JPS5851208B2 - electromagnetic balance device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an electromagnetic balance device having a solenoid,
In particular, the present invention relates to an electromagnetic balance device that compensates for errors in detected values caused by temperature changes in the solenoid.

Before explaining the electromagnetic balance device according to the present invention, a conventional electromagnetic balance device will be briefly explained.

In FIG. 1, when an object to be weighed 4 is placed on a weighing pan 2, the balance 6 becomes unbalanced and the position of the iron core 10 of the differential transformer 8 changes.

Due to this displacement of the iron core 10, the differential transformer 8, which is excited by the AC power supply 12, generates an output voltage proportional to the magnitude of the displacement.

This output voltage is rectified by a rectifier 14 and supplied to a solenoid 22 of an electromagnetic balance device 20 via an amplifier 16.

The solenoid 22 attracts the permanent magnet 24 with a force F balanced with the weight W of the object 4 to be weighed, and returns the balance 6 to an equilibrium state.

In the equilibrium state, the current supplied to the solenoid 22 is
For example, since it is proportional to the weight W of the object to be weighed 4, the weight of the object to be weighed 4 can be detected by measuring the current (2).

In this electromagnetic balance device, the solenoid 22 generates heat due to the current (2) supplied to the solenoid 22.

Now, assuming that the DC resistance value of the solenoid 22 is R, the amount of heat generated W is expressed as W=IR.

FIG. 3 shows a graph of current versus heat generation.

Here, ImaX uses a solenoid 2 to balance the balance 2 when the maximum measurable load is placed on the balance 2.
This is the current supplied to 2.

As can be seen from FIG. 3, the amount of heat generated by the solenoid 22 changes greatly from 0 to 2 R according to the load W.

By the way, the permanent magnet 24 has a property that its magnetic force decreases as the temperature rises, and this permanent magnet 24 is arranged within the solenoid 22 as shown in FIG. 1.

Now, when a large load W is applied to the balance 2, a large current is supplied to the solenoid 22 to balance it.

Then, the solenoid 22 generates heat, the temperature of the permanent magnet 24 increases significantly, and its magnetic force decreases significantly.

In such a state, the equilibrium state is disrupted, and in order to restore the equilibrium state, the current (2) supplied to the solenoid 22 is increased.

In this electromagnetic balance device, the load W is detected by measuring this current (2), so a large error occurs in the detection result.

The present invention aims to provide a temperature compensation device that keeps the temperature of a permanent magnet approximately constant despite changes in current and reduces changes in magnetic force so that the electromagnetic balance device can perform accurate detection. purpose.

This invention will be explained below based on two embodiments shown in FIGS. 4 to 7.

FIG. 4 shows the first embodiment, in which 26 is a solenoid with a DC resistance value R, 28 is a permanent magnet disposed within the solenoid 26, 30 is an operational amplifier, and 32 is a coil-shaped heating element.

The heating element 32 is provided near the permanent magnet, is made of copper or other metal, and has a DC resistance value R.

The heating element 32 is wound in a non-inductive manner.

One end of the solenoid 26 is grounded via a resistor 34 having a resistance value R.

Further, one end of the solenoid 26 is connected to a resistor 3 having a resistance value r.
6 to the non-inverting input terminal of the operational amplifier 30.

Here, between the resistance value r1 and the resistance value R, R< r 1
The relationship holds true.

The solenoid 26 is supplied with the output of the amplifier 16 of FIG.

A voltage of -e2 is supplied to the non-inverting input terminal of the operational amplifier 30 via a resistor 3B having a resistance value r1.

e2 is e 2 ” R1 ■ m aX is the value of

The inverting input terminal of the operational amplifier 30 is grounded via a variable resistor 40 having a resistance value VR, and is further connected to the heating element 3.
It is also connected to one end of 2.

The output terminal of the operational amplifier 30 is connected to the other end of the heating element 32 via a current amplifier 42.

In this electromagnetic balance device, the temperature of the permanent magnet 28 is kept substantially constant by the heat generated by the heating element 32 and the heat generated by the solenoid 26.

Now, in order to determine the amount of heat generated by the heating element 32, the current 1 flowing through the heating element is determined.

Since the voltages applied to the inverting input terminal and the non-inverting input terminal of the operational amplifier 30 are equal, the following equation holds true.

However, el is the voltage across the resistor 34.

Therefore, ■1 becomes.

The value of el is, when the load changes from O to the maximum load,
Since the current (2) flowing through the solenoid 26 changes from 0 to (2)max, it changes from 0 to R1 (2)max.

