JP3473197B2 - Electronic balance - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electronic balance of the electromagnetic force balance type, and more particularly, to a digital conversion of a displacement detection signal of a load receiving portion due to a measured load, and to a digital conversion of the digital signal. The present invention relates to an electronic balance having a so-called digital servo mechanism for controlling an equilibrium operation using digital conversion data. 2. Description of the Related Art In an electromagnetic balance of the electromagnetic force balance type,
In general, the displacement sensor detects the deviation of the load receiving part from the balance point due to the measured load, and the current corresponding to the detection result is applied to the force coil placed in the static magnetic field to respond to the measured load. Equipped with a servo system that corrects the deviation of the load receiving part from the balance point by generating the generated electromagnetic force.The force coil is in a state where the measured load and the generated electromagnetic force are balanced and the load receiving part is at the balance point. The magnitude of the load to be measured is determined from the magnitude of the current flowing through the device. Conventionally, analog servos are often used as a servo system in such an electromagnetic force balance type electronic balance. However, an amplified signal of the output of a displacement sensor is converted into a digital signal, and the digitally converted data is used. A so-called digital servo system that determines a feedback current value to be passed through a force coil by digital calculation has also been proposed (for example, Japanese Patent Application Laid-Open No. 3-65326). [0004] By the way, when the above-mentioned digital servo system is adopted for the servo system of the electronic balance, the conversion speed is high and the A-D with a large number of bits is used.
It is necessary to use a converter, and if the zero point or span of the A / D converter drifts or the linearity changes, the measured value is affected. A- which adopted a high method
A D converter is required, and an A-D
There is a problem that the converter becomes extremely expensive. An object of the present invention is to provide a simple and inexpensive A / D converter which does not affect the measured value even if the zero point or span drift or linearity of the A / D converter changes. An object of the present invention is to provide a digital servo type electronic balance with high resolution and high accuracy using a converter. In order to achieve the above object, an electronic balance according to the present invention is generated by applying a current to a force coil 2 provided in a magnetic field, as shown in FIG. In a balance that balances the electromagnetic force to be measured with the load W to be measured and obtains the load to be measured from the magnitude of the current flowing through the force coil 2 in the equilibrium state, the detection signal D of the displacement of the load receiving unit 1 due to the load to be measured W A / D converter 6 to be digitized, digital operation / control means 7 for determining a feedback current value to be supplied to force coil 2 using digital data from A / D converter 6, and digital operation / control A current supply means 8 for supplying a current corresponding to the data supplied from the means 7 to the force coil 2 is provided. The signal D and the reference point signal D ₀ of the displacement detection signal D are selectively input at a preset cycle, and the digital operation / control means 7 outputs the digital signal from the A / D converter 6. data using, as the digital conversion data of the displacement detection signal D is coincident with the digital conversion data of the reference point signal D _0, characterized by determining the feedback current value. In an electronic balance employing a normal digital servo system, the deviation becomes zero by PID calculation or the like using digital conversion data of a displacement detection signal of the load receiving portion, that is, the load receiving portion has So that the displacement detection value becomes 0,
Determine the feedback current value. Therefore, for example, when a zero point drift occurs in the AD converter that digitizes the displacement detection signal, even if the servo system effectively functions so that the deviation becomes apparently zero, the deviation of the drift actually becomes larger. The system balances in the remaining state, the balance point shifts,
Affects measured values. On the other hand, in the present invention, the analog input to the A-D converter 6, and the detection signal D of the displacement of the load receiving portion 1, periodically switched to the reference point signal D ₀ of the displacement detection signal, Digital conversion data of the displacement detection signal D and its reference point signal D ₀ are supplied to the digital operation / control means 7. Then, the digital arithmetic and control unit 7 determines a feedback current value as digital conversion data of the displacement detection signal D coincides with digital conversion data of the reference point signal D _0. That is, the digital operation / control means 7
In the above, instead of controlling the magnitude of the feedback current value so that the digital conversion data of the displacement detection signal D becomes a value that absolutely represents 0, the digital conversion data has a value representing the reference point signal D _0. The magnitude of the feedback current value is controlled so as to be as follows. Therefore, in the configuration of the present invention, even if, for example, a zero point drift occurs in the A / D converter 6 and the digital conversion data of the displacement detection signal D changes by that amount, the digital _value of the reference point signal D ₀ changes. The same A-
Since they are digitized by the D converter 6, they change by the same amount. If the feedback current value is controlled so that these two data coincide, the drift of the A / D converter 6 is canceled, There is no shift of the balance point and no effect on the measured values. Further, since control is performed so that both the digital conversion data of the displacement detection signal D and its reference point signal D ₀ are made to match, even if a span drift or a change in the linearity of the A / D converter 6 occurs, the system of the system is changed. Although it affects the gain increase / decrease, the balance point does not deviate, so that the measured value is not affected. FIG. 1 is a block diagram showing an entire configuration of an embodiment of the present invention. FIG. 2 is a diagram showing a specific example of a circuit configuration of a portion surrounded by a chain line in FIG. is there. The measuring dish 1a to which the measured load W is applied is
It is supported by a load receiving portion 1 that can be displaced vertically in the drawing. A force coil 2 is mounted on the load receiving portion 1, and the force coil 2 is arranged in a static magnetic field created by a magnetic circuit (not shown) mainly composed of a permanent magnet.
