Electromagnetic-force-balancing-type electronic balance

Disclosed is an electromagnetic-force-balancing-type electronic balance having a magnetic circuit which includes a yoke 4, a first permanent magnet 1, a pole piece 3, a second permanent magnet 2 and a cover 5. A connection member 7 made of a magnetic material is disposed in an air-gap region included in the magnetic circuit. The connection member 7 may be disposed between the second permanent magnet 2 and the cover 5 or between the cover 5 and the yoke 4 to facilitate assembling operations. Alternatively, first and second gap-defining members 8, 9 made of a non-magnetic material may be disposed, respectively, between the first permanent magnet 1 and the yoke 4 and between the second permanent magnet 2 and the cover 5, to provide symmetric properties in magnetic sub-circuits so as to counteract an imbalance in hysteresis characteristic. The present invention can suppress adverse influences of a magnetic resistance caused by an air gap existing in the magnetic circuit of the electronic balance.

TECHNICAL FIELD

The present invention relates to a magnetic circuit suitably usable in an electromagnetic-force-balancing-type electronic balance.

BACKGROUND ART

As shown inFIG. 9, a magnetic circuit incorporated in a conventional electromagnetic-force-balancing-type (or electromagnetic-force-compensation-type) electronic balance comprises first and second permanent magnets1,2disposed such that their magnetic poles of the same polarity are opposed to one another, a pole piece3sandwiched between the first and second permanent magnets1,2, a tubular-shaped yoke4having an opening only on one side thereof and an inner surface in contact with the first permanent magnet1, and a pair of covers5a,5bin contact with or adjacent to the second permanent magnet2(see, for example, the following Patent Publication 1). The electromagnetic-force-balancing-type electronic balance has a movable assembly10which comprises a movable lever11, a force-coil mounting plate12fixed to one end of the movable-section lever11, and a force coil13fixedly mounted to the force-coil mounting plate12. The electronic balance also has a weighting pan (not shown) fixed to the other end of the movable-section lever11. The electronic balance is designed to control a current to be applied to the force coil13so as to allow the force coil13to be kept at a position approximately concentric to the pole piece3, and subject a current value detected from the force coil13to various corrections so as to calculate a weight of an object placed on the weighting pan.

In a design stage of the magnetic circuit, with a view to reliably fastening between the cover pair5a,5band the yoke4through screwing or the like, regardless of machining tolerances and assembling errors in the components, an air gap A has been set between the cover pair5a,5band the second permanent magnet2, as a margin of error (see, for example, the paragraph [0024] of the Patent Publication 1).

The air gap A is likely to be entirely or partly left only on the side of the second permanent magnet2after assembling of the magnetic circuit. The remaining air gap creates a magnetic resistance in the magnetic circuit to cause a problem, such as hysteresis. This problem will be specifically described with reference toFIG. 10, whereinFIG. 10(a) is a sectional side view showing the magnetic circuit incorporated in the conventional electromagnetic-force-balancing-type electronic balance, andFIG. 10(b) is an equivalent magnetic circuit thereof. As shown inFIG. 10(a), there are two magnetic sub-circuits: a first magnetic sub-circuit21extending from one of the magnetic poles of the first permanent magnet1to the other magnetic pole of the first permanent magnet1through the yoke4and a space between the yoke4and the pole piece3; and a second magnetic sub-circuit22extending from one of the magnetic poles of the second permanent magnet1to the other magnetic pole of the second permanent magnet2through the cover pair5a,5b, the yoke4and the space between the yoke4and the pole piece3.

Respective equivalent magnetic sub-circuits23,24of the first and second sub-circuits21,22are shown inFIG. 10(b). If a certain air gap exists between the cover pair5a,5band the second permanent magnet2in the second magnetic sub-circuit22, a magnetic resistance RA proportional to the air gap will be created in the second permanent magnet2to cause an imbalance between the first and second magnetic sub-circuits21,22. As to this problem, the inventors found that the influence of hysteresis on change in current flowing through the force coil13becomes more prominent as a current to be applied to the force coil is increased or as a required torque for the movable assembly10is increased, as described in more detail below.

