Patent Application: US-47161206-A

Abstract:
a method serves to process output signal of a measuring transducer in a force - measuring device , in particular in a balance , wherein the measuring transducer produces a measuring signal representative of a load acting on the device and the measuring signal is filtered in a variable analog filter and / or , after processing in an analog / digital converter , the measuring signal is filtered in a variable digital filter , in order to remove unwanted signal components that are caused by disturbances affecting the force - measuring device , in particular by changes in the weighing load . the measuring signal is monitored in regard to the occurrence of a change in the weighing load and after a load change has been detected , at least one filter parameter of the filter is reset and then varied as a function of time in accordance with a prescribed time profile f c , so that the filter is opened after a load change has been detected and then closed again to the point where a predefined filter characteristic has been attained which is determined by the end value of the at least one filter parameter . by using this method , it is possible with simple measures to realize noticeably shortened transient settling times of the force - measuring device after a load change has occurred .

Description:
fig1 illustrates an exemplary balance 1 according to an exemplary embodiment with symbolically indicated extraneous influence factors d m and d l which have a significant influence on the time profile of the measuring signal ms . the time profile of the signal varies on the one hand as a function of changes in the weighing load d l . the signal profile further varies dependent on mechanical influence factors d m such as vibrations and shocks whereby parts of the balance , in particular the measuring transducer or its parts , are caused to oscillate . even after a change d l of the load acting on the balance has already taken place , a mechanical disturbance results each time in transient oscillations that superimpose themselves on the usable signal . the signal - processing unit in the balance 1 therefore has the task of separating that part of the signal that represents the weighing load in the best and fastest way possible from unwanted parts of the signal that are caused by vibrations , shocks and load changes . this can be of particular importance in balances in which load changes follow each other in short time intervals . particularly in balances with a high measuring resolution , it can be further desired that the noise is also reduced sufficiently well after the decay of the transient oscillations . fig2 shows as an example the block diagram of the balance of fig1 with a measuring transducer 10 which sends an analog measuring signal ms a representing the weighing load to an analog / digital converter 12 by way of a first signal - processing unit 11 serving to process analog signals ( filtered signal indicated in the drawing as ms af ). the analog / digital converter 12 directs the digitized measuring signal ms d to a second signal - processing unit 13 serving to process digital signals . from the second signal - processing unit 13 , the filtered digital measuring signal ms df is sent to a processor 16 which is connected to a keyboard 19 and an indicator 18 , for example a liquid crystal display , on which the measuring results are displayed . the signal - processing units 11 , 13 send detection signals s det serving to announce load changes through a first control line to the processor 16 which after receiving a detection signal s det returns a reset signal s rst to the signal - processing units 11 , 13 whereby the filters 113 , 133 in the signal - processing units are opened and then closed again according to the time profile of at least one closure function f c ( t ). the measure of putting the filter into a completely open state after a load change has been detected allows load changes to be followed with a rapid response . by subsequently closing the filter 113 , 133 to the point where an intended filter characteristic is obtained , for example a low - pass characteristic of small bandwidth , an optimal suppression is achieved for signal disturbances , in particular oscillations caused by the load change as well as signal disturbances occurring at a later time . fig3 shows the block diagram of fig2 in a more general form wherein it remains undefined which modules ( processor , signal processors , discrete circuits or software modules ) provide the functions and the computing power to carry out the disclosed method . in this circuit arrangement , the analog measuring signal ms a that is present at the input side of the analog signal - processing unit 11 and the digital measuring signal ms d that is present at the input side of the digital signal - processing unit 13 are directed , respectively , to detector modules 111 and 131 where the respective incoming signals ms a and ms d are compared to a threshold value th , and if the signal is found to exceed the threshold , a generator module 112 or 132 , respectively , is triggered ( as indicated by the trigger signal trg ), whereby the at least one filter parameter r of the respective filter 113 , 133 is reset and subsequently varied and returned to a given end value along a time profile in accordance with the given closure function f c ( t ). thus , with the circuit arrangement of fig3 it is possible to control analog and / or digital filters 113 , 133 in accordance with exemplary embodiments . in the analog signal - processing unit 11 , active filters 113 can be used with adjustable filter parameters of the kind described in reference [ 8 ], chapter 13 , on pages 888 to 893 . the electronic control of the filter parameters is described on page 891 . in the digital signal - processing unit 13 , digital filters 133 with adjustable filter parameters are used as described in reference [ 8 ], chapter 21 . of course , the control of the