ELECTRONIC BALANCE

Abstract: A load on the tray 18 is balanced coarsely and finely by coils 2 and 3 which move in a permanent magnetic field and carry balancing currents. Displacement detector 1 controls the current in coil 3. When the ranges of this fine control is exceeded the voltage across R applied to A to D converter 7 is input to memory 16 containing a program for control of a presettable counter 10. The counter 10 generates a first switching signal on reaching the count set by 16 and a second switching signal when reset by multivibrator 13, thereby controlling the duty factor of pulses supplied to winding 2 via change-over switch 8.

Description: The present invention relates to an electronic balance, and more particularly to an electronic balance utilizing as a balancing force an electromagnetic force which is automatically controlled so as to be balanced against a weight to be measured. In general, known electronic balances have a force equilibrating unit comprising an electromagnetic coil and a magnetic circuit to provide a static magnetic field, the electromagnetic coil being mechanically connected with a weight-receiving tray and supported movably in a vertical direction in said static magnetic field. The electromagnetic coil, supplied with an electric current, produces an electromagnetic force to oppose a weight placed on the weight-receiving tray. The weight value is given by the current which makes the electromagnetic force just balanced against the weight to be measured. In relation to this type of balance the present patent applicant has already proposed in European Patent Specification 0 039 249, first published on 4 th November 1981, after the priority date of the present application an electronic balance in which the electromagnetic coil is supplied with an "alternating" pulse current, namely, a pulse current whose pulse polarity is alternating. The frequency of the pulse is chosen sufficiently high so that the balance may be prevented from possible mechanical vibrations. The electromagnetic force opposing the weight is regulated by automatically controlling the duty factor of the pulse current, with the amplitude and frequency kept constant. The average current, which determines the electromagnetic force, depends on the duty factor of the pulse. The use of an "alternating" pulse current is to keep the Joule heat generation by the coil always constant irrespective of the average value of the pulse current. There are also disclosed some other kinds of electronic balance in which a "non-alternating" pulse current is used. In any way, the most important problem involved in an electronic balance in which a pulse current is used is to measure the duty factor of the pulse so precisely as to comply with a resolving power required for the balance. The duty factor of a pulse current is, in general, measured with a clock signal whose frequency is sufficiently higher than that of the pulse. In principle, therefore, the resolving power can be increased to any degree by increasing the frequency of the clock. In practice, however, there are some technological and economic difficulties and disadvantages in increasing the clock frequency on a large scale. In case of a balance using a 1kHz-pulse current a 1MHz-clock gives a resolving power of the order of only 10⁻³. If, as is often the case with a precision balance, a resolving power of the order of 10⁻⁶ is needed, a 1GHz-clock must be used. The use such a high frequency clock in a balance is apparently unpractical in both technological and economic aspects. U.S. Patent Specification US-A-4 189 017 proposes to avoid the use of a high frequency clock in a balance by compensating the digitalization errors which occur when a voltage signal representing the displacement of the balance pan is compared by a pulse length modulator with a sawtooth voltage and the output of the modulator is passed to a synchronizer fed with the clock pulses. The compensation is effected by means of an integration circuit which modifies the signal input to the pulse length modulator. The present invention aims at solving the problems involved in measuring the duty factor of the pulse current flowing through the electromagnetic weight-balancing coil, and it is an object of the present invention to provide an improved electronic balance which gives a precise value of measurement without incorporating such a very high frequency clock as is mentioned above. Another object of the invention is to enable such an improved electronic balance to give a precise value of measurement in a short time. A further object of the invention is to enable such an improved electronic balance to be manufactured in a simple construction and at low cost. Other objects and advantages of the present invention will become apparent from the detailed description of the invention given hereinafter in conjunction with the