JPH10160551A - Electronic balance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electronic balance having the same performance with a low-resolution A/D-converter as with a high-resolution A/D-converter, by applying a dither to the light-emitting quantity of a light emission element, and preventing the standing wave caused by the resolution of the A/D-converter from occurring. SOLUTION: The driving current of a light emission diode 41 is changed at a frequency corresponding to the frequency of an oscillator 91, and a dither is applied to the luminescence quantity of the light emission diode 41. The incident light quantities to photo-diodes 42a, 42b are momentarily changed. The output of a differential amplifier 44, i.e., displacement detected value, is momentarily changed even if the displacement quantity is constant except for the case that the displacement of a balance beam is true 0, and the input signal to an A/D-converter is momentarily changed. The PID(proportional, integral, differential) output from a PID arithmetic section is changed to various values with the elapse of time, and the balance beam is controlled to be swung according to the change and balanced at the position of the displacement 0. A specific standing wave based on resolution is resolved.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electronic balance,
More specifically, the present invention relates to an electronic balance having a digital servo mechanism.

[0002]

2. Description of the Related Art Generally, in an electronic balance, a plate is engaged with one end of a balance beam, and a force coil of an electromagnetic force generator is fixed to the beam. The magnitude of the current supplied to the force coil is controlled based on the result of the PID (proportional / integral / differential) calculation of the detected value of the rotational displacement of the balance beam so that the rotational displacement of the beam becomes zero. At the same time, the magnitude of the load to be measured acting on the plate is determined from the magnitude of the current flowing through the force coil in the equilibrium state where the rotational displacement is zero, and the weighing display value is determined.

In a conventional electronic balance, the above-described servo mechanism is configured by an analog circuit, and a current flowing through a force coil is converted into a voltage and then digitized, and a weighing display value is determined using the digital data. Was.

On the other hand, in recent years, an electronic balance having a digital servo mechanism has been proposed. In an electronic balance having this digital servo mechanism, a beam displacement detection result is digitized to perform a digital PID operation, and a force is applied. Determine the current value to be supplied to the coil,
The measurement display value is determined based on the digital PID calculation result (usually the I calculation result).

Here, the rotational displacement of the balance beam is usually detected by inserting one end of the balance beam between a light emitting element and a light receiving element facing each other and detecting the output from the light receiving element. More specifically, for example, a slit is formed at one end of the balance beam, and one light emitting element and two light receiving elements are opposed to each other with the slit interposed therebetween, and the outputs of the two light receiving elements are output. The momentary rotation position of the balance beam is detected by differential amplification.
In such an optical position sensor, the drive control of the light emitting element is performed by a fixed drive method driven by a constant power supply circuit, or the light emission amount is detected by a light receiving element, and the light emission amount of the light emitting element is determined based on the detection result. Is used to control the light amount to be constant.

[0006]

By the way, in an electronic balance provided with a digital servo mechanism which digitizes and detects the rotational displacement detection result of the balance beam, the displacement detection result is digitized depending on the target performance. An A / D converter for performing the conversion requires a resolution of 16 bits or more. That is, if the resolution of the A / D converter is low, if the balance beam does not displace beyond a certain range, the digital conversion value remains within the range of the quantization error, and P
As a result, the ID input is unique, the PID output is also unique, the current flowing through the force coil is also unique, and the control result is a state in which a specific standing wave is generated. . In such a state, no matter how the PID output is averaged, the weighing display value is not stable.

A high-speed A / D converter having a high resolution of at least a certain level as described above is expensive. Therefore, when a high-performance electronic balance is to be constructed by a digital servo mechanism, the cost is increased. There is a problem that is forced. Here, as a means for apparently improving the resolution of the AD conversion value, it is generally considered that the displacement detection value is preamplified and then guided to the AD converter. In this case, the dynamic range is accordingly increased. Is reduced.

It is an object of the present invention to provide a low-resolution A / D converter without lowering its dynamic range and substantially the same result as using a high-resolution A / D converter. To provide a low-cost, high-performance, full-digital electronic balance.

[0009]

A configuration for achieving the above object will be described with reference to FIGS. 1 and 2 showing an embodiment. The electronic balance of the present invention applies a load to be measured. And a balance beam 2 to which an electromagnetic force against the load to be measured is applied by the electromagnetic force generating device 3 and the rotational displacement of the balance beam 2 is received by the light emitting element 41. The position is detected by the optical position sensor 4 including the elements 42a and 42b, and the detection result is A-
A digital servo mechanism for digitizing by the D converter 5, determining the electromagnetic force generated by the electromagnetic force generator 3 by digital PID calculation using the digital displacement detection data, and controlling the balance beam 2 to an equilibrium state; In addition, in the electronic balance that determines the weighing display value based on the PID calculation result, the driving circuit 9 of the light emitting element 41 includes dither providing means 91 that changes the light emission amount of the light emitting element 41 at a predetermined frequency. Is characterized by

Here, in the present invention, the waveform, amplitude and frequency of the dither to be given to the light emission amount may be respectively fixed, or controlled so that all or any one of them is intentionally changed. May be.