In response to a magnetic field externally applied to a ferromagnetic material, such as iron, magnetic domain walls are moved to produce magnetization in a direction of the applied magnetic field, and the number of magnetic domains oriented in the magnetic field direction is increased to generate magnetization. Then, when the magnetic field intensity is further increased, the entire crystal structure of the material has only magnetic domains oriented in the magnetic field direction, and the magnetization reaches saturation. In this process, when the ferromagnetic material is a high-purity metal, the magnetic domain wall movement can be induced easily, or the magnetization reaches saturation only by a low intensity of magnet field. In contrast, if the ferromagnetic material contains impurities, the magnetic domain wall movement is hindered, and a higher intensity of magnetic field is required to allow the magnetization to reach saturation. Moreover, even after the external magnetic field is eliminated, the magnetization intensity will not return to zero to cause a remanent magnetization. As with the ferromagnetic material containing impurities, a remanent magnetization is caused by the air gap A existing in the magnetic circuit. The yoke4or the cover pair5a,5bis magnetized by the second permanent magnet2. Further, during the course of magnetization based on a magnetic field generated by the force coil13applied with a current, when the applied current is relatively low, a resulting magnetization intensity is limited to a small ratio relative to the magnetization intensity based on the permanent magnet, or to a small value relative to the saturation magnetization, and therefore the intensity of a resulting remanent magnetization will have a negligible small impact. In contrast, if a current is applied to the force coil13at a value allowing a magnetization intensity to reach saturation, a resulting remanent magnetization or hysteresis will have a non-negligible impact.

DISCLOSURE OF THE INVENTION

In view of the above problem, it is an object of the present invention to provide an electromagnetic-force-balancing-type electronic balance capable of suppressing adverse influences of a magnetic resistance caused by an air gap existing in a magnetic circuit.

It is another object of the present invention to provide an electromagnetic-force-balancing-type electronic balance capable of counteracting an imbalance in hysteresis characteristic.

It is another object of the present invention to provide an electromagnetic-force-balancing-type electronic balance capable of facilitating assembling operations thereof.

In order to achieve the above objects, the present invention has the following features. The first aspect of the invention provides an electromagnetic-force-balancing-type electronic balance having a magnetic circuit which includes a plurality of components consisting of a tubular-shaped yoke having an opening only on one side thereof, a first permanent magnet, a pole piece; a second permanent magnet, and a cover. In this magnetic circuit, the yoke is disposed to orient the opening in an upward direction. The first permanent magnet, the pole piece and the second permanent magnet are housed in an internal space of the yoke in this order along the upward direction, while allowing the first and second permanent magnets to be disposed such that their magnetic poles of the same polarity are opposed to one another. Further, the cover is disposed above the yoke. The electronic balance is characterized by comprising a magnetic connection member mechanically connecting between two selected from the components.

The electronic balance, according to the first aspect of the invention, has a particular function based on the above feature. Specifically, the connection member allows the two components having a gap therebetween to be connected to one another mechanically and magnetically therethrough. Thus, a magnetic resistance due to the gap included in the magnetic circuit can be eliminated.

According to the second aspect of the invention, in the electronic balance as in the first aspect of the invention, the connection member mechanically connects the second permanent magnet and the cover.

The electronic balance, according to the second aspect of the invention, has a particular function based on the above feature. Specifically, the connection member allows the second permanent magnet and the cover to be connected to one another mechanically and magnetically therethrough. In this case, the cover and the yoke are in contact with one another to allow the second permanent magnet and the yoke to be magnetically coupled to one another through the connection member and the cover, and the second permanent magnet and the pole piece are in contact with one another, so as to form a second magnetic sub-circuit extending from one of the magnetic poles of the second permanent magnet to the other magnetic pole of the second permanent magnet through the connection member, the cover, the yoke, a space between the yoke and the pole piece, and the pole piece. Further, the yoke and the first permanent magnet are in contact with one another so as to form a first magnetic sub-circuit extending from one of the magnetic poles of the first permanent magnet to the other magnetic pole of the first permanent magnet through the yoke, the space between the yoke and the pole piece, and the pole piece. Thus, a magnetic resistance due to the gap included in the first or second magnetic sub-circuits can be eliminated.

According to the third aspect of the invention, in the electronic balance as in the second aspect of the invention, the cover is formed with a through-hole, and the connection member is disposed to protrude upward from a top surface of the cover through the through-hole.

