JPH0810155B2 - Weight detector - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a weight detection device that detects the weight of an object to be measured by utilizing a change in capacitance.

2. Description of the Related Art A large number of capacitance-type pressure sensors have been put to practical use, in which electrodes are opposed to each other with a predetermined gap therebetween and the pressure applied to the electrodes is taken out as a change in capacitance.

Since the capacitance value of such a sensor is usually as small as about 20 to 50 pF, a large error occurs due to the temperature characteristics of the detection circuit and the sensor itself. Therefore, how to solve the change in characteristics due to temperature was an important issue in this type of sensor.

For this reason, a capacitance is provided in the center of the substrate where the capacitance value changes sharply according to the applied pressure, and a reference electrode whose capacitance value does not change much depending on the pressure applied to the outer periphery Mold pressure sensors are generally known.

The capacitance type pressure sensor described in Japanese Patent Laid-Open No. 58-198739 is an example of such a sensor. By charging and discharging the above two capacitors and comparing them, the temperature of the sensor and the circuit can be compared. We are trying to reduce the characteristics.

FIG. 6 shows the configuration of such a capacitance type pressure sensor, FIG. 6 (a) shows a cross section, and FIG. 6 (b) shows a state in which two substrates are developed.

Two electrodes are printed on the substrates 1 and 2 made of alumina, and the peripheral edges thereof are sealed with glass 3 so as to face each other with a constant gap. Therefore, the detection electrode 4 in the center changes sharply with respect to the external pressure, while the reference electrode 5 on the outer periphery is close to the glass and is not easily bent, and the pressure change due to the external pressure is very small compared with the former.

Since both capacitors are arranged close to each other on the same substrate, they are affected by temperature almost in the same manner. Therefore, by comparing the two, it is possible to remove only the influence of temperature and obtain only pressure information.

FIG. 7 shows such a conventional circuit configuration. The capacitance C _p of the detection electrode 4 and the capacitance C _r of the reference electrode 5 are the resistances R ₁ and R ₂ respectively.
And a charge / discharge circuit is configured. Both capacitors are turned on and off by the transistors Q ₁ and Q ₂ , and charging and discharging are repeated.
The operation and waveforms of each part are omitted because they are detailed in the cited example, but if the capacitance of the sensor is set to C _p > C _r ,
When the power supply voltage is V _CC , the output voltage V of the low pass filter 6
_out becomes V _out = V _CC (1-C _r / C _p ) ... (1). Since the temperature characteristics of the sensor are almost the same as already described, the change in capacitance due to temperature ΔC _r (ppm /
C) and ΔC _p (ppm / ° C.) are canceled by the (1-C _r / C _p ) term to remove the temperature characteristic of the sensor from the output voltage V _out .

However, at this time, R ₁ = R ₂ , and the comparators 7 and 8 and the transistors Q ₁ and Q ₂ must be selected as a pair so as to have uniform temperature characteristics. If these conditions are observed, the temperature characteristics due to circuits other than the sensor can be removed.

The problem to be solved by the invention However, when the capacitance type pressure sensor having such a conventional configuration is used as a weight detection device, it is found that the detected weight value drifts to a practically nonnegligible temperature value. It was

In pursuit of this cause, it was found that the temperature characteristic of the sensor was extremely small, but the temperature characteristic due to the circuit was relatively large.

This is generally because the operating frequency f of the oscillation circuit is expressed by the following equation, but it is presumed that this circuit constant K has a temperature characteristic.

f = K / RC (2) However, K: Circuit constant FIG. 8 shows the temperature characteristics of the operating frequency f measured by an oscillator circuit. The operating frequency f on the horizontal axis changes C or R, and the temperature characteristic Δf on the vertical axis is obtained by the following equation.

Δf = (f ₂₀ -fα) / [f ₂₀ × _(α-20)] ... (3) where, f _20: Frequency at 20 ° C. f.alpha: Frequency ie Figure 8 at alpha ° C., the temperature of the sensor It is shown that even if the characteristic is suppressed to be small, the temperature characteristic remains in the oscillation circuit, which appears according to the operating frequency, and the higher the frequency, the larger the temperature characteristic.

Experiments were carried out in various oscillator circuits, but basically the same temperature characteristics depending on the operating frequency were confirmed by any oscillator circuit, although the slope of the temperature characteristic line and the positive and negative polarities changed. .

This phenomenon occurs when an oscillation circuit is used as a means for detecting the capacitance of the sensor, when the detection frequency and the reference frequency match, that is, when the detection capacitance and the reference capacitance match.
The temperature characteristic is completely erased, but it is suggested that when the frequency shifts, the temperature characteristic due to the circuit appears.

However, in the prior art cited above, if C _p > C _r is not satisfied, operation does not occur, and therefore temperature characteristics due to this frequency shift must be prepared over the entire range of use.

The present invention intends to suppress the influence of temperature characteristics caused by such a circuit to a slight extent.

