Positioning a semiconductor wafer is a much different task than positioning an automobile engine. While both jobs require incredible accuracy, the size and weight of the two objects greatly differentiates the type of tool necessary for the positioning.
Clearly, two different types of precision drives are required for these two operations. The most widely used options are harmonic drives and rotary vector (RV) drives.
While harmonic drives and RV drives, available through Harmonic Drive Technologies, Peabody, MA, have similar benefits, the applications where each should be used differ. A full understanding of these two technologies and when each should be implemented is essential for maximizing the performance and life cycle of industrial machines.
Rotary Vector Drives
An RV drive is densely constructed with almost all of the space within its envelope occupied by steel or iron. It is a cycloidal gear system with many of its parts precisely ground to size and held rigidly in place by a solid structure -- so rigidly that every moving part can only move in its intended path.
Such restricted motion greatly contributes to torsional rigidity. Because of their design, RV drives are heavier and larger than harmonic drives. The RV drive was derived from a hydraulic gearmotor, developed by Teijin Seiki, and sold for years to manufacturers of heavy-duty excavating equipment. Designed for moving earth and banging into rocks, this ancestor to the RV drive was modified and refined for precision applications.
A pair of cycloidal gears, which are offset 180 degrees to balance the load, are driven by multiple crank shafts to share the load and increase the torsional rigidity. A spur gear pre-stage drives the crankshafts, which increases the speed ratio and minimizes vibration. The cycloidal gears mesh with a large quantity of precisely ground steel pins seated in pockets inside the housing's outer edge. The combined tooth contact area of the two cycloidal gears and pins ensures that the load is distributed almost entirely around the pitch circle.
Manufacturers of large robots have found the RV drive to offer the right combination of precision and durability. For similar reasons, RV drives are ideal for manufacturers of tool changer carousels and turret drives in machine tools, welding positioners, and index table drives. All require high torsional rigidity and overload capacity.
Harmonic Drives
If weight is a factor or space is very tight, designers should choose a harmonic drive -- a very precise, zero-backlash speed reduction system that develops its tooth mesh by flexing one of its parts. A cam with two 180-degree opposed lobes deflects a thin-walled gear with many small teeth into an ellipse, forcing teeth away from the center on its major axis. Rotating the cam causes two simultaneous waves of gear teeth to rise and mesh with a close-fitting ring gear, which has two more teeth than the flexing member, accounting for the high ratio.
Many teeth are forced into simultaneous engagement at each end of the ellipse, forming a pure torque couple that can transmit a great deal of torque for the drive's light weight. Recent improvements in tooth and wave geometry have increased tooth contact area and considerably improved stiffness.
The harmonic drive's combination of size and weight is ideal for small robot joints, semiconductor wafer handling systems, solar array and antenna positioners in space satellites, positioners for medical scanners and laser diagnostic equipment, and other applications where space is tight but high torque and accurate positioning are still required.
Choosing the Right Drive
Harmonic drives are available in rated torques ranging from 15 inch-pounds (1.7 Nm) to about 30,000 inch-pounds (3,400 Nm). RV drives have rated torques that range from 900 inch-pounds (100 Nm) to 60,000 inch pounds (7,000 Nm). On the lower end of the size/weight scale, harmonic drives are the ideal choice. Gear weights can be as little as a few ounces and can be small enough to occupy the space of a ping-pong ball -- an incredible feat when you consider that this is high ratio with no multiple stages.
From 900 inch-pounds to about 30,000 inch-pounds, both technologies have products available. However, the practical range is up to about 2,000 inch-pounds because the larger harmonic drives do not have an advantage in torque-to-envelope ratio or the stiffness of RV. In most cases above 2,000 inch-pounds, RV drives will have a cost advantage.
In the 1,000 to 2,000 inch-pound torque range, RV drives and harmonic drives, both available through Harmonic Drive Technologies, Peabody, MA, are relatively close in size and performance. However, above 1,000 inch-pounds, the weight advantage of the harmonic drive, once properly housed and supported with bearings, becomes less.
The ultimate decision depends on the customer's preference for lightweight (harmonic drives), ratios higher than 160:1 (harmonic drives), torsional rigidity (RV drives), overall length (RV drives are shorter), and overload capacity (RV drives).
Above 2,000 inch-pounds (226 Nm), the RV drives have most of the advantages. They are more compact, torsionally stiffer, and their built-in output bearings support significant external loads, thus saving considerably in system costs.
Improved Productivity and Increased Efficiency
Space savings lead to cost savings and both harmonic drives and RV drives are very compact. They also increase life in many applications because load reversals - even those due to routine accelerations and decelerations - create wear damage if backlash is not reduced or eliminated. Because both technologies are consistent mechanically (due to having low lost motion) control engineers are better able to optimize their production efficiency; i.e., spend more time controlling the process and less time compensating for mechanical errors.
The variety of harmonic drives and their ability to be customized enables retrofits to existing machines after other technologies have failed or developed errors from wear. Many positioning machines have been either upgraded to new precision levels or given new leases on life with harmonic drives less prone to failure or wear.
RV drives are reliable because of their rugged construction and low-wear characteristics. For example, a Harmonic Drive Technologies' customer had a stamping press-feeding robot mechanism with a history of many types of drive failures. It became trouble-free after RV drives were installed, even though the mechanism had to endure constant, round-the-clock high loading in an automotive stamping plant. RV drives have the kind of reliability found in cam-indexing machines, but provide increased flexibility as they are not confined to a single repetitive motion.
The Future
New harmonic drives and RV drives have been introduced with integrated brushless servo motors. These are the ultimate in compact and powerful servo actuators designed for precise positioning. Companies now using or considering high-torque direct-drive servos should consider these integrated servo motors for their lower control complexity, higher torque throughout the speed range, lower cost and, in some cases, smaller size.
Harmonic drive and RV drive technologies have also been introduced with hollow shaft versions of both gears and gearmotors. These allow users to channel vacuum lines, wiring harnesses, welding rods, concentric shafting and cooling lines etc. through the center. For those requiring large center holes (up to 5.4 inches/138mm), Harmonic Drive Technologies' RV-C gear system stands out. The RV-C gear has a hollow middle, allowing vacuum lines, wiring harnesses, concentric shafting and cooling lines to be run through its center. The RV-C also has specially designed, built-in output bearings that support large thrust and overhung loads.
A new linear actuator from Teijin Seiki, parent company of Harmonic Drive Technologies, using the RV cycloidal gearing laid out in a track, is now being tested. This low-wear device will be able to precisely move robots along a track as it interacts with several machine tools.
While similar in the benefits they offer, harmonic drives and rotary vector drives provide the best results when they are used in the proper applications. Factors such as torque range, space requirements and weight are essential in determining which of these unparalleled drives will best maximize a motion control machine's performance.
