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cies were addressed, and the upgraded device was accepted in
December, 2006.
3.2.3 Medical Device Testing Services
First Article
The Medical Device Testing Services (MDTS) first article is
shown in Figure 15. The equipment is relatively small. It con-
sists of (1) a main wheeled cabinet (26 in. wide by 76 in. high
by 24 in. deep) that houses the test chamber, the hydraulic
pump, the hydraulic actuator, and associated control elec-
tronics; (2) a separate heat exchanger (16 in. wide by 23 in.
high by 20 in. deep) that provides temperature control for the
test cell; and (3) laptop computer for controlling the machine
and collecting and analyzing test data. The heat exchanger is
Figure 13. Interlaken specimen mounted under the laptop computer table in Figure 15. An interesting
extensometer system. feature of the MDTS SPT is the hydraulic system operates on
115 V AC power, and the hydraulic pump only operates
intermittently, which makes the unit very quiet during oper-
compliance tests detailed in Version 2.0 of the specification ation. The heat exchanger requires single phase 208230 V
were performed. Table 9 summarizes the items included in AC power. The unit also requires compressed air to raise and
the specification compliance testing. The specification com- lower the test chamber and apply confining pressure.
pliance testing revealed several deficiencies with the upgraded Figure 16 shows the MDTS SPT with the test chamber
equipment summarized in Table 10. Most of the deficiencies open, and a specimen inserted for confined testing. The test
were related to the control and analysis software. Table 10 chamber is relatively small, only 10 in. in diameter by 21 in.
also summarizes the changes made to resolve the deficiencies. high. The reaction posts are located in an awkward position,
Substantial effort was expended by both the research team making installation of the instrumentation on the specimen
and Interlaken to resolve the deficiencies. All of the deficien- for dynamic modulus testing difficult. The test cell heat
exchangers and associated fans are mounted in the top of the
test chamber. The test chamber is insulated; a sight glass and
lighting are provided to allow the operator to view the speci-
men during testing. The chamber is opened and closed by a
manually controlled pneumatic actuator located at the back
of the machine as shown in Figure 17. It is locked in the closed
position by a manual ring locking mechanism that has posi-
tion switches that are interlocked with the pressure and load
control.
The specimen mounted deformation system consists of
two magnetic LVDT extensometers mounted 180° apart as
shown in Figure 18. Each extensometer includes two very
flexible springs that allow only vertical movement of the ends.
Each extensometer includes a pin that centers the measuring
system. When the pin is released, the extensometer is acti-
vated. These extensometers are similar to those developed by
Interlaken. This deformation measuring system is the third
system that was developed by MDTS for the SPT. The other
two systems did not function properly during the ruggedness
testing. To quickly and accurately mount the glued gage
points to the specimen, MDTS designed the gluing apparatus
shown in Figure 19. This system has pneumatic actuators that
press the gage points against the specimen at the correct gage
Figure 14. Interlaken gluing apparatus. length at the center of the specimen.
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Table 9. Summary of specification compliance tests.
Specification
Item Section Method
Assembled Size 4.4 & 4.6 Measure
Specimen and Display Height 4.4 Measure
Component Size 4.7 Measure
Electrical Requirements 4.5 & 4.6 Documentation and trial
Air Supply Requirements 4.8 Documentation and trial
Limit Protection 4.9 Documentation and trial
Emergency Stop 4.10 Documentation, visual inspection, trial
Loading Machine Capacity 5.1 Independent force verification
Load Control Capability 5.2 5.4 Trial tests on asphalt specimens and manufacturer provided dynamic
verification device.
Platen Configuration 5.5 Visual
Platen Hardness 6.1 Test ASTM E10
Platen Dimensions 6.2 Measure
Platen Smoothness 6.3 Measure
Load Cell Range 7.1 Load cell data plate
Load Accuracy 7.2 Independent force verification
Load Resolution 7.3 Independent force verification
Configuration of Deflection Measuring 8.1 Visual
System
Transducer Range 8.2 Independent deflection verification
Transducer Resolution 8.3 Independent deflection verification
Transducer Accuracy 8.4 Independent deflection verification
Load Mechanism Compliance and 8.5 Measure on steel specimens with various degrees of lack of
Bending parallelism
Configuration of Specimen 9.1 Visual
Deformation Measuring System
Gauge Length of Specimen 9.1 Measure
Deformation Measuring System
Transducer Range 9.2 Independent deflection verification
Transducer Resolution 9.3 Independent deflection verification
Transducer Accuracy 9.4 Independent deflection verification
Specimen Deformation System 9.5 Trial
Complexity
Confining Pressure Range 10.1 & 10.5 Independent pressure verification
Confining Pressure Control 10.2 Trial tests on asphalt specimens
Confining Pressure System 10.3 & 10.4 Visual
Configuration
Confining Pressure Resolution and 10.5 Independent pressure verification
Accuracy
Temperature Sensor 10.6 & 11.4 Independent temperature verification
Specimen Installation and 9.5, 10.7 & Trial
Equilibration Time 11.3
Environmental Chamber Range and 11.1 Independent temperature verification
Control
Control System and Software 12 Trial
Data Analysis 13 Independent computations on trial test
Initial Calibration and Dynamic 14 Certification and independent verification
Performance Verification
Calibration Mode 14.6 Trial
Verification of Normal Operation 15 Review
Procedures and Equipment
On-line Documentation 16.1 Trial
Reference Manual 16.2 Review
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Table 10. Interlaken deficiencies and solutions.
Deficiency Solution
Slow temperature recovery. Modified the temperature control software to switch control from the test
chamber probe to the bath probe when the chamber is opened, then back to
the test chamber probe when the chamber is closed.
Cooling fluid leaks. Replaced plastic hose clamps with steel band hose clamps.
Units for temperature control. Modified software to use both U.S. customary and SI units.
Test chamber binding. Lubricated actuator shaft and realign chamber.
Test chamber air leaks. Added temporary seal to affected areas.
0.01 Hz loading for dynamic Modified software to allow user to test at 0.01 Hz loading.
modulus.
Irregular first cycle data during Modified software to collect additional cycles that are not stored. Only the
dynamic modulus testing. last 10 cycles are stored and analyzed.
Incorrect computation of dynamic Modified software to correct computations.
modulus data quality statistics.
Poor dynamic load control at high Added a tuning algorithm to allow user to develop and store templates with
temperatures. servo-hydraulic gains.
Software occasionally crashes Modified strain limit shut down algorithm.
when maximum strain is reached
during flow number testing.
Incomplete documentation. Provide required documentation
Final design, fabrication, and shop testing of the MDTS rized in Table 9 was performed. The specification compliance
SPT required approximately 18 months. MDTS was given testing and the ruggedness testing revealed several deficien-
authorization to proceed with the machine on February 6, cies with the equipment, which are summarized in Table 11.
2004. The machine was delivered on September 14, 2005. Most of the deficiencies were related to the control and analy-
Upon delivery, the specification compliance testing summa- sis software. Table 11 also summarizes the changes made to
Figure 15. Overall view of MDTS SPT. Figure 16. Open test chamber for the MDTS SPT.
