Method and apparatus for measuring depth and hardness

A method and apparatus of measuring the hardness or various other characteristics of a sample surface, comprises a load cell carrying a penetrator and connected to a computer for reading signals from the load cell. The sample is advantageously mounted on the vertically moveable table of a microscope. The table can be moved by a stepper motor also connected to the computer for raising the sample into contact with the penetrator until a preselected force is reached. The force is sufficient to indent the sample surface with the penetrator. Optical or other mechanisms can then be used to measure the indentation. One other mechanism which can be used to measure the indentation is a second load cell carrying a tube which also contacts the sample surface and produces a second signal. Signals from the two load cells can be processed to measure a difference between the signals which provides a measurement of the indentation produced by the penetrator.

FIELD AND BACKGROUND OF THE INVENTION 
The present invention relates, in general, to a method and apparatus for 
measuring depth of penetration and hardness into and of a material to be 
tested. In particular, the method and apparatus of the invention utilizes 
responds to a signal from a load cell that indicates a force applied to a 
tool for making an indentation on a sample of the material. The signal 
discontinues the force when a set point has been reached and depth of 
penetration, and/or hardness is measured either optically or by using a 
second load cell in an inventive arrangement. 
Present systems used to measure the hardness or micro-hardness of, 
particularly, metallic samples are known as the Brinell, Vickers or 
Rockwell systems. Each of these requires that a known weight be placed on 
top of a ball or diamond penetrator which contacts the surface of a 
sample, to create an indentation in the sample. The weight and penetrator 
are then removed and the sample transferred to optical computation 
apparatus to calculate the hardness of the sample from an optical 
determination of the size of the indentation made in the sample by the 
weight and penetrator. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to avoid weight-operated 
indentation devices and, preferably, to automate or semi-automate the 
hardness-measuring technique. One embodiment of the invention also avoids 
optical computation used by the prior art to calculate hardness. 
To these and other ends, the invention provides a method in which a 
penetrator is mounted on a load cell apparatus. A sample surface is 
brought in contact with the penetrator. The load cell produces a signal 
proportional to an increasing force being applied between the penetrator 
and a sample surface. When the signal indicates that a predetermined force 
has been reached between the penetrator and the sample, the force is 
discontinued in response to the signal. An alternative embodiment produces 
a signal from the load cell only when the force reaches the predetermined 
value. An indentation produced in the sample by the penetrator and force, 
is then optically measured in a known manner, according to one embodiment 
of the invention. 
According to another embodiment of the invention, two load cells are 
needed. A penetrator is mounted to one of the load cells. A sample surface 
is brought in contact with the penetrator. The load cell produces a signal 
proportional to increasing force between the penetrator and the sample. 
When the signal indicates that a predetermined force is being applied 
between the penetrator and sample, the force is discontinue in response to 
the signal. Simultaneously, the other load cell which carries a ring that 
is in non-penetrating contact with the sample surface, is producing a 
second signal that is proportional to the increasing force between the 
ring and the sample. If neither load cell was carrying a penetrator, the 
two signals would be equal to each other and independent of displacement 
of the sample up or down. With a penetrator tool mounted to one of the 
load cells, as the sample is moved upwardly the tool penetrates the sample 
surface and a difference in the signals from the two cells is proportional 
to the depth of penetration created by the tool. The depth thus being 
measured, allows for a calculation of the hardness of the material without 
the need for optical measurements. 
Accordingly, a further object of the present invention is to provide a 
method and apparatus for measuring the depth of penetration and/or 
hardness of a sample material, which apparatus is simple in design, rugged 
in construction and economical to manufacture. 
An important distinction of the invention over the prior art is that force 
is being applied to the penetrator through a precise load cell rather than 
by a weight. 
The various features of novelty which characterize the invention are 
pointed out with particularity in the claims annexed to and forming a part 
of this disclosure. For a better understanding of the invention, its 
operating advantages and specific objects attained by its uses, reference 
is made to the accompanying drawings and descriptive matter in which 
multiple embodiments of the invention are illustrated.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 2 shows a microscope at M. The microscope is preferably a model Examet 
microscope manufactured by Union Optical Co., Ltd., of Tokyo, Japan, with 
a reflected light, head. However, any suitable microscope may be used. 
The microscope has a knob 2A, and by rotating of the knob, a microscope 
table 2 is moved along the optical path of the microscope, i.e. vertically 
as shown in FIG. 2. 
A stepper motor 1 is mounted on one side of the microscope together with a 
belt 1A engaged between the stepper motor and the knob 2A. Rotation of the 
stepper motor rotates the knob via the belt to move the microscope table 
2, as described above. As an example, the stepper motor may be a Phytron 
model ZSS motor. The arrangement of the belt 1A between the stepper motor 
and the knob is well understood and no further descriptions is provided 
here. 
