Why all the buzz about CANbus?

A new control technology has migrated from the automobile industry to plastics processing. It's called CANbus, and it has started to change the way some familiar injection molding equipment works.

CANbus consists of multiple microprocessors distributed in a "control area network" and built around a "bus" architecture in which several computer components share a digital communications pathway. So far, it has shown up in controls for robots, peripheral equipment, and even some molding machines. One transducer supplier, anticipating wider adoption of this type of control architecture, recently developed a CANbus interface for its linear position sensors.

Several plastics equipment suppliers actively promoted CANbus as a feature of their products at NPE in Chicago this past June. They claim several advantages for this type of control architecture: faster and more reliable data processing and communications, multitasking capabilities, simplified point-to-point wiring, and easy expandability.

SPEEDY WORKCELLS

In practical terms, these features can add up to faster operations for robot workcells, which is why Wittmann Robot and Automation Systems Inc. of Torrington, Conn., adopted CANbus for all its CNC robots 18 months ago. Sales manager Ken Heyse explains that Wittmann's previous CNC controller, which used a single microprocessor for all robot and peripheral functions, had to perform various subroutines sequentially, potentially interrupting the operation of the robot.

CANbus, by contrast, handles data locally. The robot itself, its pendant, stackers, and other downstream equipment all process data in their own microprocessors. Only then does a master controller coordinate all those efforts. In Wittmann's CANbus CNC controls, all the robot programs and related subroutines run simultaneously on the different microprocessors. The result, Heyse says, is a speedier workcell.

Wittmann is unable to quantify the increase because it has no installations for which to compare the new and old approaches. But outdoor-furniture maker U.S. Leisure in Alexandria, Ind., is using a new robot system from Wittmann and is convinced that CANbus adds speed and flexibility to a complex workcell for cooling and stacking molded chairs. Technology v.p. George Lemieux, who has worked with a variety of robotics, says the 3-D operation of CANbus controlled robots appears "much smoother, more fluid" than in the past--thanks to the simultaneous execution of subroutines. And the microprocessor in the robot's pendant has sped up teach programming as well.

Some robot makers question whether CANbus is the way to go. Sterltech Div. of Sterling Inc., Milwaukee, and AEC/Application Automation, Wood Dale, Ill. (two firms which share a corporate parent and R&D efforts), previously announced plans for robots based on CANbus (see PT, June '94, p. 89). Since then, however, these companies have abandoned those plans. "We're now investigating another type of control architecture," an AEC company spokesman said, declining to elaborate.

CANBUS IN PRESS CONTROLS

Boy Machines Inc. of Exton, Pa., has brought CANbus to injection-machine controllers with the recent introduction of the Procan control system at NPE (PT, May '94, p. 44). As the successor to Boy's Mipronic-Plus microprocessor controller, Procan appeared first on the company's 24.2-ton machine. Now the 40-, 55-, and 88-ton presses also use it.

"CANbus offers fast, reliable data transmission between the controls and the process," says national sales manager John Johnson, noting that digital communications eliminate electrical noise problems found on many shop floors.

According to Johnson, CANbus's local data processing and reliable communications facilitate closed-loop control over the entire machine--that is, on both the clamp and injection end. "CANbus provides a better link between where you compare what's happening in the process and what the pump is doing," he explains. "It essentially sends a handshake to every part of the machine."

Boy isn't alone in offering CANbus-based machine controls. Van Dorn Demag of Strongsville, Ohio, employs a CANbus architecture on Demag NC4 controllers, which first appeared in 1992.

Demag product manager Richard Shaffer, agrees that CANbus offers benefits but adds that processors need only worry about control architecture if the differences affect machine performance. Distributed processing and trouble-free communications, two hallmarks of CANbus, can be achieved in other ways--such as the VMEbus in Van Dorn's newest Pathfinder controls or the transputer-based controls sold by other vendors.

BENEFITS IN SENSORS, TOO

Other suppliers see CANbus in the future for sensors and other machine components. MTS Systems Corp. of Research Triangle Park, N.C., recently developed a version of its Temposonics LP linear position transducer with a CANbus interface and built-in microprocessor. Temposonics sensors calculate position from the flight time of an electronic pulse through the device and are inherently digital, notes marketing director Jesse Russell. In the past, however, communications took place via an analog signal. "CANbus allows us to communicate digitally and that's the most accurate way," Russell says. "It lets us extract the true performance of the sensor."

The performance gain has been substantial. Sensor accuracy on standard models is listed at [+ or -]0.05% of length; now, MTS cites an absolute figure of [+ or -]0.005 in. for the CANbus-capable unit. Russell feels that CANbus architecture, with incorporation of a microprocessor at the transducer level, will also enable new SPC functionality and diagnostics of position-related problems.

Moog Inc.'s Electronics & Systems Div., East Aurora, N.Y., has imminent plans to bring out CANbus-capable servo valves, says plastics product manager Jerry Lisowski. The company's aftermarket injection-machine controllers already have a slot to accommodate CANbus devices and components such as the new the valves.