Top scale balance

#The present invention relates to a top scale balance and more particularly to a top scale balance having a Roberval's chain and a lever mechanism for load transmission. It is noted that the present invention can be applied not only to a so-called electronic balance having a load sensing portion of the electromagnetic equilibrium type, but also to a so-called scale having a load cell or the like serving as a load sensing portion.

#FIGS. 9(A) and (B) are plan and side views of a conventional top scale balance, and FIG. 10 shows the mechanism thereof. In such a top scale balance, a sample scale 20 is supported through a Roberval's chain 10 (which is also called a parallel guide). This restrains the sample scale 20 from being transversely displaced and/or inclined such that the sample scale 20 is vertically moved as horizontally maintained. This prevents the occurrence of an error due to offset placing of a sample on the sample scale 20, i.e., a so-called offset position error (which is also called a four-corner error).

#The Roberval's chain 10 has a structure in which a movable post 13 is connected to a stationary post 14 through two parallel horizontal members, i.e., upper and lower horizontal members 11, 12 each provided at both ends thereof with flexible portions 11a, 11b, 12a, 12b which serve as hinge portions. The sample scale 20 is supported by the movable post 13. A load acting on the sample scale 20 will be transmitted, through a lever 30 connected to the movable post 13, to a load sensing portion 40 such as a load cell, an electromagnetic equilibrium mechanism or the like.

#FIG. 1 is a side view of a mechanism illustrating an embodiment in which a first invention is applied to an electronic balance of the electromagnetic equilibrium type;

#FIGS. 2(A)-(D) are views illustrating the arrangement of a specific example in which the upper horizontal member 11 is lower in rigidity than the lower horizontal member 12 in the embodiment in FIG. 1;

#FIG. 3 is a view illustrating the operation made at the time when an offset placed load is applied to the mechanism in FIG. 1;

#It is an object of the present invention to provide a top scale balance which can solve the problems above-mentioned of the conventional top scale balance, and in which, even though the mechanism is made in a compact and thin design with the lever ratio increased, a change in offset position error dependent on the magnitude of a load is small, thus enabling the top scale balance to be made highly precise in a compact and thin design with a less offset position error.

#To achieve the object above-mentioned, a first invention provides a top scale balance comprising, as shown in FIG. 1: a Roberval's chain 10 having (i) two, upper and lower, parallel horizontal members 11, 12 each provided at both ends thereof with flexible portions, (ii) a movable post 13 and (iii) a stationary post 14 connected to the movable post 13 through the horizontal members 11, 12; a sample scale 20 supported by the movable post 13; a lever 30 having an elastic fulcrum 31 and a point of force 32, the point of force 32 being connected to the movable post 13 through an elastic connection fulcrum 51 positioned between the upper and lower horizontal members 11, 12; and a load sensing portion 40 to which a load on the sample scale 20 is to be transmitted by the lever 30, and this top scale balance is characterized in that, out of the upper and lower horizontal members 11, 12, the horizontal member nearer to the elastic connection fulcrum 51 (the lower horizontal member 12 in FIG. 1) is greater in rigidity than the other horizontal member (the upper horizontal member 11 in FIG. 1).

#The rigidity of the horizontal member 11 or 12 also includes the rigidity of each of the flexible portions 11a, 11b or 12a, 12b formed at both ends thereof, and more precisely includes the rigidity of the movable or stationary post 13 or 14 in the horizontal direction. Accordingly, the first invention above-mentioned also includes an arrangement in which the rigidities of the horizontal members 11, 12 themselves including the rigidities of the flexible portions 11a, 11b, 12a, 12b at both ends thereof, are substantially equal to each other and in which the movable or stationary post 13 or 14 connected to one ends of the horizontal members 11, 12 is weakened such that the rigidity of the horizontal member further from the elastic connection fulcrum 51 (the upper horizontal member 11 in FIG. 1) is apparently weakened as compared with the rigidity of the other horizontal member (the lower horizontal member 12 in FIG. 1).