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Direct Drive Motors: Delivering Reliability, Precision, and Performance

Jul 28, 2023

Direct drive motors have a long-standing history as a technology that has continually evolved and improved over several decades. Despite the perception that it has reached its limits, innovators such as ETEL continuously evolve and improve the performance and efficiency of their direct drive products.

In this article, we will introduce the technology behind direct drive motors, explore their advantages, and discuss example applications.

Direct drive motors, whether linear or rotary torque motors, directly couple the payload to the source of motion. This eliminates the need for mechanical transmission elements like gearboxes, timing belts, speed reducers and worm gear drives. As a result, machine designs can be simplified, and more compact solutions can be achieved.

Torque motors and linear motors belong to a distinctive category of brushless permanent-magnet synchronous motors. Linear motors are a coiled assembly that travels along a straight track of permanent magnets. By leveraging the electromagnetic induction between a coil assembly (primary part) and a permanent magnet assembly (secondary part), linear motors efficiently convert electrical energy into linear mechanical energy. These motors are engineered to generate substantial force at low speeds or even in stationary conditions.

A torque motor can be viewed as a traditional servo drive featuring numerous poles that resembles a linear motor in a rotary configuration or. The substantial number of poles in conventional torque motors allows them to achieve high torque levels at moderate speeds, even without any motion. Additionally, their compact design is an appealing attribute, encompassing a slender lamination stack and a sizable hollow shaft or bore.

Both linear motors and torque motors fall under the “frameless” motor category. What this means is these motors do not incorporate a housing, bearings, or feedback device. Machine builders have the flexibility to select and optimize these components based on the required performance or procure them as part of a pre-assembled unit.

Unlike brushed rotary motors, direct drive motors have no contact between the rotor and stator. This absence of mechanical wear significantly improves reliability. The reduced number of mechanical parts minimizes maintenance requirements and lowers the overall system cost.

Direct drive motors offer several important advantages over competing solutions. First, they are more reliable, experiencing fewer issues and less downtime compared to alternatives that rely on gearboxes or belt drives, which often necessitate substantial lubrication and maintenance.

Another advantage is the absence of backlash or inertia mismatch. In systems with multiple gears, there is often a slight disconnect or lag between interconnected parts due to the tiny spaces between the gears. This can lead to a loss of precision by the time the motion is transmitted from one end to the other. Direct drive motors avoid this problem, enhancing the accuracy and repeatability of the motor motion.

The superior reliability, reduced downtime, simplified maintenance and improved precision make direct drive motors an attractive choice for engineers seeking optimal performance and efficiency in their machine designs.

Integrating a direct drive motor may require additional effort during the initial stage for those new to using it, as precise motor positioning and alignment of various components are necessary. Additionally, precautions must be taken due to the presence of strong permanent magnets in the motor.

However, the setup and integration time are consistently reduced as the process becomes more reproducible with experience gained by the customer’s staff. Direct drive motor vendors typically offer training and high levels of support to guide their customers through the installation process.

Direct drive motors are often water-cooled, with liquid flowing across its outer surface to dissipate heat. ETEL has developed what it has dubbed a “Close Cooling” sleeve that simplifies installation by serving as a housing for water, coming pre-fitted the motor with just an input and output for liquid flow. This eliminates the need for customers to build their own cooling housing.

Looking at these factors together, the total cost of ownership (TCO) of direct drive motors is lower compared to competing technologies. While there may be some additional integration efforts involved upfront, the financial downside is outweighed by the lower running costs, longer lifespan, and reduced maintenance of direct drive motors.

Direct drive motors have applications in various industrial and production scenarios. ETEL caters to many machine tool customers in industries such as gear grinding, milling and boring, among others. In these scenarios, where heavy workpieces need to be moved or rotated while being cut, the motor must deliver high torque while maintaining excellent precision.

Semiconductor manufacturing is another demanding application that requires motors with high precision, the ability to achieve rapid acceleration and accurate, repeatable positioning for super-fast motions. The semiconductor industry also upholds stringent quality standards — necessitating consistent and error-free operation.

More generally, across multiple industries, ETEL’s linear motors have been used on some of the first Coordinated Measuring Machines (CMMs) utilizing direct drives. CMM’s move a measuring probe on an X-Y-Z plane to take measurements on a machine part and using linear motors allows them to maintain the proper submicron levels of precision needed, while still enabling an increase in throughput due to reduced downtime. For CMMs, the most valuable aspect of direct drive linear motors is their ability to provide the necessary precision without any wear components. Unlike conventional machines based on belts and pulleys, CMM machines operate at peak performance for many years.

In another domain, ETEL serves Tarus, a long-time customer specializing in the development of clay milling machines for the automotive industry. These 5-axis CNC milling machines enable designers to create vehicle models by carving large blocks of clay with exceptional precision, made possible by ETEL direct drive motors.

GreenPowerUSA (GPUSA), a unique customer, employs ETEL direct drive motors in the development of low-cost, autonomous seismic sources. These sources generate low-frequency vibrations transmitted underground. With appropriate receivers to detect the vibrations, the composition of intervening rocks can be determined — an essential task in oil and gas reservoir monitoring and underground CO 2 storage applications.

Finally, an exceptional example application involves the Solar Impulse 2 airplane, which became the first plane to fly around the globe solely relying on solar energy. Throughout its 35,000-km, 500 flight-hour journey, the airplane required an optimum performance-to-weight ratio, necessitating lightweight components. ETEL contributed its expertise in direct drive technology to develop a motor with an efficiency exceeding 95 percent. Although slightly modified for extreme environmental conditions, the motor was based on standard ETEL technology.

When selecting a direct drive motor, it is important to consider whether the supplier specializes in this area or offers it as one of many products. Large companies that sell drives and controllers often provide motors as well. However, accepting bundled motors from such companies may prevent customers from obtaining the best solution for their specific applications.

Specialist motor vendors, like ETEL, invest years in refining their direct drive motors. For example, ETEL optimizes the arrangement of coils within the motor, increasing torque output for the same physical size by packing the coils more densely.

Another common issue affecting many motors is cogging, which causes slight ripples during rotation instead of smooth motion. This phenomenon arises due to the natural magnetic attraction between the magnets and the outer rotor. By carefully designing the motor and optimizing the communications with its controller, cogging can be minimized.

Direct drive motors offer reliable, low-maintenance solutions for various applications in machine tools and other industries. They deliver the precision required in industries like semiconductor manufacturing while keeping the TCO low.

This article was written by Brian Zlororzycki, Product Specialist for ETEL components at HEIDENHAIN (Schaumburg, IL). For more information, visit here .

This article first appeared in the August, 2023 issue of Motion Design Magazine.

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