Material testing machine with a control device for cable disconnection warning

There is provided a material testing machine that a control device of the material testing machine includes a detection circuit which extracts a resistance component caused by the physical quantity and a capacitive component caused by an electrostatic capacitance of the cable from a measurement signal from the detector, a memory element which stores a normal capacitive component extracted by the detection circuit, and a comparator which compares the current capacitive component extracted by the detection circuit with the normal capacitive component, and when a comparison result from the comparator indicates that a value of the current capacitive component varies beyond a predetermined allowable range with respect to the normal capacitive component, it is treated that the cable is disconnected, and a disconnection warning is provided.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority of Japan patent application serial no. 2018-103659, filed on May 30, 2018. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification.

BACKGROUND

Technical Field

The disclosure relates to a material testing machine in which a detector is connected to a control device via a cable.

Description of Related Art

A material testing machine which performs material testing to evaluate strength or properties of a material includes a detector which measures a physical quantity, such as a displacement meter for measuring displacement corresponding to elongation of a test piece or a load cell for measuring a test force given to the test piece. The displacement meter and the load cell are connected to a control device of the material testing machine, and a measuring circuit for the displacement meter and a measuring circuit for the load cell are disposed in the control device. Additionally, the control device displays data obtained from these detectors on a display device (refer to Patent Document 1).

The control device and the detectors are connected by cables. Patent Document 2 proposes an abnormality detection device including a disconnection detection circuit for detecting an abnormality such as disconnection or a short circuit of a cable.

PATENT DOCUMENTS

The disconnection detection circuit in the abnormality detection device of Patent Document 2 is present separately from a strain measurement circuit, and the strain measurement circuit is input a normal measurement signal of a detector configured with a strain gauge bridge circuit. Therefore, when it is required to detect the disconnection of the cable, it is necessary to switch an input from the strain gauge bridge circuit to the strain measurement circuit to an input from the strain gauge bridge circuit to the disconnection detection circuit.

In such a switching method in which a signal from the strain gauge bridge circuit is input to either one of the circuits as described above, the disconnection detection circuit and the measurement circuit cannot be operated at the same time. Thus, the disconnection of the cable cannot be detected during the measurement, it may take time for the user to notice the disconnection of the cable, and the test piece or test time may go to waste.

SUMMARY

There is provided a material testing machine including a load mechanism configured to apply a test force to a test piece, a detector configured to measure a physical quantity when the test force is given to the test piece by the load mechanism, a control device, a cable configured to connect the detector with the control device, and a display part configured to display a measurement result of the detector, wherein the control device includes a detection circuit which extracts a resistance component caused by the physical quantity and a capacitive component caused by an electrostatic capacitance of the cable from a measurement signal from the detector, a memory element which stores a normal capacitive component extracted by the detection circuit, and a comparator which compares the current capacitive component extracted by the detection circuit with the normal capacitive component, and when a comparison result from the comparator indicates that a value of a current capacitive component varies beyond a predetermined allowable range with respect to the normal capacitive component, it is treated that the cable is disconnected, and a disconnection warning is provided.

DESCRIPTION OF THE EMBODIMENTS

The embodiments of the disclosure provide a material testing machine capable of easily detecting disconnection of a cable at any time.

According to the embodiments of the disclosure in the material testing machine described, the control device may provide the disconnection warning by displaying a warning on the display part.

According to the embodiments of the disclosure in the material testing machine described, the detector may be a load cell which has a strain gauge bridge circuit as a physical quantity detection mechanism and measures a test force applied to the test piece, or a displacement meter which has the strain gauge bridge circuit as the physical quantity detection mechanism and measures elongation of the test piece.

According to the embodiments of the disclosure in the material testing machine described, since the comparator which extracts the resistance component caused by the physical quantity of the detector and the capacitive component caused by the electrostatic capacitance of the cable from the measurement signal from the detector by the detection circuit and compares the normal capacitive component with the current capacitive component is provided, it is possible to detect whether or not the cable is disconnected and to provide a user with the disconnection warning even during the measurement of the physical quantity by the detector. Since the configuration of the detection circuit is used as it is for the detection of the capacitive component, it is unnecessary to provide a circuit for detecting an abnormality of the cable separately from the measurement circuit like in the conventional case, and it is possible to detect the disconnection of the cable with a simple configuration.

Hereinafter, embodiments of the disclosure will be described with reference to the drawings.FIG. 1is a schematic front view of a material testing machine according to an embodiment of the disclosure.

