Signal Conditioning Handbook

Document Sample
Signal Conditioning Handbook Powered By Docstoc
					                 Signal
                      Conditioning
                 &    PC-Based Data Acquisition




                         Third Edition
Return to Main Menu
Main Menu
     Chapter 1        Intro to Data Acquisition & Signal Conditioning
     Chapter 2        Analog to Digital Conversion
     Chapter 3        Multiplexing
     Chapter 4        Electrical Measurements
     Chapter 5        Fundamental Signal Conditioning
     Chapter 6        Temperature Measurement
     Chapter 7        Strain Measurements
     Chapter 8        Vibration and Sound
     Chapter 9        Displacement and Position Sensing
     Chapter 10       Noise Reduction and Isolation
     Chapter 11       Digital and Pulse Train Signal Conditioning
     Chapter 12       Transducer Electronic Data Sheets




Return to Main Menu
Introduction

     •    Signals, Sensors, and Signal Conditioning

     •    Data Acquisition Systems

     •    PC-Based Data Acquisition Equipment




Return to Main Menu
Analog to Digital (ADC)


     •    ADC Types

     •    Accuracy and Resolution

     •    Noise Histograms

     •    ADC Output Averaging



Return to Main Menu
                                    Chapter 1
Analog to Digital Conversion
 Analog-to-digital (A/D) converters

      are used to transform analog information, such as voltage
      signals or measurements of physical variables (for example,
      temperature, force, or shaft rotation) into a form suitable for
      digital handling, which might involve any of these operations: (1)
      processing by a computer or by logic circuits, including
      arithmetical operations, comparison, sorting, ordering, and code
      conversion, (2) storage on electronic media until ready for further
      handling, (3) display in numerical or graphical form on a
      computer, and (4) transmission




Return to Main Menu
                                                                       Chapter 2
Analog to Digital Conversion

     ADC Types

     • Successive Approximation Register

     • Voltage to Frequency

     • Integrating Dual Slope

     • Sigma-Delta




Return to Main Menu
                                           Chapter 2
Analog to Digital Conversion

   •   Successive-Approximation ADC
   •   Usually associated with higher speed systems

   •   Generally 12 and 16 bit

   •   Inexpensive

   •   Conversion process starts at Maximum range (MSB)
       and steps down through a defined sequence to the
       (LSB) until correct reading is found using a DAC
       and an analog comparator circuit.




                                                          Figure 2.01

Return to Main Menu
                                                                        Chapter 2
     Analog to Digital Conversion
     Successive approximation A / D example



     •   Simple example of a 4 bit SAR A/D with 0 to 100 volt max range
     •   16 discreet steps (LSB 6.25)

     •   For example: If the input voltage is 60V and the reference voltage is 100V, in the 1st clock cycle, 60V is
         compared to 50V (the reference, divided by two.) The voltage from the comparator is positive (or '1')(because
         60V is greater than 50V). At this point the first binary digit (MSB) is set to a '1'. In the 2nd clock cycle the input
         voltage is compared to 75V (being halfway between 100 and 50V) because 60V is less than 75V, the
         comparator output is now negative (or '0'). The second binary digit is therefore set to a '0'. In the 3rd clock
         cycle, the input voltage is compared with 62.5V (halfway between 50V and 75V). The output of the comparator
         is negative or '0' (because 60V is less than 62.5V) so the third binary digit is set to a 0. The fourth clock cycle
         similarly results in the fourth digit being a '1' (60V is greater than 56.25V, the DAC output for '1001' followed by
         zeros). The result of this would be in the binary form 1001




Return to Main Menu
                                                                                                                      Chapter 2
Analog to Digital Conversion

   •   Sigma-Delta ADC

   •   Usually associated with slower speed but higher accuracy
   •   Over-sampling improves accuracy
   •   Serial bit stream fed to LPF circuit
   •   Effective sample rate output many times slower than A/D
       process loop
   •   Noise reduction by spreading noise over larger sample freq
       using over-sampling and applying LPF and integration
   •   Inherently linear
   •   Resolution: 16 to 24 Bits




Return to Main Menu                                                 Figure 2.04
                                                                                  Chapter 2
Sigma- Delta ADC
Sigma-delta ADCs are far more complex than other
ADC types.

The oversampled analog signal goes through an
integrator whose output drives a comparator (a 1-bit
ADC) that, in turn, drives a 1-bit DAC in the
feedback loop. Through a series of iterations, the
integrator, comparator, DAC, and summing junction
produce a serial bit stream that represents the
oversampled input voltage.

Once digitized, the oversampled signal goes through
a digital filter to remove frequency components at or
above the Nyquist frequency, which is one-half of
the ADCs output-sampling rate (Ref. 4). A digital
low-pass filter removes those high-frequency
components, and a data decimator removes the
oversampled data. In an ADC with 128X
oversampling, the decimator will retain 1 bit for
every 128 bits that it receives. The final output is a
serial bit stream




Return to Main Menu
    Sigma-Delta ADC continued
     •   Example: Let VIN=1.0V, VREF=2.5V.
     •   The outputs from the comparator will be: 1, 0, 1, 1, 1, 0, 1, 1.

     •   This means 6 of the 8 outputs have been a 1
     •   6/8 = output is 75% of full-scale.
     •   The allowed input range is -2.5 to +2.5 (+/-VREF) so the span is -
         2.5 to +2.5.

     •   With a 1.0V input the input is 3.5V above the bottom of the 5.0V
         span or 70% of full-scale.
     •   If we continue looping, the ones density of the above output
         stream will get closer and closer to 70%.

