U.S. patent application number 13/161337 was filed with the patent office on 2011-12-15 for methods and instruments for material testing.
This patent application is currently assigned to THE REGENTS OF THE UNIVERSITY OF CALIFORNIA. Invention is credited to Jonathan Adams, Barney Drake, Paul Hansma, Jason Lulejian, Douglas Rehn.
Application Number | 20110303022 13/161337 |
Document ID | / |
Family ID | 40156830 |
Filed Date | 2011-12-15 |
United States Patent
Application |
20110303022 |
Kind Code |
A1 |
Hansma; Paul ; et
al. |
December 15, 2011 |
METHODS AND INSTRUMENTS FOR MATERIAL TESTING
Abstract
Methods and instruments for characterizing a material, such as
the properties of bone in a living human subject, using a test
probe constructed for insertion into the material and a reference
probe aligned with the test probe in a housing. The housing is hand
held or placed so that the reference probe contacts the surface of
the material under pressure applied either by hand or by the weight
of the housing. The test probe is inserted into the material to
indent the material while maintaining the reference probe
substantially under the hand pressure or weight of the housing
allowing evaluation of a property of the material related to
indentation of the material by the probe. Force can be generated by
a voice coil in a magnet structure to the end of which the test
probe is connected and supported in the magnet structure by a
flexure, opposing flexures, a linear translation stage, or a linear
bearing. Optionally, a measurement unit containing the test probe
and reference probe is connected to a base unit with a wireless
connection, allowing in the field material testing.
Inventors: |
Hansma; Paul; (Goleta,
CA) ; Drake; Barney; (Rena, NV) ; Rehn;
Douglas; (Lompoc, CA) ; Adams; Jonathan;
(Santa Barbara, CA) ; Lulejian; Jason; (Pismo
Beach, CA) |
Assignee: |
THE REGENTS OF THE UNIVERSITY OF
CALIFORNIA
Oakland
CA
|
Family ID: |
40156830 |
Appl. No.: |
13/161337 |
Filed: |
June 15, 2011 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
12079444 |
Mar 27, 2008 |
7966866 |
|
|
13161337 |
|
|
|
|
60921788 |
Apr 3, 2007 |
|
|
|
Current U.S.
Class: |
73/862.53 |
Current CPC
Class: |
G01N 3/42 20130101; A61B
5/0002 20130101; A61B 5/441 20130101; A61B 5/0053 20130101; A61B
5/4504 20130101 |
Class at
Publication: |
73/862.53 |
International
Class: |
G01L 5/00 20060101
G01L005/00 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] This invention was made with Government support under Grant
no. RO1 GM 065354-05 from the National Institutes of Health and
Grant no. NCC-1-02037 from NASA. The Government has certain rights
in this invention.
Claims
1-44. (canceled)
45. An instrument for characterizing a material, comprising: a
housing; a three footed reference probe aligned with the housing,
wherein the three feet form a tripod base for resting the
instrument on a surface and the plane of the tripod forms a
reference plane; a test probe aligned within the reference probe; a
force generator; and; a force sensor operatively coupled to the
test probe for determining a force vs distance parameter by
measuring the force needed to insert the test probe a predetermined
distance or as a function of distance into the material relative to
the reference plane formed by the tripod; wherein the test probe
applies a force on the material of at least several Newtons.
46. The instrument of claim 45, wherein the tripod base is located
on a circle having a diameter of about 1.5 inches.
47. The instrument of claim 45, further comprising a distance
sensor.
48. The instrument of claim 45, wherein the tripod feet are
adjustable and contain permanent magnets.
49. The instrument of claim 45, wherein the tripod feet are
rounded.
50. The instrument of claim 45, wherein the tripod feet are
pointed.
51. The instrument of claim 45, wherein the test probe is coaxially
aligned within the reference probe and is not cantilevered.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of Provisional Patent
Application No. 60/921,788, filed Apr. 3, 2007, which is
incorporated herein in its entirety.
FIELD OF THE INVENTION
[0003] The invention relates to an apparatus and method for
materials testing.
BACKGROUND OF THE INVENTION
[0004] Indentation testing to determine the hardness of materials
has a long history. Conventional indentation tests include the
Brinell hardness test, the Rockwell hardness test, and the Vickers
hardness test. The Brinell and Vickers tests involve indenting at a
fixed load and then examining the diameter of the indentation. As
shown schematically in FIG. 1A, the Rockwell test, which is the
most commonly used test, involves measuring the depth of
indentation from a fixed load by measuring how far a test probe 102
goes into the material under test 104. This requires a rigid frame
106. It cannot work if there is a soft layer in the mechanical path
from the top of the material under test 104 down through the rigid
frame 106 and back to the test probe 102 that will deform during
indentation (as indicated schematically by the springs 108 in FIG.
1C) because the distance that the test probe 102 goes into the
material under test 104 cannot be distinguished from the deflection
of the soft layer. A real example of this problem would be
attempting to measure the Rockwell hardness of a bone surface
exposed during surgery. The soft tissue between the bone and the
table on which the body rested would be like the springs 108 shown
in FIG. 1C.
[0005] The development of very sensitive methods for measuring the
depth of indentations such as capacitance sensors, optical beam
deflection, laser interferometers or even very sensitive linear
variable differential transducers, LVDTs, together with the
development of sophisticated techniques for determining mechanical
parameters from force vs. distance data only, (ref. W. C. Oliver
and G. M. Pharr. Measurement of hardness and elastic modulus by
instrumented indentation: Advances in understanding and refinements
to methodology. J. Mater. Res. 19 (2004), 3. (review article)), has
made possible a new class of indentation machines called
nanoindentation testers or nanoindenters. They typically use
submicrometer indentations. Nanoindentation testors also use a
rigid frame 106 as shown schematically in FIG. 1A to enable
accurate measurement of the distance that an indenter goes into the
sample at a fixed load for macroindentation tests or variable loads
for nanoindentation tests. Again, a substantial soft layer under
the sample as shown in FIG. 1C would prevent accurate
nanoindentation testing.
