a
AN-383
APPLICATION NOTE
ONE TECHNOLOGY WAY
•
P.O. BOX 9106
•
NORWOOD, MASSACHUSETTS 02062-9106
•
617/329-4700
Whenever a package is shipped, whether it contains an
expensive piece of equipment or perishable food items,
shocks and extreme temperatures can adversely affect
its contents. Many types of equipment, no matter how
ruggedly designed or carefully packaged, can be suscep-
tible to mishandling. Sophisticated shipping recorders
are sometimes attached to the container to monitor and
time-stamp mishandling for insurance claims.
A similar shipping recorder can be embedded directly
into any microprocessor-based equipment, not only for
insurance purposes, but also for warranty, contractual
and field service reasons. When equipment arrives at a
customer’s facility, knowing that it was mishandled, and
how, will help in getting the equipment up and running
faster.
The recorder, described here, can be used for more than
just monitoring shipping problems. For example, this
function is valuable in portable medical equipment for
which calibration can be of critical importance. Shocks
during use or misuse can cause misalignment for
mechanical systems, printers, copiers, robotics, and
many other types of equipment requiring proper align-
ment for reliable operation. Even if a system has no
mechanical components, mishandling can cause cali-
bration problems or cause electrical interconnects to
come loose, creating intermittence. For these reasons
scheduled maintenance is not always adequate.
The block diagram of Figure 1 illustrates the concept of a
shock and temperature recorder that can be imple-
mented with minimum additional cost and design effort.
In some cases, all that is needed are the temperature
and acceleration sensors and battery backup; the bal-
ance of the circuitry may already be present in the exist-
ing system. The design details will depend on various
aspects of your system.
The sensing functions are accomplished using inte-
grated circuit sensors which are mounted directly onto
PC boards along with all the other electrical compo-
nents. To measure shock levels, three ADXL50
accelerometers are used, one on each axis, to determine
Embedded Shock and Temperature Recorder
by Robert Briano
Figure 1. Shock and Temperature Recorder for Shipping and Transport
AD7777
AD820 LEVEL
COMPARATOR
FOR WAKE-UP
(OPTIONAL) BATTERY
POWER
CYCLING
SWITCH
MUX T/H 10-BIT
A/D
I/O
DEVICE
REAL
TIME
CLOCK
MEMORY
µP
ADXL50
Y
ADXL50
X
ADXL50
Z
AD22100
–2–
PRINTED IN U.S.A. E1943–25–8/94
when and how the system might have been mishandled.
An AD22100 temperature sensor is used to measure
potentially damaging temperature changes.
In order to adequately couple the shock signal to the
ADXL50, some care must be used both in the placement
of the ADXL50 and the PC board as well as the securing
of the board to the chassis of the equipment. Applica-
tion Note AN-379 gives some helpful hints on this topic.
A microprocessor is at the heart of the recorder, control-
ling all the data acquisition and recording functions. The
data acquisition can be done with a microprocessor-
compatible ADC, such as the AD7777. The ADXL50s
should be sampled at a rate of approximately 100 Hz,
which is adequate to detect a signal from a drop or
bump. When a shock signal is detected, the sampling
rate can be increased to capture the transient signal with
more resolution. Sampling from the AD22100 tempera-
ture sensor can be done at a much lower rate. In order to
record the time of any shock or temperature event, a
real-time clock is necessary.
Under most conditions, transient shocks will not last
more than 1–2 seconds. Data can be stored during the
event and stopped when the transient event has ended.
Enough memory should be allocated to record a series
of damaging events.
As with any system that is battery powered, current con-
sumption is critical. The ADXL50 typically consumes
10 mA and the AD22100 consumes 0.5 mA from a 5 V
power supply. For example, sampling at 100 Hz will al-
low cycling of the power supply where the sensors are
on for only 10% of the time, reducing the average cur-
rent to 1 mA. (Application Note AN-376 describes this in
more detail.)
Another option for conserving power is to use one of the
ADXL50s (Z axis in Figure 1) to trigger system wake-up
when a shock level above a preset trigger level is de-
tected. A low power comparator can be made from a
low power, single supply op amp, such as the AD820.
This technique can be used whenever the micro-
processor has a higher power consumption than the
ADXL50.
The shock and temperature recorder can often be
embedded in your equipment using the existing elec-
tronics architecture with minimal additional compo-
nents. This inexpensive system enhancement will help
increase the reliability of equipment while being rela-
tively easy to implement.
For additional technical information, contact Chuck
Kitchin at (617) 937-1665. For additional literature call
(617) 461-3392.