     •   Example: After 27 clock tics from the comparator: (measured
         longer)
     •   101110110110111011011011101 = 19/27 = 70.3 % of full
         scale =
     •     - 2.5 + 3.515 = 1.015 volts



Return to Main Menu
        Analog to Digital Conversion
    •    Sigma-Delta ADC with Digital Filter


    •     Filter improves accuracy
    •     60 Hz Notch
    •     High Resolution
    •     Uses Oversampling
    •     Input – High frequency
    •     Output – Low Frequency
    •     This technique can therefore be used effectively to increase
          the dynamic range of the ADC at the expense of overall
          output sampling rate and extra digital hardware




Return to Main Menu                                                      Figure 2.05
                                                                                       Chapter 2
Analog to Digital Conversion


LETS Measure the same DC
   signal with:

IOtech PersonalDaq/56 -
    Sigma-Delta ADC 22 – bit

    AND

IOtech PersonalDaq/3000 –
    SAR ADC 16 - bit




Return to Main Menu
                               Chapter 2
Analog to Digital Conversion

Table of ADC Attributes




Return to Main Menu            Figure 2.06
                                             Chapter 2
   Analog to Digital Conversion

    •   A / D Resolution:
    •   ADC resolution is the number of bits used to represent the analog input signal. To more accurately replicate the
        analog signal, you must increase the resolution. Using an ADC with higher resolution also reduces the quantization
        error (the unknown value between steps)

    •   LSB (least significant bit) value = The smallest incremental voltage step of the A/D over a given range. (Not the
        smallest voltage step you can accurately measure!)

    •    Calculate LSB value by converting the number of bits to the power of 2 and then dividing that number into the total
        span of the input voltage range of the data acquisition device.
    •    16 bit system with (+ / -) 10 volt input range = 20 volts / 65536 = 305 uV steps




Return to Main Menu
                                                                                                                 Chapter 2
Analog-to-Digital Conversion

     1.   ADC Accuracy: Accuracy is the degree of conformity of a measured or
          calculated quantity to its actual (true) value.


     2.   Errors in the A/D conversion process can affect Accuracy
     3.   Total system error = square root of the sum of each individual contributing

          error squared (root summed square)

     4.   Errors: Quantization error, Full Scale error, Offset error, Linearity, Missing

          digital codes




Return to Main Menu
                                                                                           Chapter 2
        Analog-to-Digital Conversion

   1.     Factory Calibration – Recommended once every 12 months
         •     Legacy hardware requires gain and offset adjusted via trim pots
               coefficients using precision calibration equipment.
         •     Newer hardware often requires specialized software and equipment to
               digitally adjust gains and trim offsets..
         •     Customer receives new Calibration Certificate that indicating that the
               product meets the published specifications
         •     Calibration Certificate often indicates equipment traceability.
   2.     System Calibration
         •     Performed by end-user using software scaling. Does not change any
               factory calibration settings in the actual hardware device.
         •     Calibrates system End-to-End to eliminate any offset errors including
               transducer interfacing and external signal conditioning




Return to Main Menu
                                                                                        Chapter 2
Analog to Digital Conversion

•   ADC Accuracy and Resolution

     •   Quantization error – Part of the A/D
         conversion process

     •   Quantization error results from an analog
         value that falls between digital steps. Worst
         case would be exactly half way between
         digital steps. (half LSB)
     •   Quantization error shows up as noise on an
         FFT that is evenly distributed over the
         sample frequency range.




Return to Main Menu                                      Figure 2.07A
                                                                        Chapter 2
Chapter 2
Analog to Digital Conversion
ADC Accuracy and Resolution

   1.   Gain Error - Part of the A/D conversion process
   2.   The same as Full Scale Error without the offset error
   3.   An A/D conversion error that is worst at the top end of
        a given range
   4.   Each conversion step has an equal ratio of gain error
        which adds up across the measurement range as more
        bits of the A/D are used.
   5.   Factory calibration corrects this using stored linear
        equation or look up table stored in the device by
        pivoting the conversion line on zero.
   6.   Expressed in LSB




Return to Main Menu                                               Figure 2.07B
Chapter 2
Analog to Digital Conversion
•   ADC Accuracy and Resolution


     •   Linearity Error    Part of the A/D conversion process

     •   Some isolated portion along the conversion line becomes less
         accurate.

     •     Should be 1 LSB or less

     •    Some systems use
     •    calibration points stored in
     •     NVRAM lookup table.




Return to Main Menu                                                     Figure 2.07C
     Chapter 2
     Analog to Digital Conversion
     •   ADC Accuracy and Resolution

     •    Missing Codes     Part of the A/D
     •                    conversion process
     •    Some A/D architectures
     •    have missing binary conversion codes.

     •     As the input voltage is swept over its
     •     range, all output code combinations
     •     should appear at the converter output.

     •    Not as common of a problem
     •     as technology has improved

     •    Histogram would show zero samples at a
     •    missing code location




     •

Return to Main Menu                                 Figure 2.07D
Chapter 2
Analog to Digital Conversion
•   ADC Accuracy and Resolution


     •     Offset Error
     •   measured value has the same + / - offset from the actual value
         across the entire measurement range

     •    Factory calibration
     •    Offset is removed by aligning the negative most conversion code
         with zero and then adjusting for gain error by rotating the transfer
         function about the "new" zero point using a calibrated voltage
         source at full scale. The process is usually repeated.




Return to Main Menu                                                             Figure 2.07E
Chapter 2
Analog to Digital Conversion
    •   A / D Resolution:
    •   While the A/D converters used on most 16-bit boards are extremely accurate, other circuitry is usually not. 16-bit accuracy in
        analog circuits is very hard to achieve. Gains greater than one, high sampling rates, and input signals that change rapidly from
        zero volts to full-scale all tend to reduce the accuracy of the board’s analog circuitry. In handling real-world signals, analog
        circuitry sometimes becomes the limiting element on overall accuracy.

    •   ENOB (Effective Number of Bits) test is a common way to scrutinize a data acquisition system to determine the total accuracy that
        a system will have under realistic user conditions including specified interface cabling.