By this, 11 is when I=O becomes.

If you adjust the value of variable resistor 40 so that (1)? From equation (2) It can be expressed as

On the other hand, since this circuit can be regarded as linear, for the above-mentioned reason, the sum of the cotton values of the current (2) flowing through the solenoid 26 and the current (1) flowing through the heating element 32 can be expressed as follows.

Therefore, a current I is applied to the solenoid 26 and the heating element 32, respectively.
, Joule heat W generated by the flow of 11
1. W2c, and the solenoid 26 and heating element 32 are
Since they are located at approximately the same location and give the same thermal effect to the permanent magnet 2B, the amount of heat generated is the sum of equations (3) and (4), and is expressed as follows.

If this is plotted on a graph, it becomes Figure 5. Comparing Fig. 3 and Fig. 5, in Fig. 5 with the heating element 32 attached, the change in heat generation is smaller, and even if the current changes depending on the load,
Since temperature changes can be kept small, errors in detection results can be reduced.

FIG. 6 shows a second embodiment, in which switches 44, 46, a resistor 48, and an operational amplifier 50 are added to the first embodiment.

The switch 44 is connected between the solenoid 26 and the resistor 36, and the series circuit of the switch 46 and the resistor 4B is
It is connected between the non-inverting input terminal of operational amplifier 30 and ground.

The value of resistor 48 is r1/3. The inverting input terminal of the operational amplifier 50 is connected to the solenoid 26 via a resistor 52.
A voltage of e2/2 is supplied to the non-inverting input terminal via a resistor 54.

Operational amplifier 50, at its output G, switches switches 44, 46
It opens and closes.

Let the current flowing through the solenoid 26 be ■, R(<rl, r3, r.

If the value of each resistor is chosen so that the following holds true, e1=R1■
...(6) becomes.

Also, if e2 is the same as in the first embodiment, the maximum value of ■ is m
If ax, e2 is e2=R1■max ・・・・・・(
7).

Opening/closing force of switches 44 and 46, output of operational amplifier 50
If the configuration is such that the switch 44 ON1 switch 45 OFF is satisfied when G<O, the current flowing through the heating element 32 can be reduced to ■1
Then, if the variable resistor 40 is adjusted so that the following holds true, (8)
The formula (9) is as follows.

Therefore, the amount of heat generated W including the heat generated by the solenoid 26 is shown in FIG. 7 as a graph.

Compared to the case shown in Fig. 3 without a heating element, in the case shown in Fig. 7, the change in the amount of heat generated by the permanent magnet 28 is small, and even if the current changes depending on the load, the temperature change can be kept small. Therefore, the error in detection results can be reduced.

In the above two embodiments, the heating element is made of metal and has a coil shape, but in general, any material that can generate heat by passing an electric current may be used, and the heating element does not need to be in the shape of a coil.

Moreover, the installation location of the heating element is not limited to the inside of the solenoid, but may be the space between the permanent magnet and the solenoid, or the surface or inside of the permanent magnet.

As described above, in this electromagnetic balance device, the temperature of the permanent magnet is kept substantially constant, so errors in detection results can be reduced.

[Brief explanation of drawings]

Figure 1 is a schematic configuration diagram of a conventional electromagnetic balance device, Figure 2 is a wiring diagram of the electromagnetic balance device shown in Figure 1, and Figure 3 shows the amount of heat generated by the electromagnetic balance device shown in Figure 1 with respect to changes in current. Figure 4 is a diagram showing changes, and Figure 4 is a wiring diagram of the first embodiment of this invention.
The figure shows the change in heat generation amount with respect to the change in current in the first embodiment, FIG. 6 is a connection diagram in the second embodiment of the present invention, and FIG. FIG. 3 is a diagram showing changes in calorific value. 26... Solenoid, 28... Permanent magnet, 30... Operational amplifier, 32... Heating element, 42... Current amplifier, 44 ,46...
...Switch, 50...Operation amplifier.

Claims (1)

[Claims] 1. An electromagnetic balance device used in conjunction with a measuring section and an unbalance detection section that detects the magnitude of unbalance of the measuring section that occurs when an object to be measured is placed on the measuring section,
A solenoid to which a current proportional to the magnitude of the unbalance is supplied from the unbalance detection section, a magnetic body disposed within the solenoid that returns the measuring section to the balanced position, and a region near the magnetic body. An electromagnetic balance device comprising: a heating element disposed in the solenoid; and a circuit that maintains the sum of the calorific value of the solenoid and the calorific value of the heating element substantially constant.