Electromagnetic Compensatory Beam Weighing

Description

[0001] The present invention relates to an electromagnetically compensating beam balance comprising a two-armed, rotatably mounted transmission lever, each lever arm being coupled to an associated load-bearing device. An actuator integrated in a closed control loop can be arranged on at least one of the two load-bearing devices or below the lever joint.

[0002] The measurement uncertainty in high-precision mass comparison depends significantly on the mechanical system of the mass comparator. This system is subject to many disturbances, the influence of which on the measurement uncertainty can be significantly affected by the mechanism and its adjustment.

[0003] It is known that equal-arm mechanisms, such as the beam balance, offer many advantages regarding measurement uncertainties in mass comparisons due to their symmetrical design. These include: the compensation of thermal expansions, the independence from ambient pressure, symmetrically arranged heat sources, a lower sensitivity to electric and magnetic fields, a lower influence of vertical ground vibrations due to the symmetrical structure, a double-sided position measurement and substitution and transposition method for mass comparisons.

[0004] Scales for high-precision mass comparisons are almost exclusively based on the compensation principle (electromagnetic force compensation, or EMF). Asymmetrical mechanisms are frequently used for this purpose. However, these solutions forgo the aforementioned advantages of symmetrical structures.

[0005] The solution presented in patent DE 10 2016 106 695 B4 extends the concept of electromagnetic force compensation with the associated concept of the compliant mechanism to a symmetrical structure, although the associated rigid body model has one degree of freedom F=0, which is disadvantageous for its implementation as a compliant mechanism.

[0006] The mechanism of an electromagnetically force-compensating load cell known from DE 10 2016 106 695 B4 fulfills two functions: (1.) The weighing pan is typically guided quasi-linearly by a parallel crank (guiding function). This allows lateral forces to be transferred directly into the frame. This part of the mechanism is also referred to as the Roberval mechanism. (2.) A transmission lever transfers the weight force coming from the weighing pan to the opposite side of the lever (transmission function). The mechanisms of the electromagnetically compensating beam balance proposed in DE 10 2016 106 695 B4 also fulfill both functions. However, the functional integration—i.e., one part of the mechanism fulfills both the guiding and transmission functions—proves disadvantageous for a compliant mechanism, since the joint orientation is not optimized for a single function, and thus only a compromise solution can be generated for each function.The typically used solid-state joints with a semicircular contour have a preferred loading direction along their longitudinal axis. They are less suitable for lateral loads because they are less stiff in this direction. However, both loads occur in the aforementioned functional integration. Furthermore, a planar implementation and a monolithic design are not planned.

[0007] The object of the present invention is therefore to overcome the disadvantages identified in the known prior art and to provide an electromagnetically compensating beam balance which comprises an optimized compliant mechanism with solid-state joints as a mechanical basis, which is preferably planar and monolithic.

[0008] According to the invention, this problem is solved by the features of the first claim. Advantageous embodiments of the solution according to the invention are specified in the dependent claims.

[0009] An electromagnetically compensating beam balance according to the invention comprises a two-armed, rotatably mounted transmission lever, each lever arm of which is coupled to an associated load-bearing element, and which has an actuator integrated in a closed control loop on at least one side, wherein each load-bearing element is guided linearly in the vertical direction by means of a lever mechanism. According to the invention, the lever mechanism is designed as a parallel crank consisting of an upper parallel link, a lower parallel link, and the associated load-bearing element, which is fixed to the frame and pivotally connected via a coupling element.

[0010] In one embodiment, the translation lever has a horizontal actuator below its lever joint.

[0011] In one embodiment, the electromagnetically compensating beam balance has an adjustment weight for the translation lever, the center of gravity of which can be adjusted in the vertical direction close to the center of the lever joint.

[0012] In one embodiment, the electromagnetically compensating beam balance has an adjustment weight on the upper and / or lower parallel link.

[0013] In one embodiment, to compensate for the torsional stiffness of the lever mechanism, both arms of the translation lever are coupled to the respective coupling element vertically spaced from the lever joint via external lever joints.

[0014] In one embodiment, the load-bearing pans are mechanically coupled to the load-bearing surfaces, either rigidly mounted and / or gimbal-mounted.

[0015] In one embodiment, each of the closed control loops comprises an electromagnetic actuator consisting of a frame-mounted magnet system and a moving coil arrangement, and, for determining the deflection of the translation lever relative to the frame, a position sensor consisting of a frame-mounted unit and a moving unit.

[0016] In one embodiment, the double-sided lever mechanisms are designed as a monolithic, planar overall structure.

[0017] In one embodiment, the parallel cranks are coupled to the translation lever in an application-specific optimized manner and / or positioned accordingly.

[0018] In one embodiment, the parallel cranks each have two lower and upper parallel links arranged parallel to each other.