[0006] This solution to the problem of soft layers has been
previously implemented, for example, in U.S. Pat. No. 1,770,045,
with a durometer as shown in FIG. 2A. In this case a rigid frame is
not needed because the base of the durometer 202 rests directly on
the material under test 204 and indentations of the test probe 206
(sometimes called the foot) into the material are measured relative
to the position of the base of the durometer 202. However,
durometer indentation measurements only characterize the material
with a hardness number. Attempts have been made to relate hardness
measurements taken with a durometer to the elastic modulus of the
material. However, no accurate, widely accepted model is available.
This is in part due to the difficulties in theoretical analysis
arising from the complex indenter geometry, and the inability to
correct for time-dependent effects because of a lack of control of
the loading rate with the durometer [Briscoe, B. J. and Sebastian,
K. S. An analysis of the durometer indentation. Rubber Chemistry
and Technology 66 (5): 827-836 1993)].
[0007] Other prior art portable hardness testers also exist. In
particular there are many rebound testers such as the TH130 and
TH150 pocket-size hardness tester from Corvib and many ultrasonic
hardness testers such as the High Resolution SH-21 Portable
Hardness Tester from Micro Photonics Inc. Here too, however, to the
best of our knowledge there exists no portable tester that measures
more material properties beyond just hardness.
[0008] One approach to indentation measurement on soft samples is
to use, as a distance reference, the upper surface of the sample as
is found in the instrument outlined in U.S. Pat. No. 6,142,010. In
spite of this improvement, this instrument is limited in that it is
solely designed for measuring hardness and relies on an external
mechanical frame (as opposed to a reference probe) to maintain a
rigid mechanical path between the sample and the distance
measurement. The upper surface of the sample is used for a
differential measurement of the indentation depth in the CSM
Indentation Testers, which can measure more that just hardness.
Here again, however, a rigid frame is present.
[0009] Atomic Force Microscopes (AFMs) can rest on the surface of
the material under test and could, in principle at least, be used
for indentation tests [C. A. J. Putman, H. G. Hansma, H. E. Gaub,
and P. K. Hansma, Langmuir 8, 3014 (1992)]. An example of
indentation tests on bone with the AFM is James B. Thompson et al.,
Nature 414, 774, 13 Dec. 2001, though this was done with a
prototype AFM that was not capable of resting on the surface of the
material under test.
[0010] One AFM company, Asylum Research, has also produced a
nanoindenter, the MFP-3D NanoIndenter.TM. for Quantitative Surface
Characterization. This instrument eliminates the problem of angular
motions of cantilevers and goes to higher forces, up to 14
milliNewtons. It consists of a new NPS.TM. Nanopositioning sensor
for their MFP-3D.TM. Stand Alone Atomic Force Microscope. The
sample is held rigidly to the MFP-3D scanner through specialized
sample mounts. Thus it is not designed to rest on the surface of
the sample as for the present invention.
[0011] Other publications dealing with prior art systems include:
U.S. Pat. No. 5,450,745; U.S. Patent Publication Nos. 2002/0170360
and 2005/0262685; "Micro Hardness Tester (MHT) for fracture
toughness determination of brittle materials", No. 8, July 1998;
CSM Indentation Testers, four page brochure; and "ASTM Proposed
Instrumented Indentation Testing Standard", pages 1-4, October
2003.
[0012] Thus, while there have been portable hardness testing
devices and devices that measure parameters other than hardness, we
are aware of no prior device that combine the ability to be
portable with the ability to measure a wide variety of parameters
based on indentation of a probe into a sample.
BRIEF SUMMARY OF THE INVENTION
[0013] The present invention overcomes the foregoing drawbacks in
providing improvements in the technology for measuring material
properties of materials such as bone in a living person, teeth, the
leg bones of race horses, the wing of an aircraft, the surface of a
part during manufacturing or assembly and other structures that are
not easily tested in conventional mechanical testers.
[0014] The invention is designed to measure more parameters than
just hardness. This is important for many applications such as
predicting bone fracture resistance or monitoring fatigue damage in
airplane wings where measurements of hardness alone are not
sufficient. Thus, compared with the devices referred to above in
the BACKGROUND OF THE INVENTION section, the invention extends the
capabilities of previous instruments for measuring the material
properties of materials under test by making it possible to measure
more material properties than just hardness with a portable
instrument. Moreover, the instrument can be portable and hand held.
At the heart of the invention is a measurement head that contains a
reference probe that rests substantially on the surface of the
material under test and provides a reference for measuring the
distance that a test probe indents the material under test. The
invention can, optionally, measure complete force vs. distance
curves during one or multiple indentation cycles where the force is
the force that the invented instrument supplies during the
indentation cycle(s).
[0015] More particularly, in a departure from prior devices, we
provide a device and method for characterizing a material using a
test probe and constructed for insertion into the material and a
reference probe aligned with the test probe in a housing. The
housing is hand held or placed so that the reference probe contacts
the surface of the material under pressure, applied either by hand
or by the weight of the housing, causing the test probe to indent
the material while maintaining the reference probe substantially
under said pressure. This allows the evaluation of one or more
properties of the material related to indentation of the material
by the probe.
[0016] Referring again to FIGS. 1A-1D, the invention replaces the
rigid frame 106 with a reference probe 110 that rests directly on
the surface of the sample. Now, as will be further detailed below,
the relevant mechanical path will be from the material up through
the reference probe 110 and back down to the test probe 102.