filter parameters is easier to realize with this solution , for example through stepwise changes of the filter parameter in a register of the signal processor that is being used . according to reference [ 8 ], page 1133 , there is a growing trend away from analog signal processing towards digital signal processing . the advantages according to reference [ 8 ] are seen in the higher degree of accuracy and reproducibility as well as in the lower sensitivity to disturbances . for the method presented herein , the simple and precise way in which the digital filters can be controlled is of particular importance . in the following , exemplary embodiments are described based on the use of digital filters . the design structure , function and properties of digital filters are described in reference [ 8 ], chapter 21 . electronic balances with digital filters are disclosed for example in references [ 6 ] and [ 7 ]. the ways in which digital filters are realized by means of a signal processor is described in reference [ 8 ], chapter 21 . 7 . 2 , on pages 1181 to 1184 . if a signal processor is used , it constitutes essentially by itself the digital signal - processing unit 13 . the aforementioned modules , the detector module 131 , the generator module 132 and the filter 133 by which a filter stage 130 is formed are therefore based on implemented software modules . as shown in reference [ 8 ], pages 1157 and 1158 , an improved approximation of a desired frequency response is possible with higher - order filters . in balances , too , higher - order filters can be used . as an example , an eighth - order filter 133 is used in the block diagram of fig3 , wherein the filter parameters r 1 , . . . , rx of the generator module 132 can be varied in such a way that after a load change has been detected , the frequency response of the filter 133 is optimally adapted to the oscillations that occur . this has the advantageous result that the filter 133 can be controlled with only one detector module 131 and only one generator module 132 . however , the determination of the filter parameters r 1 , . . . , rx and the determination of the individual closure function profiles f c1 ( t ) to f cx ( t ) as well as their implementation present a highly demanding task . in other embodiments , the higher - order filter is therefore replaced by cascaded partial filters of lower order , such as first - order filters , each of which has a detector module 131 and a generator module 132 assigned to it . the cascading of partial filters by which a filter of n - th order can be formed is described in reference [ 8 ] on pages 1146 to 1147 and 1174 . fig4 shows a first - order digital filter 133 1 with a subtractor stage sub , an adder stage add and a multiplier stage mpr , wherein the filter 133 1 together with a generator module 132 1 forms a filter stage 130 1 which , as shown in fig5 and 6 , is part of a cascade of filter stages 130 1 , . . . , 130 n forming an n - th order filter that is controlled . the function of the digital filter 133 1 is known from reference [ 8 ], chapter 21 . it should be noted that the filter parameter r which is provided in the multiplier stage mpr and defines the transfer function g ( z ) of the filter 133 1 as in the case where the filter parameter r is reset to a value of 1 , the transfer function g ( z ) will likewise become equal to 1 , and the filter 133 1 will allow the input signal to pass through without being filtered . at the output side of the subtractor stage sub the first time derivative of the measuring signal is present , which is directed to the detector module 131 1 . thus , the output signal of the subtractor stage sub indicates the rate of change occurring in the input signal of the filter stage 130 1 . in order to determine whether the changes are the result of a load change , the output signal of the subtractor stage sub is directed to a first integrator module int 1 by way of a delimiter module lim which can be provided , and to a second integrator module int 2 by way of an inverter inv . the larger of the respective output signals of the two integrator modules int 1 , int 2 is selected by a module max and directed to a comparator cmp , which compares said signal to a threshold value th and issues a trigger signal rst if the threshold has been exceeded . the trigger signal rst activates the generator module 132 1 which follows in series , so that the generator module sets the filter parameter r for the time t = 0 at least approximately to the value of 1 and subsequently varies the filter parameter in accordance with the given closure function f c ( t ) until the filter parameter r at the time t = tx assumes the end value x and is kept at this value until the next trigger signal rst arrives . furthermore , the detector module 131 1 ( more specifically the integrator modules int 1 , int 2 ) is reset so that further load changes can be detected . for the correction of drift - related deviations , a drift compensation module dcm can be connected to input terminals of the integrator modules int 1 , int 2 , which can , for example , prevent the output signals from drifting away under a constant weighing load . a cumulative summation ( cusum ) of currently present signals as well as recursively determined signals takes place in the integrator modules int 1 , int 2 . the first integrator module int 1 monitors positive load changes , while the second integrator module int 2 monitors negative load changes . the foregoing exemplary method allows small changes in the signal , i . e ., small load changes , to be determined rapidly and precisely . as a sensible practice , load - change detection signals from the other filter stages 130 2 , . . . should likewise be considered which is why the detector module 131 1 includes a logic gate or to which the trigger signals rst , rst in of the local comparator cmp as well as of the further filter stages 130 2 , . . . can be directed . the generator module 132 1 is therefore