Practical Aspects of Digital Data Acquisition (Understanding Sources of Error in the Measurement Chain)

 
Practical Aspects of Digital Data Acquisition
  (Understanding Sources of Error in the
            Measurement Chain)
Digital Data Acquisition Agenda

             1   Introduction

             2   Measurement Chain

             3   Sources of Error

             4   Filtering

             5   How Do I Read a Spec Sheet

             6   Advanced Topics
Signals and Processing

Signal: measurable quantity carrying information about some physical phenomenon
     Pressure, displacement, acceleration, …
     Temperature, voltage, biomedical potential (EKG, EEG, ...)

The signal is generated by a sensor or transducer

     Accelerometer:     acceleration     voltage
     Microphone:        pressure     voltage
     Strain Gauge:      strain (deformation)     voltage
     Thermocouple:      temperature changes       voltage

Signal will be digitized and stored has a time history

                                                         Analog Signal
Objective Avoid Bad Data
Digital Data Acquisition Agenda

         1           Introduction

         2           Measurement Chain

         3           Sources of Error

         4           Filtering

         5           How Do I Read a Spec Sheet

         6           Advanced Topics

          5 copyright LMS International - 2011
Measurement Chain

                                                  Analog Domain             Digital Domain

                         physically measured                                 Digital signal
                               quantity                     + noise         representation

                                   Sensor supply
 Structure

                                     Signal
                      Sensor       Conditioning      Gain        Alias      ADC          DSP
                                                               Protection

             sensor            Cable       Conditioner and        Filter        ADC            Calculation
                                            sensor supply                     accuracy
              noise            noise            noise             noise                          noise

                                             6
Digital Data Acquisition Agenda

             1          Structure

             2          Sensor

             3          Wiring

             4          Signal Conditioning

             5          Alias Filter

             6          Analog to Digital Converter

             7          Time File
             7 copyright LMS International - 2011
Structure

                                 • Sensors can go just about
                                   anywhere and measure just
                                   about any physical phenomena
                                    • The structure can have an
                                      effect on the measurement

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Structure Wind Turbines

      BLADES                                                          NACELLE
       & HUB                GEARBOX           GENERATOR
                                                                      & TOWER
• High power / high current /high frequency controlled electronics e.g.
• Long power wires from inverter at the bottom to the rotor of the generator at
  the top.
• Power cables to the grid in the ground have shown serious problems for long
  microphone cables when doing outdoor sound power.
• Metal structures that are controlled by electrical engines can also present a
  high frequency antenna that easily couples to strain gauges.
Structure Examples

    Easier           Harder
Potential Structure Issue Summary

Do you have to run long lead wires?
Does the structure have large electric currents?
Common Noise Generators:

Temperature changes?
Digital Data Acquisition Agenda

             1          Structure

             2           Sensor

             3           Wiring

             4           Signal Conditioning

             5           Alias Filter

             6           Analog to Digital Converter

             7          Time File
             12 copyright LMS International - 2011
Typical Sensors

   Wheel force           Stress/Strain           Acceleration              Pressure
                                                                          Microphones
                       Strain gages            DC-accelerometers
                                                                       Pressure transducer
                                               ICP-accelerometers

           WFT
                       • Tension
                       • Compression
 • Force Fx, Fy, Fz                                                   • Sound Pressure
                       • Torque                • Body accelerations
 • Moment Mx, My, Mz                                                  • Brake pressure
                       • Local stresses        • Subsystems
 • Angle, speed                                                       • Air pressure - dampers
 Force / Moment          Displacement            Temperature                Others
    Load Cells           String pots            Thermocouple          • RPM
  Torque Sensors           LVDT                                       • GPS – Global position
                                                                      • CAN signals
                                                                      • Video

                       • Absolute displ.
 • Engine torque       • Relative displ.       • Engine
 • Drive shafts        e.g. damper, bushings   • Gearbox
Active vs Passive Sensors

 “Passive” sensors add little or no noise to the measurement chain:
     Piezoelectric (charge) transducers
     Strain gauges