[0017] The invention increases the capability of the durometer by
adding a measurement head containing electronic actuators to
generate forces and/or displacements as well as sensors for load
and displacement that are coupled to a computerized data
generation, collection and analysis system to get many parameters
beyond just hardness. Compared with other instrumented indentation
systems capable of measuring properties beyond hardness, our
invention greatly increases the ability to test samples with
complex geometries or in locations where attachment of a sample to
a rigid sample holder is impossible.
[0018] The invention is also distinct from the above described AFMs
in that the test probes of the invention are not mounted on
cantilevers as for the AFMs. Thus there is not the problem of
angular motions of cantilevers. Also, the preferred embodiments of
the present invention typically go to much larger forces, several
Newtons, compared to the microNewtons, nanoNewtons or below as
typical of AFMs. This is an advantage for testing real materials
without special surface preparation because the probed volume is
large enough to be insensitive to thin surface layers of, for
example, the water that covers most materials in ambient
environments, and surface topography.
[0019] An additional feature of the invention is an optional
wireless connection between a portable measurement head, which
contains the mechanical components necessary for the measurements
together with some electronics, and a base station, which contains
electronics including, optionally, a computer. The base station can
both supply instructions for the measurements and acquire data from
the measurements.
[0020] Another additional feature of the invention is the optional
ability to hand-hold the measurement head. This increases the ease
and speed with which measurements can be made on complex structures
such as airplane landing gear or a race horse's leg. The
combination of wireless operation and a hand held measurement head
is particularly useful for measurements in the field: outside a
testing lab.
[0021] Still another feature enabling a compact hand-holdable
instrument with the extensive capabilities of the invention is the
use of opposing flexures with a linear translation stage that
facilitate incorporation of a voice coil actuator.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] For a more complete understanding of the present invention,
reference is now made to the following descriptions taken in
conjunction with the accompanying drawing, in which:
[0023] FIGS. 1A-1D are schematic drawings illustrating the advances
in the present invention over the prior art instruments that rely
on a rigid frame for measuring the depth of indentation;
[0024] FIGS. 2A and 2B are schematic drawings illustrating the
advances in the present invention over the prior art instruments
called durometers;
[0025] FIG. 3 is a cross-sectional view of the measurement head of
this invention in accordance with one embodiment;
[0026] FIG. 4 is a cross-sectional view of the measurement head of
this invention in accordance with one embodiment;
[0027] FIGS. 5A and 5B depict a portable embodiment of the
instrument with wireless connection between a portable measurement
head of this invention, which contains the mechanical components
necessary for the measurements together with some electronics, and
a base station, which contains electronics including, optionally, a
computer;
[0028] FIGS. 6A-6C depict the flexures used in the embodiments of
FIGS. 3 and 4;
[0029] FIG. 7 depicts the finite element analysis of the flexure
shown in FIG. 6;
[0030] FIGS. 8A-8D depict an alternate design for the flexures used
in the embodiments of FIGS. 3 and 4. This flexure has lower
stiffness than the one depicted in FIG. 6;
[0031] FIGS. 9A-9E depict various reference probes that can be used
with the measurement head of this invention shown in FIG. 3;
[0032] FIG. 10 depicts various test probes for the invention;
[0033] FIG. 11 depicts probe assemblies, which are combinations of
test probes with reference probes, that can be used with the
measurement heads of this invention show in FIGS. 4, 13, 14 and 15,
showing also various ways to attach a test probe to the shaft of
the measurement head of this invention and various ways to inhibit
buckling of the test probe;
[0034] FIG. 12 depicts various support stands for the measurement
head used in the invention;
[0035] FIG. 13 is a view of the measurement head of this invention
in accordance with one embodiment that contains mechanisms for both
a coarse and fine adjustment of the relative position of a test
probe and a reference probe;
[0036] FIGS. 14A and 14B are views of the measurement head of this
invention in accordance with two embodiments that use a linear
stage rather than flexures;
[0037] FIG. 15 is a view of the measurement head of this invention
in accordance with one embodiment that uses wire flexures rather
than the disk flexures of FIGS. 3 and 4 or the plastic flexures of
Clark Synthesis force generator shown in FIG. 13;
[0038] FIGS. 16A-16C show three types of data graphs that can be
generated with this invention;
[0039] FIGS. 17A-17F show various parameters that can be extracted
from data graphs generated with this invention;
[0040] FIG. 18 shows prior art equations [W. C. Oliver and G. M.
Pharr. Measurement of hardness and elastic modulus by instrumented
indentation: Advances in understanding and refinements to
methodology. J. Mater. Res. 19 (2004), 3.] for calculating Hardness
and Elastic Modulus;
[0041] FIG. 19 shows a screenshot of the user interface of the
Labview program used in the currently preferred embodiment to
control the invention;
[0042] FIG. 20 shows a screenshot of the automated data analysis
interface of the Labview program used in the currently preferred
embodiment;
[0043] FIG. 21 shows a screenshot of the automated data analysis
curve of Energy Dissipated as a function of time from the Labview
program used in the currently preferred embodiment;
[0044] FIG. 22 shows a screenshot of the automated data analysis
curve of Adhesion force as a function of time from the Labview
program used in the currently preferred embodiment;
[0045] FIG. 23 shows a screenshot of the automated data analysis
curves of Elastic Modulus and Hardness as a function of time from
the Labview program used in the currently preferred embodiment;
and
[0046] FIG. 24 depicts a generalized measurement head used in the
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0047] The essential feature of the invention is a probe that is
inserted into a material under test a distance that is measured
relative to a reference probe, which rests substantially on the
surface of the material under test. In a preferred embodiment, the
probe consists of a steel shaft tipped with a sharpened diamond. It
slips inside a three footed reference probe with feet on a circle
of diameter approximately 1.5 inches. A typical penetration depth
is 0.05 mm.
