Abstract:
The scales include a tray ( 1 ) placed on a base ( 3 ) and lifted by an air cushion, with no mechanical connection with the base ( 3 ). The weight of an object ( 21 ) laid on the tray ( 1 ) varies the air cushion pressure, and the measurement of this pressure by an external sensor ( 19 ) is straightforward and gives the weight of the object ( 21 ) once a law of correlation has been determined. Lateral pipes enable gas to be blown against a flange ( 2 ) of the tray ( 1 ) and thus horizontal displacements of the tray and friction to be avoided. Several vertical pipes are generally provided so as to reduce tray ( 1 ) tipping after the object ( 21 ) has been thrown off centre, the weight being then given by the sum of the measurements of each vertical pipe.

Description:
DESCRIPTION 
     A pneumatic weighing device is the subject of this invention. 
     Everyday weighing devices include delicate mechanisms liable to go wrong or be damaged during use or maintenance operations, which makes them unsuitable for some situations, particularly in sealed enclosures, insulated from the outside, where only rather awkward remote operator devices are able to work: and some corrosive, humid, very hot or radiation charged atmospheres may make the operating conditions even more difficult. 
     This is why a weighing device has been designed which is entirely devoid of mechanisms and where only the force of expansion of a compressed gas is used to supply the energy necessary for operation and to measure the weight of the object being weighed. Such a device normally requires no on-the-spot adjustment or upkeep, and it is designed to be fully resistant to jarring caused by placing the object to be weighed. Moreover, the operation starts from an actual weight (that of the tray when empty), which allows the proper operation of the apparatus to be permanently monitored. The flow of compressed gas provides cleaning, anti-corrosion protection and cooling of the apparatus. 
     The device is distinguished from prior pneumatic scales, which have a conventional structure but which are fitted with a pneumatic sensor to measure the displacement of a test body under the weight of the object being weighed, since the compressed gas is only used in them for measurement. They are moreover old-style scales, the pneumatic displacement sensors having now been replaced by sensors of another kind. 
     The gas used for weighing is here used to produce a levitation of the tray on which the object is laid by creating an air cushion without a test body; but as such a system is particularly unstable, additional arrangements must be made to ensure that the tray is kept in balance, confronting at one and the same time its lateral displacements, its rotation and any tipping movements produced by an object throwing it off centre when being placed on it, and without ever allowing friction between the tray and the fixed base of the device, which would prejudice the accuracy of measurement. 
     A satisfactory pneumatic weighing device according to the most general definition of the invention includes in these conditions a base on which is placed a flanged tray surrounding the base, a system for blowing gas at constant pressure finishing in pipes passing through the base, one of the pipes, used for weighing, being vertical and emerging under the tray, others of the pipes being lateral, and emerging through chambers facing the tray flange. The gas cushions thus formed between the base and the tray by the blown gas ensure its centring relative to the base. The device also includes a measurement means of a magnitude correlated (beforehand) with the weight of an object laid on the tray. 
     The gas pressure prevailing in the cushion located at the end of the vertical pipe balances the weight of the tray and of the object placed upon it. This pressure is an increasing function of the weight of the whole. 
     The pressure is easy to measure provided a pressure inlet is installed in this pipe and a pressure sensor is installed, even in a remote place. The purpose of the lateral pipes is to centre the tray relative to the base by removing any potential for friction. The stability of the tray against tipping is better if there are N vertical pipes similar to the previous ones, preferably arranged at the apex of a regular polygon of the same centre as the tray. A pressure inlet may then be installed on each vertical pipe, and the sum of the N measurements after processing the signals gives the weight of the object. 
     In a first approximation, this function may be presented as follows:              (     M   +   m     )        g     =     n                 S                     ∑     i   =   1       i   =   n                     Pi         ,   where                          
     M=Tray mass 
     m=Object mass 
     n=Number of chambers 
     S=Surface of a chamber 
     Pi=Pressure prevailing in  th  chamber. 
     Precautions may also be taken against rotational movements of the tray, in the frequent situation where it is circular like the flange and the base: the base and the flange are then shaped with a vertical groove and a pin engaged in the groove, leaving two clearances in front of opposite lateral faces of the pin, and the gas blowing system also includes two lateral pipes leading respectively to clearances in opposite directions. The pin slides freely in the tray groove without contact with it. dr 
     A Preferred embodiment of the invention will now be described using the following figures: 
     FIG. 1 is a general view of the invention; 
     FIG. 2 is a horizontal cross-section of the upper part of the scales; 
     and FIG. 3 is a detail of FIG.  2 . 
