JPH01233338A - Load detector using piezoelectric sensor - Google Patents

Abstract

PURPOSE:To eliminate the effect of a drift and thereby to detect a cutting load accurately, by disposing two piezoelectric sensors so that a load applied to one piezoelectric sensor is increased by the cutting load and that a load applied to the other piezoelectric sensor is thereby decreased. CONSTITUTION:Sheet-shaped piezoelectric sensors 1a and 1b are set on a feed screw mechanism which moves a work table or a tool stage of a machine tool. The piezoelectric sensors 1a and 1b are disposed on the opposite sides of a feed nut, for instance, so that a load applied to the sensor 1a is increased in accordance with a cutting load and that a load applied to the sensor 1b is decreased, to the contrary, in accordance with the cutting load. Electric charges generated by the sensors 1a and 1b are amplified by charge amplifier circuits 2a and 2b and converted into voltages. An output of the amplifier circuit 2b is inverted by an inverter circuit 6 and added to an output of the amplifier circuit 2a by an addition circuit 5. Then the charges of the sensors 1a and 1b leak through input impedances of the amplifier circuits 2a and 2b, and therefore a drift to be generated is offset.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a load detection device using a piezoelectric sensor for measuring cutting force or cutting resistance of a machine tool.

[Prior Art] The present inventors used a film sheet-like piezoelectric sensor as a device for detecting the cutting force or cutting resistance of a machine tool, and used this piezoelectric sensor to detect the cutting force or cutting resistance of a ball screw mechanism of the machine tool. We have proposed a load detection device that is installed at a position that receives the load caused by the load.

FIG. 1 shows a circuit configuration of a load detection device using a piezoelectric sensor.

In Fig. 8, 1 is a piezoelectric sensor as a transducer that detects the load due to cutting force or cutting resistance, and the piezoelectric sensor 1 is in the form of a film sheet with a thickness of about 0.2III11. .

As is well known, the piezoelectric sensor 1 generates an electric charge between the electrodes according to the load applied to the piezoelectric sensor. Therefore, in order to convert the electric charge generated according to the load of the piezoelectric sensor into a voltage signal, a charging circuit is used as an external circuit. A charge amplification circuit 2 known as an amplifier is provided.

The charge amplification circuit 2 has a capacitor C connected to the feedback circuit of the operational amplifier 3, and the piezoelectric sensor 1 can be regarded as a variable sound ff1cX since the generated charge ff1qx changes depending on the load.

Here, if the amplification factor A of the operational amplifier 3 is infinite, the charge generated by the piezoelectric sensor 1 is transferred as is, qx, to the feedback capacitor C due to the imaginary short effect of the operational amplifier 3, and ideally, the charge generated by the piezoelectric sensor 1 is ff1q
An output voltage EO proportional to x can be obtained. In reality, the amplification factor of operational amplifier 3 does not become infinite, so
The output voltage EO is EOζ-qX/C.

Further, a reset switch 4 is connected to the feedback circuit of the operational amplifier 3 in parallel with the capacitor C. That is, since a predetermined assembly load is applied to the piezoelectric sensor 1 in the initial state, the capacitor C is discharged and reset to zero before the load is detected.

[Problem to be Solved by the Invention 1] However, in a conventional load detection device using such a piezoelectric sensor and a charge amplification circuit, as shown in FIG. 9, the output voltage EO of the charge amplification circuit changes over time. Drift that changes over time poses a problem, making it impossible to accurately detect the load.

That is, drift means that when the charge generated between the electrodes of the piezoelectric sensor 1 is amplified and converted into a voltage signal by the charge amplification circuit 2,
Since the input impedance of the charge amplification circuit 2 cannot be made infinite, this means that the charge generated in the piezoelectric sensor 1 leaks through the input impedance, causing the output to fluctuate.

Such drift is not a problem when detecting AC components such as vibrations, but when detecting DC components such as cutting loads.
This is a big problem when detecting specific components.

For this reason, in load detection using conventional piezoelectric sensors,
Phenomena such as vibrations with very short periods do not have the problem of drift, so they are effectively used to measure forces such as vibrations, but they can be used effectively to measure forces such as vibrations, but phenomena where a nearly constant load is applied over a relatively long period of time. (phenomena that last for more than one second) are not often used.

