Gravity compensation device

Abstract

To provide a device for a gravity compensation designed to compensate for the unbalance of a rotary element (1) pivoting on a support body (4).SOLUTION: A device of the present invention comprises a compensating moving body (2) pivotally mounted on a support body (4) and kinematically connected to a rotary element (1). The device further comprises an elastic return element (3), a first end of which is mounted eccentrically and directly on the compensating moving body (2) so as to exert force on the compensating moving body, and the intensity and the lever arm vary as a function of the angular position of the rotary element. In a creative way, a second end of the elastic return element (3) is mounted on the support body (4) or mounted on a second compensation moving body (2) pivotally mounted on the support body (4) and kinematically connected to the rotary element (1). The characteristics of the device are selected so that the torque produced on the rotary element compensates for the torque due to the imbalance over the functional angular range.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a gravity compensation device, in particular for a machine tool or measuring machine. More particularly, the invention relates to a device for compensating for imbalances in rotating elements of such machines. The invention further relates to a machine tool or measuring machine equipped with such a device. [Background technology]

[0002] Conventional Technology Many devices are known for compensating for the effects of gravity on moving parts that are not balanced about their axis of rotation.

[0003] U.S. Patent No. 4,768,762 describes a device for compensating for the weight of a rotary mobile body by means of a spring attached to a band wrapped around a cam integral with the mobile body. The torque exerted by the mobile body's weight varies as a sine function of the angle formed between a vertical line and a line passing through the mobile body's axis of rotation and its center of gravity. The spring's return force varies as a function of its extension. The cam profile is designed so that the return torque exerted by the spring compensates for the mobile body's weight as a function of the mobile body's angle and the spring's extension.

[0004] U.S. Patent No. 4,500,251 discloses a robot equipped with a device for compensating for the weight of an end segment pivoted around a middle segment, which itself is pivoted on a frame. A first wheel, pivoted to the frame on the same axis of rotation as the middle segment, is connected by a chain or toothed belt to a second wheel of the same diameter, coaxial with, and integral with the end segment. Since the two wheels always have the same orientation, the angular position of the first wheel relative to the frame corresponds to the angular position of the end segment relative to the frame, regardless of the angular position of the middle segment. A spring is fixed between the frame and the first wheel and exerts a restoring torque that compensates for imbalance in the end segment. When the end segment is in a vertical position, the spring's restoring force passes through the axis of the first wheel, and the restoring torque is zero. Conversely, the lever arm and restoring torque are maximized when the end segment is in a horizontal position. It is important that the fluctuations in the spring tension and orientation remain low to ensure that the fluctuations in the direction and strength of the restoring force remain low and that the restoring torque approaches a pure sine function of the imbalance. To achieve this, their attachment points to the frame must be as far apart as possible. By using two connected wheels of the same diameter, information about the orientation and compensation torque of the end segments can be transmitted. It is not possible to attach a spring directly to the middle segment, whose orientation changes. A robot can have any number of segments and transmit the orientation information of the end segments to the frame using a series of identical links.

[0005] In these first two compensation mechanisms, the length of the return spring is quite long, which makes it impossible to imagine integrating these systems into the limited space of a machine tool, especially if the support to which the return spring is attached is itself a moving body. Furthermore, the direct force acting on the rotating element containing the imbalance to be compensated induces forces with a radial component on the rotation axis, which adversely affect the positioning accuracy of the rotating element and cause premature wear of the pivot.

[0006] U.S. Patent No. 20100294173 discloses a gravity compensation mechanism for a machine tool vibration table. Two pinions of the same size are meshed together to rotate in opposite directions at the same speed. The two pinions are pivoted on a frame, one of which is integral with the vibration table. A constant compression or tension force is applied between two pins fixed to each pinion using various devices, including hydraulic cylinders, pneumatic pistons, springs with one end attached to a winder to prevent their length from changing, weights, and pulley systems.