[0019] The solution according to the invention is based on separating the functions of guidance and translation within the mechanism (mechanical part) of the beam balance, thereby enabling its implementation as a compliant mechanism with solid-state joints without the aforementioned disadvantages. By introducing nonlinearity into the mechanism, its stiffness can be minimized, resulting in very high sensitivity. Furthermore, the mechanism can be manufactured as a planar (flat) structure, advantageously simplifying production and allowing flexibility in the selection of the manufacturing process and thus also the material.

[0020] The invention is explained in more detail below with reference to the drawings. They show: Figure 1 - the basic structure of the compliant mechanism of the electromagnetically compensating beam balance according to the invention; Figure 2 - a planar embodiment of the compliant mechanism; Figure 3 - a first, space-optimized embodiment of the compliant mechanism; Figure 4 - a second, space-optimized embodiment of the compliant mechanism; Figure 5 - a third embodiment of the compliant mechanism; Figure 6 - a fourth embodiment of the compliant mechanism; Figure 7 - a fifth embodiment of the compliant mechanism; and Figure 8 - a sixth embodiment of the compliant mechanism.

[0021] The present invention introduces an electromagnetically compensating beam balance which is Figure 1It is initially depicted as a rigid body mechanism. The focus is on designing the mechanism as a flexible mechanism that can preferably be manufactured monolithically, i.e., from a single semi-finished product.

[0022] The beam balance can have weighing pans, each arranged on one side of the transmission lever (1). A distinction can be made between fixed weighing pans (110a,b) and gimbal-mounted, suspended weighing pans (111a,b). However, any combination of the weighing pan coupling is also conceivable. The weighing pans are each mechanically attached to the so-called load-bearing element (70a,b), which is guided relative to a frame (15) by means of a lever mechanism. This lever mechanism consists of four joints and four links and is referred to in gear engineering as a parallel crank. These are the frame (15), the lower parallel link (60a,b), the upper parallel link (61a,b), and the load-bearing element (70a,b), whereby the parallel links are each fixed to the frame and articulated via a coupling element (50a,b).With the help of this arrangement of the parallel crank, a quasi-linear guidance of the weighing pans in the vertical direction can be achieved, following the example of the Roberval platform scale.

[0023] The force exerted by the weight of the weights (100a,b or 101a,b) on the balance pans (110a,b or 111a,b) is transmitted on each side via the coupling element (50a,b) to the transmission lever (1). With equal lever lengths to the lever joint (10), the moments of weights with equal masses should exactly cancel each other out. If the weights have a mass difference, the static equilibrium is disturbed.

[0024] According to the principle of electromagnetic force compensation, the mechanism is held in equilibrium by one or more additional forces, so that the deflection of the transmission lever (1) is minimal (ideally = 0). These additional forces can be generated, for example, by an electromagnetic actuator. Its main components are a frame-mounted magnetic system (21a,b,c) and a moving coil (20a,b,c). When an electric current flows through a coil of one of these magnetic systems, a force proportional to the current is generated. The magnitude of the required current or additional force can be defined using a closed-loop control system. A fundamental component of such a control loop is a position sensor (200a,b and 201a,b), which can be used to determine the deflection of the transmission lever (1) relative to the frame (15).The position sensor (200a,b and 201a,b) consists of two parts: a frame-mounted unit (200a,b) and a moving unit (201a,b). It is often implemented as an optical sensor. The actuator consisting of 20c and 21c is an option available only in [various configurations]. Figure 1 This is illustrated. However, all embodiments shown in the following figures can also be supplemented with this actuator variant. This applies equally to the adjustment weights described below.

[0025] In a preferred embodiment, the beam balance has a lever adjustment weight (300). This weight is positioned on the lever joint (10) such that the center of gravity of the rotatably moved transmission lever (1) can be adjusted vertically close to the center of the lever joint (10). The horizontal center of gravity of the transmission lever (1) should remain as unaffected as possible. By using and additionally fine-tuning this adjustment weight (300), the sensitivity to inclination, i.e., the influence of floor inclinations on the measured quantity, is minimized.

[0026] A similar effect is achieved with the parallel link adjustment weights (360a,b and 361a,b), which can be attached to the lower parallel link (60a,b) and / or the upper parallel link (61a,b). In conjunction with the lever adjustment weight (300), systematic adjustment can achieve a state in which the tilt sensitivity is minimal and the scale's sensitivity to mass differences is maximized. This is not possible without this combination.

[0027] To further increase (maximize) the sensitivity of the beam balance, a preferred embodiment provides for the vertical joint distance between the lever joint (10) and the outer lever joints (51a,b) to be designed in such a way that the initial torsional stiffness of the compliant mechanism is compensated.

