JP6135777B2 - Material testing machine - Google Patents

Description

  The present invention relates to a material testing machine for executing a material test, and more particularly to a material testing machine equipped with a physical quantity detection means having a strain gauge type transducer.

  Such a material testing machine has, for example, a configuration in which a pair of screw rods are rotatably supported on a table in synchronization with each other, and both end portions of the crosshead are supported on these screw rods via nuts. Then, the crosshead is moved along the pair of screw rods by rotating the pair of screw rods in synchronization with each other by the rotation of the motor. A jig such as a gripping tool is connected to the crosshead and the table. And it is comprised so that a test force may be applied with respect to a test piece by moving a crosshead in the state which hold | gripped both ends of the test piece with jigs, such as these pair of grips.

  In such a material testing machine, the test force acting on the test piece is detected by the load cell. Moreover, the displacement amount of the distance between the gauge marks in the test piece is measured by a displacement meter. Load cells and displacement meters used in material testing machines have so-called strain gauge transducers that electrically convert various physical quantities such as test force and displacement by a bridge circuit consisting of four strain gauges. The thing is known (refer patent document 1). These load cells and displacement meters are connected to an AC amplifier on the material tester main body side by a cable.

JP 2011-169765 A

FIG. 4 is a schematic diagram showing the connection between the conventional load cell 14 and the load amplifier 41. FIG. 5 is an electric circuit diagram for detecting a test force by the load cell 14 in a conventional material testing machine. Capacitors C ₁ and C ₂ indicate stray capacitance between cable A and cable C and stray capacitance between cable B and cable C, respectively. Further, a stray capacitance (not shown) exists between the cables, as in C ₁ and C ₂ .

  The load cell 14 has a strain gauge type transducer that electrically converts a test force by a bridge circuit 24 composed of four strain gauges. The load amplifier 41 is an AC amplifier and provides an AC signal as an excitation signal to the bridge circuit 24 of the load cell 14.

  Conventionally, as shown in FIG. 4, a four-core shielded wire is used as the cable CL for connecting the load cell 14 and the load amplifier 41. At both ends of the cable CL, cable-side connectors 51 including cable-side connection terminals A, B, C, and D and a shell are disposed. The load cell 14 is provided with a load cell side connector 25 comprising load cell side connection terminals A, B, C, D and a shell, and the load amplifier 41 is comprised of load amplifier side connection terminals A, B, C, D and a shell. A load amplifier side connector 45 is provided. Then, one end of the cable side connector 51 is connected to the load cell side connector 25, and the other end of the cable side connector 51 is connected to the load amplifier side connector 45, whereby the cable CL is connected between the load cell 14 and the load amplifier 41. Signals are input and output via the. In addition, when connecting the strain gauge type displacement meter and the displacement meter amplifier, they are similarly connected by the cable CL.

  In this connection example, assuming that the core wires of the shield wires connecting the corresponding alphabetic terminals of the load cell side connector 25 and the load amplifier side connector 45 are A, B, C, and D, respectively, from the load amplifier 41 to the load cell 14. A and B lines are wired to terminals that output the excitation signal of C, and C and D lines are wired to terminals that input strain signals from the load cell 14 to the load amplifier 41.

  In such a shielded wire, stray capacitance exists between the core wires of AB, AC, AD, BC, BD, and CD. Furthermore, if the shield of the shield wire is S, stray capacitance also exists between the core wires of AS, BS, CS, and DS and the shield. And the magnitude | size of these stray capacitances is 300-500 pF (picofarad) as the capacity | capacitance between lines between each core wire, when the cable length is 3 meters, It is about 1000 pF (picofarad).

  If the stray capacitance is a constant value, the value may be handled as an offset amount for the measurement result, and the influence of the stray capacitance can be canceled before the measurement is started. However, in the material test, the stray capacitance fluctuates even during execution of the material test due to a change in the position of the shield wire due to the movement of the crosshead or a user's hand touching the shield wire.