[0007] Patent document No. 2000301405 describes another mechanism for compensating for gravity on a machine tool tilting table. The end of a cylinder acts on a crank kinematically connected to the table to compensate for table imbalance. Due to space constraints near the table's rotation axis, the crank is not attached directly to the table, but to an offset moving body connected to the table by a transmission belt or gears with a 1:1 drive ratio.

[0008] U.S. Patent No. 2,584,921 describes a mechanism for counteracting imbalance in a rotating element, such as a crane jib, that oscillates between 0° and 180° relative to the vertical. The first end of a spring is attached to a pivoting mobile unit kinematically linked to the rotating element by a chain that meshes with two sprockets of the same diameter. The second end of the spring is connected to a cable wound around a drum on the rotating element. In the angular range of the rotating element from 90° to 180°, the restoring torques exerted by the spring on the drum and the pivoting mobile unit are both opposite to the torque due to the imbalance and proportional to the extension of the spring. In the range of 0° to 90°, the extension of the spring and the restoring torque acting on the drum are almost unchanged, and the torque exerted by the pivoting mobile unit is in the same direction as the torque due to the imbalance. This device is limited to an angular range of 0° to 180° relative to the rotating element and is not suitable for tilting tables of machine tools, which typically pivot between -110° and +110°. Furthermore, in order to make the change in spring orientation negligible, the pivoting body must be placed at a distance from the rotating element, which makes the device not very compact.

[0009] U.S. Patent No. 8,220,765 describes a mechanism for compensating for imbalance of a rotating element on a support. A first flexible strand is wrapped around three pulleys, one of which is pivotally attached to the rotating element. A return spring has one end attached to the support and the other end attached to a second strand. The first and second strands are attached to the same pulley, such that rotation of the rotating element causes rotation of the pulley, extension of the spring, and tension in the first and second strands, generating a return torque on the movable pulley attached to the rotating element. The generated torque completely compensates for the sinusoidal torque due to the imbalance. However, this system is cumbersome and not easily integrated into constrained environments.

[0010] All of these mechanisms present various drawbacks in terms of size, complexity, reliability and inertia, especially when the support to which the rotating element is attached is itself a moving body and when the rotating element is intended to be subjected to large angular accelerations.

[0011] Generally speaking, any imbalance in a rotating element of a machine without a compensating device must be supported by the motor driving the element's rotation. This results in premature motor wear, increased electrical consumption, motor overheating, and thermal distortion, all of which are detrimental to machine accuracy and require additional cooling. Motor servo control and regulation are also more sensitive in the presence of imbalance. Finally, working in undesirable fixed positions close to 90° requires a brake to prevent the rotating element from rotating. Adding a counterweight to balance a rotating element is undesirable because it increases its inertia, affecting the speed and accuracy of machining or measurement.

[0012] It is an object of the present invention to ameliorate various shortcomings of the prior art and to provide a gravity compensation system that is accurate over at least one functional angular range, simple, reliable, compact, and has low inertia. Summary of the Invention

[0013] Disclosure of the Invention The object of the present invention is achieved by a gravity compensation device, in particular for machine tools or measuring machines, designed to compensate for the imbalance of a rotating element pivoting on a support. The compensation device comprises a compensating body pivotally mounted on the support and kinematically connected to the rotating element, the device further comprising an elastic return element, the first elastic end of which is eccentrically mounted on the compensating body so as to exert a force on the compensating body, the strength and lever arm of which vary as a function of the angular position of the rotating element. According to one aspect of the invention, the second end of the elastic return element is attached to the support or to a second compensating body pivotally mounted on the support and kinematically connected to the rotating element. This configuration makes it possible to select the characteristics of the device so that the torque generated on the rotating element compensates for the torque due to the imbalance over a functional angular range.

[0014] The device makes it possible to reproduce a quasi-sinusoidal compensation torque without applying radial forces to the rotating elements, while remaining particularly simple, reliable, compact and with low inertia.

[0015] According to an advantageous embodiment, the stable equilibrium position of the rotating element, where the torque due to the imbalance is zero, corresponds to the unstable equilibrium position of the compensator.

