Electronic balance

An electronic balance including a first Roberval's chain, a second Roberval's chain connected to the first Roberval's chain, and a detector. The first Roberval's chain includes a first fixed pillar, a plurality of first beams, and a first movable pillar supported by the first fixed pillar via the plurality of first beams. The first beams extends substantially in parallel with each other and in a first extending direction. The first movable pillar is configured to receive a load to be measured. The second Roberval's chain includes a second fixed pillar, a plurality of second beams, and a second movable pillar supported by the second fixed pillar via the plurality of second beams. The plurality of second beams extends substantially in parallel with each other and in a second extending direction substantially perpendicular to the first extending direction of the plurality of first beams. The second movable pillar is connected to the first movable pillar. The detector is configured to detect the load electrically via a movement of the first movable pillar.

CROSS-REFERENCE TO RELATED APPLICATIONS
 The present application claims priority under 35 U.S.C. .sctn.119 to
 Japanese Patent Application No. 10-338,954, filed Nov. 30, 1998, entitled
 "Electronic Balance." The contents of that application are incorporated
 herein by reference in their entirety.
 BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to an electronic balance which includes a
 plurality of Roberval's chains.
 2. Discussion of the Background
 Many of conventional electronic balances and scales utilize a Roberval's
 chain (also called a "parallel guide") to support a balance pan in order
 to limit the movement of the balance pan on which a mass is placed. The
 Roberval's chain includes a fixed pillar, a movable pillar, and upper and
 lower beams. The fixed pillar is fixed to or integrated into a frame of
 the scale. The movable pillar is supported by the fixed pillar via the
 upper and lower beams which are substantially in parallel with each other.
 Each beam is connected to the fixed pillar at one end and to the movable
 pillar at the other end through each fulcrum. The movable pillar supports
 the balance pan. Thus, the load applied to the balance pan is transmitted
 to an electric load detector through the movable pillar and a lever.
 Japanese Unexamined Utility Model Publication (Kokai) No. 63-35,924
 discloses a Roberval's chain which is constructed by assembling separate
 pieces of a fixed pillar, a movable pillar, and upper and lower beams.
 Japanese Unexamined Patent Publication (Kokai) No. 63-277,936 discloses a
 Roberval's chain which has a single piece construction carved out from a
 single flat plate. The contents of these references are incorporated
 herein by reference in their entirety.
 In these balances, however, it is difficult to make an adjustment because
 an adjustment of an offset error in the longitudinal direction of the
 upper and lower beams affects an offset error in the transverse direction
 of the upper and lower beams or vise versa.
 SUMMARY OF THE INVENTION
 According to one aspect of the invention, an electronic balance includes a
 first Roberval's chain, a second Roberval's chain connected to the first
 Roberval's chain, and a detector. The first Roberval's chain includes a
 first fixed pillar, a plurality of first beams, and a first movable pillar
 supported by the first fixed pillar via the plurality of first beams. The
 first beams extends substantially in parallel with each other and in a
 first extending direction. The first movable pillar is configured to
 receive a load to be measured. The second Roberval's chain includes a
 second fixed pillar, a plurality of second beams, and a second movable
 pillar supported by the second fixed pillar via the plurality of second
 beams. The plurality of second beams extends substantially in parallel
 with each other and in a second extending direction substantially
 perpendicular to the first extending direction of the plurality of first
 beams. The second movable pillar is connected to the first movable pillar.
 The detector is configured to detect the load electrically via a movement
 of the first movable pillar.