Here, the influence of the stray capacitance in the conventional cable CL on the measurement result will be considered. First, the stray capacitance (interline capacitance) between the core wires of the shield wire will be considered. FIG. 6 is an equivalent circuit diagram for considering the stray capacitance between the core wire that transmits the excitation signal and the core wire that transmits the strain signal. Figure 7 is a graph showing the change in the measured voltage V _M when the capacitance is balanced, FIG. 8 is a graph showing the change in the measured voltage V _M when capacity is unbalanced. In addition, the vertical axis | shaft of the graph of FIG. 7 shows output voltage value (mV), and a horizontal axis shows distortion. In addition, the vertical axis of the graph of FIG. 8 indicates the output voltage value (μV), and the horizontal axis indicates the amount of change in capacitance.

In the circuit of FIG. 6, assuming that the angular frequency of the power source V _S is ω, the impedances R ₁ , R ₂ , C ₁ , and C ₂ , ZR ₁ , ZR ₂ , ZC ₁ , and ZC ₂ are expressed by the following equation (1). expressed. C ₁ is the stray capacitance between A and C, C ₂ is imitating the stray capacitance between B and C, and R ₁ and R ₂ are imitating the left half of the strain gauge bridge (FIG. 5).

_Further, R 1 _{C 1} parallel _circuits, each impedance of the R 2 _{C 2} parallel circuits _Z 1, _{Z 2} is represented by the following formula (2).

Using the expression (2), the output voltage V _O from this circuit is expressed by the following expression (3).

Here, since the load amplifier 41 is an AC amplifier, the measured value has a function of extracting only the real part of the equation (3) at the same angular frequency as ω. Therefore, the measurement voltage V _M of the measurement result is expressed by the following equation (4).

Here, when V _S = 5 V, ω = 2 × π × 10 k rad / s, R (resistance) and C (capacitance) are changed from the initial state of R ₁ = R ₂ = 350Ω and C ₁ = C ₂ = 500 pF. when examining the change of the measured voltage V _M at the time of change, the graphs of FIGS.

When a rated test force is applied to a general load cell 14, the strain gauge of the bridge circuit 24 changes by 3000 με (microstrain). The relationship between the strain ε and the gauge constant K is expressed by ε = ΔR / R / K (ΔR: the strain gauge resistance change amount, R: the strain gauge resistance value). Then, a gauge constant K is 2, the capacitance is balanced, distortion measured voltage V _M at the time of change in a range of ± 3000Myuipushiron, as shown in FIG. 7, changes ± 15mV. Incidentally, when given a general strain rated displacement gauge displacement meter also measures the voltage V _M is similar changes.

Meanwhile, and resistance to equilibrium, the measured voltage _{V M} of one of the capacitance _C 1, _{C 2} when the _{(C 1)} is changed in a range of 500 ± 5 pF (when the capacitance unbalance), FIG. As shown in FIG. This is a first size of 5000 minutes of the output of the full scale of the measurement voltage V _M at the time of giving the rated test force to the load cell 14.

The load cell 14 has a test force range with a guaranteed accuracy range of 1 / 1000th to 1/1000 of the rated test force, and guarantees the measurement accuracy at ± 0.5% of the digital indication value. In this case, the resolution required for the load cell 14 is 1/4000 of full scale. However, change in the measured voltage V _M which has been described with reference to FIG. 8 is a 1 in the magnitude of 5000 minutes of the output of the measuring voltage V _M of the full scale, 400,000 parts of the full scale is needed resolution 1 It corresponds to 80 times the size. From this, it is assumed that the fluctuation of the line capacitance has an influence on the measurement result by the load cell 14.

Next, the stray capacitance (capacity between shields) C between each core wire of the shield wire and the shield will be considered. FIG. 9 is an equivalent circuit diagram for considering the stray capacitance between the core wire and the shield for transmitting the excitation signal and between the core wire and the shield for transmitting the distortion signal. Figure 10 is a graph showing changes in excitation voltage V _E when capacity is unbalanced, Figure 11 is a graph showing the change in the measured voltage V _M when capacity is unbalanced. 10 and 11, the vertical axis represents the output voltage value (mV), and the horizontal axis represents the amount of change in capacitance.

In the circuit of FIG. 9 composed of a combination of a resistor (R), a capacitor (C), and an inductor (L), the angular frequency of the power source V _S is ω, and the same procedure as the above-described equations (1) to (3). Thus, when the output voltage V _O is obtained from the impedance, the excitation voltage V _E given to the bridge circuit 24 is expressed by the following equation (5).

