JPH07218324A - Electronic balance - Google Patents

Abstract

(57) [Abstract] [Purpose] Current flowing in force coil (coil current)
Even if the reference resistance for converting the voltage signal to a voltage signal has a temperature coefficient of about several ppm, a voltage signal that is accurately proportional to the coil current can be obtained, and high precision is achieved by using an inexpensive reference resistance. Provide an electronic balance that can be used. [Structure] The heat generating resistor 13 is arranged in a positional relationship where the heat resistance is substantially zero with respect to the reference resistor 7, and the heat generating resistor 1 is arranged.
The circuit means 14 for controlling the current i flowing through 3 based on the magnitude of the coil current I is provided, and the sum of the heat generation amount of the reference resistor 7 and the heat generation amount of the heat generation resistor 13 due to the coil current I becomes substantially constant. Thus, the current i flowing through the heating resistor 13 is controlled.

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.

[0002]

2. Description of the Related Art In general, an electromagnetic force balance type electronic balance has a movable force coil arranged in a magnetic field of a permanent magnet and a mechanism in which a weighing dish is connected to the force coil via a lever or the like. The displacement sensor detects the displacement of the lever or the like due to the measurement load acting on the weighing pan, and the current flowing through the force coil is controlled so that the displacement becomes zero. Then, the load to be measured is calculated from the magnitude of the current flowing through the force coil.

Here, in determining the measured value to be displayed on the display device from the magnitude of the current flowing through the force coil, in practice, the coil current is converted into an analog voltage signal by flowing it through a reference resistor, and then the analog voltage signal is converted. A-
The data is digitized by a D converter, loaded into a microcomputer as load data, subjected to averaging processing, etc. to obtain a measured value to be displayed.

[0004]

By the way, among the conventional electronic balances as described above, the ones with high precision are as follows:
The temperature coefficient of the reference resistance is as close to zero as possible,
Moreover, it is necessary to use the allowable power which is several times to several tens of times higher than the actually used power.

That is, the reference resistance generates heat due to the coil current flowing therethrough, and its temperature changes, but if the resistance value of the reference resistance changes due to this temperature change, load data representing the coil current can be obtained correctly. I can't.
Therefore, as the reference resistance, use a resistor whose temperature coefficient is as close to zero as possible and has a large allowable power.
It is necessary to reduce the temperature change due to the coil current flowing, and to use the one whose resistance value does not easily change even if the temperature changes.

Here, in order to pursue high precision of the electronic balance, it is necessary to have a reference resistance having a temperature coefficient of zero, but a resistor having a temperature coefficient of absolute zero is substantially used. This does not exist, and this point is the limit for improving the precision of the balance.
Further, there is a problem that the cost is high even when a resistor which is not absolute zero but is as close to zero as possible and has a large allowable power is used.

The present invention has been made in view of such circumstances, and the load data accurately representing the coil current can be obtained even if the temperature coefficient of the reference resistance is about several ppm and the allowable power is not so large. It is an object of the present invention to provide an electronic balance that can be obtained and can achieve high accuracy by using an inexpensive reference resistor.

[0008]

A structure for achieving the above object will be described with reference to FIG. 1 which is an embodiment drawing, and an electronic balance of the present invention has a current flowing through a force coil 4 in a magnetic field. The heating resistor 13 is arranged at a position where the thermal resistance becomes substantially zero with respect to the reference resistor 7 for converting the voltage signal into a voltage signal, and the current flowing through the heating resistor 13 is controlled based on the magnitude of the coil current. The circuit means 14 has a circuit means 14 for controlling the heat generation resistance 13 so that the sum of the heat generation quantity of the reference resistance 7 and the heat generation quantity of the heat generation resistance 13 due to the coil current flowing becomes substantially constant. Characterized by controlling the current flow.

[0009]

According to the present invention, the amount of change in temperature due to self-heating caused by the coil current flowing through the reference resistor 7 and the amount of heat generated by the heating resistor 13 placed in a positional relationship in which the thermal resistance becomes substantially zero are calculated. By adjusting the amount, it is intended to cancel each other out.