 “Active” sensors have on-board electronics that do add noise:
      ICP sensors

        PCB
       333Bxx

     DC accelerometers

   PCB
   3711B03

                          14
Isolated vs Non Isolated Sensors
A sensor is isolated form the structure when current cannot flow
between the sensor and structure
Sensors that are Electrically Isolated from the Structure
    Strain Gages are usually Isolated from Structure
    Some commercial sensors provide electrical Isolation
       • Accelerometers – PCB J Suffix
Sensors that are commonly not isolated from structure
    Thermocouple
    Some Accelerometers
    These sensors require floating or isolated grounds to prevent ground loops
When taking Operational Measurements it is recommended to Isolate sensors
from the structure
Ground Loops

• When installing accelerometers onto electrically conductive surfaces, there
  exists the risk of ground noise pick-up.
• If Sensor is grounded at a different electrical potential than the signal
  conditioning and readout equipment, ground loops can occur
• Noise from other electrical equipment and machines that are grounded to the
  structure can enter the ground path of the measurement signal through the
  base of a standard accelerometer
Possible Causes / How Do Ground Loops Appear in Data

                                             Although usually 50 or 60 Hz
                                             Ground Loop can be any frequency
                                             created large power devices like
                                             frequency controlled electric motors

                                            Possible sources are:
                                               • motors, pumps, generators,…
                                               • mains supply, electric power
                                                    to machines
                                               • other EM field sources

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How Do We Fix Ground Loops
•   The easiest solution is to break the ground loop is by electrically isolating or
    "floating" the accelerometer from the test structure:
•   In case no manufacturer provided isolation is available, one can make use of
    insulating materials.
       These include:
       • isolating tape
       • a piece of Bakelite
       • gluing paper between sensor and structure can already improve the situation
       substantially
Other Options to Deal With Ground Loops

• Other tips for decreasing ground loop effects:
   • If possible, make sure all parts of the loop are grounded on the same
   physical ground. This also includes the amplifier.
   • Try to switch off possible EMC sources in the vicinity of the test table.
   •Unplug data acquisition system from AC power
   •If using AC power make sure grounding plug is working
   •Use grounding strap
   •Use Isolated Ground Transformer
   •Use Clean Power – Orange plug

 • Try using a signal conditioner with a floating ground
Digital Data Acquisition Agenda

             1          Structure

             2           Sensor

             3           Wiring

             4           Signal Conditioning

             5           Alias Filter

             6           Analog to Digital Converter

             7          Time File
             20 copyright LMS International - 2011
Wiring

         •   Wiring can be complicated sometimes
         •   Connectors can be critical for field data acquisition
               • Rough environments can cause intermittent
                   connections
         •   Unshielded cable can act as antennas for Electro Static
             Interference
         •   Cables can act as low pass filters
         •   Cross Talk
Cabling Noise Sources – Electro-Magnetic Interference EMI

Electrostatic
    Electrostatic Fields - generated by the presence of
    voltage with, or without current flow
    Mechanism: capacitive coupling, by which charges
    of correspondingly alternating sign are developed
    on any electrical conductors subjected to the field
    Example Fluorescent Lights

Magnetic
    created either by the flow of electric current or by
    the presence of permanent magnetism
    In order for noise voltage to be developed in a
    conductor, magnetic lines of flux must be “cut” by
    the conductor
    Examples Electric Motors and Transformers
Countermeasure Electrostatic - Shielding

Transducer Lead wires become
antennas
The simplest and most effective barrier
against electrostatic noise pickup is a
conductive shield, sometimes referred to
as a Faraday cage
It functions by capturing the charges that
would otherwise reach the signal wiring
Must be provided with a low resistance
drainage path (ground)
Popular types of cable shields are
braided wire and conductive foil.
Countermeasure for Electro-Magnetic Noise

Electrostatic shield wires ineffective, requires different shielding principle that bends
or shunts the magnetic field
Ensure that noise voltages are induced equally in both sides of the amplifier input
     Common Mode Noise Reduction: covered in Signal Conditioning
The noise voltages (V1 and V2) induced in the signal wires will therefore depend
greatly upon their distances from the current-carrying conductors
     Twisting the signal conductors together tends to make the distances equal, on
     the average, thereby inducing equal noise voltages which will cancel each
     other.
Special attention required for cables running in parallel with high current lines.
(Current produces magnetic fields).
Cabling Best Practices

