Making a personal CNC router with closed-loop reliability

Case summary: Stepper and driver package increases positioning and cutting speeds of small, personal CNC routers to that of much larger machines.

End User: ShopBot Tools Inc., Durham, N.C.

Founded: 1996

Employees: 30

Engineering activity: ShopBot builds personal CNC routers, called Personal Robotic Tools (PRTs), for individual woodworkers in small shops or at production-line stations in bigger facilities.

Design goals:
• Increase positioning speeds from 200 to 300-ipm range to 1,500 to 2,000 ipm.
• Increase cutting speeds to 500 to 600 ipm, as available in larger, more expensive CNC routers.

Proposed solution: Combine power and speed advantages of stepper motors with feedback advantages of servomotors. Use hybrid stepmotor and driver package to eliminate missed steps and provide closed-loop system reliability.

Selection criteria:
• Two-phase stepper motor delivers speed and precision, but feedback device constantly monitors motor shaft to detect and correct for loss of synchronism.
• Closed-loop control only comes into play when a position deviation of 1.8° or higher is detected.

Enabling technology: AlphaStep Series stepping motor and driver package from Oriental Motor USA Corp., Torrance, Calif.

Design in action: The system works like a large XYZ plotter, with one AlphaStep motor on the Y and Z axes, and two on the X. Using CNC technology, the machine cuts wood and plywood to make furniture, signs, musical instruments, crafts, and boats.