Force measuring device with disturbance compensation

Abstract

Force measuring device for detecting machining forces between a tool (3) and a workpiece (4), comprising a multi-axis force-torque sensor (1) having a first stationary part (13) fixedly connected to the machine body, and a second movable part (14.1) fixedly connected to the tool (3) or the workpiece (4), and a structure located between the first and second parts, which is configured to detect forces and / or moments acting between the first and second parts and convert them into electrical signals, and at least one measuring amplifier (17.1) which processes the electrical signals and transmits them to an evaluation unit (7), and a disturbance sensor (2) which also has a first stationary part (13) fixedly connected to the machine body, and a second movable part (14.2), which is rigidly connected to a substitute mass (6), and a structure located between the first and second part, which is configured to detect forces and / or moments acting between the first and second part and to convert them into electrical signals, and at least one second measuring amplifier (17.2), which processes the electrical signals of the disturbance sensor (2) and transmits them to the same or a separate evaluation unit (7), characterized in that the disturbance sensor (2) can detect at least as many force components as the force-torque sensor (1).

Description

Technical field

[0001] In many manufacturing machines, machine tools, and testing equipment, the forces occurring between the tool and the workpiece are measured using special, usually multi-axis, force-torque sensors. A common problem is distinguishing the forces acting between the tool and workpiece (useful signals) from interfering forces (noise signals). Interfering signals are primarily caused by accelerated movements acting on the mass attached to the multi-axis force-torque sensor. Due to inertia, these movements generate a disturbing force that the force-torque sensor cannot readily distinguish from the forces occurring between the tool and the workpiece. Furthermore, changes in the direction of the gravitational force acting on the multi-axis force-torque sensor can lead to measurement errors. To differentiate between useful and disturbing signals, various sensors are available in the field, e.g.,...Accelerometers, gyroscopes, tilt sensors and structure-borne sound sensors, which detect the relevant disturbances and transmit the measured values ​​in real time to an evaluation unit that is capable of cleaning the raw signal supplied by the force-torque sensor of disturbance-related influences based on the measured values ​​supplied by the disturbance sensors. Background of the invention

[0002] Accelerated movements of the machine axes, vibrations of the machine components, excited natural vibrations of the machine body or foundation, tilting of the machine body due to weight shifts, and much more cause forces acting on the force-torque sensor, generating disturbance forces that must be distinguished from the forces acting between the tool and the workpiece. Disturbance sensors provide compensation values ​​that can be used in a common evaluation unit to separate useful and disturbance signals.

[0003] The task of interference signal compensation becomes increasingly complex as the force-torque sensor is expected to detect more force components. For example, if a six-axis force-torque sensor is to detect the force components Fx, Fy, Fz, as well as the torques Mx, My, and Mz, the disturbance sensor must measure the accelerations along the three spatial axes, the change in rotation rate about the three principal axes, and the inclination of the force-torque sensor relative to the direction of gravity along three axes. All these sensors must have the same sampling rate and at least the same sensitivity to the disturbance forces and torques as the force-torque sensor itself. Furthermore, the disturbance sampling must be perfectly synchronized with the measured value sampling and exhibit minimal zero-point drift.Furthermore, the natural frequency and damping of the disturbance sensors should be greater than or equal to the natural frequency and damping of the force-torque sensor to be compensated. A sensor system that combines all these properties, even if it can be implemented with commercially available components, can cost many times more than the force-torque sensor and cause significant problems during integration into the target machine. The object of the invention is therefore to create a method and a system for disturbance detection that meets the requirement for synchronous detection of disturbance signals caused by accelerations, oscillations, inclinations, and vibrations with the necessary accuracy and reliability, and can be integrated into the target machine with reasonable effort.

[0004] From DE 102008015005 A1, a chuck-integrated force measuring system for determining cutting forces at the cutting edge of a rotating tool, for example a drill or milling cutter, is known, comprising at least one sensor and a measurement processing station. The at least one sensor is designed as a strain gauge that can be placed on a tool holder of a machine tool. Furthermore, a measuring hub is provided that can be inserted into a machine tool and is designed with its own chuck for receiving the tool, wherein the at least one sensor is integrated into the adapter.