The electronic balance, as in the third aspect of the invention, has a particular function based on the above feature. Specifically, the connection member disposed to protrude upward from the top surface of the cover through the through-hole formed in the cover can be fixed while being kept in contact with a wall surface defining the through-hole.

According to the fourth aspect of the invention, in the electronic balance as in the first aspect of the invention, the connection member mechanically connects the yoke and the cover.

The electronic balance, as in the fourth aspect of the invention, has a particular function based on the above feature. Specifically, the connection member allows the yoke and the cover to be connected to one another mechanically and magnetically therethrough. In this case, a second magnetic sub-circuit is formed to extend from one of the magnetic poles of the second permanent magnet to the other magnetic pole of the second permanent magnet through the cover, the connection member, the yoke, a space between the yoke and the pole piece, and the pole piece. Further, a first magnetic sub-circuit is formed to extend from one of the magnetic poles of the first permanent magnet to the other magnetic pole of the first permanent magnet through the yoke, the space between the yoke and the pole piece, and the pole piece.

According to the fifth aspect of the invention, in the electronic balance as in the fourth aspect of the invention, the connection member mechanically connects the cover and an outer periphery of the yoke.

The electronic balance, as in the fifth aspect of the invention, has a particular function based on the above feature. Specifically, the connection member allows the cover and the outer periphery of the yoke to be connected to one another mechanically and magnetically therethrough.

According to the sixth aspect of the invention, the present invention also provides an electromagnetic-force-balancing-type electronic balance having a magnetic circuit which includes a plurality of components consisting of a tubular-shaped yoke having an opening only on one side thereof, a first permanent magnet, a pole piece, a second permanent magnet, and a cover. In this magnetic circuit, the yoke is disposed to orient the opening in an upward direction. The first permanent magnet, the pole piece and the second permanent magnet are housed in an internal space of the yoke in this order along the upward direction, while allowing the first and second permanent magnets to be disposed such that their magnetic poles of the same polarity are opposed to one another. Further, the cover is disposed above the yoke. The electronic balance is characterized by comprising a gap-defining member disposed between two selected from the components, so as to reduce a difference in magnetic resistance between a first magnetic sub-circuit formed of the first permanent magnet, the yoke and the pole piece, and a second magnetic sub-circuit formed of the second permanent magnet, the cover, the yoke and the pole piece.

The electronic balance, as in the sixth aspect of the invention, has a particular function based on the above feature. Specifically, the two components are mechanically connected to one another through the gap-defining member disposed therebetween with a given magnetic resistance of the gap-defining member. Thus, a difference between respective magnetic resistances included in the first and second magnetic sub-circuits can be effectively reduced.

According to the seventh aspect of the invention, in the electronic balance as in the sixth aspect of the invention, the gap-defining member includes a first gap-defining member disposed between the first permanent magnet and the yoke, and a second gap-defining member disposed between the second permanent magnet and the cover.

The electronic balance, as in the seventh aspect of the invention, has a particular function based on the above feature. Specifically, in the case where the second permanent magnet and the cover are connected to one another through the second gap-defining member disposed therebetween with a given magnetic resistance of the second gap-defining member, and the cover and the yoke being in contact with one another, the second permanent magnet and the cover can be magnetically coupled to one another through the second gap-defining member and the cover, with a predetermined magnetic resistance. Further, the first permanent magnet and the yoke can be mechanically connected to one another through the first gap-defining member disposed therebetween with a given magnetic resistance of the first gap-defining member. In this case, a second magnetic sub-circuit is formed to extend from one of the magnetic poles of the second permanent magnet to the other magnetic pole of the second permanent magnet through the second gap-defining member, the cover, the yoke, a space between the yoke and the pole piece, and the pole piece. Further, a first magnetic sub-circuit is formed to extend from one of the magnetic poles of the first permanent magnet to the other magnetic pole of the first permanent magnet through the first gap-defining member, the yoke, the space between the yoke and the pole piece, and the pole piece.

As above, the first aspect of the invention has the aforementioned function. Thus, a magnetic resistance due to the gap included in the magnetic circuit can be eliminated to achieve an effect of being able to reduce the influence of hysteresis.

The second aspect of the invention has the aforementioned function. Thus, the connection member can be simply inserted between the second permanent magnet and the cover to obtain the effect of the first aspect of the invention. After the first permanent magnet, the pole piece and the second permanent magnet are fixedly housed in the internal space of the yoke, and a force coil is inserted into the internal space of the yoke, the cover may be fixedly attached to the yoke while interposing the connection member therebetween to achieve an effect of being able to readily perform an adjustment for eliminating a gap.