 According to another aspect of the invention, a scale includes a housing
 and an electronic balance provided in the housing. The housing has a
 circumferential portion, and first and second side portions at both ends
 of the circumferential portion. The electronic balance includes a first
 Roberval's chain, a second Roberval's chain connected to the first
 Roberval's chain, and a detector. The first Roberval's chain includes a
 first fixed pillar fixed to the housing, a plurality of first beams, and a
 first movable pillar supported by the first fixed pillar via the plurality
 of first beams. The first beams extends substantially in parallel with
 each other and in a first extending direction. The first movable pillar is
 configured to receive a load to be measured. The second Roberval's chain
 includes a second fixed pillar fixed to the housing, a plurality of second
 beams, and a second movable pillar supported by the second fixed pillar
 via the plurality of second beams. The plurality of second beams extends
 substantially in parallel with each other and in a second extending
 direction substantially perpendicular to the first extending direction of
 the plurality of first beams. The second movable pillar is connected to
 the first movable pillar. The detector is configured to detect the load
 electrically via a movement of the first movable pillar.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 The preferred embodiments will now be described with reference to the
 accompanying drawings, wherein like reference numerals designate
 corresponding or identical elements throughout the various drawings.
 FIG. 1 is a perspective view of first and second Roberval's chains (or
 parallel guides) of an electronic balance according to a first embodiment
 of the present invention, and FIG. 2 is a top plan view of the first and
 second Roberval's chains shown in FIG. 1. Referring to FIGS. 1 and 2, the
 first Roberval's chain 1 includes a first fixed pillar 11, a first movable
 pillar 12, and first upper and lower beams (13a and 13b). The first upper
 and lower beams (13a and 13b) extend substantially in a first direction
 (X) and substantially in parallel with each other. The first upper and
 lower beams (13a and 13b) connect the first fixed pillar 11 and the
 movable pillar 12. A fulcrum (E) is formed at both ends of each of the
 first beams (13a and 13b). The first Roberval's chain 1 of the first
 embodiment is formed by carving or making slits in a single flat plate
 which has an even cross-section. Similarly, a lever 2, a fulcrum (2a) for
 the lever 2, and a connecting member 3 are formed by carving or making
 slits in the single flat plate. The connecting member 3 connects the lever
 2 and the first movable pillar 12.
 A balance pan receiver 4 for supporting a balance pan 9 (see FIG. 7) is
 provided on, for example, fixed to the top of the first movable pillar 12.
 The first fixed pillar 11 has an extended portion (11a) which extends
 along the first direction (X). A load sensor 5 is provided between the
 lever 2 and the extended portion (11a) of the first fixed pillar 11. The
 load sensor 5 is, for example, a tuning fork type, a string vibration type
 or the like. Further, an electromagnetic restoring system may be used as
 the load sensor 5. In the electromagnetic restoring system, feedback
 control of a servomotor is carried out based on an output from a
 displacement sensor. In the string vibration type, a vibrator vibrates a
 string. A load applied to the balance pan 9 (see FIG. 7) is transmitted to
 the first movable pillar 12 and then tilts the lever 2 via the connecting
 member 3. Subsequently, the tilt of the lever 2 is detected by the load
 sensor 5 which in turn sends an electric signal in proportion to the load
 which is applied to the balance pan 9 (see FIG. 7).
 The second Roberval's chain 6 also includes a second fixed pillar 61, a
 second movable pillar 62, and second upper and lower beams (63a and 63b).
 The second upper and lower beams (63a and 63b) extend substantially in a
 second direction (Y) and substantially in parallel with each other. The
 second upper and lower beams (63a and 63b) connect the second fixed and
 movable pillars (61 and 62). The second Roberval's chain 6 is likewise is
 formed by carving or making slits in a single piece of a flat plate which
 has an even cross-section. The first and second movable pillars (12 and
 62) are connected by screws 64 such that the longitudinal direction (the
 first direction (X)) of the first upper and lower beams (13a and 13b) and
 the longitudinal direction (the second direction (Y)) of the second upper
 and lower beams (63a and 63b) are substantially in perpendicular.
 Each fulcrum (e) is formed at each end of each of the second beams (63a and
 63b). Two thin portions (t) which have narrow widths in the first
 direction (X) are formed between two fulcrums of each of the second beams
 (63a and 63b).
