During operation of the test machine, the function generator produces an input voltage signal that is fed into a voltage-to-current amplifier.  The current from the amplifier drives the voice coil actuator.  Thus, the net result is the function generator directly controls the force (not displacement) generated by the test machine.  A multimeter monitors the current into the actuator.

The outputs from the machine are the displacement of the actuator (and sample if that is loaded into the machine) and the forces associated with this displacement.  These outputs are measured by an LVDT and a load cell/strain gauge and fed into the computer via a NI DAQ card and a program written in LabView.

 
 
 

General Setup	Turn on power strip, computer, etc.  Make sure that the voltage-to-current amplifier is not connected.  Otherwise, the actuator may move suddenly.
Function Generator .	The signal generator is the input to the system.  It controls the force output of the actuator. Set amplitude of the output voltage to zero (turn knob all the way to the left) Set DC offset to pen-mark near knob.  This produces approximately zero actuator offset on the test machine. Make sure that the range of the machine is 0-2 V.  If it is higher, damage to the machine may result. Turn function generator on.
Multi-Meter	The multi-meter is used to monitor the current out of the voltage-to-current amplifier and to ensure that the bias (offset) current out of the amplifier is zero. Set range of multi-meter to 10 amps. Adjust the DC offset on the function generator to bring the multi-meter reading to approximately zero.
Strain Indicator	The strain indicator is used to read the output of the load cell/strain gauge.  It is hooked up as a full-bridge. Push "amp zero."  Ensure that it is zero and turn the silver, flat knob to adjust if the reading is nonzero. Press "run" (green button).  The reading on the screen is the pre-loaded offset. Turn the "balance" knob to adjust the value and bring it to zero.
Software	The software is written in Labview.  It takes readings from the strain gauge/load cell and the LVDT to monitor the displacement and forces associated with sample deformation.  Software for testing the dog bones are in c:\users\testing\dog bone. There are 2 versions - one with software filter and one without software filter. Use the non-filter version for calibration The calibration setting is used to calculate the linear fit of the strain gauge and LVDT readings (no data is recorded).  The testing setting is used to record data.
Calibration	It is assumed that hte displacement and the force readings are linear.  This means that the equations used to describe them are: displacement = a1(voltage_displacement) + b1 force = a2 (voltage_force) + b2 a1 and b1 can be calculated from the voltage readings taken at two positions a known distance apart.  We are interested in the change in displacement, not absolute displacement. b2 = 0.  It is known that when force = 0, voltage_force = 0. a2 = force/voltage_force. To find a2, we can place a known mass/weight on the test machines and measure the voltage_force outputs.

 

Last updated February 27, 2000 by chengw@stanford.edu.
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