[0048] The method is particularly suited to evaluating one or more
properties of a living human bone of a subject, the test probe
being inserted through the periosteum and/or soft tissue on the
bone so that the test probe contacts the subject's bone. Such an
evaluation is described in our previous work on developing a bone
diagnostic Instrument, filed in U.S. patent application Ser. No.
11/417,494 filed May 3, 2006 titled Methods and Instruments for
Assessing Bone Fracture Risk, the disclosure of which is
incorporated herein by reference.
[0049] In one class of embodiments, the probe and reference probe
are connected to a measurement head. This measurement head can be
connected to a base station either with wires or with a wireless
connection. This measurement head can be held on the material under
test with a stand or it can be hand held or it can be hand held
with the help of an optional support. This optional support can,
for example, serve to hold the measurement head at a fixed angle
relative to the surface of the material under test during the test
process.
[0050] The test process consists of one or more indentation cycles
during which the force applied to the probe and the distance that
the probe is inserted into the material under test are measured.
These measurements can be analyzed to give material parameters such
as Elastic Modulus, Hardness, Adhesion (that is the maximum force
needed to pull the probe out of the material), Elastic Energy
Dissipation, Plastic Energy Dissipation, Total Energy Dissipation,
Maximum distance of insertion, and Maximum Force during the cycle.
All of these parameters can be measured as a function of time
through a series of cycles. For example, we sometimes measure bone
parameters over a series of 80 cycles at 3.5 Hz. Additional useful
parameters are the ratios of the final to initial values of the
individual parameters: for example, the ratio of the Plastic Energy
Dissipation on the final cycle to the Plastic Energy Dissipation on
the first cycle. It has turned out that the ratio of the final
value of Hardness to the initial value of the Hardness on bone is
correlated with resistance to bone fracture.
[0051] FIG. 3 is a detailed drawing of the measurement head of a
currently preferred embodiment of our invention. A test probe 302
consisting of a shaft 304 and a sharp tip (often a diamond) 306 is
attached to a shaft 308 that connects to the core 310 of an LVDT
312 (for example Measurement Specialties MHR 025). This in turn
connects to a load cell 314 (for example Futek LSB 200) and then,
with a shaft 315, to a force generator 316 consisting of two
flexures of novel design 318, (which will be described in more
detail in FIGS. 6, 7 and 8) together with a voice coil actuator (a
modified version of BEI Kimco Magnetics LA16-27-000A) which
consists of a moving coil 320 in a magnetic field assembly 322. The
flexures 318 are attached with screws 323. This force generator 316
is anchored in an inner shell 324 that is capped by the flexures
318. The force generator 316 is held in an outer shell 326. The
outer shell is connected to a nose piece 328, which supports the
LVDT body 312. The position of the LVDT body 312 can be adjusted to
zero or otherwise adjust the signal from the LVDT 310, 312 with a
fine screw 330 and is locked into place with set screws 332. The
nose piece 328 also rigidly supports a reference probe 334 that
rests on the surface of the sample under test 336. The measurement
head can be hand held during the test, which has the advantage that
it can be forced against the sample under test with greater force
than its own weight. In general the largest force that can be
applied by the force generator 316 to the sample under test 336
must be less than the force with which the measurement head is
pressed against the surface. Otherwise the measurement head will
lift off the sample under test 336. This greater force with hand
held operation allows greater maximum force during the measurement
of force vs. distance curves. Hand held operation also allows
measurements on surfaces that are not substantially horizontal and
measured from above. Alternately, elastic elements such as springs
or rubber tubes or bungee cords can used to hold the measurement
head against the sample under test.
[0052] The electrical signals to actuate the force generator 316 as
well as the force signal from the load cell 314 and the distance
signal from the LVDT 310, 312 pass through an electrical connector
338 (AMP 28 pin connector). An optional adjustment of the initial
position of the test probe 302 relative to the reference probe 334
can be made with an optional spring 340 that is pulled with an
optional screw 342 that is threaded through a cap 344. The
bandwidth of both the LVDT 310,312 and the load cell 314 and both
their amplifiers and the data acquisition system is 1 kHz or above.
Thus the instrument can be operated to obtain complete force vs.
distance curves in cycle times as fast as 0.1 second. For maximum
resolution, cycle times as fast as 1 second are more typical. For
maximum speed, cycle times as fast as 0.01 second have been used,
but the force vs. distance curve is not accurately captured. Fast
cycle times can, however, be used to test for damage during cyclic
loading with, optionally, slower, more accurate curves taken before
and after the fast cyclic loading.
[0053] FIG. 4 is a detailed drawing of another version of the
measurement head of a currently preferred embodiment of our
invention. This version is designed to be used as Bone Diagnostic
Instrument (Methods and Instruments for Assessing Bone Fracture
Risk U.S. patent application Ser. No. 11/417,494) for which soft
tissue overlying the bone must be penetrated in order to measure
the properties of the underlying bone. In this version the test
probe 402 is a sharpened steel rod of diameter 0.015'' held in a
mounting pin 404 which attaches magnetically to a permanent magnet
406 that is attached to a shaft 408 that connects to the core 410
of an LVDT 412 (for example Measurement Specialties MHR 025). This
in turn connects to a load cell 414 (for example the Futek LSB 200
or the Sensotec Model 34 precision miniature load cell) and then to
a force generator 416 consisting of two flexures of novel design
418, which will be described in more detail in FIGS. 6, 7 and 8)
together with a voice coil actuator (a modified version of BEI
Kimco Magnetics LA16-27-000A) which consists of a moving coil 420
in a magnetic field assembly 422. This force generator 416 is
anchored in an inner shell 424 that is capped by the flexures 418.