    •   The results of this test might state something like – This 16 bit Daq system has a Total Accuracy of 14.5 bits when used in my
        exact application setup




Return to Main Menu
           Chapter 2
           Analog to Digital Conversion
•    ENOB:
•    Effective Number of Bits -        FFT
     display

•    Typical distribution of samples from an
     ENOB test. A software utility could be
     developed to perform this test

•    Software can calcluate SINAD with this
     data and then compute ENOB




    Return to Main Menu                        Figure 2.08
A / D Resolution
ENOB test example:
•   Install or plug the data acquisition system to be tested into a computer. Because tests involve
    the
•   entire board, not just IC-level subsystems purchased from other vendors, everything that can
    affect
•   performance is covered, including configuration of the subsystems, how they are
    interconnected and
•   operated.
•   2. Connect a test signal to one of the modules analog input channels. This signal should
    come from a precision generator, accurate to at least .001%, and providing a very low
    noise, very low distortion
•   sine wave. Any noise or distortion detected after the signal is acquired is contributed by the
    board or
•   module itself. Set the test signal’s voltage level to within 1 dB of the maximum input range of
    the
•   board. Set the frequency to a known value, typically 1kHz, providing a reference point when
    comparing different modules.
•   3. Ground a second input channel on the module. This represents the lowest signal level
    the module will see.
•   4. Make connections to the module using the cables and screw terminal panels
    recommended by the
•   manufacturer. The tests occur under real-world conditions, in the environment that you would
    run the system under normal test conditions.
•   5. Run a testing program that represents the most demanding input conditions the module is
    designed to see:
•   a. Operate the Data Acquisition system at full rated sample speed.
•   b. Alternate between sampling the test signal and the grounded input.
•   c. Going rapidly from no input to full-scale input at full rated throughput exposes slewing,
    bandwidth, and distortion errors, and fully exercises the analog input circuitry on the module.
•   d. Capture 1024 samples on each input channel. A large number of samples is required so the
    sophisticated FFT algorithm in the test program can compute ENOB with statistical accuracy.
•   e. Calculate and display ENOB – the number of bits of effective accuracy. A Programming
    interface software package such as Dasylab or Labview could be used to perfrom the FFT and
    calculate and display ENOB.
Return to Main Menu
•   The ratio of good samples to bad samples translates to the number of
    good bits versus bad bits to calculate the Effective Number of Bits.
    Chapter 2
    Analog to Digital Conversion
•      ENOB example:
•      From the acquired FFT of 1024 points of the test signal .
•      ‘A-signal’ represents the RMS value of the peak of the fundamental
       frequency.
•      ‘A-noise’ is the RMS value of each of the remaining points within
       half of the sample frequency range.
•      As en example, a tested Data Acquisition Board has a SINAD of 64
       dB when measuring a 1 MHz sinusoid at full scale range. Thus,
       effective number of bits for this signal is:




    Return to Main Menu
•
•
     ENOB:
     Effective Number of Bits
                                                    Chapter 2
•

•
     Histogram Display

     A histogram is the graphical version of a
                                                    Analog to Digital
     table which shows what proportion of cases
     fall into each of several or many specified
     categories
                                                    Conversion
•    Typical distribution of samples from an
     ENOB test on a histogram display.

•    This histogram illustrates how 12-bit ADC
     samples in a set were distributed among the
     various codes for a 2.5-V measurement in a
     range of 10 volts. Most codes ended up at
     1024 (2.5v) but others fell under a Gaussian
     distribution due to white noise content




    Return to Main Menu



                                                                        Figure 2.08
•    Real Histogram test from Labview
•

                                            Chapter 2
     Troubleshooting missing code problem




    Return to Main Menu
Chapter 2
Analog to Digital Conversion
•   The ENOB Test     •   Test Results
•      ADC                 Slewing
•      MUX                 Harmonic Distortion
•      PGA
                           Analog Circuits
•      SSH Amp
•      Cross talk
                           ADC Accuracy
•      FFT                 Noise Pickup
                           Channel Cross Talk
                           Nonlinearities
                           Channel Offset




Return to Main Menu
Chapter 2
Analog to Digital Conversion
     ADC Output Averaging

     •     Improved Accuracy

     •     More Stable Readings

     •     Gaussian Distribution




Return to Main Menu
 Chapter 2
 Analog to Digital Conversion
      ADC Output Averaging

     SOURCES OF NOISE

     - The Data Acquistion System itself.

     - SIGNAL WIRES are subject to EMI ((electromagnetic interference) (60Hz noise) and RFI (Radio frequency
          interference) (computer noise), both of which can induce noise on the wires carrying the transducer signal to
          the A/D board.

     - SENSOR NOISE - Sometimes it is necessary to electrically isolate the sensor from the measured medium


     TYPES of Noise

     - Periodic noise: (such as sine waves, especially 50 or 60 Hz noise)
Return to Main Menu
     - Random noise: broad range of frequencies of random amplitude – (Gaussian noise) (white noise)
   Chapter 2
   Analog to Digital Conversion
        ADC Output Averaging                         Averaging Types

Software running average: Reduces RANDOM noise using a smoothing factor controlled by you through the
      software can be increased or decreased to specify the number of actual waveform data points or samples that the

      moving average algorithm will use. A waveform can be thought of as a long string or collection of data points. The

      algorithm accomplishes a moving average by taking two or more of these data points from the acquired waveform,

      adding them, dividing their sum by the total number of data points added, replacing the first data point of the

      waveform with the average just computed, and repeating the steps with the second, third, and so on data points until

      the end of the data is reached. The result is a generated waveform consisting of the averaged data and having the same

      number of points as the original waveform.

- Noise is typically reduced by the square root of the number of averaged samples.
  Return to Main Menu
Remember -The sample rate has to be increased behind the scenes to accommodate this !!!!

             You cannot exceed the highest sample rate of the system
   Chapter 2
   Analog to Digital Conversion
         ADC Output Averaging

Averaging Types

Line cycle averaging

- A mode that is typically enabled in software which adjusts the selected sample rate of the data acquisition system to fit
      the number of samples taken perfectly within one cycle of either a 50 Hz or 60 Hz waveform (evenly spaced).

- synchronizes a series of readings with the power line frequency and averages the series—nulls out the effect of most AC
      line noise on the signals.




  Return to Main Menu
   Chapter 2
   Analog to Digital Conversion
         ADC Output Averaging

Averaging Types

Over Sampling (per channel)

- Forces a multiplexed data acquisition system to sit on a designated channel for X number of samples and
      then averages those samples together to create the measured value.


- Acts like a LPF because it will average out spikes or noise that would show up during high speed mux transitions.

- Increases settling time on each channel so that signal can stabilize.