[0016] According to an advantageous embodiment, the position at which the torque due to the imbalance is zero corresponds to the position of maximum tension of the elastic return element.

[0017] According to a particularly advantageous aspect of the invention, the compensating mover is kinematically connected to the rotating element with a drive ratio of less than 1, so that a given angular displacement of the rotating element produces a smaller angular displacement of the compensating mover.

[0018] According to an advantageous embodiment, the elastic return element is mounted between two compensation wheels having the same drive ratio as the rotary element, so that the angular displacements of the two compensation wheels are equal or opposite.

[0019] According to an advantageous embodiment, the elastic return element is mounted between two compensation movers pivoting in the same direction, the drive ratio being between 0.6 and 0.9.

[0020] According to another advantageous embodiment, the elastic return element is mounted between two compensation movers pivoting in opposite directions, the drive ratio being between 0.2 and 0.8. [Brief explanation of the drawings]

[0021] Further details of the invention will become apparent from the following description, which refers to the accompanying drawings. [Figure 1] 1 shows a kinematic diagram of a compensation device according to the invention; [Figure 2] 1 shows a schematic diagram of a first variant of a compensation device according to the invention; [Figure 3] 1 shows a schematic diagram of a second variant of the compensation device according to the invention; [Figure 4] For a given configuration of the first variant, the values ​​of the lever arm, tension and compensation torque as a function of the angle of the rotating element are shown, respectively. [Figure 5] For a given configuration of the first variant, the values ​​of the lever arm, tension and compensation torque as a function of the angle of the rotating element are shown, respectively. [Figure 6] For a given configuration of the first variant, the values ​​of the lever arm, tension and compensation torque as a function of the angle of the rotating element are shown, respectively. [Figure 7] 4 shows different compensation torque curves obtained with different drive ratios of a compensation device according to the first or second variant. [Figure 8] 4 shows different compensation torque curves obtained with different drive ratios of a compensation device according to the first or second variant. [Figure 9] 4 shows different compensation torque curves obtained with different drive ratios of a compensation device according to the first or second variant. [Figure 10] 3 shows a schematic diagram of a third variant of the compensation device according to the invention; [Figure 11]4 shows different compensation torque curves obtained at different drive ratios for a compensation device according to the third variant. [Figure 12] 4 shows different compensation torque curves obtained at different drive ratios for a compensation device according to the third variant. [Figure 13] 4 shows different compensation torque curves obtained at different drive ratios for a compensation device according to the third variant. DETAILED DESCRIPTION OF THE INVENTION

[0022] MODE FOR CARRYING OUT THE INVENTION FIG. 1 shows a kinematic diagram of a gravity compensation device according to a first embodiment of the present invention. The compensation device is mounted on a support 4, in particular a machine tool or measuring machine. A rotating element 1 is pivotally mounted about axis X of the support 4. The rotating element 1 is unbalanced on its axis of rotation and has an imbalance that the compensation device is designed to compensate for. The compensation device is designed to compensate for the imbalance of the rotating element for rotations not exceeding + / - 180° around the equilibrium position of the rotating element, i.e., when the center of gravity G is vertical below the axis of rotation X. In practice, the angular amplitude of the rotating element is preferably limited by a stop and is less than 360°. The device is adapted to provide a compensation torque over a functional angular range encompassing the angular amplitude of the rotating element.

[0023] Rotating element 1 has a mass whose center of gravity G is located a distance a from the axis of rotation X, as shown in the diagram in Figure 2. The unbalance torque of rotating element 1 is a sine function of the angle β between a perpendicular line and a plane containing the axis of rotation X and the center of gravity G. If the mass of rotating element 1 is m and gravity is g, then the unbalance torque Cb is equal to: Cb=mgasinβ

[0024] 2, 3 and 10, the torque Cb due to imbalance is zero when the center of gravity is perpendicular to the X-axis and is maximum when the center of gravity is horizontal. The compensation device according to the present invention aims to provide a compensating torque Cc opposite to the torque Cb in order to eliminate the disadvantages of imbalance without significantly increasing inertia.

