Practical Aspects of Digital Data Acquisition (Understanding Sources of Error in the Measurement Chain)
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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 8 copyright LMS International - 2011
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 17 copyright LMS International - 2011
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 31
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 22.214.171.124 1.20 126.96.36.1991.28 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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