More twists per unit
length better

If must have excess
cable avoid coiling fold
instead
Long Wires on Strain Gauges

•    Lead-wire causes voltage drop – so you do not
     get requested excitation
•    This de-sensitizes the Bridge cause in an error
     in calibration (sensitivity)
•    Two Countermeasures:
        1. Lead wire compensation: measure lead
             wire resistance and mathematically
             calculate error in sensitivity
        2. Sense Line: Non current carrying line
             that allows system to adjust excitation
             voltage to ensure you get the specified
             excitation
•    When long reaches of multiple conductors are
     run adjacent to each other, problems with
     crosstalk between conductors can be
     encountered. With runs of 50 feet [15 m] or
     more, significant levels of noise can be induced
     into sensitive conductors through both magnetic
     and electrostatic coupling
        •    Countermeasure individually shielded
             pairs one pair for excitation, and one pair
             for the signal
Long Wires on Accelerometers
Capacitance in wire acts as low pass filter (rc-circuit)
Cable Length Example

                   Example:
                   •100 ft. cable
                   •capacitance of 30 pF/ft, the total
                   capacitance is 3000 pF.
                   •This value can be found along the
                        diagonal cable capacitance lines.
                   •Maximum output range of 5 volts
                   •constant current signal conditioner is set
                   at 2mA,
                   •The ratio on the vertical axis can be
                   calculated to equal 5.
                   •The intersection of the total cable
                   capacitance and this ratio result in a
                   maximum frequency of approximately
                       10.2 kHz
Frequency Dependent Calibration

                                                 FRF    Inverse FRF

Short Cable

Long cable

                                                       Impulse Response

                            Long cable compensated

              Supporting long cable or measurement distortion
                               compensation
Digital Data Acquisition Agenda

             1          Structure

             2           Sensor

             3           Wiring

             4           Signal Conditioning

             5           Alias Filter

             6           Analog to Digital Converter

             7          Time File
             30 copyright LMS International - 2011
Sources of Noise Electronics noise

Analog electronic circuits are built with resistors,
capacitors, inductors, semiconductors and
switches. Each electronic component has a
certain noise contribution

Signal conditioning and anti alias filtering adds
thermal noise

Sensor supply circuits can also produce noise

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Signal Conditioning - Components
 Signal conditioning means manipulating an analog signal in such a
 way that it meets the requirements of the next stage for further
 processing.
     Can Include AC Coupling - ICP
 Sensor Supply:
     Provides low noise excitation source either current or voltage to
     transducer
 Amplifier:
     Scales input voltages to input of ADC
     Single ended and Differential
Hardware AC Coupling Filter

                              freq (Hz)   Amplitude

                                   0.01         0.019996001
                                   0.02         0.039968038
                                   0.03         0.059892291
                                   0.04         0.079745222
                                   0.05         0.099503719
                                   0.06         0.119145221
                                   0.07         0.138647845
                                   0.08         0.157990501
                                   0.09         0.177152998
                                    0.1         0.196116135
                                    0.2         0.371390676
                                    0.3         0.514495755
                                    0.4         0.624695048
                                    0.5         0.707106781
                                    0.6          0.76822128
                                    0.7         0.813733471
                                    0.8         0.847998304
                                    0.9         0.874157276
                                      1         0.894427191
                                      2           0.9701425
                                      3         0.986393924
                                      4         0.992277877
                                      5          0.99503719
Instrumentation Amplifiers
Single-ended. An unbalanced input, non-isolated. Suitable for measurements where common mode
voltages are zero, or extremely small.
Differential. A balanced input, non-isolated. Suitable for measurements where the sum of common
mode and normal mode voltages remains within the measurement range of the amplifier.
       Helps common mode voltage noises
Single-ended, floating common. An isolated and quasi-balanced input (the floating common is
typically connected to the (-) input of a differential amplifier). Suitable for off-ground measurements up
to the breakdown voltage of the isolation barrier, and exhibits very good common mode rejection (100
db typical).
Differential, floating common. An, isolated and balanced input. Suitable for off-ground
measurements to the breakdown voltage of the isolation barrier, and exhibits superb common mode
rejection (>120 db).
Common Mode Noise Rejection