 The first fixed pillar 11 of the first Roberval's chain 1 and the second
 fixed pillar 61 of the second Roberval's chain 6 are fixed onto a fixed
 part such as a base or a frame of a scale.
 Generally, the Roberval's chain has a function to keep the balance pan from
 turning over or tilting, and to reduce an "offset error," which is an
 error caused by an offset load applied on the balance pan. The offset
 error is reduced by precisely adjusting the parallel degree between the
 upper and lower beams of the Roberval's chain. In other words, in order to
 reduce the offset error, the parallel degree between the upper and lower
 beams is adjusted such that the distances between fulcrums disposed at the
 both ends of the upper and lower beams are in agreement. The parallel
 degree described above depends upon the degree of an allowable offset
 error, i.e., the accuracy of a balance. However, it is usually in the
 order of 1 .mu.m to 10 .mu.m. It is difficult to obtain an acceptable
 parallel degree simply by processing its parts with precision. Hence,
 after an electronic balance is assembled, it is necessary to adjust the
 offset errors while a position of a mass on the balance pan is changed.
 Conventionally, to adjust the offset error, with respect to a single
 Roberval's chain, an offset error in the longitudinal direction of the
 beam as well as in a direction perpendicular to the longitudinal direction
 are adjusted, as the position of a mass is changed on the balance pan.
 For example, in a Roberval's chain of the single piece construction type,
 such an adjustment can be achieved by shaving off a part of each fulcrum
 at the ends of the upper and lower beams, which corresponds to front and
 back as well as right and left. The Roberval's chain which is provided
 with adjustment mechanisms which slightly move the positions of fixed
 points between the fixed pillar and each fulcrum. In such a Roberval's
 chain, the adjustment mechanisms at corresponding points are adjusted
 accordingly. Because the single piece construction Roberval's chain has a
 thin width in the transverse direction of the beams, its strength in the
 transverse direction is less than that in the longitudinal direction.
 Accordingly, the single piece construction Roberval's chain is more
 susceptible to offset errors in the transverse direction. As a result, the
 Roberval's chain of the single piece construction has a difficulty in
 dealing with a large weight or supporting a large balance pan.
 According to the first embodiment, a tilt or a movement of the balance pan
 supported by the balance pan receiver 4 is limited by both the first and
 second Roberval's chains (1 and 6). Accordingly, the offset errors caused
 by an offset load applied on the balance pan are significantly reduced by
 the function of the first and second Roberval's chains (1 and 6).
 In the first Roberval's chain 1, a distance between the two fulcrums (E)
 along the first direction (X) (longitudinal direction of the first beams
 (13a and 13b) is long. Hence the parallel degree between these first beams
 (13a and 13b) can be adjusted with relative ease, thereby reducing the
 offset error in the first direction (X) easily. On the contrary, it is
 difficult to adjust the offset error in the second direction (Y) (a
 direction perpendicular to the longitudinal direction (X) of the first
 beams (13a and 13b)), because each first beams (13a and 13b) does not have
 a large width in the second direction (Y). In the second Roberval's chain
 6, it is difficult to adjust the offset error in the first direction (X)(a
 direction perpendicular to the longitudinal direction of the second beams
 (63a and 63b)), because each of the first beams (63a and 63b) does not
 have a large width in the first direction (X). However, because the
 distance between the two fulcrums (e) along the second direction (Y)
 (longitudinal direction of the second beams (63a and 63b) is long, the
 parallel degree between these beams (63a and 63b) can be adjusted with
 relative ease, thereby reducing the offset error in the second direction
 (Y) easily.
 Therefore, by adjusting only the offset error in the first direction (X)
 with respect to the first beams (13a and 13b) of the first Roberval's
 chain 1 and only the offset error in the second direction (Y) with respect
 to the second beams (63a and 63b) of the second Roberval's chain 6, the
 first and second Roberval's chains (1 and 6) effectively function to
 reduce the offset errors caused by the offset load applied to the balance
 pan. Accordingly, the electronic balance of the first embodiment
 significantly reduces the offset errors in all directions.
