Arrival of Full-Scale Linear Motor Age-Linear Shaft Motor
Published on : Thursday 01-06-2023
The application of Linear Shaft motors to various fields is rapidly expanding, says Mamoru Takabayashi.
I feel that linear motors have become increasingly used around the world these days. Users began to adopt linear motors from the viewpoint of differentiation and superiority from products of other companies in the business strategy.
Traditionally, linear motion systems for industrial machinery and measuring instruments have mainly been using a rotary stepping motor or servo motor with a combination of ballscrew converting rotary motion into linear motion. However, with the enhancement of performance of industrial machines and measuring instruments, higher speed and higher positioning accuracy are required. There is a limit in linear motion system by ballscrew, and it is said that the drastic enhancement of performance over the past has not been fundamentally expected. This has become a limit to the contradictory high speed and resolution due to the one which depends on the resolution of the motor and the ballscrew pitch.
It is very difficult to reduce the velocity stabilities to less than 1% in 10mm/sec by the conventional method.
Here, I introduce a Linear Shaft Motor developed and manufactured by GMC Hillstone Co Ltd.
Linear Shaft Motors are innovative motors that fundamentally change motion control with these conventional configurations.
Structure and operating principle of linear shaft motor
It is simple and consists of a shaft containing magnet and forcer containing coil (Figs. 1 and 2).
The shaft is a stainless-steel tube filled with equally spaced Nd-Fe-B magnets. The forcer is arranged in three phases (U/V/W) with the coil surrounding the shaft. Since it is three-phase, it is possible to use drivers from other manufacturers. The forcer and shaft are non-contact, so that there is a gap when installed in the system. At this time, there is no effect on thrust even if it is installed eccentrically, and mounting is simple. This is a very important point in practical use.
The operation principle is that the thrust force is generated by the action of the magnetic flux generated from the permanent magnet and the current flowing in the coil (Fleming's left hand law).
Features of linear shaft motor
It has the following features due to the structure:
1. Positioning accuracy depends on linear scale. High-precision positioning is realized by using high-precision linear scale. No backlash
2. Because it is coreless, cogging hardly occurs. Very good speed ripple at low speed (see Fig. 3)
3. Simple structure with large degree of freedom, and
4. Magnetic flux generated by the permanent magnet is wrapped 360 degrees around the outer coil, making effective use of the magnetic flux.
Comparison with other linear motors
The positioning between the Linear Shaft motor and other linear motors is shown below (see Figs. 4 and 5).
As a linear motor, it is common to have a core. But the ripple due to cogging is large and the magnetic attraction force is also large. As a result, improvement of expected accuracy, reliability and maintenance-free are inhibited. On the other hand, Linear Shaft Motor has almost no thrust ripple and realises high accuracy. Assembly becomes simple because of no attracting force. These are features of a coreless linear motor.
As for flat type coreless linear motor or U-shaped linear motor, heat is easily trapped in a state where the coil is sandwiched by the magnet (see Figs. 6 and 7). The coil of the Linear Shaft Motor is exposed and easily radiates heat.
Applications of linear shaft motor
Fig. 8 shows the assembly example of Linear Shaft Motor.
Users can choose different stroke lengths to suit each application. At the same time, diameter of the shaft can be selected according to required thrust. As a rough indication, I will show differences in application with differences in size of shaft diameters.
Small diameter (about Φ4 to 12 mm): mainly suitable for desktop-type devices. Small diameter shafts are often adopted in the stage related to microscopes, and there are observation applications such as industrial products and bio-related products. In this industry, high resolution image and its image processing are required; high resolution (scale) at the same time high precision with low ripple performance of Linear Shaft motors are required. In this type, a cross roller guide is used to draw out the features of a Linear Shaft motor such as high accuracy and low ripple at low speed. Stepping motors are also often adopted for desktop-type equipment at low cost and easy control, but the stepping motors have unique noise. Low noise is required in the analyser field, and quiet Linear Shaft motors meet this requirement.
Medium diameter (~Φ35mm): This size is often adopted in the field such as machine tools and semiconductors where high thrust, high speed and high precision are required. It is also a big advantage that it is non-contact and maintenance-free.
Large diameter (~Φ60mm): Shaft with the largest 60.5mm diameter has a 3,100 N acceleration thrust force, and they are for special applications.
Unique driving method is possible with Linear Shaft Motor (see Fig. 9). For example, a plurality of forcers can be driven independently on a single shaft. This operation cannot be performed with a ballscrew.
Also it is possible to double the thrust by tandem drive connecting the forcer on one shaft. And there is parallel drive using a Linear Shaft motor in parallel. Parallel drive is mainly effective for system construction in guntray style. Furthermore, by fixing the forcer and moving the shaft side, it is also possible to operate without considering the durability of cable. Various configurations are possible depending on the engineer's ideas.
There are many applications mainly on the horizontal axis (XY axis). However, for use on the gravitational axis (z axis), counter balance is required when power is turned off.
Summary
The application of Linear Shaft motors to various fields as coreless type linear motors is rapidly expanding. Engineers are selecting linear motors from the first development stage.
In the past, some niche markets required sub-nano level control. But they have expanded into various fields, and Linear Shaft Motors have begun to be used here. In addition, further evolution is expected, such as built-in scale and actuators using rotary and Linear Shaft motors.
Mamoru Takabayashi is Managing Director of Nippon Pulse Motor Co Ltd.
Nippon Pulse is a global manufacturer and Tier 1 supplier of precision motion control products for original equipment manufacturers. The company provides a wide array of motion control solutions to meet various needs, including industry-leading stepper motors, the innovative Linear Shaft Motor, controllers and drivers.
