Submission to the New Journal of Student Research Abstracts
Building a Probe Drive for a Plasma Device
Roland Hwang, Max Praglin (The Buckley School, Sherman Oaks, CA), Evan Warfel
(Palisades High, Pacific Palisades, CA), Bob Baker (University High, Los Angeles, CA),
Joe Wise (New Roads School, Santa Monica, CA), Walter Gekelman (UCLA)
In an effort to more accurately take measurements with a Langmuir probe, students at
the LAPTAG high school plasma physics lab designed and machined a probe drive with
a computer monitored control system. The drive is mounted on top of a movable cart
with adjustable height having shelves for a computer, control box, and monitor. The
probe drive is designed with two movable parts, a rotational axis for turning the probe
while inside the plasma device and a horizontal axis for moving the probe in and out of
the machine. The horizontal axis consists of a worm gear set upon base blocks at each
end with a motor attached in the back. The worm gear moves the cart at 0.5 cm per turn,
with the motor turning at increments of 1.8 degrees (.0314 radians), thus giving the
possibility of 0.0025 cm steps in and out of the plasma machine. The rotational motor,
mounted with the probe atop the cart, provides an accuracy of 0.045 degree turns
because of our 40:1 step converter attached from the motor to the probe. Although using
a pre-fabricated worm gear and carriage, the students designed and fabricated the
system to allow for horizontal and rotational control by machining various gear systems,
mounting blocks and plates for the motors and gears, anti-vibration rubber gaskets, and
stabilizing wheels for the carriage’s precise movements. To control this drive, students
built a control/circuit box consisting of two speed controls that determine how fast or how
long each motor runs. Students wired a 68 to 25 pin converter so that wires within the
box can be connected to a computer - two sets of wires run from the box, one to each
motor and the other to the adjustable limit switches placed to stop the drive to prevent
the cart or probe from moving too far in any one direction. (The limit switches are
adjustable so that any size or shaped probe can be placed on the drive without having to
machine completely new parts.) The control box provides all the power to both the speed
controls and the motors - the motors require roughly 0.9 amps of current and 20-25
volts. Students use LabVIEW, a measurement and automation controller software by
National Instruments, to change the speeds, velocities, and accelerations of the motors.
Using LabVIEW with the oscilloscope, students take data with Langmuir probes at
precise intervals inside the machine so that they know, when interpreting data, at what
position within the plasma certain phenomena, such as Electron Bernstein waves, occur.