Device for measuring the magnitude of a force applied to the free end of a cantilever beam
This is a continuation, of application Ser. No. 056,268 filed July 10, 1979, now abandoned. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The present invention concerns a device for measuring the magnitude of a force applied to the free end of a cantilever beam. In particular, the present invention concerns the incorporation of a cantilevered beam in a weighing scale and measuring the strain in the beam when the same is subjected to the weight of an object being weighed. The measured strain is then converted to a digital read out. 
     2. Prior Art 
     U.S. Pat. No. 3,863,724 describes such a measuring circuit, which uses the signal originating from the sensor of a strain-gauge to yield a numerical result equivalent to the strain undergone. In particular, the device utilizes a conventional analog-to-digital converter. 
     When implementing an electronic circuit of this high-quality type, a major complication appears in the analog amplifying circuits. This is particularly true, if one must maintain great precision in terms of ambient-temperature variations, or of supply-voltage variations (if the equipment must be supplied from a battery), as well as in terms of stability in time. Indeed, this requires the insertion of voltage-stabilizing circuits and of drift-compensating circuits, and the making of many adjustments at the end of the manufacture. All these factors have an unfavorable effect on the cost of these electronics. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is precisely to remedy these shortcomings at least in part, by use of an inexpensive device which makes it possible to eliminate all stabilizing and compensating circuits, as well as the adjustments otherwise necessary. 
     To this end, the device of the present invention includes a cantilevered beam and two pairs of strain gauges. One pair of said strain gauges is positioned on the surface of the cantilevered beam that is in tension during use (i.e. when a force is applied to the free end of the cantilevered beam) and the other of said two pairs of strain gauges is positioned on the surface of the cantilevered beam that is in compression when in use. The four strain gauges are connected together to form a bridge, which bridge produces an output voltage that is a function of the magnitude of the force supplied to the beam when the bridge is supplied with a voltage. A voltage source is supplied for the bridge and an amplifier is connected to the output of the bridge. An analog-to-digital converter is connected to the output of the amplifier. The converter is also connected to a microprocessor, which microprocessor has an output connected to a digital display which in response to the signals from the microprocessor will display a value representative of the magnitude of the force applied to the end of the cantilevered beam. Importantly, the converter is of the two-slope type and includes a pair of switches, one of which is connected to the output of the amplifier and the other of which is connected to the current input to the bridge. Both of these switches are under the control of the microprocessor by virtue of which the supply voltage itself is used as a reference voltage. Additionally, a third switch also under the control of the microprocessor is connected in the line supplying current to the bridge so that current to the bridge may be cut off during the measuring operation in order to then measure the offset voltage at the output of the amplifier. 
     In the device according to the invention, the only remaining adjustments are in connection with the sensor utilized, and not with the electronics. Such a device resorts simultaneously to an analog part and to a digital part, forming an indivisible whole to the extent to which the operation can only be viewed in an overall manner. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic vertical sectional view of a scale of the invention; 
     FIG. 2 is a top view, enlarged, of a detail of FIG. 1; 
     FIG. 3 is a schematic of the electronic circuit used in the scale of FIG. 1; and 
     FIG. 4 is a chart explaining the various operating sequences. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The weighing device represented in FIG. 1 encompasses a stand 1, at the top of which there is a horizontal tray 2, designed to support the article 3, whose weight one wishes to determine. Within stand 1 there is a cantilever beam 4, one end of which is fixed in the side wall of stand 1. Beam 4 is fashioned in a manner such as to extend in substantially a horizontal position below tray 2, its free end 4c being on the same vertical centerline as the midpoint of tray 2. Between tray 2 and cantilever beam 4 there are four levers 5 (of which two only are represented in the drawing), which are mounted in articulated fashion with one of their ends on the upper part of stand 1, and with the other end on the free end 4c of beam 4; in fact, tray 2 rests on intermediate points 5a of said levers 5, by means of supporting arms 6. With this construction, the free end 4c of cantilever beam 4 is subjected to a downward vertical force which is proportional to the magnitude of the weight of an article 3 supported by the tray 2. The proportionality is a function of the relative lengths of the lever arms of the levers 5. 
     Cantilever beam 4 has mounted thereon four strain gauges 8 with identical resistance R. A first pair of gauges, 8 1  and 8 3  (FIG. 2), is mounted side by side on the upper surface 4a of beam 4, in a manner such as to work in tension, and a second pair of gauges 8 2  and 8 4  , is mounted side by side on the lower surface 4b of beam 4, in a manner such as to work in compression. These strain gauges 8 1  to 8 4  (whose electrical resistance R varies as a function of the weight of article 3 placed on tray 2) are designed to be integrated into electronic circuit 12 (FIG. 3). The electronic circuit of FIG. 3 generates a digital signal representing the weight of the article, which signal is then transmitted to display unit 13. 
     For that purpose, strain gauges 8 1  to 8 4  are electrically connected among themselves in a manner such as to form a resistance bridge 9 (FIG. 3), the connections being such that gauges 8 1  and 8 3  make up two of the opposite sides of said bridge 9, while gauges 8 2  and 8 4  the two other opposite sides of the bridge. Two of the opposite vertices 9 14  and 9 23  of bridge 9 are thus designed to operate as input terminals of said bridge, while the two other opposite vertices 9 12  and 9 34  are designed to operate as output terminals of the bridge. 