Means for Solving the Problems The present invention, in order to solve the above problems, a mounting table on which an object to be measured is mounted, is opposed to the mounting table with a predetermined gap, and a detection electrode is provided in the central portion and an outer peripheral portion thereof. A control unit having an electrostatic capacitance type pressure sensor formed of a pair of flat plates having a reference electrode, a detection means for reading the capacitances of the detection electrode and the reference electrode, and an arithmetic means, for switching and measuring the capacitances of the both electrodes. It has and.

Action The weight detection device of the present invention is configured to be mechanically engaged with the mounting table so that the capacitance value of the detection electrode changes depending on the weight of the object to be measured mounted on the mounting table. The electrode shape is set so that the capacitances of the detection electrode and the reference electrode are substantially the same when the object to be measured having a weight of approximately one half of the weight is placed. Then, the control unit performs measurement through the detection unit while switching the capacitances of the detection electrode and the reference electrode, arithmetically processes the measurement result using the arithmetic unit, and calculates the weight of the object to be measured based on the solution.

With such a configuration and operation, the weight can be detected on the mounting table from no load to the maximum weighing without extremely increasing the temperature characteristics.

Example Hereinafter, a weight detection device according to the present invention will be described with reference to the drawings.

FIG. 2 shows an example in which a weight detecting device according to the present invention is mounted on a mounting table of a heating device such as a microwave oven, and detects the initial weight of the object to be heated and the weight change during heating.

A heat source 10 is coupled to the heating chamber 9, and an object to be measured 12 is placed on a mounting table 11. Reference numeral 13 is a motor that drives the mounting table 11 to rotate, and improves the occurrence of uneven heating when the object 12 to be measured is heated by the heat source 10.

The rotating shaft of the motor 13 freely moves in the thrust direction, and the tip end thereof is supported by the capacitance type pressure sensor 14. Therefore, the weights of the mounting table 11 and the object 12 to be measured are stored in the capacitance type pressure sensor 14. Transmitted.

The structure of the sensor is the same as that of the conventional one shown in FIG. 6 and has already been described. Therefore, the description will be omitted here by avoiding duplication.

FIG. 3 is an embodiment of a block diagram showing the system configuration of the weight detecting apparatus according to the present invention.

The single detection means is the reference capacitance C _r, which is the capacitance value of the reference electrode.
And a CR oscillation circuit 15 including a detection capacitance C _p which is the capacitance value of the detection electrode and a resistor R. Switching means
16 is controlled by a switching gate signal control means 18 incorporated in the control unit 17, switches between the reference capacitance C _r and the detection capacitance C _p and is connected to the oscillation circuit 15, and the control unit 17 sets the oscillation frequencies f _r and f _p. Input to the counter means 19 inside. Then, the output of the counter means 19 is stored in a predetermined address of the RAM 20, respectively, transferred to the arithmetic means 21 and subjected to division processing, and the ratio r is obtained.

Next, the essential points of the present invention will be described with reference to FIG. FIG. 1 is a diagram showing a capacitance relationship between a detection electrode and a reference electrode. The horizontal axis represents the weight w of the object to be measured placed on the mounting table,
The vertical axis represents the output frequency of the oscillation circuit as the detection means and the ratio r thereof. The point w = w _pl represents the state where only the mounting table is mounted, and the point w = w _max represents the maximum weighing amount.

The heavier the weight w, the greater the deflection of the substrate,
Since the distance between the electrodes becomes smaller, the capacitance value becomes larger from the following equation.

C = εS / d (4) where ε: Permittivity in air C: Interelectrode capacitance S: Electrode area d: Electrode distance Frequency f is calculated from equation (2), so if weight w increases, frequency On the contrary, f becomes lower.

Here, if the ratio r of both is calculated, Becomes Since a single oscillator circuit is used as the detection circuit, the circuit constant K is the same for both f _r and f _p , and the resistance R
Since the frequency ratio r is calculated, it is the same as the ratio between the detection capacitance C _p and the reference capacitance C _r . Therefore, the temperature characteristic of the sensor can be canceled by obtaining the ratio r.

Here, the reference capacity C _r and the detection capacity C _p are further calculated as w _pl to w _max.
Between, i.e., when a measured object of a certain weight w _z , which is lighter than the maximum weight, is placed on the mounting table, it is natural that if the two objects are matched at two times, naturally, the two The frequencies f _r and f _p will intersect at the point w = w _z .

Therefore, the ratio r becomes 1 at the point of w = w _z . As described above, when the operating frequencies are the same, the temperature characteristic caused by the circuit can be completely eliminated. That is, w _{z where the} ratio r = 1
In principle, the temperature characteristic at the point is zero.

From the relationship between the ratio r and the weight w, it can be seen that the weight w can be obtained by calculating a high-order approximation formula, for example, the following quadratic formula.

w = ar ² + br + c (5) However, a, b, c: constants As described above, the ratio r between the two frequencies f _r and f _p is obtained, and C _r and C _p are calculated at w = w _z point. By making them coincident, the temperature characteristic of the sensor can be removed and the temperature characteristic of the circuit can be erased.