The force generator 416 is held in an outer shell 426. The outer
shell is connected to a nose piece 428, which supports the LVDT
body 412. The position of the LVDT body 412 can be adjusted to zero
or otherwise adjust the signal from the LVDT 410, 412 with a fine
screw 430 and is locked into place with set screws 432. The nose
piece 428 also rigidly supports a reference probe 434 that rests on
the surface of the sample under test 336. In this version the
reference probe 434 consists of small diameter stainless steel
tubing 436 held in a brass body 438 that is threaded into the nose
piece 428 and held rigidly in position with a knurled locking nut
440
[0054] The electrical signals to actuate the force generator 416 as
well as the force signal from the load cell 414 and the distance
signal from the LVDT 410, 412 pass through an electrical connector
442 (AMP 28 pin connector). An optional adjustment of the initial
position of the test probe 402 relative to the reference probe 434
can be made with an optional spring 444 that is pulled with an
optional screw 446 that is threaded through a cap 448.
[0055] FIGS. 5A and 5B shows how the currently preferred
embodiments of our invention can be made wireless. The measurement
head 503 (shown in more detail in FIGS. 3 and 4) is combined with a
wireless adapter/power pack module 500 for the instrument. This
module 500 has many functions including: supplying power for the
instrument, amplifying and conditioning the signals from the
transducers, the transmission of data, the reception of input
signals, and the amplification of input signals.
[0056] The transmission of the data starts at the connector 501,
where the interface with the instrument is located. The signal for
the distance sensor (from the instrument) is sent through the
connector 501 to the PC board with distance sensor amplifier and
signal conditioner 504. Here the signal is amplified and then sent
to the wireless module 508. The data is then sent to the partner
computer(s) 513 (FIG. 5B) with electromagnetic radiation 509. The
signal from the load cell (from the instrument) goes through the
connector 501 to the PC board with load cell amplifier and signal
conditioner 502. Here the signal is amplified and conditioned and
then sent to the wireless module 508 where it is transmitted to the
partner computer(s) 513 with electromagnetic radiation 509.
[0057] Input signals are received via electromagnetic radiation 509
by the wireless module 508. The signals are then transmitted to and
amplified by the PC board with amplifier 506 that drives the force
generator 416. The amplified signal is then sent to the force
generator 416 in the instrument via the connector 501.
[0058] The whole system, including the instrument, may be powered
from the rechargeable battery 510. The battery itself has an energy
port 511 where external power can be introduced into the system to
either recharge the battery or power the wireless module
externally. The battery 510 may also be wired such that it is
easily removable or replaceable. Batteries such as found in small
hand tools such as cordless drills are suitable. For the force
generator 416 in the currently preferred embodiment, we typically
use average currents of less than 1 A at voltages of a few Volts
for times of order 20s thus requiring of order 0.0056 Ah per test.
This can easily be supplied by the type of NiMH rechargeable
batteries used in cordless drills, which can supply 3 Ah, enough
for over 500 tests.
[0059] The wireless adapter/power pack module 500 can (optionally)
also contain a keypad 512 to set test parameters and access
selected test data and analysis on a display screen 514. For
wireless use, the switch 516 can be added to the measurement head
503 (shown in more detail in FIGS. 3 and 4) to trigger test cycles
conveniently.
[0060] The measurement head 503 is connected to the wireless
adapter/power pack module 500 with cable 518. As shown in FIG. 5A,
the cable can be external, joining the measurement head 503 to the
wireless adapter/power pack module 500 that is mounted on top of
the measurement head. This permits the removal of the wireless
adapter/power pack module 500 if it is desired to have the
measurement head connected directly to control electronics and a
computer. If the unit is designed only for wireless use, the cable
can, of course, be internal. Alternately the wireless adapter/power
pack module 500 can be separate from the measurement head 503 as
shown in FIG. 5B. This has the advantage of making the hand held
part lighter and the wireless adapter/power pack module 500 more
capable in terms of battery capacity, data processing and data
storage.
[0061] FIGS. 6A-6C show the circular flexures used in the currently
preferred embodiment of the invention. The flexures are included to
guide the motion of the force generator (for example 416, FIG. 4)
and ensure that there is no substantial off-axis motion. The
flexures design consists of a large, horizontal, thin inner
membrane 602 connected to a outer, thin, horizontal membrane 604
through a vertical ring 606. The design of the flexures was
improved through the use of finite element analysis. FIG. 7 shows
the results of simulating the deformation of one of the flexures
under an axial load.
[0062] The softness of the circular flexure may be increased by
cutting out radial sections 802 of the flexure, as shown in FIGS.
8A-8C. Further softening could be achieved by cutting away more of
the circular flexure, for example, leaving only thin radial strips
like spokes on a wheel.
[0063] FIGS. 9A-9E show various reference probes that can be used
in place of the reference probe 334 shown in FIG. 3. 9A has three
rounded feet 900 to minimize marring of the material under test. 9B
has three pointed feet 902 to minimize lateral slipping. 9C has
three adjustable feet 904 that contain permanent magnets 906 to
minimize slipping and marring of magnetic surfaces such as steel.
These magnets 906 could also be electomagnets or mechanically
switchable magnets as used in magnetic bases (see, for example,
1228 in FIG. 12). 9D shows one of many possible variants of fixed
908 and adjustable feet 910. The motion of the adjustable foot 910
is demagnified at the location of the test probe 912 to give more
precise positioning. 9E shows a reference probe that is suitable
for use with a standard diamond indenter 914. As an example, a
Rockwell Diamond Indenter with a Versitron shank 914 is shown, but,
of course many others could be used with a suitably designed
reference probe.
