www.ni.com chapter 1 transducers, signals, and signal conditioning topics data acquisition overview...
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CHAPTER 1
Transducers, Signals, and Signal Conditioning
Topics• Data Acquisition Overview• Transducers• Signals• Signal Conditioning
Lesson 8Data Acquisition and Waveforms
System Overview
Transducer Overview
Topics• What is a Transducer?• Types of Transducers
What is a Transducer?
A transducer converts a physical phenomena into a measurable signal
SignalPhysical
Phenomena
Signal Overview
Topics• Types of Signals• Information in a Signal
– State, Rate, Level, Shape, and Frequency
Signal Classification
Your Signal
AnalogDigital
Two possible levels:• High/On (2 - 5 Volts)• Low/Off (0 - 0.8 Volts)
Two types of information:• State• Rate
Digital Signals
Digital
Your Signal
Digital Signal Information
Digital
Your Signal
Analog Signals
Your Signal
Analog
Continuous signal• Can be at any value with
respect to timeThree types of information:
• Level• Shape• Frequency (Analysis required)
Analog Signal Information
Your Signal
Analog
Analysis
Required
Signal Conditioning Overview
Topics• Purpose of Signal Conditioning• Types of Signal Conditioning
Why Use Signal Conditioning?
• Signal Conditioning takes a signal that is difficult for your DAQ device to measure and makes it easier to measure
• Signal Conditioning is not always required– Depends on the signal being measured
Noisy, Low-Level Signal Filtered, Amplified Signal
Amplification• Used on low-level signals (i.e. thermocouples)
• Maximizes use of Analog-to-Digital Converter (ADC) range and
increases accuracy
• Increases Signal to Noise Ratio (SNR)
Low-Level Signal ExternalAmplifier DAQ Device
Lead Wires
InstrumentationAmplifier
Noise
ADC+_
DAQ Hardware Overview
Topics• Types of DAQ Hardware• Components of a DAQ device• Configuration Considerations
Data Acquisition Hardware
DAQ Hardware turns your PC into a
measurement and automation system
Computer
Your Signal
DAQ Device
Terminal Block
Cable
Terminal Block and Cable
Your Signal
Terminal Block
Cable
• Terminal Block and Cable route your signal to specific pins on your DAQ device
• Terminal Block and Cable can be a combination of 68 pin or 50 pin
50 pin connector
DAQ Device
ComputerDAQ Device
• Most DAQ devices have:– Analog Input– Analog Output– Digital I/O– Counters
• Specialty devices exist for specific applications– High speed digital I/O– High speed waveform generation– Dynamic Signal Acquisition (vibration, sonar)
• Connect to the bus of your computer• Compatible with a variety of bus protocols
– PCI, PXI/CompactPCI, ISA/AT, PCMCIA, USB, 1394/Firewire
Configuration Considerations
• Analog Input– Resolution– Range– Gain– Code Width– Mode (Differential, RSE, or NRSE)
• Analog Output– Internal vs. External Reference Voltage– Bipolar vs. Unipolar
Resolution
• Number of bits the ADC uses to represent a signal• Resolution determines how many different voltage
changes can be measured
• Example: 12-bit resolution
• Larger resolution = more precise representation of your
signal
# of levels = 2resolution = 212 = 4,096 levels
100 200150500
Time (s)
0
1.25
5.00
2.50
3.75
6.25
7.50
8.75
10.00
Amplitude(volts)
16-Bit Versus 3-Bit Resolution(5kHz Sine Wave)
16-bit resolution
3-bit resolution
000
001
010
011
100
101
110
111
| ||||
Resolution Example• 3-bit resolution can represent 8 voltage levels• 16-bit resolution can represent 65,536 voltage levels
Range
• Minimum and maximum voltages the ADC can digitize• DAQ devices often have different available ranges
– 0 to +10 volts – -10 to +10 volts
• Pick a range that your signal fits in• Smaller range = more precise representation of your signal
– Allows you to use all of your available resolution
Range
100 200150500Time (s)
0
1.25
5.00
2.50
3.75
6.25
7.50
8.75
10.00
Amplitude(volts)
Range = 0 to +10 volts(5kHz Sine Wave)
3-bit resolution
000
001
010
011
100
101
110
111
| ||||
100 20015050Time (s)
0
-7.50-10.00
-5.00
-2.50
2.50
5.00
7.50
10.00
Amplitude(volts)
Range = -10 to +10 volts(5kHz Sine Wave)
3-bit resolution
000
001
010
011
100
101
110
111
| ||||
Proper Range• Using all 8
levels to represent your signal
Improper Range• Only using 4
levels to represent your signal
Gain• Gain setting amplifies the signal for best fit in
ADC range
