Abstract:
An electromagnetically force-compensating force-measuring apparatus ( 100 ) that includes a force dependent support coil ( 2 ) and an integrating analog/digital converter ( 10; 10 ′) that converts the coil current (I S ) into a digital output signal. A current/voltage converter ( 6 ) is connected downstream of the support coil ( 2 ), the output of the current/voltage converter being connected to a measurement voltage input ( 14 ) of the analog/digital converter ( 10; 10 ′) and to the input of a voltage amplifier ( 8 ). The resistance value of a first heating resistor (R SH ) is equal to the resistance value of the support coil ( 2 ), the resistance value of a second heating resistor (R WH ) is equal to the conversion factor (R W ) of the current/voltage converter ( 6 ) and the gain factor of a voltage amplifier ( 8 ) is equal to the ratio of the resistance value of the first heating resistor to the resistance value of the second heating resistor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a Continuation of International Application PCT/EP2013/000838, with an international filing date of Mar. 20, 2013, which in turn claims priority to German Patent Application 10 2012 103 037.5, filed Apr. 10, 2012. The entire disclosures of both these related applications are incorporated into the present application by reference. 
    
    
     FIELD OF AND BACKGROUND OF THE INVENTION 
     The invention relates to an electromagnetically force-compensating force-measuring apparatus, comprising
         a support coil which is mounted in a permanent magnet arrangement and through which a coil current generated by a controller, depending on a force, flows during operation, and   an integrating analog/digital converter which is designed to convert an electrical signal, which is representative of the coil current and is applied to the measurement signal input thereof, into a digital output signal,
 
wherein the analog/digital converter is connected at the reference voltage input thereof to a reference voltage source which has two reference voltages which have the same magnitude and are oppositely poled relative to one another, and alternately connects a reference voltage switch to an integrator of the analog/digital converter, wherein a ratio of the intervals in which the individual reference voltages are connected to the integrator within a measuring clock cycle is a measure of the presently flowing coil current.
       

