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

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