An electromagnetic force corresponding to the magnitude of the feedback current flowing therethrough is generated, and the balance is made so that the load receiving portion 1 is positioned at a predetermined position against the load W to be measured as described below. The displacement of the load receiving portion 1 is detected by a displacement sensor 3 using, for example, two photo sensors. The output of the displacement sensor 3 is amplified by the preamplifier 4 and input to the A / D converter 6 via the switch 5 as a displacement detection signal D. The switch 5 has a signal D corresponding to a reference point of the displacement sensor 3 in addition to the displacement detection signal D from the preamplifier 4.
₀ , that is, 2 constituting the displacement sensor 3 in the example of FIG.
Potential signal at the middle point of two photosensors, ie, basically 0
A signal D _{0 which} is V is led. The switch 5 is switched at a constant cycle by a control signal from the digital operation / control unit 7, and therefore, the analog input of the A / D converter 6 has a displacement detection signal D and a reference point signal D at a constant cycle. ₀ will be input alternately. The A / D converter 6 has a sawtooth wave generating circuit 61 for generating a sawtooth wave whose voltage varies between -V and + V in response to the input of a trigger pulse _{PT having} a constant period, and a sawtooth wave generating circuit for the sawtooth wave. 61 and the A / D converter 6
Comparator 62 for comparing the analog input to, an AND gate 63 which receives the output of the comparator 62 and the clock pulse P _C, counts the clock pulses P _C which has passed through the AND gate 63, the arrival of the reset pulse P _R The counter 64 outputs the count value and resets the same at the same time. As shown in a time chart showing the signal waveform of each part in FIG. or voltage value of the reference point signal D ₀ is counted by a counter 64 to count clock pulses in a time that exceeds the voltage value of the sawtooth wave, and outputs the counted value as a digital conversion value. According to the AD converter 6, the analog input is 0
In the state of V, the digital conversion output becomes 1/2 full scale value. Incidentally, the trigger pulse P _T and the reset pulse P _R or the like is supplied from the digital arithmetic and control unit 7. The drive of the switch 5 is also controlled by a command from the digital operation / control unit 7. [0014] Digital conversion output of the A-D converter 6, i.e. the digital conversion data displacement detection signal D, digital conversion data of the reference point signal D ₀ are respectively incorporated into the digital computing and control unit 7, the force coil 2 To determine the magnitude of the feedback current value to be supplied to the power supply. The digital operation / control unit 7 determines a feedback current value by a method exemplified below, supplies the determined result to the current supply unit 8, averages the feedback current value every moment, and then sets a constant coefficient. (Span coefficient)
, And the measured value (measurement display value) is determined, and the value is displayed on the display 9. The current supply unit 8 includes a digital operation / control unit 7
For generating a current having a magnitude corresponding to the data supplied from the controller and supplying the current to the force coil 2 using, for example, a DA converter and a voltage / current conversion circuit for converting the output voltage thereof into a current. Or a switching circuit for switching the constant current source and its output current, and by controlling the operation of the switching circuit according to the input data, a pulse current of a constant peak value and a constant cycle, and the duty is input. A known circuit such as one using a so-called pulse width modulation method that changes according to the size of data can be employed. The output of the temperature sensor 10 for measuring the temperature near the permanent magnet for generating a static magnetic field in which the force coil 2 is placed is introduced into the current supply unit 8.
The magnitude of the feedback current is corrected to compensate for a change in the generated electromagnetic force due to a change in the temperature of the permanent magnet. [0016] Now, in the digital computing and control unit 7, the digital conversion data of the displacement detection signal D and the reference point signal D ₀ is input alternately through A-D converter 6, using these, the displacement detection signal D as digital conversion data matches the digital conversion data of the reference point signal D _0, to determine the feedback current value. That is, for example, when the feedback current value is determined by calculating the displacement detection value by PID (proportional / integral / derivative), the calculation is not performed simply by using the digital conversion data of the displacement detection signal D as a parameter, but the displacement detection signal D to PID calculation of the difference between the digital conversion data of the digital input data and the reference point signal D ₀ as a parameter. Here, the reference point signal D ₀ is supplied to the AD converter 6.
