Linear encoder

Abstract

A linear encoder for a numerically controlled machine tool comprises: a substantially linear profile, the profile being suitable for being fixed on a structural body of the machine tool; a substantially linear scale bar, the scale bar being fixed on the profile so as to extend along the profile parallel to the longitudinal axis of the profile; a movable slider, the movable slider being assembled / installed on the profile so as to be able to move along the profile, parallel to the longitudinal axis of the profile and across the scale bar, and the movable slider being suitable for being rigidly fixed to a movable part of the machine tool; an electronic reading device, the electronic reading device being at least partially arranged on the movable slider and being suitable for reading the position of the movable slider on the scale bar; and finally a thermal stabilization device, the thermal stabilization device being suitable for keeping the entire scale bar substantially stably at a predetermined target temperature.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This patent application claims the priority benefit of Italian Patent Application No. 102019000003813, filed on March 15, 2019, the disclosure of which is incorporated herein by reference in its entirety. Technical Field

[0003] The invention relates to a linear encoder.

[0004] More specifically, the present invention relates to a linear encoder for large CNC machine tools, particularly suitable for large CNC gantry milling machines, and the following disclosure will explicitly mention its use without losing its general nature. Background Art

[0005] As is well known, on large CNC gantry milling machines, the tool holder head is fixed on a straight horizontal load-bearing beam with a high rigidity structure, which extends horizontally above the workpiece clamping platform and has two axial ends slidingly resting on two elevated side shoulders rising from the base plate on opposite sides of the workpiece clamping platform.

[0006] The tool holder head can move along the load-bearing beam parallel to the longitudinal axis of the beam, i.e. in a first horizontal direction. In turn, the load-bearing beam extends perpendicular to the two side shoulders and can move horizontally along the two side shoulders perpendicular to its longitudinal axis, i.e. in a second horizontal direction orthogonal to the first horizontal direction.

[0007] The position of the tool holder head along the load beam and the position of the load beam on the two side shoulders are usually detected by a series of linear encoders, also commonly called linear position sensors, which are arranged on the load beam and the two side shoulders.

[0008] The most commonly used linear encoders in CNC gantry milling machines include: a linear profile of appropriate length, which is usually made of metal material and has a basically U-shaped cross-section; a linear scale bar with a strip structure, which is firmly fixed to the bottom of the groove defined by the linear profile and extends basically parallel to the longitudinal axis of the profile over the entire length of the same profile; a movable slider, which can move along the groove of the linear profile, parallel to the longitudinal axis of the profile and across the linear scale bar; and an electronic reading device, which can directly read the slider position on the linear scale bar and output an analog or digital signal indicating the position of the movable slider along the linear profile.

[0009] More specifically, in the most common types of linear encoders, the scale strip is provided with a series of notches or reference marks, usually spaced apart on the top surface throughout the length of the same strip of the scale strip, and the electronic reading device is provided with a photoelectric sensor located on the movable slide and capable of detecting the presence of the notches or reference marks on the portion of the scale strip facing / aligned with the movable slide.

[0010] Traditionally, the straight profile is rigidly fixed to a load-bearing beam or a side shoulder parallel to the direction of movement of the workpiece, while the movable slide is rigidly fixed to the tool holder head or the end of the load-bearing beam, i.e. the moving part.

[0011] Unfortunately, the accuracy of the above-described linear encoders decreases as the length of the device increases and reaches unacceptable values ​​above 5 to 6 meters in length.

[0012] In fact, the rectilinear profile is usually fixed in a rigid manner to the metal structure of the CNC machine and is therefore subject to thermal expansions of various natures which, unfortunately, can reach significant values ​​parallel to the longitudinal axis of the profile. Since it is traditionally made in one piece, the scale strip is obviously subject to the same disadvantages, with all the problems that entails.

[0013] To at least partially overcome these problems, in the best linear encoders on the market, the linear profile is usually made of a metal material with a thermal expansion coefficient comparable to that of the metal structure of the CNC machine tool, while the scale bar is usually made of carbon steel or glass.

[0014] Despite these techniques, the large gantry milling machines currently on the market cannot achieve machining accuracy above one hundredth of a millimeter.