• Gain settings are 0.5, 1, 2, 5, 10, 20, 50, or 100 for most devices
• You don’t choose the gain directly– Choose the input limits of your signal in LabVIEW– Maximum gain possible is selected– Maximum gain possible depends on the limits of your
signal and the chosen range of your ADC
• Proper gain = more precise representation of your signal
– Allows you to use all of your available resolution
Gain Example
100 200150500
Time (s)
0
1.25
5.00
2.50
3.75
6.25
7.50
8.75
10.00
Amplitude(volts)
Different Gains for 16-bit Resolution(5kHz Sine Wave)
Gain = 2
| ||||
Your Signal
Gain = 1
– Input limits of the signal = 0 to 5 Volts– Range Setting for the ADC = 0 to 10 Volts– Gain Setting applied by Instrumentation Amplifier = 2
• Code Width is the smallest change in the signal your system can detect (determined by resolution, range, and gain)
• Smaller Code Width = more precise representation of your signal
• Example: 12-bit device, range = 0 to 10V, gain = 1
code width = range
gain * 2 resolution
101 * 212
= 2.4 mVrange
gain * 2 resolution=
20
1 * 212= 4.8 mVIncrease range:
10
100 * 212= 24 VIncrease gain:
Code Width
Grounding Issues
• To get correct measurements you must properly ground your system
• How the signal is grounded will affect how we ground the instrumentation amplifier on the DAQ device
• Steps to proper grounding of your system:– Determine how your signal is grounded
– Choose a grounding mode for your Measurement System
Measurement System
SignalSource
VS
-
+
VM
Signal Source Categories
Grounded
+
_Vs
Floating
+
_Vs
Signal Source
Grounded Signal Source
• Signal is referenced to a system ground
– earth ground– building ground
• Examples:– Power supplies– Signal Generators– Anything that plugs into
an outlet ground
Grounded
+
_Vs
Signal Source
Floating Signal Source
Floating• Signal is NOT
referenced to a system ground
– earth ground– building ground
• Examples:– Batteries– Thermocouples– Transformers– Isolation Amplifiers
+
_Vs
Signal Source
Measurement System
• Three modes of grounding for your Measurement System
– Differential– Referenced Single-
Ended (RSE)– Non-Referenced Single-
Ended (NRSE)
• Mode you choose will depend on how your signal is grounded
Measurement System
-
+
VM
ACH (n)
ACH (n + 8)
+
_
Instrumentation
Amplifier
+
_
VS
+
_
AISENSE
AIGND
Measurement System
Differential Mode
Differential Mode• Two channels used for each signal
– ACH 0 is paired with ACH 8, ACH 1 is paired with ACH 9, etc.
• Rejects common-mode voltage and common-mode noise
VM
ACH (n)
ACH (n + 8)
+
_
Instrumentation
Amplifier
+
_
VS
+
AISENSE
AIGND_
RSE Mode
Referenced Single-Ended (RSE)• Measurement made with respect to system ground• One channel used for each signal• Doesn’t reject common mode voltage
Measurement System
VM
ACH (n)
ACH (n + 8)
+
_
Instrumentation
Amplifier
+
_
VS
+
_ AISENSE
AIGND
NRSE ModeNon-Referenced Single-Ended (NRSE)
• Variation on RSE• One channel used for each signal• Measurement made with respect to AISENSE not system ground• AISENSE is floating• Doesn’t reject common mode voltage
Measurement System
Choosing Your Measurement SystemSignal Source
Differential RSE NRSE
Measurement System
Grounded
+
_Vs
Floating
+
_Vs
Differential RSE NRSE
Measurement System
Options for Grounded Signal Sources
RSE
NRSE
Differential
BETTER
+ Rejects Common-Mode Voltage
- Cuts Channel Count in Half
NOT RECOMMENDED
- Voltage difference (Vg) between the two grounds makes a ground loop that could
damage the device
GOOD
+ Allows use of entire channel count
- Doesn’t reject Common-Mode Voltage
Options for Floating Signal Sources
RSE
NRSE
Differential
BEST
+ Rejects Common-Mode Voltage
- Cuts Channel Count in Half
- Need bias resistors
BETTER
+ Allows use of entire channel count
+ Don’t need bias resistors
- Doesn’t reject Common-Mode Voltage
GOOD
+ Allows use of entire channel count
- Need bias resistors
- Doesn’t reject Common-Mode Voltage
DAQ Software Overview
Topics• Levels of DAQ Software• NI-DAQ Overview• Measurement & Automation
Explorer (MAX) Overview
Levels of Software
DAQ Device
User
What is NI-DAQ?• Driver level software
– DLL that makes direct calls to your DAQ device
• Supports the following National Instruments software:– LabVIEW– Measurement Studio
• Also supports the following 3rd party languages:– Microsoft C/C++– Visual Basic– Borland C++– Borland Delphi
What is MAX?
