GB2040479A

GB2040479A - Weighing Device

Info

Publication number: GB2040479A
Application number: GB7930087A
Authority: GB
Original assignee: Geosource Inc
Current assignee: Geosource Inc
Priority date: 1979-01-19
Filing date: 1979-08-30
Publication date: 1980-08-28
Also published as: JPS5598312A; FR2447015A1; DE2935311A1; AU5024379A; AR222187A1

Abstract

A device for detecting the weight of an article by measuring the pressure required to support the article is characterized by a weighing table 18 having a plurality of apertures 28 and a chamber 24 for receiving compressed air. The pressure required to support the gross weight of an article passing over the apertures is sensed by a pressure transducer 34 and translated into an electrical signal to represent the weight of the article. <IMAGE>

Description

SPECIFICATION Weighing Device and Method This invention relates to a weighing device and method, and more particularly to a weighing device capable of receiving articles to be weighed at high speeds.

In many manufacturing processes, especially in the canning industry, quantity control is a very important parameter. To achieve efficiency in the weighing function in a canning process, for example, it is necessary to eliminate manual weighing of the individual articles. Automatic high speed weighing systems are utilized to move large quantities of product over a conventional scale to separate the product falling outside a predetermined weight range.

In some high speed weighing systems conveyor belts are used to transport products. The conveying system transports the product to a weigh conveyor having a lightweight belt material, or an endless chain system, and operating in a fashion to move over a conventional scale. Thus, the weight of that product will be determined as it moves over the scale.

Since a conventional scale having either torsion or compression springs are utilized in these systems many disadvantages result. For example, there is a required damping time or settling time after each individual product is weighed. Since there must be an interruption in time between weighing individual products, a further problem of spacing the objects to be weighed is encountered. By moving a conveyor belt over the scale there is also mechanical vibration to overcome. Although dash-pots and other damping elements have been used to solve many of these problems, spring loaded scales tend to either overact or counteract.

Thus, the major disadvantage in existing weighing devices utilized for high speed weighing is the time for settling and spacing.

The weighing device provided in the present invention is capable of weighing articles at a high rate of speed. In one embodiment of the invention the weight of the article is calculated from the proportional relationship of the pressure needed to support the article and the area surfaced of the article to its numerical weight. The weighing device further provides a means of weighing products having a wide range of sizes and weights. The signal output of the weighing device is representative of the instantaneous displacement of a weigh transducer. This provides a means for calculating, via circuitry for performing differentiating functions, the velocity and acceleration of the signal. These latter signals are utilized to compute the weight of an article causing the displacement in the weigh transducer.

The weighing device in accordance with the present invention utilizes a weighing table having an air chamber which is connected to a compressor supplying pressurized air. Pressurized air forced through the support holes in the top of the weighing table forms an air cushion for supporting the article to be weighed. A portion of the pressure required to support the article as it passes over the weighing table may be sensed with a transducer which translates the support into an electrical signal for numerical weight calculation.

An outstanding feature of the present invention is the use of the weighing table to receive articles from a transport mechanism and carry the article across the weighing table in a frictionless manner, where the required pressure to support the article may be sensed and delivered to a pressure transducer. The transducer utilizes a linear variable differential transformer having a movable transformer core which may be attached to a diaphragm reacting to the pressure sensed under the article being weighed. The transformer core is thus displaced by the diaphragm motion and delivers a voltage signal to circuitry designed to derive the numerical weight from the magnitude of that signal.

One embodiment of the invention includes a single action diaphragm whereby the pressure sensed below the article being weighed is fed through a conduit to the diaphragm to directly move the transformer core. In an alternative embodiment, a differential diaphragm may also be utilized to perform this function, however, a reference pressure must be tapped from the air chamber unbalancing the diaphragm such that the pressure required to support the article moves the diaphragm toward a balanced condition, moving the transformer core in the process. In an alternative embodiment, the pressure transducer either single ended or differential may be any device suitable to convert a pressure to an electrical signal.

The present invention overcomes the disadvantages of damping, spacing requirements, and overall mechanical wear by its independence of the transporting mechanism. It is a general object of the invention to weigh a high volume of products independent of the means for transporting the products to the weighing device.

Further, the problems associated with spring loaded scales do not exist and vibrations from the transport mechanism are not factors to be considered in the accuracy of the weighing function. Further, since the scale is capable of transporting the article, the transport mechanism does not come in contact with this scale and thus a more conventional belt may be utilized to transport to and from the scale reducing mechanical wear problems.

