01 - Overview of Signals

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The ur ose of a Data Ac uisition s stem is to measure a h sical henomenon

© National Instruments Corporation 2 DAQ & SC Course Instructor Manual

such as light, temperature, pressure, sound, etc. The building blocks of a DataAcquisition system are as follows:

• Transducer

• Signal• Signal Conditioning eXtensions for Instrumentation (SCXI)

• Data Acquisition (DAQ) device

• Driver level and application level software

These five building blocks allow you to bring the physical phenomena you want tomeasure into your computer for analysis and presentation. In the following pages,we will discuss each one of these blocks individually to give you knowledge of eachbuilding block, and how they fit together to make up your Data Acquisition system.


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Definition for DA



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In our discussion of transducers, ou will learn what a transducer does, and what


types of transducers to use for measuring the following physical phenomena:

• Temperature

• Light

• Sound

• Force

• Pressure

• Position

• Fluid flow

• pH levels


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The ur ose of a transducer is to convert a h sical henomena li ht, tem erature,


pressure, position, sound, etc.) into a measurable electrical signal, such as voltage orcurrent.


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Transducers exist for a variet of a lications from measurin tem erature to


pressure to fluid flow. The above list is only a sample of the types of transducers thatexist and the types of applications they can be used for. If you have a physicalphenomena to measure, a transducer probably exists to measure it. For moreinformation on transducers and where to get them please visithttp://www.ni.com/sensors

Different transducers have different requirements for converting a physicalphenomena into a measurable signal. For instance, a Resistance TemperatureDetector (RTD) needs an excitation current in order to measure the temperature. Athermocouple doesn’t need any sort of excitation current, but it does need cold

junction compensation. Strain gages use a configuration of resistors called aWheatstone Bridge to measure strain.


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With the hel of a transducer we have converted a h sical henomena li ht,


temperature, pressure, sound, etc.) into a signal. Not all signals are measured in thesame manner, so we will need to learn how to categorize our signal as one of twotypes:

• Digital• Analog

Once we have categorized our signal we need to figure out what type of informationwe want out of that signal. The possible types of information we can obtain from asignal are:

• State

• Rate

• Level

• Shape

• Frequency

The next section will discuss all five types of information that can be obtained froma signal and give real world examples.

Note : Our discussion of signals assumes that we are acquiring the signal. However,most of the points apply to generating a signal as well. The only exception beingthat you don’t need to do analysis to generate a signal with a specific frequency.


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A si nal can fall into one of two cate ories:


• Digital

• Analog

Next we will see what makes a signal either digital or analog. We will also see howthe distinction of either digital or analog affects the way we will measure our signal.


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Unlike a di ital si nal, an analo si nal can be at an volta e level with res ect to


time. Since an analog signal can be at any state at any time, the physical quantitieswe want to measure differ from those of a digital signal. We can measure the level,shape, or frequency of an analog signal.


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As we ust learned, we can measure three uantities of an analo si nal: level,


shape, and frequency. We will go through these options one by one.

Level

Measuring the level of an analog signal is similar to measuring the state of a digitalsignal. The only difference is that an analog signal can be at any voltage state,whereas a digital signal can only be at one of two states.

Shape

Because analog signals can be at any state with respect to time, the shape of thesignal is often important. For instance, a sine wave has a different shape than asawtooth wave. Measuring the shape of a signal opens the door to further analysison the signal itself such as peak values, slope, integration, etc.

Frequency

Measuring the frequency of an analog signal is similar to measuring the rate of adigital signal. However, you cannot directly measure the frequency of an analogsignal. Software analysis of the signal is required to extract the frequencyinformation. The analysis is usually done by an algorithm called a FourierTransform.


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Lets look at some real world exam les of measurin the level of an analo si nal.


Normally when we measure the level of a signal, the signal does not change muchwith respect to time. However, we usually need to measure the signal with a highlevel of accuracy. Using a variety of different transducers we could measure thevoltage of a power supply, the temperature of a mixing tank, the pressure inside ahose, or the load on a piece of machinery, just to name a few.


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Next let us examine some exam les of measurin the sha e of an analo si nal.


