Single-drive motion control

South Bend, IN-By eliminating the need for a motor at each axis, a new motion control system promises to cut costs, improve reliability, and promote ease of use. The MX Robot uses a single motor to achieve three axes of CNC motion. Because it eliminates so much traditional hardware, the MX Robot is said to cost approximately half as much as conventional systems. It's also 57% lighter and uses 67% fewer parts than comparable systems.

Key to the new system is the elimination of all but one of the motors and other hardware traditionally associated with three-axis systems. Larry J. Costa, inventor of the MX Robot, explains that studies show approximately 60% of all three-axis material-handling applications operate only one axis at a time.

Such systems could achieve the same goals with a single motor, he says. "If you look at most applications in welding, manufacturing, and material handling, the extra hardware adds no value," Costa asserts.

To achieve three-axis CNC control with a single motor, Costa designed a patented cable, drum, and pulley system that moves one carriage at a time in each axis. The system's drum attaches to a BDS5 servo motor from Industrial Drives Division of Kollmorgen Corp., Radford, VA.

During operation, the motor rotates the drum using a trochoidal zero-backlash cam reducer-made by Dojen Inc., Woburn, MA-for speed reduction. Two cables are routed via idler pulleys to each of the system's movable carriages. Cortland Cable Co., Cortland, NY, makes the nylon-jacketed, DuPont Kevlar(TM) cables. Two cables wind around the drum in opposing directions. Each cable is anchored to opposite ends of the z-axis. As the drum rotates in the clockwise or counter-clockwise direction, it winds one cable in and feeds the other out. As a result, the drum moves the carriages to the precise point specified by the controller. "We make the cable drum do the work of a timing belt," Costa says.

To actuate one carriage at a time, the MX Robot uses pneumatic brakes made by Festo Corp., Hauppauge, NY. It moves the x-carriage, for example, by applying brakes to the y- and z-axes, then rotating the drum in the correct direction. In this way, the carriages move orthogonally, a single axis at a time. The brakes, kept in a normally engaged position, are activated by SMC solenoid valves and Festo pneumatic switches to verify each brake's release.

To maintain alignment while it reels the cables in and out, the drum traverses along a ball spline made by THK America, Inc., Schaumburg, IL. Employing two pairs of cam followers from The Torrington Co., Torrington, CT, achieves proper cable alignment on the drum. The carriages slide on Linear Modules with integral Star Ball Rail systems, made by Star Linear Systems, Charlotte, NC. Position feedback can be supplied by the motor's resolver, or by other means.

By dramatically simplifying the design of three-axis motion control, Costa has created a new niche in motion-control hardware. Until now, manufacturers have created a wide array of motion control systems using belts, cables, ball screws, hydraulics, and other means. Those systems proved particularly handy for applications requiring circular or helical interpolation in their motion profiles. At the same time, however, they required one motor for each axis of motion. "The bottom line was that the customer who needed one axis of motion at a time ended up paying for motors on all axes," Costa says.

The need for additional hardware didn't end with motors. Because the motors became part of the motion payload, conventional systems typically required heavier structural components to supply greater rigidity. They also needed larger power supplies and separate servo amps to drive the additional motors.

The greater complexity of those systems also called for more powerful controllers. In turn, the controllers added programming complexity.

Costa's MX Robot uses single-task controllers and closes the loop on only one axis. This setup, combined with orthogonal motion profiles and single-motor hardware, simplifies the programming process. "If you can program the channels on your car radio, you can program this robot," Costa states.

He says the system combines the cost effectiveness of pneumatics with the speed and flexibility of CNC control. "It's not for every application," Costa explains. "But if you're moving only one axis at a time, you can have CNC control for a much lower cost."

Additional details...Contact Larry Costa, Costa Automation Technology, Box 6524, South Bend, IN 46660, (219) 674-0859.