A system for weighing an article in accordance with the above-noted outstanding features of the invention includes circuitry for interpreting the signal obtained from the transducer.

In one embodiment of the circuitry, the oscillatory motion of the transducer is sensed and signals representative of the instantaneous displacement, of the instantaneous velocity and of the instantaneous acceleration of the transducer are produced. These signals are used in a circuit to compute the weight of an article from the second order differential equation of the motion of the oscillating transducer responding to the presence of the article. In an alternative embodiment, the transducer's signal is filtered to produce a signal representative of the weight of the product.

The above noted outstanding features and other features and advantages of the present invention will become more apparent upon reading the following detailed description in reference to the drawings in which: Figure 1 is a diagrammatic front elevational view of a weighing system using a single action diaphragm in accordance with the principles of the present invention; Figure 2 is a diagrammatic top view of a weighing table and transport mechanisms in accordance with the principles of the present invention; Figure 3 is a partial schematic partial diagrammatic view of a pressure transducer utilized in the present invention; Figure 4 is a diagrammatic front elevational view of a weighing device utilizing a differential diaphragm in accordance with the principles of the present invention;; Figure 5 is a partial block, partial schematic diagram of a preferred form of the electrical portion of a system.

Referring now to the drawings and more particularly to Figure 1, a weighing system 10 is illustrated.

The weighing system 10 comprises a weighing device 12 and input and output transport mechanisms such as conveyances 14 and 1 6.

The pneumatic weighing device 12 includes a weighing table 1 8 having top and bottom planar members 20 and 22 defining an air chamber 24 therebetween. The weighing table 1 8 is further laterally enclosed with contoured sides 26 to facilitate movement of articles 11 from input conveyor 14 onto the weighing table 1 8 and ultimate discharge of articles 11 onto output conveyor 16.

As shown in Figure 2 the top planar member 20 of the weighing table 18 has a plurality of apertures 28. One of the apertures 30 is utilized for sensing the pressure under the article 11 and may be a different diameter than the remainder of the apertures 28. The sense hole 30 must be large enough so not to restrict the pressure under the article 11 from passing through. Guide rails 32 are also provided to ensure that the article 11 passes over the aperture 30.

The pneumatic weighing device 1 2 further comprises a pressure transducer 34 electrically connected by conductor 36 to circuitry 38 for deriving the numerical weight of the article 11 by the proportional relationship of the pressure to support the article 11 to the article weight.

Operationally, pressurized air is supplied to the air chamber 24 from a compressor 40 by way of a flexible conduit 42. As a result of the pressurized air being fed to air chamber 24, air is forced out of the weighing table 1 8 by way of apertures 28.

This escaped air forms an air cushion under the article 11 as it passes over the air table 1 8.

A conduit 44 connects the apertures 30 in the top member 20 of the weighing table 18 to the pressure transducer 34. The pressure transducer 34 comprises a linear variable differential transformer 46 having its transformer core attached to a diaphragm 48 which is directly connected to sense hole 30 by way of conduit 44.

Figure 3 iliustrates the pressure transducer 34 with a schematic view of the linear variable differential transformer 46. The linear variable differential transformer 46 is powered by a DC power source, Vi 49, which may be on the order of 1 5 volts. This DC signal is received by an oscillator circuit 50 which drives a first transformer coil 52. The transformer core 54 is rigidiy attached to a diaphragm 48. The diaphragm 48 may be a single action diaphragm as depicted in Figure 1 requiring a single conduit 44 for determining pressure from the sense hole 30, or a differential diaphragm requiring a reference pressure to be delivered from the air chamber 24 by a conduit 64 depicted by the dotted lines in Figure 3.As the transformer core 54 is moved between the first transformer coil 52 and a second transformer coil 56 the oscillating signal is coupled between coils and delivered to a detector circuit 57 where the signal is AC filtered and yields a DC output voltage, VO,58.

Calibrating the pneumatic weighing device 12 requires that the initial output voltage 58 from the low voltage differential transformer 46 be equal to zero.

As the article 11 is passed over the aperture 30 in the weighing table 1 8 an amount of pressure is forced through the conduit 44 to the diaphragm 48, which is in the form of a single action diaphragm in Figure 1. The diaphragm 48 in turn displaces the transformer core 54 shown in Figure 3 causing an imbalance in the linear variable differential transformer 46 resulting in an output voltage 58. This output voltage signal 58 is then delivered by way of conductor 36 to circuitry 38 for deriving the numerical weight of the article 11 based upon the proportional relationship of the area and pressure to support the article 11 to the article weight.