- Very common with Sigma Delta ADC systems to increase accuracy by reducing noise. These systems are usually slower
       scanning systems that
   Return to Main Menu are designed for higher accuracy. Excellent for Thermocouples and slow changing voltage
       signals such as pressure sensors.
 Chapter 2
 Analog to Digital Conversion
      ADC Output Averaging




IOtech PersonalDaq/3000 – Software Running Average Example




 Return to Main Menu
Chapter 2
Analog to Digital Conversion


     Return To Main Menu

     Continue to Next Chapter




Return to Main Menu
Chapter 3
Multiplexing and Sampling Theory
•   The Economy of Multiplexing

•   Fundamental Concepts




Return to Main Menu
Chapter 3
Multiplexing and Sampling Theory
     •   Multiplexers
     •   Scans Numerous Channels
     •   Uses Single ADC
     •   Reduces Cost
     •   Reduces Acquisition Rate
     •   Will probably disappear in future due
     •      due to technology improvements and
     •      low cost A/D. You will have an A/D per channel




Return to Main Menu                                          Figure 3.01
Chapter 3
Multiplexing and Sampling Theory
     •   Mux With Selectable Gain


     •   Selects Channel
     •   Sets Gain
     •   All Channels Scanned at Same Rate
     •   Typical: 512 Scan/Gain combinations and each
         channel can have different gain




Return to Main Menu                                     Figure 3.03
Chapter 3
Multiplexing and Sampling Theory
•   Sample and Hold
•   Time Skew Problem
•   Sample & Hold Circuit
•   Works With Multiplexer
•   Instrumentation Amp
•   Low-Pass Filter
•   Sample & Hold Circuit
•   Low Impedance Amps




Return to Main Menu          Figure 3.05
Chapter 3
Multiplexing and Sampling Theory
•   Inadequate Nyquist sample rate
•   Nyquist = The Sampling Theorem states
    that a signal can be exactly reproduced if it
    is sampled at a frequency F, where F is
    greater than twice the maximum frequency
    in the signal. (usually 2.56 x)
•   This is important when selecting a data
    acquisition system for applications that
    involve waveform capture and dynamic
    signals.
•   Aliasing is a false waveform that appears
    in the data due to connecting too few
    sample points.




Return to Main Menu                                 Figure 3.06
Chapter 3
Multiplexing and Sampling Theory
    •   Acceptable Nyquist Sampling Rate

    •   Original Signal Preserved after being digitized by A/D




    •   IOtech PersonalDaq/3000 Aliasing live example

    •




Return to Main Menu                                              Figure 3.07
    Chapter 3
    Multiplexing and Sampling Theory
•      Mux Settling Time in multiplexed A/D systems

•      Source Impedance – The output impedance of the transducer or circuit you
•                 are connecting needs to have low impedance when interfacing to a DAQ system that
•          multiplexed without buffering.   - A good rule of thumb is to stay below 1 K ohms -
•
•          The multiplexer needs to discharge a small electrical build up charge back through
•          the signal connection              - Otherwise errors result in the measurement -

•          Systems with fast channel to channel scan times do not have enough settling time per channel to allow
•         for proper mux charge bleed off. Some systems allow you to increase the settling time per channel which
•                 will slow down the overall scan rate of the system (trade off)

•      Stray Capacitance – Likewise, connecting large external capacitance is not recommended

•      Frequency Range – High external R/C will also reduce frequency response with the mux
•      Buffer Amplifier – Most manufacturers of these types of DAQ systems will have a tech tip
•         on how to add your own external buffer circuit between your high R / C device and the
•         un-buffered DAQ system or have an add on module to add to the system with buffering




    Return to Main Menu
Chapter 3
Multiplexing and Sampling Theory


     Return To Main Menu

     Continue to Next Chapter




Return to Main Menu
Chapter 4
Electrical Measurements

     •   Valid Measurements
     •    DC Voltage
     •    AC Voltage
     •    Current
     •    Resistance
     •    Wheatstone Bridges
     •    Single-Ended & Differential Measurements
     •    Kelvin Connections




Return to Main Menu
Chapter 4
Electrical Measurements
•   Making Valid Measurements


•   Calibration – Factory and possible system cal in the field
•   Shielding – Twisted shielded pair with the shield tied to ground at only one end
•   Filtering – LPF if available set to just above the frequency of interest
•   Grounding – slight ground differences between unit under test and DAQ system are a
    setup for ground loops.




Return to Main Menu
        Chapter 4
        Electrical Measurements
    •     DC Voltage
    •     Instrument Level – most DAQ systems have max of + / - 10 v on their base voltage inputs.
    •                   Usually the mfg will offer additional add on signal conditioning that converts signal types out side of this range and then feeds them into the base + / - 10 volt
          inputs of the system.

    •                  Gain ranges can be set per channels from + / - 10 down to + / - 156mv

    •     High Voltage – Either voltage divider (be careful of high resistance on input)
    •         Or use a designated converter option or 5B module.

    •     Low Voltage – Remember ENOB and noise follow issues when trying to measure millivolt level signals. Usually the highest gain range on a base DAQ system is
          not high enough gain for transducers like strain gages and thermocouples which output millivolts

    •                  In this case a special high gain module needs to be used or 5B modules.
    •                  In some cases 22 bit Sigma Delta A/D systems can make these measurement if you are careful

    •     Range Switching – Most DAQ systems cannot change the range during a measurement.




Return to Main Menu
Chapter 4
Electrical Measurements
     •   AC Voltage
     •   Low Voltage
     •   pay attention to input channel range and p-p of your AC signal to measure
     •   Most DAQ systems will capture the actual waveform and will not report RMS
     •   5B modules can report RMS directly
     •   Some advanced software can convert to RMS on PC (Dasylab, Labivew)

     •
     •   High Voltage
     •   Divider circuit with Diff Buffers
     •   IOtech offers high voltage converter cards




Return to Main Menu                                                                  Figure 4.01
Chapter 4
Electrical Measurements
•   Shunt Resistor Current
    Measurement


•   4-20 mA Current Loop




Return to Main Menu          Figure 4.06
        Chapter 4
        Electrical Measurements
    •      Current Measurements
    •      4-20ma current loops – A common instrumentation signal that uses a changing current value to represent
           transducer or process values. A DAQ system can easily decode this by placing a 250 Ohm resistor across the
           HIGH and LOW inputs of a base differential voltage input. The DAQ system will read the voltage across the
           resistor and report a voltage between 1 volt and 5 volt which is directly proportional to 4 – 20 ma.
    •        - Differential inputs should be used so as to not accidentally tie the DAQ ground to a non ground part of the
           4-20ma loop.


