One of the most important choices engineers make while developing new platforms for machine tools is deciding which type of motor to use. Direct drive motors have always been seen as a technological step up from standard transmission devices, with torque and linear motors perceived as higher-end alternatives to gearboxes and ball screws, respectively. Although the basic design has been around a long time—permanent magnets on the rotor and copper coils on the stator—motor makers are still fine-tuning different aspects of the design to increase overall performance and quality. Here are the important trends direct-drive designers are pursuing to ensure their customers gets the most out of their machine tools.
1. Increasing their force density. One of the key advantages of direct drive motors is the high performance they deliver relative to their footprint. Torque and force, for torque and linear motors respectively, let machines take up less space overall without having to sacrifice power. Outside of size, power is the most important consideration when selecting a motor. Technological advancements, a competitive environment that demands efficiency, and ever-more complex parts have the machine tool industry, and in turn motor manufacturers, looking into how to maximize force density within a volume.
A common approach is to fit more copper coils within a volume, either with unique shapes or different proprietary manufacturing methods. The more densely packed the copper coils are, the more strands can fit, leading to a stronger magnetic flux.
Outside of the motor itself, heat dissipation is an issue that affects motor performance; the more effectively a motor is cooled, the better it performs. As this is a key aspect when selecting motors, designers will likely always be working on improving it through better designs and materials.
When ETEL released the TMB+ a few years ago, increasing the torque density of the previous models was one of the design priorities when it came to advancing direct drive motor design.
2. More size options. Direct drive motors are key components in machine tools from a performance and product quality point of view because they must provide the tool-turning power profile within a certain volume. Increasing geometrical variation in motor design gives machine builders more flexibility in fitting in the best motor for particular applications in the space available.
Torque motors have the variables of diameter, length, and power. But it’s not as challenging as it once was to find the ideal mix. For example, if the outer diameter of a motor is limited then there are often many height options available to still maximize torque.
The variety of sizes available also offers cost flexibility. For example, the ability to increase the length of a linear motor can reduce width and, in turn, the amount of costly magnet material along the length of travel. Having a large variety of sizes lets machine makers more easily maximize the space and efficiency of the machine, so much so that it may start to feel like the motors were made just for the machine and not the other way around.
ETEL’s line of TMB+ direct drive motors range from 160 mm (6.3 in) to 1288 mm (4.2 ft).
3. Simplified integration. Liquid cooling has become standard in machine tools, and it’s not difficult to see why. Properly dissipating the heat can essentially double a motor’s torque capabilities, letting much smaller motors do the job. Still, liquid cooling can be intimidating in certain applications. To help ease these concerns, motor manufacturers often provide the option of standalone, built-in, and enclosed-cooling jackets. These all but eliminate coolant leaks and the challenge of designing the best cooling channels. The responsibility for providing these accessories is increasingly shifting from machine builder to motor manufacturer.
One way to simplify integration for linear motors is to keep the magnetic way consistent. This can become complex when, for instance, a builder works with different sizes of linear motors across their machine and the linear motor tracks don’t have a consistent bolt-hole pattern or width increments. This becomes especially complicated if an engineer wants to upgrade the motor and two parts must be adjusted. If a similar magnetic way track can be used across several motor models, as is possible with ETEL’s magnetic way-compatible LMG and LMS motors, adjustments are easier as the motor changes. The benefits are even greater if a next-generation model can still use the original tracks, as it lets customers upgrade machines with minimal effort.
4. More winding options. Although a motor may generate a certain peak torque or force (e.g., continuous and peak values), it may still be limited by how much of that is available at different speeds. The faster a motor runs, the lower its torque output can become due to back-EMF voltage, Eddy currents, and other factors. A common way of reducing these effects is by adjusting the winding of the copper coils to raise the speed threshold at the cost of requiring more current. This is an aspect that can be adjusted with minimal impact on the motor’s torque/force capabilities. Most motors have at least two options, but there is the possibility of having up to four—as with ETEL’s size range offering, which gives designers more flexibility in terms of what can be done within a fixed motor size. The idea is to propose several torque vs. speed profiles within each motor footprint to satisfy as many operating ranges as possible.
This torque vs. speed graph is for a direct drive motor. Changing the windings moved the diagonal line out the X-axis.
5. New ways of integrating. Direct drive motors have had standard applications for a while, but as machine makers have gotten more familiar with the technology, they’ve come up with ways of taking fuller advantage of them. For example, in a traditionally configured linear motor, the magnet rather than the motor moves along a magnet track. This makes for either short, quick strokes with the magnetic way or a single magnetic piece moving across a track of linear motors. This lets the payload be lighter and also not be attached to a cable, giving it more free reign when it moves.
In addition, designers can take a linear motor and adjust the magnetic way to move on a curve rather than a straight line for moving a payload in a circle or “stadium configuration.” This not only takes up less space than a torque motor, but is also more economical because customers avoid paying for a large torque motor with unnecessarily high torque capabilities. Combine that with the modularity of a separate linear motor that can be used in parallel, and it’s also easier to integrate and maintain.
ETEL has developed a curved linear motor track for its direct drive linear motors and it will be released this year.
6. Greater efficiency. An increase in demand for greater energy efficiency due to environmental standards and concerns has many companies seeking out “greener” technological pastures. To meet these standards, motor makers have been looking into innovative design ideas to keep power losses as low as possible. This includes segmented magnets and wrapping the lamination stack with coils of Litz wires in incredibly small layers (~0.1 mm) to reduce the potential for Eddy currents. Just as with increasing power density, motor makers are looking at all aspects of the materials and how they’re assembled to get the most efficient performance out of the same basic design first conceived for direct drive technology.
Brian Zlotorzycki is a business development specialist for ETEL motors at Heidenhain Corp. If you have questions about direct drives, click here.
