Direct drive is a decades-old idea. But this old idea has been dressed up with new features designed to capture experienced users as well as win converts to an attractive but sometimes-problematic technology.
Most rotary machines run on servomotors that are connected to rotating members by transmissions. But direct-drive rotary motors connect directly to the load.
Because it requires no wear prone transmission parts, direct drive reduces system downtime, lifecycle costs, and maintenance requirements. What's more, there's no transmission-related loss of accuracy. "If you use a direct-drive motor with a high-resolution encoder, the accuracy and repeatability are an order of magnitude better than they would be with a belt-drive/gear-drive solution," says Robert Mastromattei, research associate for Motion Tech Trends, a network of motion control consultants.
Frameless direct drive
Mastromattei is also co-owner of Applimotion Inc.
(http://rbi.ims.ca/3845-520), which designs so-called "frameless" direct-drive rotary motors. Frameless motors don't come in housings with a shaft, bearings, and feedback devices. Instead, frameless kits include only a rotor and stator, making the motors easy to customize and fit into available spaces.
Though Applimotion specializes in custom-designed products, some of these have been turned into general-purpose direct-drive motors. One is the ULT series motor, which includes a single-piece magnet ring rotor. According to Mastromattei, this ring differs from conventional direct-drive rotors, which are made by gluing individual magnets onto a steel ring.
The ULT rotor is magnetized with a proprietary multiple-pole pattern that can include dozens of poles. With no attached magnets, the rotor offers a larger hole for the passage of cabling or fluids through the motor, and easier assembly Mastromattei notes.
The motor's other major component is the stator, which features single-slot winding that reduces the amount of copper on the ends. As a result, Applimotion can produce motors with axial heights measuring just 8 mmóa good fit in the crowded interiors of semiconductor fabrication equipment, Mastromattei adds.
Another frameless option is the IndraDyn T from Bosch Rexroth
(http://rbi.ims.ca/3845-521). This features a pole and winding arrangement that reduces torque ripple to less than 0.1 percent, compared to about 3 percent for similar motors on the market, claims Douglas Bruss, a product support engineer at the company. "If there's a positive torque ripple in one place, we'll cancel it out with a negative torque ripple in another place," Bruss explains.
This motor is so smooth that it produces motion that can't be felt by humans, according to NeuroKinetics Inc., which uses the motor in a rotational chain system used by hospitals to test human balance.
Pluses and minuses
Give me the shaft: Danaher's new
Cartridge DDR motor attaches directly to a machine's shaft, eliminating
the need for transmission components.
The custom-designed direct-drive motor is much more expensive than a conventional motor/transmission combination, says Alex Kiderman, chief technology officer at NeuroKinetics. But a motor and transmission can't match the performance of the direct-drive unit, which "lets us control the motion profile exactly," he says. The motor also helps NeuroKinetics minimize system noise and vibration. The result: "After a few minutes, the patient won't know if the chair is rotating or not," Kiderman reports.
On the downside, frameless part sets often require costly and time-consuming integration into the user's system. So some system designers opt for fully housed alternatives like the Goldline DDR (Direct Drive Rotary) motor from Danaher Motion
(http://rbi.ims.ca/3845-522). The housed Goldline includes a rotor and stator, as well as a feedback device and bearings. But these bearings must be aligned with bearings on the shaft being driven.
So earlier this year, Danaher introduced the patented Cartridge DDR motor. The Cartridge DDR package includes a rotor, stator, and feedback device, but has no bearings. Instead, it uses the machine's bearings to support the rotor, saving space and eliminating bearing alignment. "You mount the motor onto the shaft, plug in the cables, and you're up and running in minutes," says Tom England, Danaher's business unit manager for direct-drive products.
Besides easy installation, the Cartridge DDR features 30-50 poles. The high pole count and a new lamination and winding structure combine to produce torque output up to 50 percent greater than that of a comparably sized conventional servomotor, says England.
The Cartridge DDR is designed for any machine with its own set of bearings. One such machine is a prototype tire wheel balance machine developed by ITW Micro-Poise, which manufactures test and measurement equipment for the tire and automotive industries. The company has found that direct-drive motors improve the measurement quality of a machine. With direct drive, "you don't get backlash and other external influences that you would get with belts and pulleys," notes Tom Williams, engineering group manager.
So Williams and his colleagues turned to a frameless direct-drive motor. But they were put off by difficult assembly-related tasks. "We basically had to build the motor ourselves," Williams recalls. "It probably took a guy a day to put it together."
Not so when installing the Cartridge DDR. "It's a bolt-on, so a guy can do it in about half an hour," Williams says. The motor also offers much higher torque density than that of its frameless predecessor.
Unlike a standard motor, the Cartridge DDR "sits right on the customer's machine," England notes. But that means the heat from the motor can flow into the machine. This can be undesirable in semiconductor and medical device applications, so users in these fields must use cooling devices to dissipate the motor's heat.
In addition, England notes that the Cartridge DDR usually costs about 20 percent more than a standard motor and transmission. But Danaher estimates that the motor can save users thousands of dollars a year by eliminating the maintenance, replacement, and downtime costs of a transmission system. "We're typically saving $10,000 to $30,000 per axis over the lifecycle of the machine," he says.