    •      Common Mode Limits
    •       - If using non isolated inputs you must make sure that the common mode voltage to system ground
           does not exceed the specification of the DAQ system. This involves careful placement of the shunt
           resitor.
    •      - Isolated inputs such as 5B modules would eliminate this concern




Return to Main Menu
Chapter 4
Electrical Measurements
•   Data Acquisition Common Mode
    Voltage
•   Common Mode Limits
•        In most non-isolated DAQ systems, the
    voltage present from the + or – input of any
    channel in the system should never exceed
    the highest voltage that system can measure
    on its highest range – even though the signal
    between + and – of the differential input might
    be very small.
•   (actually it’s the + / - rails of the Op Amps in
    the system, however the measurement
    accuracy deteriorates as you approach the
    rails with common mode. Staying below the
    maximum range is safest
•    -Current shuts
•   - Batteries in series ****




Return to Main Menu                                    Figure 4.04
Chapter 4
Electrical Measurements
     •   Ground Referenced
         Differential Measurements
     •   Current Measurement
     •   Shunt Resistors
     •   Separate localized differential measurements across an individual shunt for
         each motor will eliminate the chance of error from the common ground point
         not being exactly the same exact ground potential at every motor location.
         Especially if there is long distance between the physical locations of each of
         these motors




Return to Main Menu                                                                       Figure 4.05
Chapter 4
Electrical Measurements
•   Current
    Transformers
•   Isolation – non contact
•   Current to Voltage Transformation
•   Large squeeze clamp that hangs
    around wire / cable.
•   Remember the output is a miniature
    version of the original sine wave
    and not an RMS value.

•   -   Often times the output of these
    are in the 100’s of millivolts range.
    Make sure your DAQ system has
    an ENOB to accurately reproduce
    the smallest value that you need to
    measure.




Return to Main Menu                         Figure 4.07
Chapter 4
Electrical Measurements
•   Ohmmeters
•   Current Injection
•   Digital Multimeters
•   Data Acquisition System Inputs




Return to Main Menu
Chapter 4
Electrical Measurements
•   Resistive Voltage Divider to measure
    resistance


•   Single-Ended Inputs or Differential-
    Ended Inputs
•   Precise resistor value for Known Rk
•   DAC output of DAQ system could
    supply known DC voltage
•   DAC voltage could be fed into
    another DAQ channel and measured
    to allow PC software to automatically
    compute and display Ru unknown
    resistance
•   This type of test fixture could be
    created very quickly using Dasylab
    icon based interface software




Return to Main Menu                         Figure 4.10
Chapter 4
Electrical Measurements
     •   Single vs. Differential Measurements


     •   Number of Channels – Differential channels use 2 each single ended channels os you end up with half as many total channels.
     •   Noise – Differential inputs inherently cancel out Common Mode Noise that is present on both input leads.
     •   Ground Loops – Differential input low side is not DAQ analog ground so there is no attempt by the measurement input circuit to tie independent grounds
         together.
     •             Single Ended inputs all reference the measurement to the same DAQ system analog ground which might be different than the remote
         measurement ground.
     •              There might also be unrealized small current paths when many channels are wired into the DAQ system and sharing the same common point.
         These cause ground loops.




Return to Main Menu
Chapter 4
Electrical Measurements
    •   Differential Thermocouple Measurements
    •   Common-Mode Voltages
    •   Long TC wire runs often pick up noise along the way through
        inductive coupling. Noise that is below the Common Mode limit of
        this system will be canceled out by the differential input.
    •   DC voltage that is present on the TC measurement surface can
        also be rejected up to the system CM limit.
    •   Try to use shielded TC wire or long runs




Return to Main Menu                                                        Figure 4.14
Chapter 4
Electrical Measurements
    •   Making small voltage measurement with large Common Mode




    •   IOtech PersonalDaq/3000 use thermocouple to measure temperature of 12 volts + battery
        terminal with same ground reference
    •   Common mode demo




Return to Main Menu
Chapter 4
Electrical Measurements
•   Differential Mode
    Noise Rejection


•   Large Common Mode
    Voltage
•   Phase and Amplitude
•




Return to Main Menu       Figure 4.16
Chapter 4
Electrical Measurements


     Return To Main Menu

     Continue to Next Chapter




Return to Main Menu
Chapter 5
Fundamental Signal Conditioning
•   Amplification
•   Instrumentation Amplifiers
•   Filtering
•   Attenuation
•   Isolation
•   Linearization
•   Circuit Protection




Return to Main Menu
Chapter 5
Fundamental Signal Conditioning
•   Data Acquisition
    Block Diagram
•   Multiple Input
    Channels
•   Multiplexer
•   Instrumentation
    Amplifier
•   Analog-to-Digital
    Converter (ADC)




Return to Main Menu        Figure 5.01
Chapter 5
Fundamental Signal Conditioning
•   Parasitic RC Time Constant
•   Accuracy
•   Source Resistance – In general, try
    to keep your connection impedance below
    1 K ohms when connecting to a non
    buffered DAQ – otherwise see if they
    have buffered options for high impedance
    loads

•   Sampling Rates – Some DAQ
    systems allow for oversampling which
    allows the mux to sit on a channel for a
    specified number of samples to allow
    settling




Return to Main Menu                            Figure 5.02
Chapter 5
Fundamental Signal Conditioning
•   Input & Source
    Impedance
•   Voltage Divider
•   Piezoelectric
    Transducers
•   Amplifier Reduces
    Error
•    - Specialized signal
    conditioners added on
    to DAQ system




Return to Main Menu         Figure 5.03A
Chapter 5
Fundamental Signal Conditioning
•   MUX Charge Injection Effects


•   Low Source Impedance
•   Voltage Spikes
•   Can carry over to adjacent
•    channels if high impedance load
•   Is connected




Return to Main Menu                    Figure 5.03B
Chapter 5
Fundamental Signal Conditioning
     •   Operational Amplifiers
     •   Inverting Op Amps
     •   Non-Inverting Op
         Amps
     •   Thermocouples
     •   Strain-Gage
         Bridges
     •   - Plug and Play signal conditioning cards
         are available for many DAQ systems




Return to Main Menu                                  Figure 5.04
    Chapter 5
    Fundamental Signal Conditioning
     •   Low-Pass Filters
     •   Low-Frequency Signals
     •   Filtered/Buffered Data Acquisition System
     •   Set just above the frequency you want to measure.
     •   Usually offered as an add on or plug in feature to DAQ systems.
     •   Some specific measurement fields require filtering for accurate
         and industry standard measurements such as vibration monitoring
         with accelerometers.