First, the influence of the stray capacitance C (the stray capacitance between A and S) between the core wire that transmits the excitation signal and the shield will be considered. In the circuit of FIG. 9, V _S , L, R ₁ , C, R ₂ are respectively
V _S : Excitation voltage source L: Excitation signal line inductance R ₁ : Excitation signal line resistance C: Excitation signal line and shield stray capacitance R ₂ : Strain gauge bridge resistance. V _S = 10 V, ω = 2 × π × 10 k rad / s, R ₁ = 10 mΩ, R ₂ = 350 Ω, L = 5 μH, C = 500 pF, distortion voltage measurement voltage when C changes ± 1% V _M, as shown in FIG. 10, changes ± 1 uV. This is only a variation of ± 0.000001% with respect to the excitation voltage of 10V.

Next, the influence of the stray capacitance C (stray capacitance between CS) between the core wire transmitting the strain signal and the shield will be considered. Here, V _S , L, R ₁ , C, and R ₂ in the circuit of FIG.
V _S : Strain gauge bridge electromotive voltage (bridge differential output)
L: Strain signal line inductance R _1: Resistance strain signal line C: the stray capacitance of the distortion signal line and the shield R _2: are simulating the input resistance of the strain amplifier (receiver). Here, when V _S = 15 mV, ω = 2 × π × ₁₀ k rad / s, R ₁ = 350Ω, R ₂ = 100 kΩ, L = 5 μH, C = 500 pF, and the distortion signal when C changes ± 1% measured voltage _{V M,} as shown in FIG. 11, changes ± 35 nV. This is only a variation of ± 0.00023% for a strain voltage of 15 mV. That is, it is considered that the fluctuation of the inter-shield capacitance is such that it can be ignored with respect to the measurement accuracy of the load cell 14 currently required in the material test.

  As described above, the influence of the fluctuation of the stray capacitance on the measurement result of the load cell 14 is large between the core wire that transmits the excitation signal and the core wire that transmits the strain signal (between A and C and between A and D). It has been found that there is a small distance between the core wire transmitting the shield and the shield (between A and S) and between the core wire transmitting the strain signal and the shield (between CS). From these considerations, the inventor of the present application has found that it is effective to reduce fluctuations in line-to-line capacitance in reducing noise in measurement by a physical quantity detection means having a strain gauge transducer.

  The present invention has been made on the basis of the above consideration, and an object thereof is to provide a material testing machine capable of reducing the influence of the stray capacitance of a cable and accurately measuring a physical quantity.

  The invention according to claim 1 comprises a physical quantity detecting means having a strain gauge transducer, an AC amplifier, and a cable connecting the physical quantity detecting means and the AC amplifier, and a material that gives a test force to a test piece. In the material testing machine for performing the test, the cable is output from the AC amplifier to the first shield line for transmitting an excitation signal input to the physical quantity detection means, and from the physical quantity detection means to the AC amplifier. And a second shield wire for transmitting a strain signal.

  According to a second aspect of the present invention, in the first aspect of the invention, the first shield line and the second shield line are twisted pair shield lines.

  According to a third aspect of the present invention, in the first or second aspect of the present invention, the physical quantity detection means is a load cell that detects a test force applied to the test piece.

  According to a fourth aspect of the present invention, in the first or second aspect of the present invention, the physical quantity detection means is a displacement meter that detects a displacement amount given to the test piece.

  According to the invention described in claim 1 to claim 4, the cable connecting the physical quantity detection means and the AC amplifier, the first shield line for transmitting the excitation signal input from the AC amplifier to the physical quantity detection means, Because the cable consists of a second shielded wire for transmitting the strain signal output from the physical quantity detection means to the AC amplifier, even if the cable is being moved or the user touches the cable during the test, It is possible to reduce the influence of stray capacitance and accurately measure physical quantities.

  According to the second aspect of the present invention, since the first shield wire and the second shield wire are twisted pair shield wires, the influence of external electrostatic coupling noise and electromagnetic coupling noise can be reduced, and the physical quantity can be more accurately determined. Can be measured.

  According to the third aspect of the present invention, in the material test, it is possible to more accurately execute test force measurement that requires high detection accuracy.