[0025] The compensation device comprises a compensating mobile 2 pivotally mounted on a support 4 and kinematically connected to the rotating element 1. In the illustrated embodiment, the compensating mobile 2 has teeth Z2 that mesh with teeth Z1 of the rotating element 1, but other connection means, such as a chain or a toothed belt, are also suitable. A first end of a resilient return element 3 is eccentrically mounted to the compensating mobile 2. Typically, the resilient return element consists of a tension spring, one end of which is preferably pivotally mounted to the compensating mobile 2. In a first variant shown in FIGS. 1 and 2, a second end of the resilient return element 3 is preferably pivotally mounted to the support 4. The resilient return element 3 exerts a tension force on the mounting point D of the compensating mobile 2 in the direction of the second mounting point E, generating a restoring torque Cra on the compensating mobile, which transmits a compensation torque Cc to the rotating element 1 via the gear teeth Z1, Z2.

[0026] The axis of rotation of the compensating mover 2 does not have to be parallel to the axis of rotation of the rotating element without departing from the scope of the invention. For example, in terms of saving space, it is preferable to arrange the compensating mover perpendicular to the rotating element, preferably using a bevel gear.

[0027] When the rotating element 1 is in a stable equilibrium position, its center of gravity is at a low position perpendicular to the X-axis, i.e. the torque due to the angle β and the imbalance Cb is zero. This position corresponds to the unstable equilibrium position of the compensator, where the tension in the elastic return element 3 is at a maximum and the direction of the restoring force passes through the rotation axis of the compensating mobile body 2. This means that the return torque Cra and the compensation torque Cc are also zero.

[0028] The diagram in Figure 2 shows the geometric elements of the compensating device shown in Figure 1. The first end of the elastic return element 3 is fixed at D at a distance r from the center of rotation O of the compensating mobile 2. The second end is fixed at E to the support 4 at a distance R from point O.

[0029] The return torque Cra acting on the compensation movable body 2 is the product of the lever arm d and the tension T of the tension spring, and this depends on the length L between the spring attachment points E and D. Cra=Td

[0030] When Z1 and Z2 are the numbers of teeth of the Z1 and Z2 toothing sections, respectively, the drive ratio Re between the rotating element and the compensation moving body is

[0031]

number

[0032] The rotation angle α of the compensation movable body can be estimated from the tilt angle β of the rotation element 1. α=Re.β

[0033] In triangle EDD', the following relationships exist:

[0034]

number

[0035] From this, we estimate the value of Ω as a function of α, and hence as a function of β.

[0036]

number

[0037] This allows us to calculate the lever arm d. d=RcosΩ

[0038] The length L can be estimated from Ω.

[0039]

number

[0040] The tension T of a spring depends on its stiffness K, its elongation, the difference between its length L and its unloaded length Lo, and its unload tension To. T=(L-L0)K+To

[0041] This allows the compensation torque Cc to be calculated as a function of the angle β.

[0042]

number

[0043] In all the examples shown in Figures 4 to 9 and 10 to 13, some dimensions of the devices are arbitrarily set to show the influence of certain parameters on the compensation torque. Thus, for the rotating element 1, an imbalance of 5 kg with an eccentricity a of 40 mm was set, and for the compensation device, distances R and r of 50 mm and 22.5 mm, respectively, were set.

[0044] In the first example shown in FIGS. 4 to 6, the drive ratio

[0045]

number

[0046]

number

[0047] [Table 1]

[0048] Surprisingly, it is possible to reproduce a pseudo-sinusoidal compensation torque that is very close to the sinusoidal torque due to the imbalance, as shown in Figure 6. In the example shown, over a range of + / - 160°, the maximum value of the residual torque represents only 0.7% of the maximum torque without compensation. It will be appreciated that these figures represent the compensation torque for positive values ​​of β, and that the device behaves symmetrically for negative values ​​of β.

