Electronic Test Instrumentation with Lab VIEW

ETE 420

Course Description:

Fundamentals of electronic test instrumentation and computer data acquisition systems, theory and function of electronic measurements, op-amp applications and signal conditioning, sensors applications such as strain gage and temperature. Computerized data acquisition and programmable instrument control (IEEE - 488) utilizing LabVIEW graphical programming software.

Strain Gauges        2

Thermistor, RTD, Thermocouples        2

ADC Converters        3

Resistor Measurements        3

Wheatstone Bridge        4

Operational Amplifier and single supply        5

Strain Gauges

        In the demonstration we learned about strain gauges and witnessed tensile test on coupon. Bill placed the coupon of the specific material being tested into the tensile tester, which pulls the piece from each side. If it is being pull at low forces another device will be placed on the coupon to measure extension. If test is checking for breaking period the additional device will be removed at a certain point so when the coupon does break it will not damage the device. It plots the data stress vs extension. This is important to make sure that the material will be compliant to the needs of the product. We also took a look at strain gauges and different orientations and number of gauges can produce similar results with less error and more accuracy.

Thermistor, RTD, Thermocouples

First we took a look at the Semiconductor sensors LM34 and LM35; where the LM35 measures 10MV per Celsius and the LM34 is 10mV per Fahrenheit, AD590 sensor measured 1mA per kelvin. Station 6 proved to us that at reference temperature of 0 degrees Celsius is 100 ohms as the datasheet. The Lakeshore diode temperature unit is good for cryogenic work. A Peltier cooler is an inverse thermocouple, whereas current flows through the device it produces heat. A thermocouple turns delta temperature into a voltage. One can also measure temperature using non-contact methods such as using IR. Calibrators are used to generate temperature type signals for testing and prototyping.

ADC Converters

        We  investigated the characteristics of an Analog-to-digital converter. More specifically we had the transfer function of an 8-bit ADC. Using LabVIEW we emulated an ADC with a full scale range of 5 volts and determined the quantization(Q) value for an 8 bit system As can be seen in the image of the XY Graph taken from LabVIEW. We also used an ADC Chip connected to Leds to see an IC that did the same as are simulation. In depending on the voltage the LEDs would display a binary number representing its scaled value.

lab view lab 1-2.PNGlab view lab 1.PNG

Resistor Measurements

        We measured 200 resistors to see what our 3.5 digit meter would record their value as. And than again with a 6.5 digit meter to compare the values of the same resistors read on two different systems. On the 3.5 digit meter the values were read quite quick and within a 5% error of the said value. However when using the more accurate 4 wire method, which is to supply current with one set of wires and measure voltage with the other thus eliminating the lead resistance giving us better accurate values of the resistors. With the 6.5 digit meter we noticed it took a longer time for the value to settle on the meter. This can be caused due to the actual heating of the resistor. With this data we see a normal distribution of the resistors and how consistent the manufacturer produced each resistor within the tolerance allotted.

Wheatstone Bridge

        A Wheatstone bridge is used for determining an unknown resistance. This works but setting the bridge in either one of four configurations with each have its own pros and cons. A quarter, half, or full bridge. Where a quarter bridge all three resistors are known except the Rx, our unknown resistance we measure the voltage across the bridge. Knowing the other resistance and the voltage as well as a lookup table for the component we are trying to determine, we can find the value. With half bridge we have two of the components ideally change in resistance at the same time giving us a voltage of zero across the bridge thus the difference in voltage can be related to the resistance with a lookup table. Where a full bridge is the most accurate it is the most costly, as we can see in the figure below of each of the configurations we can remove linearity error which is a systematic error that are circuit creates.

Operational Amplifier and single supply

        We used a TLC 274L4 quad op-amp as a single supply op amp, when using a single supply op-amp we generally have to offset our signal so that it does not clip, due to the low being grounded. If we have a AC signal based around 0 volts we will  loose the negative portion of the signal. Using a dual supply op amp this can be prevented however we need to apply a negative voltage to the the lower rail of the opamp which is some circuits is more work to have an additional power supply thus using a single supply op amp will eliminate the need for the negative supply however we lose resolution because we have to shift the signal up to be able to see the full amplified signal. We can see in the data below that we get some linearity of the amplifier in a signal supply as the voltage signal being amplified such as that of a Wheatstone bridge where the voltage difference is typically really small.single rail lab.PNG