The present invention has been made in view of such conventional problems, and provides a load detection device using a piezoelectric sensor that allows the piezoelectric sensor to accurately detect the cutting load without being affected by drift. The purpose is to

[Means for Solving the Problems] In order to achieve this object, the present invention has a method in which the load applied to one piezoelectric sensor increases and the load applied to the other piezoelectric sensor reverses the cutting load. It has a load detection mechanism in which two piezoelectric sensors are arranged, and a charge amplification circuit amplifies and converts each of the charges generated according to the load applied to each piezoelectric sensor into a voltage signal, and the output of one of the charge amplification circuits is inverted. After that, the output of the other charge amplification circuit and the addition circuit are added together.

A load detection mechanism with two piezoelectric sensors arranged can be used, for example, to detect the feed screw of a machine tool. Assemble and fix the piezoelectric sensors to both ends of the nut member screwed to the foot using presser feet, and connect a work table or tool stand to the side of the presser foot to which the piezoelectric sensors are fixed.

In another example of the load detection mechanism, a feed screw shaft of a machine tool is rotatably supported, and piezoelectric sensors are tightened and fixed on both sides of seven outer rings of a bearing that receives thrust force.

[Function] In the load detection device of the present invention having such a configuration, a cutting load is applied to one piezoelectric sensor in a direction that increases the assembly load, and at the same time, a cutting load is applied to the other piezoelectric sensor in a direction that increases the assembly load. Since the applied load is applied in the direction of decreasing the applied load, the amount of charge generated according to the increase or decrease of the load of the two piezoelectric sensors is amplified and converted into a voltage signal by a charge amplification circuit, and then the amplified output of one is inverted and the amplified output of the other is converted. By adding them together using an adding circuit, it is possible to offset the drift that changes over time, and even if a constant load is continuously applied to the piezoelectric sensor, it can respond to the load without being affected by fluctuations due to drift. A detection signal can be obtained accurately.

In addition, since the detection outputs of two piezoelectric sensors are added together, the output voltage can be increased according to the load, and S
A detection signal with a good /N ratio can be obtained.

[Embodiment] FIG. 1 is a circuit block diagram showing an embodiment of the present invention.

In FIG. 1, la and 1b are film sheet-like piezoelectric sensors, and as will be explained later, they are assembled into a load detection mechanism, and when a cutting load is applied, for example, the piezoelectric sensor 1a receives a cutting load. A cutting load is applied to the piezoelectric sensor 1b in a direction that increases the tightening load, and conversely, a cutting load is applied to the piezoelectric sensor 1b in a direction that decreases the assembly load.

Charge amplification circuits 2a and 2b known as charge amplifiers are provided following the piezoelectric sensors 'la and 1b. The charge amplification circuits 2a, 2b have operational amplifiers 3a, 3b,
Capacitor Ca in the feedback circuit of operational amplifiers 3a and 3b,
Cb is connected, and a reset switch 4a for performing discharge reset in parallel with capacitors Ca and Cb;
4b is connected.

Here, the principle of the charge amplification circuits 2a and 2b will be explained as follows, taking the charge amplification circuit 2a as an example.

Since the piezoelectric sensor 1a generates an electric current 1b q x according to the applied load, it can be regarded as a variable capacitor ff1cX.

Here, the input voltage of the operational amplifier 3a is El, and the output voltage is E.
Assuming that the O1 amplification factor is A and the charge of the feedback capacitor is qa, the following relational expression is obtained.

EO = -A-Ei Ei -Eo = qa / Ca Ei = (QX - qa) / (Cx + Cc)
However, CG is the signal cable capacitance between the piezoelectric sensor 1a and the charge amplification circuit 2a.

According to these relational expressions, qX=Qa ・(1+(Cx 0Cc)/(Ca(1
+A>) Ei = qa /Ca -1/ (1+A>E
o = -qa /Ca -A/ (1+A>).

Here, since the amplification factor A of the operational amplifier 3a is A>>1, qx~qa EiζO EO - qa/Ca.