[0028] The realization of the previously described basic structure of an electromagnetically compensating beam balance with a monolithic compliant lever mechanism is in Figure 2 The diagram shows that this lever mechanism can be very effectively implemented as a planar structure. This offers several advantages in terms of manufacturing, as all essential machining steps can be performed from one direction and in a single setup. This allows the full positioning accuracy of the machine tool to be utilized. Furthermore, the simple cutouts allow for a high degree of flexibility in the choice of manufacturing process and, if necessary, material.

[0029] In contrast to the compliant lever mechanism known from DE 10 2016 106 695 B4, the presented lever mechanism allows for a separation of the functions of guidance and transmission. As shown in Figure 2As can be seen, the solid-state joints for force transmission are arranged vertically and subjected to a tensile force. The joints that, as part of the parallel cranks, fulfill the guiding function and are subjected to horizontal forces are horizontally oriented. Thus, the aforementioned sub-functions can be implemented with an optimized design of the lever mechanism.

[0030] A further advantage of the solution according to the invention lies in the degree of freedom F that can be realized (cf. VDI 2127). This should be F = 1 in order to obtain a positively driven transmission. If F < 1, an ideal transmission is immobile. This applies to the transmissions known from DE 10 2016 106 695 B4. For the one described in Figure 1 The gear ratio shown results in F = -1 (2x overdetermined) and for the gear ratio according to Figure 2F = 0 (overdetermined by 1x). According to theory, these gears are therefore immobile. However, their mobility can be ensured through precise manufacturing and / or elastic construction. Nevertheless, the mobility caused by unavoidable manufacturing deviations is restored by additional elastic deformation, which leads to an increase in stiffness, considered a disadvantage here.

[0031] For the in Figure 2In the illustrated embodiment of the planar compliant mechanism, only a single frame connection (15) is provided in the upper region of the structure, where the main force flow is transferred into the frame via the lever joint (10). The entire mechanism can also be suspended via an additional support structure (not shown). To minimize stiffness for the nominal load of the beam balance, the outer (lateral) joints on the lever (51a,b) are preferably arranged at a defined vertical distance from the lever joint (10). Furthermore, the load-bearing elements (70a,b) can be stiffened by connecting them to additional structural elements (not shown), and the topology of the transmission lever (1) can be optimized for high stiffness with minimal mass.Finally, the moving parts of the symmetrically arranged position sensors (201a,b) can be arranged at a large horizontal distance from the center of rotation, which further increases the sensitivity of the scale.

[0032] In Figure 3 A further, space-optimized embodiment of the compliant mechanism is disclosed. However, a disadvantage of this embodiment is the requirement for an additional connection (15a,b) to the frame. This connection can be either monolithic, achieved through an out-of-plane structure, or through two coupling points to the frame. The latter embodiment has the disadvantage that positioning errors can occur between the frame components. The monolithic variant increases the complexity of the geometry and thus also the difficulty of manufacturing such a mechanism.

[0033] The disadvantages of the in Figure 3The illustrated embodiment for the compliant lever mechanism (restricted frame attachment) can be combined with a Figure 4 The illustrated embodiment avoids this problem. Here, the coupling element (50a,b) is connected separately by means of two detachable mechanical coupling points (31). Of course, the problems of mechanical coupling points known to those skilled in the art must be taken into account; however, the detachable connection of the coupling element (50a,b) also allows for adjustment of the vertical distance between the joint (10) and the outer lever joints (51a,b) (stiffness compensation).

[0034] Figure 5 Figure 1 shows another embodiment in which a planar frame connection of all components is possible. This is achieved by relocating the translation lever (1) below the entire lever structure.

[0035] However, it is also within the scope of the invention to arrange the translation lever (1) above the overall structure ( Figure 6 This also allows for an advantageous frame connection. However, a planar implementation requires a stationary lever joint (10). This is subjected to compressive stress, which could negatively affect its function.

[0036] At the in Figure 7 In the illustrated embodiment of the compliant mechanism, the arrangement of the parallel cranks (60a,b and 61a,b) was varied. This prevents an increase in tilt sensitivity due to the symmetrical deformation of the transmission lever (1) and the elongation of the coupling elements (50a,b), since the reaction forces on side a compensate for those on side b.

[0037] At the in Figure 8In the disclosed embodiment, the linear guidance properties can be improved by means of a so-called double parallel crank. In the rigid body model, this results in perfect linear motion of the load-bearing element and the components rigidly connected to it (coil (20a,b), position sensor (201a,b) and weighing pan (110a,b)).

[0038] The presented embodiments of a planar, compliant lever mechanism do not preclude its design in the third spatial direction. For example, the parallel cranks of the quasi-linear guide can lie symmetrically outside the central plane of the lever mechanism.