[0064] FIG. 10 shows different possibilities for the tips of test
probes. Test probe 1000 is patterned on the diamond indenter used
in Knoop hardness testing. It has a pyramid-shaped diamond 1002
with apical angles of 130.degree. and about 170.degree., mounted on
a tungsten carbide shank 1004. Test probe 1006, has a diamond 1008
in the shape of a square-based pyramid whose opposite sides meet at
the apex at an angle of 136.degree. as used in Vickers hardness
testing of metals and ceramics, mounted on a ceramic shaft 1010.
Test probe 1012 is a disk that can be rotated for measuring
friction, .PHI.=0, or viscosity of tissue near a bone surface, at
.PHI.=0 or O>0 as in conventional viscosity measurements. Test
probe 1014 is wedge shaped and is used for assessing the fracture
resistance of materials. Test probe 1016, designed for testing the
material properties of bone and teeth, has a cone at its end. In a
preferred embodiment .theta.=90 and the test probe is tool steel.
In other embodiments the test probe can have angles .theta.=70 and
50 and can have a tip 1018 of a different material, such as
diamond. Test probe 1020 is patterned after the indenters used in
some Rockwell and Brinell hardness testing, and has a half sphere
of tungsten carbide 1022 bonded to a steel shank 1024. Test probe
1026 is a tube that can be rotated for measuring friction on the
surface of a material. Test probe 1028 is a screw that can test
bone by measuring the torque necessary to screw it into the bone.
These are only intended as representative examples. Many other
geometries and test probe materials could be used.
[0065] FIG. 11 shows details of three probe assemblies (test probes
in reference probes) in the top row. In the first example, the test
probe 1102 is held in a mounting pin 1104 that is a 1/16'' diameter
steel rod with a hole in the end into which the test probe is glued
or soldered. The reference probe 1106 is composed of sharpened,
small diameter tubing 1108 joined to a threaded body 1110. In the
second example, the test probe 1112 is shorter, but, when mounted
on a longer mounting pin 1114, gives the same overall length of
mounted test probe as the previous example. In this case the
reference probe is a hypodermic syringe needle 1116 that is
removably mounted in a threaded Luer adaptor 1118. In the third
example, the test probe 1120 is mounted in a mounting pin 1122.
Here the reference probe 1124 has no tubing projecting from the
end, but is suitable for use when the material under test is not
covered with a layer that must be penetrated (as in the case of
skin covering bone).
[0066] FIG. 11 also shows, in the bottom row, some details of an
alternate to the magnet shown as 406 in FIG. 4 for holding mounted
test probes. The collet 1126 holds the mounting pin 1128 for the
test probe 1130. This collet 1126 is attached to a shaft 1132,
shown as 408 in FIG. 4. The collet 1134 holds the test probe 1136
directly. The tube 1138, which can optionally be attached to the
test probe 1136, functions to minimize buckling of the test probe
1136. The collet 1140 holds a tube 1142 in which the test probe
1144 is mounted.
[0067] Finally, the magnet 1146 holds the mount 1148 for the test
probe 1150. This test probe and all the test probes in FIG. 11 can
have many tip shapes, as shown in FIG. 10.
[0068] FIG. 12 shows various support stands for the instrument. The
instrument can be hand held, resting on the three feet of the
reference probe 334 that rest on the surface of the sample under
test 336 shown in FIG. 3. More reference probes for hand held use
were shown in FIGS. 9A-9E. For some applications, however, it is
useful to supplement or eliminate the hand holding the instrument.
The goal is to stabilize the instrument more than is possible when
just hand held. As some examples, the measurement head 1202 is
attached with a removable mount 1204 (for example, a 1/4-20 screw
mount such as used for cameras) to a rail 1206 which is, in turn
held in a guide block 1208 (for example, the Miniature
Corrosion-Resistant Versa-Mount Guide Blocks and Rails form Mc
Master Carr can be used) which is, in turn, attached with a
removable mount 1210 (for example, a 1/4-20 screw mount such as
used for cameras can be used here also) to a support arm 1212
attached to a base 1214. This support stand allows the measurement
head 1202 to move freely up and down while being constrained
laterally and held vertically.
[0069] The measurement head 1216 is also mounted via a removable
mount 1218 to a rail 1220 which is, in turn held in a guide block
1222 (for example, the Miniature Corrosion-Resistant Versa-Mount
Guide Blocks and Rails from Mc Master Carr can be used) which is,
in turn, attached with a removable mount 1224 (for example, a
1/4-20 screw mount such as used for cameras can be used here also)
to an adjustable arm 1226 attached to a magnetic base 1228.
[0070] The measurement head 1230 is permanently mounted by
attachments at each end to a rail 1232, which is, in turn, mounted
to an articulating arm system 1234 (such as the FlexArm available
from Midwest Specialties Inc.). The probe assembly 1236 is shown
schematically penetrating soft tissue of a leg 1238 to reach the
tibia 1240. The leg is held in a modified V block 1242 to stabilize
it during the measurements.
[0071] The measurement head 1244 is held in a microscope stand
1246. The material under test is in a fluid cell 1248 that is
filled with fluid 1250. One of the advantages of all the
embodiments shown is that it is easy to work with samples under
aqueous buffers to, for example, simulate physiological conditions.
It is also easy to put in a heating stage or hot plate under the
fluid cell 1248 or under a material under test that is not in a
fluid cell because there is no rigid frame that limits the space
below the measurement head. Though FIG. 12 shows a particular
microscope stand 1246, a wide variety of microscope stands are
available for mounting stereo microscopes including ones for
operating rooms that roll on the floor and allow the surgeon to see
parts of a patient's body on the operating table. This type of
rolling microscope stand could hold the measurement head 1244 for
testing the bone or teeth of a patient on a table. The core
assemble is mounted in a disk of the correct diameter for the
particular microscope stand (typically about 3'' in diameter). This
mounting in the disk can be rigid (as shown) or via a rail and
guide block system as shown in the other support stands. Conversely
the other support stands can be used without a rail and guide block
system. The advantage of the rail and guide block system is that
the force of the probe assembly on the material under test is
constant: the weight of the moving parts (for example the
measurement head 1202, the removable mount 1204 and the rail
1206).
