Servos make plasma cutter light and nimble

Jan. 26, 2006
Brushless servomotors let a maker of plasma and oxyfuel cutters cut intricate designs in metal. The machines coming out of Dynatorch Inc.

Visible on this view of a Dynatorch Inc. cutting machine are the SmartMotors powering each of the device's axes. The precision gearboxes, custom made for Dynatorch, allow use of relatively small but fast servomotors. The cutting machines let artists turn out such intricate pieces as this weather vane by Sam Dennison of Green Mountain Enterprises.

Brushless servomotors let a maker of plasma and oxyfuel cutters cut intricate designs in metal. The machines coming out of Dynatorch Inc. in Paducah, Ky., are capable of such detailed work that they have a following among metal sculptors and artists, as well as industrial firms interested in high throughput.

Key to the system's performance, say Dynatorch officials, is the use of servomotors rather than the stepping motors generally found on competing devices. Step-pers are heavier and slower than servos of similar capacity. Moreover, they provide less power at high speeds. These qualities limit steppers to gear-reduction ratios of about 3.5:1.

In contrast, servos can be geared up tremendously. They can thus run at high speed and sit on a much lighter framework than stepmotors with the same performance.

The servos are SM2315D SmartMotors from Animatics Corp., Santa Clara, Calif. SmartMotors have the controller, amplifier, and encoder built into the motor itself. Thus, gearing boosts the effective resolution of the builtin encoder. The motor package, about 2.3-in. long and 2.25-in.-wide, conforms to NEMA frame and shaft dimension standards and provides about 7,000 rpm. The system uses three or four SmartMotors, two precision gearboxes, and an integrated arc voltage torch-height control system that Dynatorch devised. The company says it designed the gantry system to be light but stiff. The reason is that the cutting machine is often called upon to create intricate features that involve rapid reversals and direction changes at speeds of up to 200 ipm. Thus, a stiff frame and the accel/decel performance of the motors are both extremely important for making these moves accurately. Moreover, there are no side forces on plasma cutters, so there was no need for a massive gantry structure to compensate.

The SmartMotors operate from a single microcomputer that handles the PID loop, trajectory generation, I/O control, communication, and runs the cutting program. On the cutting machine, the motors run in host mode where they process motion commands in parallel to effect coordinated motion.

SmartMotors in the system are interchangeable. For example, host controller software will sense a swap between the X axis and torch-height motors. Motors still get the correct instructions in this case. Likewise, the host recognizes when the system gets an additional X-axis motor for a dual-drive gantry. Here it automatically sets itself up for an X-master/slave configuration. For similar reasons, damaged motors can be swapped out without a special service call.

Dynatorch software performs CNC functions and controls the height of torch on the fly so the system can more closely follow the contours of plates. Software optimizes DXF files for plasma and oxy-fuel cut paths and implements other features such as resizing objects.

Animatics Corp., (408) 748 8721,
Dynatorch Inc., (877) 260-2390,

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