Direct-drive rotary motors streamline machine design


W hen electric motors are used in low-speed rotary applications, a transmission is often installed to gear down motor speed for higher output torque. This reduces cost because gearing allows small high-speed motors to produce high torque at low-speeds. However, in high-accuracy applications, such as film-coating lines and integrated circuit test machines, designers avoid gearing because it causes a host of problems like position error, lost motion (backlash), more maintenance, and audible noise.

Most designers would specify so-called direct-drive rotary (DDR) motors if they could afford them. Until recently, this option was usually reserved for high-end commercial and military applications since direct drive was too expensive for most industrial machines. Fortunately, this has changed.

Several manufacturers produce DDR motors for applications that just a few years ago could not consider this technology. Manufacturers include Kollmorgen (Radford, Va.); Compumotor (Rohnert Park, Calif.), a division of Parker Hannifin Corp.; and NSK (Ann Arbor, Mich.; Tokyo, Japan).

Direct-drive rotary motors, sometimes called torquer motors, develop high torque at relatively low speeds, usually just a few hundred rpm. Two types of DDR motors are available. In frameless motors, customers purchase components such as a rotor, stator, and feedback device. These parts are then assembled with the rest of the machine.

Housed DDR motors integrate the rotor, stator, and feedback device into one assembly. Housed torquer motors do not have an intermediate shaft coupling. Instead, the load is attached directly to the DDR rotor. The rotor has a through-hole, typically about 50-mm in diameter, which allows plumbing and wiring to pass through the center. Housed motors have independent bearings, while frameless motors rely on the bearings of the machine.

Why use DDR motors?

Increasing a machine's accuracy is the main reason to choose DDR motors. Since the load is rigidly coupled to the motor, error caused by transmission components is eliminated: there is no backlash, belt stretch, or gear-tooth error. The main limitation is the accuracy of the feedback device, but feedback devices for DDR motors are very accurate.

Also, stick-slip is usually eliminated. Stick-slip is a condition in which moving a load over very small distances cannot be done with accuracy. It often comes from transmission components that bring high-friction and high-compliance. Because DDR motors reduce friction and virtually eliminate coupling compliance, they are often not subject to stick-slip.

Another advantage is that the high stiffness between motor and load effectively removes mechanical resonance-the phenomenon in which a compliant load generates instability under high servo gains. This means the servo gains of DDR systems can be set very high, allowing faster servo response and greater resistance to torque disturbances.

Audible noise is also reduced because of fewer moving parts. Maintenance is reduced because the only wearing component in the system is the bearing. If the bearings are permanently lubricated, the assembly can achieve zero maintenance. Machines using DDR motors are often simpler and smaller because the transmission is eliminated. And DDR motors can actually reduce cost in cases where highly accurate transmission components or feedback devices would otherwise be needed.

Not right everywhere

Direct-drive motors are not right for every application. They're usually more costly than conventional rotary motors using transmissions, especially when high gear ratios (>10:1) are used to gain mechanical advantage. Feedback devices for DDR motors are usually also more costly. Low friction of DDR-based machine systems-normally an advantage-can be a problem in some designs that rely on friction to bring motion to rest when power is removed.

Finally, for engineers who are familiar with conventional rotary motor design, time is required to learn how to apply DDR technology.

George Ellis, senior scientist at Kollmorgen.

Comments? E-mail:

No comments
The Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by...
The System Integrator Giants program lists the top 100 system integrators among companies listed in CFE Media's Global System Integrator Database.
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
This eGuide illustrates solutions, applications and benefits of machine vision systems.
Learn how to increase device reliability in harsh environments and decrease unplanned system downtime.
This eGuide contains a series of articles and videos that considers theoretical and practical; immediate needs and a look into the future.
Controller programming; Safety networks; Enclosure design; Power quality; Safety integrity levels; Increasing process efficiency
Additive manufacturing benefits; HMI and sensor tips; System integrator advice; Innovations from the industry
Robotic safety, collaboration, standards; DCS migration tips; IT/OT convergence; 2017 Control Engineering Salary and Career Survey
Featured articles highlight technologies that enable the Industrial Internet of Things, IIoT-related products and strategies to get data more easily to the user.
This article collection contains several articles on how automation and controls are helping human-machine interface (HMI) hardware and software advance.
This digital report will explore several aspects of how IIoT will transform manufacturing in the coming years.

Find and connect with the most suitable service provider for your unique application. Start searching the Global System Integrator Database Now!

Infrastructure for natural gas expansion; Artificial lift methods; Disruptive technology and fugitive gas emissions
Mobility as the means to offshore innovation; Preventing another Deepwater Horizon; ROVs as subsea robots; SCADA and the radio spectrum
Future of oil and gas projects; Reservoir models; The importance of SCADA to oil and gas
Automation Engineer; Wood Group
System Integrator; Cross Integrated Systems Group
Jose S. Vasquez, Jr.
Fire & Life Safety Engineer; Technip USA Inc.
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
click me