 Furthermore, in the second Roberval's chain 6, the thin portions (t) which
 have narrow widths in the transverse direction (the first direction (X))
 of the second beams (63a and 63b) are formed in each of the second beams
 (63a and 63b). Thus, when a load which is offset in the first direction
 (X) is applied on the balance pan and forces the first movable pillar 12
 to tilt in the first direction (X), the second beams (63a and 63b) flex at
 the thin portions (t) and prevent distortion between the first and second
 Roberval's chains (1 and 6). Hence, the effect on the second Roberval's
 chain 6 due to the load which is offset in the first direction (X) is
 reduced.
 In the first embodiment, the offset error in the transverse direction (the
 second direction (Y)) of the first Roberval's chain 1 is reduced despite
 the use of the first Roberval's chain 1 having a relatively low stiffness
 in the second direction (Y), and the offset errors is easily adjusted.
 In the first embodiment, an electronic balance is provided with the second
 Roberval's chain in addition to the first Roberval's chain. The second
 Roberval's chain 6 extends in the second direction (Y) substantially
 perpendicular to the longitudinal direction (the first direction (X)) of
 the first Roberval's chain 1. The movable pillar 62 of the second
 Roberval's chain 6 is connected to the first movable pillar (12) of the
 first Roberval's chain 1. By constructing an electronic balance as such,
 the first Roberval's chain 1 mainly supports an offset load in the first
 direction (X), and the second Roberval's chain 6 mainly supports an offset
 load in the second direction (Y). Thereby, offset errors in the first and
 second directions (X and Y) are individually adjusted by adjusting the
 first and second Roberval's chains (1 and 6), respectively. Consequently,
 the offset errors is easily adjusted, and the stiffness in the transverse
 direction (the second direction (Y)) is effectively reinforced.
 Thus, when a load which is offset in the transverse direction (the second
 direction (Y)) of the first Roberval's chain 1 is applied on the balance
 pan, the second Roberval's chain 6 receives the torsional force exerted on
 the first Roberval's chain 1. Namely, even if the first Roberval's chain 1
 has a relatively low stiffness in its transverse direction (the second
 direction (Y)), the second Roberval's chain has enough stiffness in that
 second direction (Y) to support the offset load. Thus, even if an
 electronic balance utilizes Roberval's chains of the single piece
 construction type, the offset errors in the first and second directions (X
 and Y) are reduced, and so the electronic balance with highly accuracy can
 be obtained.
 Furthermore, because the first Roberval's chain 1 mainly supports an offset
 load in the first direction (X) and the second Roberval's chain 6 mainly
 supports an offset load in the second direction (Y), an adjustment of the
 offset error in each direction is independently made with respect to the
 first and second Roberval's chains (1 and 6) by correcting offset error in
 their respective longitudinal directions. Hence, the offset errors in the
 first and second directions (X and Y) are adjusted, and their adjustments
 do not affect each other.
 Additionally, because the second Roberval's chain 6 does not need to have
 an enough stiffness in its transverse direction (the first direction (X))
 as described above, the second Roberval's chain 6 may be made of the
 single plate. Thus, the cost and size of the second Roberval's chain 6 or
 in turn, the electronic balance, becomes small.