In order to initialize the proper proportional relationships for the circuitry 38 to evaluate, a further step in calibration is required. An article 11 having a predetermined numerical weight is placed on the device 1 2 the output voltage 58 at the linear variable differential transformer 46 is set to represent that numerical weight. Other articles having the same surface area will be weighed using a linear relationship between the pressure and weight based on the initial calibration parameters. The electronic circuitry 38 utilized in performing the calculation of the numerical weight may be the type utilized in the Dynachek weighing devices made and manufactured by the assignee of the present invention.

Considering now the preferred form of the electrical portion of the system in accordance with the invention as shown particularly in Figure 5, where weight calculating circuitry 38 is illustrated along with timing control circuitry with respect to the products delivered to the scale and display of the actual numerical weight of those products.

The output voltage signal 58 generated when an article 11 moves over the weigh table, is provided to a signal conditioning circuit 38 for deriving the numerical weight of the article. The incoming signal 58, is filtered by a two-stage low pass filter 70 of the type generally known in the art. The two-stage filter will then deliver the signal 58 to amplifier 72 for proper gain control over the signal before differentiation. The resistor network including resistors 74, 76 and 78 enable zeroing the amplifier 72 by applying a DC offset voltage and further enabling setting the system gain. The signal is then differentiated twice in differentiator circuits 82 and 84 which comprise amplifiers formed in a two-stage a.c. coupled network.The first amplifier (not shown) generates a first derivative, that is, the change of position divided by the change in time computed as the change in time approaches, but does not reach zero. This derivative produces the velocity dx/dt based upon displacement of the pressure transducer 34. The second amplifier (not shown) in the two-stage differentiater network generates the second derivative of the displacement, that is the instantaneous change of velocity divided by the change of time as time approaches, but does not reach zero. This derivative provides an acceleration signal d2x/dt2. The weight calculating circuit provides for the summing of the displacement, velocity, and acceleration signals provided by the differentiaters networks 82 and 84.The velocity and acceleration signals 86 and 88 are delivered to an analog multiplier 90 which calculates the force applied to the weight cell according to the Newtonian equation F=MA where A is the acceleration computed by the differentiater circuit 84 and M is a linear combination of the displacement, velocity, and acceleration and may be calculated using the following relationship: Mg=g(Rdx/d t+Kx/-(dxd t+g)-Mo) For any given system, the constants R, K and Mo are readily determined with R being the damping constant of the entire mechanically resonant circuit, K the effective spring constant and Mo the weight of the parts. The displacement velocity and acceleration along with the output of the analog multiplier 90 are delivered to summing amplifier 92.The summing amplifier provides an analog output signal that is converted to a frequency or rate output proportional to the amplitude. This signal is further amplified in amplifier 94 and rectified in full wave rectifier 96, before delivered to the voltage-to-frequency conversion circuitry 98 and rate multiplier 100 providing a signal proportional to the amplitude of the original weight cell signal 58.

Amplifier 94 and full wave rectifier 96 provides a nulling amplification effect which in turn provides a polarity sensor for the zeroing circuit and a unipolar voltage for the voltage/frequency converter 98.

The rate multiplier 100 has the purpose of scaling the frequency from the voltage frequency converter 98 such that one count will equal one unit of product weight.

The presence or absence of articles 11 on the weighing device is determined by a photocell reaction using photocell 102. Timing for the circuit is provided with a selectable output counter which generates a precise sampiing interval selectable from 0 to 99 milliseconds.

Thus the counter enable 104 reacts to the photocell's 102 determination of product on the weighing device and triggers a timer 106 providing input into the basic delay system.

The delay counter 108 receives a timing pulse from timer 106 and further is connected to the rate multiplier 100 for receiving the frequency signal proportional to the amplitude of the initial weigh signal 58. A display 110 is further provided to have a continual showing on a front panel display of the count which is indicative of the weight of the article 11. This signal is further compared in a comparator 112 to a predetermined range of numerical weights and using circuitry for providing an overweight comparison 114 and an underweight comparison 11 6 the product may be rejected as failing either of these criteria.