Most signals that we are interested in measuring the shape of change rapidly withrespect to time. We also need a high level of accuracy in how we measure thesignal. Examples of measuring shape range are abundant in the fields of medicine,electronics, and automotive, and range from measuring a heartbeat to a video signalto the vibration of a spring. Once we have acquired the signal we can then analyzeit to extract the specific information we need about the shape. For instance whenyou are measuring a blood pressure you are concerned with the peak value, whereaswith RC Circuit response you are more concerned with how the amplitude variesover time.


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Now we will examine some exam les of measurin the fre uenc of a si nal. When


we measure the frequency of a signal we do not need to acquire the signal as quicklyas we do when we measure shape. However, we still need a good deal of accuracy.We also need some form of software analysis to convert the time signal we haveacquired into a frequency signal. This is usually done in the form of a FourierTransform. Examples of measuring frequency can be found in a variety of fieldssuch as geophysical studies, acoustics, and telecom, and range from measuring anearthquake to speech analysis as show above.


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A di ital si nal has onl two ossible states: ON or OFF. ON is also called hi h


logic and OFF is also called low logic. Digital signals are often referred to as a TTL(Transistor-to-Transistor Logic) signal. The specifications for a TTL signal statethat a voltage level between 0 - 0.8 Volts it is considered low logic, and a voltagelevel between 2 - 5 Volts is considered high logic. Most digital devices in industryaccept a TTL compatible signal.

Since a digital signal only has two states, we can only measure two quantities of adigital signal: state or rate. The following pages will discuss measuring state andrate as well as give some real world examples of both.


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Let us consider a real world exam le of measurin the state of a di ital si nal.


Assume we have a switch that we want to monitor. Lets say our switch turns a lighton and off. As we can see above, when the switch is open we are going to measure 0Volts (OFF). When the switch is closed we are going to measure 5 Volts (ON). Bymeasuring the state of the digital signal we can tell if our light is on or off.


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Now lets look at a real world exam le for measurin the rate of a di ital si nal.


Assume we have a motor and we want to tell how fast the shaft of the motor isspinning. We will use an encoder to convert the rotary motion of the shaft into adigital signal. When an encoder rotates it produces two digital signals. Each digitalsignal is a series of alternating ON and OFF states otherwise known as a pulse train.For each increment of rotation we will get a pulse. The increment of rotationdepends on the encoder. For instance, the DAQ Signal Accessory that is used withthe course has an encoder that gives 24 pulses/revolution. We can then measure therate of one of the pulse trains to determine how fast the shaft is rotating.

Note : We could measure both pulse trains to determine the direction that the shaftwas rotating as well as how fast. We will discuss encoders later in this course.


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Now that we have learned the five different t es of information that we can extract


from a signal we will take a look at deriving all five types of information from thesame signal. Let us look at a pulse train. If we treat our signal as a digital signal,we can measure the state of the signal as either ON or OFF, and the rate at which arestates are changing. If we treat our signal as an analog signal we can measure thelevel of the signal at any point in time, the shape of the signal as it rises or falls fromone state to another, and we can determine the frequency of the signal throughsoftware analysis. Measuring both state and level may seem redundant, however thetwo measurements are different. When we measure the state we only know if thesignal is ON of OFF. As we learned earlier, ON can be a range from 2 - 5 Volts andOFF can be a range from 0 - 0.8 Volts. By measuring the level of our signal we candetermine the exact voltage of our signal. Frequency and rate are also very similar.However, the difference here is that when we measure rate we are measuring howoften a portion of our signal occurs. For instance we can count the number of risingedges as shown above. When we measure the frequency of a signal, we are gettingthe frequency content of our signal. We need to perform analysis such as an FFT toaccomplish a frequency measurement. As we will learn later, we can accomplish allfive of these measurements using a DAQ device and LabVIEW. We will now do anexercise that will measure all five types of information in our signal.


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In this exercise, the students will open a pre-built LabVIEW VI and study the state, rate, level,


shape, and frequency of a signal.