In this material testing machine, a test piece10is placed in a test space of a material testing machine body surrounded by a crosshead13, a base15, and covers14mounted upright on the right and left sides of the base15, and a tensile test is performed as a material test. The test piece10is disposed in the test space by an upper gripper11disposed on the crosshead13side and a lower gripper12fixed to the base15grasping both ends thereof. Further, the material testing machine includes a load cell16which is a detector for detecting a test force applied to the test piece10, and a displacement meter17which detects elongation occurring in the test piece10. Each of the load cell16and the displacement meter17has a strain gauge bridge circuit as a detection mechanism which converts a physical quantity in the test piece10into an electric signal. Additionally, cables CL connect the load cell16to the control device20and the displacement meter17to the control device20.

A nut portion (not shown) which is screwed with a pair of screw rods disposed in the cover14provided upright on the right and left sides of the base15is disposed at both ends of the crosshead13. Additionally, the pair of screw rods rotate in synchronization with driving of a motor disposed in the base15, and thus the crosshead13moves in the vertical direction. When the crosshead13is raised, a tensile load (the test force) is applied to the test piece10.

The upper gripper11for gripping an upper end of the test piece10is mounted on the crosshead13. On the other hand, the lower gripper12for gripping a lower end of the test piece10is mounted on the base15. When the tensile test is performed, the crosshead13is raised in a state in which the both ends of the test piece10are gripped by the upper gripper11and the lower gripper12, and thus a tensile test force is applied to the test piece10.

The test force applied to the test piece10is detected by the load cell16disposed in the crosshead13. An amount of displacement between upper and lower reference points of the test piece10is detected by a displacement meter17which is a contact type elongation detector. Signals from the load cell16and the displacement meter17are input to the control device20via a cable CL. The control device20creates a driving control signal of the motor for raising and lowering the crosshead13, rotates a servo motor (not shown) through a servo amplifier (not shown) disposed inside the base15, and thus operates a load mechanism. Accordingly, the crosshead13moves along a load axis, and various material tests, such as the tensile test, are performed. The displacement meter17may be a contact type or a non-contact type.

The control device20is configured by a computer, a sequencer, and peripheral devices thereof, has a micro-processing unit (MPU)51or a memory for temporarily storing an operation program or data necessary for controlling the machine, and controls the entire machine. The control device20is connected to an operation unit19used for starting and stopping a test, performing an operation for raising and lowering the crosshead13, or the like, and a display part18for displaying the test force measured by the load cell16and the amount of displacement measured by the displacement meter17.

FIG. 2is a block diagram showing a main functional configuration of the control device20. In the control device20of the material testing machine, a plurality of units are provided as constituent units corresponding to functions of measuring instruments, sensors, and so on selected according to the test.FIG. 2shows a carrier wave type (AC type) strain measurement circuit including a function block which outputs a signal to the strain gauge bridge circuit constituting a physical quantity detection mechanism of the load cell16or the displacement meter17and receives signal input from the strain gauge bridge circuit.

A memory element35which stores carrier wave data for exciting the strain gauge bridge circuit, a D/A converter32which performs digital-analog conversion of carrier waves, a power amplifier31which amplifies an analog-converted waveform, a synchronization signal generation circuit36which controls a timing of transmitting the carrier wave data from the memory element35to the D/A converter32, an instrumentation amplifier33which amplifies a measurement signal input from the strain gauge bridge circuit, an A/D converter34which performs analog-digital conversion of the measurement signal, a detection circuit41which extracts a resistance component A and a capacitive component B from the measurement signal, a memory element45which stores a normal capacitive component BN, a comparator47which compares a current capacitive component B with the normal capacitive component BN, and the MPU51which performs display control of measurement results or the like on the display part18are disposed in the control device20.

The carrier wave data which is sent from the memory element35to the D/A converter32in accordance with a timing signal from the synchronization signal generation circuit36and is analog-converted is amplified by the power amplifier31, and then output from the control device20and input to the strain gauge bridge circuit of the detector via the cable CL. Carrier waves amplitude-modulated by strain are output from the strain gauge bridge circuit excited by the carrier waves and then input to the control device20via the cable CL. The signal received by the control device20is amplified by the instrumentation amplifier33, digitized by the A/D converter34and then input to the detection circuit41.

The synchronization signal which is the same as the timing at which it is input from the synchronization signal generation circuit36to the memory element35is input to the detection circuit41. The detection circuit41is configured by a digital calculation circuit and extracts the resistance component A matching a phase of the carrier waves and the capacitive component B of which a phase is different by 90 degrees from the carrier waves among the components synchronized with the carrier waves of the input measurement signal. That is, in the strain measurement circuit shown inFIG. 2, the detection circuit41is provided to extract the resistance component A caused by the physical quantity, such as a force and a displacement, and the capacitive component B caused by an electrostatic capacitance which is parasitic on the cable CL irrespective of a force and a displacement. The resistance component A is proportional to an amount of change in the force or the displacement. The embodiments of the disclosure adopt a configuration in which the disconnection of the cable CL is detected using the fact that a value of the capacitive component B caused by the electrostatic capacitance which is parasitic on the cable CL changes when the cable CL is disconnected.

