Developing an Automated Instrument for Elastic Material Characterization
Basic Information
Students: Aleksi Lankinen, Antti Laitinen, Lassi Pikkarainen, Ville Synkkänen
Project manager: Ville Synkkänen
Instructor: Shahriar Haeri
Starting date: 27.1.2023
Completion date: 5.6.2023
Introduction
Material characterization is the process of determining the characteristics of a material. Material characterization is necessary for many kinds of different development work and choosing the right material for a product or a component can make a significant difference.
In this project, we were specifically interested in determining the Young’s modulus of material samples. Young’s modulus, also known as elastic modulus, is a characteristic of materials that defines the relationship between stress (force per unit area) and strain (proportional deformation) in a material. In other words, it is a measurement of stiffness. By measuring the force applied to an elastic material along with its displacement, the elastic modulus of the material can be calculated using Hertzian theory.
The project involved developing a measurement device used to perform material characterization. The device has been used on the ELEC-C1310 laboratory course for one of the laboratory exercises. The intention was to update the hardware of the measurement setup and improve the scripts to make the system more user-friendly. The device will be used on the laboratory course in the future.
Objectives
The main objective of the project was to update the existing force measurement setup. It was be divided into the following areas: prototyping the new setup, setting up all hardware interfaces and characterizing the force sensor, assembling the final setup, updating the scripts used for material characterization and data post processing, and documenting the project. The expected users of the measurement setup are students taking the automation laboratory course. The goal was to improve the user experience over the older setup. An additional objective of creating a material characterization simulator was also added partway through the project. The students could use the simulator to learn about the hardware and the software without actually using the measurement setup itself.
A large part of the project was creating extensive documentation, such as a sensor characterization report, material characterization report and final project report. These documents can aid in future development and facilitate maintenance of both the software and the hardware. As a part of the project course, a business case document was also created.
Project planning
The project was divided into five high-level phases in the project plan:
- Introduction to the project and creation of project plan.
- Prototyping: Testing the equipment required for the new setup, creating a test setup and performing characterization of the new force sensor.
- Creating a business pitch for the project and the required documents. Presenting the created pitch.
- Building the final setup, updating the measurement scripts and testing the functionality. Validating results and postprocessing.
- Creating documentation on the characterization of the sensor and the material characterization results. Creating material for presenting the project. Compiling documentation into the final report.
Test setup
Setup description
In the first phase of the project, a test setup was built to verify the functionality of the new parts and to get familiar with interfacing with the components. The test setup can be seen in the picture below. The plan was to replace the PC (Windows 11), the motor controller (PI C-863), the micro-translation stage (PI M-110), the force sensor (TEI FSB101), and the amplifier (EFE AMP320) with new parts. In addition, the force sensor needed a new measuring tip. The data acquisition (DAQ) system did not need a replacement. A spare DAQ was used for the test setup.
The test setup forms a measurement feedback loop where the PC controls the micro-translation stage via the motor controller and reads the force measurement from the force sensor through the amplifier and DAQ. A visualization of this can be seen in the figure below. The goal is to execute all actions on the PC with MATLAB scripts. The connections through the force sensor to the DAQ were made with electrical cabling. Other connections between devices used appropriate cables. The amplifier and force sensor were powered with an external power supply unit. The electrical connections were made with electrical clips. The power supply was set to 10 VDC when testing the functionality of the parts.
To mount the force sensor, a holder was designed and 3D printed with PLA. The sensor measurements were taken from the vendor provided documents. With a half of a millimeter excess tolerance, the sensor holder kept the force sensor in place. A new revision of the sample platform was also designed and 3D printed with high infill PLA. This design solved the problem of round edges along the bottom of the platform.
Findings
The force sensor was tested with a piece of paper to ensure forces that would not exceed the ± 50-gram range. The measurement data was read and plotted in MATLAB. By pressing the piece of paper on the sensor, a change of voltage was observed in the measurement. The amplifier functioned as expected by amplifying the force sensor signal to be readable for the DAQ.
