JPH0968455A - Electronic balance - Google Patents

Abstract

(57) 【Abstract】 PROBLEM TO BE SOLVED: To provide a pulse width ratio-current conversion circuit that can operate using a reference voltage generation circuit that generates a constant voltage with reference to a circuit reference potential (common potential). Provide an electronic balance. A resistor Rs is inserted between an input terminal of an operational amplifier circuit 2 and a signal output terminal of a reference voltage generation circuit 1 for generating a required voltage signal with respect to a reference potential of the circuit, and an operational amplifier circuit 2 is provided. While operating as an integrator that integrates the input current from the input end, the current feedback circuit of the operational amplifier circuit 2 includes loads 4a and 4b and switching elements 3a and 3b.
By inserting a series circuit with b in parallel with each other and exciting each switching element 3a, 3b with a PWM signal of an opposite phase, the operational amplifier circuit 2 is balanced in a state where its input current and feedback current match. , The load 4a, 4b each P
A DC current proportional to the pulse width ratio of the WM signal is passed, and at least one of the loads 4a and 4b is used as an electromagnetic coil to oppose the load to be measured.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electromagnetic force balance type electronic balance adopting a pulse width modulation method.

[0002]

2. Description of the Related Art In an electronic balance of the electromagnetic force balance type,
Generally, the displacement detection signal from the balance point of the balance rod due to the load to be measured is PID (proportional / integral / derivative) calculated, and the current according to the calculated result is applied to the coil (electromagnetic coil) placed in the static magnetic field. Electromagnetic force generated by
The balance is acted on the balance rod against the load to be measured to balance it, and the size of the load to be measured is obtained from the amount of current flowing through the coil in the balanced state of the balance rod.

In an electromagnetic force balance application device such as such an electromagnetic force balance type electronic balance which requires relatively high precision control, a coil of a current having a magnitude exactly proportional to the PID calculation result is used. PI for the purpose of
A method in which a pulse width modulation is performed on the D calculation result and a switching element excited by the pulse width modulation signal (hereinafter referred to as a PWM signal) is provided, and a constant current signal is switched by the switching element and supplied to the coil is adopted. ing.

As a so-called pulse width ratio-current conversion circuit that supplies a current having a magnitude corresponding to such a pulse width ratio to a load, a circuit as illustrated in FIG. 1 or 2 has been conventionally used. There is.

In the example of FIG. 1, the switching elements 12a and 12b are excited by two kinds of PWM signals of opposite phases which are synchronized with each other from the pulse width modulation circuit 11, and one end of each of the switching elements 12a and 12b is loaded. It connects to one end of 13a, 13b and supplies a constant current signal to the other end of the switching elements 12a, 12b. The other ends of the loads 13a and 13b are connected to the positive side of the power source of the circuit so that a pulse current flows through the loads 13a and 13b by the switching operation of the switching elements 12a and 12b.

Further, in the example of FIG. 2, the pulse width modulation circuit 2
One end of the switching element 22 excited by the PWM signal from 1 is connected to the input end of the integrator 24, a constant current signal is supplied to the other end, and the load 23 is inserted in the feedback circuit of the integrator 24. Switching element 22
By the switching operation of, the DC current proportional to the pulse width ratio of the PWM signal flows through the load 23.

[0007]

By the way, in the conventional pulse width ratio-current conversion circuit as described above, as shown in FIG.
As shown in FIG. 2, a constant current circuit is configured with a reference voltage based on the positive or negative side (negative side in FIGS. 1 and 2) of the power source of the circuit. That is, the reference power supply for generating the constant current is in a floating state with respect to the reference potential of the circuit (common potential, 0 potential of the circuit operating power supply). Therefore, there is a problem in that the configuration of the reference voltage generating circuit is more complicated than that of a normal reference voltage generating circuit using the common potential as a reference. Further, it is not possible to share the reference voltage source with other circuits that require the reference voltage source for the common potential, such as the reference voltage source of the AD converter for digitizing the load detection signal. It is economical.

The present invention has been made in view of the above circumstances, and its object is to provide a reference voltage generating circuit for generating a constant voltage with respect to a reference potential (common potential) of the circuit. By using the generated constant current signal, a current according to the pulse width ratio of the PWM signal can be made to flow through the electromagnetic coil, which has a simple structure and can be used in other circuits such as an AD converter. An object of the present invention is to provide a highly accurate and inexpensive electronic balance provided with a pulse width ratio-current conversion circuit that can operate using a reference voltage generation circuit that can be shared as a reference voltage source.