• MAX stands for Measurement & Automation Explorer• MAX provides access to all your National Instruments
DAQ, GPIB, IMAQ, IVI, Motion, VISA, and VXI devices
• Used for configuring and testing devices
• Functionality broken into:
– Data Neighborhood
– Devices and Interfaces
– Scales
– SoftwareIcon on your
Desktop
Data Neighborhood• Provides access to the DAQ
Channel Wizard
• Shows configured Virtual Channels
• Includes utilities for testing and reconfiguring Virtual Channels
DAQ Channel Wizard• Interface to create
Virtual Channels for:– Analog Input– Analog Output– Digital I/O
• Each channel has:– Name and Description– Transducer type– Range (determines Gain)– Mode (Differential, RSE,
NRSE)– Scaling
Devices and Interfaces• Shows currently installed
and detected National Instruments hardware
• Includes utilities for configuring and testing your DAQ devices
– Properties– Test Panels
Properties
• Basic Resource Test– Base I/O Address – Interrupts (IRQ)– Direct Memory Access
(DMA)
• Link to Test Panels• Configuration for:
– Device Number– Range and Mode (AI)– Polarity (AO)– Accessories– OPC
Test Panels
• Utility for testing– Analog Input– Analog Output– Digital I/O– Counters
• Great tool for troubleshooting
Scales
• Provides access to DAQ Custom Scales Wizard
• Shows configured scales
• Includes utility for viewing and reconfiguring your custom scales
DAQ Custom Scales Wizard
• Interface to create custom scales that can be used with Virtual Channels
• Each scale has its own:
– Name and Description– Choice of Scale Type
(Linear, Polynomial, or Table)
Sampling Considerations
• Analog signal is continuous
• Sampled signal is series of discrete samples acquired at a specified sampling rate
• Faster we sample the more our sampled signal will look like our actual signal
• If not sampled fast enough a problem known as aliasing will occur
Actual Signal
Sampled Signal
Aliasing
Adequately
Sampled
Signal
Aliased
Signal
Nyquist Theorem
Nyquist Theorem• You must sample at greater than 2 times the
maximum frequency component of your signal to accurately represent the FREQUENCY of your signal
NOTE: You must sample between 5 - 10 times greater than the maximum frequency component of your signal to accurately represent the SHAPE of your signal
Nyquist Example
Aliased Signal
Adequately Sampled
for Frequency Only
(Same # of cycles)
Adequately Sampled
for Frequency and
Shape
100Hz Sine Wave
100Hz Sine Wave
Sampled at 100Hz
Sampled at 200Hz
Sampled at 1kHz100Hz Sine Wave
Data Acquisition Palette
Analog Input
Analog Output
Calibration and Configuration
Signal Conditioning
Digital I/O
Counter
DAQ Channel NameConstant
DAQ Channel Name Data Type• Allows you to use numeric channels
(0, 1, etc.) or virtual channels• Automatically detects all currently
configured virtual channels
Control Terminal Constant
Analog Input Palette
• Intermediate VIs– Built out of
Advanced VIs
+ Highly recommended
+ Very flexible
• Easy VIs– Built out of
Utility VIs
+ Easy to use
- Less flexible
• Utility VIs– Convenient
groupings of Intermediate VIs
• Advanced VIs– Building blocks
for other levels
Easy VIs
Intermediate VIs
Advanced VIs
Utility VIs
Single-Point AI VIs
• Perform a software-timed, non-buffered acquisition
+ Good for battery testing, control systems
- Not good for rapidly changing signals due to software timing
AI Sample Channel– Acquires one point on one channel
AI Sample Channels– Acquires one point on multiple channels
Multiple-Point (Buffered) AI VIs• Perform a hardware-timed, buffered acquisition• Highly recommended for most applications• Allows triggering, continuous acquisition, different input limits for
different channels, streaming to disk, and error handling
AI Config– Configures your device, channels, buffer
AI Start– Starts your acquisition, configure triggers
AI Read– Returns data from the buffer
AI Clear– Clears resources assigned to the
acquisition
AI Config• Interchannel Delay
– Determines the time (in seconds) between samples in a scan
• Input Limits– Max and Min values for your signal
– Used by NI-DAQ to set gain
• Device– Number of the device (from MAX)
you are addressing
• Channels– Chooses what channel(s) you are
addressing
• Buffer Size– Number of scans the buffer
can hold
– A scan acquires one sample for every channel you specify
– 1000 scans x 2 channels = 2000 total samples
• Task ID– Passes configuration
information to other VIs
• Error In/Out– Receives/Passes any errors
from/to other VIs
Different Gains for Different Channels
• AI Config allows different gains for different channels• The first element of the input limits array corresponds to the
first element of the channel array
Gain = 2Gain = 20
Range = 0 to +10V