[0049] Figures 4 and 5 show the lever arm d and tension T, respectively, as a function of angle β. The lever arm value d is zero when angle β is zero, increases up to a value r where segment [OD] is perpendicular to segment [DE], and then decreases. The tension T reaches a maximum when angle β is zero and then decreases steadily. The product dT, which is proportional to the compensation torque, has a zero value at zero angle β, passes through a maximum value, and then decreases as the tension in the elastic return element relaxes. The compensation torque is preferably closest to the unbalance torque when it is highest, i.e., when angle β is close to 90°. Therefore, it is important to center the pseudo-sine wave so that its maximum value is obtained for angle β of 90°. The parameters that most significantly affect the location of this maximum value are the open length Lo of the spring, the drive ratio, and the distance R, which defines the position of the second end of the elastic return element. The unloaded length also affects the decrease in tension T as a function of angle β. The greater the no-load length, the faster the spring tension decay occurs and the peak of the quasi-sine wave moves toward lower values ​​of β. The drive ratio defines the position of the maximum lever arm as a function of β. The lower the drive ratio, the later the maximum lever arm occurs and the peak of the quasi-sine wave moves toward larger values ​​of β. When the quasi-sine curve is centered at a β value of 90°, the no-load tension and stiffness can be adjusted to approximate the sinusoidal curve of the torque due to imbalance.

[0050] Prior art devices aim to replicate the sinusoidal torque due to gravity by applying a constant force, or a force that varies as little as possible, to a point of application attached directly to the rotating element, or to a moving body kinematically connected to the rotating element with a 1:1 drive ratio. In an inventive way, the device of the present invention makes it possible to simulate sinusoidal torque by utilizing joint variations in lever arm and tension as a function of the angular position of the rotating element.

[0051] The compensating device of the present invention can be used to compensate for imbalances of rotating elements with a limited angular amplitude of no more than + / - 180° around the equilibrium position. For fixed geometrical characteristics of the device and fixed angular amplitude of the rotating elements, the drive ratio can be used to define the minimum length the spring reaches when the rotating elements are in abutment. This determines the effective stroke of the spring element and its maximum unloaded length, which should preferably be less than this minimum length so that the spring always remains under tension.

[0052] Figures 7 to 9 show the compensation obtained for drive ratios of 2 / 3, 4 / 5, and 5 / 6, respectively, for which the properties of the elastic return element, i.e., stiffness, unloaded length, and unloaded tension, have been adapted. Note that the peak of the compensation curve is obtained for values ​​of β slightly greater than 90° in the case of Figure 7, and slightly smaller in the case of Figure 10.

[0053] In the example shown corresponding to the first variant of the invention, values ​​of the drive ratio Re between 0.7 and 0.8 provide the best compensation curve. However, while the values ​​of R and r are arbitrarily fixed, it is possible to vary these parameters, so that it is not possible to characterize the invention in a simple way by providing a range of values ​​for each parameter. Generally speaking, for the first two variants of the invention, it is possible to obtain gravity compensation over at least part of the angle range + / - 180° for drive ratio values ​​between 0.6 and 0.9.

[0054] FIG. 3 shows a schematic diagram of a second variant with two compensating bodies 2 carrying Z2 teeth of the same diameter that mesh with the Z1 teeth of the rotating element 1. The two compensating bodies have the same drive ratio relative to the rotating element, resulting in their angular displacement being equal, i.e., of the same magnitude and direction. The second end of the spring element 3 is not attached to the support 4 as in the previous variant, but to the second compensating body. Mounting a single spring between points D1 and D2 is equivalent to mounting two springs at the center point E, making this variant kinematically equivalent to the previous variant. The single-spring solution saves a second pin for attachment to the support 4 and increases the useful length of the spring, which would otherwise be lost due to the attachment means to the support 4.