[0039] Furthermore, the coupling strap does not necessarily have to be connected to the other links via two joints. It is sufficient to incorporate a flexible element with distributed compliance.

[0040] Likewise, the representation as an upper-tray weighing system does not preclude simultaneous or exclusive use as a lower-tray weighing system.

[0041] Finally, the position, number and wiring of the components of the electromagnetically compensating beam balance can also be varied analogously to the embodiments described in DE 10 2016 106 695 B4.

[0042] The two parallel cranks and / or the two parallel links (60a,b, 61a,b) can be movable separately and independently of each other in the embodiments shown above. Reference symbol list

[0043] 1- Translation lever 10- Lever joint 15- Frame 20a, b, c- Coil (moving) 21a, b, c- Magnet system (frame-fixed) 25- Rigid connection 30- No mechanical connection of intersecting lines 31- Detachable mechanical coupling points 50a, b- Coupling element 51a, b- Outer lever joints (possibly with vertical distance to the lever joint) 60a, b- Lower parallel link 61a, b- Upper parallel link 70a, b- Load bearing 100a, b- Weights 101a, b- Test weights 110a, b- Weighing pan (e.g., substitution weighing pan) 111a, b- Suspended weighing pan 120a, b- Cardan suspension 200a, b- Frame-fixed unit of the position sensor 201a, b- Moving unit of the position sensor 300 - Adjustment weight for translation lever 360a, b - Adjustment weight for lower parallel link 361a, b - Adjustment weight for upper parallel link

Claims

1. Electromagnetically compensating beam scale, comprising a two-armed, rotatably mounted transmission lever (1), each lever arm of which is coupled to an associated load receiver (70a,b) and which has, at least on one side, an actuator (20a,b; 21a,b) integrated in a closed control loop, wherein each load receiver (70a,b) will be or is guided linearly in the vertical direction by means of a lever mechanism, characterized in that the lever mechanism is in each case embodied as a parallel crank comprising an upper parallel link (61a) fixed to the frame and connected in an articulated manner via a coupling element (50a,b), a lower parallel link (60a), the load receiver (70a) and an upper parallel link (61b), a lower parallel link (60b) and the load receiver (70a).

2. Electromagnetically compensating beam scale according to Claim 1, characterized in that the transmission lever (1) has a horizontal actuator (20c, 21c) below a lever joint (10).

3. Electromagnetically compensating beam scale according to either of Claims 1 and 2, characterized in that it has an adjusting weight (300) for the transmission lever (1), the centre of gravity of which transmission lever can be adjusted close to a centre of a lever joint (10) in the vertical direction.

4. Electromagnetically compensating beam scale according to any of Claims 1 to 3, characterized in that it has an adjusting weight (360a,b and 361a,b) on both the upper and / or lower parallel link (60a,b and 61a,b).

5. Electromagnetically compensating beam scale according to any of Claims 1 to 4, characterized in that, in order to compensate for the rotational stiffness of the lever mechanism, both arms of the transmission lever (1) are coupled by means of outer lever joints (51a,b) to the respective coupling element (50a,b) in a manner vertically spaced apart from a lever joint (10).

6. Electromagnetically compensating beam scale according to any of Claims 1 to 5, characterized in that scale pans (110a,b) rigidly mounted to the load receivers (70a,b) and / or cardanically suspended scale pans (111a,b) are mechanically coupled.

7. Electromagnetically compensating beam scale according to any of the preceding claims, characterized in that each of the closed control loops has a respective electromagnetic actuator made up of a magnet system (21a,b,c) fixed to the frame and a coil arrangement (20a,b,c) which moves with it, and, in order to determine the deflection of the transmission lever (1) relative to the frame (15), comprises a position sensor made up of a unit (200a,b,c) fixed to the frame and a unit (201a,b,c) which moves with it.

8. Electromagnetically compensating beam scale according to any of the preceding claims, characterized in that the double-sided lever mechanisms are designed as a monolithic, planar overall structure.

9. Electromagnetically compensating beam scale according to any of the preceding claims, characterized in that the parallel cranks are coupled to and / or positioned on the transmission lever (1) in an optimized manner specific to the application.

10. Electromagnetically compensating beam scale according to Claim 8, characterized in that the parallel cranks each have two lower and upper parallel links arranged parallel in relation to each other.

11. Electromagnetically compensating beam scale according to any of Claims 7 to 10, characterized in that the position sensor is designed as an optical sensor.

12. Electromagnetically compensating beam scale according to any of Claims 5 to 11, characterized in that the vertical distance between the lever joint (10) and the outer lever joints (51a,b) is configured such that an initial rotational stiffness of the lever mechanism is compensated for.

13. Electromagnetically compensating beam scale according to any of Claims 2 to 12, characterized in that the lever joint (10) and / or the outer lever joints (51a,b) are in the form of solid body joints.