Therefore, the input voltage Ei of the operational amplifier 3a is maintained at approximately zero, and all the charges generated in the piezoelectric sensor υ1a are stored in the feedback capacitor +j-Ca, and the charge amplification circuit 2a
The output voltage EO is uniquely determined by the ratio between the capacitance of the feedback capacitor Ca and the electric charge qx generated by the piezoelectric sensor 1a.

The principle of the charge increasing circuit 2a is the same as that of the charge amplifying circuit 2b provided in the piezoelectric sensor 1b.

The output of the charge amplification circuit 2a that converts and amplifies the charge generated by the piezoelectric sensor 1a into a voltage signal is input to the addition circuit 5, and the output of the charge amplification circuit 2b that converts and amplifies the charge generated by the piezoelectric sensor 1b is inverted by the inversion circuit 6. After that, it is added to the adder circuit 5. As a result, the adder circuit 5 outputs a signal obtained by adding the inverted output of the charge amplification circuit 2b to the amplified output of the charge amplification circuit 2a.

FIG. 2 shows piezoelectric sensors 1a and lb shown in the embodiment of FIG.
It is an explanatory view showing one example of a load detection mechanism provided with.

In FIG. 2, 7 is a feed screw shaft, the right end of which is rotatably supported by a ball bearing 8.
A pair of support bearings 9a receives both radial load and thrust load at the left end.

The output shaft of a pulse motor (not shown) is connected to the shaft end taken out to the left from the bearing portion formed by the support bearings 9a and 9b.

A nut member 10 is screwed onto the feed screw shaft 7. Piezoelectric sensors 1a and 'lb are arranged at both ends of the nut member 10, and the nut member is tightened with bolts at positions shown by dashed lines using presser feet 11a and 11b. Piezoelectric sensor l at 10
a.

1b is tightened and fixed.

Figure 3 shows the nut member 1 for the screw shaft 7 in Figure 2.
FIG. 2 is a cross-sectional view showing an attached portion of the device 0;

In FIG. 3, the nut member 10 is connected to the feed screw shaft 7.
The piezoelectric sensors 1a and 1b are fitted into the shaft portions on both sides of the nut member 10, and the presser feet 11a and 11b are fitted from the outside and the nut is tightened by tightening the bolts at the positions indicated by the dashed lines. It is fastened and fixed to the member 10.

Such a piezoelectric sensor 1a shown in FIG. 2.3.

1b, the nut member 10
A presser foot 118 that tightens and fixes the piezoelectric sensors Ia and Ib to
．． The llb side is connected to the work table or tool stand of the machine tool, and the feed screw shaft 7 is connected by a pulse motor.
By rotating the nut member 10, the rotational movement is converted into a linear movement in the left-right direction, and the work table or tool stand is moved via the presser feet 118 and 11b.

At this time, for example, if the feed screw shaft 7 is rotated so as to send the nut member 10 to the right, the nut member 1
The axial load received by the piezoelectric sensor 1b placed on the right side of the nut member in response to the cutting load via the work table or tool stand acts in the direction of increasing the tightening load of the piezoelectric sensor 1b, and conversely increases the tightening load of the piezoelectric sensor 1b. An axial load acts on the piezoelectric sensor 1a mounted on the left side of the piezoelectric sensor 10 in a direction that reduces the tightening load of the piezoelectric sensor 1a. On the other hand, when the feed screw shaft 7 is rotated to move the nut member 10 to the left, a load is applied to the left piezoelectric sensor 1a in the direction of increasing the tightening load, and the right piezoelectric sensor 1b is affected by a load that increases the tightening load. The axial load acts in the direction that reduces the tightening load.

FIG. 4 is an explanatory diagram showing another embodiment of the load detection mechanism provided with the piezoelectric sensors 1a and 1b shown in FIG. 1.

In FIG. 4, piezoelectric sensors la and Ib are incorporated into support bearings 9a and 9b that rotatably support the feed screw shaft 7 and receive thrust. That is, the piezoelectric sensor 1a is installed on the left side of the long port bearing 9a installed in the bearing hole 13, and the piezoelectric sensor 1b is installed on the right side of the support bearing 9b, and the presser foot 14 is inserted into the bearing hole 13 from the outside of the piezoelectric sensor 1a. It is attached and fixed with bolts. The piezoelectric sensors 1a, 1b are suppressed by the sides of the support bearings 9a, 9bkT () and the actuator ring, and the inner ring is fitted into the feed screw shaft 7 side and rotates, so the piezoelectric sensors 1a, 1b Provided so that it does not come into contact with the Of course, the right end of the feed screw shaft 7 is rotatably supported by a ball bearing 8, and a nut member 10 for converting rotational motion into linear motion is screwed together.

