Outer loop torque control

Abstract

To provide a material testing device that uses an algorithmic approach to implement an outer loop control software algorithm for control of a plurality of motors imparting different forces on a material testing specimen.SOLUTION: In particular, a torsion motor is controlled by outer loop control software to control rotational force applied to a material testing sample.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] This application claims priority under 35 U.S.C. §119(e) to U.S. patent application Ser. No. 16 / 160,151, filed Oct. 15, 2018, the contents of which are incorporated herein by reference in their entirety for all purposes.

[0002] The present disclosure relates to an outer control loop that controls torque as an additional motion in a materials testing environment. [Background technology]

[0003] Universal testing machines (frames) typically house sophisticated electronics, including printed circuit boards (PCBs), housing connections, processors, and other components to control them. When packaging these electronics for use in such machines, several factors must be considered, including cooling, shock protection, and compliance with electromagnetic immunity and emissions (EMI) and other standards, all of which must be considered in conjunction with minimizing the footprint occupied by the machine and optimizing maintainability.

[0004] In the prior art of materials testing, it is known to apply stress using a single force, such as an axial force, and materials testing involves measuring strain in response to the stress.

[0005] In the prior art, strain pacing may be used as part of the material testing process. Strain pacing is an outer loop software algorithm that monitors current data and modifies the position velocity to achieve a user-defined tensile strain rate determined from a tensile strain sensor. The pacing function in the software allows the strain rate of the sample to be controlled to a predefined level. It is intended for use with materials that exhibit smooth, continuous deformation characteristics in the elastic region, such as metals and rigid or semi-rigid plastics. Many such materials are highly sensitive to the strain rate applied during material testing, making it desirable (and sometimes essential) to control or at least limit the strain rate. The actual maximum strain rate that can be controlled will depend on the characteristics of the sample being tested and the strain rate accuracy requirements. The less stiff the sample is and the more gradual the yield, the higher the strain rate that can be used.

[0006] Electromechanical testing machines are typically run at a constant rate of crosshead displacement (i.e., crosshead speed) by using a position control servo loop. Characteristics of test systems and specimens may result in non-constant strains in the specimen, especially in the yield region, even at a constant rate of crosshead displacement. Therefore, the strain rate must be controlled.

[0007] In strain pacing mode, the software achieves the requested speed by constantly varying the crosshead speed. The extensometer monitors the actual speed of the specimen, and the software calculates the crosshead speed required to maintain the requested speed as the test progresses. Due to the fact that most of the crosshead displacement is converted into permanent specimen deformation, in order to maintain a constant strain rate, the crosshead speed is reduced as the material yields.

[0008] Strain pacing mode is intended for monotonic (unidirectional) testing only and is not suitable for controlling transient effects such as up and down yielding, discontinuous yielding phenomena, or strain rate when using multiple motors to apply forces in multiple axes or degrees of freedom that may be orthogonal. Summary of the Invention

[0009] It is therefore an object of the present disclosure to provide improvements in materials testing, including the ability to apply multiple forces via multiple axes or degrees of freedom to a test sample, and to provide appropriate feedback to enable this functionality.

[0010] This objective, and others, is accomplished by providing multiple motors and drives in the materials testing machine, allowing for the application of multiple forces in multiple axes or degrees of freedom that may be orthogonal (including but not limited to axial and rotational axes in a cylindrical coordinate system). An outer loop is used to control the torque to apply a torsional or rotational force to the materials test sample.

[0011] Control of a rotary motor or drive by an outer loop typically requires no additional hardware: axis control is achieved entirely through software, using torque cells as feedback devices, providing machine expansion capabilities to add controlled axes to hardware originally designed for single axis machines.

[0012] Further objects and advantages of the present disclosure will become apparent from the following description and the accompanying drawings. [Brief description of the drawings]

[0013] [Figure 1] FIG. 1 is a perspective view of one embodiment of a materials testing apparatus of the present disclosure. [Diagram 2] 2 is a schematic diagram of an external loop used to control the torsional or other force applied to a material test specimen by the apparatus of FIG. 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Referring now to the drawings in detail, it can be seen that Figure 1 is a perspective view of a materials testing apparatus or mechanical test frame 100. The test frame 100 has a base 102 to which a biaxial load cell 104 is attached. A lower grip 106 is attached to the biaxial load cell 104. The biaxial load cell 104 measures the load (typically an axial load and a rotational torque) applied to a materials test sample (not shown) engaged between the lower grip 106 and an upper grip 108, and communicates information or raw data regarding the axial load and the rotational torque via a cable 109 to a control unit 122 which includes a manual input unit 124 and a screen 126.

[0015] The rail 110 extends upward from the base 102 and supports an upper assembly 112 with a crosshead 113 that engages the upper grip 108. The upper assembly 112 includes a drive assembly 114 that drives the crosshead 113 to apply a vertical force (i.e., linear or axial force, or stress) to a material test sample (engaged between the lower grip 106 and the upper grip 108). In addition, changes in the axial translation of the crosshead 113 along the rail 110 while applying an axial force to the material test sample can be interpreted as a change in distance or length (Δy) for strain calculations. Information or raw data regarding the vertical or axial position of the crosshead 113 (and thus the vertical position of the upper grip 108) is provided by an encoder mounted on a motor located within the base 102 and in communication with the control unit 112 (which includes a manual input unit 124 and a screen 126). Information about the rotational position of upper grip 108 is provided by an encoder attached to a torsion motor housed within upper assembly 112 and transmitted via cable 120 to control unit 122 .