[0015] In fact, in the case of load-bearing beams and / or shoulders exceeding 5 to 6 meters in length, errors due to thermal expansion of the linear encoder, or more precisely errors in the scale bar of the linear encoder, can significantly affect the accuracy with which the CNC machine control system can determine the absolute position of the tool. Summary of the invention

[0016] The object of the invention is to produce a linear encoder for numerically controlled machine tools which guarantees very high accuracy even when the linear encoder length exceeds 5 to 6 meters.

[0017] In accordance with these aims, according to the present invention, there is provided a linear encoder as defined in claim 1 and preferably, but not necessarily, in any of the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The present invention will now be described with reference to the accompanying drawings, illustrating non-limiting embodiments:

[0019] Figure 1 an isometric view of a linear encoder made in accordance with the teachings of the present invention, with portions broken away and portions removed for clarity;

[0020] Figure 2 for Figure 1 Front view of the linear encoder shown with portions removed for clarity;

[0021] Figure 3 for Figure 1 and Figure 2 A side view of the linear encoder shown is taken along section line AA; and

[0022] Figure 4 for Figure 1 , Figure 2 and Figure 3 Cross-sectional side views of various embodiments of linear encoders are shown with portions removed for clarity. DETAILED DESCRIPTION

[0023] Reference Figure 1 , Figure 2 and Figure 3 , the number 1 as a whole represents a linear encoder, also known as a linear position sensor, which is particularly suitable for large CNC gantry milling machines, and more generally, for large CNC machine tools.

[0024] The linear encoder 1 first includes: a roughly linear load-bearing profile 2, the profile 2 is preferably made of metal material, suitable for being fixed to the structure of the machine tool (not shown) in a basically rigid manner, and its longitudinal axis L is basically parallel to the moving direction d of the machine tool workpiece, preferably fixed by a series of transverse anchor bolts (not shown) appropriately distributed along the entire length of the profile 2; a roughly linear scale bar 3, the scale bar 3 preferably has a strip structure, and is firmly fixed to the profile 2 so as to extend along the profile 2 parallel to the longitudinal axis L of the profile, preferably basically extending over the entire length of the profile 2; and a movable slider 4, the movable slider 4 is assembled / installed on the profile 2 so as to be able to move along the profile 2, parallel to the longitudinal axis L of the profile and across the scale bar 3, and is suitable for being rigidly fixed to a movable part (not shown) with a known position on the machine tool, preferably fixed by transverse anchor bolts (not shown).

[0025] More specifically, the central portion of the profile 2 is preferably configured to be rigidly fixed / anchored to a machine tool structure (not shown), while both ends and other intermediate portions of the profile 2 are preferably configured to be floatingly fixed / anchored to a machine tool structure (not shown).

[0026] Furthermore, the linear encoder 1 comprises an electronic reading device 5 which is at least partially located on the movable slide 4 so as to move across the scale strip 3 and is suitable for directly reading the position of the movable slide 4 on the scale strip 3. Preferably, the electronic reading device 5 is further suitable for outputting an analog or digital signal indicating the position of the movable slide 4 along the profile 2.

[0027] More specifically, the scale strip 3 preferably extends seamlessly along the profile 2. In other words, the scale strip 3 is preferably integral.

[0028] Furthermore, the scale strip 3 is preferably provided with a series of reference marks, logos and / or markings (not shown in the figures), preferably of the optical or magnetic type, which are distributed substantially evenly and seamlessly along the entire length of the scale strip 3 .

[0029] In turn, the electronic reading device 5 is configured to detect the presence of reference marks, symbols and / or markings present on the facing portion / segment of the scale strip 3 and, based on the presence / spatial arrangement of said reference marks, symbols and / or markings, to determine the position of the movable slider 4 along the scale strip 3 and therefore along the profile 2.

[0030] Reference Figure 1 , Figure 2 and Figure 3 In the example shown, in particular, the profile 2 is preferably shaped to form / define a straight longitudinal groove / channel 6 inside it, which is parallel to the longitudinal axis L and extends over the entire length of the profile 2, and the scale bar 3 is preferably accommodated in the said straight groove 6 which is locally parallel to the profile axis L.

[0031] Preferably, the two axial ends of the profile 2 are also closed / sealed by two terminals 7 preferably made of metal material.