[0072] FIG. 13 shows a previous embodiment of this invention. The
position of the test probe 1302 relative to the reference probe
1304 can be coarsely adjusted by screwing the threaded Luer adaptor
1306 into or out of the frame arm 1308. Fine adjustment comes from
turning the screw 1310 with the knob 1312. The frame arm 1308 is
held against the tip of the screw 1310 by a spring 1314. In this
embodiment the force and motion are generated by a transducer 1316
(Clark Tactile Sound Transducer, U.S. Pat. No. 5,473,700) that
consists of two dome shaped disks that are joined at their edges.
One supports a voice coil and the other supports a magnet
structure. This figure illustrates that the force generator of this
invention is not restricted to just the type of voice coil system
shown in the other figures. Other alternatives for a force
generator have been shown in FIG. 13 of Methods and Instruments for
Assessing Bone Fracture Risk U.S. patent application Ser. No.
11/417,494). This embodiment used a load cell 1318 and an optical
position detector 1320.
[0073] FIGS. 14A and 14B show two embodiments of this invention.
These embodiments are based on a commercially available compact
positioning system (VCS-10 Voice Coil Linear Stage from Equipment
Solutions, Inc.) In the left embodiment the test probe 1402 is held
in a mounting pin 1404 which attaches magnetically to a permanent
magnet 1406 that is attached to a shaft 1408 on which is mounted an
arm 1410 whose motion is detected by the optical position detector
1412. The shaft 1408 continues to a load cell 1414 and then to a
support block 1416 that is screwed to the movable platform 1418 of
a one axis stage with a guide block 1420 under the platform 1418.
The force and motion are generated by a voice coil in magnet
structure 1424. In the right embodiment a shaft 1426 is directly
mounted in support block 1416. In this case the force is monitored
as proportional to the current to the voice coil 1428. In practice
this is very close to being an accurate proportionality. If
necessary, however, it can be corrected with a correction factor of
the moving mass times the acceleration. For example, for a moving
mass of 0.1 kg and a maximum acceleration of 100 microns in ten
milliseconds, the maximum force correction would be of order) 0.1
kg.times.100 microns/(0.01 sec).sup.2=0.1 Newton. Since the
embodiment in FIG. 14B does not have the compliance of the load
cell 1414 to deal with, the motion of the test probe can be
monitored using the built in position detector 1430 in the VCS-10
Voice Coil Linear Stage from Equipment Solutions, Inc. Alternately,
higher position resolution can be achieved with a high resolution
LVDT (for example Measurement Specialties MHR 025) or other
supplemental distance detectors such as capacitance sensors,
optical beam deflection detectors, or laser interferometers to
measure the motion of the movable platform 1418 and thus the test
probe 1402 relative to the reference probe 1432, which is
stationary relative to the guide block 1420, which is attached to
the case 1434 on which the mount 1436 for the reference probe is
attached. Thus the body of the LVDT or other distance detector
would be fastened to the case 1434 and the core of the LVDT would
be attached to the test probe.
[0074] This commercial unit can also be used in feedback mode to
run the invention with position control. In this case, for example,
the force needed to indent the material under test to a fixed
maximum depth could be monitored as a function of cycle number.
This was not, however, our preferred embodiment because the force
and position noise with the VCS-10 Voice Coil Linear Stage and the
SCA824 Linear Servo Controller were much larger than in our
preferred embodiment. We believe that some of the problem was due
to friction in the one axis stage. Feedback can be more easily used
with the much smaller friction from flexures such as in the other
embodiment shown in this document. We note that feedback control
could also be used to run in a force controlled mode.
[0075] FIG. 15 shows another previous embodiment of this invention.
In this embodiment the flexure support of the voice coil 1502 in
the magnet structure 1504 is provided by two wires. The upper wire
1506 attaches to the shaft 1508 with a cylindrical block 1510. The
outer ends of the upper wire 1506 are attached to blocks 1512 which
are, in turn, mounted on flexures 1514 which are, in turn, mounted,
with blocks 1516 to the support shell 1518. The lower wire 1520
attaches to the shaft 1508 with blocks 1522. These blocks 1522 are
mounted via flexures 1524 as shown above (as 1514) but rotated 90
degrees around the axis of the lower wire 1520 so the flexures are
not as visible as above. These flexures 1524 are mounted on a
movable stage 1526 that slides on two rods 1528 and 1530. This
movable stage can be moved by turning the knob 1532 which turns the
screw 1534 which connects the movable stage 1526 to frame element
1536 which is held stationary relative to the magnet structure 1504
and the rods 1528 and 1530. Thus turning the knob 1532 lowers the
shaft 1508 and the test probe 1538 relative to the reference probe
1540. In this embodiment the force sensor 1542 is again a load
cell. The position sensor 1544 was optical. A capacitance sensor
for position could also be used in this and other embodiments.
[0076] FIG. 16A shows the force measurement for a cyclic
indentation cycle test on PMMA taken with the invention as
described in FIG. 13. The corresponding distance measurement over
the same set of indentation cycles is shown in FIG. 16B. FIG. 16C
shows a single indent-retract cycle on PMMA taken with the
invention as described in FIG. 15. The loading cycle consists of an
indentation at a fixed rate of voltage drive increase to the force
generator, a pause at fixed voltage drive to the force generator,
and a retraction at a fixed rate of voltage drive decrease to the
force generator.
