by Adrian Glasser
This blog will describe the development of a computer controlled squeezing instrument to be used in laboratory research for measuring mechanical stiffness of small samples.
The author of this blog has no commercial relationship with Futek, the company from which the load cell used in this application was bought.
A squeezer applies a force through a sample onto a load cell. The applied force can be based on a known magnitude of movement of a sturdy linear actuator driving down onto the sample with a fixed magnitude of linear displacement or it could be through recording the force from the load cell continuously while the linear actuator moves down onto the sample until a pre-defined load is measured.
Testing of this nature has been performed with a prior version of such a squeezer. Various different samples can be tested in this way. Below is a video and data collected from testing on a spherical hydrogel sample. The video shows the linear actuator moving down onto the sample in 6 sucessive steps at 10 second intervals and then up again in 6 sucessive steps. The video is created by capturing an individual image after each step-displacement has been applied by the linear actuator, so the video plays over a much shorter time than the time over which the actual test was performed. The force data is collected during the test by continuously recording the load output from the load cell.
To build the squeezer, first, it is necessary to identify an appropriate load cell that can be used to measure force and from which data can be read into a computer via custom-developed software. The software used for this process will be MATLAB as that is the programming language that I am most familiar with, especially for allowing a computer to communicate with hardware. In this case, the hardware will ultimately consist of several different things, including a linear actuator, one or two video cameras and the load cell.
The load cell that will be used is the LSB200 from Futek. This is a load cell rated to 10g which should prove suitable for this application. The LSB200 connects to a computer via the Futek USB220 which, as the name implies, is a USB output device which sends signals from the load cell to the computer. The USB220 can output data at about 1000 samples per second. Simple MATLAB code is able to read from the load cell at about 500 samples per second. However, a sampling rate of only about 30 to 100 samples per second will be required for this application.
The load cell has two M3 threaded holes, one on top and the other on the bottom. A custom designed, 3D printed stand and platform have been added to the top and bottom of the load cell so the load cell stands upright and so test samples can be placed on the top platform to change the force on the load cell so the changes in the force values being read from the load cell can be seen in the software in real-time. This 3D printed stand and platform are simply for ease-of-use while developing the software and they will be replaced later with other attachments to the load cell.
There is some minimal example MATLAB code on the Futek web site of how to read from the load cell and there is extensive documentation on the Futek web site on the software functions that are available to interact with the load cell. However, to accomplish this task will require development of MATLAB software that is capable of performing all the functions required of the squeezer. There is remarkably little in the way of publicly available information or example MATLAB code of reading from Futek load cells. Futek refer user of their products to the own SENSIT software to read from their sensors. I will not be able to use the SENSIT software for this application as the squeezer will need to interact with other hardware, which the SENSIT software cannot do.
The custom developed software will be a Graphic User Interface (GUI) based application written in MATLAB that can be installed and run on a Windows based computer.
Here below is a short video of the initial GUI based software application, showing some elementary capabilities for interacting with the load cell. The video is best viewed in full-screen so the details can be seen. The connection to the load cell can be opened, data can start to be read from the load cell in real-time, a tare function has been implemented to tare the recorded force (i.e., set it to zero), reading from the load cell can be stopped and the load cell connection can be closed. The load cell is extremely sensitive and, in this recording, all that is being recorded from the load cell are finger taps on the table on which the load cell is resting. At the start of the video, the load cell is recording about 2 grams and this is the weight of the 3D printed platform that is mounted above the load cell.
The MATLAB code for reading from the load cell has now been partially integrated with a larger application that will ultimately run and control the squeezer. This software application does a lot of things, all of which will not be described here. The primary point of this development stage is to integrate the real-time load recording from the load cell with a live-streamed video feed from a USB video camera. Both of these capabilities have been integrated in the software and are demonstrated in the video below.
View the video in full-scree mode to see the details. First the video camera is connected and then a live video feed is started with the Grab button. Once the live video is started, the load cell is connected, the real-time load reading is started, the tare button is pressed to set the load cell reading to zero and then a tablet is placed on the platform above the load cell and is then removed a few seconds later so the real-time response from the load cell can be observed on the graph. Note from the real-time recording from the load cell that the load cell is extremely sensative and that load recorded from placing the tablet on the platform is far greater than the load created from just the static weight of the tablet.
There is still a lot more that needs to be done to the software to get it fully functional, including integrating reading from the load cell and the live video feed with movement of the linear actuator that will actually do the sample squeezing.
This is how far we have progressed so far. This site will be updated as progress is made. Stop by again some time to see how things are proceed.