[0032] More specifically, in the example shown, the profile 2 preferably has a substantially U-shaped cross section.

[0033] Furthermore, the substantially rectilinear profile 2 is preferably made of steel and is preferably divided into a series of integral segments 8, which are preferably all of the same length and are arranged to be aligned one to the other and connected, preferably with the interposition of connecting elements 9, which are preferably made of polymeric material.

[0034] In the example shown, in particular, the length of the integral section 8 is preferably less than 1.5 m (meters).

[0035] Special reference Figure 1 , Figure 2 and Figure 3 Preferably, the linear encoder 1 also includes a pair of longitudinal gaskets 10, which are preferably made of elastomeric material, extending parallel to the longitudinal axis L of the profile, fixed to the profile 2 at the side of the upper opening of the linear groove 6, and are basically end-face turned over the entire length of the profile 2 and fully fastened to each other to close the opening of the linear groove 6, thereby preventing dust and other contaminants from reaching the scale bar 3.

[0036] Preferably, the longitudinal gasket 10 also has a lip-shaped structure and is fixed on two longitudinal edges of the profile 2 so that they protrude from each other and contact each other at the center line of the opening of the linear groove 6 .

[0037] Special reference Figure 1 and Figure 3 On the other hand, the scale bar 3 is preferably firmly fixed to the bottom of the linear groove 6.

[0038] Furthermore, the graduated strip 3 preferably consists of a strip of metal material, which is preferably made of carbon steel and which preferably extends substantially without interruption along the entire length of the profile 2 .

[0039] More specifically, in the example shown, the scale strip 3, or more precisely a strip-like strip, is preferably placed / located on top of a longitudinal rib 11, which in turn protrudes from the bottom of the rectilinear groove 6 towards the opening of the groove 6 and extends parallel to the longitudinal axis L of the profile essentially over the entire length of the profile 2, or more precisely, over the entire length of each integral segment 8.

[0040] Preferably, the exposed face of the tape strip also has a series of graphic marks or engravings evenly spaced without interruption along the entire length of the tape strip.

[0041] In other words, the scale strip 3 is provided with a visible micrometric graduated scale and the electronic reading device 5 is adapted to read said graduated scale.

[0042] In different embodiments, the ribbon strip may also be made of glass and / or composite materials. In addition, the ribbon strip may have a series of transverse magnetic strips in addition to or in place of the graphic logo.

[0043] Reference Figure 1 , Figure 2 and Figure 3 On the other hand, the movable slide 4 is preferably movable along the profile 2 without contacting the profile 2 .

[0044] Preferably, the movable slider 4 further comprises: a reading head 12, which is accommodated in the longitudinal groove 6 of the profile 2 in an axially sliding manner and also partially passes over the scale bar 3; a fixing block 13, which is permanently arranged outside the profile 2 and is constructed to be rigidly fixed / fixable on the movable part of the CNC machine tool; and finally comprises a connecting arm 14, which rigidly connects the external fixing block 13 to the reading head 12, so that the reading head 11 and the fixing block 13 move together in the direction d. Preferably, the connecting arm 14 also has a plate-like structure parallel to the longitudinal axis L of the profile, so that the longitudinal gasket 10 can be more easily deformed / opened when moving along the upper opening of the linear groove 6.

[0045] Furthermore, in the example shown, the reading head 12 , the fixing block 13 and the connecting arm 14 are preferably made in one piece, preferably made of a metal material.

[0046] Special reference Figure 3On the other hand, the electronic reading device 5 preferably comprises a photoelectric sensor 15 which is arranged in the reading head 11 of the movable slider 4 to pass over the scale bar 3 and is suitable for detecting the presence and / or arrangement of graphic marks or engravings present on the scale bar 3.

[0047] However, in different embodiments, in addition to or instead of the optoelectronic sensor 15, the electronic reading device 5 may include a magnetic sensor capable of detecting the presence and / or arrangement of magnetic strips.

[0048] Reference Figure 1 , Figure 2 and Figure 3 Unlike currently known linear encoders, the linear encoder 1 also includes an active thermal stabilization device 20, which is suitable for keeping the entire scale bar 3 basically continuously at a predetermined and basically constant target temperature T0, the value of the target temperature is preferably in the range between +20°C and +90°C.

