The motion-control puzzle has many pieces. Generally, the more pieces, the larger the device, and the more compliance or "play" in the system. Such compliance, or "tolerance stack up," dictates system performance in terms of accuracy and repeatability, and ultimately depends on the machining tolerances of the components that marry together. To simplify designs, provide more compact power-transmission solutions, and solve tolerance-stack-up and alignment issues, gear head and motor suppliers are teaming up to simplify motion-system design with compact integrated products that decrease the number of pieces in the motion control puzzle.
For example, at last September's International Machine Tool Show in Chicago, sister companies Torex Corp. and Exlar Corp. (Chanhassen, MN) teamed up to introduce what they claim is the first fully integrated gear reducer and brushless ac-servo-motor actuator for use in precision-positioning applications. Then, Bayside Motion Group (Port Washington, NY) tied the knot with MFM Technologies and introduced the Stealth servo gearmotor (See Design News 6/21, p.62) in March at the 1999 National Design Engineering Show in Chicago. More recently, Thomson Micron (Ronkonkoma, NY) introduced the True Planetary gearmotor (See Design News 6/21, p.153.)
|Internal servo simplifies design|
|With oil cooling, the Torex gearmotor operates at up to three times the nominal torque and pow er rating on a continuous basis.|
Sun gear = motor shaft. Whether the gear is actually hobbed onto the motor shaft, uses a self-locking taper, or attaches in some other way, fully integrated gearmotors essentially make the sun gear a part of the motor shaft. This is important because servo systems typically use pinion clamps or couplings to join these parts together. Such mounting mechanisms add compliance and length to the design in order to accommodate centerline misalignment between motor and gearhead. Moreover, torquing the collar fasteners incorrectly can lead to failure under load. "Fully integrated gearmotors use no clamps at all to improve reliability," says Torex Engineering Manager Ben Hoffmann. "To decrease size and increase rigidity the designs generally tuck the front motor bearing into the planet carrier, and tuck the rear motor bearing into the motor armature."
Engineers can expect the integration trend to continue with even more integrated products emerging in the near future to help improve performance and reliability, while simplifying motion- system design. For example, while Torex plans to expand its gearmotor line to include more sizes, Bayside launches its Luge direct-drive rotary stage and MicroPlus linear stage that have MFM's brushless dc motor technology built in. Thomson Micron (Ronkonkoma, NY) and Thomson Airpax (Cheshire, CT) have just teamed up to launch an integral stepper gearmotor.
|Integral stepper saves space|
|Initially available in NEMA 17 and 23 frame sizes, Thomson Micron mounts the pinion gear to the motor shaft to shorten the stepper gearmotor's configuration.|
Although direct drive rotary tables are nothing new, Bayside's Luge direct-drive rotary stage is a good example of how integration can improve performance. The design eliminates the worm gear to increase smoothness, accuracy, and speed.
The problem with worm-gear designs is that they depend on friction between worm and drive gear for table rotation. Pre-loading the worm gear to increase stiffness increases wear rate and breakaway torque. High-wear rates and breakaway torque limit speed and precision. But the direct-drive rotary stage eliminates the worm gear altogether by placing the rotor/magnet assembly directly on the steel hub that mounts to the rotating top plate. The hub has an integral hardened and ground bearing race. Mating bearing races, fabricated of bearing steel, are preloaded and the entire hub assembly is mounted in a one- or two-piece aluminum housing. The motor winding is pressed into the aluminum housing, with a hall- effect commutation board provided at the bottom.
"Accuracy on our direct-drive unit is based solely on the motor's encoder," explains Feinstein. "Repeatability is within 62 encoder counts, and backlash is near zero. Our worm-driven design's positioning accuracy is within 2 arc-min, it achieves positioning repeatability of 3 arc-sec, and backlash is at least 6 arc-min." In addition, the direct drive design offers finer indexing resolutions in both the clock- wise and counterclockwise direction, while the worm-gear design is unidirectional.
|The Luge direct-drive rotary stage, eliminates the worm gear to increase smoothness, accuracy, and speed.|
Ballscrew = motor shaft. Bayside's Micro Plus integrated linear stage follows the same general "less is more" design concept. "Traditional linear-slide designs use bearing- and motor-mounting plates to mate with the motor," Feinstein explains, "and a coupling connects the motor shaft to the ballscrew. We eliminate the motor, in terms of its own bearing structure, the motor mounting, and the coupling."
The integrated design consists of a linear-bearing structure, a precision-grade ballscrew with integral motor, and housing. "In our configuration," says Feinstein, "the ballscrew is the motor shaft." In essence, the design uses a longer screw, with motor-shaft dimensions turned right into it. The ballscrew's angular-contact bearings support the motor to eliminate the need for additional motor bearings.
|Integral slide eliminates motor mounting and coupling|
|The Micro Plus integrated linear slide uses a longer screw, with motor-shaft dimensions turned right into it. The ballscrew's angular-contact bearings support the motor to eliminate the need for additional motor bearings.|
Today, everyone is making products smaller and better. Essentially that's what these companies are doing, achieving results by decreasing the footprint, reducing the number of components, and increasing performance.