March 23, 1998 Design News
DESIGN APPLICATIONS
From the regional editors
Controller makes motors follow the
leader
Single-box solution simplifies
machine operations, regulating both speed and web tension
by Laurie Peach, Associate Editor
Minnetonka, MN--Ever
notice wavy patterns in the ceiling cloth of your car?
If you own an expensive one, probably not. But for those
of us who drive the lower-end models, the alternating
light and dark bands are often distinguishable. Although
they could be considered part of the decor, the moiré
pattern is the result of uneven rolling of the headliner
cloth prior to dying.
Mack Johnson, field service and installation engineer
for D.R. Kenyon (Bridgewater, NJ), solved the problem
using a precise motor speed control device, Microspeed,
from Drive Control Systems, a unit of Electro-Sensors.
Microspeed acts as a "smart speed-pot," or
potentiometer, for ac and dc variable-speed drivers
to trim tension. This closed-loop system is especially
useful for wind and unwind applications where specific
parameters, such as line speed, web tension, and roll
diameter, need to be monitored and the data used as
inputs to determine the correct following speed.
The dyeing of headliner material is one such application.
Textile manufacturers wind the uncolored material onto
hollow, perforated cores. After winding, dye is forced
into the core and out through the material. If the fabric
isn't wound at a constant tension, the finished product
looks like a tie-dyed tee shirt. The take-up roll, a
large metal cylinder, resembles an empty toilet paper
roll. Without fabric, it may be only 18 inches in diameter.
After taking up the cloth, the roll expands to 48 inches.
To maintain correct tension and surface speed, the winding
speed of the core must slow as the fabric roll diameter
increases.
"It is difficult to maintain tension on a center
hyperbolic winder when the core is slowing down as more
fabric is added," says Richard Sabol, head electrical
engineer at D.R. Kenyon. "We found an easy way
to do something very difficult with the Microspeed."
"We can make a motor precisely follow another
motor anywhere," says Brad Slye, president and
CEO of Electro-Sensors Inc. Microspeed acts like the
cruise control in a car with one major difference: Cruise
control works only in the master mode while Microspeed
typically works in a follower mode. "Instead of
setting the cruise control at a set speed, the cruise
is programmed to match the exact speed of the car in
front. If it slows down, you slow down. If it speeds
up, you speed up, while maintaining a constant distance
of two car lengths," says Slye.
The web tension required by the headliner fabric will
vary depending on material thickness, weight, and type
of fabric. "We chose Microspeed because of the
diversity and ease of changing parameters," says
Johnson. "Microspeed allows the operator to change
up to 50 different input variables with the touch of
a keypad. Other systems are more difficult for in-house
people to operate."
The Microspeed is easily programmed just like your
microwave oven, with a color-coded keypad and displays
that feature both numbers and names. Slye says, "I
knew we had it right when my four-year-old could program
the device, and he can't read." There is also an
additional display that shows what variable is being
changed, with arrows to scroll through all possible
parameter changes.
Usually fabric wind-up machinery requires a smart drive,
plus a tension control as well as a load cell to maintain
speed and web tension. "[Microspeed] gives all
start and stop features in one unit," says Johnson.
"I don't know of another device on the market that
is a closed-loop system with tension control."
"Our biggest engineering challenge," says
Slye, "was trying to integrate all of the solutions
into one box."
To wind the headliner material at the proper tension,
the Microspeed uses encoder feedback to obtain the line
speed and surface speed of the core. These encoders
are similar to the magnetic wheel speed sensors used
in the cruise control of a car. The lead encoder, by
definition, is the one to follow. It provides the actual
line speed, or how fast the cloth is entering the process;
typically measured in feet/minute or yards/minute. The
feedback encoder gives the surface speed of the building
core, which should closely track the incoming lead line
speed of the material. The Microspeed monitors the lead
encoder and controls the speed of the motor that drives
the wind-up roll to match the line speed. The Microspeed
verifies the roll is winding at the correct speed using
feedback from the surface speed encoder on the roll.
A shaft mounted on load cells is used to measure the
tension of the cloth web. The load cells translate the
tension in the web to a 0 to 10V analog signal. The
signal is fed to the Microspeed. An operator programs
the desired tension into the motor control device. As
the fabric is wound, the Microspeed simultaneously examines
the lead line speed, the web tension, and the surface
speed of the roll and adjusts the rate of the take-up
core to maintain the programmed tension.
The Microspeed replaces the manually operated potentiometer
used for setting the desired speed of the drive. A 16-bit
controller from Intel serves as the microprocessor.
Because the system is completely digital, it is not
DSP based. No analog conversion is required.
Additional details?Contact Brad Slye at
Drive Control Systems, (800) 323-0504.
Other Applications
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