[0049] More specifically, the graduated strip 3 is preferably fixed to the profile 2 with the ability to expand freely parallel to the longitudinal axis L relative to the latter, and vice versa.

[0050] On the other hand, the thermal stabilization device 20 is preferably adapted to selectively and actively heat and / or cool the rectilinear profile 12 so as to keep the temperature of each single point of the scale bar 3 substantially within a tolerance range of a predetermined width spanning the above-mentioned target temperature T0.

[0051] Preferably, the target temperature T0 also lies in the middle of the abovementioned tolerance range and / or the width of the tolerance range is preferably below 2°C and more conveniently below 1°C.

[0052] In other words, the thermal stabilization device 20 is preferably adapted to maintain the temperature of each single point of the scale strip 3 substantially around the target temperature T0 with a tolerance smaller than ±1°C and optionally also smaller than ±0.5°C.

[0053] In the example shown, in particular, the profile 2 is preferably divided longitudinally into a series of sampling areas, which preferably all have the same length and each accommodates a respective sampling portion of the scale strip 3 .

[0054] In turn, the thermal stabilization device 20 is preferably configured to selectively and independently heat and / or cool various sampling areas of the profile 2, so that the temperature of each sampling portion of the scale bar 3 is continuously maintained near the above-mentioned target temperature T0.

[0055] More specifically, the active thermal stabilization device 20 preferably includes: a series of temperature sensors 21, which are appropriately distributed along the profile 2 to detect the temperature at different points of the scale bar 3 in real time; a series of thermally active elements 22, which are separated and different from each other, appropriately distributed along the profile 2, and are configured to cool and / or heat the sampling area of ​​the profile 2 to which it is fixed and the scale bar 3 integral therewith; and a control unit 23, which is preferably located outside the profile 2 and is suitable for controlling each thermally active element 22 independently of each other according to the signal from the temperature sensor 21.

[0056] More specifically, the control unit 23 is programmed / configured to selectively turn on and off each thermally active element 22 to restore the local temperature of the scale bar 3 to near the target temperature T0 when the temperature sensor 21 on the same sampling area of ​​the profile 2 detects that the local temperature of the scale bar 3 deviates too much from the target temperature T0.

[0057] In other words, the control unit 23 is preferably programmed / configured to switch each thermally active element 22 on and off if the local temperature of the scale bar 3 exceeds said tolerance range centered around the target temperature T0.

[0058] In the example shown, in particular, the temperature sensors 21 are preferably substantially evenly spaced along the profile 2 and the scale strip 3 , and each sampling area of ​​the profile 2 accommodates at least one temperature sensor 21 and more conveniently a plurality of temperature sensors 21 .

[0059] On the other hand, the thermally active elements 22 are preferably arranged so as to be in contact with the corresponding sampling area of ​​the profile 2 so as to be able to transfer heat to the profile 2 / absorb heat from the profile 2 by conduction, and preferably extend on the profile 2 substantially along the entire length of the same sampling area of ​​the profile 2.

[0060] In other words, the individual thermally active elements 22 have a rectangular structure and preferably extend aligned one behind the other on the profile 2 .

[0061] Reference Figure 1 , Figure 2 and Figure 3 In the example shown, in particular, the profile 2 and the scale strip 3 are preferably divided into a series of sampling areas / portions with a length of less than 0.5 m (meter).

[0062] Furthermore, the thermal stabilization device 20 is preferably configured to maintain the scale bar 3 at a target temperature T0 which is higher than the ambient temperature.

[0063] In other words, the thermal stabilization device 20 is preferably configured to heat each sampling area of ​​the profile 2 individually.

[0064] In the example shown, in particular, the target temperature T0 is preferably equal to +50°C.

[0065] As a result, the thermoactive element 22 is preferably a heating member which, on command, is suitable for locally heating the profile 2 and the scale strip 3 integral therewith.

[0066] In turn, the control unit 23 is preferably programmed / configured to selectively turn on and off each thermally active element 22 with a heating function when the temperature sensor 21 on the corresponding sampling area of ​​the profile 2 detects that the local temperature of the scale bar 3 drops excessively below the target temperature T0, thereby locally heating the profile 2 and returning the local temperature of the scale bar 3 to near the target temperature T0.