The surface of the microscope table 2 in the optical path of the 
microscope, i.e., the upper side of the table in FIG. 2, has a motorized, 
movable stage 6. The motorized stage 6 moves a sample 3 of material to be 
tested, from one indentation position to the next, according to a computer 
program and for a position aligned with a penetrator 4, to a position in 
the optical path for optical observation of the indentation(s). For 
example, the stage 6 can be the one manufactured by Markhauser, as its 
Model MT mot. 50.times.50. 
The penetrator 4 is mounted on a load cell 5. The penetrator 4 may be 
diamond shaped according to the Vickers and Knopp systems or rod -shaped 
according to the Brinell and Rockwell systems. For example, such 
penetrators are manufactured by Albert Ernehm. 
The load cell 5 produces an analog signal proportional to force applied to 
the penetrator in the direction of movement of the microscope table 2, 
i.e. vertically in FIG. 2. For example, the load cells manufactured by 
Huntleigh, Model 505H are suitable. 
Referring to FIGS. 2 and 3, a second load cell 8 produces an analog signal 
proportional to force applied to a tube D in the direction of movement of 
the microscope table 2, i.e., vertically in FIG. 2. This load cell may 
also be one manufactured by Huntleigh (Model 505H). Tube or ring D engages 
a large surface of sample 3 and does not, penetrate the sample. Tube D is 
a surface contactor for contacting the sample surface. 
The microscope M has a lens and light arrangement L by which the sample 
surface may be observed. 
The stepper motor 1, motorized stage 6, load cell 5, and load cell 8, are 
all connected by respective cables to a personal computer C as illustrated 
in FIGS. 1 and 6, such as the IBM 386 shown in FIG. 1. 
In operation, the sample 3 is placed on the motorized stage 6 and the 
motorized stage 6 is positioned, if necessary, by control from the 
personal computer, for the sample 3 to be under the penetrator 4 and tube 
D before the microscope table 2 is moved upwardly. The computer C then 
operates the stepper motor 1 to move the table 2 upwardly. The upward 
movement of the microscope table 2 then causes the sample 3 to contact the 
tube D, The force of contact between the sample and the tube D continues 
to increase as the stepper motor continues to operate. At the precise 
moment when the sample 3 is in contact with the penetrator 4, the readings 
from both cells are recorded by computer C. 
The corresponding analog signal from load cell 5 to the computer continues 
to increase with the increasing force until the signal reaches a value 
which is preset in the computer to correspond to a predetermined desired 
force. Data for calibrating the signal from the load cells to the 
particular force between the sample and the penetrator and tube is 
supplied by the manufacturer of the load cells. In addition, force to 
signal calibration techniques are well known to those skilled in the art 
to require further description. The analog signal from the load cell 5 can 
be converted by standard methods into a digital signal for use by the 
computer in its calculations. 
When the signal from load cell 5 reaches the predetermined value, the 
computer is programmed to stop the stepper motor. The program of the 
personal computer further reverses the stepper motor to lower the 
microscope table at least sufficiently to avoid further contact between 
the sample 3 and the penetrator 4 and tube D. The program in the personal 
computer then causes the motorized table to move the sample to the next 
indentation site, or to the optical path for visual observation. 
The program in the personal computer takes the readings of both load cells 
5 and 8 when the load cell 5 with the penetrator 4 reached the 
predetermined value set in the computer. A routine of the program of the 
personal computer calculates the hardness of the material of the sample 
from the initial readings of both load cells and the later readings of 
both load cells when the predetermined value was reached. The difference 
in values is proportional to the depth created by the penetrator in the 
sample surface. The exact relationship depends on the configuration of the 
penetrator. 
Both load cells 5 and 8, have base portions 5A and 8A respectively, which 
are fixed to the microscope M, and operating ends which are free to move 
vertically where sending a signal to computer C, which is proportional to 
the force being exerted against the load cell. In this way, the pressure 
being exerted on the surface of sample 3, whether it is by the penetrator 
4, or the nonpenetrating ring D, can be measured precisely. As noted 
above, if both load cells 5 and 8 were carrying a single ring D, the 
signal from the two load cells would be exactly the same. Because one of 
the load cells 5 is actually connected to the penetrator 4, the signals 
are different and this difference is a measurement of the amount of 
penetration of penetrator 4 into the surface of sample 3. 
The exact relationship between the readings from the load cells and the 
depth of penetration can be ascertained empirically. The load cells of the 
present invention are linear devices. If you read one volt for a load of 
1,000 grams, you would get 2 volts from a load of 2,000 grams. They are 
also mechanically linear. If on a particular load cell, 1,000 grams would 
result in a 1 mm deflection, then a load of 2,000 grams would result in a 
2 mm deflection. In the present example of the device, the load cells have 
a 0.5 mm deflection at 2,000 grams. Thus, if we measured a signal at 1 
gram and later took a reading at 2,000 grams, imposed by a stepper motor, 
we can divide the number of steps by 2,000 and know the number of steps 
per gram for the machine. 