When no product is in view of the photocell 102, the output of the rate multiplier 1 00 is used to clock a counter 108 whose outputs are converted to a zero-correcting voltage with a digital-to-analog converter (not shown).

With reference again to Figure 4, since the conduit 44 connecting the aperture 30 to the diaphragm 48 is held at atmospheric pressure, foreign particles are free to form a blockage in the conduit thus reducing the accuracy of the device.

To prevent any particles from falling into the conduit 44, pressurized air is supplied from the air chamber 24 by way of a flexible conduit 60 coupled to the conduit 44. This supply of pressurized air to the conduit 44 performs a dual function. As stated above, foreign particles are prevented from falling into the conduit 44; further, air is supplied to the diaphragm 48 thus speeding the weighing action since the diaphragm is pre-pressurized to a point closer to the pressure received from beneath the article 11.

In an alternative embodiment shown in Figure 4, a differential diaphragm 62 may be used in place of the single action diaphragm 48 shown in Figure 1. The differential diaphragm 62 requires a reference pressure which may be received by way of a conduit 64 tapped into the air chamber 24.

The reference pressure received from the air chamber 24 holds the differential diaphragm 62 in an unbalanced position with pressure received from beneath the article moving the differential diaphragm 62 to a balanced position, thereby moving the transformer core 54 shown in Figure 3, and resulting in an output voltage 58 which may be translated into the numerical weight of the article 11 by circuitry as described above.

Since it is still necessary to prevent blockage of the conduit 44 in the alternative embodiment, air supplied to the diaphragm 62 to effect a reference pressure is partially passed through the diaphragm to the sense hole 30 through the conduit 44 to prevent foreign particles from entering.

Although the invention has been described in its preferred form with a degree of specificity, it is understood that this description has been made only by way of example. Changes in the specifics of this embodiment will be apparent to those skilled in the art without departing from the spirit and scope of the invention. For example, instead of the linear variable differential transformer 46 utilized in the pressure transducer 34, a capacitor may be used to provide a voltage signal in response to the back pressure at the sense hole due to the article weight.

Claims

1. A weighing device comprising: a support; a weighing table disposed on said support, with said table having a first member with a plurality of spaced apertures therein, and a second member located beneath said first member in spaced parallel relationship, said first and second members laterally enclosed defining an air chamber therebetween; a compressor providing pressurized air to said chamber through a conduit, thereby enabling an article to be weighed to move across said first member in a frictionless manner; and a pressure transducer pneumatically connected to one of said plurality of apertures for translating a pressure proportional to the pressure necessary to support said article into an electrical signal.

2. A weighing device as set forth in Claim 1, wherein said aperture connected to said pressure transducer has a diameter different than the remaining plurality of apertures in said first member.

3. A weighing device as set forth in Claim 1, wherein said pressure transducer comprises a linear variable differential transformer and a diaphragm connected thereto.

4. A weighing device as set forth in Claim 3 wherein said diaphragm is a differential diaphragm pneumatically connected by a first conduit to said air chamber to obtain a reference pressure, and pneumatically connected by a second conduit to one of said apertures in said first member to sense a pressure proportional to the pressure required to support said article to be weighed as it passes over said first member.

5. A weighing device as set forth in Claim 3 wherein said diaphragm is a single action diaphragm pneumatically connected by a first conduit to one of said plurality of apertures in said first member to sense a pressure proportional to the pressure required to support said article to be weighed as it passes over said first member, and where said first conduit is coupled to a second conduit connected to said air chamber for supplying air to said first conduit thereby preventing blockage by foreign particles.

6. A weighing device as set forth in Claim 1, wherein said pressure transducer comprises a capacitor element.

7. A weighing system comprising: a support; a weighing table disposed on said support, with said table having a first member with a plurality of spaced apertures therein, and a second member located beneath said first member in placed parallel relationship, said first and second members laterally enclosed defining an air chamber therebetween; a compressor providing pressurized air to said chamber through a flexible conduit, thereby enabling an article to be weighed to move across said first member in a frictionless manner; a pressure transducer connected to one of said plurality of apertures for translating the pressure necessary to support said article to a voltage signal; input and output transports for moving said articles to be weighed onto and off of said weighing table, and disposed in adjacent relationship and operably associated with said weighing table; and circuit means for providing the numerical weight of said article derived from said voltage signal.