Cancels Common Mode Voltages: appears simultaneously and in phase on each of the
instrument's inputs with respect to power ground.
Common Mode Noise Rejection property of Differential amplifier typical spec 80dB
Example: Assume that you want to measure a 3VDC normal-mode signal in the presence
of a +6VDC CMV, and assume that
      the normal-mode signal gain is 1.
      80dB = 20 log (VCMV in / VCMV out)          Product Spec:
                                                  Common Mode Rejection (60Hz):
      80dB = 20 log (6VDC / VCMV out)             79dB@10V input range, 99dB@1V input
      4 = log (6VDC / VCMV out)                   range and 109dB@£100mV input range
      10,000 = 6VDC / VCMV out
      VCMV out = 0.6mV
Quarter vs Full Bridge

 • Full Bridge has common mode noise rejection because of 2 wire
   (differential) connection to amp
     • Differential Voltage
 • Quarter and Half Bridges have a one wire connection
     • Single ended Voltage

              Quarter Bridge                         Full Bridge

           36 copyright LMS International - 2011
Common Mode Noise Reduction

         1.00
                       F           AutoPow er Quarter Bridge      Curve 10.00 600.00 RMS Hz
       500e-3          F           AutoPow er Full Bridge

       300e-3
                                                                          0.02 9.34e-3 0.35 muE
       200e-3                                                           1.38e-3 1.01e-3 0.02 muE
       100e-3
        70e-3
        50e-3

        30e-3
        20e-3
 muE
 Log

        10e-3
         7e-3
         5e-3

         3e-3
         2e-3

         1e-3
       700e-6
       500e-6

       300e-6
       200e-6

       100e-6 10.00        60.00                                                                           600.00
             10.00    50           100        150         200   250   300    350   400   450   500   550   600.00
                                                                        Hz

                           37 copyright LMS International - 2011
Digital Data Acquisition Agenda

             1          Structure

             2           Sensor

             3           Wiring

             4           Signal Conditioning

             5           Alias Filter

             6           Analog to Digital Converter

             7          Time File
             38 copyright LMS International - 2011
Antialiasing Filter

 Purpose : To prevent folding of frequency content above Nyquist
 frequency into Measurement Bandwidth
                        Nyquist frequency
                          (Bandwidth)
                                     fs
                           f max =
                                     2

 LMS System prevents Aliasing by the combination of an Analog anti-
 aliasing filter and Over Sampling in the Sigma-Delta Converter
Scadas Anti-Aliasing FIlter
ALIASING

 Are you sure you’re getting
 what you think you’re getting?
Alisasing - Sampling = only look from time to time …

        ∆t
                         1
                    Fs =
                         ∆t

       Are you getting the right amplitude?
        Are you getting the correct frequency???
Beneficial use of Aliasing   Need to gemovie in

                                                  Something
                                                  strange?
                                                  Glass vibrates
                                                  at 608 Hz,
                                                  while we see it
                                                  vibrating at 2
                                                  Hz!
Sampling – Potential Source of Trouble
                   1.5

                     1

                   0.5

                     0

                   -0.5

                    -1

                   -1.5

                                 Sample     Actual  Observed
                                   Hz     Frequency Frequency
                                   100        25        25
                                   100        50        50      Nyquist Frequency   f max
                                   100        60        40
                                   100        75        25
                                   100       100         0
                                   100       125        25
                                   100       150        50
                                   100       160        40
                                   100       175        25
                                   100       200         0
“Observed”
frequencies f /2
             s

                                                                          True
                                                                          frequencies

              0           fs/2   fs                   2fs               3fs
How Do I Minimize Aliasing

Sample Extremely high: If there is no frequency
content above Nyquist Frequency then there is
no Aliasing
  This is not always practical or possible:
     • Large files sizes,
     • Limitations of data acquisition equipment
Anti-Aliasing Filter
  Low Pass Analog and Digital filters

           45 copyright LMS International - 2011
Practical Sample Rate Considerations

 Anti-aliasing filters are a necessary part of most data acquisition
 system systems.