embodiments and appended drawings. To accomplish the above mentioned objects an electronic balance as claimed provided comprising two weight-balancing systems: a coarse balancing system and a fine balancing system. The coarse balancing system comprises an electromagnetic coil whose attainable maximum force reaches the maximum scalable weight of the balance, a pulse current supplier to the coil, and a pulse current duty factor controller which coarsely regulates the duty factor of the pulse according to a relatively rough digital signal. The value of the digital signal changes in accordance with a predetermined program until the tray loaded with a weight to be measured is brought to a coarsely balanced position within a predetermined range. The fine balancing system also contains an electromagnetic coil, which gives the tray a fine balancing in accordance with an output from a tray displacement detector provided to detect the displacement of the tray. The current flowing through this coil is digitalized, and then added to said digital signal concerned with the duty factor regulation in the coarse balancing system, to give the precise result of weight measurement. Brief Description of the Drawing Fig. 1 shows a block diagram of a first embodiment of the invention. Fig. 2 shows a block diagram of a second embodiment of the invention. Fig. 3 shows the currents flowing through the electromagnetic coils in the embodiment shown in Fig. 1. Fig. 4 shows the currents flowing through the electromagnetic coils in the embodiment shown in Fig. 2. Fig. 5 shows a block diagram of a third embodiment of the invention. Fig. 6 shows the circuit in the electromagnetic coil in the embodiment shown in Fig. 5. Fig. 7 is a flow chart illustrating the function of the embodiment shown in Fig. 1. Figs. 8, 9, 10 and 11 are flow charts illustrating different ways of determining the presetting signal in the embodiment shown in Fig.1. Detailed Description of the Invention Referring to Fig. 1 which shows a block diagram of a first embodiment of the present invention, a force equilibrating unit 1A contains two electromagnetic coils 2 and 3, and a tray displacement detector 1 shown in the vicinity of a weight receiving tray 1B. The unit 1A is devised so that a weight loaded to the tray 1B may be balanced coarsely in the first place by the force produced by the coil 2 and then balanced precisely by that produced by the coil 3. The coils 2 and 3, wound on the same bobbin and mechanically connected with the tray, are designed to be movable freely in a vertical direction in a static magnetic field provided by a permanent magnet assembly (not shown in the figure). The electromagnetic coil 2, a change-over switch 8, a constant-current supply 9 and a presettable up-and-down counter 10 constitute a coarse balancing system. The current outputted from the constant-current supply 9 is converted to an "alternating" pulse current by the change-over switch 8 which is driven by the digital output from the presettable up-and-down counter 10. The coil 2 is thus supplied with an alternating pulse current. In the figure the change-over switch 8 is illustrated only schematically, but it is really made up of any known electronic change-over switching circuits. On the other hand the displacement detector 1, a displacement signal amplifier 4, a PID controller 5, a power amplifier 6, the electromagnetic coil 3 and a resistor R constitute a fine balancing system. The coarse balancing system covers the whole weighing range of the balance coarsely, whereas the fine balancing system covers only a limited range but precisely. On receiving on the tray 1B a weight to be measured the tray displacement detector 1 outputs a tray displacement signal to the PID controller 5 through the displacement signal amplifier 4. This signal causes the PID controller 5 to develop a current in the system to make the coil 3 produce a force opposing the weight. If the weight is small so as to be overcome by a current within the maximum value that the PID controller 5 can control, the balance comes to be precisely balanced only by the fine balancing system. The resistor R develops a voltage which corresponds to the balancing current flowing through the coil 3. This voltage is converted to a digital signal by an A-D converter 7, and then outputted therefrom toward a weight value indicator 15 through an adder 14 without any value added to the digital signal in the present case. In case the weight overwhelms the control capacity of the PID controller 5, a saturation signal is inputted to a memory 16 from the A-D converter 7. On receiving the saturation signal the memory 16 outputs a presetting signal to the presettable up-and-down counter 10 in accordance with a predetermined program stored in the memory 16. With the presetting signal inputted the counter 10 starts counting the clock signal sent from a