introduction to sensors and data acquisitiondsclab/leks/dsc_intro_sensors_and_daq.pdf ·...

Lab Overview Sensors DAQ Modeling Experimentation Summary

Introduction to Sensors and Data AcquisitionExperimenting with a compound pendulum

Prof. R.G. Longoria

Department of Mechanical EngineeringThe University of Texas at Austin

Fall 2014

ME 144L Dynamic Systems and Controls Lab (Longoria)


Lab Overview

In the laboratory, you will experiment with a compound pendulum setupequipped with a potentiometric sensor to measure rotational displacementabout a fixed support shaft.

Sensor and digital data acquisition (or ‘DAQ’) concepts are introduced.

Learning LabVIEW continues, now programming to control DAQ hardwarefor signal analysis.



When released from an initial angle, the pendulumoscillates and eventually stops.

Decay of pendulum motion is likely due to energy dissipated by friction in the

pivot and/or in the air. We will determine which dominates by making

measurements from an angle sensor in combination with results from modeling

and simulation.



Lab Objectives

1 Learn sensor concepts, using sensors, and signal conditioning

2 Become familiar with the National Instruments myDAQ data acquisitionhardware

3 Continue learning how to program in LabVIEW, now for data acquisition

4 Develop a LabVIEW program to measure sensor signals using myDAQ

5 Calibrate rotational potentiometer for angle measurement

6 Write LabVIEW programs that analyze signals to generate useful data

7 Run experiments with the compound pendulum and save angular positionmeasurements over time as the pendulum comes to rest after being released.

8 Use measured data to answer questions about the system (e.g., estimatesystem parameters, system energy stored or dissipated, etc.)



Most modern sensors are electromechanical

We can classify by the sensingmechanism.

Resistive (potentiometers,strain gauges, thermistors,light, etc.)

Capacitive (Very common inMEMS; accelerometers, studsensors, etc.)

Inductive and Magnetic(proximity, distance, ...)

Piezolelectric (force, ...)



Resistive sensors rely on changes in resistance

The resistance of a uniform conductor is given by, R = ρL/A, with ρthe resistivity, L the length and A the constant cross-sectional areathrough which current flows.

Resistance changes either by a geometric (A, L) or material change(ρ) in the resistive element.

Resistance can be directly measured (by an ohmmeter) or inferredthrough a signal conditioning circuit (e.g., a voltage-divider)



Signal conditioning for resistive sensors converts resistancechange to voltage change

Signal conditioning refers to thedevices and processes we use tomodify and/or improve the natureof a sensor signal. Examplesinclude filters, amplifiers, etc.

Consider a basic voltage divider,where

vout =

[R2

R1 +R2

]vin By using a voltage divider, we can

transform the resistance changeinto a voltage change which ismore readily measured.



Calibration of the potentiometric sensor

Effectively, the potentiometric sensor is configured like a voltage divider where theoutput voltage is related to the change in shaft position.

Calibration builds a relation between the output voltage and angular position.

We seek relation θ = f(vout), where vout is the measurable output voltage. It isdesirable to have a sensor that has a linear relation between the measurand (hereθ) and the measured voltage.



Why we like linear sensor models

Linear model relations between measured voltages, say vm, and ameasurand of interest, ym, make it easy to represent calibration with asingle constant or line (e.g., from regression),

ym = K · vm

Another advantage is that if the relation between a measured voltagesignal and the measurand is linear then when you look at the temporaltrends in the measured signals these are the same for the actual physicalvariable(s) of interest.

Having a nonlinear sensor is tolerable, especially since modern computingcan easily represent the model.NOTE: It is expected that when calibrations are conducted, the regression may introduce a ‘y-intercept’ (i.e.,ym = K · vm + b). This model is more generally called affine, meaning there is a linear relation with some translation (orrotation).



Pre-Lab 1 – sensing circuit

Review the sensing circuit (potentiometer) discussed in the lecture slides. Submita description of how the basic potentiometric circuit is used to measure pendulumangular position. Use diagrams and equations in this description.



Most modern voltage measurements are made using A/Dconverters

Most basic electrical measurementsrely on an analog-to-digital (A/D)converter, which are even included aspart of modern microcontrollers.

In a DMM, signal conditioners inferother electrical quantities from ameasurement of voltage.

Note the signal conditioners needed toallow measurement of current andresistance.



For more general purpose measurement andinstrumentation applications, data acquisition devices offermore functionality

Analog Output (AO)I Generate DC VoltagesI General waveforms (Function Generator)

Digital I/OI General low (0V) and high (5V) pulsesI Read digital pulses

Timing I/OI Generate pulse trains (square waves)I Read frequency, time values

Always critically evaluate DAQ specifications to determine if your needs can be

met by a particular DAQ device.



The NI myDAQ connects via USB

Form factor: 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 rightway

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.



Analog-to-Digital (A/D) Conversion

The A/D converter (ADC) converts an analog voltage into a binarynumber through the process of quantization.

The ADC will have a full-scale voltage range (VFS) over which it canoperate.

Example: For the NI myDAQ device, there are two analog inputs withdifferent FS range. What is difference between DC and AC coupled?

The number of bits dictates how many discrete levels will be used torepresent measured voltages.

Example: An 8-bit converter with a VFS = 10 V gives a resolution of10V/256 = 39.1 mV.