If w _z is selected as follows, for example, the weight can be detected in the entire detection weight range without extremely increasing the temperature characteristic.

w _z = (w _pl + w _max ) / 2 (6) FIG. 4 shows an example of a concrete circuit configuration of such a system. The control unit 17 is formed by the microcomputer 1, and is provided with an output E ₀ as a switching gate signal control means and a TC as an input terminal of a built-in counter means.

The detection means 15 is formed by a combination of a sawtooth wave generation circuit of an operational amplifier and a waveform shaping circuit. The switching means 16 is composed of an analog switch,
This can be achieved with other semiconductor switching means as well as with relays.

Reference numeral 22 denotes a level shift circuit that performs voltage conversion and waveform shaping, and may be added as needed.

For example, analog switch is μPC4066, operational amplifier is
The TL082 and the microcomputer can be realized by MB88515, but it goes without saying that they can be used as long as they have a function equivalent to this.

FIG. 5 is a flowchart showing a control program of the microcomputer.

When the weight measurement is started, the gate signal E ₀ is first changed to the H level (a). And after a slight delay time is appropriately inserted (b), built-in counter which is connected to the TC terminal is started (c), the measurement of the reference frequency f _r is started.

Then, the gate time of the counter, for example, 1 second, is managed using a timer interrupt or the like (d), and when this predetermined time has elapsed, the counter is stopped (e). The counting result _fr is transferred to a predetermined address of the RAM and stored (f).

Then the gate signal E ₀ is turned the L level (g), the measurement of the f _p is carried out in exactly the same procedure as below f _r (h)
~ (L).

After such processing, f _r and f _p stored in the RAM are then subjected to division processing to calculate r first (m), and then the weight w is calculated by the quadratic approximation formula based on the ratio r. (N).

The weight w obtained by the above procedure can cancel the temperature characteristic of the sensor as described above.

As described above, the weight detecting device of the present invention is
There is no restriction that it is used in the condition of C _p ＞ C _{r, and} the point where C _p = C _{r at} which the temperature characteristic of the circuit becomes zero can be effectively used, and the measured object between zero load and maximum weighing can be mounted. By matching them when placed, the temperature characteristics could be minimized over the entire detection weight range.

Further, since the ratio r does not have the power supply voltage V _CC term as in the conventional case, it is not affected by the temperature characteristic of the power supply. Further detection capacity
Since the resistance R and the oscillation circuit 15 used to detect C _p and the reference capacitance C _r are exactly the same, there is no need to pair those having the same temperature characteristics as in the conventional case.

Furthermore, the output frequency of the oscillator circuit can be input to the counter as it is, and it is not necessary to convert it to DC voltage with a low-pass filter as in the past, so the circuit configuration is simple and inexpensive.

[Brief description of drawings]

FIG. 1 is a diagram showing the weight and each frequency and its ratio according to the present invention, FIG. 2 is a sectional view showing an embodiment of the configuration of the weight detecting device according to the present invention, and FIG. 3 is a block diagram thereof. Fourth
FIG. 6 is a circuit diagram showing a specific example of the present invention, FIG. 5 is a flow chart showing the structure of a control program, FIG. 6 (a) is a sectional view of a capacitance type sensor, and FIG. 6 (b) is the same development. FIG. 7 is a circuit diagram of a conventional example, and FIG. 8 is a temperature characteristic diagram according to the operating frequency of the circuit when an oscillation circuit is used as the detection circuit. 4 ... Detection electrode, 5 ... Reference electrode, 11 ... Mounting table, 15 ...
… Detecting means, 16 …… Switching means, 17 …… Control section,
19 ... Counter means, 21 ... Calculation means.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kenzo Ochi 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (56) Reference JP-A-58-198739 (JP, A) JP-A-61- 40517 (JP, A)

Claims (1)

[Claims] 1. A capacitance type which is formed of a pair of flat plates facing a mounting table on which an object to be measured is placed with a predetermined gap, a detection electrode in the center, and a reference electrode in the outer periphery. The electrostatic capacitance type pressure sensor comprises a pressure sensor, a detection unit for reading out the capacitances of the detection electrode and the reference electrode, and a control unit for switching and measuring the capacitances of the detection electrode and the reference electrode via the detection unit. Is mechanically engaged with the mounting table, and is configured so that the capacitance value of the detection electrode changes depending on the weight of the object to be measured mounted on the mounting table, and approximately half of the maximum weighing amount on the mounting table. When one object to be measured is placed, the electrode shape is set so that the capacities of the detection electrode and the reference electrode are substantially equal to each other, and the control unit has a calculation means, and the detected detection The capacitance value of the electrode and the reference electrode Determine the specific by using an arithmetic unit, the weight detection apparatus configured to calculate the weight of the object that is placed on the mounting table on the basis of the ratio.