[0067] Reference Figure 1 and Figure 3 In the example shown, in particular, the temperature sensor 21 is preferably located in the longitudinal groove 6 of the profile 2 and is preferably distributed on the profile 2 so that each integral segment 8 of the profile 2 accommodates at least one temperature sensor 21 and more conveniently accommodates multiple temperature sensors 21.

[0068] Furthermore, each temperature sensor 21 is preferably housed / inserted in a longitudinal rib 11 of the profile 2, just below the graduated strip 3. Preferably, the temperature sensor 21 finally communicates with the control unit 23 via an optical fiber 24 which, in the example shown, extends inside a rectilinear groove 6 of the profile 2, substantially over its entire length.

[0069] More specifically, the temperature sensor 21 is in turn passed through an optical fiber 24 which terminates in a control unit 23 .

[0070] On the other hand, the thermally active element 22 is preferably fastened directly to the profile 2 outside the longitudinal groove 6 .

[0071] Furthermore, the thermally active element 22 is preferably arranged outside the profile 2 along the wall of the profile directly supporting the scale strip 3 .

[0072] In the example shown, in particular, the thermally active element 22 is fixed to the outer surface of the profile 2 , preferably along a wall forming the bottom of the rectilinear groove 6 and having the longitudinal ribs 11 on the opposite sides.

[0073] Finally, each thermally active element 22 is preferably composed of a resistor suitable for generating heat by the Joule effect, and the control unit 23 is preferably suitable for supplying electrical energy to each resistor separate and independent from the other resistors, so as to power each resistor separate from the other resistors according to the signal from the temperature sensor 21.

[0074] More specifically, with particular reference to Figure 3In the example shown, each thermally active element 22 preferably comprises: a strip 26 made of a thermally conductive material and optionally an electrically insulating material, attached to and extending on the outer surface of the linear profile, preferably made of a polymer material; and one or more wires 27 made of an electrically conductive material, embedded in the strip 26 so as to pass over the surface of the profile 2 and be able to transfer the generated heat directly to the profile 2.

[0075] In other words, each thermally active element 22 preferably has a ribbon-like structure.

[0076] In turn, the control unit 23 is electrically connected to the electric wires 27 of each heater or resistor 22 so as to circulate the current in the same electric wires 27 .

[0077] The operation of the linear encoder 1 is easily inferred from the above and requires no further explanation.

[0078] As for the thermal stabilization device 20 , during operation of the CNC machine, the control unit 23 activates and deactivates each thermally active element 22 according to the signals from the temperature sensor 21 present in the corresponding sampling area of ​​the profile 2 , so as to maintain the local temperature of the scale bar 3 around the target temperature T0 .

[0079] The advantages associated with the presence of the thermal stabilization device 20 are significant.

[0080] Firstly, experimental tests have highlighted that, even if the length of the linear encoder 1 exceeds 8 meters, keeping the scale bar 3 at a predetermined and substantially constant temperature (i.e. the target temperature T0) during operation of the CNC machine tool allows to practically eliminate the errors caused by the thermal expansion of the scale bar 3 and the profile 2.

[0081] Furthermore, since it is always kept near the target temperature T0, the scale bar 3 can be made of a less rigid and brittle and cheaper material than currently used for the same component, thus simplifying and reducing the production costs of the linear encoder. In fact, a low coefficient of thermal expansion is no longer an essential requirement for the material used to produce the scale bar 3.

[0082] Finally, it is clear that modifications and variations may be made to the linear encoder 1 described above without departing from the scope of the present invention.

[0083] For example, in Figure 3 In a different embodiment of middle hatching, the thermally active element 22 of the thermal stabilization device 20 , more specifically a resistor, instead of being arranged outside the profile 2 , can be arranged inside the longitudinal groove 6 of the profile 2 , preferably against the longitudinal rib 11 .

[0084] Reference Figure 4In another embodiment, each thermally active element 22 of the thermal stabilizing device 20 comprises one or more pipes 30, preferably made of metal material, arranged in contact with the profile 2 so as to exchange heat with the profile 2 and suitable for being passed through by a heat transfer fluid (such as water or oil) at a given temperature, preferably higher or lower than the target temperature T0, thereby being able to heat or cool the profile 2 and the scale bar 3.