[0055] The diagram in FIG. 10 shows a third variant of the compensation device according to the invention, also comprising two compensating bodies 2. In this case, the two compensating bodies are mounted in series rather than in parallel so that they rotate in opposite directions, with one compensating body acting as an inverter for the other. In the illustrated example, the drive ratios of the rotating elements are identical for the two compensating bodies, resulting in opposite angular displacements, i.e., the same value but opposite direction. It will be understood that the two compensating bodies do not necessarily have to be directly coupled; a reversing wheel could be mounted between the rotating element 1 and one of the compensating bodies, or any other arrangement that reverses the direction of rotation of this compensating body relative to the first moving part. An elastic return element 3 is mounted between the two compensating bodies 2, so that the mounting points D1 and D2 are aligned with the centers O1 and O2 of the compensating bodies 2 when the angle β is zero. In this configuration, the segment [D1D2] is always parallel to the center line [O1O2].

[0056] The lever arm d is simply expressed as: d=rsinα

[0057] If the distance between the centers O1 and O2 is 2R, the length L between the hook points D1 and D2 is expressed as follows: L=2(R+rcosα)

[0058] For the same drive ratio, the increase in the lever arm d and the decrease in the length L as a function of the angle β are faster in this third variant than in the previous two variants. The maximum value of the lever arm r occurs at an angle α equal to 90° in this third variant, whereas in the first two variants it is greater than 90°. As a result, this variant is suitable for lower drive ratios Re, which limit the elongation of the elastic return element 3 and optimize its fatigue resistance. On the other hand, lower drive ratio values ​​do not allow compensation over a wide angular range. Generally speaking, in the case of this third variant of the invention, it is possible to obtain gravity compensation over at least part of the angular range of + / - 180° for drive ratio values ​​between 0.2 and 0.8.

[0059] 11 to 13 show examples of compensation torques obtained with the third variant for different drive ratios while maintaining the same values ​​of R and r as in the previous example.

[0060] In the example shown, the two compensating gears are identical, but they may have different drive ratios and different eccentricities r without departing from the scope of the invention.

Claims

1. 1. A gravity compensation device, in particular for machine tools or measuring machines, intended to compensate for imbalance of a rotating element (1) pivoting on a support (4), said device comprising a compensating mover (2) pivotally mounted on said support (4) and kinematically connected to said rotating element (1), said device also comprising an elastic return element (3) having a first end eccentrically mounted directly on said compensating mover (2) for applying a force to said compensating mover, the strength and lever arm of said compensating mover varying as a function of the angular position of said rotating element, a second end of said elastic return element (3) mounted on said support (4) or on a second compensating mover (2) pivotally mounted on said support (4) and kinematically connected to said rotating element (1), said characteristics of said device being selected such that the resulting torque on said rotating element compensates for the torque due to the imbalance over a functional angular range.

2. 2. The device according to claim 1, wherein the stable equilibrium position of the rotating element (1), where the torque due to the imbalance is zero, corresponds to an unstable equilibrium position of the compensation device.

3. 2. The device according to claim 1, wherein the stable equilibrium position of the rotating element (1), where the torque due to the imbalance is zero, corresponds to a position of maximum tension of the elastic return element (3).

4. 2. The device according to claim 1, wherein the drive ratio between the rotating element (1) and the compensating mover (2) is less than 1, such that a given angular displacement of the rotating element (1) produces a smaller angular displacement of the compensating mover (2).

5. 2. The device according to claim 1, wherein the elastic return element (3) is mounted between two compensation movers (2) having the same drive ratio as the rotation element (1), so that the angular displacements of the two compensation movers are equal or opposite.

6. 2. Device according to claim 1, wherein the elastic return element (3) is mounted between two compensation movers (2) pivoting in the same direction.

7. 2. The device according to claim 1, wherein the drive ratio between the rotating element (1) and the compensation mover (2) is between 0.6 and 0.

9.

8. 2. Device according to claim 1, characterized in that the elastic return element (3) is mounted between two compensating bodies (2) pivoting in opposite directions.

9. The device according to any one of the preceding claims, wherein the drive ratio between the rotating element (1) and the compensation mover (2) is between 0.2 and 0.

8.

10. 2. Device according to claim 1, characterized in that the elastic return element (3) is mounted between two compensation movers (2) having different drive ratios with the rotating element (1).

11. A machine tool or measuring machine comprising the compensation device according to claim 1.

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