Even though the micro-translation stage functioned well with the test software, there were difficulties in accessing the motor controller C# libraries in MATLAB. After seeking support from the manufacturer, we eventually managed to get the stage used in the old setup working on the new PC. We did not manage to get the new stage working. As a result, we decided that the micro-translation stage and the motor controller would not be updated on the final setup. Only the force sensor, the measurement tip, and the amplifier would be changed from the final setup, along with the new PC.
Sensor characterization
After validating the functionality of the new force sensor and amplifier, the system had to be characterized to find the gain and offset of the new sensor. This was necessary to convert the output voltage from the amplifier into a force reading.
Characterization setup
For the tip of the force sensor, a ruby ball tip was used. It is a Stylus M3 ruby ball Ø3.0mm carbide stem Ø1.5mm, base stainless steel Ø4.0mm L 20.0mm, ML 15.6mm by Mitutoyo. The ruby tip is much harder compared to the sample materials used in the measurements. Additionally, the ruby measurement tip makes the calibration process easier by extending the reach of the force sensor.
For the sensor characterization setup, an inverted mount for the micro-translation stage was used. An L-bracket and a 3D-printed sensor holder were used to affix the stage to the edge of a test bench. The stage was mounted to allow the sensor to be at an appropriate height in relation to the test bench. To measure the force applied to the sensor, an A&D EK-400H laboratory scale with a resolution of 0.01 g was used. The force sensor characterization setup can be seen in in the picture below.
Characterization procedure
Before the characterization process, the accuracy and repeatability of the scale was tested using set weights. The effect of the position of the force on different parts of the scale tray was also investigated and it was apparent that the position of the force on the tray had no measurable effect. Similarly, the function of the installed sensor was quickly tested using a piece of paper to apply minimal force and plotting voltages during a few seconds of operation. After turning on the scale, we waited for a while to make sure the weight reading settles to obtain accurate and repeatable results.
A sensor characterization MATLAB script was used to collect the measurements and input weight information from the scale at set intervals. This was done to build a dataset of voltages and weights during the process. This data was used to calculate the gain and offset of the sensor. The script includes a clause that ensures that the maximum force value is not reached to protect the sensor from damage. During post processing a first order polynomial is fitted on the recorded data to get the gain of the force sensor.
The characterization script works by first positioning the sensor at a point where it is close to making contact with the scale and taking a static reading from the sensor for one second to establish the sensor output without any contact. After the reading without contact has been recorded, the sensor is moved down towards the scale by steps of predefined size. Between steps, the sensor value is read, and when the sensor output increases by more than 0,01 V compared to the static reading, indicating that the sensor is in contact with the scale, the movement is stopped.
After the exact contact location has been found, a measurement loop is started. Similarly to finding the contact location, the stage moves down in predefined steps. After each step the script takes the weight value read from the scale as user input. After each step it takes a few seconds for the value shown by the scale to settle to a constant value. Therefore, it is recommended to wait a few seconds to see if the weight fluctuates before inputting the value. The weight and the corresponding voltage are saved. This loop repeats until the maximum allowed weight, voltage or a predefined end position is reached. Finally, the stage is moved up to the maximum height and the measurement data from the sensor and the weight values input by the user are saved in a file.
The effect of the sensor excitation voltage on the measurement results was tested by comparing 10 V and 18 V. With a 10 V power supply, the gain is higher than with an 18 V power supply. In practice, this means that the 50-gram threshold is reached at a lower voltage and a part of the voltage range is left unused. A suitable amplifier gain was chosen by consulting the user manual and testing different configurations. With a 1.0 mV/V gain, the output voltage would cap before reaching 50 grams of weight. With a 2.0 mV/V gain, 50 grams was reached at a lower voltage, as with the 10 V power supply. 1.5 mV/V was determined to be a suitable gain. The output voltage does not cap too early and the whole voltage range is used. The measurement data plots for 10 V and 18 V power supply voltages and 1.0 mV/V and 1.5 mV/V amplifier gains can be seen below.