In the above configuration of the present invention, the light emitting element 4
When dither is given to the light emission amount of No. 1, the light receiving elements 42a,
The displacement detection result of the balance beam 2 based on the output of the A-D converter 42b changes every moment according to the dither of the light emission amount even if the displacement amount (deviation from the equilibrium state) is constant. Output changes every moment. Therefore, the AD
The PID calculation result of the digital displacement detection data from the converter 5 also changes to various values every moment, and a specific standing wave caused by the A / D conversion resolution disappears. The weighed display value obtained by the conversion is stable without being affected by the resolution of the AD converter 5. In this state, the electromagnetic force generated by the electromagnetic force generating device 3 changes every moment in response to the dither of the light emission amount. As a result, the digital servo mechanism moves the balance beam 2 to a predetermined equilibrium state while shaking. Although it will operate, there is no particular problem by appropriately setting the waveform, amplitude, and frequency of dither according to the balance mechanism, or by appropriately controlling it according to the state of the system.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is an overall configuration diagram of an embodiment of the present invention, in which a schematic diagram showing a mechanical configuration and a block diagram showing an electrical configuration are shown together.

A plate 1 for applying a load to be measured is supported on one end of a balance beam 2 which is rotatable about a fulcrum 21. A force coil 31 of the electromagnetic force generator 3 is fixed to the balance beam 2. The electromagnetic force generating device 3 has a structure in which the force coil 31 is movably arranged in a static magnetic field generated by the magnetic circuit 32.
1 generates an electromagnetic force corresponding to the magnitude of the current flowing through the balance 1 and acts to balance the balance beam 2 against the load to be measured, as described later.

The rotational displacement of the balance beam 2 is detected by detecting the position of the slit 22 formed at the tip of the other end with the optical position sensor 4 every moment. The result of the displacement detection is digitized by the A / D converter 5, and thereafter, is taken into the arithmetic unit 6 every moment. Arithmetic unit 6
Is actually composed of a microcomputer and its peripheral devices, but in this figure, a block diagram is shown for each main function based on the program written in the ROM, and the PID is added to the digital displacement data. The PID calculator 61 includes a PID calculator 61 that performs calculations, and a metric value calculator 62 that determines a weighing display value by an averaging process or the like using the calculation results from the PID calculator 61. The calculation result by the weighing value calculation unit 62 is displayed on the display 7. Also, P
The magnitude of the current flowing through the force coil 31 of the electromagnetic force generator 3 is controlled based on the calculation result by the ID calculation unit 61, whereby the balance beam 2 is applied to the balance beam 2 against the load applied to the plate 1. An electromagnetic force is applied to reduce the rotational displacement of the balance beam to zero, and the balance beam 2 is balanced to the equilibrium position. In this example, a pulse current flows through the force coil 31, and the duty of the pulse current changes based on the PID calculation result.

That is, the operation result from the PID operation unit 61 is introduced into a pulse width modulation circuit 81, and is converted into a pulse signal having a constant frequency and a duty according to the PID operation result. This pulse signal is output to switch 8
2 is driven to chop the DC current from the constant current source 83, whereby a pulse current having a constant peak value, a constant cycle, and a duty of which changes according to the PID calculation result flows through the force coil 31. It is.

The optical position sensor 4 is mainly composed of one light emitting diode 41 and two photodiodes 42a and 42b as shown in FIG. These are arranged to face each other with a slit formed at the tip of the balance beam 2 interposed therebetween. Each photodiode 42a, 4
The output of 2b is amplified by the amplifiers 43a and 43b, respectively, and then differentially amplified by the differential amplifier 44. The instantaneous differential amplification value is input to the AD converter 5 as the displacement detection result of the balance beam 2. You.

