A linear motor is a transmission device that directly converts electrical energy into linear motion mechanical energy without the need for intermediate conversion machinery. It is also commonly referred to as a linear motor or linear engine. Yaskawa Electric offers a wide variety of linear motors, which are widely used in the aforementioned industries due to their advantages such as high thrust, high precision, and low heat generation.
1. Structure and Principle of Linear Servo Motor
A linear motor mainly consists of three parts: a stator (primary), a mover (secondary), and a support for linear motion. Similar to rotary motors, when a linear motor is connected to three-phase alternating current, a magnetic field is formed in the air gap between the primary and secondary windings. Ignoring end effects, this magnetic field should be sinusoidally distributed in the linear direction; however, it translates rather than rotates, hence it is also called a traveling wave magnetic field. The interaction between the traveling wave magnetic field and the secondary winding generates electromagnetic thrust.
The linear motor position measurement section supports optical or magnetic scales, typically using differential sine wave signals with a peak value of 1Vpp. These signals are converted into serial data by an 8-bit or 12-bit serial converter and input to the servo driver. The resolution is 1/256 or 1/4096 of the wavelength of the input two-phase sine wave. For higher accuracy requirements, in addition to selecting a high-precision optical scale, a high-resolution serial converter can also be chosen.
The serial conversion unit can be selected with or without a magnetic pole sensor. If a magnetic pole sensor is selected, it must be connected to the lead wire of the magnetic pole sensor; otherwise, an alarm will be triggered.
Figure 1 shows that Yaskawa linear servo motors have the advantages of high speed and high precision.
Figure 2 shows that the Yaskawa linear servo motor has the advantages of high acceleration and simple structure.
Figure 3 shows that the Yaskawa linear servo motor has the advantage of being easy to use.
2. Advantages of Yaskawa linear servo motors
In terms of speed: the servo motor + ball screw method may cause resonance due to load changes and mechanical design and installation when running at high speed, which will limit the speed; while the linear servo motor is a direct drive, so the only factor affecting the speed is the linear servo motor itself.
Regarding precision: Conventional ball screw drives may fail to meet customer requirements due to limitations in the ball screw's machining and installation precision; direct drive via a linear servo motor reduces mechanical errors. Higher precision of the grating ruler also results in higher actual load accuracy, as shown in Figure 1.
In terms of acceleration: even with a lighter load, the acceleration improvement rate of a conventional transmission mechanism is still relatively low; a linear servo motor significantly improves acceleration under the same conditions.
Regarding heat generation: The heat generated by long-term high-speed operation may cause the ball screw to expand, resulting in deviations in positioning accuracy; however, the linear servo motor has less heat conduction to the surrounding environment, which can ensure high positioning accuracy.
In terms of construction: In linear motion, when the stroke is relatively large, it is difficult to extend the ball screw; however, the linear servo motor can be expanded by adding multiple stators, or multiple movers can be installed on a single stator, so that multiple actions can be performed on the same axis, as shown in Figure 2.
Regarding noise: Ordinary transmission mechanisms tend to generate significant noise when running at high speeds; linear motors, due to their direct drive eliminating most of the mechanical contact, generate less noise when running at high speeds.
In terms of maintenance: ordinary transmission mechanisms have more mechanical contact parts and require regular maintenance; linear motors have fewer mechanical parts besides the stator, rotor and guide rail, so maintenance is easier, as shown in Figure 3.
3. Types of Yaskawa linear servo motors
Yaskawa linear motors are divided into three types: coreless type (also known as U-type), cored flat type (also known as F-type), and cored T-type.
Since the coreless U-shaped coil has no iron core, there is no attraction between the magnet and the coil. The attraction and thrust between the magnets are small. The advantages of the coreless U-shaped coil are high precision, low speed ripple, and low noise.
The cored F-type coil has an iron core, which gives it an attractive force. Relatively speaking, it is more prone to fluctuations. Its small gap results in a smaller magnet and better positioning. The advantages of the cored F-type coil are its flat, space-saving design, high thrust, and high rigidity.
The cored T-shaped slider also has an attractive force, but with two magnets placed on each side of the iron core, the magnetic forces cancel each other out, so its attractive force has a smaller impact on the slider's load. The advantage of the cored T-shaped slider is its lower requirement for mechanical rigidity, as shown in Figure 4.
Figure 4. Classification of linear motors
Figure 5
Figure 6 shows a system consisting of a Yaskawa MP3000 motion controller, a Σ-7 servo driver, and a linear servo motor, which can be used in gantry cranes.
4 Industry Application Cases
When Yaskawa linear servo motors are paired with Yaskawa controllers, single-axis, multi-axis, and multi-axis synchronous mechanisms can be constructed, which greatly improves the function and performance of the machinery. The constructed mechanisms are suitable for a variety of device applications, as shown in Figure 5.
The system consisting of Yaskawa MP3000 motion controller, Σ-7 servo driver and linear servo motor can be used in gantry moving mechanisms, such as LCD sealing and dispensing machines, LCD slicing machines, LCD coating machines, inkjet printing and coating equipment for light distribution films, etc.
The MP3000 motion controller enables synchronous control and inter-axis error compensation; the Σ-7 servo driver boasts a high speed response frequency and is easy to adjust; the linear servo motor replaces the ordinary transmission mechanism, directly driving the load movement, and its long stroke ensures rigidity. This mechanism guarantees the customer's final speed and accuracy requirements, as shown in Figure 6.