     Bridge 9 is designed to be supplied by a direct-current-source, 10, whose positive pole 10a is connected to input 9 14  of the bridge by means of switch 11. Switch 11 consists of a transistor, whose emitter is connected to said positive terminal of the source, and whose collector is connected to said input 9 14  of the bridge. The midpoint 10 [of the voltage-source] is connected to the ground, as is the other input 9 23  of the bridge. The negative pole 10c of said voltage-source 10, as well as its positive pole 10a, are furthermore connected to the various constituent elements of circuit 12, in a manner such as to supply these elements (the connections are not represented in the drawing, but are simply sketched out by means of arrows issuing from poles 10a and 10c). So far as the two output terminals 9 23  and 9 34  of bridge 9 are concerned, they are connected to the two inputs of an amplifier 14, which is equipped with a negative-feedback resistor 15, connected between its output and that input which is connected to terminal 9 34 . 
     The output of amplifier 14 is connected to one of the voltage-inputs 16a of a two-slope analog-to-digital converter 16, whose other voltage input 16b is connected to input terminal 9 14  of bridge 9. Said converter 16 comprises, in known fashion, an integrator 17, a comparator 18, and two electronic switches 19 and 20, all of which are arranged in the following manner: switches 19 and 20 are connected, respectively, by one of their terminals, to the respective voltage-inputs 16a and 16b of converter 16, while their other terminal is connected (through a resistor 21) to one of the inputs of integrator 17; the other input being connected to the ground. The output of integrator 17 (said integrator being equipped with a capacitor 22 connected between its output and that input that is connected to switches 19 and 20) is, for its part, connected to one of the inputs of comparator 18, whose other input is connected to the ground, and whose output constitutes the output 16c of converter 16. 
     Processing circuit 12 furthermore encompasses a microprocessor 24, the function of which is to control the entire circuit. The microprocessor may be, for example, the microprocessor sold under the designation TMS-1000 by Texas Instruments, Inc. The microprocessor is programable to perform the functions detailed below and includes, in known manner, a clock, program memories, data memories, and a control unit with elements capable of effecting arithmetical operations (elements which are not represented in the drawing). Said microprocessor 24 is connected, through its input 24a, to output 16c of converter 16, and by its output 24e to the display unit 13. Microprocessor 24 furthermore includes three control outputs 24b, 24c and 24d, respectively connected to the base of transistor 11, to the control of switch 20 and to the control of switch 19. Control outputs 24c and 24d have the function of triggering--at well-defined moments, to be explained further below--the opening or closing of the respective switches, 20 and 19. Control output 24b, specifically, has the function (again at well-defined moments) of either making transistor 11 conducting, so as to apply a voltage U AL  to input-terminal 9 14  of bridge 9 (voltage U AL  being substantially equal to one-half the voltage of source 10); or of blocking transistor 11, so as to apply a voltage that is substantially zero, to said input-terminal 9 14 . (The reason that the voltage is substantially zero is that the other input 9 23  is grounded, and gauges 8 1   to 8 4  have low impedance). 
     The function of bridge 9 is to generate, whenever supplied with voltage U AL , a voltage ΔV between its outputs 9 12  and 9 34 , such that: 
     ΔV=kU AL  f(ΔR)=kU AL  f(P) 
     where k designates a constant, P is the magnitude of the weight of the article placed on tray 2, and ΔR is the variation in the resistance of gauges 8, caused by the presence of weight P (the value of ΔV is substantially proportional to ΔR, for reasonable strains). That output voltage ΔV becomes zero whenever bridge 9 is no longer supplied (transistor 11 is blocked). 
     The task of amplifier 14 is to amplify voltage ΔV, injected at its input, by providing at its output a signal U M  such that: 
     
         U.sub.M =G(ΔV+U.sub.of) 
    
     where G and U of  represent, respectively, the gain and the offset voltage of the amplifier. (As is known, all real amplifiers feature an offset voltage that varies as a function of a number of parameters such as supply voltage of the amplifier, ambient temperture, aging of amplifier). 
     Whenever bridge 9 is no longer supplied, amplifier 14 thus provides at the output a signal U&#39; M  such that: 
     
         U&#39;.sub.M =G U.sub.of 
    
     The function of analog/digital converter 16, in turn, is to handle, jointly with processor 24, the conversion of the analog voltages U M  and U&#39; M  into digital values, and to work out the difference (U M  -U&#39; M ), so as to yield a result that is independent of the variable offset-voltage U of  of the amplifier 14. In totally unexpected fashion (as will be shown further below), the use of voltage U AL  as a reference voltage for converter 16 allows one to get rid of the error that may derive from the voltage drift of source 10; consequently, an ordinary commercial battery may be used as that voltage source, instead of having to use costly stabilized voltage supply sources. (For all practical purposes, the result of the measurement becomes independent of the voltage drift of source 10). 