The third aspect of the invention has the aforementioned function. Thus, in addition to the effect of the first aspect of the invention, the connection member disposed to protrude upward from the top surface of the cover and fixed while being kept in contact with the wall surface defining the through-hole can achieve an effect of being able to provide an electromagnetic-force-balancing-type electronic balance having enhanced adjustability and high process yield.

The fourth aspect of the invention has the aforementioned function. Thus, the same function as those in the invention as in the second aspect of the invention can be achieved.

The fifth aspect of the invention has the aforementioned function. For example, the yoke may be formed to have a height allowing the second permanent magnet to reliably protrude from the yoke, and the connection member may be formed to have a height coming into contract with an outer peripheral surface. Thus, in addition to the effects obtained by the invention as in the fourth aspect of the invention, the fifth aspect of the invention can achieve an effect of being able to provide an electromagnetic-force-balancing-type electronic balance having enhanced adjustability and high process yield.

The sixth aspect of the invention has the aforementioned function. Thus, a thickness and/or material of the gap-defining member disposed in the magnetic circuit can be appropriately selected to achieve an effect of being able to desirably adjust hysteresis characteristics in the magnetic circuit.

The seventh aspect of the invention has the aforementioned function. Thus, the magnetic resistances of the first and second gap-defining members in the magnetic circuit are inserted relative, respectively, to the first and second permanent magnets to achieve an effect of being able to counteract an imbalance in hysteresis characteristic.

BEST MODE FOR CARRYING OUT THE INVENTION

First Embodiment

As shown inFIG. 1, a magnetic circuit for an electromagnetic-force-balancing-type (or electromagnetic-force-compensation-type) electronic balance according to a first embodiment of the present invention comprises first and second permanent magnets1,2disposed such that their north (N) poles are opposed to one another, a pole piece3sandwiched between the first and second permanent magnets1,2, a tubular-shaped yoke4having an opening only on one side thereof and an inner surface in contact with the first permanent magnet1, a pair of covers5a,5bin contact with or adjacent to the second permanent magnet2, and a connection member7disposed between the second permanent magnet2and the cover pair5a,5b. The electronic balance has a movable assembly10which comprises a movable lever11, a force-coil mounting plate12fixed to one end of the movable-section lever11, and a force coil13fixedly mounted to the force-coil mounting plate12. The electronic balance also has a weighting pan (not shown) fixed to the other end of the movable-section lever11. The connection member is made of a magnetic material. While the connection member7is preferably made of the same magnetic material as that of the cover pair5a,5band the yoke4, the material of the connection member7is not necessarily identical to the material of the cover pair5a,5band the yoke4, but may be any other suitable magnetic material. Further, the first and second permanent magnets1,2are not necessarily disposed such that their N poles are opposed to one another, but may be disposed such that their south (S) poles are opposed to one another. In either case, a polar direction should be appropriately set depending on a control circuit of the force coil13and other factor, because a direction of a torque to be produced in the force coil13is determined by the polar direction and a direction of a current to be applied to the force coil13. This electromagnetic-force-balancing-type electronic balance is assembled according to the following process.

Firstly, the first permanent magnet1is adhesively attached onto a central region of an inner surface of a bottom wall of the yoke4. Then, the pole piece3is adhesively attached onto a top surface of the first permanent magnet3. In this embodiment, each of the yoke4and the pole piece3is made of a ferromagnetic material, such as soft iron, and thereby magnetically attracted to the first permanent magnet1. Thus, each of the yoke4and the pole piece3can be adequately positioned only by an extremely low adhesive force. Then, the second permanent magnet2is adhesively attached onto a top surface of the pole piece3while orienting the second permanent magnet2relative to the first permanent magnet1such that their poles of the same polarity are opposed to one another. When the pole piece3has a thickness enough to avoid magnetic saturation, the first and second permanent magnets1,2are strongly attached to the pole piece3based on a magnetic force even through their poles of the same polarity are opposed to one another. Thus, the second permanent magnet2can be adequately positioned only by an extremely low adhesive force. In this step, the first permanent magnet1, the second permanent magnet2and the pole piece3should be disposed such that their vertical axes are aligned with a vertical axis of the yoke4, to prevent a magnet field from being inhomogeneously formed in the internal space of the yoke. Further, a special jig should be prepared to prevent the first permanent magnet1, the second permanent magnet2and the pole piece3from being magnetically attached onto an inner surface of a side wall of the yoke4. For example, the above attaching operations may be performed while inserting the jig through a through-hole formed in respective centers of the first permanent magnet1, the second permanent magnet2, the pole piece3and the bottom wall of the yoke4.