[0005] From DE102016114193A1, a force measuring device for the at least three-axis detection of forces Fx, Fy, and Fz and / or moments Mx, My, and Mz is known, consisting of a first flange-like part through which the forces / moments to be measured are introduced, a second flange-like part through which the introduced forces / moments are received and transmitted, and a tubular deformation zone connecting these two parts, through which all tensile, compressive, torsional, and shear forces acting on the first flange-like part are transmitted to the second flange-like part. At least the inner or outer surface of the tubular deformation zone serves as an application surface for deformation sensors, and the tubular deformation zone is subdivided circumferentially into at least eight sectors S1 to S8. The force measuring device further comprises an evaluation unit for generating the measured value of at least one force component.The force measuring device is characterized by the fact that at least one deformation sensor is arranged on each individual sector S1 to S8, all deformation sensors of a sector are interconnected in a common Wheatstone bridge, and each sector on the tubular deformation zone is opposite a second sector offset by 180° around the central axis of the deformation zone with a similar arrangement of deformation sensors.

[0006] From DE 102016214434B3 a force-torque sensor for a robotics unit is known, comprising a one-piece carrier body which has a cylindrical basic shape and is provided with two mutually parallel connecting discs and with several bridge elements which connect the connecting discs in one piece.

[0007] However, none of the force measuring devices known from the prior art have integrated means for disturbance compensation. Description of the invention

[0008] This problem is solved by providing a force measuring device according to the invention with a disturbance sensor which is largely a technological copy of the multi-axis force-torque sensor, but is completely decoupled from the effects of the forces and moments acting between the tool and the workpiece. The disturbance signals are detected by measuring the forces acting on a substitute mass, which is rigidly connected to the moving part of the disturbance sensor and thus forms part of it. The ratio between substitute mass M s and the stiffness C s The disturbance sensor is chosen such that it corresponds to the ratio of the attached movable mass M. k and the stiffness C kThe natural frequency of the disturbance sensor should correspond as closely as possible to that of the force-torque sensor along the same spatial axis. This ensures that the natural frequency of the disturbance sensor is approximately the same as the natural frequency of the multi-axis force-torque sensor in the unloaded state and in the same load direction. If the natural frequency of the disturbance sensor were lower than that of the force-torque sensor, it would limit the frequency range of the force measuring device. Increasing the stiffness of the disturbance sensor relative to the attached equivalent mass, for example, to raise its natural frequency, would, using otherwise identical technology, result in a lower sensitivity of the disturbance sensor compared to the force-torque sensor. Consequently, small and minute disturbances could not be detected with the same accuracy and therefore could not be used for disturbance compensation.

[0009] When a force impulse acts on the force-torque sensor, it begins to oscillate at its natural frequency. The decay time is determined by the force-torque sensor's inherent damping, which is direction-dependent. Ideally, the damping factor of the disturbance sensor's inherent damping should correspond to that of the force-torque sensor in the same direction to achieve the most accurate disturbance detection possible during the decay phase of an oscillation triggered by an external impulse.

[0010] In general, the more similar the disturbance sensor is to the force-torque sensor, the more similar its behavior towards disturbances will be, and the more meaningful the signal provided by the evaluation unit through differential calculation will be. This consideration includes the entire measurement chain, including the cabling, measuring amplifiers, and digital-to-analog converters. Ideally, the disturbance sensor should be located as close as possible to the multi-axis force-torque sensor and preferably positioned at its center. Furthermore, the stationary part of the disturbance sensor should be rigidly connected to the stationary part of the multi-axis force-torque sensor.

[0011] According to an advantageous embodiment, the disturbance sensor is temperature-controlled and maintained at the same temperature level as the multi-axis force-torque sensor. This is preferably achieved by circulating the same cooling medium through both the multi-axis force-torque sensor and the disturbance sensor in a common temperature control circuit.