  According to the invention described in claim 4, in the material test, it is possible to more accurately execute the measurement of the elongation amount and the compression amount of the test piece that requires high detection accuracy.

1 is a schematic diagram of a material testing machine according to the present invention. 3 is a wiring diagram showing a connection between a load cell 14 and a load amplifier 41. FIG. It is a graph which shows the influence on the test force measurement result by the difference in a cable. It is a wiring diagram which shows the connection of the load cell 14 and the load amplifier 41 in the conventional material testing machine. It is an electric circuit diagram for the test force detection by the load cell 14 in the conventional material testing machine. It is an equivalent circuit diagram for considering the stray capacitance between the core wire which transmits an excitation signal, and the core wire which transmits a distortion signal. Is a graph showing the change in the measured voltage V _M when the capacitance is balanced. Capacity is a graph showing the change in the measured voltage V _M at the time of unbalance. It is an equivalent circuit diagram for considering the stray capacitance between the core wire for transmitting an excitation signal and a shield, and between the core wire for transmitting a distortion signal and the shield. Capacity is a graph showing changes in excitation voltage V _E at the time of unbalance. Capacity is a graph showing the change in the measured voltage V _M at the time of unbalance.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic diagram of a material testing machine according to the present invention.

  This material testing machine includes a table 16, a pair of support posts 19 erected on the floor, and a pair of screw rods erected so as to be rotatable on the table 16 in the vertical direction inside each support column 19. 11, a crosshead 13 movable along the screw rods 11, and a load mechanism 30 for moving the crosshead 13 to apply a test force to the test piece 10. FIG. 1 shows a state in which the left column 19 of the pair of columns 19 is removed.

  The cross head 13 is connected to the pair of screw rods 11 via nuts (not shown). The lower end portion of the screw rod 11 is connected to the load mechanism 30, and the power from the drive source of the load mechanism 30 is transmitted to the screw rod 11. As the pair of screw rods 11 rotate in synchronization, the cross head 13 moves up and down along the pair of screw rods 11.

  The crosshead 13 is provided with an upper gripping tool 21 for gripping the upper end portion of the test piece 10. On the other hand, the table 16 is provided with a lower gripping tool 22 for gripping the lower end portion of the test piece 10. When performing a tensile test, the test force (tensile load) is applied to the test piece 10 by raising the cross head 13 in a state where both ends of the test piece 10 are held by the upper gripping tool 21 and the lower gripping tool 22. ).

  At this time, the test force acting on the test piece 10 is detected by the load cell 14 disposed on the crosshead 13. Further, the displacement amount of the distance between the gauge points in the test piece 10 is measured by the displacement meter 18. Since the displacement amount measured in this embodiment is the elongation amount of the test piece 10, the displacement meter 18 is also referred to as an extensometer. Thus, this material testing machine includes the load cell 14 and the displacement meter 18 as physical quantity detection means.

  The control unit 23 includes a computer, a sequencer, and peripheral devices thereof. The control unit 23 is connected to the display unit 26 having a touch panel and the load mechanism 30, and operates the load mechanism 30 according to the test conditions set via the display unit 26. In addition, the control unit 23 is provided with a load amplifier 41 connected to the load cell 14 via the cable CL1, and takes in a signal output from the load cell 14 and executes data processing. The control unit 23 is provided with a displacement meter amplifier connected to the displacement meter 18 via the cable CL2. The controller 23 receives the signal output from the displacement meter 18 and executes data processing.

  FIG. 2 is a wiring diagram showing the connection between the load cell 14 and the load amplifier 41.

  The load cell 14 has a so-called strain gauge type transducer that electrically converts test force by a bridge circuit 24 composed of four strain gauges. The load cell 14 is provided with a load cell side connector 25 including load cell side connection terminals A, B, C, and D and a shell.

  The load amplifier 41 disposed in the control unit 23 is an AC amplifier, and outputs an excitation signal to be supplied to the bridge circuit 24 of the load cell 14 and receives a distortion signal from the bridge circuit 24. The load amplifier 41 is provided with a load amplifier side connector 45 including load amplifier side connection terminals A, B, C, and D and a shell.

