Compensated Mechanical Testing System

Abstract

A mechanical testing system having a frame, and a stage for holding a sample. An arm for pressing a tool against a surface of the sample. A primary actuator is connected to the frame and applies a primary force and drives the tool relative to the sample, thereby causing the frame to flex. A displacement sensor measures a displacement value comprised of two components, the first component including a distance traveled by the probe into the sample as the primary force is applied, and the second component including a measure of a degree of flex of the frame as the primary force is applied. A compensating actuator is connected to the frame and applies a compensating force that reduces the second component of the displacement value.

Claims

1. A mechanical testing system, comprising: a frame, a stage for holding a sample, an arm for pressing a tool against a surface of the sample, a primary actuator connected to the frame for applying a primary force and driving the tool relative to the sample, thereby causing the frame to flex, a displacement sensor for measuring a displacement value comprised of two components, the first component including a distance traveled by the tool relative to the sample as the primary force is applied, and the second component including a measure of a degree of flex of the frame as the primary force is applied, and a compensating actuator connected to the frame for applying a compensating force that reduces the second component of the displacement value.

2. The mechanical testing system of claim 1, wherein the compensating actuator comprises multiple compensating actuators disposed at different positions on the frame.

3. The mechanical testing system of claim 1, wherein the compensating actuator is disposed on an opposite side of the stage from the primary actuator.

4. The mechanical testing system of claim 1, wherein the primary actuator is at least one of electromagnetic, electrostatic, piezoelectric, hydraulic, mechanical, and pneumatic.

5. The mechanical testing system of claim 1, wherein the compensating actuator is at least one of electromagnetic, electrostatic, piezoelectric, hydraulic, mechanical, and pneumatic.

6. The mechanical testing system of claim 1, wherein the displacement sensor uses at least one of a capacitor and a laser to measure the displacement.

7. The mechanical testing system of claim 1, wherein the compensating force completely erases the second component of the displacement value.

8. The mechanical testing system of claim 1, wherein the compensating force corrects an apparent stiffness of the frame to about ten million newtons per meter.

9. A mechanical testing system, comprising: a frame, a stage for holding a sample, an arm for pressing a tool against a surface of the sample, a primary actuator connected to the frame for applying a primary force and driving the tool into the sample, thereby causing the frame to flex, wherein the primary actuator is at least one of electromagnetic, electrostatic, piezoelectric, hydraulic, mechanical, and pneumatic, a displacement sensor for measuring a displacement value comprised of two components, the first component including a distance traveled by the tool into the sample as the primary force is applied, and the second component including a measure of a degree of flex of the frame as the primary force is applied, and a plurality of compensating actuators connected to the frame for applying a compensating force that reduces the second component of the displacement value, wherein the compensating force corrects an apparent stiffness of the frame to about ten million newtons per meter.

10. The mechanical testing system of claim 9, wherein compensating actuator is at least one of electromagnetic, electrostatic, piezoelectric, hydraulic, mechanical, and pneumatic.

11. The mechanical testing system of claim 9, wherein the displacement sensor uses at least one of a capacitor and a laser to measure the displacement.

12. The mechanical testing system of claim 9, wherein the compensating force completely erases the second component of the displacement value.

13. A method for testing at least one of a hardness and an elastic modulus of a sample, the method comprising the steps of: placing a sample to be tested on a stage that is supported by a frame, using a primary actuator that is supported by the frame to apply a primary force and drive a tool into a surface of the sample, thereby causing the frame to flex, measuring a displacement value with a displacement sensor, the displacement value comprised of two components, the first component including a distance traveled by the tool into the sample as the primary force is applied, and the second component including a measure of the flex of the frame as the primary force is applied, and applying a compensating force with a compensating actuator that is connected to the frame, where the compensating force reduces the second component of the displacement value.

14. The method of claim 13, wherein the compensating actuator comprises multiple compensating actuators disposed at different positions on the frame.

15. The method of claim 13, wherein the compensating actuator is disposed on an opposite side of the stage from the primary actuator.

16. The method of claim 13, wherein the primary actuator is at least one of electromagnetic, electrostatic, piezoelectric, hydraulic, mechanical, and pneumatic.

17. The method of claim 13, wherein the compensating actuator is at least one of electromagnetic, electrostatic, piezoelectric, hydraulic, mechanical, and pneumatic.

18. The method of claim 13, wherein the displacement sensor uses at least one of a capacitor and a laser to measure the displacement.

19. The method of claim 13, wherein the compensating force completely erases the second component of the displacement value.