clock 11, and outputs a first switching signal when it completes counting the clock signal up to the number determined by the presetting signal. The counter 10 is further devised also to output a second switching signal when reset by the reset signal from a monostable multivibrator 13, which is driven at a frequency made reduced from that of the clock 11 through a frequency divider 12. The change-over switch 8, driven by these first and second switching signals, alternates the direction of the current to be supplied to the coil 2, so that the coil 2 is supplied with an alternating pulse current. The frequency of the pulse, determined by the reset signal, is equal to the frequency of the monostable multivibrator 13, while the duty factor is determined by the counted number of the clock pulses. The presetting signal to determine the number of the clock pulses to be counted is successively renewed in accordance with the program prepared in the memory 16 until the duty factor comes to make the average of the pulse current give the coil 2 a force balancing with the weight. Once the coarse balancing is thus attained positioning the weight-receiving tray 1B at a coarsely balanced point, the PID controller 5 becomes free from saturation, and the fine balancing system again starts functioning. Then the coil 3 is controlled finely and the tray 1B is positioned at a precisely balanced point. At the adder 14 the output from the A-D converter 7 is added to the presetting signal outputted from the memory 16. The output of the adder 14 is displayed at a displayer 15 as a precisely measured weight value. Fig.7 shows a flow chart explaining the performance of the abovedescribed first embodiment. It is judged at Step 91 if the fine balancing system is saturated. If the system is judged not to be saturated, at Step 92 the adder 14 takes in a fine balancing value W₂ obtained by the fine balancing system, namely the output from the A-D converter 7. Step 92 is followed by Step 93 at which the adder 14 adds the value W₂ to a value W₁ given by the coarse balancing system, namely the presetting signal sent from the memory 16. A tare is subtracted from the sum W = W₁ + W₂ at Step 94, and then a final measured weight is displayed at the displayer (Step 95). On the other hand, if at Step 91 the fine balancing system is judged to be saturated, the presetting signal is renewed at Step 96 according to the program. At Step 98 it is judged if the coarse balancing system normally produces a: balancing force. If the system is judged working normally, Step 98 is followed again by Step 91. This process is repeated until Step 91 judges the fine balancing system made free from saturation. In the process described above there are considered four kinds of process of renewing the presetting signal. These are explained by the partial flow charts shown in Figs. 8, 9, 10 and 11. The partial flow charts correspond to a part defined between the points A and B in Fig. 7. Fig. 8 shows the first way of renewing the presetting signal. If the fine balancing system is judged to be saturated on the positive side, the presetting signal W₁ is replaced by W₁ + 1 (Step 97A). If the system is saturated in the negative side the presetting signal W₁ is replaced by W₁ - 1 (Step 97B). In each case it is judged at Step 97C or 97D if the renewed value makes the coarse balancing system produce a force less than the maximum controlling force of the fine balancing system. The process is repeated until Step 91 (Fig. 8) judges the fine balancing system made free from saturation. Fig. 9 shows the second way of renewing the presetting signal. If the fine balancing system is judged to be saturated on either the positive side or the negative side, the presetting signal W₁ is replaced by W₁ + N or W₁ - N, respectively, where N is a predetermined integer larger than unity. In repetition of the process if the saturation polarity of the fine balancing system changes to the opposite (negative or positive respectively) sign, a value of unity is, respectively, subtracted from or added to the presetting signal. Fig. 10 shows the third way of renewing the presetting signal. An integer N to be added to or subtracted from the presetting value W₁ is initially predetermined at a value equal to the decimal number corresponding to the number of bits of the presettable counter 10. In the repetitional process of renewing the presetting signal, the integer N is successively reduced by half until the fine balancing system is judged saturated either on the positive side or the negative side. This way enables the whole balancing system to be settled in the shortest time. Fig.11 shows the fourth way of renewing the presetting signal. In this example, a comparator 27 shown with a dotted line in Fig. 1 also is put into action. As is shown by the flow chart, the presetting signal is increased or decreased by N pulses