[0085] In this embodiment, the control unit 23 is suitable for regulating the flow of heat transfer fluid along one or more ducts 30 of each thermally active element 22 according to the signals from one or more temperature sensors 21 present on the same sampling area of ​​the profile 2, so as to keep the local temperature of the scale bar 3 close to the target temperature T0.

Claims

1. A linear encoder (1), comprising: A substantially rectilinear profile (2), the profile (2) being suitable for being fixed to a structure of a machine tool; a substantially rectilinear scale bar (3) fixed to the profile (2) so as to extend along the profile (2) parallel to the longitudinal axis (L) of the profile; a movable slide (4), the movable slide (4) being assembled / mounted on the profile (2) so as to be movable along the profile, parallel to the longitudinal axis (L) of the profile and passing over the scale bar (3), and the movable slide (4) being suitable for being rigidly fixed on a movable part of a machine tool; an electronic reading device (5), the electronic reading device (5) being at least partially placed on the movable slide (4) and being suitable for reading the position of the movable slide (4) on the scale bar (3); The linear encoder (1) is characterized in that it also includes an active thermal stabilization device (20), wherein the active thermal stabilization device (20) is suitable for keeping the entire scale bar (3) substantially stably at a predetermined target temperature (T0); The profile (2) is longitudinally divided into a series of sampling areas, each sampling area carrying a corresponding sampling portion of the scale bar (3); the active thermal stabilization device (20) is configured to selectively and independently heat and / or cool each sampling area of ​​the profile (2) to maintain the temperature of each sampling area of ​​the scale bar (3) near the predetermined target temperature (T0).

2. A linear encoder according to claim 1, wherein the active thermal stabilization device (20) is suitable for selectively heating and / or cooling the profile (2) to maintain the temperature of each individual point of the scale bar (3) substantially within a predetermined tolerance range spanning the predetermined target temperature (T0). The linear encoder according to claim 2 , wherein the width of the tolerance range is less than 2° C. .

4. The linear encoder according to claim 1, wherein the active thermal stabilization device (20) comprises: a series of temperature sensors (21), the temperature sensors (21) being distributed along the profile (2) so as to detect the temperature at different points of the scale bar (3); A series of thermoactive elements (22) distributed along the profile (2) and configured to heat and / or cool the sampling area of ​​the profile (2) to which they are fixed, as well as the scale bar (3) integral therewith; and a control unit (23) adapted to control each of the thermoactive elements (22) independently of one another based on the signal from the temperature sensor (21).

5. The linear encoder according to claim 4, wherein the thermally active element (22) is a resistor.

6. A linear encoder according to claim 4, wherein each thermally active element (22) is rectangular in shape and extends on the profile (2) over substantially the entire length of the sampling area of ​​the profile (2), and the same thermally active element (22) is fixed to the profile (2).

7. A linear encoder according to claim 5, wherein the profile (2) is shaped to form / define a straight groove / channel (6) inside and extending parallel to the longitudinal axis (L) of the profile; and the scale bar (3) is accommodated in the straight groove / channel (6).

8. The linear encoder according to claim 7, wherein the temperature sensor (21) is arranged in the linear groove / channel (6).

9. Linear encoder according to claim 7, wherein the scale bar (3) is arranged on a longitudinal rib (11) protruding from the bottom of the linear groove / channel (6).

10. The linear encoder according to claim 9, wherein the temperature sensor (21) is located on the longitudinal rib (11).

11. The linear encoder according to claim 8, wherein the thermally active element (22) is arranged in the linear groove / channel (6).

12. The linear encoder according to claim 6, wherein the profile (2) has a substantially U-shaped cross section.

13. A linear encoder according to claim 8, wherein the upper opening of the linear groove / channel (6) is closed by a pair of longitudinal gaskets (10) extending parallel to the longitudinal axis (L) of the profile, and the longitudinal gaskets (10) are fixed to the profile (2) at the side edges of the upper opening of the linear groove / channel (6) and are fully fastened to each other.

14. The linear encoder according to claim 1, wherein the scale bar (3) has a belt-like structure.

15. The linear encoder according to claim 1, wherein the predetermined target temperature (T0) is in the range between +20°C and +90°C.

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