GB2032631A

GB2032631A - An automatic weighing machine

Info

Publication number: GB2032631A
Application number: GB7937632A
Authority: GB
Original assignee: Pitney Bowes Inc
Current assignee: Pitney Bowes Inc
Priority date: 1977-06-17
Filing date: 1979-10-31
Publication date: 1980-05-08
Also published as: DE2826511C2; DE2826511A1; FR2406190A1; GB2032632B; GB2000305B; FR2406190B1; FR2406189B1; FR2406189A1; GB2032632A; GB2032631B; GB2000305A

Abstract

An automatic weighing scale for measuring the weight of a load 30 comprises a base 10, a movable load support structure 11 and a beam mounted for pivoting in a substantially vertical plane and operatively connected to the load support structure. The beam supports a balance weight 22 movable therealong, and is pivotable in response to movement of either the load support structure or the balance weight. Drive means 21 is connected to the balance weight for moving the balance weight along the beam and a balance detector is supported by the base to determine when the movement of the balance weight pivots the beam to a condition in balance with a weight supported by the load structure. A measuring means is operatively associated with the balance weight for measuring the movement thereof. <IMAGE>

Description

1 GB 2 032 631 A 1

SPECIFICATION An automatic weighing machine

This invention relates to an automatic weighing scale. This application is divided out of Patent Application No. 27153/78, (Serial No. 2000305).

According to the invention, there is provided an automatic weighing scale measuring the weight.of a load comprising:

a base; a load support structure carried by the base and 75 movable in response to a load to be weighed being placed thereupon; a shaft or rod mounted for pivoting in a substantially vertical plane and operatively connected to the load support and supporting a balance weight movable therealong, the shaft or rod being pivotably supported upon the base_ and being pivotablelin response to movement of either the load support structure of the balance weight, the movement of the load support structure being arranged to pivot the shaft in a sense opposite to that induced by the movement of the balance weight on one direction along the shaft orrod; drive means operatively connected to the 90 balance weight for moving the balance weight along the shaft or rod; a balance detector supported by the base to determine when the movement of the balance weight has pivoted the shaft or rod to a condition 95 in balance with a weight supported by load support structure; and measuring means operative associated with the balance weight for measuring the movement of said balance weight.

The balance detector may be supported upon a bendable connecting strip disposed between the base and said lead screw, the bendable connecting strip being caused to flex when the lead screw is caused to pivot, whereby the balance detector senses flexure in the strip and detects when the lead screw achieves a balanced condition.

The balance detector preferably includes a strain gauge means mounted on the bendable connecting strip.

The measuring means may comprise an encoder disc and a detector for measuring lead screw rotation.

The invention will be better understood from 115 the following particular and non-limiting description, given with reference to the accompanying drawings, in which:

Figure 1 is a schematic side view of one example of the analytical balance of the invention; 120 Figure 2 is a functional block diagram for the analytical balance shown in Figure 11; Figure 3 is a perspective view of the analytical balance illustrated in Figure 11; Figure 4 is a side view of the analytical balance 125 depicted in Figure 3; Figure 5 is a perspective view of a null indicating transducer shown in Figure 3; and Figure 6 is a perspective cutaway view of a balance weight and lead screw construction for eliminating backlash, as shown in the analytical balance of Figure 3.

Referring to Figure 1, 3 and 4, schematic and constructional views of an exemplary and non- limiting analytical balance according to the invention are shown. The balance has a base and frame member 10, to which a flexural load support structure 11 is attached. The flexural support 11 can be of a twin leaf spring construction, or it may be a flexural pivot assembly, etc. A pan 12 is attached to the movable wall 14 of the flexural support 11 (Fig. 1) and will deflect (arrow 13) when a load 30 to be weighed is placed thereupon.

A pivotable shaft 15 extends horizontally through a portal 16 in flexural wall 14. A flange member 17 attached to, and extending from, the wall 14 is attached to the pivotable shaft 15 via a thin metal band 18. Thus, as the pan 12 deflects (arrow 13), the pivotable shaft 15 will be caused to pivot in a counterclockwise direction as shown by arrow 19. A null indicating transducer shown generally by arrow 20, and illustrated in greater detail in Figure 5, senses the imbalance from the horizontal position that is experienced by the shaft 15. This balance sensing transducer 20 generates a signal which is sent to a motor 2 1. The motor 21 is attached to the shaft 15, and causes the shaft 15 to rotate in response to the unbalanced signal sent by transducer 20.