     Operation 
     The operation of the device just described is as follows: when article 3, to be weighed, is place on tray 2 of the device, tray 2 applies to the free end 4c of cantilever beam 4 a downward vertical force that is proportional to weight P of said article 3. In this fashion, gauges 8 1  to 8 4  are subjected to variation in resistance, ΔR, that is a function of that weight P. The electric circuit associated with the gauge bridge 9 thereupon functions in the following manner, over an entire measuring cycle. Let us, first, call respectively, in FIG. 3 and FIG. 4, ALPONT, SWR and SWM, the signals supplied by the respective control-outputs 24b, 24c and 24d, of processor 24; U S  is the signal provided at the output of integrator 17; COMP is the signal generated as the output of comparator 18; and U M  is the voltage that appears at the output of amplifier 14. 
     At time T 0  =0 (start of the measuring cycle), microprocessor 24 commands, respectively, the cutoff of the supply of bridge 9, and the closing of the measuring-switch 19 (switch 20 has remained open ever since the end of the preceding cycle). During a fixed period of time T 1  that had previously been set for integrator 17 (see the variation of U S  in the diagram of FIG. 4), said integrator 17 thereupon proceeds to integrate the value U&#39; M , delivered by amplifier 14. 
     At the end of said time-period T 1  --i.e., at time t 1  --microprocessor 24 then commands, respectively, the supplying of bridge 9, the opening of the measuring switch 19, and the closing of the reference switch 20. Integrator 17 then proceeds, during a variable time-period T&#39; 2 , to discharge the stored value U S  ; that discharge stops automatically when comparator 18 toggles at the end of said time-period T&#39; 2  (i.e., at time t 2  ). At time t 2 , microprocessor 24 commands the opening of reference-switch 20, while simultaneously proceeding to store in the memory the value T&#39; 2  --all this until time t 3 . 
     At that moment t 3 , microprocessor 24 commands the closing of measuring-switch 19; integrator 17 then proceeds, within the same fixed time-period T 1 , to integrate the value U M , supplied by amplifier 14. 
     At the end of that time-period T 1  --i.e. at time t 4  --microprocessor commands the opening of switch 19 and the closing of switch 20; integrator 17 proceeds, for a variable time-period T 2 , to discharge the stored value U S . That discharge stops automatically when comparator 18 toggles at the end of time-period T 2  (i.e., at time t 5 ). At time t 5 , the microprocessor commands the opening of switch 20, and simultaneously takes into account the value T 2  supplied to it by comparator 18. The microprocessor then works out the difference (T 2  -T&#39; 2 ), and, after processing, sends the appropriate signal to display 13, which signal is precisely representative of the weight of article 13 located on the tray 2 of the device. A short time t 6  after time t 5  (an offset necessary to let microprocessor 24 effect above-mentioned operations), the microprocessor commands the cutoff of supply to bridge 9, and the closing of measuring-switch 19, so that the circuit is ready to carry out another measuring cycle. 
     The signal supplied by microprocessor 24 to display 13 is quite representative of the weight P of article 3, since above-mentioned operations made it possible to suppress the parasite signal due to the offset voltage U of  of amplifier 14. This signal is particularly representative of weight P, since it turns out to be completely independent of any possible fluctuations in supply voltage U AL  (and hence of the voltage of source 10), as we shall now prove. 
     Let use call R 1  and C, respectively, the values of resistor 21 and the capacitance of capacitor 22, associated with integrator 17. The value U S  worked out by integrator 17 then follows, respectively, during the time period contained between times t 0  and t 2 , the equation: ##EQU1## and, during the time period contained between times t 3  and t 5 , the equation: ##EQU2## 
     Since the frequency of repetition of the measuring cycle is chosen so as to be significantly greater than the drift of the values U AL  and U of  (hence that of the values U M  and U&#39; M ), equation (1) may also be written: ##EQU3## 
     In analogous fashion, equation (2) may also be written: ##EQU4## 
     Subtracting one equation from the other, one obtains: ##EQU5## and by replacing U M  and U&#39; M  by their previously indicated expressions: ##EQU6## Hence the final equation: ##EQU7## 
     Equation (5) clearly shows that the value (T 2  -T&#39; 2 ) that is supplied to the display is representative of weight P, and that it no longer depends either on the offset voltage U of  of amplifier 14, or on the supply voltage U AL  (hence on the voltage of source 10). 
     The device just described offers numerous advantages, among which one could cite the following: 
     total absence of devices for adjusting the offset voltage of the amplifiers; 
     unaffected by any variation in such offset voltages, caused by variations in the temperature, or in the supply voltage, or by a long-term drift in the amplifiers; 
     elimination of stabilizing circuits for the voltage supplied to the measuring bridge, due to the self-compensating nature of the measurement. 
     Such a device is therefore particularly suited for equipment that is to be supplied by batteries. 
     Of course, the device according to the invention is not limited to above-mentioned weighing application; instead, it can be used in any application utilizing resistive sensors (which could be other than strain gauges), whose output voltage is proportional to the effect to be measured. Thus, the use of temperature sensors, or of any other appropriate type of sensor, could be considered.