 In order to ensure alias free data in the band of interest sample
 rates are often:
                          f s ≥ 2.5 f max

 This places the filter roll-ff outside the band of interest

 Example for 100 Hz Alias free band
                   250
 100 * 2.5 = 250 →     = 125 → 125 * .8 = 100 Hz
                    2
Digital Data Acquisition Agenda

             1          Structure

             2           Sensor

             3           Wiring

             4           Signal Conditioning

             5           Alias Filter

             6           Analog to Digital Converter

             7          Time File
             47 copyright LMS International - 2011
The Analog to Digital Converter (ADC, A/D or A to D)

Converts a continuous quantity to a discrete time digital
representational
      Typically device that converts an input analog
      voltage or current to a digital number proportional to
      the magnitude of the voltage or current
      Reverse is called Digital to Analog Converter (DAC)
Resolution of the converter indicates the number of
discrete values it can produce over the range of analog
values.
The values are usually stored electronically in binary
form, so the resolution is usually expressed in bits. In
consequence, the number of discrete values available,
or "levels", is a power of two

     http://en.wikipedia.org/wiki/Analog-to-digital_converter
Some ADC Formulas
The number of voltage intervals is given by

      where M is the ADC's resolution in bits.
The voltage resolution of an ADC is equal to its overall
voltage measurement range divided by the number of discrete
values:

      where M is the ADC's resolution in bits and EFSR is the
      full scale voltage range (also called 'span'). EFSR is
      given by

 # of Bits      # of Voltage steps (+/-10 Volts)/Bit
      8                255             7.81E-02
     12                4095            4.88E-03
     16               65535            3.05E-04
     24              16777215          1.19E-06
  http://en.wikipedia.org/wiki/Analog-to-digital_converter
ADC

 Not only the resolution, but also the type of
ADC can be crucial

 The ADC resolution (in # of bits) doesn’t always
reflect its dynamic range (in dB)

 In general, Σ∆ A/D converters are a good
compromise for NVH and durability test
scenarios
Types of ADC’s

A direct-conversion ADC
A successive-approximation ADC
A ramp-compare ADC
The Wilkinson ADC
An integrating ADC (also dual-slope or multi-slope ADC)
A delta-encoded ADC or counter-ramp
A pipeline ADC (also called subranging quantizer)
A sigma-delta ADC (also known as a delta-sigma ADC)
A time-interleaved ADC
An ADC with intermediate FM stage

http://en.wikipedia.org/wiki/Analog-to-digital_converter
ADC Errors
Aliasing. A precondition of the sampling theorem is that the signal be bandlimited. However, in
practice, no time-limited signal can be bandlimited. Since signals of interest are almost always time-
limited (e.g., at most spanning the lifetime of the sampling device in question), it follows that they are
not bandlimited. However, by designing a sampler with an appropriate guard band, it is possible to
obtain output that is as accurate as necessary.
Integration effect or aperture effect. This results from the fact that the sample is obtained as a time
average within a sampling region, rather than just being equal to the signal value at the sampling
instant. The integration effect is readily noticeable in photography when the exposure is too long and
creates a blur in the image. An ideal camera would have an exposure time of zero. In a capacitor-
based sample and hold circuit, the integration effect is introduced because the capacitor cannot
instantly change voltage thus requiring the sample to have non-zero width.
Jitter or deviation from the precise sample timing intervals.
Noise, including thermal sensor noise, analog circuit noise, etc.
Slew rate limit error, caused by an inability for an ADC output value to change sufficiently rapidly.
Quantization as a consequence of the finite precision of words that represent the converted values.
Error due to other non-linear effects of the mapping of input voltage to converted output value (in
addition to the effects of quantization).

http://en.wikipedia.org/wiki/Analog-to-digital_converter
Sigma Delta ADC Process

 24 Bit Sigma-Delta Converter Process on Scadas Mobile
     Over Sample - Max Sample Rate
         • 32 x 204.8 KHz = 6.5 MS/sec (MS= Million Samples)
     Decimate to ADC Rate 204.8 KHz
         • Resampling requires digital re-sampling filter to prevent aliasing
         • digital filter of 150dB/Oct roll-off
         • 100dB alias protection
         • Provides Alias free bandwidth of 92kHz -
     Further decimation to software selectable sample rate in steps of 2 and 2.5
         • Each decimation requires re-sampling filter
Decimation to User defined Rate
Phase Match Basics of Sine Waves - Phase

                 x(t ) = A sin( 2πft + θ )                                                         Phase Match: Maximum
                                                                                                   time stamp error of a
                                                                                                   given frequency sine
                                                                                                   wave at a specified input
       1.00
                                                                                                   range:
       0.70