To know the indentation per gram of force, we can us a laser measuring 
device to measure the deflection when the load reaches 1 gram and another 
when the load cell reaches 2,001 grams. The difference of the two 
distances, divided by 2,000, is the distance per gram. Accordingly, if we 
know the number of steps per gram and the distance per gram, we could also 
know the distance per step. By concentrically mounting the diamond-shaped 
support of penetrator 4, inside tube D, approximately the same surface 
area of sample 3 is being touched to avoid errors due to inconsistencies 
across the surface as the penetrator 4 moves into the sample, the load 
cells are studied at different lengths and a signal showing the difference 
in voltage between the two load cells and thus be representative of the 
depth of penetration. When used with a stylus instead of a diamond and 
pressing table under smaller loads, the present invention can also be used 
for measuring flatness, roundness, roughness or layer thickness of the 
sample 3 
Thus, the depth measurement can be taken by simply subtracting the 
amplitudes of the signals of load cells 5 and 8. Hardness can be 
calculated as a known function of-depth of penetration (as measured above) 
and load applied (which is the load measured by load cell 5). For example, 
if a calculation of hardness was desired using the Vickers system, it 
would be noted that Vickers diamonds have 136.degree. angle between faces. 
Applying basic trigonometry, the relationship between the diagonals and 
depth is 7, i.e., the depth is seven times smaller than the length of one 
diagonal. Normal calculations of hardness with the Vickers system is: 
##EQU1## 
Where: D1 and D2 are diagonals of indentation. 
d: is indentation depth. 
##EQU2## 
FIG. 6 shows the functional and hardware aspects of the invention connected 
to achieve the method of the invention. 
It is noted that in all of the figures, the same reference numerals are 
utilized to designate the same or functionally similar parts. 
FIG. 4 also illustrate microscope M set up to practice another embodiment 
of the invention. 
The microscope has a knob 2A. By rotation of the knob, microscope table 2 
is moved along the optical path of the microscope, i.e. vertically as 
shown in FIG. 4 as in the embodiment of FIG. 2. 
As before, the load cell 5 (FIG. 5) is connected to the computer C and 
produces an analog signal that is proportional to force applied to the 
penetrator in the direction of movement of the microscope table 2, i.e. 
vertically. 
The microscope has a lens and light arrangement L by which the optical path 
of the microscope may be observed. The microscope also has a video camera 
7 that also observes the optical path of the microscope. The video camera 
can be of the type manufactured by Hitachi. 
The stepper model 1, motorized stage 6, load cell 5 and video camera 7 are 
all connected by respective cables to personal computer C. 
Referring to FIGS. 4 and 7, in operation of the apparatus according to the 
method, the sample 3 is placed on the motorized stage 6 and the motorized 
stage 6 positioned, if necessary, by control from the personal computer 
for the sample 3 to contact the penetrator 4 when the microscope table 2 
is moved upwardly. The computer then operates the stepper motor to move 
the table upwardly. 
The upward movement of the microscope table 2 then causes the sample 3 to 
contact the penetrator 4. The force of the contact between the sample aid 
the penetrator continues to increase as the stepper motor continues to 
operate. The corresponding analog signal from the load cell to the 
computer continues to increase with the increasing force until the signal 
reaches a value preset in the computer to correspond to a predetermined 
force. Data calibrating the signal from the load cell to the particular 
force between the sample and, the penetrator is supplied by the 
manufacturer of the load cell, but force to signal calibration techniques 
are well known to those skilled in the art. 
When the signal from the load cell reaches the predetermined value set in 
the computer, the computer is programmed to stop the stepper motor. The 
program of the personal computer further reverses the stepper motor to 
lower the microscope table at least sufficiently to avoid further contact 
between the sample 3 and the penetrator 4. The program for the personal 
computer then causes the motorized stage 6 to move the sample into the 
optical path of the microscope. The sample may then be observed through 
the lens L. 
The program for the personal computer also causes the video camera 7 to 
observe the sample. The program causes the motorized table to move the 
sample until the video camera observes a field that includes the 
indentation made in the sample. A field grabber routine allows the 
computer to obtain information as to the width of penetration from the 
video image of the indentation. The field grabber program uses standard 
image analysis and measuring techniques to supply a digital signal 
corresponding to the width of the penetration. As noted previously, due to 
the fixed angle of the penetrator 4, this corresponds to the depth of 
penetration. The personal computer analyzes the resulting digital 
information as to depth to calculate the hardness of the material of the 
sample. 
While specific embodiments of the invention have been shown and described 
in detail to illustrate the application of the principles of the 
invention, it will be understood that the invention may be embodied 
otherwise without departing from such principles.