Flashcards
Introduction to Digital Data Acquisition
Cards in this deck (22)
- 1What does it mean for experimental data to be acquired digitally?-This means that the measured quantity, whatever it may be, is transformed into a number stored in digital form
- 2Analog signal-an analog signal is one that, at least in principle, can take on any value whatsoever and can vary continuously with time
- 3What must a measured quantity be converted to before a digital signal?-an analog signal!
- 4Difference between analog and digitalAn analog signal is one that, at least in principle, can take on
any value whatsoever and can vary continuously with time. In contrast, digital representations of
analog signals, as we shall see, can take on only certain discrete values and are not continuous
functions of time - 5Transducers-A device that converts the quantity of interests (ex: pressure, displacement etc) into an electrical voltage
-Frequently, the output of a transducer will be linear in the quantity of interests, at least over a certain range - 6Sensitivity of the transducer-k is the constant of proportionality
- 7How to convert analog signal or digital form-We sample the analog signal at fixed intervals of time (this means that we take the value of analog signal at certain times and convert this value to digital form)
-The effect of this is that we take a continuous analog signal and replace it with discrete digital representation (so we know the value of the signal only at certain instants of time) - 8Sample Rate-the rate at which we sample an analog signal (
-usually expressed as a number of samples per sec, also called Hertz)
-The higher the sampling rate, the more accurate our representation of the analog signal - 9A/D converter-Conversion of the analog signal to digital form is done by a device known as a analog-to-digital converter
-In practice, the typical A/D converter is simply an integrated circuit (IC) board
which takes one or more analog voltages which one wishes to place in digital form and outputs
their digital representations into one of the ports of a personal computer (PC). - 10What does it mean for a A/D conconvertor to have only 1 bit-This means that any analog signal can be only represented as a 0 or as a 1.
For the sake of illustration, suppose that we wish
to sample an analog signal which varies between 0 and 100 mV, but that our A/D converter has
but a single bit. This means that any analog signal can be only represented as a 0 or as a 1. Thus
any signal between 0 and 50 mV would be represented by a 0 and any signal between 50 and 100
mV would be represented by a 1. - 11Resolution of A/D Convertor (example)- For the sake of illustration, suppose that we wish
to sample an analog signal which varies between 0 and 100 mV, but that our A/D converter has
but a single bit. This means that any analog signal can be only represented as a 0 or as a 1. Thus
any signal between 0 and 50 mV would be represented by a 0 and any signal between 50 and 100
mV would be represented by a 1. Obviously, this converter can only distinguish between two
signal values if they are greater than 50 mV apart. Thus we say that the resolution of the
converter is 50 mV. - 12What does it mean to have a two bit A/D convertor-Now suppose that we have a two-bit A/D converter. Then our signal can be represented
by one of four digital combinations: (0 0), (0 1), (1 0), and (1 1). In this case, (0 0) would
represent signals between 1 and 25 mV, (0 1) signals between 25 and 50 mV, and so on. In this
case, the converter would have a resolution of 25 mV. - 13Resolution equation
- 14Resolution (example)
- 15Purpose of Introduction to Digital Data Acquisition lab-We want to use a digital data acquisition system (the LabVIEW package) in conjunction
with a standard 16-bit A/D converter to determine the sensitivity of the LVDT (the slope of the
linear portion of the output voltage vs. displacement curve). We will then use this information to
measure the deflection at the tip of several cantilever beams. - 16LVDT device-Device that measures linear displacement
-The LVDT operates by sensing the difference in voltage
induced in two adjacent coils by a moving inner rod.
-Unfortunately the output of the LVDT cannot be used in raw
form; it must first be passed through a signal conditioner in order to be converted to usable form.
The output of the signal conditioner is then fed directly into the A/D converter. - 17What does the output of signal conditioner-the output voltage at the signal conditioner linear function of displacement of the displacement of the LVDT Core
-Over the
entire range of motion of the LVDT,
however, the behavior of the device usually departs somewhat from linearity, and might look somewhat like the sketch at right - 18What does it mean to calibrate the LVDT?-Determining the slope of the linear portion of the voltage vs displacement curve (denote this number by k)
-We have to be careful. The slope k depends on the voltage supplied by the power supply to the LVDT. As long as this remains constant, the k will remain constant
- 19Procedure for calibrating he LVDT
- 20Procedure for measuring the stiffness of the beams
- 21What should the ratio of the two aluminum beam of different radii's stiffness be?
- 22What should the ratio of the steel and aluminum beams of equal diameter be?
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- ME 021 Flashcards on Introduction to Digital Data Acquisition , created by Samina Forbath on 21/11/2018.
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