Shaft 15 has at least a portion thereof, that is threaded. In other words, shaft 15 can be thought of as a lead screw. When the lead screw (shaft 15) is rotated, a displaceable weight 22, that is threaded upon shaft 15, is caused to be advanced upon the shaft. The shaft 15 is rotated, and the weight 22 is moved, in such a manner as to bring shaft 15 into a horizontally balanced condition. In other words, the weight 22 is advanced along shaft 15 indirection 23 in orderto cause a clockwise pivoting 24 of shaft 15. The weight 22 is advanced until the transducer 20 senses a balanced condition, at which time, the motor 21 receives a signal from transducer 20 to stop rotating shaft 15.

The distance from the initial starting position from which the weight has moved, is indicative of the weight of the load that has been placed on pan 12. This distance can be measured from the amount of turns given to shaft 15 by the motor 11. For this purpose, the shaft 15 and motor 21 are operatively connected to a shaft encoder 25. This encoder 25 measures the number of revolutions of the shaft. This measurement is then fed to a counter, and is converted to a weight reading. The encoder used for this purpose may be of the type manufactured by Disc Instruments, Inc., Costa Mesa, California; Model EC80 Rotaswitch.

Figure 2 shows in block diagram how the analytical balance system operates. When an unknown weight is placed on the pan 12, the pan weight 30 causes a pivoting of shaft 15 -resulting in transducer 20 sensing an unbalance. The 2 transducer 20 will actuate the motor 2 1, which will rotate lead screw (shaft) 15. Shaft 15 will rotate to advance the known balance weight 22, which will pivot the shaft 15 in an opposite sense, to eliminate the imbalance being sensed by 70 transducer 20.

When a balanced condition is finally achieved, the encoder 25 will measure the total rotation of the lead screw 15, and will feed this information to counter (conversion unit) 26 to convert this measurement into a direct weight reading.

The calculation necessary for this conversion is well known, and involves a summation of the moments about shaft 15, i.e., the known weight 22 multiplied by the distance it has traveled along 80 shaft 15, equals the unknown weight 30 multiplied by the given distance it acts about the flexure pivot point of shaft 15. Solution of this moment equation will provide the weight of unknown weight 30.

Referring to Figures 3, 4, and 6; the movable weight 22 and shaft 15 are shown in greater detail. The shaft 15 is actually part of a pivotable assembly 75 comprising side arms 31 and 32, which form an elongated box-like frame. The assembly or frame 75 comprises a cross-member 34 connected across arms 31 and 32, respectively. Shaft 15 is rotatablyjournalled within this frame, as can be seen from bearing 35, which is shown in cross-member 34 depicted in Figures 3 and 4. The shaft 15 and the frame pivot is a unit assembly, and are supported in pivotable movement by bands 36 and 37 of transducer 20, as will be explained in more detail hereinafter. The frame is secured to block 40 which in turn sandwiches bands 36 and 37 between blocks 38 and 39 (see Figure 5).

In order that a fine adjustment be possible in the analytical balance, it is necessary to eliminate friction and backlash between the lead screw (shaft 15) and weight 22. To this, end the weight 22 has been designed to be primarily supported by rolling friction. Weight 22 is designed in box-like shape as seen in Figures 3 and 6. The weight itself carries three wheels 41, 42 and 43, which support weight 22 in rolling upon the arms 31 and 32 as shown. Since the arms 31 and 32 support the weight 22, very little friction is obtained at the lead screw connection between the weight 22 and shaft 15. Thus, the shaft 15 can very accurately and precisely move weight 22.

However, because a given amount of friction is still desirable to prevent backlash, a scheme was devised to load the lead screw with a given or predetermined amount of force. A leaf spring 44 is 120 secured to journal members 45 and 46, respectively, as shown in Figure 6. Journal members 45 and 46 are threaded to lead screw 15. The leaf spring 44 is secured to weight 22 via a shaft 48 which is journalled within weight 22. 125 The shaft 48 can be rotated by the head of a screw driver placed in slot 49, in order to put tension in leaf spring 44. This tension will load the lead screw with a desired or given force via journals 45 and 46. A lock nut 51 and set screw 130 GB 2 032 631 A 2 will hold shaft 48 in place in order to maintain the given tension on leaf spring 44. Thus, it will be seen that while the force of the weight 22 has been removed from the lead screw connections in journals 45 and 46, a predetermined force is put back in order to eliminate backlash via leaf spring 44.

