Cooper Union Mechanical Tester

Biomomentum's MACH-1 Micromechanical Testing System is an off the shelf solution for evaluating the mechanical properties of biomaterials. However its hefty price tag and relative simplicity make it a target for homebuilt duplication by labs like Colombia's Biomedical Engineering Lab. Constructing a mechanical tester requires the marriage of a load cell to track the force applied, a linear variable differential transformer (LVDT) to track position, and a linear actuator to manipulate the sample. Two existing Colombia designs were studied for the basis of an improved design to be shared in an open source fashion. The first design is a relatively low cost (less than $1500) design that utilizes low cost sensors that need little to no signal conditioning or external power supplies. The disadvantage of this approach is the sensors lack the accuracy of a commercial mechanical tester and rely on communication ports, like a parallel port, newer computers have eliminated. The second design costs approximately twice as much but has equal accuracy (7µm resolution and .4g load sensitivity) to the MACH-1 tester thanks to more expensive and complicated sensor technology. To develop a new open source design, these high-end sensors were combined with a stiffer redesigned frame to hold the sensors and a stepper motor based linear actuator. The new tester has the flexibility to use different position and load cell sensors to reduce cost or reconfigure the device to test different materials. For example, a user could select a load cell rated for high loads to test knee cartilage. An easy to use open source program was created to set up different types of mechanical tests and collect data. The move towards more complicated sensors and a stand alone program meant more effort went into designing the tester, but a user who adopts the design will be spared the work and gain the benefits.

The structure of the tester is created from two pillars of 3030 style 80/20 extrusion, a proprietary system of T-slotted aluminum. These slots allow the entire bottom of the loading assembly to be slid downwards to increase the range of travel for the actuator or give easy access to the sample tray. The overall height of the machine is 12 inches so that one piece of 24 inch extrusion could be used to make the machine but the height can easily be increased. The motor used is an Ultramotion Digit Stepper Actuator which when configured to 1600 steps per inch of travel, has 16µm of positioning resolution. An optional tube mount is used to mount the motor to the front plate of the tester. The availability of several lead screw configurations makes it easy to customize the tester for a faster rate of travel. The miscellaneous parts used to build the machine are designed to be made as easily as possible. With some creativity the design could be constructed without the most complex tool needed, a lathe. A mill however, is not required. The bottom platen is dished to allow for a hemisphere to be rested in the cup which allows the stage to self-align to be perpendicular to the load. This, like much of the design, can be customized to the user's manufacturing capabilities.

The sensors and the motor driver are connected to the computer over USB with a TI-6211 data acquisition unit (DAQ), though any TI DAQ with a counter output and pulse generation could be used. The TI-6210 is the cheapest option ($630) with these specifications. A similar setup using a more complicated TI DAQ was used in conjunction with Labview in the more expensive Colombia design. In general the higher end TI DAQs with pulse generation have sensitivities far more precise (5µV) than the output of the sensors (in the mV range). Driving the motor is accomplished by using the counter output of the DAQ to produce a pulse that is fed into a Gecko 6203V stepper driver, which produces the high output voltages used by the motor. By using the Python programming language to write a stand-alone program for the new design, the need for an expensive software package was eliminated, though more programming knowledge is needed to customize the software. A library of Python tools for interfacing with TI DAQs called PyDAQTools was used to write the program.

In order to maximize the potential usefulness of the new tester, it was designed to have more accuracy at a higher cost. The cheaper Colombia design uses a US digital PE-500-2-N-S-D linear probe encoder which has a 1 inch stroke and 12.7µm resolution and costs only $200. It also is conveniently packed in an easy to mount box with a spring loaded probe. The sensor could be clamped off to the side and the probe was rested on a convenient spot on the actuator. To read the sensor with a computer, a quadrature to serial adapter is needed. For position sensing on the more expensive Colombia design and the new design, a Macrosensor PR-750-200 LVDT with its LVC-2401 signal conditioner was chosen for a cost of $500 and a resolution of 7µm. The limiting factor in accuracy is mainly the noise from the signal conditioner, which an upgrade to an LVC-2500 conditioner would reduce by a factor of 10. The large available area on the front plate can be used to mount a different sensor if desired. For example a Macrosensor GSA-750 is a budget priced LVDT, would be pin compatible with the PR-750, and is a spring loaded package, simplifying construction of the testing rig by eliminating the LVDT core clamp part.

Similar customization is available in the load cells. Loosening the top screw of the load cell assembly allows the load cell to be removed without wrapping the cables around the tester, a problem the cheaper Colombia design had. The cheaper Colombia model used an iLoad 10lb load cell that had a convenient USB interface but it sacrificed accuracy for a higher capacity. Additionally it was observed that the load cell got “stuck” when loaded and had a delay in returning to an unloaded state. The new tester is designed with the 50 or 250 gram load cell (Honeywell parts 060-1426-02 and 060-1435-03) used by the expensive Colombia design but the user can make changes by using a different load cell with equivalent wiring. A model GM signal conditioner and calibration unit reports a differential voltage to the DAQ and is compatible with various load cells up to 1,000lbs.

start/projects/mechanical_loaderdale/mechanicaltester/narrative.txt · Last modified: 2012/10/11 20:05 by daleshort
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