Is input, the digital arithmetic and control unit 7 is not supplied with information on the displacement detection signal D at that time. Therefore, in a state where the data of the reference point signal D ₀ is taken in, the digital operation / control unit 7 sends the data to the current supply unit 8 so as to continuously supply the feedback current value immediately before that to the force coil. The switching of the switch 5 may be performed every AD conversion cycle (for example, 1 ms). However, the displacement detection signal D is mainly AD-converted to improve the stability of the control, and the reference point signal D It is desirable that ₀ is set so as to perform A-D conversion at relatively low frequency. According to the above-described embodiment of the present invention, even if the A / D converter 6 has a zero point, a drift of a span, or a change in linearity, the servo system always detects displacement. The digital conversion value of the signal D is the reference point signal D
_In order to balance the load receiving unit 1 by functioning to match the digital conversion value of _0, the drift amount and the like included in the digital conversion data are canceled by each digital conversion data, the balance point is always constant, and the measured point is measured. No error occurs in the measured value of the load W. The A / D converter 6
When the span or linearity of the system changes, the gain of the system changes, but since the balance point does not shift, the measured value itself is not affected. Here, in the above embodiment, the reference point signal D ₀ of the displacement detection signal D is set to a signal which is basically 0 V and is taken out from the middle point of the two photo sensors constituting the displacement sensor 3. However, the present invention is not limited to this,
As shown the circuit diagram of a main part in FIG. 4, a half of the value and the output of the power supply V may be used as the reference point signal D ₀ of the displacement sensor 3. In this case, when using the above-mentioned sawtooth-wave A / D converter 6, it is assumed that the sawtooth wave changes between 0 and + V. As apparent from this, by employing the reference point signal D ₀ of a 1 / 2V as shown in FIG. 5,
It becomes possible to use an AD converter having a unipolar input. In the configuration of the present invention, the preamplifier 4 is not a linear amplifier, but a non-linear amplifier in which the gain increases as the amount of displacement of the load receiver 1 with respect to the balance point decreases, or the preamplifier 4 has a variable gain. Is used to switch the gain so that the smaller the amount of displacement becomes, the higher the gain becomes, the A / D conversion resolution of the displacement detection signal D and the reference point signal D ₀ is relatively improved near the balance point. As a result, the resolution of the balance can be improved. In order to obtain the stability of the system, when performing this processing, it is preferable to lower the gain by digital operation and keep the gain of the system constant. Further, the electromagnetic force of the force coil 2 is directly applied to the load receiving portion 1 in the example of FIG. 1, but the present invention is applied to one side of the lever which is rotatable about a fulcrum. It goes without saying that a balance mechanism can be employed in which a force coil is provided on the other side of the measuring dish and the rotational displacement of the lever is detected by a displacement sensor. As described above, according to the present invention,
In a digital servo type electronic balance in which the displacement detection result of the load receiving portion is digitized by an AD converter and the feedback current value is determined by digital operation, A
Since the displacement detection signal of the load receiving unit and the reference point signal thereof are selectively input to the D-converter and digitized, and the feedback current value is determined so that these two digital values match, Even if the zero point or span drift of the AD converter or the linearity changes, the balance point of the load receiving portion does not shift, and an accurate measurement value can be obtained. Therefore, the A / D converter has a high resolution, but has some problems in stability such as zero point drift and span drift. The A / D converter has a simple method of comparing a sawtooth wave and an analog input, and an A / D converter having a unipolar input. A highly accurate electronic balance can be obtained using an inexpensive AD converter such as a -D converter.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing an overall configuration of an embodiment of the present invention. FIG. 2 is a circuit diagram showing a specific configuration example of a portion surrounded by a dashed line in FIG. FIG. 4 is a timing chart showing an example of signal waveforms at various parts in FIG. 2; FIG. 4 is a circuit diagram showing a main part of another embodiment of the present invention; Preamplifier 5 Switch 6 A / D converter 7 Digital operation / control unit 8 Current supply unit

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-55-40906 (JP, A) JP-A-49-66157 (JP, A) Fully open 1987-193528 (JP, U) Really open 1979 75531 (JP, U) (58) Fields investigated (Int. Cl. ⁷ , DB name) G01G 7/04 G01G 23/36 G01G 23/37 G01D 3/00

Claims (1)

(57) [Claims 1] The magnitude of the current flowing through the force coil in a state where the electromagnetic force generated by flowing the current through the force coil provided in the magnetic field is balanced with the load to be measured. In a balance for obtaining a measured load from the above, the detection signal of the displacement of the load receiving portion due to the measured load is digitized.
A digital converter for determining a feedback current value to be passed through the force coil using digital data from the A / D converter; and a current corresponding to data supplied from the digital arithmetic unit. With current supply means for supplying to the force coil,
A displacement detection signal of the load receiving section and a reference point signal of the displacement detection signal are selectively input to the A / D converter at a preset cycle. A feedback current value is determined so that digitally converted data of the displacement detection signal coincides with digitally converted data of a reference point signal of the displacement detection signal, by using digital data from the D converter. Electronic balance.