Patent Application
20120279321
The present invention applies to the field of the tribology. More specifically, it relates to a device for measuring the force exerted in one or two directions on a surface to be studied.
There are currently different techniques for characterizing mechanical, tribological, physicochemical or other properties of materials, surface coatings and lubricants, which use some form of controlled mechanical action on the surface to be characterized in a special atmosphere, for example, in ultra-high vacuum or in an atmosphere of controlled gas and pressure. In these techniques, a burin or punch which exerts a pressure force in the direction normal to the surface to be characterized is normally used. Also, in some cases as the result of the traction of the burin or punch on the surface to be characterized a friction force is produced in the direction tangential to the surface to be characterized. The control of the normal force and the measurement of the tangential force with high precision and reproducibility are conditions necessary for many characterization techniques. Furthermore, to characterize different materials such as ceramics, metals, metal alloys, polymers, composite materials, hard surface coatings, solid or liquid lubricants, greases, etc., which have very different mechanical properties such as hardness, modulus of elasticity, free surface energy, etc., the device has to have a broad range of measurement.
There are several indirect methods and devices for measuring forces described in patents U.S. Pat. No. 5,115,664, U.S. Pat. No. 5,212,657, U.S. Pat. No. 7,000,451, in Stachowiak G. Experimental Methods in Tribology.—Amsterdam: Elsevier, 2004; and in Liu H. Bhushan B. Adhesion and friction studies of microelectromechanical systems/nanoelectromechanical systems materials using a novel microtriboapparatus. J. Vac. Sci. Technol. A21(4), 2003, pp. 1528-1538. These methods use a flexible member with a well determined stiffness constant. This flexible member is usually located between a stiff base and the sample or between a stiff base and the punch or burin which exerts force on the sample. The normal force exerted by the burin or punch on a surface is determined by the deflection of the flexible member in the direction normal to the surface of the sample. The tangential force resulting from the friction between the burin or punch and the surface to be characterized is determined by the deflection of the flexible member in the direction tangential to the surface. In both cases it is necessary to determine the stiffness constants of the flexible member in every direction by means of prior calibration of a sensor.
In the devices described in the previously mentioned patents and publications, the deflection of the flexible member is measured by means of a sensor, which by way of illustration can be one of the following types: fiber optic, capacitive, inductive, laser interferometric sensor or the like. One of the most widely used ones is the fiber optic sensor. In the mentioned devices, the distance between the sensor and the flexible member is pre-established before creating the vacuum or controlled atmosphere and cannot be changed or adjusted without breaking the vacuum. In that of the publication of Liu H., Bhushan B. Adhesion and friction Studies of microelectromechanical systems/nanoelectromechanical systems materials using a novel microtriboapparatus. J. Vac. Sci. Technol. A21(4), 2003, pp. 1528-1538, a fiber optic sensor is used in which the distance between the end of an optical fiber and a reflective surface can be adjusted by means of a manually operated external micropositioner. This external micropositioner allows selecting a near range or a far range, depending on the necessary measurement resolution. In this device there are two piezoelectric motors used for moving the sample in two coordinates. However, these piezoelectric motors are not used for the movement and adjustment of the fiber optic sensors.
The manual adjustment of the position of the sensors by means of an external positioner is a common feature of all previously disclosed devices and constitutes a significant obstacle when performing the fine adjustment of the position of the sensor or calibrating it again in vacuum or controlled atmosphere applications when, in order to perform this adjustment or calibration, it is necessary to break the vacuum or controlled atmosphere. In these applications it is not possible to adjust the position or calibrate the sensors used for measuring the deflection of the flexible members remotely. In ultra-high vacuum systems which require heating of the system for the degassing thereof at a temperature normally comprised between 100° C. and 400° C., deformations of the structural members of the system may occur as the result of the high temperature or the pressure. This can affect the position of the sensors installed within the system, cause their misalignment, be detrimental to the measurement and, in some cases, prevent their use for measuring forces. In existing systems, in order to adjust the position of the sensors again it is necessary to open the system and break the vacuum or the controlled atmosphere, which entails losses of time (up to several days) and considerable economic costs.