FIG. 5 is a sectional view showing a piezoelectric sensor used in the present invention, and FIG. 6 is a partially cutaway plan view thereof.

5 and 6, the piezoelectric sensor has a piezoelectric material layer 2 in the center.
1, and electrode layers 22a, 22 on both sides of the piezoelectric material layer 21.
b, and the piezoelectric material layer 21 and the electrode layers 22a, 2
The piezoelectric sensor 2b is produced by cutting out a piezoelectric film in the form of a film sheet that constitutes a piezoelectric element typified by PZT. The detection sensitivity of a piezoelectric sensor is determined by the piezoelectric constant,
In other words, it is determined by the amount of electric charge generated per unit force, and the piezoelectric constant is constant regardless of the size of the area receiving the force of the piezoelectric sensor, so increasing the area reduces the load applied per unit area and increases detection sensitivity. without reducing
The rigidity of the sensor can be increased. In addition, the thickness of the film sheet-shaped piezoelectric sensor is about 0.2 mm, and even if the Young's modulus is small, the deformation (deformation strain) due to force is minute, and as shown in Figure 2-4, it can be rotated with a machine tool. Even if the piezoelectric sensors 1a and 1b are directly incorporated into the portion that converts motion into linear motion, the machining accuracy will not be affected.

Furthermore, a piezoelectric sensor 1a having a structure shown in FIGS. 5 and 6.

1b, the capacitance of the feedback capacitors Ca and Cb of the charge amplification circuits 2a and 2b shown in FIG. 1 can be changed from 1kc+f' to 1000.
To! The load can be detected over a wide range up to jf, and a high resolution of about 100 (llf) can be obtained.

Referring again to FIGS. 5 and 6, insulating sheets 23a and 23b are further fixed with adhesive or the like on both sides of the film sheet-like piezoelectric sensor consisting of one layer 21 of piezoelectric material and electrode layers 22a and 22b on both sides. Lead wires 24a and 2/ib are drawn out from the electrode layers 22a and 22b.

Next, the operation of the embodiment shown in FIG. 1 will be explained with reference to the signal waveform diagram shown in FIG.

FIG. 7(a) shows the output voltage of the charge amplification circuit 2a that converts and amplifies the charge generated by the piezoelectric sensor 1a, and FIG. 7(b)
represents the output voltage of the charge amplification circuit 2b which converts and amplifies the charge generated by the piezoelectric sensor 1b.

Here, FIGS. 7(a) and 7(b) show a case where an increasing cutting load is applied to the piezoelectric sensor +J1a, and a decreasing cutting load is applied to the piezoelectric sensor 1b.

First, until the time to when the machine tool starts cutting, the reset switches 4a and 4b provided in the charge amplification circuits 2a and 2b are
are switched on, and both the feedback capacitors Ca and Cb are placed in a discharge reset state, and the electric charge generated by the assembly load of the piezoelectric sensors la and 1b is discharged and reset to make the output voltage zero. In this state, reset switch 4 at time to start cutting of the machine tool.
When a and 4b are switched off, the output voltage increases and decreases due to drift over time, and cutting starts at time t1, and a cutting load is applied to the piezoelectric sensor 1a in the direction of increasing the assembly load. , the output voltage of the charge amplification circuit 2a increases according to the cutting load;
The output voltage of b decreases in accordance with the cutting load.

The output voltage of the charge amplifying circuit 2a in FIG. 7(a) is inputted as is to the adding circuit 5, while the output voltage of the charge amplifying circuit 2b is inverted by the inverting circuit 6 to produce an inverted signal as shown in FIG. 7(C). and is applied to the additive circuit 5. As a result, the adder circuit 5 produces the tightening output shown in FIG. 7(d), which is the sum of the output voltage in FIG. 7(a) and the inverted voltage in FIG. 7(C), and the drift over time is canceled out. Furthermore, the output voltage obtained by adding the output voltage due to the change in cutting load can be increased by 1q.

