Why does PlasmaCAM use servo motors instead of stepper motors?

It's true that this machine could be made a lot cheaper if we used stepper motors and drives instead of high-tech servo motor systems. But when it comes to something this critical, quality cannot be compromised. Experienced machine tool users know that servo motors are vastly superior to stepper motors, because newer, high-performance machines (like CNC mills) use servo motors, whereas older, more troublesome machines used stepper motors. Yet the real reason for the performance difference requires some explanation.

PlasmaCAM's servo motors use optically-encoded feedback, so the controlling software always knows the true position of the machine. A simple stepper motor controller must "trust" that the motor has moved exactly as requested each time a step current is made. Without feedback, the controller cannot identify and correct whenever the motor misses a few steps (like during jolts, vibrations, hang-ups, etc). Since a single shape may require millions of steps to trace, errors in position will continue to accumulate unbeknownst to the controller, until the machine is finally re-zeroed against a physical stop. Hence, errors in stepper position are both unpredictable and unreported.

To solve this problem, optical encoders can be coupled to stepper motors so the controller can determine the true position and make corrections when needed. However, the added cost and complexity prevents the system from being a low-budget alternative to servo systems. The more common solution involves overrating the motors (using larger, higher-geared motors that are under utilized) to reduce the probability of slipping. This extra rotating inertia (typically much greater than the mass of the moving parts) brings undesirable side effects: diminished acceleration and speed. For plasma cutting, the machine's ability to cut intricate shapes (which require abrupt changes in direction during high-speed cutting) would be greatly limited.

Powerful, lightweight motors are a must, because their torque is needed for speeding up and slowing down all the moving mass - including the spinning motor armatures. (To picture the importance of this, imagine pushing a 100 lb cart at a walking pace and trying to make a sharp 90 degree turn without overshooting at all.) That's why the Model DHC2™ fully utilizes the 300 oz-in capability of its servo motors, providing over 1.5 G of acceleration. (An object traveling at 100 inches per minute can totally reverse direction in only 0.002 inch at this acceleration.) See the photos of intricate sample parts throughout this web site and watch the demo video to see what this machine can do for you.

The Model DHC2 does provide some cost savings by utilizing a stepper motor for the Z axis only. This motor is slower and weaker than the X/Y axes motors. However, it's not nearly as critical because the motor only has to control the height of the torch. Accuracy is not affected, less speed is required, and the position is automatically reset by the controller between each cut.


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