Return to Main Menu                                                        Figure 5.13A
Chapter 5
Fundamental Signal Conditioning
     •   Amplifier/Filter/MUX Combination
     •   Maximum Signal-to-Noise Ratio
     •   Common in higher end DAQ systems
     •   More expensive as a system but much easier to
         interface to transducers, much quieter readings, lower
         noise floor.
     •   IOtech Wavebook / Zonicbook




Return to Main Menu                                               Figure 5.13C
Chapter 5
Fundamental Signal Conditioning
     •   High-Pass Filter
     •   Low-Frequency Interferrence
     •   50/60 Hz Power Lines
     •   Also Blocks DC through capacitive coupling
     •   Present on higher end systems or as an add on feature
     •   Also known as AC coupling
     •   Some dynamic transducers require it such as accelerometers.
         Blocks DC offsets and only passes dynamic signals




Return to Main Menu                                                    Figure 5.14
Chapter 5
Fundamental Signal Conditioning
•   Optical Isolation Amplifier
•   LED Transmitter and
    Receiver Pair


•   Digital to Analog
    Conversion


•   No actual physical
    electrical connection
    between measured
    system and DAQ system




Return to Main Menu               Figure 5.23
Chapter 5
Fundamental Signal Conditioning
•   Transformer-Type
    Isolation Amplifier


•   Pulse Generator
•   Modulator-
    Demodulator
•   Isolated Grounds
•   5B modules use this type of
    isolation (show example)




Return to Main Menu               Figure 5.24B
Chapter 5
Fundamental Signal Conditioning
•   Isolated Sensor – Current
    Transformer
•   AC Voltage and Current

•   Ground Connections




      Ratio 1:1                 Ratio 2:1   Ratio 4:1




Return to Main Menu                                     Figure 5.27
Chapter 5
Fundamental Signal Conditioning
•   Isolated Sensors –
•   Strain Gage


•   Many physical measurement          Full-Bridge Circuit

•     transducers are inherently
•     electrically isolated
•    - more details in later chapter




Return to Main Menu                            Figure 7.01
Chapter 5
Fundamental Signal Conditioning
•   Overload
    Protection         Circuit A


•   Current Limiting
    Resistors
•   Diode and FET
    Voltage Clamps
•   Electrostatic
    Discharge          Circuit B
•   Shielding &
    Grounding
•   Humidity Control




Return to Main Menu                Figure 5.35
Chapter 5
Fundamental Signal Conditioning


     Return To Main Menu

     Continue to Next Chapter




Return to Main Menu
Chapter 6
Temperature Measurement
•   Thermocouple Basics


•   Thermocouple Measurement Pitfalls
•   RTD Measurements
•   Thermistor Measurements




Return to Main Menu
Chapter 6
Temperature Measurement
•   Thermocouple Basics


•   Type T Thermocouple Circuit

•   Dissimilar Metals

•   Reference Junction

•   Seebeck Coefficient




Return to Main Menu               Figure 6.01
    Chapter 6
    Temperature Measurement
    •   NIST Table
    •   Common Thermocouple Types




Return to Main Menu
Chapter 6
Temperature Measurement
•   Alternate
    Thermocouple Ice
    Bath

•   Copper Terminals
•   NIST EMF Output




Return to Main Menu       Figure 6.03
Chapter 6
Temperature Measurement
•   Chromel/Alumel
    Thermocouple


•   Copper Terminals
•   Ice-Bath
    Compensation




Return to Main Menu       Figure 6.05
Chapter 6
Temperature Measurement
•   Eliminating the Ice Bath


•   Isothermal Block




Return to Main Menu            Figure 6.06
Chapter 6
Temperature Measurement
•   Hardware Ice
    Bath
    Replacement


•   Generates Voltage
•   Electronic Ice-Point
    Reference




Return to Main Menu        Figure 6.07A
Chapter 6
Temperature Measurement
•   Hardware Ice Bath Replacement
•   Typical Commercial Scanning Module




       to Main 6.07B
Return Figure Menu
Chapter 6
Temperature Measurement
•   Thermocouple Linearization

•   Analog Linearization
•   Software Linearization




Return to Main Menu
Chapter 6
Temperature Measurement
•   Virtual Junction
•   Welded Junctions
•   Twisted Wire Junctions
•   Soldered Junctions
•   Insulation Resistance
•   Short circuit causes junction




Return to Main Menu                 Figure 6.09
Chapter 6
Temperature Measurement
•   Auto-Zero Correction


•   Shorted Channel
•   Drift
•   Reference Node
•   Stored Error Voltage




Return to Main Menu        Figure 6.10
Chapter 6
Temperature Measurement
•   Open Thermocouple
    Detector
•   Resistance Change
•   Value usually rails or
    some software will report
    an Open TC alarm
    message




Return to Main Menu             Figure 6.11
Chapter 7
Strain Measurements
•   Strain Basics
•   Strain Measurement Configurations
•   Calibration Approaches




Return to Main Menu
     Chapter 7
     Strain Measurements




     •   Wheatstone Bridge - Resistance Changes to Voltage
     •   Balanced Bridge - When all four resistors in the bridge are absolutely equal, the bridge is perfectly balanced and Vout = 0. This is rarely the case
         in a real world mounting of a strain gage. The DAQ system with strain gage capability will typically have an offset adjustment to allow you to balance
         the bridge as part of the calibration.
     •   When used with an instrument, a strain gauge replaces one or more of the resistors in the bridge, and as the strain gauge undergoes dimensional
         changes (because it is bonded to a test specimen), it unbalances the bridge and produces an output voltage proportional to the strain.