That is, two photodiodes 42
The light receiving surfaces a and 42b are arranged such that their light receiving surfaces are adjacent to each other, and the output light from the light emitting diode 41 enters each light receiving surface through the slit 22 of the balance beam 2. The center of the slit 22 is located at each photodiode 42a,
In a state where it is located on the boundary line of each light receiving surface of 42b,
The amount of light incident on each light receiving surface is equal, so that the outputs of the photodiodes 42a and 42b are equal to each other, and the output of the differential amplifier 44 is zero. When the balance beam 2 is displaced from this state, a difference occurs in the amount of light incident on each light receiving surface, thereby causing a difference in the output of each of the photodiodes 42a and 42b, and the differential amplifier 44 responds to the direction of the displacement. A signal having a polarity and a magnitude corresponding to the amount of displacement is output.

The driving circuit 9 of the light emitting diode 41 includes an oscillator 91, an amplifier 92 in which a voltage signal from the oscillator 91 is introduced to an inverse input via a resistor, and an output from the amplifier 92 is supplied to a base. Further, the transistor 93 has an emitter connected to the light emitting diode 41, and the driving current of the light emitting diode 41 changes at a frequency corresponding to the oscillation frequency of the oscillator 91. Granted. In addition, the fluctuation of the light emission amount other than the dither of the light emitting diode 41 is suppressed by the feedback circuit returned to the amplifier 92 from the midpoint between the two photodiodes 42a and 42b. The oscillation frequency of the oscillator 91 is, for example, 1 to several kHz.

According to the above-described embodiment of the present invention, since the light emission amount of the light emitting diode 41 is dithered, even if the position of the slit 22 of the balance beam 2 does not change, each of the photodiodes 42a and 42b The amount of light incident on the light changes every moment.

The output of the differential amplifier 44 having the output of each of the photodiodes 42a and 42b as a differential input, that is, the displacement detection value, is, therefore, except for the case where the displacement of the balance beam 2 is true 0. Even if the displacement amount of 2 is constant, its value changes every moment, the input signal to the AD converter 5 changes every moment, and the value of the digital conversion output changes variously. As a result, the PID output from the PID calculation unit 61 also changes to various values with time, so that the magnitude of the current flowing through the force coil 41 changes every moment, and the balance beam 2 is displaced while swinging according to the change. It is controlled so as to balance to the position of 0. At this time, the AD converter 5
Even if a low-resolution one is used, the output changes in various ways, so that a specific standing wave based on the resolution is eliminated. The weighing display value calculated by the above becomes a smooth value without a habit, and a result equivalent to the case of using a high-resolution AD converter is obtained.

In the embodiment described above, the dither given to the light emission amount of the light emitting diode 41 is an example in which the waveform, frequency and amplitude are constant, and these are appropriately set according to the characteristics of the balance mechanism. By doing so, there is no particular problem. However, better control can be achieved by intentionally controlling any or all of the dither waveform, frequency, and amplitude of this light emission amount in accordance with the control situation of the system. Is expected to be achieved.

[0022]

As described above, according to the present invention, dither is given to the light emission amount of the light emitting element of the optical position sensor for detecting the displacement of the balance beam, and the displacement detection value is digitized. A specific standing wave based on the resolution of the A / D converter is prevented from being generated in the control system. Therefore, even if a low-resolution A / D converter is used as the A / D converter, It is possible to obtain a digital servo type electronic balance having substantially the same performance as that using an A / D converter with high resolution without impairing the dynamic range, and it is an inexpensive and high performance full digital electronic balance. Is obtained.

[Brief description of the drawings]

FIG. 1 is an overall configuration diagram of an embodiment of the present invention, showing both a schematic diagram showing a mechanical configuration and a block diagram showing an electrical configuration.

FIG. 2 is a detailed configuration diagram of the optical position sensor 4 and a driving circuit 9 of the light emitting diode 41 thereof.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 plate 2 balance beam 3 electromagnetic force generator 31 force coil 4 optical position sensor 41 light emitting diode 42a, 42b photodiode 44 differential amplifier 5 A / D converter 6 arithmetic unit 61 PID operation unit 62 weighing operation unit 81 pulse Width modulation circuit 82 Switch 83 Constant current source 9 Light emitting diode driving circuit 91 Oscillator 92 Amplifier 93 Transistor

Claims (1)

[Claims] 1. A balance beam for supporting a plate for applying a load to be measured and receiving an electromagnetic force against the load to be measured by an electromagnetic force generator, and a rotational displacement of the balance beam. Is detected by an optical position sensor comprising a light emitting element and a light receiving element, and the detection result is digitized by an A / D converter. A digital servo mechanism that determines the generated electromagnetic force and controls the balance beam to be in an equilibrium state, and in the electronic balance that determines the weighing display value based on the PID calculation result, wherein the driving circuit of the light emitting element includes: An electronic balance including dither providing means for changing the amount of light emitted from a light emitting element at a predetermined frequency.