controlled by the signal that is present at the output side of the logic gate or and is further directed to the other filter stages 130 2 , . . . the signal rst out which is present at the output side of the logic gate or further controls the switch sw which is indicated symbolically in fig4 and is provided in exemplary embodiments to short out the filter 133 1 , for example , during at least one measurement cycle ts after a load change has occurred . the closure function f c ( t ) which is used after a load change has been detected is derived , for example , from the transient settling time of the filter which depends on the magnitude of the noise a ( see fig7 ) that can be expected to occur at the input side of the filter and which also depends on the resolution a of the force - measuring device . a suitable closure function f c ( t ) can be found as follows . based on the amplitude a of the noise and the resolution a of the balance , a factor ρ is determined : with the measurement cycle time ts and the elapsed time ti since the load change , the filter parameter r can be selected as follows : accordingly , the closure function f c ( t ) describing the variation that the filter parameter r is subjected to depends only on the amplitude a of the noise that is present at the input side of the filter 133 1 and on the desired resolution a . the time profile of the closure function f c ( t ) is being followed only to the point where an end value x ( r = x { 0 & lt ;×≦ 1 }) has been reached that was calculated for a bandwidth that needs to be maintained . since the factor ρ is small ( ρ & lt ;& lt ; 1 ; ρ & gt ; 0 ), the logarithm lnρ takes on a negative value ( for example − 10 ) which at the time of the load change determines the value of the exponent [ lnρ ×( ts / ti )], because ts / ti at that time is approximately equal to 1 . consequently , the expression 1 − e [ lnρ ×( ts / ti )] takes on a value that is close to zero , and the value of r will be close to 1 , so that as a result the transfer function is approximately equal to 1 . fig5 shows an exemplary digital signal - processing unit 13 with n filter stages 130 1 , . . . , 130 n which together form an n - th order filter and which have filter parameters r 1 , ..., r n individually assigned to them . the afore - described modules of the filter stages 130 1 , . . . , 130 n are realized by means of a software program which is stored in a memory 1301 and executed by a signal processor 1300 . fig6 shows the digital signal - processing unit 13 of fig5 with an optimizing module 135 by means of which the filter parameters r 1 , . . . , r n or more specifically their end values r 1x , .. . , r nx and closure functions f c1 ( t ), . . . . , f cn ( t ), can be individually optimized . in the following , the optimization module 135 which is implemented by means of a program p opt will be explained in more detail with reference to the signal profile s - z that is shown in fig7 . for the duration of the transient settling period of the measuring system , the closure functions f c1 ( t ), . . . , f cn ( t ) are of primary relevance . in the execution of the optimization program popt , the closure functions f c1 ( t ), . . . , f cn ( t ) are therefore during a first optimization phase subjected to a stepwise variation , and the varied closure functions in each step are applied to the stored signal profile s - z , whereupon a test is made to determine the time t 1 at which the decaying transient signals lie within the boundaries of a first window nlw 1 . the test step in which the shortest time t 1 is registered therefore indicates that the respective set of signal profiles of the closure functions f c1 ( t ), . . . , f cn ( t ) represents an optimum , and these closure functions are subsequently used in the filter stages 130 1 , . . . , 130 n . the end values x 1 , . . . , x n of the filter parameters r 1 , . . . , r n are relevant for the noise level of the measuring signal that is present after the transient settling phase of the measuring system . in a second optimization phase of the optimization program p opt , the end values x 1 , . . . , x n of the filter parameters r 1 , . . . , r n are therefore subjected to a stepwise variation , and the varied end values in each step are applied to the stored signal profile s - z , whereupon a test is made to determine which set of end values x 1 , . . . , x n allows a noise - limiting second window nlw 2 to be closed best . the optimization of the balance can be performed on the basis of several signal profiles that were recorded under favorable operating conditions of the balance 1 . the methods and balance 1 have been described and illustrated as various exemplary embodiments . the exemplary force - measuring device has been described in the form of a balance 1 . however , exemplary embodiments can also be used in other force - measuring devices such as gravimetric measuring devices , weighing modules , load cells and force sensors which in some cases can form part of a balance . further , the described exemplary embodiments can be used in combination with different technologies such as analog technology or digital technology , or can be realized as a software solution in conjunction with a signal processor . it will be appreciated by those skilled in the art that the present invention can be embodied in other specific forms without departing from the spirit or essential characteristics thereof . the presently disclosed embodiments are therefore considered in all respects to be illustrative and not restricted . furthermore , the invention is of course not limited to the filters presented herein but can be realized with any desired variable filters of any desired order . the scope of the invention is indicated by the appended claims rather than the foregoing description and all changes that come within the meaning and range and equivalence thereof are intended to be embraced therein . 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