[0077] FIG. 17A and FIG. 17B show respectively the measured maximum
indentation distance and maximum force after pausing at the maximum
drive to the force generator. The pause is included to reduce the
effect of viscoelasticity on measurements of the retraction slope
and thus the increase the accuracy of the measured elastic modulus.
FIG. 17C shows a linear fit to the initial part of the retraction
curve, called the retraction slope, which may be used as a material
characterization parameter, or in subsequent analysis to determine
the elastic modulus of the sample. FIG. 17D shows the first and
last indent-retract cycles for a series of several indentations
taken with the invention as described in FIG. 15, plotted together
for comparison purposes. Change in any measured property over a
series of indentation cycles may be measured, as illustrated in
FIG. 17E. The change in maximum force is measured between the first
and last indentation cycles over a series of 30 cycles. FIG. 17F
shows the measurement of work during the indent-retract cycle that
may be used to characterize a sample. The area beneath the loading
and pause cycle is quantified as the work of indentation. The
elastic energy recovery is defined as the area beneath the
retraction curve. The difference between the work of indentation
and the elastic recovery is defined as the energy dissipated in the
indent-retract cycle.
[0078] FIG. 18 (prior art) shows the measured parameters that are
pertinent to the measurement of elastic modulus and hardness in the
invention. The variables and equations used in the calculation are
listed. The analysis method used is that of Oliver and Pharr (ref.
W. C. Oliver and G. M. Pharr. Measurement of hardness and elastic
modulus by instrumented indentation: Advances in understanding and
refinements to methodology. J. Mater. Res. 19 (2004), 3. (review
article)).
[0079] The operation of the invention is aided by computer
interfacing. FIG. 19 shows a screenshot of the Labview program used
to run the invention. The force and distance measurements are
collected and plotted both versus time and as a force versus
distance graph in real-time. There are several controllable
parameters to alter the indentation protocol, including:
indentation frequency, indentation amplitude, number of indent
cycles, and the half-angle of the conical indenter, called the
probe half angle.
[0080] Automated data analysis upon completion of the indentation
cycles is achieved through the computer interface. A screenshot of
the current data analysis interface is shown in FIG. 20. The main
analysis screen shows several measured quantities that may be used
to characterize a material, as well as the raw force and distance
data, the identified transition points in each indent-retract
cycle, and a comparison of the first and last indentation curves
for multiple-cycle testing. FIG. 21, FIG. 22 and FIG. 23 show
additional analysis screenshots of the change in the measured
energy dissipated, the change in maximum adhesion force during
retraction, and the change in both elastic modulus and hardness as
a function of time through the cyclical test.
[0081] FIG. 24 shows a generalized measurement head for this
invention. The test probe 2402 consisting of a shaft 2404 and a
sharp tip (often a diamond) 2406 is attached to a shaft 2408 which
is, in turn, connected to an optional torque and angular
displacement sensor 2410 then to an optional torque generator 2412,
then to an optional linear displacement sensor 2414, then to an
optional force sensor 2416, and finally to an optional force
generator 2418. The reference probe 2420 is connected to the
housing 2422 that holds the transducers and generators. The housing
2422 could be supported and positioned on the sample under test by
a support such as those drawn in FIG. 12. The optional torque and
angular displacement sensor 2410 together with the optional torque
generator 2412 can be used to measure friction with test probes
such as 1012, 1022 and 1026 (FIG. 10) or the torque necessary to
screw a test probe like 1028 (FIG. 10) in or out of a material
under test. This might, for example, be useful in determining
whether a patient's bone is suitable for holding screws for
mounting orthopedic appliances. With the optional force sensor 2416
and the optional force generator 2418 the force to pull out a screw
could be measured as a test of bone quality.
[0082] Although the present invention has been described in
connection with the preferred embodiments, it is to be understood
that modifications and variations may be utilized without departing
from the principles and scope of the invention, as those skilled in
the art will readily understand. Accordingly, such modifications
may be practiced within the scope of the following claims.
REFERENCES
[0083] The following references are each incorporated herein by
reference. [0084] 1. W. C. Oliver and G. M. Pharr. Measurement of
hardness and elastic modulus by instrumented indentation: Advances
in understanding and refinements to methodology. J. Mater. Res. 19
(2004), 3. [0085] 2. C. A. J. Putman, H. G. Hansma, H. E. Gaub, and
P. K. Hansma, Langmuir 8, 3014 (1992). [0086] 3. Briscoe, B. J. and
Sebastian, K. S. An analysis of the durometer indentation. Rubber
Chemistry and Technology 66 (5): 827-836 1993). [0087] 4. James B.
Thompson et al., Nature 414, 774, 13 Dec. 2001. [0088] 5. Paul K.
Hansma, Patricia J. Turner, and Georg E. Fantner, Bone Diagnostic
Instrument, REVIEW OF SCIENTIFIC INSTRUMENTS 77, 075105 (2006).
[0089] 6. U.S. Pat. Nos. 1,770,045, 5,450,745, 5,463,897,
5,473,700, 6,142,010, and 6,405,599, and U.S. Patent Publication
Nos. 2002/0170360 and 2005/0262685. [0090] 7. U.S. patent
application Ser. No. 11/417,494 filed May 3, 2006 titled Methods
and Instruments for Assessing Bone Fracture Risk. [0091] 8. "Micro
Hardness Tester (MHT) for fracture toughness determination of
brittle materials", No. 8, July 1998. [0092] 9. CSM Indentation
Testers, four page brochure. [0093] 10. "ASTM Proposed Instrumented
Indentation Testing Standard", pages 1-4, October 2003.
* * * * *