Download Data acquistion and LabVIEW

Data Acquisition and LabVIEW
Prof. R.G. Longoria
Department of Mechanical Engineering
The University of Texas at Austin
June 2015
ME 144L Dynamic Systems and Controls Lab (Longoria)
Lab Objectives
1
Become familiar with the National Instruments myDAQ data acquisition
hardware
2
Begin/continue learning to program in LabVIEW, especially for data
acquisition
3
Develop a LabVIEW program to measure sensor signals using myDAQ
4
Calibrate rotational potentiometer for angle measurement
5
Write LabVIEW programs that analyze signals to generate useful data
6
Run experiments with the compound pendulum and save angular position
measurements over time as the pendulum comes to rest after being released.
7
Use measured data to answer questions about the system (e.g., estimate
system parameters, system energy stored or dissipated, etc.)
ME 144L Dynamic Systems and Controls Lab (Longoria)
Most modern voltage measurements are made using A/D
converters
Most basic electrical measurements
rely on an analog-to-digital (A/D)
converter, which are even included as
part of modern microcontrollers.
ME 144L Dynamic Systems and Controls Lab (Longoria)
In a DMM, signal conditioners infer
other electrical quantities from a
measurement of voltage.
Note the signal conditioners needed to
allow measurement of current and
resistance.
For more general purpose measurement and
instrumentation applications, data acquisition devices offer
more functionality
Analog Output (AO)
I
I
Generate DC Voltages
General waveforms (Function Generator)
Digital I/O
I
I
General low (0V) and high (5V) pulses
Read digital pulses
Timing I/O
I
I
Generate pulse trains (square waves)
Read frequency, time values
Always critically evaluate DAQ specifications to determine if your needs can be
met by a particular DAQ device.
ME 144L Dynamic Systems and Controls Lab (Longoria)
The NI myDAQ connects via USB
Form factor:
ME 144L Dynamic Systems and Controls Lab (Longoria)
Connections:
What should you know about A/D conversion?
General concepts:
Resolution and range
How fast to sample
How many times to sample
Hardware specific:
Device and configuration
(using NI MAX)
Connecting signals the right
way
What channels to sample
How to deal with the data
There are many different types of software and hardware commercial products for
DAQ. National Instruments products have seen increased application and
adoption in industry, research, etc., including areas that were once considered the
domain of very ‘high-end’ systems.
ME 144L Dynamic Systems and Controls Lab (Longoria)
Analog-to-Digital (A/D) Conversion
The A/D converter (ADC) converts an analog voltage into a binary
number through the process of quantization.
The ADC will have a full-scale voltage range (VF S ) over which it can
operate.
Example: For the NI myDAQ device, there are two analog inputs with
different FS range. What is difference between DC and AC coupled?
The number of bits dictates how many discrete levels will be used to
represent measured voltages.
Example: An 8-bit converter with a VF S = 10 V gives a resolution of
10V/256 = 39.1 mV.
ME 144L Dynamic Systems and Controls Lab (Longoria)
A/D Conversion: Quantization
A signal entering a computer must be discretized in amplitude and time
(sampling). Amplitude quantization depends on the number of bits in the A/D
converter.
Comparing A/D resolution for n = 3 vs 16:
∆n=3 = VF S /23 = 1.25 V compared to ∆n=16 = VF S /216 = 0.15 mV
ME 144L Dynamic Systems and Controls Lab (Longoria)
Choosing a sampling or scan rate (scans/sec, or Hz)
The ADC samples according to a scan rate.
How fast you sample should minimally satisfy the Nyquist sampling
theorem.
Nyquist: the sampling rate should be at least two times the highest
frequency present in the signal.
Satisfying the Nyquist criterion helps ensure the signal can be
reconstructed properly.
You need to balance how fast you sample, how many samples you
store, etc.
ME 144L Dynamic Systems and Controls Lab (Longoria)
In selecting a sample rate, think about time resolution also
Depending on your objective, you might choose scan rate to satisfy Nyquist
criterion, but remember accuracy in time measurements.
ME 144L Dynamic Systems and Controls Lab (Longoria)
“All grounds are not the same the world ’round.”
Understanding grounds is important in making proper signal connections.
Can you connect them?
Circuit or signal common
Earth ground
Chassis ground
ME 144L Dynamic Systems and Controls Lab (Longoria)
Ground symbols:
Types of signal sources
Grounded source:
Referenced to system ground
(e.g., earth, building)
Share a common ground with a
DAQ board, oscilloscope, etc.