Here, in the strain gauge bridge circuit, a received signal f(t) can be expressed by the following equation (1) due to the resistance component A and the capacitive component B.
f=Asin ωt+Bcos ωt(1)

In Equation (1), ω is a frequency of the carrier signal, and for simplicity, a phase difference from the carrier waves is zero. In general, the Fourier transform F(ω) of the received signal f(t) is expressed by the following Equation (2).
F(ω)=∫−∞∞f(t)e−jωtdt(2)

In the detection circuit41of the strain measurement circuit shown inFIG. 2, convolution integration shown by the following Equation (3) is performed for each cycle of the carrier waves, and thus the resistance component A and the capacitive component B are extracted.

In Equations (2) and (3), the component of which a phase coincides with sin ωt in Equation (1) is the resistance component A, and the component of which a phase coincides with cos ωt in Equation (1) is the capacitive component B. The resistance component A and the capacitive component B are extracted respectively from the following Equations (4) and (5).

The capacitive component B is removed by Equation (4), and only the resistance component A is extracted. Similarly, the resistance component A is removed by Equation (5), and only the capacitive component B is extracted.

FIG. 3is a flowchart showing a notice procedure of providing a disconnection warning.

In the strain measurement circuit shown inFIG. 2, a check of whether or not the cable CL connecting the load cell16or the displacement meter17to the control device20is disconnected is always possible as long as a power source is input to the material testing machine and the signal from the strain gauge bridge circuit of each of the detectors can be received on the control device20side. When the signal input from the strain gauge bridge circuit of each of the detectors to the control device20via the cable CL is input to the detection circuit41, the resistance component A and the capacitive component B are extracted from the above-described Equations (4) and (5) (Step S11). The resistance component A is input as a signal of the physical quantity to the MPU51and is displayed as the test force detected by the load cell16or the displacement detected by the displacement meter17on the display part18.

The capacitive component B is sent to the comparator47and is compared with the normal capacitive component BNstored in the memory element45in advance. Here, the normal capacitive component BNstored in the memory element45is a value of the capacitive component B obtained by the above-described Equation (5), for example, when it can be recognized that it is normal, such as at the time of start of the first use of the cable CL, calibration of the detector, or the like. The comparison in the comparator47is to determine an extent to which the capacitive component B extracted from the signal of the detector at the current time varies with respect to the normal capacitive component BN. Therefore, the comparator47compares the current capacitive component B extracted by the detection circuit41with the normal capacitive component BNand outputs a comparison result thereof to the MPU51. The comparison result is a result derived based on a comparison between the current capacitive component B and the normal capacitive component BN, such as the determination result of whether or not the capacitive component B varies beyond an allowable range with respect to the normal capacitive component BN(Step S12), or the like.

The MPU51which has received the comparison result provides a disconnection warning in accordance with the comparison result output from the comparator47(Step S13). For example, when the current capacitive component B varies beyond the allowable range with respect to the normal capacitive component BN, the disconnection warning indicating “the disconnection of the cable” is provided to the display part18by the MPU51which has received the result. The disconnection warning is not limited to a warning display on the display part18, and other methods, such as a warning sound, may be adopted.

As described above, in the embodiment, when the signal (the resistance component A) indicating the physical quantity is taken out from the input signal by digitizing the signal from the detector input to the control device20using the A/D converter34and inputting it to the detection circuit41, the signal (the capacitive component B) caused by the electrostatic capacitance of the cable CL can also be taken out without providing an independent disconnection detection circuit. Then, the signal caused by the electrostatic capacitance of the normal cable CL is stored in advance in the memory element45, and when a difference from the stored value becomes large, it is determined that the cable CL is disconnected, and a user is warned. Therefore, the user can always be aware of the disconnection, and it is possible to prevent unnecessary continuation of the test.

Further, since the signal caused by the electrostatic capacitance of the cable CL extracted at predetermined time intervals by the detection circuit41can also be taken out as a continuous quantity, it is possible to determine not only complete disconnection of the cable CL but also a state of being almost disconnected from a tendency of change of the capacitance component B with time. If the cable CL can be replaced or repaired before the cable CL is completely disconnected, waste of the test piece10and the test time can be prevented.

In the above-described embodiment, the detector having the strain gauge bridge circuit has been described as an example. However, the capacitance component caused by the electrostatic capacitance of the cable CL from the signal input via the cable CL can also be extracted and used to determine the presence or absence of the disconnection of the cable CL in detectors having other AC measurement circuits by adopting the detection circuit41and the comparator47similar to the embodiment. For example, the embodiments of the disclosure can be applied to detection of disconnection of a cable CL in a differential transformer type displacement meter in which a primary coil is excited with AC and displacement by an induced voltage generated in a secondary coil due to a movable iron core moving in conjunction with the elongation of the test piece10is detected.