8. A weighing system as set forth in Claim 7 wherein said input and output transports comprise endless belt conveyors.

9. A weighing system as set forth in Claim 7 further including a motor for driving said input and output transports.

10. A weighing system as set forth in Claim 7 wherein said weighing table is laterally enclosed with contoured side members for receiving said input and output transports.

11. A weighing system comprising: a weighing device having a weighing table having a surface with a plurality of apertures therein, a compressor for supplying pressurized air to said apertures, and a pressure transducer for translating the pressure required to support an article to be weighed, as it passes over said weighing table, into an electrical signal, wherein said pressure transducer oscillates at its natural resonance with the fluctuations in weight of the articles passing over said weighing table; and circuit means for providing the calculated numerical weight of said article derived from said electrical signal.

12. A weighing system as set forth in Claim 11 wherein said circuit means comprises: means responsive to an oscillatory motion, for producing a first signal, a second signal, and a third signal, respectively representative of the displacement of said pressure transducer, of the velocity of said pressure transducer, and of the acceleration of said pressure transducer during said oscillatory motion thereof; and computing means responsive to said first, second and third signals for solving the second order differential equation of motion of a vibrating damped system for said pressure transducer to produce an output representative of said weight.

13. Apparatus in accordance with claim 12, in which said computing means is operative to produce said output signal in response to the values of said first, secondand third signals occurring during a period of time less than the time of one cycle of said oscillatory motion.

14. Apparatus in accordance with claim 12, comprising integrating means supplied with said output signal for discriminating against interference signal components present therein.

1 5. Apparatus in accordance with claim 12, comprising indicator means responsive to said output signals for producing visual indications of the value of said weight.

16. Apparatus in accordance with claim 12, comprising actuator means and level sensing means responsive to said output signal for actuating said actuator means when said output signal has a value in a predetermined range.

17. Apparatus in accordance with Claim 12, in which said equation is of the form Mg=g( (Rdx/dt+Kx/-(dx/dt+g)-Mo) where Mis the mass of said article, Mo is the effective mass of said pressure transducer, R is the damping factor for said pressure transducer, K is the spring-constant for said pressure transducer, g is the acceleration of gravity, and x is the instantaneous value of the displacement of said pressure transducer produced by the weight of said article.

1 8. Apparatus in accordance with claim 12, comprising means for moving a plurality of articles to be weighed in succession onto said surface, whereby said output signal represents successively the weights of said articles.

1 9. A weighing system comprising: a support; a weighing table disposed on said support, with said table having a first member with a plurality of spaced apertures therein, and a second member located beneath said first member in spaced parallel relationship, said first and second members laterally enclosed defining an air chamber therebetween; a compressor providing pressurized air to said chamber through a flexible conduit, thereby enabling an article to be weighed to move across said first member in a frictionless manner; an oscillating pressure transducer connected to one of said plurality of apertures for translating a pressure proportional to the pressure necessary to support said article to a voltage signal; and circuit means for providing the numerical weight of said article derived from said voltage signal, wherein said circuit means comprises: means responsive to an oscillatory motion, for producing a first signal, a secondsignal, and a third signal, respectively representative of the displacement of said pressure transducer, of the velocity of said pressure transducer, and of the acceleration of said pressure transducer during said oscillatory motion thereof; and computing means responsive to said first, second and third signals for solving the second order differential equation of motion of a vibrating damped system for said pressure transducer to produce an output representative of said weight.

20. Apparatus in accordance with Claim 19, in which said equation is of the form of Mg=g((Rdx/d t+xl-(dxld t+g)-M,) where M is the mass of said article, m0is the effective mass of said pressure transducer, R is the damping factor for said pressure transducer member,k is the spring-constant for said pressure transducer, g is the acceleration of gravity, and x is the instantaneous value of the displacement of said pressure transducer produced by the weight of said article.

21. A method for weighing an article comprising the steps of: supplying pressurized air to a weighing table, having a plurality of spaced apertures therein, for forming an air cushion thereon; transporting said article to be weighed to said weighing table; sensing the pressure required to support the weight of said article by means of a diaphragm; translating said pressure into an electrical signal by means of a pressure transducer; and calculating the numerical weight of said article from said electrical signal by circuit means.

22. A weighing device comprising a weighing table having a surface with a plurality of apertures therein, means for supplying pressurized air to said apertures in said weighing table, and a pressure transducer for translating a pressure proportional to the pressure required to support an article to be weighed, as it passes over said weighing table, into an electrical signal.