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We have now taken our h sical henomena, converted it into a si nal with our


transducer, and decided the type information in our signal we want to measure.However, it is not always possible to connect our signal directly to our DataAcquisition device. We might need to alter the signal to make it suitable for ourData Acquisition device to measure. We can alter our signal with signalconditioning hardware. National Instruments main signal conditioning product lineis referred to by the acronym SCXI which stands for Signal Conditioning eXtensionsfor Instrumentation. For more information on National Instruments SCXI productsas well as other signal conditioning hardware please visit http://www.ni.com/sigconIn the following section we will discuss the purpose of signal conditioning, and thefollowing common types of signal conditioning:

• Amplification

• Excitation

• Linearization

• Isolation

• Filtering


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As we learned in our discussion of transducers, most transducers need some sort of


external hardware in order to perform their job. For instance, RTD’s need excitationcurrent, and strain gages need a configuration of resistors called a Wheatstonebridge.

In addition to needing external hardware, not all transducers produce a perfectvoltage for our Data Acquisition device to measure. The signal from the transducercould be noisy, or if could be too small or too large for the range of our DAQ device.For instance, thermocouples, strain gages, and microphones all produce a voltage inthe millivolt range making it hard to detect changes in the signal.

Most transducers need some form of signal conditioning whether it is to provide anexcitation current or to turn the signal from the transducer into one that can be easilymeasured by a DAQ device. We will now discuss some common types of signalconditioning and their uses.


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Am lification is a wa of increasin a si nal from a transducer that is too small for


your DAQ device to accurately measure. A common example is a thermocouple.Thermocouples output a voltage in the millivolt range. If you were to send thesignal from your thermocouple straight to your DAQ device, it is feasible that achange of a degree or two in temperature would not be detected by your system.However, if we amplify the signal we will be measuring a signal that is better suitedto the range of our DAQ device. Your signal can either be amplified on the DAQdevice or externally. The problem with amplifying the signal on the DAQ device isthat we also amplify the noise the signal has picked up on its way to the DAQdevice. In order to minimize the amount of noise that is amplified it is best to placethe amplifier as close to the signal source as possible. Thus it is usually best to usesome form of external amplification. As we will see next, we can show the benefitof external amplification with an index called the Signal to Noise Ratio.


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The Si nal to Noise Ratio SNR is a measure of how much noise exists in our


signal compared to the signal itself. It is defined as the voltage level of your signaldivided by the voltage level of the noise. The larger the Signal to Noise Ratio thebetter. As you can see above, the Signal to Noise Ratio is the best when onlyexternal amplification is used on your signal, and the worst when the signal is onlyamplified on the DAQ device.


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Transducer Excitation


As we learned in our discussion of transducers, most transducers need some externalvoltage or current in order to perform their job. The excitation voltage or currentcan be provided by your signal conditioning hardware.

LinearizationA good deal of transducers do not produce voltages in a linear manner. For instance,a change in voltage of 10 millivolts for a thermocouple is usually not a change of 10degrees. Most transducers have linearization tables that map out how to scale yourtransducer. The linearization of your transducer can either be done in hardware orsoftware.

Isolation

Often your signal will exceed the limits that your DAQ device can handle. Trying tomeasure a signal that is to small for your DAQ device can only result in an

inaccurate reading, but trying to measure a signal that is too large for your DAQdevice can damage the device. With large voltages we apply a signal conditioningtechnique called isolation. The signal conditioning hardware is designed to handlehigh voltages and attenuate them to a voltage your DAQ device can handle.

Filtering

Filtering is used to remove unwanted portions of your signal. The most commonapplication of this is to remove unwanted noise from your signal. Most noise iscreated from lights or the power supply of your computer and will show up around

60Hz. Using a low pass filter with a cutoff frequency below 60Hz will help toremove that noise from your signal. Filtering can be done in hardware or software.


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Above is a listin of some common transducers and the t es of si nal conditionin


that are often necessary to make the signal easy to measure for a DAQ device.


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1. A DA S stem consists of five com onents: Transducers, Si nals, Si nal


Conditioning, DAQ Hardware, and DAQ Software

2. d. Measureable Signal


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3. Level, Sha e, Fre uenc


4. State, Rate


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5. All of the above

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01 - Overview of Signals

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