Return to Main Menu
      Chapter 7 Strain Measurement
•   Typical Strain Gage
•   Small Size – strain gages are paper thin membranes that mount flush on a material or structure in order to measure the amount of deformation due to an applied force .

•   Bonded to Specimen – Usually a specialized epoxy is used to adhere the gages to the material surface. Measurement accuracy can be affected by the quality of this
    mechanical bond.
•   Fragile Wiring Usually the epoxy is applied in a way so that it helps strain relief the small wires that are soldered in.
•   Strain Measurements - Gages change resistance at their output terminals when stretched or compressed.
•   Units of Measure - We need to remember that strain is the fractional change in length in a material when the material is stressed. It is normally measured in inches/inch
•      Normally, in most metals, for instance, the strain will not exceed .005 inch/inch
•      - The material will elongate no more than .5 inches in a 100 inch long piece of material


•   microstrain (µstrain)
      •      a common engineering unit measuring strain. An object under strain is typically deformed (extended or compressed), and the strain is measured
             by the amount of this deformation relative to the same object in an undeformed state. One microstrain is the strain producing a deformation of one
             part per million (10-6).




Return to Main Menu
     Chapter 7 Strain Measurement
     Strain Measurement

     Strain is a dimensionless unit, defined as a change in length per unit length. For example, if a 1-m-long bar stretches to 1.000002 m, the
          strain is defined as 2 microstrain (µε). Strain gauges have a characteristic gauge factor (value is included on the packaging for each
          gage when purchased), defined as the fractional change in resistance divided by the strain. For example, 2 µε applied to a gauge
          with gauge factor of 2 produces a fractional resistance change of 2 × 2 e10-6 = 4 e10-6, or 4 µΩ. Common gauge resistance
          values typically range from 120 to 350 Ω, but some devices can be as low as 30 or as high as 3 kΩ.




        To obtain accurate strain data, extremely small resistance changes must be measured. A Wheatstone bridge circuit is widely used to
         convert the gauge's microstrain into a voltage change that can be fed to the input of the A/D converter (ADC)




Return to Main Menu
        Chapter 7 Strain Measurements
•   Typical commercial Strain Gage Signal Conditioning
•   -   Adjustable high gain (much higher than base analog DAQ inputs), to allow for amplifications of small signal changes associated with strain gages. Gains of 1000 or more are
    typcial so that 1 millivolt on the bridge is presented as 1 volt to the DAQ analog input – A/D. Gain is adjustable to allow various bridge measurement ranges to be fitted to a base
    analog input range such as + / - 5 volts.
•   -   Offset adjustment compensates for unbalanced bridge issues. Lets you force zero strain to exactly zero volts.
•   -   Excitation adjustment. Usually 10 volts for 350 ohm gages for high signal output and 5 volts for 120 ohm gages to minimize self heating of the gage. Accurate measurements
    depend on a stable, regulated, and adjustable low-noise excitation source usually built into the strain gage conditioner.
•   -   Filtering – Low Pass filter can be engaged to block out high frequency noise that can make these small voltage readings appear jumpy. / High Pass filter (AC Coupling) available
    on some systems to allow for dynamic strain measurement such as dynamic pressure or acceleration. Static readings decay back to zero.
•   -   Calibrating – Lower cost systems are usually adjusted by a manual calibration process which involves turning knobs or potentiometers to adjust the above parameters. Higher
    end systems request setup parameter from the user through software menus and then performs the calibration automatically with out any manual adjustments by the user. This
    usually involves direct conversion from millivolts to any required Engineering units such as microstrain.
•
•




Return to Main Menu
     Chapter 7
     Strain Measurements




     •   Full Bridge Cofiguration
     •   Although half-bridge and quarter-bridge circuits are often used, the full bridge is optimal for strain gauges. This circuit has the highest sensitivity, the
         fewest error components, and the highest output that reduces the effects of noise on the measurements
     •   A full-bridge circuit contains four strain gauges mounted on a test member: two on the surface under tension and the other two on the opposite surface
         under compression. As the member deflects, the two gauges in tension increase in resistance while the other two decrease, unbalancing the bridge and
         producing an output proportional to the displacement.
     •   Vo = Vex / x        vo = bridge output, Vex = excitation, x = change in R




Return to Main Menu
Chapter 7
Strain Measurements
    •   Half-Bridge Circuit
    •   - More suitable for locations that can’t fit 4 gages.
    •   Uses Two Gages with two Bridge Completion
        Resistors
    •   Slightly more Non-Linear than full bridge
    •   More susceptible to temperature drift in data since
        resistors are separated
    •   Vo = Vex ( X / 2)
    •   Vo = bridge output, Vex = excitation, X = relative
        change is resistance




Return to Main Menu                                             Figure 7.02
Chapter 7
Strain Measurements
    •   Quarter-Bridge Circuit

    •   One Strain Gage and three completion resistors -
        easiest to setup, but hardest to measure with A/D
    •   Small voltage Output – noise is a potential problem
    •   Vo = Vex ( X / 4)
    •   Vo = bridge output
    •   Vex = excitation, X = delta R




Return to Main Menu
Chapter 7
Strain Measurements
•   Kelvin Strain-Gage
    Bridge Circuit


•   Excitation Voltage
•   Bridge Output Voltage
•   Voltage Errors
•   Improves accuracy




Return to Main Menu         Figure 7.04
    Chapter 7
    Strain Measurements
    •   Resistive Heating
    •   Resistive heating from the excitation source in strain gauges also should be considered because the gauges respond to temperature as well as
        stress. In most standard circuits, the heat that each gauge dissipates is <100 mW, so it's usually not a problem. This is especially true when the
        strain gauge is bonded to a material that conducts heat quickly, such as metal.