     Digital force-measuring devices operating according to the principle of electromagnetic force compensation, for example digital balances, have long been known. EP 2 253 944 A1 (which corresponds to US 2010/0294573 A1) discloses a digital balance of this type. 
     According to the measuring principle of electromagnetic force compensation, a support coil connected to a load arm is arranged axially movable in the air gap of a permanent magnet. Current flow through the support coil generates a magnetic field which interacts with the magnetic field of the permanent magnet and leads to a deflection of the coil and of the load arm connected thereto. The position of the load arm is detected by suitable position sensors. The current source for the coil current and the position sensors are components of a control circuit, the control variable of which is the position of the load arm and the manipulated variable of which is the current flow through the support coil. If the load arm is deflected by a force that is to be measured, this deflection is measured by the position sensors and is communicated to a controller which adjusts the current flow through the support coil such that the deflection is counteracted. The current through the support coil is therefore a direct measure of the force acting on the load arm. The coil current or a variable which is representative thereof is digitized in an analog/digital converter (abbreviated: A/D converter) connected downstream. In particular, the principle of the integrating A/D converter is well-known in the art. 
     A circuit diagram showing the principle of an electronic measurement value detection device  100  of a force-measuring apparatus of this type with a support coil  2 , a controller  4  and an A/D converter  10  is shown in  FIG. 1 . 
     The heart of the A/D converter  10  is the integrator  12  which comprises an operational amplifier with an inverting input  122 , a non-inverting input  123  and an output  124 , as well as a capacitor  125  which is connected between the inverting input  122  and the output  124  of the operational amplifier  121 . The non-inverting input  123  of the operational amplifier  121  is connected to a reference voltage, particularly to ground. The inverting input  122  is connected via the measuring resistor R M  to the measurement voltage input  14  where the measurement voltage U M , which is representative of the coil current I S  flowing through the support coil, is applied during operation. In particular, the coil current I S  can be converted by a current/voltage converter  6  into the measurement voltage U M . Furthermore, the inverting input  122  is connected via a reference resistor R Ref  to the reference voltage switch  16  which, depending on the switch setting, electrically connects either the first reference voltage input  18  or the second reference voltage input  20 . A reference voltage U Ref1  or U Ref2  is applied to each of the reference voltage inputs  18 ,  20  which typically have inverse polarity relative to one another and can have the same voltage level. The integrator output  126  is connected to the test voltage input  221  of a comparator  22 , the reference voltage input  222  of which is connected to a comparator reference voltage, which can be, for example, ground. The comparator  22  outputs a signal or a signal change at its output  223  when the test voltage applied to the test voltage input  221  corresponds to the reference voltage applied to the reference voltage input  222 . The comparator output signal is fed back as the switching signal, via a control device  40 , to the reference voltage switch  16 . 
     An A/D converter of this type operates as follows: in a first phase of a measuring clock cycle T, the reference voltage switch  16  is switched such that the first reference voltage input  18  is connected. During this phase, the integrator integrates the sum of the measurement current I M , which results from the drop in the measurement voltage U M  across the measuring resistor R M , and the reference current I Ref1 , which results from the drop in the first reference voltage U Ref1  across the reference resistor R Ref . After a time pre-defined by the control device  40 , specifically the duration of a first measuring phase t1, which thus represents an integration phase, the reference voltage switch  16  switches over, so that the first reference voltage input  18  is disconnected and the second reference voltage input  20  is connected. Now the integrator integrates the sum of the measurement current I M  and the reference current I Ref2 , which results from the voltage drop in the second reference voltage U Ref2  across the reference resistor R Ref . In this example, the polarities of the measurement voltage U M  and the first reference voltage U Ref1  are opposite and the polarities of the measurement voltage U M  and the second reference voltage U Ref2  are the same. The integrated and deintegrated voltage respectively lie at the integrator output  126  and therefore at the test voltage input  221  of the comparator  22 . 
     This second measuring phase which thus represents a deintegration phase is denoted herein as τ. As soon as the integrator voltage is fully deintegrated, a comparator signal is output which is used by the control device  40  to switch over the reference voltage switch  16  once more and to begin anew measuring clock cycle. Furthermore, the control device  40 , which during the preceding measuring clock cycle has measured the durations of the two measuring phases t1=T−τ and τ and, in particular, has calculated the ratio of the duration of the second measuring phase τ to the measuring clock cycle duration T of the preceding measuring clock cycle, i.e. the duty factor δ=τ/T, can output a corresponding numerical value which is a measure of the measurement voltage U M  applied during the measuring clock cycle and thus of the coil current I S  flowing through the support coil  2 . 
     A disadvantage of the necessary dependence of the coil current I S  on the present measurement value is that the power loss arising in the support coil is also measurement value-dependent. The power loss leads to heating of the overall device so that thermal influences (faults) are also measurement value-dependent. This is not tolerable for precision measurements. The above-mentioned application EP 2 253 944 A1 (which corresponds to US 2010/0294573 A1) discloses a possibility for compensating for power losses in the support coil. It is proposed, in particular, to provide the support coil with a double winding, each partial winding being connected to a separate coil current source. The coil current sources are controlled in such a way that the total of their currents generates the electromagnetic forces required to compensate for the weight force, wherein at the same time, the power loss absorbed in the support coil remains constant. 
     DE 31 49 990 A1 (which corresponds to U.S. Pat. No. 4,450,923) discloses another approach. Herein, in addition to the direct current, an alternating current is also applied to the support coil, in order to generate an overtemperature in the support coil. A direct current and an alternating current proportional to the direct current and the alternating current in the support coil is passed through a strongly temperature-dependent resistor of a voltage divider circuit, in particular a glow wire, the resistance value of which is kept constant by a controller. Based on the proportionality of the currents in the support coil and in the glow wire, a constant power loss is caused in the support coil. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an alternative compensation for the power loss arising in the support coil. 
     This object is addressed in the context of an electromagnetically force-compensating force-measuring apparatus as recited above, in that the output of the controller is connected, via a first heating resistor which is thermally coupled to the support coil, to the output of a voltage amplifier, the input of which is connected to the output of the reference voltage switch and that a current/voltage converter is connected downstream of the support coil, the output of said current/voltage converter being connected to the measurement voltage input of the analog/digital converter and, via an inverter and a second heating resistor, to the input of the voltage amplifier 
     wherein 
     
         
         
           
             the resistance value of the first heating resistor is equal to the resistance value of the support coil, 
             the resistance value of the second heating resistor is equal to the conversion factor of the current/voltage converter, and 
             the gain factor of the voltage amplifier is equal to the ratio of the resistance value of the first heating resistor to the resistance value of the second heating resistor. 
           