That is, a coil current proportional to the magnitude of the load W to be measured flows through the reference resistor 7, and the temperature changes according to the amount of heat generated according to the magnitude of the current.
If the heat generating resistor 13 is arranged so that the heat resistance becomes substantially zero with respect to the reference resistor 7, and the current flowing through the heat generating resistor 13 is controlled so that the sum of the heat generation amounts of both is always constant, The temperature of the resistor 7 becomes constant regardless of the magnitude of the coil current. Therefore, even if a resistor having a temperature coefficient of several ppm is used as the reference resistor 7, a temperature change due to self-heating hardly occurs, so that a voltage signal accurately proportional to the coil current can be obtained.

[0011]

1 is a block diagram showing the configuration of an embodiment of the present invention. The measured load W is placed on the weighing dish 1. The weighing dish 1 is connected to one end side of a lever 2 supported by a fulcrum 2a, and the displacement of the tip portion of the lever 2 is detected by a displacement sensor 3. A winding frame 4a is fixed to the other end of the lever 2, and the force coil 4 is wound around the winding frame 4a.

The output of the displacement sensor 3 is introduced into the PID control circuit 5, and in this PID control circuit 5, a current corresponding to the magnitude of the output of the displacement sensor 3 is passed through the force coil 4.
The force coil 4 is arranged in a static magnetic field created by a magnetic circuit 6 mainly composed of a permanent magnet, and when the current from the PID control circuit 5 flows through the force coil 4, the magnitude of the current and the strength of the magnetic field are increased. An electromagnetic force corresponding to the strength is generated. This electromagnetic force is applied to the force coil 4 and the winding frame 4.
It is transmitted to the lever 2 via a and acts so that the displacement of the lever 2 becomes zero against the measured load W to balance the system. Therefore, in the equilibrium state of the system, the magnitude of the current flowing through the force coil 4 is proportional to the measured load W if the strength of the magnetic field by the magnetic circuit 6 is constant.

The current flowing through the force coil 4 is passed through the reference resistor 7, converted into a voltage signal here, and then introduced into the AD converter 8. Therefore, the digital data from the A / D converter 8 becomes data representing the magnitude of the current flowing through the force coil 4 and, in turn, the magnitude of the measured load W acting on the weighing pan 1, that is, load data, and becomes a microcomputer. Adopted in 9.

In addition to the load data, the microcomputer 9 uses the AD converter 1 to detect the temperature detection signal from the temperature sensor 10 placed near the permanent magnet in the magnetic circuit 6.
The temperature data digitized in 1 is also incorporated, and the temperature data is used to compensate the magnetic field change caused by the temperature change of the permanent magnet, and the load data is displayed on the display 12 by averaging the load data. Determine the weight value to be measured.

A heating resistor 13 is provided close to the reference resistor 7 in such a positional relationship that the thermal resistance with the reference resistor 7 becomes substantially zero. And this heating resistor 1
A current i supplied from the arithmetic circuit 14 is passed through 3. This current i is uniquely obtained by the arithmetic circuit 14 according to the magnitude of the coil current I which is the current flowing through the reference resistor 7.

That is, the arithmetic circuit 14 receives the coil current I, which is the output of the PID control circuit 5, and inputs the coil current I, the resistance value R of the reference resistor 7, and the heating resistor 13.
Magnitude of the current i such that the sum of the heat generation amount of the reference resistor 7 due to the current I flowing and the heat generation amount of the heat generating resistor 13 due to the current i flowing becomes constant from the resistance value R ′ of To decide. Expressed as a formula,

[0017]

[Equation 1]

A current i that satisfies the above condition is calculated and supplied to the heating resistor 13. That is, the arithmetic circuit 14 calculates from the coil current I

[0019]

[Equation 2]

The current i is calculated by
Shed on. As a result, regardless of the magnitude of the current I flowing through the reference resistor 7, the sum of the amounts of heat generated by the reference resistor 7 and the heat-generating resistor 13 is always constant, and the thermal resistances of these two are almost zero. Since they are arranged in a positional relationship, the temperature of these two can be kept constant by setting the sum of the calorific values of both to an appropriate value.