The object of the invention is to palliate the technical problems mentioned in the previous section. To that end, it proposes a measurement device for measuring the force applied by a contact member on the surface of a sample, comprising a vacuum or controlled atmosphere chamber in which there are housed the sample and the contact member, mechanical means for transmitting the force exerted by the contact member in the form of the displacement of at least one flexible member and at least one sensor suitable for measuring said displacement and furthermore comprising at least one motor-driven linear micropositioner incorporated inside the vacuum or controlled atmosphere chamber for positioning an associated sensor. The device preferably comprises a control system for each micropositioner located outside the vacuum or controlled atmosphere chamber for controlling the movement of the sensor remotely. The device is suitable for measuring a force that is tangential or normal to the surface or both forces at the same time. The sensors are optionally optic sensors. There is preferably a microactuator capable of operating the micropositioner and the latter is located in linear guides.
For the purpose of aiding to better understand the features of the invention according to a preferred practical embodiment thereof, a set of drawings is attached to the following description in which the following is depicted with an illustrative and non-limiting character:
The invention consists of using one or several motor-driven linear micropositioners incorporated in the force measurement device and located inside the vacuum chamber. The device (
When two forces in two different directions between the burin or punch and the surface, for example in the direction normal and tangential to the surface, are measured, two assemblies formed by supports 10 and 30, flexible members 20 and the sensor 42 are used. These assemblies are located inside the vacuum or controlled atmosphere chamber and are connected to one another such that the normal to the surface of the first assembly coincides with the direction of the first force to be measured and the normal to the surface of the second assembly coincides with the direction of the second force to be measured. The displacement of the flexible members of each assembly is measured by a corresponding sensor in the same manner as in the previously described case of the device for measuring a single force. A corresponding micropositioner incorporated in the device and located inside the vacuum or controlled atmosphere chamber is used to position each sensor. The control and power supply system of the micropositioners can be a single system for two micropositioners or independent for each micropositioner and is located outside the vacuum or controlled atmosphere chamber.
The device for measuring a single force allows measuring the force exerted between the burin or punch and the solid surface in the direction normal or tangential to the surface. The device needs no modifications in its structure to measure either force, it must simply be placed and adjusted in a suitable manner with respect to the solid surface.
When the force is applied to the burin or punch 5 in the tangential direction with respect to the surface (y axis), the device is positioned and fixed in the stiff base 1 with the normals of the planes of the strips 20 in the x direction.
When the force is measured in the direction normal (x axis) to the surface, the device is positioned and fixed in the stiff base 1 such that the planes of the parallel strips are parallel to the solid surface in which the force is exerted by means of the burin or punch 5 (
When the force is exerted between the burin or punch 5 and the solid surface in the pre-selected direction (normal or tangential to the surface) depending on the orientation of the strips 20 with respect to the surface as previously described, the parallel leaf springs 20 connected with their ends between the supports 30 and 10, and being the members of lesser stiffness, are elastically deformed. The movement of support 30 with respect to support 10 which occurs due to the deformation of the parallel leaf springs 20 is measured using a sensor 42.
The force F is determined from the movement value Δz measured with the sensor and a constant k, the value of which is determined by means of prior calibration, according to the following formula:
F=Δz k.