[0016] Similarly, the drive assembly 114 includes a torsion motor or drive that applies a torsional force to the material test sample via the upper grip 108. The torsion motor is controlled by a torsion motor control 130 that is mounted on the rails 110 of the test frame 100.

[0017] The torsion force application mechanism, including the torsion motor control 118, is an electromechanical assembly that, when added to the materials testing machine or test frame 100, provides the torsional capability to the axially moving crosshead 113. The torsion force application (TAO) mechanism can be added to a single column frame or a dual column frame. A user of the materials testing machine can rotate the test specimen before, during, or after application of an axial load. Rotation is monitored and controlled using an incremental encoder attached to the torsion motor in the upper assembly 112. Torque and axial load values ​​are recorded and controlled using a dual axis load cell 104 that monitors the force applied to the test specimen.

[0018] The torsion force application mechanism and the mechanical test frame 100 are controlled by software. A user working through a software interface, such as a manual input unit 124, enters test parameters and thereby defines how the materials testing machine 100 will perform a test. All movements of the materials testing machine 100 are typically accomplished with closed loop control, which means that the hardware and firmware of the materials testing machine 100 is dedicated to controlling the axial movements of the crosshead 113.

[0019] The mechanism can rotate the material test sample being tested (engaged between the lower grip 106 and upper grip 108) until a desired torque value is reached. At this point, the mechanism can enter an outer loop torque hold mode, in which the rotational speed of the torsion motor, controlled by an external controller, is updated at regular intervals to maintain the torque rate of the material test sample being tested at zero (or typically a preselected positive or negative torque rate). A gain factor multiplier is incorporated into the algorithm that can hold a constant and desired torque for materials with different stiffnesses.

[0020] The algorithmic approach is illustrated in more detail by flow chart 300 in Figure 2. A current torque rate is calculated in step 302 while a current rotational speed is calculated in step 304. The outputs from steps 302, 304 are used by step 306 in the algorithm to maintain the torque rate at zero (or a typically pre-selected positive or negative torque rate). The output from step 306 is used to calculate a new rotational speed in step 308. The output from step 308 is used to apply the new rotational speed to the spinning motor in step 310. The output from step 310 is received by steps 302, 304 and the closed loop control system is completed.

[0021] This technique can be used to apply forces to a material test sample using two or more motors.

[0022] Thus, the several above-mentioned objects and advantages are most effectively attained. Although the preferred embodiment of the present invention has been particularly shown and described herein, it is to be understood that the invention is in no way limited thereto.

Claims

1. 1. A material testing apparatus comprising: a first grip and a second grip for holding a material test sample therebetween; a first actuator configured to apply a first force via at least one of the first grip and the second grip; a second actuator configured to apply a second force via at least one of the first grip and the second grip; a feedback system for controlling the second drive; A material testing apparatus comprising:

2. 2. The material testing device of claim 1, wherein the first grip is a lower grip and the second grip is an upper grip located above the first grip.

3. The materials testing device of claim 2 , wherein the first force is orthogonal to the second force.

4. The materials testing apparatus of claim 3 , wherein the first actuator applies an axial force to the materials test sample.

5. The materials testing apparatus of claim 4 , wherein the second drive applies a rotational force to the materials test sample.

6. 6. The materials testing device of claim 5, wherein the lower grip is attached to a load cell for measuring axial and rotational stresses on the sample.

7. 7. The material testing apparatus of claim 6, further comprising a rail, the second grip being driven by the first drive, and the displacement of the second grip being measured to calculate an axial strain in the material test sample.

8. 8. The material testing apparatus of claim 7, wherein the second drive comprises an incremental encoder for measuring rotational strain of the material test sample.

9. 9. The materials testing apparatus of claim 8, wherein the feedback system uses a current torque rate and a current rotational speed to calculate the rotational force required to maintain a torque rate of the material test sample at zero.

10. 10. The material testing apparatus of claim 9, wherein a new rotational speed is calculated using the calculated rotational force necessary to maintain a torque rate of the material test sample at zero, and the calculated new rotational speed is transmitted to the second drive.

11. 1. A method of materials testing comprising the steps of: providing a first grip and a second grip for holding a material test sample therebetween; providing a first actuator configured to apply a first force via at least one of the first grip and the second grip; providing a second actuator configured to apply a second force via at least one of the first grip and the second grip; providing a feedback system for controlling the second drive; A method comprising:

12. The method of claim 11 , wherein the first grip is a lower grip and the second grip is an upper grip located above the first grip.

13. The method of claim 12 , wherein the first force is orthogonal to the second force.

14. The method of claim 13 , wherein the first actuator applies an axial force to the material test sample.

15. The method of claim 14 , wherein the second actuator applies a rotational force to the material test sample.

16. 16. The method of claim 15, further comprising providing a load cell attached to the lower grip for measuring axial and rotational stresses on the sample.

17. 17. The method of claim 16, further comprising providing a rail on which the second grip is driven by the first drive, and the displacement of the second grip is measured to calculate an axial strain of the material test sample.

18. The method of claim 17 , wherein the second drive comprises an incremental encoder that measures rotational strain on the material test sample.

19. 20. The method of claim 18, wherein the feedback system performs the step of calculating a rotational force required to maintain a torque rate of the material test sample at zero using a current torque rate and a current rotational speed.

20. 20. The method of claim 19, wherein the feedback system performs the steps of calculating a new rotational speed using the calculated rotational force necessary to maintain a torque rate of the material test sample at zero, and transmitting the calculated new rotational speed to the second drive.

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