Referring now to Figure 5, the transducer 20 will be explained in greater detail. As aforementioned, the pivotable frame 75 is secured to bands 36 and 37, respectively, and thus the weight of the pivotable frame 75 is carried by these bands. Bands 36 and 37 are each in turn secured and supported by hollowed out frame 33, which is rigidly attached to the main frame 10 via clamp 57, block 56, and wall 64. Thus, it is seen, that bands 36 and 37 support the major portion of the weight of the pivotable assembly 75, which support is directly traceable back to the main frame 10.

Bands 36 and 37 are made very thin relative to center band 70 of the transducer 20. This allows the pivotable assembly 75 to pivot easily, because bands 36 and 37 provide very little in the way of flexural resistance to this assembly.

Centre band 70, is designed to be relatively thick and somewhat resistive to bending of the pivotal frame 75, and is connected to L-shaped flange 54, which is bendably supported by twin leaf springs 71 and 72. Leaf spring 71 and 72 are anchored to cross-bar 80, which in turn is secured to hollowed-out frame 33. Band 70 is connected on its other end to the pivotable assembly 75 via block 40.

1004 Thus is can be seen, that the center band 70, while resistive to bending gives way to both bending and tension.

Thinner bands 36 and 37 are stiff or unyielding in tension or axial loading while the thicker center band is relatively soft and yieldable in tension or axial loading.

Thus, the two outer bands 36 and 37, while - shouldering most of the weight of pivotable assembly 75, are almost completely yieldable to the pivoting moments of the assembly 75.

The center band 70, on the other hand, is yieldable to bending moments induced by assembly 75, and almost completely yieldable in the tension or axial loading produced by the assembly75.

On the back of band 70 are mounted four strain gage elements 90, 90a, 91 and 91 a. respectively. These strain gage elements detect the pivoting of assembly 75 in either a clockwise or a counterclockwise direction, respectively.

The accuracy of the strain gages 90, 90a, 91 and 91 a for detecting pivoting of assembly 75 is greatly enchanced by the structural arrangement of bands 36, 37, and 70. These gages will not be distorted by the stresses induced by the weight of assembly 75, because this loading is supported by bands 36 and 37. These gage elements 90, 90a, 91 and 91 a will also be free from deflecting errors because of the bendable supporting bridge comprising leaf springs 71 and 72. The strain 3 GB 2 032 631 A 3 gages, which are electrically arranged in a 25 Wheatstone bridge will, however, be sensitive to the moments of the assembly 75.

Because of the unique construction of transducer 20 and the weight 22, it will be evident that a very sensitive automatic analytical balance 30 has been obtained.

Claims

1. An automatic weighing scale for measuring 35 the weight of a load comprising:

a base; a load supporting structure carried by the base and movable in response to a load to be weighed being placed thereupon; a shaft or rod mounted for pivoting in a substantially vertical plane and operatively connected to the load support, and supporting a balance weight movable therealong, the shaft or rod being pivotably supported upon the base and being pivotable in response to movement of either the load support structure or the balance weight, the movement of the load support structure being arranged to pivot the shaft in a sense opposite to that induced by the movement of the balance 50 weight in one direction along the shaft or rod; drive means operatively _connected to the balance weight for moving the balance weight along the shaft or rod; a balance detector supported by the base to determine when the movement of the balance weight pivoted the shaft or rod to a condition in balance with a weight supported by load structure; and measuring means operatively associated with:the balance weight for measuring the movement of said balance weight.

2. A scale according to claim 1 in which the balance detector is supported upon a bendable connecting strip disposed between the base and said lead screw, the bendable connecting strip being caused to flex when the lead screw is caused to pivot, whereby the balance detector senses flexure in the strip and detects when the lead screw achieves a balanced condition.

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3. A sea ' le accordirlg to claim 2 in which the balance detector includes strain gauge means mounted on the bendable connecting strip.

4. A scale according to claim 1, 2 or 3 in which the measuring means comprises an encoder and a detector for measuring lead screw rotation.

Printed for Her Majesty's Stationery Office by the Courier Press, Leamington Spa, 1980. Published by the Patent Office. 25Southampton Buildings, London, WC2A lAY, from which copies may he obtained.