         
       
    
     This object is further addressed in conjunction with a further electromagnetically force-compensating force-measuring apparatus in that the output of the controller is connected via an inverter, a first heating resistor which is thermally coupled to the support coil, and a second heating resistor to the output of a voltage amplifier, the input of which is connected to the output of the reference voltage switch and that a passive current/voltage converter with a shunt resistor connected to ground is connected downstream of the support coil, the output of said current/voltage converter being connected to the measurement voltage input of the analog/digital converter, wherein:
         the resistance value of the first heating resistor is equal to the resistance value of the support coil,   the resistance value of the second heating resistor is equal to the resistance value of the shunt resistor, and   the gain factor of the voltage amplifier is equal to the ratio of the total of the resistance values of the first and second heating resistor to the resistance value of the second heating resistor.       

     Preferred embodiments of these two types of apparatuses are also disclosed and claimed herein. 
     Underlying the two variants of the invention is the same basic inventive concept of compensating for the power loss in the support coil using an additional heating resistor which is fed with a current which is representative of the duty factor, i.e. the measurement value. The duty factor-dependency is found by detecting the respective voltage applied, which varies temporally exactly with the duty factor. The support coil power loss is therefore not precisely compensated for at each infinitessimal time point, but always averaged over a measuring clock cycle. However, this is fully sufficient since typical measuring frequencies are in the region of several hundred Hertz and are therefore significantly faster than any thermal influences. Nevertheless, in order to realize the basic inventive concept, further elements which themselves provoke a power loss which is measurement value-dependent and therefore requires compensation are needed. In order to realize the basic inventive concept, it is therefore required that a large number of elements be considered and taken into account together, and this leads to the above-mentioned two different variants of the invention. 
     In a preferred embodiment of the first variant of the invention, it can be provided that the current/voltage converter has a converter resistor which is connected in series between the support coil and the input of the analog/digital converter and a difference amplifier connected in parallel thereto, the second input of which is preferably connected to ground. An embodiment of a current/voltage converter of this type is known in principle to a person skilled in the art. The conversion factor corresponds to the resistance value of the converter resistor. 
     In a further development of the first variant of the invention, it is provided that the inverter is a component of a power compensation circuit within the analog/digital converter. Also, within the A/D converter, the problem arises that a measurement value-dependent current flows through the measuring resistor R M , which leads to a measurement value-dependent power loss in the converter. Various compensation measures which are independent of the support coil power loss compensation are conceivable and require, inter alia, the inversion of the measurement voltage. This inverted signal also plays a part in the context of the support coil power loss compensation, so that the relevant component, i.e. the inverter, can be used jointly and a component can be spared. 
     Further features and advantages of the invention are disclosed in the following description and the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIG. 1  is a sketch showing the principle of an electronic measurement value detection device of a conventional force-measuring apparatus, 
         FIG. 2  is a sketch showing the principle of a first embodiment of an electronic measurement value detection device of a force-measuring apparatus according to the first variant of the invention, 
         FIG. 3  is a sketch showing the principle of a second embodiment of an electronic measurement value detection device of a force-measuring apparatus according to the first variant of the invention, 
         FIG. 4  is a sketch showing the principle of a first embodiment of an electronic measurement value detection device of a force-measuring apparatus according to the second variant of the invention, 
         FIG. 5  is a sketch showing the principle of a second embodiment of an electronic measurement value detection device of a force-measuring apparatus according to the second variant of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     The same reference signs in the figures relate to the same or similar components. 
       FIG. 1  has already been described in detail above in the context of the outline of the prior art. 
       FIG. 2  shows a sketch of the principle of a further development according to the invention of the device of  FIG. 1 . In this embodiment, the current/voltage converter  6  is configured as a converter resistor R W  connected in series downstream of the support coil  2  with a difference amplifier  7  connected in parallel. The current/voltage converter  6  converts the coil current I S  with a conversion factor which corresponds to the resistance value of the converter resistor R W  into the measurement voltage U M  which is applied to the measurement voltage input  14  of the A/D converter  10 . This measurement voltage U M  is applied by the inverter  9  to one side of a heating resistor R WH , the resistance value of which is equal to the resistance value of the converter resistor R W . The other side of the heating resistor R WH  is connected to the output of the reference voltage switch  16  so that the difference between the measurement voltage and the respective present reference voltage U Ref1  or U Ref2  lies across the heating resistor R WH . It should be noted herein that the time intervals within which U Ref1  and/or U Ref2  are applied corresponds to the duty factor δ. The reference voltage U Ref1 , U Ref2  is also applied to the input of a voltage amplifier  8 , the output of which is applied to one side of a further heating resistor