Plural number of the connection members7different in thickness are prepared in advance. A specific one of the connection members7which has a thickness allowing a height of the specific connection member7when placed on a top surface of the second permanent magnet7to become equal to that of an upper edge of the yoke4is selected, and adhesively attached onto the top surface of the second permanent magnet7. The above assembling steps 1 and 2 may be performed for the magnetic circuit independently.

Then, the assembled magnetic circuit is incorporated into a body of the electromagnetic-force-balancing-type electronic balance, and a position of the magnetic circuit is finely adjusted to allow the force coil13to be adequately located in a space between the pole piece3and the yoke4.

Lastly, the cover pair5a,5bis fixedly attached onto a top surface of the connection member7, for example, by fastening the cover pair5a,5bto the upper edge of the yoke4using screws. In this step, the force-coil mounting plate12should be inserted through two notches6a,6bformed in each of the cover5aand the cover5bwhile paying attention to preventing the force-coil mounting plate12from coming into contact with the cover pair5a,5b.

Second Embodiment

As shown inFIG. 3, except that the cover pair5a,5bis formed with a cutout for the connection member7, and the connection member7is disposed to protrude upward from a top surface of the cover pair5a,5bthrough the cutout, an electromagnetic-force-balancing-type electronic balance according to a second embodiment of the present invention has the same structure as that in the first embodiment. This electronic balance is assembled according to the following process (Assembling steps 1 and 3 are the same as those in the first embodiment, and their descriptions will be omitted.).

A specific one of the connection members7which has a thickness allowing the second permanent magnet2to adequately protrude upward from the top surface of the cover pair5a,5bis adhesively attached onto the top surface of the second permanent magnet2. The above assembling steps 1 and 2 may be performed for the magnetic circuit independently.

The cover5aand the cover5bare formed, respectively, with two cutout portions which form a through-hole in a state after the cover5aand the cover5bare combined together as the cover pair. The cover pair5a,5bis fixedly attached to the upper edge of the yoke4, for example, through screwing, while clamping the connection member7between the cutout portions. Preferably, the cutout of the cover pair5a,5bis formed to have a curvature accurately conforming to that of an outer periphery of the connection member7. However, after combined, the cover5aand the cover5bare not necessarily in contact with one another. Thus, the cutout of the cover pair5a,5bmay be formed to have a curvature slightly greater than that of the outer periphery of the connection member7to an extent that allows the cover pair5a,5band the outer periphery of the connection member7to have a contact area enough to suppress the influence of hysteresis due to a magnetic resistance of the contact region therebetween, at a negligible level in view of the entire magnetic circuit, and each of the cover5aand the cover5bmay be formed in a relatively small size to prevent the cover5aand the cover5bfrom coming into contact with one another. While a cover in this embodiment is divided into the cover5aand the cover5b, it may be a single-piece cover without dividing. In this case, a through-hole may be formed directly in the single-piece cover instead of the cutout portions. Further, the through-hole and the outer periphery of the connection member7may be machined to allow these to be fitted to one another. This machining can be performed with a small variation or error as compared with a sum of tolerances of the components, to achieve a required accuracy relatively easily. If the through-hole formed in the cover has an area greater than that of a sectional area of the connection member7, the cover may be fixed while displacing the center of the through-hole from that of the connection member7, so as to allow a part of a wall surface defining the through-hole to come into contact with the outer periphery of the connection member7.

Third Embodiment

As shown inFIG. 4, except that the yoke4is formed to have a height less than that of the second permanent magnet after assembling, and the connection member7is formed in a ring shape and sandwiched between the cover pair5a,5band the yoke4, an electromagnetic-force-balancing-type electronic balance according to a third embodiment of the present invention has the same structure as that in the first embodiment. This electronic balance is assembled according to the following process (Assembling steps 1, 3 and 4 are the same as those in the first embodiment, and their descriptions will be omitted.).

