The error was statistically assessed:N = number of tests X = difference in reading Y = Length of test piece post test 

 

The Design and Build of an External Extensometer for DIT’s Material Testing Machine

Developing the ProgramDeveloping the Solution

Morven Gannon, 3rd Year, Automation Engineering - DT003/A

Abstract & Objectives

Read displacement at an extremely high resolution during a tensile or compression test, data logs the results and acts as a mobile microscope

Present transmission of the displacement information

Objective 1 Define the systems present deficiencies, the operation and procedure of a tensile test using D.I.T’s Lloyd LR30K, and the test samples used.

Objective 2 Compare the contact and non-contact extensometer systems on the market, and assess an ideal solution

Objective 3 Upon determining an alternative, investigate its properties and applications.

Objective 4 Design and build a fully functional non-contact extensometer for D.I.T’s tensile test machine at a reasonable price, using available software.

Objective 5 Develop an interface that is comprehensive to use, with a calibration capability and user defined functionality.

Objective 6 Evaluate the performance of the system and refine the output to gain comparable and if possible improved readings than the present system.

Objective 7 Enable the user to observe the difference between the new systems displacement readings and Nexygen output.

Test Piec

eHolding

Apparatus Crossbar

Twin Drive

Screws

Servo

Motor

Drive

BeltEncoder Disc

Nexygen Software

Defining the ProblemThe project benefitted from having regular access to the D.I.T Materials Lab. This enabled a continuous assessment of the needs and wants of the customer. An initial ‘Voice of Customer’ questionnaire helped define the problem and continual input helped to define the solution.

The internal optical decoder works on the standard principle of a digital rotary encoder. It is mounted in its own unit and rotated by a belt drive attached to the main shaft of the large servo motor that drives the twin screws that lower or heighten the cross bar. 

The Lloyds LR30K Materials Testing Machine located in DIT Bolton Street materials laboratory has been proven to give inaccurate extensometer readings. This generates inaccurate strain data that has an adverse effect on materials tests results. The alternative extensometers available from the vendor or on the market are beyond the materials labs budget. The principle objective of this project and the report is to resolve this problem by analysing the problem, investigating alternative extensometer systems, developing an alternative and then building, programming and installing it. This solution would take the form of a vision system.The resulting ‘MTVS’ system is capable of reading a displacement as small as 1.8 micrometres. It has an inbuilt calibration function, a comprehensive UI and a comparison graph that represents both the Nexygen and the MTVS displacement readings. As an extra feature, it can also be used as a mobile microscopic measuring device. The system has been validated and the primary user is satisfied with the end product. In order to achieve this end, a list of objectives was set:

The imaging device used is a TE70 Microscopic USB camera. It is the best available way to read displacement to the required resolution.It is clamped to the cross bar of the Lloyds LR30k Materials Testing Machine gantry and translates a change in displacement of the target image. This is converted into a numerical value and written to file and drawn to graph as a stress and strain graph. There is also a mobile microscope interface function that can be used as a microscopic measuring device. The software used is NI IMAQ Vision and the core function of the system for the displacement tests is the IMAQ Optical Flow VI, set to track the movement of a single pixel on the Y axis.

The decision to mount the camera on the crossbar was vital to the entire direction of the project, and the only viable solution without purchasing purpose built cameras, housing and lenses.Initially the project was directed at observing the change in position of a mark on the test subject held in the jaws of the Lloyds LR30K. If the camera to be used is a USB Microscopic camera then in order to keep the image onscreen, the camera would have to move with it. This is not an option, so directing the camera at a static background and measuring the movement of the camera on the Y axis became the only viable solution. By mounting the camera on the crossbar of the LR30K and reading an upward or downward displacement, the user could determine with fewer degrees of separation between the test piece and the software output (see Firgure.18) a displacement in micrometres. If the degrees of separation between the test piece and software are diminished, and the extra element of no mechanical contact is introduced to the system, then it should be possible to reduce an error in reading.

The research into available solutions and the results from the customer survey and ongoing input indicated that the best way to attain more accurate readings for D.I.T’s Lloyds LR30K materials testing machine, without purchasing the expensive vendor specific plug and play unit, or another extensometer system was to design and build a PC based optical vision system extensometer to determine displacement.

Transmission of the displacement information with MTVS System

Test Piece Holding Apparatus

Crossbar

CameraClamp Camera

MTVS Softwar

e

The Physical BuildThe physical placement of the camera required a clamp to hold it in place. This clamp had to absorb the maximum amount of vibration the LR30K would commonly produce. As the field of vision would only cover 1mm², and the distance from the subject image would be 3mm maximum, a slight jolt could knock the image off target and out of focus very easily. The moment of fracture of a typical dog bone steel sample is the most commonly occurring event where vibration would be a real concern.Tests where carried out to determine the extent of this jolt, but the image retained its position, so the materials chosen for the build are inexpensive overcompensation.

With over 24 purpose built SUB VI’s the programming is extensive and detailed. The heart of the tensile and compression testing function is the IMAQ Optical Flow LKP VI. This calculates the optical movement (velocity flow) in the image using the Lucas and Kanede algorithm. The method is a widely used differential calculation to estimate that the movement is equal in the pixels close to the particle selected to track, and it assumes that the movement between frames is consistent and steady. It statistically estimates the location of the particle. The Calibration Gauge VI function is one of many sub structures in the program . This is the function that overlays the 5 reference lines that are overlaid in proportion to the 100 micrometre graticule slide markings by the user to calibrate the system.

Building the UIThe design of the UI is both functional and intuitive. The extra feature of the mobile microscope function was added and fulfils the adaptability brief of the project.To operate for a simple tensile test the user will run through a designated sequence in tandem with the Nexygen test set-up procedure.The results will be defined in the output graphs on the next tab and user designated text files.The image can be calibrated remotely provided the same model of the camera is used as the one mounted on the LR30KThe user can run the MTVS from any of the PC’s within the materials lab and potentially transfer the program to any PC with the required LabVIEW software.

The particle starts here

Change in displacement

The image moves in an

upward direction

Nexygen Load vs Displacement

Nexygen Load vs Displacement

System ValidationIn order to prove the success of the project a series of tests where undertaken to match the output of the MTVS system with the output of the Nexygen system by doing 6 simple compression test using exactly the same settings. The test selected was a ‘Compress to Limit General Purpose Test’, set to move 0.5mm at a speed of 3mm per second. This very slow and slight movement helped to prove the resolution of the MTVS system and offered a comparative reference between systems. The graph image, taken from the ‘Document and Graph’ tab shows the variability in signal of the MTVS as opposed to the steady state reading of the Nexygen suggests two possibilities:

1. The MTVS system is giving far greater accuracy and precision in its reading, indicating that the Nexygen software is acting solely on the basis that the motor is turning.

2. The nature of the statistical approximations of the IMAQ LKP Optical Flow are fluctuating the position of the target particle. 

But if the linear Nexygen output is correct, then at this resolution, why vibration is not accounted for or even represented. The similarity in the signals is a clear indication that they are reading the same result, but in two areas specifically (the first rise of load and the final signal at the end of the displacement) there is a clear discrepancy. This was repeated in 5 of the 6 tests. This clearly warrants further investigation.