R SH , the other side of said further heating resistor being connected to the output of the controller  4 . The heating resistor R SH  is thermally coupled, as indicated by a thermal coupling arrow Θ, to the support coil  2  and has the same resistance value as said support coil. The heating resistor R WH  is thermally coupled to the converter resistor R W , as also indicated by a thermal coupling arrow Θ. With a suitable selection of the gain factor of the voltage amplifier  8 , specifically g=R SH /R WH  or g=R S /R W , a constancy results, as can be shown mathematically, i.e. the duty factor-independence of the power loss which arises at the support coil  2 , the converter resistor R W , the heating resistor R WH  and the heating resistor R SH  as a total and averaged over a measuring clock cycle. The power loss is therefore no longer measurement value-dependent, so that thermal effects always have the same influence regardless of the actual measurement value. 
       FIG. 3  shows a further development of the device of  FIG. 2 . A compensation for the power loss in the measuring resistor R M  and a direct portion compensation of the integrator voltage are provided here in the region of the A/D converter  10 ′. These will be described in greater detail below. With regard to the support coil power loss compensation, this variant is particularly interesting in that the inverter  24  (described below), which is a component part of the power loss compensation circuit for the measuring resistor R M , simultaneously serves as an inverter  9  of the support coil power loss compensation, so that merely a single component is required here. 
     The A/D converter  10 ′ of  FIG. 3  differs in two respects from the A/D converter  10  of  FIG. 2 . The first respect concerns the configuration of the comparator reference voltage applied at the reference voltage input  222  of the comparator  22 . Whereas in the A/D converter  10  of  FIG. 2 , only ground is connected here, in the case of the A/D converter  10 ′ of  FIG. 3 , the output of an additional integrator  30  is connected to the comparator reference voltage input  222 . The additional integrator  30  comprises an operational amplifier  301  with an inverting input  302 , a non-inverting input  303  and an output  304 . Whereas the non-inverting input  303  is connected to ground, the inverting input  302  is connected via a capacitor  305  to the output  304 . The input of the integrator  30  is applied, via an input resistor  32 , to the output  126  of the integrator  12 . The mode of operation of the additional integrator  30  lies therein that it averages and inverts the output signal of the integrator  12  and makes this averaged signal available to the comparator  22  as the comparator reference voltage, in other words, in the comparator  22 , the output signal of the integrator  12  is no longer compared with ground, but with its own average value. Therefore, a voltage signal with no direct component is applied at the capacitor  125  of the integrator  12 . In this manner, the direct voltage-related faults of the capacitor  125 , such as leakage currents and dielectric absorption are prevented or at least reduced. It is thereby possible without any sacrifice of functionality in the integrator  12 , to use less high quality capacitor types as the capacitor  125 , and this results in a marked cost reduction for the circuit, or alternatively, with unchanged capacitor quality, to an improvement in measuring quality. 
     The second respect in which the A/D converter of  FIG. 3  differs from that of  FIG. 2  is that the measurement voltage input  14  is connected via an inverter  24 , i.e. via a voltage amplifier with a gain factor of “−1”, and the heating resistor R MH  is connected to the output of the reference voltage switch  16 . The resistance value of the heating resistor R MH  is equal to the resistance value of the measuring resistor R M . Similarly, the resistance value of the reference resistor R ref  is equal to the resistance value of the measuring resistor R M . This results in a constancy of power loss, i.e. a duty factor-independence of the power loss, for the whole A/D converter  10 ′. 
       FIG. 4  shows a further development of the device of  FIG. 1 . The current/voltage conversion is carried out here by a passive current/voltage converter  6  with the shunt resistor R W ′ thereof. This is provided with the same reference sign as the converter resistor R W  in  FIGS. 2 and 3 , by reason of their comparable tasks. The shunt resistor R W ′ is connected to ground between the support coil  2  and the measurement voltage input  14  of the A/D converter  10 . A voltage amplifier  8  is provided, as in the embodiment of  FIGS. 2 and 3 , at the input of which the respective present reference voltage U Ref1  or U Ref2  is applied. The output of the voltage amplifier is applied to one side of the heating resistors R SH  and R WH  which are connected in series and, to their other side, the output voltage of the controller  4  which is inverted by the inverter  9  is applied. The resistance value of the heating resistor R WH  corresponds to the resistance value of the shunt resistor R W ′; the resistance value of the heating resistor R SH  corresponds to the resistance value of the support coil  2 . The heating resistor R SH  is thermally coupled, as indicated by a thermal coupling arrow Θ, to the support coil  2 . With a suitable selection of the gain factor of the voltage amplifier  8 , specifically g=(R WH +R SH )/R WH  or g=(R W +R S )/R W , a constancy results, as can be shown mathematically, i.e. the duty factor-independence of the power loss which arises at the support coil  2 , the heating resistors R SH  and R WH  and the shunt resistor R W ′ as a total and averaged over a measuring clock cycle. The power loss is therefore no longer measurement value-dependent, so that thermal effects always have the same influence regardless of the actual measurement value. 
     Finally,  FIG. 5  shows a further development of the device of  FIG. 4 , which has, in particular, a modified A/D converter  10 ′ according to  FIG. 3 . 
     The embodiments covered by the detailed description and shown in the figures are merely illustrative exemplary embodiments of the present invention. A broad spectrum of possible variations will be evident to a person skilled in the art, based on the present disclosure. 
     REFERENCE SIGNS 
     