[0012] According to another advantageous embodiment, the natural oscillations of the force-torque sensor and the disturbance sensor are dampened in the region of their natural frequency by a vibration damper. A so-called damper mass is elastically coupled to the moving part of the force-torque sensor or to the equivalent mass of the disturbance sensor. Additional damping elements increase the damping factor and lead to a faster decay of the force-torque sensor and the disturbance sensor after the excitation has subsided. Here, too, a high degree of similarity in the vibration behavior of the disturbance sensor and the force-torque sensor is advantageous.

[0013] Changing workpieces, clamping devices, and tools can lead to significant differences in the mass attached to the force-torque sensor and its center of gravity, resulting in variable vibration behavior. In contrast, the disturbance sensor essentially always operates under the same conditions. In particular, the attached mass and the center of gravity do not change. Therefore, the signal response of the disturbance sensor is much more predictable than that of the force-torque sensor. This predictability can be used to deduce the actual force effects from the disturbance sensor's signal response using suitable algorithms, thus distinguishing between natural and externally excited vibrations. This could enable reliable measurements even near or beyond the natural frequency.A well-designed algorithm can therefore significantly mitigate the similarity requirement described above between the disturbance sensor and the force-torque sensor with respect to natural frequency and damping factor. This would, in particular, enable a more sensitive disturbance sensor, allowing for more precise disturbance compensation in the lower sensitivity range of the force-torque sensor. Reference symbol list 1 Force-torque sensor 2 disturbance sensor 3 tools 4 workpieces 5 Base plate 6 replacement mass (M S ) 7 Evaluation unit 8 Mounting plate 9 Motor spindle 10 Headstock 11.1 Absorber mass (force-torque sensor) 11.2 Absorber mass (disturbance sensor) 12 Elastomer 13 First immovable part 14.1 Second moving part (force-torque sensor) 14.2 Second moving part (disturbance sensor) 15.1 Tubular deformation zone (force-torque sensor) 15.2 Tubular deformation zone (disturbance sensor) 16 Temperature control circuit 17.1 Measuring amplifier (force-torque sensor) 17.2 Measuring amplifier (disturbance sensor) Character description

[0014] Fig. Figure 1 shows an example of a portal milling machine equipped with a force measuring device according to the invention. The clamping plate 8 rests on a combination of several force-torque sensors 1 which operate in conjunction and are connected together to an evaluation unit 7 via a multi-channel measuring amplifier 17.1. The forces and torques acting between the tool 3 and the workpiece 4 are detected by the force-torque sensors 1, which are arranged on a common base plate 5. A disturbance sensor 2 is mounted on the same base plate 5 under the clamping table 8 in the center of the force-torque sensors 1. A substitute mass 6 is mounted on this sensor. The forces and torques not resulting from the contact between the tool 3 and the workpiece 4, e.g.,Forces caused by lateral accelerations, vibrations, or tilting act on the substitute mass 6 of the disturbance sensor in the same way as they act on the mass attached to the force-torque sensors 1. The ratio between substitute mass M. s and the stiffness C s The disturbance sensor is chosen such that it corresponds to the ratio of the attached movable mass M. k and the stiffness C kThe force-torque sensor's position along the same spatial axis corresponds as closely as possible. A damping mass 11.1 is located below the clamping table and is rigidly coupled to it via an elastomer 12. The damping mass 11.1 and the elastomer 12 together form a vibration damper that reduces and dampens the disturbance vibrations occurring in the region of the clamping table's natural frequency. A vibration damper with a damping mass 11.2 and an elastomer 12 is also preferably attached to the disturbance sensor 2 to achieve the greatest possible similarity in signal behavior with the force-torque sensors.

