ME 021

Components of Digital Data Acquisition system

Transducer, Signal Conditioner, Analog to Digital Converter, Computer, Software

Types and representations of transducers

Accelerometer - acceleration

Strain Gage/load cell - strain

Thermocouple - temperature

LVDT - displacement (all due to a voltage change)

Torque transducer - type of strain gage

What does LVDT stand for?

Linear Variable Differential Transformer

Graph paper for lines of best fit

Linear -> ax + b

LogLog -> Power fit: cx^a

SemiLog -> exponential: ce^ax

Goal of curve fit

minimize sum of squares of differences between measured value and expected value from line

standard deviation

measure of the amount by which individual measurements differ from the mean

sources of error

human error

systematic error

random error

What does a wheatstone bridge do

convert small changes in strain gage resistance to voltage changes

How do you 'zero' a wheatstone bridge

You zero the voltage across the bridge (voltmeter in middle) to make sure that changes in voltage are due to changes in strain. R3 (top left) varies while 1 and 2 are help constant until reading is 0

why use load cells?

because elastic bodies return to their original shape after loading, allowing repeatable and reversible deformation measurements. It results in a linear relationship between voltage and strain

Tension and compression vs torque

Stress states are different in tension and compression, whereas torque strain is the same in loading and unloading

The Bauschinger Effect

a decrease in the yield strength of the material in compression as a result of prior deformation in tension (lower)

Cyclic hardening

If one now once again reverses the direction of twisting, the plot will continue on as shown. Now, however, when the material begins to yield again, the yield stress will be higher than that experienced on initial loading (not really seen in lab)

Instruments used in torsion experiment

Vernier Caliper, angular indication dial, measuring from chuck to chuck on the machine

Why use a compensating gage for bending

To cancel out the effects of temperature changes. The compensating gage is not under mechanical strain but experiences the same temperature as the active gage, so when used in a Wheatstone bridge, temperature-induced resistance changes cancel out. (This is R3)

Number of gages in a rosette

3

angle between gages

45

types of strain we measured in beam bending

(i) Maximum principal strain

(ii) Minimum principal strain

(iii) Maximum shear strain

Method of integration used for measured signal of accelerometer

The Trapezoidal Rule (for pretty much all integration)

Types of accelerometers

Piezoelectric accelerometer (extremely fast changes, generates field when strained)

microelectromechanical systems (single microchip)

Three-axis accelerometers

What is k as it relates to acceleration?

k is the constant of proportionality between the acceleration a and the voltage output e of the accelerometer, via a = ke

Piezoelectric as a "high frequency device"

It means it can accurately measure signals that change very rapidly, it has a high frequency response range and can capture fast dynamic events (why we use it for fast changes like dropping something)

what is a gage factor

how much the electrical resistance of the gage changes relative to the mechanical strain it experiences. A higher gage factor means a larger resistance change for the same strain

While loops in LabVeiw

By a Stop button (Boolean control) connected to the loop's conditional terminal, which stops execution when it returns TRUE

Bad wire

dashed line with x in the middle

Timer with number

The loop will execute (iterate) once every x milliseconds when within a while loop

Waveform plots

variable vs time

Types of pallets in lab view

controls, tool, functions

broken arrow

program cannot run

Official names for programs in LabView

Virtual Instruments

types of loops

while - grey loop with stop button in corner

for - N in corner

sampling rate

Sampling rate per channel = Total sampling rate / Number of channels

types of gages

Foil gage (not used in lab)

wire gage (resistors within wheatstone bridge are gages)

rosette strain gages - used for shear strain

What is the equation that relates thermocouple voltage output V to temperature T using calibration constants a and b?

T = aV + b, where a is the slope and b is the intercept determined during calibration.

Given an A/D converter with B bits and a voltage range of V, what is the equation for the code width (resolution)?

Code width = V / 2^B. For example, a 12-bit converter with a 5V range has a code width of 5/2¹² = 1.22 mV

What is the equation for the bending moment M at the strain gage location in the beam bending experiment?

M = P(L-a)b/L (where P is the load, L is the beam length, a is the distance from the left end to the load, and b is the distance from the left end to the strain gage)

What is the constant that describes the sensitivity of a strain gage called?

Gage factor