Plural number of the connection members7different in thickness are prepared in advance. A specific one of the connection members7which has a thickness allowing a height of the specific connection member7when placed on the upper edge of the yoke4to become equal to that of the second permanent magnet7is selected, and adhesively attached onto the upper edge of the yoke4.

Fourth Embodiment

As shown inFIG. 5, except that the yoke4is formed to have a height less than that of the second permanent magnet after assembling, and the connection member7is formed as a pair of half-ring-shaped connection members which are fixedly attached, respectively, onto the cover5aand the cover5bin such a manner that respective inner peripheries of the half-ring-shaped connection members7come into contact with an outer periphery of the yoke4, an electromagnetic-force-balancing-type electronic balance according to a fourth embodiment of the present invention has the same structure as that in the third embodiment. This electronic balance is assembled according to the following process (Assembling steps 1 and 3 are the same as those in the first embodiment, and their descriptions will be omitted.).

The half-ring-shaped connection members7are adhesively attached, respectively, onto the cover5aand the cover5b. Each of the half-ring-shaped connection members7is formed to have a thickness (height) allowing a contact between the respective inner peripheries of the connection members7and the outer periphery of the yoke4to be reliably ensured even if a difference between respective heights of the yoke4and the second permanent magnet2is varied.

After the cover5aand the cover5bare disposed at a height allowing the cover5aand the cover5bto come into contact with the top surface of the second permanent magnet2, the outer periphery of the yoke4and the connection members7are fixedly fastened, for example, through screwing. In this step, each of the inner peripheries of the connection members7is preferably formed to have a curvature completely identical to that of an outer periphery of the yoke4. However, after combined, the cover5aand the cover5bare not necessarily in contact with one another. Thus, each of the inner peripheries of the connection members7may be formed to have a curvature slightly greater than that of the outer periphery of the yoke4to an extent that allows each of the inner peripheries of the connection members7and the outer periphery of the yoke4to have a contact area enough to suppress the influence of hysteresis due to a magnetic resistance of the contact region therebetween, at a negligible level in view of the entire magnetic circuit, and each of the cover5aand the cover5bmay be formed in a relatively small size to prevent the cover5aand the cover5bfrom coming into contact with one another.

Each of the electromagnetic-force-balancing-type electronic balances according to the first to fourth embodiments operates as follows. When an object is placed on the weighting pan, the movable lever11is inclined to vertically move the force coil13. Thus, a current is applied to the force coil13to hinder the movement. As shown inFIG. 2, in response to the current applied to the force coil13, a vertical torque is produced in the force coil13based on an interaction between a first magnetic sub-circuit21formed of the first permanent magnet1, the yoke4, the pole piece3and a space between the pole piece3and the yoke4, and a second magnetic sub-circuit22formed of the second permanent magnet2, the connection member7, the cover pair5a,5b, the yoke4, the pole piece3and the space between the pole piece3and the yoke4. In this process, the vertical torque of the force coil13is produced approximately in proportion to the current applied to the force coil13. Thus, a weight of the object counterbalancing the vertical torque produced in the force coil13can be calculated from a value of the current. As seen inFIG. 2, an equivalent magnetic sub-circuit23of the magnetic circuit according to the above embodiments does not include the magnetic resistance RA due to the air gap A as in the equivalent magnetic sub-circuit23of the conventional magnetic circuit illustrated inFIG. 10. Thus, during weighting, a hysteresis phenomenon due to the gap A can be suppressed. Generally, it is generally difficult to use a special tool after insertion of the force coil13due to obstructions, such as frames, the movable lever11and other components of the electronic balance. In the above embodiments, an adjustment as measures against the air gap A can be performed before insertion of the force coil13. Further, after the force coil13is inserted into the interior space of the yoke4, the assembling process can be completed only by fixing the cover pair5a,5b, for example, through screwing. This makes it possible to facilitate the assembling operations.

