Engineers at Bell-Everman, Inc., Goleta, Calif., have created a precision, economical belt-drive linear actuator alternative to linear motor actuators. The ServoBelt is a customizable precision belt-drive axis for use on large format machines (greater than 2 m travel) driven by linear motors or rack and pinion drives. A moveable carriage or block rides on a Rexroth linear Ball Rails® linear rail mounted on a length of Rexroth aluminum framing. Servomotors drive a polyurethane steel reinforced T5 belt, which power the carriage. A static-bonded lower belt engages with the upper dynamic belt so that the active belting under tension is less than a few inches long. The belt mechanism enhances stiffness, accuracy, and repeatability.
Developed as a low cost alternative to linear motor systems, the ServoBelt mimics a linear motor by inhabiting the same force regime. The device offers more drive-line rigidity than conventional belt drives and can improve a machine’s settling time.
The bi-directional repeatability is four to five times better than a conventional belt or rack drive (±60 μm), with a unidirectional repeatability of ±10 μm. By using a linear encoder, it can match the repeatability of a linear motor.
A moveable carriage powered by a servo motor and belt rides on Rexroth’s linear rail and aluminum framing. Standard range of travel is from 0.15 to 50 m with almost no length limitations.
The standard range of travel is from 0.15 to 50 m with almost no length limitations. Acceleration is in excess of 4 g while speeds are up to 4 m/s with standard grade bearings, and up to 10 m/sec with a bearing upgrade. Bell-Everman’s life test units are recording 40,000 out-and-back cycles per day, with at least 16 million stops and starts already tabulated.
Michael Everman, CTO of Bell-Everman, said that key components of the design come from Rexroth’s linear Ball Rails and aluminum framing. Using Rexroth linear Ball Rails allowed them to focus on overcoming their main hurdle — creating a belt that is rigid as opposed to having the flexibility of a “guitar string.”
Everman said they experimented with a moving-motor belt drive predecessor design, but stiffness, although better than a conventional belt run, was an issue.
The ServoBelt deliversthe speed and accuracy of a linear motor at a value comparable to belt-driven actuators.
“Using the T-slot extrusions with our own belt carrier extrusion allows us to deliver standard stages in one to two weeks,” said Everman. “We can cut-to-length all of the major components, with no long-travel machining operations. Having multiple independent carriage capability, without forcer cross-talk is also a huge plus. We’re slated to make stages with ten forcers on the same axis for
an automated aircraft wing-fixture system,” continued Everman.
Versatile Rexroth linear Ball Rails and aluminum framing make it easy to create subassemblies. Other Rexroth components such as motors, pneumatic grippers, valves, and a motion logic controller with robotic kinematics can be integrated to produce a complete robot.
In addition to its versatility, the ServoBelt offers more energy efficiency than a conventional belt. “When the driveline is stiffer, you spend less time trying to maintain position when the machine is shaking around due to other processes or other axes in motion,” said Everman. “It is definitely a more efficient use of energy, so smaller motors can be used.”
Bosch Rexroth Corporation
Filed Under: Material handling • converting, Encoders • linear, Linear motion • slides, Motion control • motor controls