[0009]

FIG. 3 is a diagram showing a circuit diagram of a main part of an embodiment of a configuration for achieving the above object.
The electronic balance of the present invention excites a switching element using a PWM signal obtained by pulse-width modulating a control operation signal generated based on a displacement detection signal of a balance rod. A pulse width ratio-current conversion circuit for supplying a current proportional to the pulse width ratio of the PWM signal to the electromagnetic coil to balance the balance rod to a predetermined position by switching the constant current signal to supply the electromagnetic coil to the electromagnetic coil. In the electronic balance provided with, the pulse width ratio-current conversion circuit is configured so that the output of the reference voltage circuit 1 that generates a required constant voltage with respect to the reference potential (common potential) of the circuit is passed through the resistor Rs. To an input terminal of the operational amplifier circuit 2, and the operational amplifier circuit 2 constructs an integrator that integrates the input current from the input terminal and outputs it in the form of voltage and current. As well as, the operational amplifier circuit 2
Between the input and output ends of the switching element 3a,
Two loads 4a and 4b connected in series to 3b are connected in parallel, and the switching elements 3a of the respective loads 4a and 4b,
3b is PWM signals e _A and e _{B of} opposite phases which are synchronized with each other.
And is characterized in that at least one of the two loads 4a and 4b is the electromagnetic coil.

[0010]

In the pulse width ratio-current conversion circuit having the above structure, the integrator formed by the operational amplifier circuit 2 has the current Iin = Vs / Rs input through the resistor Rs, the switching elements 3a and 3b and the load. The difference from the current If fed back through 4a and 4b is integrated, and the integrated result is output from the operational amplifier circuit 2. The equilibrium condition of this circuit is

[0011]

[Equation 1]

## EQU1 ## Therefore,

[0013]

[Equation 2]

This circuit is balanced. Such a feedback current If is excited in opposite phases in synchronization with each other in accordance with the two types of PWM signals, and through the two switching elements 3a and 3b connected in parallel to each other,
When the loads 4a and 4b connected in parallel to each other are switched while being supplied, a DC current having a magnitude corresponding to the pulse width ratio of the PWM signal corresponding to each load 4a and 4b flows. Therefore, a pulse width ratio-current conversion circuit that operates using the constant current signal generated based on the output of the reference voltage generation circuit 1 that generates a constant voltage with the reference potential of the circuit, that is, the 0 potential as the reference is obtained. .

Here, of the two loads 4a and 4b, one may be an electromagnetic coil which is a target load and the other may be a dummy load, or both may be a differential load (differential coil) which is a target. .

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 3 is a circuit diagram of a main part of a preferred embodiment of the present invention, which is a pulse width ratio-current for determining a current to be passed through an electromagnetic coil of an electromagnetic balance type electronic balance. It is a figure which extracts and shows a conversion circuit. is there.

The reference voltage generation circuit 1 outputs a signal Vs having a constant voltage with respect to the common potential (0 potential) of the circuit. The signal output terminal of the reference voltage generating circuit 1 is connected to the input terminal of the operational amplifier circuit 2 via the resistor Rs. The operational amplifier circuit 2 has a feedback capacitor 2a, constitutes an integrator which integrates an input current and outputs it in the form of a voltage and a current, and its output stage has a current amplifier 2b having a high output resistance.
It has.

Two loads 4a and 4b are inserted in parallel in the current feedback circuit of the operational amplifier circuit 2, and switching elements 3a and 3b are connected in series to the respective loads 4a and 4b. . Further, a capacitor Cs is connected in parallel to the connection ends of the switching elements 3a and 3b with the loads 4a and 4b, and they are in a short-circuited state in terms of AC.

The switching elements 3a and 3b are driven and driven by the PWM signals e _A and e _B from the pulse width modulation circuit 5, respectively.
PWM signal e _A for switching a and 3b,
e _B is a signal of opposite phase synchronized with each other.

In the above circuit configuration, the operational amplifier circuit 2
The current Iin = Vs / Rs is supplied from the resistor Rs to the input terminal of the operational amplifier circuit 2 and the operational amplifier circuit 2 feeds back the current Iin fed through the loads 4a and 4b and the switching elements 3a and 3b.
The difference with f will be integrated and output. Therefore, the equilibrium condition of this circuit is

[0021]

(Equation 3)

,

[0023]

(Equation 4)

As

[0025]

(Equation 5)

The circuit is balanced. The instantaneous value of the current flowing from the switching elements 3a and 3b to the input terminal of the operational amplifier circuit 2 is determined by selecting the output resistance of the current amplifier 2b at the output stage of the operational amplifier circuit 2 and the value of the capacitor Cs sufficiently large. , Iin can be set to be constant if Iin is constant.

