1. Background
Linear motors are widely used in various industrial fields, including flat panel displays (FPDs), semiconductors, automation, medical diagnostics, 3D printing, and machine tools. Linear motors are similar to rotary synchronous motors where the stator and rotor are "spread out," but unlike rotary motors which output rotational torque, linear motors output thrust along their travel direction.
KOVERY Corporation, located in Gyeonggi Province, South Korea, is a manufacturer of ultra-precision motors. A leading supplier of high-precision linear motor systems, the company is committed to continuous technology and product development. KOVERY's high-precision motors utilize proprietary technology to reduce or even eliminate cogging forces that cause speed ripple, as well as accelerated bearing wear caused by the electromagnetic attraction between the stator and mover of the linear motor.
KOVERY linear motors utilize Renishaw's QUANTiC™ series incremental gratings for commutation and position control. KOVERY also uses other Renishaw measurement products, including EVOLUTE™ absolute gratings and the XL-80 laser interferometer for machine calibration. Gratings are key components enabling reliable position feedback in harsh environments.
2 Challenges
Most linear motors are "flat linear motors", characterized by permanent magnet tracks arranged below a series of electromagnetic coils (movers).
Motion control applications in fields such as semiconductor and FPD manufacturing have stringent requirements, necessitating precise control of the movement of the actuator.
Constant speed control is crucial for many advanced industrial processes, but traditional motor designs generate cogging forces, which cause ripples in motor speed.
Designers can also choose to use slotless (coreless) flat-plate motors, which offer excellent control performance but suffer from a significant reduction in thrust output. Therefore, KOVERY developed a series of motors with a "unique" design that reduces cogging forces without sacrificing thrust.
KOVERY President Kim Houng-joong explained their patented technology: “There are many types of linear motors on the market, each with its own strengths and weaknesses. Our KOVERY's uniquely developed linear motor is the first to arrange permanent magnets vertically, allowing them to pass between the pole pieces of the mover coil. This design ensures consistent magnetic flux, and each magnetic track has an independent magnetic circuit. In our design, the more magnetic tracks there are, the greater the motor's output power. This design also effectively reduces the normal attraction between the slotted iron core and the magnetic tracks without sacrificing thrust. In other words, the overall thrust of the motor is enhanced, reaching at least twice that of traditional linear motors. Other advantages include light weight, high design freedom, easy assembly, and low cost of ownership.”
Position encoders help linear motors complete commutation, enabling smooth motion and position control. Gratings with lower jitter and smaller subdivision errors are preferred because these characteristics contribute to more precise position control and reduce speed ripple.
The gratings operating in linear motors need to have two key design characteristics: a small minimum bending radius and long cable life for the readhead (sensor) cable to ensure that the machine's cable track has excellent resistance to compression, kinking and bending; and a high maximum operating temperature to withstand the heat expected to be generated when the motor coil is energized.
If the reading head is installed near the electromagnetic coil, the grating must also be able to withstand the effects of a strong magnetic field.
3 Solutions
KOVERY uses a range of Renishaw gratings in its linear motors, including the advanced QUANTiC grating.
For semiconductor and FPD process equipment applications, KOVERY's linear motors are equipped with the QUANTiC series gratings, which feature a resolution of 0.2 µm, use the RTLC40 incremental grating scale, and can output analog or digital signals. For other applications, KOVERY's motors are equipped with Renishaw EVOLUTE absolute gratings and RTLA50-S linear grating scales.
Mr. Kim continued, “KOVERY linear motors are available in a wide variety of specifications, with over ten different stroke lengths alone. Sometimes, we also require custom products with strokes several meters long. Furthermore, we anticipate continued growth in market demand for long-stroke linear motors. QUANTiC gratings offer RTLC40 steel strip grating rulers in rolls, allowing us to easily cut the rulers to the required length for each motor, thus improving production flexibility. In addition, Renishaw's short lead times and excellent cost-performance ratio are also attractive advantages. Renishaw's gratings help us enhance our product competitiveness.”
The advantages of QUANTiC gratings include: compatibility with industry-standard digital or analog output formats; relaxed installation and operating tolerances, with a clearance of ±0.3 mm and a yaw tolerance of ±0.9°; operating speeds up to 24 m/s; electronic subdivision error (SDE) as low as ±80 nm; equipped with a single-shielded flexible cable with EMI protection; and built-in LED installation indicators to assist with installation.
More detailed diagnostic data can be obtained through the optional advanced diagnostic tool ADT i-100 and the free ADT View software. ADTi-100 is ideal for applications where installation is difficult and can be used for on-site diagnostics and troubleshooting.
In particular, the QUANTiC grating is equipped with LED installation indicators and has extremely lenient installation tolerances, helping users complete the installation process quickly and intuitively, thereby reducing machine assembly costs and downtime during machine maintenance. Furthermore, the low electronic subdivision error (SDE) and high resolution of the QUANTiC grating also help improve the accuracy of linear motors, reduce speed ripple, and thus achieve smoother motion control.
Mr. Kim explained in detail: "When selecting a grating system, in addition to ensuring that the grating specifications meet the customer's application requirements, we also consider the difficulty and reliability of installing the grating in the production environment. Equipment manufacturers generally require very short product delivery cycles; from receiving the order, design, manufacturing, testing to delivery, we usually only have 3 to 6 months."
"Efficient component installation is absolutely one of the key factors in ensuring on-time product delivery. The QUANTiC grating has a loose installation tolerance, making installation very convenient. Moreover, by observing the color of the LED installation indicator on the reading head, we can quickly determine whether the signal strength meets the standard and whether the installation is successful. These features of QUANTiC help us save time, reduce costs, and increase our confidence in the product."
KOVERY linear motors are equipped with two permanent magnet tracks.
Renishaw's EVOLUTE absolute grating shares many of the advantages with the QUANTiC series, including relaxed mounting tolerances and excellent resistance to contamination. More detailed diagnostic data on the EVOLUTE is also available through the advanced diagnostic tool ADTa-100 hardware and ADT View software.
KOVERY uses a Renishaw XL-80 laser interferometer for testing and calibration of its motor products before they leave the factory. The XL-80 is used for machine calibration and quality control; it is fast, accurate, with a linear measurement accuracy of up to 0.5 ppm, and is also lightweight and portable.
“Most of our customers are precision equipment manufacturers with stringent quality requirements, so the motors must undergo rigorous inspection before leaving the factory. We use the Renishaw XL-80 laser interferometer to measure the linearity, straightness, and perpendicularity of each linear motor; we also perform dynamic measurements, including speed ripple analysis. The XL-80 laser interferometer is quick to install and easy to use, making it the ideal choice for our product testing,” said Mr. Kim.
4 Results
Renishaw's grating products and laser calibration systems have helped KOVERY create high-performance linear motors that are used in advanced process equipment in fields such as semiconductor manufacturing. The close collaboration between Renishaw and KOVERY will continue to support the development of new linear motors.
Mr. Kim summarized: "We focus on the research and development of ultra-high precision and high-speed linear motors, and it is impossible for us to know every single component inside the product. Renishaw's after-sales support service is excellent. They often communicate closely with our R&D team, not only providing relevant training when necessary, but also solving many application problems for us."