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/Dconverter.

Comparing A/D resolution for n = 3 vs 16:

∆n=3 = VFS/23 = 1.25 V compared to ∆n=16 = VFS/2

16 = 0.15 mV



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 samplingtheorem.

Nyquist: the sampling rate should be at least two times the highestfrequency present in the signal.

Satisfying the Nyquist criterion helps ensure the signal can bereconstructed properly.

You need to balance how fast you sample, how many samples youstore, etc.



In selecting a sample rate, think about time resolution also

Depending on your objective, you might choose scan rate to satisfy Nyquistcriterion, but remember accuracy in time measurements.



“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

Ground symbols:



Types of signal sources

Grounded source:

Referenced to system ground(e.g., earth, building)

Share a common ground with aDAQ board, oscilloscope, etc.

Some signal generators, power supplies

Floating source:

Isolated from absolute referencesuch as earth or building ground

Neither terminal is connected toa ground

Batteries and battery-powered sources, many sensorssuch as thermocouples, etc.



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



Pre-Lab 2 (1) – learning about LabVIEW DAQ

Read about how data acquisition is accomplished using LabVIEW in GettingStarted with LabVIEW tutorial. Create a NI-DAQmx Simulated Device. Whendeciding 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, oryou 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 DAQhardware.

3 Using a NI-DAQmx Simulated Device: study from page 4-1 to 4-6 ofChapter 4 in the Getting Started with LabVIEW tutorial. This exampleshould simulate collection of 2 channels of data; when the “while” loop isstopped the data should be saved to a LabVIEW measurement file. Here iswhat the menu sequence might look like.



Pre-Lab 2 (2) – learning about LabVIEW DAQ

4 Here is a sample screen shot of a front panel for a VI you should create forcapturing two (simulated) signals. Here is what the block diagram mightlook like.

5 Submit your VI via email to your TA before going to lab



Model for determining pendulum moment of inertia

The model for a compound pendulumwas previously derived (seeIntroduction slides) as a 2nd orderODE,

J0θ̈ +mglC sin θ = 0.

with J0 the (mass) moment of inertiaabout the axis of rotation (or pivot),O, J0 = J +ml2C , m is the totalmass, and lC the distance from thepivot to the CG. It was assumed thatthere are no damping torques.If the angle of oscillation about θ = 0is small (< 10 degrees), sin θ ≈ θ thenthe ODE becomes linear,

J0θ̈ +mglCθ = 0.

In standard 2nd order form,

θ̈ + ω2nθ = 0,

where, ωn =√mglC/J0 is the

undamped natural frequency. In this

way, a measurement of the undamped

natural period, Tn = 2π/ωn, can be

used to experimentally determine J0.



Experimentation

Before closing, consider the what can be found out by use of thependulum 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 onamplitude of oscillation - it is known that as amplitude increases, thenso must period

estimate stored energy, and how energy decreases after each cycle

estimate the total energy over time - this requires that you estimatethe potential energy as well as the kinetic energy. Estimating kineticenergy requires estimating the velocity from the measured position.

Any one of these motivates the need to analyze the signals and the data ina certain way.



Pre-Lab 3 – estimating moment of inertia

In lab, you will make period measurements to experimentally estimate thependulum moment of inertia (about the pivot) based on a model of thependulum. You will need pendulum geometric data found on the course log (useany of these values and update/verify when you go to lab) and assume a nominalvalue for aluminum density. Submit answers to the following items:

1 What are two key assumptions made in relating pendulum moment of inertiato the undamped natural period?

2 Determine the distance to the center of gravity from the pivot, the totalmass of the pendulum, and the mass moment of inertia about the pivot.

3 Calculate the theoretical undamped natural period of the pendulum (inseconds).

4 One factor in the uncertainty in your measurement of the measured periodwill depend on how many samples you measure every second (sample rate).Say you wanted to be able to say that the uncertainty was no more than1%. Given your estimate of the undamped period, what would yourecommend for a sampling rate?



Some advice:

Make notes on how to connect power, sensors, and measured signalsproperly. Simple circuit knowledge is all that is needed, and it canhelp you make sure you collect the signals correctly and don’t damageequipment.

Keep separate issues of software from hardware, but understand theywork together. LabVIEW does not measure signals – instruments dothat. LabVIEW is software that controls hardware. The hardwaredoes the actual data collection.

Similarly, we’ll use LabVIEW to numerically solve equations, butLabVIEW does not “model a physical system”– you do that!



Summary

Use this lab to build experience using simple sensors

Use this known physical problem for purposeful learning of DAQusage, signal processing, etc.

Take opportunity to experiment with very basic LabVIEW VI for datacollection.

Experiment with myDAQ for quick data acquisition, testing, andmodel improvement

Data collected in this week’s experiments will be used in the followingweek and compared to results from simulation of the model


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 rangethrough the screw terminal connectors or +/- 2 volt range through the3.5mm 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 forsimple 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 selectablefor input or output.

60 Volt Digital Multimeter (DMM) for Measuring Voltage, Current, andResistanceThe isolated DMM includes the capability to measure both AC and DCvoltage and current as well as resistance, diode voltage, and continuity.


NI myDAQ block diagram


introduction to sensors and data acquisitiondsclab/leks/dsc_intro_sensors_and_daq.pdf ·...

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