostrictive displacement sensor can meet the requirements, but the vibration resistance is poor, and the blind areas at both ends are large during installation, which is not suitable for use in workstations with limited space. Therefore, from a technical point of view, non-contact, wear-free, durable, easy to install, compact, and with fieldbus-connectable displacement sensors are indispensable for the current position control system.

Turck has been targeting the market demand and working hard to continuously develop new products to perfectly solve user problems. The inductive linear displacement sensor adopts a new detection principle. It not only inherits the many advantages of magnetostrictive linear displacement sensors, but also has better performance in terms of anti-magnetic field interference, anti-vibration, and compact appearance. Due to the design of signal acquisition and processing on two circuit boards, the flexibility of the sensor's output type is greatly improved. The sensor output type can be changed by simply replacing different signal processing circuit boards, which reduces product costs. The current product output types include analog current/voltage output, SSI high-resolution output, IO-link output, and customizable bus output types, such as Profibus, DeviceNet, CANopen, etc.

Turck's inductive linear displacement sensor is currently being used on injection molding machines in the workshop of Escha, a German connector manufacturer. Each injection molding machine uses four of these sensors, and the sensors perform very well in harsh working environments. The sensors accurately and stably ensure position detection at the four stations of each machine: mold clamping, injection molding, shooting, and ejection. "One of the most important advantages of Turck's IP67 protection grade sensors is that the product's sturdy and sealed housing has good protection against humid environments." Mr. Markus Hühn, head of Escha's plug-in production department, mentioned: "The application of sensors using non-contact detection has reduced our maintenance work. Turck's inductive linear displacement sensor has fully realized our expectations."