Biomedical Engineering Reference
In-Depth Information
requirements in the SRS and also can be influenced by
the accuracy of the test system. The test is then designed
to vary the battery voltage to cross through the threshold
and to see that the alarm is tripped within the tolerance
band. Once the alarm has been tripped, the next part of
the threshold test is to vary the parameter (in this case,
the battery voltage) in the reverse direction to ensure
that the alarm no longer enunciates.
helpful because it can uncover problems in the system, or
it can detect flaws in the overall use of the product.
System testing
System testing occurs to validate the entire medical
device, not just the software that is embedded in the
device. Depending on the way a test station is instru-
mented for software testing, there can be significant
overlap between software and system testing. For ex-
ample, software embedded in a ventilator usually re-
quires other mechanical-electrical subsystems in order
to function. It is difficult to write sophisticated simula-
tions to trick the ventilator software into thinking that all
of the other subsystems of the ventilator are present.
Therefore, a software test system might resemble a
complete-system. It is up to the responsible parties to
ensure that the degree of overlap is made known. More
importantly, it is vital that everything is tested at some
point in the product development.
Boundary testing
Boundary testing is the exercise of testing a parameter at
its limits, and of trying to exceed those limits. A mea-
sured value of O
2
that is defined as not to exceed 110%
would be a good example of this. (An O
2
reading can
exceed 100% if the O
2
sensor is out of calibration or
calibrated incorrectly.) The test then would be designed
to prove that 110% O
2
could be displayed. Additionally,
the test then would see how the device would react if the
measured O
2
value were to exceed 110%.
Stress testing
Stress testing is the process of subjecting the unit to
a bombardment of random inputsdoften keypad presses
or knob turnsdto try to cause a software failure. This can
be done manually or with an automated test system.
Manual stress testing and automated stress testing
each have their own advantages. The advantage of manual
stress testing is that the tester can test a certain aspect of
the device and observe anomalous behavior in other areas
of the device. The advantage of automated stress testing
is that the test system can provide inputs into the device
much more quickly than a human tester can.
Test execution and reporting
Test plan
For software validation, a test plan is written prior to
executing a test pass. The format follows that defined in
ANSI/IEEE Std 1012. The key aspects of the test plan
are the description of the software changes, and the tests
that will be run to prove that those software changes are
implemented correctly. The test plan is written when-
ever a full validation or a regression test is run.
Volume testing
Volume testing is the process of exercising the unit for an
extended period of time. In our medical applications, we
have found that running a volume test for a 72-hour
period can uncover problems not found elsewhere.
Volume testing is almost always done with an auto-
mated test system. Volume testing does not necessarily
use random fast, slam-bang key presses like stress testing
does. Volume testing uses a logical approach to testing all
of the paths in a software program. Once the device is in
a certain state, probabilistic algorithms like Probability
Density Function (PDF) are used in determining the
next state.
Test-configuration form
Because it is very important to be able to repeat a test,
it is necessary to document the configuration of the
equipment that is used to run a test against. This
documentation is written on a test configuration form.
The contents of these forms can vary. However, the
primary fields are date, item, and item serial number,
and there is a place for the test engineer to sign his
name. The test configuration form becomes part of the
test-documentation suite.
Executing manual protocol
Scenario testing
Scenario testing is the act of writing a test that emulates
the actual use of the software from the user's perspec-
tive. For example, in anesthesia machines, a test could be
written to cover a clinical situation. Scenario testing is
Executing manual protocol for a formal test involves
printing out the test procedures and going through the
steps in the procedures. Because these printouts are
a part of the official documentation, the test engineer
must sign and date the documents. If the tester finds