[0015] Fig. Figure 2 shows a force measuring device according to the invention with integrated disturbance compensation, which is arranged between the headstock 10 and the motor spindle 9. This design offers the fundamental advantage that the ambient conditions are less severe than those described in Figure 2. Fig. The configuration described in point 1, in which the force-torque sensors are located below the table, is significantly more stable. For example, the mass of the tools being changed typically changes less than that of the clamping devices and workpieces during different machining operations. The [configuration] Fig.The force-torque sensor shown in Figure 2 consists of a first stationary part 13, which has the shape of a ring and is rigidly flanged to the spindle head 10, and a second movable part 14.1, to which the motor spindle 9 is flanged. Between parts 1 and 2 is a thin-walled, tubular deformation zone 15.1, which is provided on its inner surface with strain gauges (not shown) that detect the deformations of the tubular deformation zone 14.1 and convert them into electrical signals. Also shown is the damping mass 11.1, which is connected to the movable part 14.1 of the force-torque sensor 1 by means of an elastomer 12. Above the stationary part 13 is the movable part 14.2 of the disturbance sensor 2, which is connected to the stationary part 13 via a tubular deformation zone 15.2 in the same way as the force-torque sensor 1. The deformations of the tubular deformation zone 15.Here too, the two moving parts are measured via internal strain gauges. The substitute mass 6 of the disturbance sensor is arranged above the stationary part. A vibration damper in the form of a damper mass 11.2 connected via an elastomer 12 is also implemented here. Furthermore, a temperature control circuit 16 is shown, which, in the interest of higher measurement reliability and lower zero-point drift, establishes a thermal equilibrium between the two moving parts 14.2 and 14.1 of the disturbance sensor 2 and the force-torque sensor 1, as well as the common stationary part 13.

Claims

[1] Force measuring device for detecting machining forces between a tool (3) and a workpiece (4), comprising a multi-axis force-torque sensor (1) having a first stationary part (13) fixedly connected to the machine body and a second movable part (14.1) fixedly connected to the tool (3) or the workpiece (4), and a structure located between the first and second parts, which is configured to detect forces and / or moments acting between the first and second parts and convert them into electrical signals, and at least one measuring amplifier (17.1) which processes the electrical signals and transmits them to an evaluation unit (7), and a disturbance sensor (2) which also has a first stationary part (13) fixedly connected to the machine body and a second movable part (14.2), which is rigidly connected to a substitute mass (6), and a structure located between the first and second part, which is configured to detect forces and / or moments acting between the first and second part and to convert them into electrical signals, and at least one second measuring amplifier (17.2), which processes the electrical signals of the disturbance sensor (2) and transmits them to the same or a separate evaluation unit (7), . characterized by , that the disturbance sensor (2) can detect at least as many force components as the force-torque sensor (1). [2] Force measuring device according to claim 1, characterized by , that the disturbance sensor (2) is based on the same measurement technology as the force-torque sensor (1). [3] Force measuring device according to claim 1, characterized by, that the structure of the disturbance sensor (2) located between the first (13) and second movable part (14.2) for detecting the effective forces and moments is of the same shape, design and orientation as the corresponding structure of the force-moment sensor (1). [4] Force measuring device according to claim 1, characterized by , that in the force-torque sensor (1) and the disturbance sensor (2) the structure located between the first stationary and the second movable part consists of at least one tubular deformation zone (15) which is firmly connected to the first and second part and on whose inner or outer surface strain gauges are applied to detect the load-dependent deformation of the tubular deformation zone (15). [5] Force measuring device according to any of the preceding claims, characterized by that the ratio M s / C s between substitute mass (M s ) (6) and the stiffness (Cs ) of the disturbance sensor (2) of the ratio M k / C k the movable mass (M) attached to the force-torque sensor (1) in the unloaded state k ) and the stiffness (C k ) of the force-torque sensor (1) deviates by less than ±50%. [6] Force measuring device according to any of the preceding claims, characterized by , that identical measuring amplifiers are used for the force-torque sensor (1) and the disturbance sensor (2). [7] Force measuring device according to any of the preceding claims, characterized by , that the force-torque sensor (1) and the disturbance sensor (2) have similar systems for damping the natural oscillation. [8] Force measuring device according to any of the preceding claims, characterized by , that the force-torque sensor (1) and the disturbance sensor (2) have a common temperature control circuit.

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