Final setup and material characterization
After all parts of the system had been tested and characterized, the final setup was built. The new force sensor, the measurement tip, and the amplifier were installed. Finally, the updated system was tested by performing material characterization measurements.
Modifications to the final setup
The test setup force sensor holder was not designed to fit the original setup. Therefore, a new revision had to be made where the sensor is mounted sideways to a wall. A new part was designed, and 3D printed with high infill PLA. The sensor was mounted with the measurement tip attached. The original MATLAB scripts used for the material characterization needed to be updated. The scripts were not efficient and clean, the plotting was simple and the outputs were uninformative. Additionally, there was no automatic Young’s modulus calculation.
Several changes were made to the material characterization scripts, such as making the scripts more automated by requesting information from the user when needed, adding feedback control in measurement loops, and making more efficient and robust measurements. Especially, a new function was created that finds the position where the sensor contacts with the sample and the maximum displacement the force sensor tolerates. The found limits are saved so that they don’t need to be searched for every experiment with the same sample. Additionally, the scripts and the output plots and text were formatted better. Young’s modulus is now calculated in a new script. It uses data gathered from the continuous measurement experiment and solves the Young’s modulus of the sample material.
Material characterization
For each of the material samples (EcoFlex, PDMS and EVA), three different measurements were performed. The measurement plots for EVA can be seen below. In the first measurement, the sample was moved up in steps. After each step, four measurements were taken with an interval of one second. This was done to observe the stress relaxation phenomenon. This measurement is referred to as step relaxation in the figures. In the second measurement, the sample was moved up at a constant velocity. Measurements were taken between certain intervals to obtain a desired number of measurements. This measurement is referred to as continuous measurement in the figures. In the third measurement, the sample was moved up in steps and the measurement was performed continuously. Stress relaxation could be observed here as well. This measurement is referred to as continuous read in the figures.
The Young’s modulus quantifies the tensile and compressive stiffness of the sample material. It can be calculated by using the coefficient of the stress-strain curve proportionality. The ruby tip of the force sensor is much harder compared to the sample materials used in the measurements. In other words, the elastic modulus of ruby is much larger than the sample’s . Therefore, the effective elastic modulus , used in the Young’s modulus calculation, is simplified into a linear equation:
Where is the Poisson’s ratio of the material, assumed to be 0.5. The relationship between the force exerted on the sample and its displacement can be modeled with the following equation:
where is the radius of the ruby ball and is the displacement of the ruby ball tip from the undisturbed sample surface. The slope can be obtained by fitting a first order polynomial to the data obtained from the continuous measurement. The plots obtained from this fitting can be seen in Figures 5-7 in the bottom right pictures. Once the slope is obtained, the effective elastic modulus can be calculated with the following equation:
Once the effective elastic modulus is obtained, the elastic modulus for the material sample can be calculated with equation 3.
Simulator
Using data obtained from the material characterization, a MATLAB-script for simulating the behavior of the device was created. The script simulates the second measurement by interpolating the position- and force values obtained from material characterization using MATLAB’s own interpolation function. This simulator allows the users to understand how the system works before actually being in physical contact with the test setup.
Additional documentation
To document the new setup more accurately and ensure better maintainability, a document concerning the sensor characterization was made. A separate document detailing the material characterization results was also made.
Conclusion
The main objectives of the project were fulfilled in a satisfactory manner. The updated measurement setup is functional and easier to use than the previous one. The results of the sensor and material characterization were documented in sufficient detail and should be useful for future users of the setup. The project code is modular to a certain degree, which should make modifying it straightforward. In addition to the requirements laid out in the project plan, a material characterization simulator was developed. It can be used to simulate how the real setup works without having access to it.