Fifth Embodiment

As shown inFIG. 6, a magnetic circuit for an electromagnetic-force-balancing-type electronic balance according to a fifth embodiment of the present invention comprises first and second permanent magnets1,2disposed such that their poles of the same polarity are opposed to one another, a pole piece3sandwiched between the first and second permanent magnets1,2, a first gap-defining member in contact with the first permanent magnet1, a second gap-defining member in contact with the second permanent magnet2, a tubular-shaped yoke4having an opening only on one side thereof and an inner surface in contact with the first gap-defining member8, and a pair of covers5a,5bin contact with the second gap-defining member9. The electronic balance has a movable assembly10having the same structure as that in the first embodiment. Each of the first and second gap-defining members is made of a non-magnetic material. This electromagnetic-force-balancing-type electronic balance is assembled according to the following process.

The first gap-defining member8is placed on an inner surface of a bottom wall of the yoke4. Then, on a top surface of the first gap-defining member8the first permanent magnet1, the pole piece3and the second permanent magnet2are adhesively attached in this order in the same manner as the assembling step 1 in the first embodiment. Further, the second gap-defining member9is adhesively attached onto a top surface of the second permanent magnet2. The subsequent operations for assembling the cover5a, the cover5band the movable assembly10are the same as those in the first embodiment. In this step, plural number of the first gap-defining members8different in thickness and plural number of the second gap-defining members9different in thickness are prepared in advance. A specific one of the first gap-defining members8and a specific one of the second gap-defining members9which allow a height of the specific second gap-defining members9when placed on the second permanent magnet2to become equal to that of an upper edge of the yoke are selected and used. Preferably, the first gap-defining member8has a thickness approximately equal to that of the second gap-defining members9. However, as long as a difference between respective hysteresis characteristics of magnetic resistances RA, RB illustratedFIG. 7is negligibly small in view of the entire hysteresis characteristic, the thicknesses of the first and second gap-defining member8may be different from one another. This magnetic circuit makes it possible to reduce a difference between the respective magnetic resistances includes in a first magnetic sub-circuit21and a second magnetic sub-circuit22so as to counteract an imbalance in hysteresis characteristic.

Sixth Embodiment

As shown inFIG. 8, except that the second gap-defining member9is formed as a pair of half-ring-shaped gap-defining members which are fixedly attached, respectively, onto the cover5aand the cover5b, and a pair of half-ring shaped magnetic connection members are fixedly attached, respectively, to the half-ring-shaped second gap-defining members9, an electromagnetic-force-balancing-type electronic balance according to a sixth embodiment of the present invention has the same structure as that in the fifth embodiment. This electronic balance is assembled according to the following process.

The first gap-defining member8is placed on the inner surface of the bottom wall of the yoke4. Then, on a top surface of the first gap-defining member8the first permanent magnet1, the pole piece3and the second permanent magnet2are adhesively attached in this order in the same manner as the assembling step 1 in the first embodiment. Further, the half-ring-shaped second gap-defining members9are adhesively attached, respectively, onto bottom surfaces of the cover5aand the cover5b, and the half-ring shaped magnetic connection members are adhesively attached, respectively, onto bottom surfaces of the second gap-defining members9. The subsequent operations for assembling the cover5a, the cover5band the movable assembly10are the same as those in the fourth embodiment. In this embodiment, the first and second gap-defining members8,9are different in shape from one another. Thus, respective thicknesses of the first and second gap-defining members8,9are pre-set to allow their magnet resistances to become approximately equal to one another.

Each of the electromagnetic-force-balancing-type electronic balances according to the fifth and sixth embodiments operates in the same manner as the first to fourth embodiments. As seen inFIG. 7, an equivalent magnetic sub-circuit23of the magnetic circuit according to the fifth and sixth embodiments includes a magnetic resistance RA due to the second gap-defining member8as with the second equivalent magnetic sub-circuit23of the conventional magnetic circuit illustrated inFIG. 10. However, a magnetic resistance RB approximately equal to the magnetic resistance RA is created in a first equivalent magnetic sub-circuit22by the second gap-defining member to counteract an imbalance in hysteresis characteristic.

While the specific embodiments for suppressing the influence of hysteresis due to an air gap included in the magnetic circuit have been described, the present invention is not limited to the specific embodiments, but various modification and changes may be made therein, as long as the connection member is disposed between two components which is likely to have an air gap therebetween, or the gap-defining member is disposed to achieve matching between first and second magnetic sub-circuits.

Further, while the cover in the above embodiments is formed as a two-piece cover desirable in terms of assembling performance, the cover may be divided into three pieces or more, or may be formed as a single-piece cover without dividing.