       0.50

       0.20
                                                                                                           θ = 2πft
Real

       0.00
 /

       -0.30

       -0.50
                                                                                                           OR
       -0.80
       -1.00
               1.00
                      1.02 1.05
                       1.041.06 1.08 1.10 1.12 1.141.161.18 1.20 1.221.241.261.28 1.30 1.32 1.35
                                                                                                           θ = ωt
                                                       s
Channel to Channel Skew Eliminated

Simultaneous Sampling
   Eliminates time skew between
   channels
   Simplifies both time and
   frequency based analysis
   techniques

Multiplexed Sampling
   Channels are sampled
   sequentially
   May require software correction
   for detecting certain patterns
What is Quantization?

In analog-to-digital conversion, the difference between the actual analog
value and quantized digital value is called quantization error or
quantization distortion.
This error is either due to rounding or truncation. The error signal is
sometimes considered as an additional random signal called quantization
noise because of its stochastic behavior.
The continuous amplitude of the real time signal will be split up in discrete
levels (= 2NUMBER of BITS)  QUANTIZATION
Quantization refers to the precision of amplitude conversion

                                                                    Time
Properties of Quantization Error (Noise):

Random (stochastic)
      If not at max range
      Considered as an additional random signal called quantization noise because of its
      stochastic behavior
Proportional to input range
Proportional to the number of bits in the systems
At lower amplitudes the quantization error becomes dependent on the input signal, resulting
in distortion
In an ideal analog-to-digital converter, where the quantization error is uniformly distributed
between −1/2 LSB and +1/2 LSB, and the signal has a uniform distribution covering all
quantization levels, the Signal-to-quantization-noise ratio (SQNR) can be calculated from

        • Where Q is the number of quantization bits
        • The most common test signals that fulfill this are full amplitude triangle waves and
          saw tooth waves.
                               Ideal SQNR            dB
                                    12             72.24
                                    16             96.32
                                    24             144.48
Analog to Digital Converter (ADC)

        Voltage
Bits
        Levels
 8        256
12        4096
16       65536
24     16777216

Precision of
conversion is
controlled by the
number of bits of
resolution in the
Analog to Digital
Converter.
What does Quantization Noise look like

                                  Signal looks Blurry
                                  Bit Noise
What Can Causes a Quantization Noise - Underload

                                       Under-load AKA
                                       improper Input
                                       Range Setting
How to Mitigate Quantization Errors

    Optimize the Input Range on your acquisition system
Result of Optimized input Range

                                  Clean clear signal
                                  High dynamic
                                  Range
Instrument specifications

• System Specification Terms
   • Dynamic Range
      • Spurious Free Floor or Noise Floor
      • Spurious Free Dynamic Range
      • Signal to Noise
   • Harmonic Distortion
   • Common Mode Noise Rejection
   • Phase Match
                      Signal
   • Cross Talk     Conditioning Gain   Alias
                                      Protection
                                                 ADC          DSP

                        Conditioner and   Filter     ADC            Calculation
                         sensor supply             accuracy
                             noise        noise                       noise

                       64
Summary – Considerations for Getting a Better Measuement

 Measurement uncertainty in a function of the entire measurement chain
    Spec sheets quantify system performance only
 Ways to reduce noise and signal reduction:
    Differential amplifier – Common Mode Noise Rejection (CMR)
    Properly grounded measurement system
    Good anti-aliasing system                 5.40

                                                5
                                                                                                                                                                   1.00

    High # of bits in A/D                       5

                                                4
                                                                                                                                F           Time 1:+Z

    Cable shielding                             3

                                                3

    Good resampling filter                      3

                                                                                                                                                                   Amplitude
                                    Real
                                     g
                                                2

    Individual A/D per channel                  2

                                                1

    Set range correctly                    500e-3

                                                0

                                           -500e-3

                                             -1.20                                                                                                                 0.00
                                                     0.00   10e-3   20e-3   30e-3   40e-3   50e-3       60e-3   70e-3   80e-3       90e-3          100e-3   0.11
                                                                                                    s
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