That is, since the charge amplification circuits 2a and 2b have the same circuit configuration, the drift that occurs over time also matches, and by inverting one of them and adding them, it is possible to cancel out the drift almost completely. .

Although the adder circuit 5 adds the inverted output of the charge amplification circuit 2b to the output of the charge amplification circuit 2a, it is of course possible to add and average the two.

Moreover, in the embodiment of FIG. 2.4, the piezoelectric sensor 1a,
1b is the workpiece' feed screw mechanism feed screw mechanism 1~
Alternatively, although it is provided on the port bearing, it may be similarly provided on the feed nut of the feed screw mechanism of the main shaft system or the support of the rear boat 11.

[Effects of the Invention] As explained above, according to the present invention, the load detection mechanism applies a cutting load to one piezoelectric sensor in a direction that increases the number of assembled carts, and at the same time applies a cutting load to the other piezoelectric sensor. is added in a direction that reduces the assembly C load,
After converting and amplifying the amount of charge generated by the two piezoelectric sensors in accordance with the increase or decrease of the cart into a voltage signal in a charge amplification circuit, the amplified output of one is inverted and added to the amplified output of the other in an adder circuit to calculate the passage of time. Even if a constant cutting load is continuously applied to the piezoelectric sensor, the detection signal corresponding to the cutting load can be accurately increased by 1 without being affected by the fluctuation due to the drift. Can be done.

Furthermore, since the detection outputs of the two piezoelectric sensors are added or averaged, the output voltage can be increased in accordance with the load, and a detection signal with a good S/N ratio can be obtained.

[Brief explanation of the drawing]

Figure 1 is a circuit block diagram showing an embodiment of the present invention. Figure 2 is an explanatory diagram showing an embodiment of the load detection mechanism of the present invention. Figure 3 is a cross-sectional diagram of the nut portion in Figure 2. Figure; Figure 4 is an explanatory diagram showing another embodiment of the load detection mechanism of the present invention; Figure 5 is a sectional view of a piezoelectric sensor used in the present invention; Figure 6 is a partially cutaway view of Figure 5; Fig. 7 is a signal waveform diagram showing the operation of Fig. 1; Fig. 8 is a circuit block diagram showing the conventional device; Fig. 9 is a signal waveform diagram showing the conventional drift problem. be. Ia, 1b: Piezoelectric sensor 2a, 2b: Charge amplification circuit (chi V-amp) 3a, 3
b: Operational amplifiers 4a, 4b: Reset switch 5: Addition circuit 6: Reversing circuit 7: Feed screw shaft 8: Ball bearings 9a, 9b: Support bearing 10: Nut members 11a, 11b, 14: Presser foot 13: Bearing hole 21: Piezoelectric material layers 22a, 22b: i-pole layers 23a, 23b: insulating sheets 24a, 24b: I, J-wires Ca, Cb: capacitors Fig. 1 Fig. 2 Fig. 3 Fig. 4 Fig. 5

Claims (1)

[Claims] 1. Two piezoelectric sensors in the form of film sheets; said 2 piezoelectric sensors so that when a load is applied, the load applied to one piezoelectric sensor increases and the load applied to the other piezoelectric sensor decreases. a load detection mechanism installed with one piezoelectric sensor; a charge amplification circuit provided for each of the two piezoelectric sensors that converts a generated charge corresponding to a load applied to the piezoelectric sensor into a voltage signal; one of the charge amplification circuits. 1. A load detection device using a piezoelectric sensor, comprising: an addition circuit that inverts the output of the charge amplification circuit and then adds the output of the charge amplification circuit to the output of the other charge amplification circuit. 2. The load detection mechanism has the two piezoelectric sensors assembled and fixed at both ends of a nut member screwed onto the feed screw shaft of the machine tool using a presser foot, and a work table or tool stand is connected to the presser foot side. Claim 1 characterized in that
A load detection device using the piezoelectric sensor described above. 3. The load detection mechanism rotatably supports a feed screw shaft of a machine tool, and the two piezoelectric sensors are installed on both sides of an outer ring of a bearing that receives thrust force and are fixed by tightening. A load detection device using the piezoelectric sensor according to 1.