when not only the fine balancing system but also the comparator 27 is saturated, and increased or decreased by one pulse when only the fine balancing system is saturated. In this case a set level of the comparator 27 is made larger than N times the saturation input of the fine balancing system so that, when the comparator 27 comes to output a non-saturated signal, the fine balancing system still remains saturated. Therefore, the presetting signal changes toward a final value with the changing direction kept unchanged. A second embodiment of the invention is shown in Fig. 2. In this embodiment the fine balancing system comprises a tray displacement detector 1, a displacement signal amplifier 4, a PID controller 5, a voltage-pulse width converter 26, a constant-current supply 9A, a switching circuit 17 and an electromagnetic coil 3. In such a construction of the fine balancing system, the coil 3 also is supplied with a pulse current through the switching circuit 17 which, driven by the pulse from the voltage-pulse width converter 26, outputs a pulse current whose average value is controlled by the PID controller 5. The output of the fine balancing system is inputted to the same adder 14 as is used in Fig. 1 through an AND gate 25 and a counter 18. Fig. 4 shows the currents flowing through the coil 3 and the coil 2 in the coarse balancing system. A third embodiment of the invention is shown in Fig. 5. This embodiment has one electromagnetic coil 2A which is commonly used for both the fine balancing system and the coarse balancing system. The coil 2A being commonly used this embodiment comprises a voltage-current converter 20. Fig. 8 shows a current flowing through the coil 2A, in which the dotted line means a current increase within the maximum controllable range of the fine balancing system. Further, all the embodiments described above can be modified by using a non-alternating pulse current in the coarse balancing system. In this case it is necessary to provide any suitable countermeasure to overcome the Joule heat variations which arise in the electromagnetic coil depending on the weight value to be measured. The present invention, as is made clear, dissolves many disadvantages and difficulties involved in the prior art of improving a resolving power and measuring speed of electronic balances in which a pulse current is used to produce a balancing force.

Claims: 1. An electronic balance comprising: a force equilibrating unit (1A) which electromagnetically compensates a weight to be measured, consisting of a weight receiving tray (1B), a displacement detector (1) for detecting the displacement of said weight receiving tray, a magnetic circuit to provide a static magnetic field, and a force coil assembly (2, 3) mechanically connected with said weight receiving tray and movably located in said static magnetic field; a clock signal source (11); a first current-supplying means (8, 9) for supplying to said force coil assembly a pulsed first current; means (12, 13) for dividing the clock frequency to establish the pulse repetition frequency of the first current; means (10) for counting clock signal pulses to determine the duty factor of the first current, the number of pulses to be counted being determined by a first digital value represented by a presetting signal applied to the counting means (10); a second current-supplying means (5, 6) for supplying to said force coil assembly a second current corresponding to the output of said displacement detector (1); a digital value determining means (16) for determining said first digital value when the output of said displacement detector reaches a predetermined upper or lower limit; a digital value generator (7) for generating a second digital value which corresponds to the average value of said second current; and an farithmetic unit (14) which derives a weight value by adding said second digital value to said first digital value. 2. An electronic balance as claimed in claim 1 wherein the force coil assembly comprises a first coil (2) to which the first current is supplied and a second coil (3) to which the second current is supplied. 3. An electronic balance as defined in claim 1 or 2, wherein the polarity of said pulsed first current is alternating. 4. An electronic balance as defined in claim 1 or 2, wherein said digital value determining means is constructed so as to function in accordance with a predetermined program. 5. An electronic balance as claimed in claim 1 in which the force coil assembly comprises a first coil (2) to which the first current is supplied and a second coil (3) to which the second current is supplied, said second current being controlled by a control pulse input to a current supply means (17) from a pulse generating means (26).

Inventors: KOMOTO, AKIRA
Applicants: SHIMADZU CORPORATION
Application year: 1982
Main classification: G01G7
IPC codes: G01G7