Before beginning the measurement of the force, the sensor is located by means of the micropositioner 45 in the initial measuring position of the sensor. When an optical sensor is used, the measurement can be performed in two ranges of measurement: the near range and the far range. Specifically, when the near range is chosen for the measurement, the optical sensor is located at the distance z0=OC (
When the measurement is performed in a controlled atmosphere or in the vacuum, the adjustment of the position of the sensor is done before or after creating the vacuum or controlled atmosphere. When the force measurement device is used in an ultra-high vacuum which requires heating the entire system for the degassing thereof, it is preferable to perform the adjustment of the position of the sensor after the heating so that the possible thermal deformations do not affect the adjustment of the position of the sensor. Furthermore, the micropositioner allows remotely changing the range of measurement between several tests without needing to break the vacuum or controlled atmosphere, which offers more advanced measurement flexibility and reproducibility and the broadest range of measurement with respect to existing devices. For their use in a vacuum, all the mechanical members of the device are made from materials with a low rate of desorption and emission of gases. Furthermore, for their use in an ultra-high vacuum, these materials have to allow heating up to 150° C. For example, by way of illustration and without limiting the scope of the invention, the materials for manufacturing the components of the device belong to the following group: stainless steel, copper and copper alloys, titanium and titanium alloys, aluminum and aluminum alloys, etc. The sensor and the micropositioner also have to be suitable for their use in a vacuum. The design of the device allows quickly removing the gases from the inside thereof, which allows its use in an ultra-high vacuum.
The device for the simultaneous measurement of two forces in two perpendicular directions (x and y axes) is presented in
The operation of the device for measuring two forces is similar to that of measuring one force.
When a force tangential to the surface is exerted between the burin or punch 5 and the solid surface, the support 30 together with the strips 60, supports 50 and 80 move along the y axis parallel to their initial position. The movement of the support 30 is measured using a sensor, in this case an optical sensor 42.
When a force normal to the surface is exerted between the burin or punch 5 and the solid surface, the strips 60 are elastically deformed and support 50 together with support 80 moves along the x axis parallel to their initial position. In the case when two forces, the normal and the tangential, are exerted simultaneously, the movement of the components 50 and 80 along the x axis occurs independently of their movement along the y axis. The movement of the support 50 in the y direction is measured using a sensor 52.
The micropositioner 54 is assembled on a Z-shaped support 12 which, where appropriate, is fixed to the support 10 with two screws 14. The micropositioner 54 is thus fixed with respect to the support 10.
Before beginning the measurement of the forces, the optical sensors 52 and 42 are located by means of the micropositioners 54 and 45 in their initial positions, corresponding to the ranges of measurement chosen for each sensor. Specifically, when the near range is chosen for the measurement, the optical sensor 45 or 54 is located at the distance z0=OC (
The force FT in the direction tangential to the surface is determined from the movement value ΔzT of the support 30 measured with the sensor 42 and a constant kN, the value of which is determined by means of the prior calibration of the strips 60, according to the following formula:
F
N
=Δz
N
k
T.
The force FN in the direction normal to the surface is determined from the movement value ΔzN of the support 50 in the x direction measured with the sensor 52 and a constant kT, the value of which is determined by means of the prior calibration of the strips 20, according to the following formula:
F
T
=Δz
T
k
T.
When the measurement is performed in a controlled atmosphere or in a vacuum, the adjustment of the position of one or two sensors is carried out before or after creating the vacuum or controlled atmosphere. When the force measuring device is used in an ultra-high vacuum which requires heating the entire system for the degassing thereof, it is preferable to perform the adjustment of the positions of the sensors after the heating so that the possible thermal deformations do not affect the adjustments of the positions of the sensors, Furthermore, the micropositioners allow remotely changing the range of measurement of one or two sensors between several tests without needing to break the vacuum or controlled atmosphere, which offers more advanced measurement flexibility and reproducibility and the broadest range of measurement with respect to existing devices. For their use in a vacuum, all the mechanical members of the device are made from materials with a low rate of desorption and emission of gases. Furthermore, for their use in an ultra-high vacuum, these materials have to allow heating up to 150° C. For example, by way of illustration and without limiting the scope of the invention, the materials for manufacturing the components of the device belong to the following group: stainless steel, copper and copper alloys, titanium and titanium alloys, aluminum and aluminum alloys, etc. The sensors and the micropositioners also have to be suitable for their use in a vacuum. The design of the device allows quickly removing the gases from the inside thereof, which allows its use in an ultra-high vacuum.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/ES2009/070635 | 12/29/2009 | WO | 00 | 7/26/2012 |