20. The method of claim 13, wherein the compensating force corrects an apparent stiffness of the frame to about ten million newtons per meter.

Description

DRAWINGS

[0012] Further advantages of the invention are apparent by reference to the detailed description when considered in conjunction with the figures, which are not to scale so as to more clearly show the details, wherein like reference numbers indicate like elements throughout the several views, and wherein:

[0013] FIG. 1 is a schematic representation of mechanical testing system according to an embodiment of the present invention.

[0014] FIGS. 2-4 depict different locations for a compensating actuator on mechanical testing system according to embodiments of the present invention.

DESCRIPTION

[0015] According to the embodiments of the present invention, load frame stiffness approaching infinite stiffness are achieved by maintaining a fixed (zero or non-zero) relative displacement between a reference point in the displacement sensor and a point on the sample, by using one or more compensating actuators. This is accomplished in the manner as described below.

[0016] With reference now to the figures there are depicted various embodiments of a mechanical testing system 100. The mechanical testing system 100 has a load frame 102 to which the various other components of the tester 100 are mounted. For example, a sample 112 is placed onto the tester 100, such as on a stage 113 that is useful for positioning various parts of the sample underneath a probe 108. On the end of the probe 108 is a tip 110 that is placed in contact with the upper surface of the sample 112 in a resting state with no exerted force, and the position of the tip 110 is measured, such as with a displacement sensor 106. This is the baseline position for the mechanical property measurement.

[0017] A primary actuator 104 starts to apply a load on the sample 112, by forcing the probe 108 down toward the sample 112, thereby pushing the tip 110 into the surface of the sample 112. The amount of force that is required to do this varies according to the hardness of the sample 112 and the depth of penetration. While the force is applied by the primary actuator 104, the displacement sensor 106 measures the distance from the baseline that the probe 108 has moved. The displacement sensor 106 can measure the displacement such as by using a variable capacitor or a laser, for example.

[0018] As introduced above, a portion of the movement measured by the displacement sensor 106 is due to the flexing of the frame 102, which occurs because of the resistance of the sample 112 to the penetration of the tip 110. The degree to which the frame 102 will flex is dependent upon a number of different factors, such as the construction of the frame 102 and the applied force, but is present to some degree or another in virtually all mechanical testing systems.

[0019] In addition to the flexing of the frame 102, other components of the system 100 might also flex or compress, further compounding the apparent error that arises by the displacement sensor 106 measuring a distance that is not entirely accounted for by the penetration of the tip 110 into the sample 112.

[0020] To compensate for these factors in the system 100, one or more compensating actuators 114 are placed somewhere within the circuit of the load frame 102. The purpose of the compensating actuator 114 is to apply a compensating force in an opposing direction to that which is applied by the primary actuator 104, such that the distance as measured by the displacement sensor 106 accurately reflects the actual distance that the tip 110 has penetrated into the surface of the sample 112. FIG. 2 shows a schematic representation of one configuration of the system 100 using mass, springs and dashpots.

[0021] The amount of force that is required to be exerted by the compensating actuator 114 is different, depending at least in part upon the construction of the mechanical testing system 100. The amount of compensating force that is required can be determined in various ways, such as empirically or theoretically or a combination of the two, for a given instrument, then written into the operational programming of the instrument 100. In some embodiments the compensating force that is applied by the compensating actuator or actuators 114 varies with the force that is applied by the primary actuator 104. In other embodiments the compensating force that is applied by the compensating actuator or actuators 114 is relatively uniform, regardless of the force that is applied by the primary actuator 104.

[0022] As built, most mechanical testing systems 100 have a frame 102 with an inherent stiffness of somewhere on the order of one to two million newtons per meter. Using the apparatus and method described herein, a frame 102 can be compensated to have an apparent stiffness of about tens of millions of newtons per meter, or more. However, in most applications, compensation to about ten million newtons per meter is adequate to reduce the errors in the hardness and other mechanical property calculations to a relatively insignificant level.

[0023] The primary actuator 104 and the compensating actuators 114 can be electromagnetic, electrostatic, piezoelectric, hydraulic, mechanical, pneumatic, or so forth. A schematic of one possible configuration of the system 100 is shown in FIG. 1. It shows a primary actuator 104 and a compensating electromagnetic actuator 114, a displacement sensor 106, the load frame 102, tip 110, and the sample 112. The compensating force can be applied synchronously or asynchronously, statically or dynamically with respect to the primary actuator 104.

[0024] The foregoing description of embodiments for this invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiments are chosen and described in an effort to provide illustrations of the principles of the invention and its practical application, and to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.