Plasma Cutter Utilizes Motor for Gantry Control
- Tuesday, 01 August 2006
To cut through metal and other materials, a plasma cutter sends an arc of electricity through a high-speed jet of an inert gas. When electricity and gas meet, plasma is formed, hot enough to melt through sheet metal, and still moving fast enough to blast molten metal away from the cut. When engineers at Dynatorch (Padukah, KY) began to design a line of plasma and oxy-acetylene cutting machines, they utilized the SM2315D SmartMotor from Animatics (Santa Clara, CA) to operate the gantry, where the torch is mounted. To avoid the warping, kinking, or scorching that can occur when metal is improperly heated, the gantry and torch must maintain a fixed distance from whatever is being cut.
Conforming to NEMA shaft and frame dimensions and CE certified, the servo motor weighs one pound, measures 2.3 x 2.25", and has a shaft diameter of 0.25". To move the gantry, the unit delivers a radial load of 7 pounds and an axial thrust load of 3 pounds. The small dimensions, minimal weight, and intelligent nature of the motor are what made it appealing to Dynatorch engineers and makes it vital to the gantry’s integrated arc voltage height control system.
Keeping the plasma cutter’s gantry lightweight was a key factor, observed Leon Drake, Dynatorch’s engineer on the project, who noted a vicious cycle when it comes to gantries: bigger gantries need larger motors, which in turn add more weight, which in turn requires a bigger, stronger, and heavier gantry, which would then require an even stronger, larger, and heavier motor, and so on. The end result is a gantry that can’t maintain proper height, and is too big, too cumbersome, too expensive to retrofit, and in the end, delivers a sub-par product.
There are no side forces on the gantries of plasma cutters, so the design did not require a large, heavy system from the outset. Additionally, with a lightweight gantry, servomotors do not need as much torque.
Utilizing Dynatorch’s in-house developed software, the motors operate from a single microcontroller that handles all processing functions including the PID loop, trajectory generator, user program execution, I/O control, and all communication over as many as three high-speed serial channels simultaneously. The controller, amplifier, and encoder are integrated into the motor. Torque vs. speed curve data is derived under dynamometer testing. In Dynatorch systems, all motion instructions are buffered to the motors, where they are parallel processed to maintain coordinated motion. Since the operation is performed as a software routine, no additional hardware is needed between the command PC and the motors.
Field service needs are reduced because each motor is addressed at power up and check to make sure it has the proper function instructions in it. That is, if the X-axis and the torch height motors are swapped, the software in the host controller would recognize the change and update the switched motors with their correct function instructions. This is also the case if an additional motor was added to the X-axis, whereupon the host controller would engage a master/slave configuration. All of this has combined to promote Dynatorch’s strength; the company has grown since the Animatics-powered Dynatorch plasma cutter went on the market three years ago, shipping products world-wide.