    
    
     The pneumatic scales are therefore composed of two main components, with no contact during operation: a circular and flat tray  1 , fitted on its periphery with a downward flange and a base  3 . The tray  1  is placed in non-operating mode on the base  3 , and the flange  2  surrounds it. The base  3  is passed through by a certain number of gas blowing pipes necessary for the invention, and its bottom is hollowed out with a chamber  4  into which a pneumatic connector  5  is driven in normal circumstances (it is shown extracted here for reasons of clarity). The base  6  of the pneumatic connector  5  always remains exposed however and receives the gas pipelines leading to the feed and measurement apparatus. There is thus a compressed gas source  7 , fitted with a regulator  8  to produce a constant pressure of gas, and one output pipeline  9  of which leads to a central bore  10  in the connector  5 , which emerges in the bottom of the chamber  4  and extends by a distribution network  11  leading to vertical pipes  12 , here numbering three and arranged in an equilateral triangle in the apex of the base  3 , until emerging under the tray  1 . Each of the vertical pipes  12  includes an input jet  13  and an output chamber  14 . The chamber  14  is a nozzle designed to provide the levitation of the tray  1 , i.e. to form a gas cushion. A pressure inlet pipe  15  emerges in each of the vertical pipes  12  between the jet  13  and the chamber  14 ; it extends through the base  3  to a respective neck  16  of the connector  5 , then communicates to a pipe  17  provided through the connector  5  and an external pipeline  18  leading to a pressure sensor  19 . The pressure measured by each of the sensors  19  is transmitted to an electronic calculator  20  which adds up these pressure measurements. 
     The pressure prevailing in the vertical pipes  12  depends in fact on the total weight of the tray  1  and of an object  21  to be weighed which is laid upon it. A preliminary calibration having been undertaken prior to actually using the device, the pressure difference measured by the sensors  19  may be correlated directly with the weight of the object  21 . The object  21  may typically be a small hermetically sealed jug containing dangerous liquid and which is put onto the tray  1  by a not shown manipulator apparatus before being picked up again to be taken to a measurement, packaging or other installation, depending on the industry in which the scales are being used. The separation of the tray  1  and the base  3  makes it possible to withstand the manipulator placing the object  21  while knocking against the tray  1  or on the contrary dropping it from a certain height without causing damage, and measurement is not appreciably affected if the object  1  is laid away from the centre of the tray  1  since the pressure prevailing in each cushion  14  is representative of the load it supports. 
     In the event of the object to be weighed being outside the triangle formed by the three cushions  14 , the tray tips, supporting itself on the cushion or cushions  14  nearest the object to be weighed. The cushion or cushions  14  located on the opposite side then give a negative weight. The sum of the whole still gives the weight of the object to be weighed but with less accuracy. 
     Another external pipeline  22 , coming from the gas source  7 , leads to the connector  5 , in a pipe  23  hollowed out in it, and from there to a final neck  16  connecting to a pipe  24  hollowed out in the base  3 . The pipe  24  ends facing a collar  25  set at the apex of the base  3  and which is surrounded by the flange  2  with a slight clearance; it includes a neck  26  on its inner face, in which the pipe  24  emerges. Three lateral 120° pipes  27  pass radially through the collar  25  from the neck  26  and make it possible to feed three jets with air. The three air cushions thus created on the inner flange of the tray  1  make it possible to centre it without friction on its base  3 . Moreover, an additional bore  28  extends in the same way to a vertical pin  29  of the collar  25 , which extends in a groove  30  corresponding in shape to the flange  2 , yet wider than the pin  29  so as to leave sufficient clearance  32  on either side of it. The pipe  28  forks into two opposite branches  31  which each feed gas to a jet  33  emerging in a chamber  34  allowing a gas cushion to be created between the pin  29  and the groove  30  of the tray  1 ; this arrangement locks the tray  1  against rotation without friction with the body  3 . 
     It may be noted that the great structural simplicity of these scales allow them to be cleaned and decontaminated without concern for their integrity and with great ease; all that is required when necessary is for the tray  1  to be removed for access to be gained to all its faces, although there is no reason why impurities should get under the tray  1  given the protection offered by the flange  2  and the permanent flow of gas.