AI Start• Task ID In/Out
– Receives/Passes configuration information to/from other VIs
• Number of Scans to Acquire– Total number of scans acquired before the acquisition completes– Default value (-1) sets # of Scans to Acquire = Buffer Size (AI Config)– A value of 0 acquires continuously
• Scan Rate– Chooses the number of scans per second
• Error In/Out– Receives/Passes any errors from/to other VIs
AI Read & AI Clear• Number of Scans to Read
– Specifies how many scans to retrieve from the buffer– Default value (-1) sets # of Scans to Read = # of Scans to Acquire (AI Start)– If # of Scans to Acquire (AI Start) = 0, default for # of Scans to Read is 100
• Scan Backlog– Number of unread scans in the buffer
• Waveform Data– Returns t0, dt (inverse of scan rate), and Y array for your data
• Clears resources assigned to the device
Error Cluster• Cluster containing:
– Boolean - tells if an error occurred– Numeric - tells the error code– String - tells the source of the error
• Right-click on edge of cluster and select Explain Error for dialog box (see below) with more information
Terminal
Indicator
Clear Resources
Return Data from the Buffer
Display
Errors
Configure the Device
Start the Acquisition
Buffered Acquisition Flowchart
Buffered Acquisition• AI Start begins the acquisition
• Acquisition stops when the buffer is full
• AI Read will wait until the buffer is full to return data
• If error input is true then Config, Start, and Read pass the error on but don’t execute; Clear passes AND executes
Start the Acquisition
Return Data from the Buffer
Display
Errors
Clear Resources
Configure the Device
Continuous Acquisition Flowchart
Done?NO
YES
Continuous Buffered AcquisitionDifferences from a buffered acquisition
• # of scans to acquire = 0• While loop around AI Read• Number of Scans to read does not = buffer size• Scan backlog tells how well you are keeping up
Analog Output Architecture
DAC
Channel 0
Channel 1
DACChannel 0
Channel 1
• Most E-Series DAQ devices have a Digital-to-Analog Converter (DAC) for each analog output channel
• DACs are updated at the same time
• Similar to Simultaneous Sampling for Analog Input
Analog Output Palette
• Intermediate VIs– Built out of
Advanced VIs
+ Highly recommended
+ Very flexible
• Easy VIs– Built out of
Utility VIs
+ Easy to use
- Less flexible
• Utility VIs– Convenient
groupings of Intermediate VIs
• Advanced VIs– Building blocks
for other levels
Easy VIs
Intermediate VIs
Advanced VIs
Utility VIs
Single-Point AO VIs
• Perform a software-timed, non-buffered generation
+ Good for generating DC voltages, or control systems
- Not good for waveform generation because software timing is slow
AO Update Channel– Generates one point on one channel
AO Update Channels– Generates one point on multiple channels
AO Update Channels
• Device– Number of the device (from
MAX) you are addressing– Ignored if using virtual channel
• Channels– Chooses what channel(s) you
are addressing– Can either be a number or a
virtual channel name– Uses the DAQ Channel Name
control
• Values– 1-D array of data – The first element of
the array corresponds to the first channel in your channels input
Multiple-Point (Buffered) AO VIs• Perform a hardware-timed, buffered generation• Highly recommended for most applications• Allows continuous generation, triggering, and error handling
AO Config– Configures your device, channels, buffer
AO Write– Writes data to the buffer
AO Start– Starts your generation
AO Wait– Waits until the generation is complete
AO Clear– Clears resources assigned to the
generation
Clear Resources
Display
Errors
Buffered Generation Flowchart
Wait Until Generation Completes
Configure the Device
Start the Generation
Write Data to the Buffer
Buffered Generation• AO Write fills the buffer with waveform data• AO Start begins the generation• Without AO Wait the generation would start (AO Start) and
then end immediately after (AO Clear)• If error input is true then Config, Write, Start, and Wait pass
the error on but don’t execute; Clear passes AND executes
AO Write One Update
• Your analog output channel will continue to output the last value written to it until either:
– The device is reset (power off, reset VI)
– A new value is written
• Use AO Write One Update at the end of your generation to set the channel back to 0
Clear Resources
Start the Generation
Configure the Device
Write Data to the Buffer
Display
Errors
Continuous Generation Flowchart
Done?NO
YES
Continuous GenerationDifferences from a buffered generation
• number of buffer iterations = 0• No AO Wait
– AO Wait would hang because the generation never completes
• While loop with AO Write– The second AO Write is used for error checking ONLY