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Plasma Cutter Utilizes Motor for Gantry Control
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.
More Information
For more information, contact Robert Bigler of
Animatics at rbgle@animatics.com, or visit http://info.ims.ca/5786-318.
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