Therefore, even if a resistor having a temperature coefficient of a certain magnitude of about several ppm is used as the reference resistor 7, the AD converter 8 is always exactly proportional to the current I flowing through the force coil 4. The supplied voltage signal is supplied.

The temperature coefficient of the heat generating resistor 13 is arbitrary, and as a method of disposing the heat generating resistor 13 and the reference resistor 7 so that the heat resistance becomes substantially zero, for example, both of them are made of ceramic. When the resistors are used, a method of patterning these resistors 7 and 13 on the substrate so as to be intricate with each other, or a coil-like heat-generating resistor 13 is wound around the reference resistor 7, or a line or plate shape is used. It is possible to adopt a method such as attaching the heat-generating coil 13 of FIG.

Further, the method of calculating the current i by the arithmetic circuit 14 can be replaced with a simpler calculation instead of the above-mentioned strict calculation. For example, when the resistance values of the reference resistor 7 and the heat generating resistor 13 are set to be substantially the same and the maximum value (full scale current, 100%) of the coil current I flowing in the reference resistor 7 is set in the arithmetic circuit 14, A method of performing a complement operation can be adopted so that the total of the currents flowing through the reference resistor 7 and the heating resistor 13 is always the maximum value. In this case, the amount of heat generated by the resistor is proportional to the square of the current flowing therethrough. Therefore, the relationship between the sum of the amounts of heat generated by both resistors and the coil current I depends on whether the coil current I is zero or the full-scale value. When the sum of the heat generation amounts of the resistors is the same and the coil current I is 50%, the sum of the heat generation amounts of the both resistors is smaller than this. Compared with the conventional electronic balance provided with only the resistor 7, the temperature change due to the self-heating of the reference resistor 7 is significantly reduced.

[0024]

As described above, according to the present invention,
Convert the current flowing through the force coil into a voltage signal and
The heat generating resistor is arranged in such a positional relationship that the thermal resistance becomes substantially zero with respect to the reference resistor for supplying to the D converter, and the magnitude of the coil current flowing through the reference resistor is set in the heat generating resistor. On the basis of the above, since a current is made to flow so that the sum of the heat generation amounts of the heat generation resistance and the reference resistance becomes substantially constant, a coil that flows in the reference resistance according to the size of the load on the weighing pan. Even if the magnitude of the current changes and the amount of heat generated by the reference resistance alone changes, the heat-generating resistor generates heat to cancel the change, and the temperature of the reference resistance is independent of the coil current. It will always be constant.

Therefore, by adopting the present invention, an inexpensive resistor having a temperature coefficient of a certain size, for example, several ppm, and an allowable power slightly larger than the actual power used is adopted as the reference resistance. However, since the temperature change due to the change of the coil current is zero or hardly occurs, it is possible to always obtain the voltage signal which is exactly proportional to the coil current. Therefore, compared to the conventional electronic balance of this type, the cost can be reduced in obtaining the same performance. Also, in pursuing the high performance of electronic balances, we were able to solve the problems that were unavoidable unless there was a resistance with a temperature coefficient of absolute zero, and the occurrence of errors due to self-heating of the reference resistance. .

[Brief description of drawings]

FIG. 1 is a block diagram showing the configuration of an embodiment of the present invention.

[Explanation of symbols]

 1 Weighing pan 2 Lever 3 Displacement sensor 4 Force coil 5 PID control circuit 6 Magnetic circuit 7 Reference resistance 8 A-D converter 9 Microcomputer 12 Display 13 Heat generation resistance 14 Arithmetic circuit

Claims (1)

[Claims] 1. An electromagnetic force generated by passing a current through a coil placed in a magnetic field is balanced with a load to be measured, and the coil current is passed through a reference resistance to be converted into a voltage signal, and then A- In an electronic balance that obtains load data by digitizing with a D converter, a heating resistor is arranged at a position where the thermal resistance is substantially zero with respect to the reference resistance, and the current flowing through this heating resistor is The circuit means has a control means, and the circuit means has a current flowing through the heating resistor so that the sum of the heating value of the reference resistor and the heating value of the heating resistor due to the coil current flowing becomes substantially constant. An electronic balance characterized by controlling the.