COMPUTER CONTROL and
DATA ACQUISITION
by David A. Ward
Part 5: Building A Thermal Cycling System
On a recent plant tour of a manufacturer of cellular
phone amplifiers, I noticed a thermal cycling chamber the
company uses to test their products in. Photo 1 shows a
similar system. It can heat its chamber up to a specified
temperature and hold that temperature for a specified
period of time. It can also cool the chamber down in the
same manner and repeat this cycle as many times as the
user desires. Their amplifiers are mainly used in cars
where the interior can easily reach 130°F in the summer
and go well below 0°F in the winter. Thermally testing
their products will help them to find any weaknesses in
them due to temperature extremes before the products go
to market. I thought this would be a good project to build
and control using NI’s LabVIEW software and the USB-
6008 hardware. Although the commercial thermal cycling
system can refrigerate the chamber well below room
temperature, the “poor man’s” thermal cycling system
illustrated here will only be able to heat with light bulbs
up to about 130°F and cool it down to room temperature.
To build the chamber, acrylic plastic was used so that
observers could see everything that was occurring inside of
the chamber (see Photo 2). The chamber uses two 120 VAC
100W light bulbs as the heating elements which are controlled
by a digital output terminal from the USB-6008 unit, P0.0. The
digital output terminal then controls a two amp AC SSR. To
make the system closed loop controlled, a LDR was placed
facing the light bulbs and its voltage was sent back into one of
the analog input terminals of the USB-6008 unit, AI2 (see
Photo 3). A muffin fan was placed at one end of the chamber
to pull in the cooler room temperature air (see Photo 4). It is
controlled by a digital output terminal, P0.1, that drives another
AC SSR.
To make its control closed loop as well, a dryer vent was
placed at the opposite end of the chamber and a photo interrupter was connected to one of the shutters and connected to
PHOTO 1
PHOTO 2
terminal P1.0 (see Photo 5). Two LM34 temperature sensors
are used in the system: one on the interior of the chamber
connected to terminal AI1 (Photo 6) and one on the outside
connected to terminal AI0. A limit switch is placed under the lid
and wired into a digital input terminal on the USB-6008 unit,
terminal P0.1 (see Photo 7).
Many of the items used to make the chamber can be
purchased at your local home improvement center and most of
the electronics were purchased from Jameco Electronics
(
www.jameco.com). We won’t be spending any more time
on the construction of the chamber itself; the photos should be
self-explanatory. We will need to spend most of the article
explaining how the VI operates. Wiring and schematic diagrams for all of the components included in the chamber were
published in the previous articles and will not be reprinted here.
When planning a complicated software program,
techniques such as flow charting and pseudo-coding may be
used. However, for visual types of programming
such as Visual Basic and LabVIEW, these
methods don’t really work very well. What does
seem to work well is to first sketch what you
want the final user interface to look like and
then describe how the system should function.
Figure 1 shows the front panel of our system
and what controls and features it has.
Now let’s describe how a user would
operate the system. When the VI is run, the user
will need to set the upper temperature and
lower temperatures, the duration of each of
PHOTO 3
50 SERVO 07.2009