Therefore, the average value I of the current flowing through the load 4a
The average values I _{BA of the} currents flowing through _AA and the load 4b are I _AA = kIf = kIin I _BA = (1-k) If = kIin, respectively. Here, k and (1-k) are PWM for excitation supplied to the respective switching elements 4a and 4b.
It is the pulse width ratio of the signals e _A and e _B.

Therefore, for example, if one of the loads 4a is an electromagnetic coil coil placed in a static magnetic field, the above-mentioned current I _AA flows in this coil to generate a balancing electromagnetic force of Ff = K · kIin. . It should be noted that K is a constant determined by the strength of the static magnetic field, the number of turns of the coil, etc. Therefore, Ff is exactly proportional to the pulse width ratio k of the PWM signal supplied to the switching element 3a, and P for excitation is used.
If the WM signal e _A is generated from the PID calculation result of the displacement detection signal of the balance rod in the electromagnetic balance type electronic balance, the balance rod can be balanced against the displacement due to the load to be measured.

Here, when only one of the loads 4a is an electromagnetic coil as described above, the other load 4b is a dummy load, but as shown in FIG.
The coils a and 4b may be coils on both sides of the electromagnetic coil having the midpoint, and both of them may be controlled. In this case, each coil 4a, when the current shown flowing at I _AA and I _BB described above, respectively 4b, the occurrence electromagnetic force as a whole Ff = K {kIin- (1- k) Iin} = K (2k- 1)
It becomes Iin.

In the embodiment shown in FIG. 3, the current amplifier 2b is provided at the output stage of the operational amplifier circuit 2 and the capacitor Cs is inserted between the switching elements 3a and 3b. However, in the present invention, The current amplifier 2b and the capacitor Cs are not necessarily required, and depending on the required conversion accuracy, the response speed of the system, and the characteristics of the operational amplifier circuit 2 for integration, a case where both or one of them is not provided is included. Then, the output resistance, the amplification degree, the capacitance, or the like is selected.

[0031]

As described above, according to the present invention, the pulse width modulation signal generated based on the displacement detection signal of the balance rod is used to switch the constant current signal and supply it to the electromagnetic coil. In an electronic balance equipped with a conversion circuit, the pulse width ratio-current conversion circuit can be operated by using a reference voltage generation circuit that generates a required voltage with reference to the circuit reference potential (common potential or 0 potential). Therefore, the configuration of the reference power supply can be simplified, and at the same time, the reference voltage generation circuit can be shared with other circuits, for example, the bias power supply and the reference power supply of the AD converter, The cost of the entire device can be reduced. Furthermore, it is possible to construct a pulse width ratio-current conversion circuit using such a reference voltage generation circuit,
There is also an advantage that the microcomputer circuit of the reference power supply voltage can be easily operated.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a configuration example of a pulse width ratio-current conversion circuit used in a conventional electronic balance.

FIG. 2 is a circuit diagram showing a configuration example of a pulse width ratio-current conversion circuit used in another conventional electronic balance.

FIG. 3 is a circuit diagram of a main part of an embodiment of the present invention, which is a circuit diagram showing a configuration of a pulse width ratio-current conversion circuit.

FIG. 4 is a circuit diagram showing a main configuration of an application example of an embodiment of the present invention.

[Explanation of symbols]

 1 Reference Voltage Generation Circuit 2 Operational Amplifier Circuit 2a Capacitor 2b Current Amplifier 3a, 3b Switching Element 4a, 4b Load 5 Pulse Width Modulation Circuit Cs Capacitor Rs Resistor

Claims (1)

[Claims] 1. A constant current signal is switched by exciting a switching element by using a PWM signal obtained by pulse-width modulating a control operation signal generated based on a displacement detection signal of a balance rod, thereby switching a constant current signal. In an electronic balance equipped with a pulse width ratio-current conversion circuit for supplying a current proportional to the pulse width ratio of the PWM signal to the electromagnetic coil to balance the balance rod to a predetermined position, the pulse The width ratio-current conversion circuit guides the output of a reference voltage circuit that generates a required constant voltage with respect to the reference potential of the circuit to the input end of the operational amplification circuit via a resistor, and the operational amplification circuit inputs the output. An integrator that integrates the input current from the end and outputs it in the form of voltage and current is configured, and a switch is connected between the input and output ends of this operational amplifier circuit. Two loads connected in series to the pendulum element are connected in parallel, and the switching elements of the respective loads are configured to be excited by PWM signals of mutually opposite phases, and at least one of the above two loads. An electronic balance, characterized in that one is the electromagnetic coil.