Direct-drive motors have been around for years, but this still-underutilized motion technology will likely get a big boost next year. Rockwell Automation plans to introduce a brand new line of rotary direct-drive motors in the second quarter of 2008, according to David Hansen, a Rockwell product manager for motion control. Initially available in both bearing-less and fully-housed designs with custom frameless designs to come later on, the motors will be part of Rockwell’s Kinetix integrated motion control porfolio.
This inclusion in Kinetix has some important design and development implications. Hansen explains that the new direct-drive motors will launch with the same level of applications support and optimization tools already enjoyed by users of the company’s servo technologies. For example, he envisions Rockwell’s Motion Analyzer sizing and optimization software helping engineers evaluate whether direct-drive or traditional servo is the best fit for a given application given its positioning, torque, speed and inertia matching requirements.
And would-be users of direct-drive technology can use all the help they can get. “Direct-drive has been something of a black art,” Hansen says. “The technology is out there, but a broad understanding of it in the engineering community is not.”
Because it eliminates the need for most, if not all, of the power transmission components found in a motion system, direct-drive technology can usher in some big changes to machine design practices. “It’s a very different way to build a machine,” Hansen says. “But once you design for it and apply it to the right applications, it can offer significant benefits to the machine builder.”
In a presentation here at Rockwell’s Automation Fair, Kinetix engineering software manager Rob Schmidt summarized some of those advantages: Because they eliminate mechanical components—such as gear boxes, ball screws and belts—direct-drive motors can have a positive impact on overall efficiency, reliability and total cost of ownership. “There are less mechanical components to fail,” he notes.
And because they usually offer better stiffness and disturbance rejection than traditional servo systems, direct-drive motors can eliminate tracking errors, resonances and other factors that sometimes result in servo de-tuning. “Compliance in particular is a major limitation in many traditional servo systems,” Schmidt says, noting that he’s seen direct-drive systems bring about ten- to hundred-fold improvements in stiffness.
Schmidt’s presentation (we’ll link to it as soon as it’s available) also goes into the potential downsides of direct-drive motors, including torque-versus-speed tradeoffs that may be more constraining than those imposed by traditional servo systems. In general, though, he makes the case that the advantages of direct-drive will be compelling in a growing number of applications. “Rotary direct drive motors are getting more and more attention,” he says.