 FIG. 3 is a perspective view of first and second Roberval's chains of an
 electronic balance according to a second embodiment of the present
 invention. Although the first embodiment described above uses the single
 piece construction for the second Roberval's chain 6, the present
 invention is not limited as such. Referring to FIG. 3, the second
 Roberval's chain 6 includes a second fixed pillar 61, and second upper and
 lower beams (63a and 63b). In the second embodiment of the present
 invention, the second fixed pillar 61 as well as the second upper and
 lower beams (63a and 63b) are separate parts, and the second Roberval's
 chain 6 is assembled from those separate parts. Namely, one end of each of
 the second upper and lower beams (63a and 63b) is connected to the second
 fixed pillar 61 via a screw (64a), and the other end of each of the second
 upper and lower beams (63a and 63b) is connected to the first movable
 pillar 12 of the first Roberval's chain 1 via a screw (64b). In the second
 embodiment, the first movable pillar 12 of the first Roberval's chain 1
 also operates as a second movable pillar of the second Roberval's chain 6.
 The second Roberval's chain 6 in the second embodiment is as effective as
 that of the first embodiment.
 Although the second beams (63a and 63b) in FIG. 3 do not have the thin
 portions (t), they may include the thin portions (t) as shown in FIGS. 1
 and 2. However, if the tilting of the movable pillar 12 in the first
 direction (X) does not cause and problems, the thin portions (t) are not
 necessary in both the first and second embodiments.
 When the offset load is applied on the balance pan in the first direction
 (X), the offset load tends to tilt the movable pillar 12 in the first
 direction (X), By constructing the second Roberval's chain 6 as such, the
 thin portions absorb such a tilting force and thus prevent its effect on
 the second Roberval's chain 6.
 Likewise, the first Roberval's chain 1 does not need to be limited to the
 single piece construction. FIGS. 4 and 5 show first and second Roberval's
 chains (1 and 6) of an electronic balance according to a third embodiment
 of the present invention. In the third embodiment, the first Roberval's
 chain 1 includes a first fixed pillar 11, a first movable pillar 12, and
 first upper and lower V-shaped beams (13a and 13b). Similar to the second
 embodiment, the first fixed pillar 11, the first movable pillar 12, and
 the first upper and lower V-shaped beams (13a and 13b) are separate parts
 which are assembled into the first Roberval's chain 1. Namely, two end
 portions (13c) of each of the first upper and lower V-shaped beams (13a
 and 13b) are connected to the first fixed pillar 11 via fulcrums (E) to
 form a triangle. The top end portion (13d) of each of the first upper and
 lower V-shaped beams (13a and 13b) is connected to the first movable
 pillar 12 via a fulcrum (E). In the third embodiment, because of the
 second Roberval's chain 6, the first Roberval's chain 1 does not have to
 have enough stiffness in the second direction (Y) nor the offset error in
 the second direction (Y) does not have to be adjusted with respect to the
 first Roberval's chain 1. Accordingly, even if the angle .theta. at the
 top end portion (13d) of each of the first upper and lower V-shaped beams
 (13a and 13b) is reduced, or even if the first upper and lower beams (13a
 and 13b) are straight instead of the V-shape, the scale has small offset
 errors in the first and second directions (X and Y), resulting in high
 performance.
 FIGS. 6 and 7 show a scale containing an electric balance according to a
 fourth embodiment of the present invention. Referring to FIGS. 6 and 7,
 the electric balance is provided in a housing 7. The housing 7 has a
 circumferential portion (7a) and first and second end portions (7b and
 7c). For example, the housing 7 has a square shape and formed of a square
 pipe. The first Roberval's chain 1 similar to that shown in FIG. 1 is
 provided in the housing 7, while the second Roberval's chain 6 is mounted
 on the second end of the housing 7. A support 81 for holding a balance pan
 receiver 82 is connected to the second movable pillar 62 and to the first
 movable pillar 11 by screws 64. The balance pan receiver 82 has a flat
 plate form and is fixedly provided on the top of the support 81. A balance
 pan 9 is provided on the top of the balance pan receiver 82.