         
           100  Measurement value detection device 
           2  Support coil 
           4  Controller 
           6  Current/voltage converter 
           7  Difference amplifier 
           8  Voltage amplifier 
           9  Inverter 
           10 ,  10 ′ A/D converter 
           12  First integrator 
           121  Operational amplifier of  12   
           122  Inverting input of  121   
           123  Non-inverting input of  121   
           124  Output of  121   
           125  Capacitor of  12   
           126  Output of  12   
           14  Measurement voltage input 
           16  Reference voltage switch 
           18  First reference voltage input 
           20  Second reference voltage input 
           22  Comparator 
           221  Test voltage input of  22   
           222  Reference voltage input of  22   
           223  Output of  22   
           24  Inverter 
           30  Second integrator 
           301  Operational amplifier of  30   
           302  Inverting input of  301   
           303  Non-inverting input of  301   
           304  Output of  301   
           305  Capacitor of  30   
           32  Input resistor before  30   
           40  Control device 
         R S  Support coil resistor 
         R SH  Heating resistor 
         R W  Converter resistor 
         R W ′ Shunt resistor 
         R WH  Heating resistor 
         R M  Measuring resistor 
         R MH  Heating resistor 
         R ref  Reference resistor 
         I S  Coil current 
         U M  Measurement voltage 
         I M  Measurement current 
         U Ref1  First reference voltage 
         I Ref1  First reference current 
         U Ref2  Second reference voltage 
         I Ref2  Second reference current 
         T Duration of measuring clock cycle 
         t1 First measuring phase (integration phase) 
         τ Second measuring phase (deintegration phase) 
         Θ Coupling arrow 
         δ Duty factor 
         g Gain factor