  The cable CL1 connecting the load cell 14 and the load amplifier 41 is composed of two shield lines S1 and S2. At both ends of the cable CL1, cable-side connectors 51 including cable-side connection terminals A, B, C, and D and a shell are disposed.

  The shield line S1 is composed of two core wires connecting the connection terminals A and B of the load cell side connector 25 and the load amplifier side connector 45, respectively, and the excitation signal given from the load amplifier 41 to the bridge circuit 24 of the load cell 14 Is for transmitting. The shield line S2 is composed of two core wires that connect between the connection terminals C and the connection terminals D of the load cell side connector 25 and the load amplifier side connector 45, and transmits the strain signal output from the load cell 14 to the load amplifier 41. Is to do. The shield portions of the shielded wires S1 and S2 are electrically connected to the case of the cable-side connector 51. When the cable CL1 is coupled to the load amplifier 41 via the connector, the shield portions S1 and S2 are electrically connected through the case of the load amplifier-side connector 45. Will be connected to ground. In this embodiment, twisted pair shielded wires in which two internal core wires are twisted together are used as the shielded wires S1 and S2. As a result, the influence of noise propagated from the external space can be reduced. Further, the two shielded wires S1 and S2 may be further covered with a common outer cover so that the whole appears as a single cable.

  FIG. 3 is a graph showing the influence on the test force measurement result due to the difference in cable. The vertical axis represents the test force value, and the horizontal axis represents time. Also, in the graph, a broken line (Single Cable) indicates a variation in the detected test force value when one conventional four-core shielded wire is used as the cable CL, and a solid line (Double Cable) indicates two of the present invention. The fluctuation of the detected test force value when the shielded wires S1 and S2 are used as the cable CL1 is shown.

  Here, the core wires of the shield wires connecting the corresponding alphabetic terminals of the load cell side connector 25 and the load amplifier side connector 45 are A, B, C and D, respectively. In the present invention, the A and B lines that transmit the excitation signal and the C and D lines that transmit the distortion signal are separate shield lines S1 and S2. Compared with the case where the conventional cable CL shown in FIG. 4 is used by inserting a shield with low impedance between the excitation signal line and the strain signal line, AC, AD, BC, B The influence of the change in the stray capacitance between −D can be reduced. For this reason, as shown in FIG. 3, the fluctuation of the test force value caused by the fluctuation of the stray capacitance that occurs when the user's hand touches the cable CL (broken line) is caused by the two shield wires S1 of the present invention. , S2 is used as the cable CL1 (solid line), which is significantly reduced compared to the prior art.

  In the above-described embodiment, the cable CL1 for connecting the load cell 14 and the load amplifier 41 has been described. However, the same cable CL1 as that for the cable CL2 for connecting the displacement meter 18 and the displacement meter amplifier is used. can do. When the displacement meter 18 has a strain gauge type transducer, it is possible to reduce the influence of the stray capacitance of the cable and to improve the displacement detection accuracy by adopting the same cable CL1 as the cable CL2. Become.

DESCRIPTION OF SYMBOLS 10 Test piece 11 Screw head 13 Cross head 14 Load cell 16 Table 18 Displacement meter 21 Upper grip 22 Lower grip 23 Control part 24 Bridge circuit 25 Load cell side connector 26 Display part 30 Load mechanism 41 Load amplifier 42 Load amplifier side connector 51 Cable Side connector S1 1st shielded wire S2 2nd shielded wire CL1 cable CL2 cable

Claims (4)

In a material testing machine that includes a physical quantity detection means having a strain gauge converter, an AC amplifier, a cable connecting the physical quantity detection means and the AC amplifier, and executes a material test that gives a test force to a test piece.
The cable is
A first shield line for transmitting an excitation signal input from the AC amplifier to the physical quantity detection means; a second shield line for transmitting a distortion signal output from the physical quantity detection means to the AC amplifier; A material testing machine comprising: The material testing machine according to claim 1,
The material testing machine, wherein the first shield wire and the second shield wire are twisted pair shield wires. In the material testing machine according to claim 1 or 2,
The physical quantity detection means is a material testing machine which is a load cell that detects a test force applied to the test piece. In the material testing machine according to claim 1 or 2,
The physical quantity detection means is a material testing machine which is a displacement meter that detects the amount of displacement given to the test piece.

Images