 In the fourth embodiment, the second fixed pillar 61 of the second
 Roberval's chain 6 extends along the periphery of the square second
 Roberval's chain 6 to surround the outsides of the second upper and lower
 beams (63a and 63b) and the outside of the second movable pillar 62. As
 such, the second fixed pillar 61 has a quadrilateral form and is connected
 to the first end (7b) of the housing 7 by screws. The first fixed pillar
 11 of the first Roberval's chain 1 is fixedly mounted on a lid 71 which is
 fixed onto the first end (7b) of the housing 7. In addition, the housing 7
 includes apertures 72 which are designed to accommodate a file. Thus, when
 the first Roberval's chain 1 requires an adjustment of the offset errors,
 a file is inserted from the apertures 72 in order to grind off some
 portions of the fulcrums (E).
 A balance case 101 has a box shape and an open top. The housing 7 with the
 first and second Roberval's chain (1 and 6) is inserted into and fixed to
 the balance case 101. The support 81 extends to the outside of the balance
 case 101 from its opening at the top. The balance pan receiver 82 and
 balance pan 9 is provided over the balance case 101 so that the open top
 of the balance case 101 is covered entirely. According to the fourth
 embodiment of the present invention, the offset errors in the second
 direction (Y) are reduced by the second Roberval's chain 6,
 notwithstanding both the balance pan 9 with a large surface and the first
 Roberval's chain 1 having a relatively small width in the second direction
 (Y). Further, because the housing 7 which is made of a square pipe has
 high torsional rigidity, the relative positions between the first and
 second fixed pillars (11 and 61) are maintained even if a large offset
 load is exerted on the scale. As a result, the first and second Roberval's
 chains (1 and 6) possess high torsional rigidity, and effectively reduce
 the offset errors in their respective longitudinal directions.
 To make the second Roberval's chain 6 effectively function, the second
 Roberval's chain 6 may resort to the strength of a square pipe housing 7.
 Thus, the first Roberval's chain 1 is provided inside a square pipe
 housing 7 along its axis while the second Roberval's chain is mounted on
 the second end (7c) of the square pipe housing 7. Accordingly, the
 strength of the square pipe housing 7 against distortion prevents the
 first and second Roberval's chains (1 and 6) from shifting from their
 relative positions when an offset load is applied to the electronic
 balance. As a consequence, the electronic balance becomes compact and has
 high strength as well as performance.
 FIG. 8 is a cross-sectional side view of a square pipe provided with a
 Roberval's chains of an electronic balance according to a fifth embodiment
 of the present invention. In the fourth embodiment, the first fixed pillar
 11 of the first Roberval's chain 1 is fixed to the lid 71. In the fifth
 embodiment, referring to FIG. 8, the first fixed pillar 11 of the first
 Roberval's chain 1 is fixed to an inner wall of the circumferential
 portion (7a) of the square pipe housing 7. Also referring to FIG. 8, the
 first Roberval's chain 1 includes an additional lever 20 in addition to
 the lever 2. The additional lever 20 is connected to the lever 2 by screws
 66 and engaged with the load sensor 5. The tilt of the lever 2 is
 transmitted to the load sensor 5 via the additional lever 20. Along this
 line, every embodiment described thus far may obviously adapt the second
 lever 20 as well as other modifications and variations.
 FIG. 9 is a cross-sectional side view of a scale which contains an
 electronic balance according to a sixth embodiment of the present
 invention. Referring to FIG. 9, the electronic balance includes an
 electromagnetic restoring system to detect a load applied to the balance
 pan 9. The electronic balance includes a servomotor 92 and a displacement
 sensor 93. When a mass is placed on the balance pan 9, the displacement
 sensor 93 detects a displacement of the lever 2 and send a displacement
 signal to the control circuit 94 that generates a correction current. This
 correction current flows through the servomotor 92. The correction current
 is controlled such that the servomotor 92 restores the lever 2 to its
 initial position.
 Obviously, numerous modifications and variations of the present invention
 are possible in light of the above teachings. It is therefore to be
 understood that, within the scope of the appended claims, the invention
 may be practiced otherwise than as specifically described herein.