Floating source:
Isolated from absolute reference
such as earth or building ground
Neither terminal is connected to
a ground
Some signal generators, power supplies
Batteries and battery-powered sources, many sensors
such as thermocouples, etc.
ME 144L Dynamic Systems and Controls Lab (Longoria)
Types of Measurement Systems
You may see these connection options on DAQ hardware.
1
Differential measurement system
2
Referenced single-ended (RSE)
3
Non-referenced single-ended (NRSE)
Example: myDAQ analog input
ME 144L Dynamic Systems and Controls Lab (Longoria)
Experimenting in the lab
Before closing, consider the what can be found out by use of the
pendulum setup, the sensor(s) provided, and DAQ measurement.
Here are some suggestions:
estimate pendulum moment of inertia
show that for large oscillations, the pendulum period depends on
amplitude of oscillation - it is known that as amplitude increases, then
so must period
estimate stored energy, and how energy decreases after each cycle
estimate the total energy over time - this requires that you estimate
the potential energy as well as the kinetic energy. Estimating kinetic
energy requires estimating the velocity from the measured position.
Any one of these motivates the need to analyze the signals and the data in
a certain way.
ME 144L Dynamic Systems and Controls Lab (Longoria)
Suggestions for lab practice
Make notes on how to connect power, sensors, and measured signals
properly. Simple circuit knowledge is all that is needed, and it can
help you make sure you collect the signals correctly and don’t damage
equipment.
Keep separate issues of software from hardware, but understand they
work together. LabVIEW does not measure signals – instruments do
that. LabVIEW is software that controls hardware. The hardware
does the actual data collection.
Similarly, we’ll use LabVIEW to numerically solve equations, but
LabVIEW does not “model a physical system”– you do that!
ME 144L Dynamic Systems and Controls Lab (Longoria)
Summary
Use this lab to build experience using simple sensors
Use this known physical problem for purposeful learning of DAQ
usage, signal processing, etc.
Take opportunity to experiment with very basic LabVIEW VI for data
collection.
Experiment with myDAQ for quick data acquisition, testing, and
model improvement
Data collected in this week’s experiments will be used in the following
week and compared to results from simulation of the model
ME 144L Dynamic Systems and Controls Lab (Longoria)
Appendix: NI myDAQ Specifications
Two Differential Analog Input and Analog Output Channels
(200 kS/s, 16 bit, +/- 10 Volts)
Access matched analog input and output channels in a +/- 10 volt range
through the screw terminal connectors or +/- 2 volt range through the
3.5mm audio jacks.
+5 , +15, and -15 Volt Power Supply Outputs (up to 500m Watts of Power)
USB powered for maximum mobility, myDAQ supplies enough power for
simple circuits and sensors.
Eight Digital Input and Digital Output Lines (3.3 Volt TTL-Compatible)
Use software-timed digital lines for interfacing both Low Voltage TTL
(LVTTL) and 5 volt TTL digital circuits. Each line is individually selectable
for input or output.
60 Volt Digital Multimeter (DMM) for Measuring Voltage, Current, and
Resistance
The isolated DMM includes the capability to measure both AC and DC
voltage and current as well as resistance, diode voltage, and continuity.
ME 144L Dynamic Systems and Controls Lab (Longoria)
Appendix: NI myDAQ block diagram
ME 144L Dynamic Systems and Controls Lab (Longoria)
Side bar – learning more about LabVIEW DAQ
Read about how data acquisition is accomplished using LabVIEW in Getting
Started with LabVIEW tutorial. Create a NI-DAQmx Simulated Device. When
deciding on a type of device to simulate, choose E series (e.g., PCI-6025E).
Refer to and/or follow the following instructions:
1
Refer to online note that explains how:
http://zone.ni.com/devzone/cda/tut/p/id/3698
2
If you did not install NI-DAQmx device drivers on your own computer, or
you prefer not to, then you need to use the METER lab for this purpose.
The NI-DAQmx drivers are required if you will use LabVIEW to control DAQ
hardware.
3
Using a NI-DAQmx Simulated Device: study from page 4-1 to 4-6 of
Chapter 4 in the Getting Started with LabVIEW tutorial. This example
should simulate collection of 2 channels of data; when the “while” loop is
stopped the data should be saved to a LabVIEW measurement file. Here is
what the menu sequence might look like.
ME 144L Dynamic Systems and Controls Lab (Longoria)