    •   However, because materials such as wood, plastic, or glass do not conduct heat away as rapidly, it is a good idea to use the lowest excitation
        voltage possible without introducing noise problems. Heat can become a problem when the strain gauges are uncommonly small, or numerous
        gauges occupy a limited space

    •   A good rule of thumb is 5 volts Exc with 120 ohm gages and 10v Exc with 350 ohm gages




Return to Main Menu
Chapter 7
Strain Measurements
•   Wheatstone-bridge
    Circuit

•   Common-Mode
    Voltage
•   Errors
•   Commercial signal
    conditioner will
    accommodate this and
    adjust out the error




Return to Main Menu        Figure 7.06
Chapter 7
Strain Measurements
     •   Shunt Bridge Circuit

     •   Shunt Resistors
     •   Software Control
     •   The most often used
     •     calibration method


     •   High precision resistor is used
     •   to simulate a known strain condition
     •   to allow the system to be calibrated


     •   Used when test specimen cannot be bent
     •   for calibration purposes.




Return to Main Menu                               Figure 7.07
Chapter 7
Strain Measurements


     Return To Main Menu

     Continue to Next Chapter




Return to Main Menu
Chapter 10
Noise Reduction and Isolation

•   Ground Loops
•   Shielded Wiring
•   Single Ended and Differential inputs




Return to Main Menu
    Chapter 10
    Noise Reduction and Isolation
     •   Interfering Ground Loop
     •   -   Long wire runs


     •   - Ground point at measurement
     •   system (SE) is not same potential as
     •   ground out at the sensor location
     •   Measured value has error
     •   because the single ended input is
     •   referenced to a shared ground with
     •   a power supply.




Return to Main Menu                             Figure 10.03
Chapter 10
Noise Reduction and Isolation
     •   Bypassed Ground Loop
     •   Run separate Power Leads
     •   Use Differential Inputs




     •   Differential input measurement
     •   is not directly tied to power supply
     •   ground




Return to Main Menu                             Figure 10.04
 Chapter 10
 Noise Reduction and Isolation

     A grounded signal source is one in which the voltage signals are
    referenced to a system ground, such as earth or building ground. Note
    that the negative terminal of the signal source shown above is referenced
    to ground. The most common examples of grounded signal sources are
    devices, such as power supplies, oscilloscopes, and signal generators that
    plug into the building ground through the wall outlet.

    The grounds of two independently grounded signal sources generally
    will not be at the same potential. The difference in ground potential
    between two instruments connected to the same building ground system
    is typically 10mV to 200mV, or even more



   Ungrounded or Floating Signal Sources
   A floating or ungrounded signal source is one in which the voltage signal
   is not referenced to a system ground, such as earth or building ground.
   Note on Figure 2 above that neither the positive nor the negative terminal
   is referenced to ground for the ungrounded source. Common examples of
   floating signal sources are digital multimeters, batteries, thermocouples,
   transformers, and isolation amplifiers.
Return to Main Menu                                                              Figure 10.04
 Chapter 10
 Noise Reduction and Isolation
      Measuring Grounded Signal Sources
      A grounded signal source is best measured with a
      differential input. There can exist up to 200 mV
      difference between two ground connections (Daq
      system and measurement). This difference causes a
      current called ground loop current to flow in the
      interconnection which can greatly affect
      measurements causing offset errors, especially
      when measuring low level signals from sensors.

      In a differential measurement system neither input
      to the instrumentation amplifier is referenced to a
      system ground

     Additionally, the shield wire from shielded cable can be
     tied to system ground at one of the cable ends. This will
     reduce noise in the measured signal even further.
Return to Main Menu
     Chapter 10
•
     Noise Reduction and Isolation
    Measuring Floating (Non-referenced) Sources

•      Floating signal sources can be measured with both differential and single-ended measurement systems. In the case of the differential measurement system,
    however, care should be taken to ensure that the common-mode voltage level of the signal with respect to the measurement system ground remains in the common-
    mode input range of the measurement device.

•       To anchor this voltage level to some reference, resistors are used. These resistors, called bias resistors, provide a DC path from the instrumentation amplifier inputs
    to the instrumentation amplifier ground. These resistors should be of a large enough value to allow the source to float with respect to the measurement reference and not
    load the signal source, but small enough to keep the voltage in the range of the input stage of the device. Typically, values between 10 kΩ and 100 kΩ work well with low-
    impedance sources such as thermocouples and signal conditioning module outputs. These bias resistors are connected between each lead and the measurement
    system ground. Failure to use these resistors may result in erratic or saturated (positive full-scale or negative full-scale) readings

•       A standard technique is to first try the measurement without the bias resistors and only engage them if determined to be needed.
•
•
•   The differential input would yield the quietest measurement due to its noise cancellation ability




Return to Main Menu
        Chapter 10
        Noise Reduction and Isolation
    •    SUMMARY:
    •    In general, a differential measurement system is preferable because it rejects not only ground loop-induced errors, but also the noise picked up
         in the environment to a certain degree. The single-ended configurations, on the other hand, provide twice as many measurement channels but
         are justified only if the magnitude of the induced errors is smaller than the required accuracy of the data. Single-ended input connections can be
         used when all input signals meet the following criteria.

    •    - Input signals are high level (greater than 1 V as a rule of thumb)

    •    - Signal cabling is short and travels through a noise-free environment or is properly shielded
    •     - All input signals can share a common reference signal at the source

    •    - Differential connections should be used when any of the above criteria are violated




Return to Main Menu
Chapter 10
Noise Reduction and Isolation
•   Single-Ended
    Measurement:
    Unprotected Wires

•   Shorted Inputs
•   Shielded Wires




Return to Main Menu        Figure 10.05
Chapter 10
Noise Reduction and Isolation
•   Single-Ended
    Measurement:
    Shielded Wires

•   Shorted Inputs
•   Shielded Wires




Return to Main Menu        Figure 10.06
Chapter 10
Noise Reduction and Isolation
•   Differential-Input
    Amplifiers

•   Algebraic Difference
•   Common Mode
    Signals




Return to Main Menu        Figure 10.17
Chapter 10
Noise Reduction and Isolation


     Return To Main Menu

     Continue to Next Chapter




Return to Main Menu
Return to Main Menu

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:34
posted:10/5/2011
language:English
pages:112