inductive non-contact position/displacement sensing: technology-application-options
DESCRIPTION
Inductive Non-Contact Position/Displacement Sensing: Technology-Application-OptionsTRANSCRIPT
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Inductive Non-Contact Position/Displacement Sensing:
Technology-Application-Options
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q This webinar will be available afterwards at www.designworldonline.com & email
q Q&A at the end of the presentation q Hashtag for this webinar: #DWwebinar
Before We Start
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Moderator Presenter
Leslie Langnau Design World
Dan Spohn Kaman Precision Products / Measuring
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WELCOME Inductive Sensing Technology, Application Concerns, and Options
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Non-contact, high precision, high resolution options: • Inductive • Laser • Capacitance
Linear Displacement Technologies
Linear Displacement Technologies
LVDTs25%
Encoders32%
Magnetostrictive9%
Potentiometers14%
Laser8%
Ultrasonic3%
Inductive6%
Capacitance3%
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Conductive Target
Sensor
Cable
Oscillator
AC current Coil
EM field
AC “Eddy” current
Opposing EM field
Electronics
Linear Inductive Technology
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Basic bridge circuit
§ Fixed crystal oscillator, typically 500KHz or 1MHz § Balanced bridge circuit, target motion imbalances bridge § Single or dual coil sensors § User calibration accessibility
Linear Inductive Technology
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Differential bridge circuit
§ Fixed crystal oscillator, typically 500KHz or 1MHz § Balanced bridge circuit, target motion imbalances bridge (twice the bridge imbalance per unit displacement over single ended) § Two single coil sensors § User calibration accessibility, but factory calibration typical
Linear Inductive Technology
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Phase circuits
§ Fixed crystal oscillator, typically 500KHz or 1MHz § Relies on coil impedance change, detection and demodulation in a phase detection circuit § Extraordinarily low noise circuit § No linearization circuitry § Can optimize for thermal stability or linearity (sacrificing the other)
Linear Inductive Technology
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Mounting
Performance
Range
Target
Speed Environment
Application Concerns
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Target material
§ Electrically conductive § Non ferrous (non-magnetic) § Ferrous (magnetic) § Lower resistivity is better § Thickness = 3 skin depths
Nonmagnetic Material
Electrical Resistivity (_ohm-cm)
Magnetic Permeability
Minimum Thickness
@1MHz
Minimum Thickness @500KHz
Aluminum 4.5 1 13 mils 18 milsBeryllium 4.3 1 12 mils 17 mils
Brass 7.4 1 16 mils 23 milsCopper 1.7 1 9 mils 13 mils
Gold 2.35 1 9 mils 13 milsGraphite 1050 1 192 mils 272 milsInconel 127 1 67 mils 95 milsSilver 1.59 1 7 mils 11 mils
Titanium 113 1 63 mils 89 milsTungsten 5.15 1 14 mils 20 mils
304/316 SS 72 1.02 50 mils 71 mils
Magnetic Material
Electrical Resistivity (_ohm-cm)
Magnetic Permeability
Minimum Thickness
@1MHz
Minimum Thickness @500KHz
17-4 PH SS 100 151 5 mils 7 milsCarbon Steel 17.5 213 2 mils 3 milsChrome Steel 29 144 3 mils 4 mils
Cobalt 6.24 250 1 mil 2 milsCast Iron 65 5000 1 mil 2 mils
Molybdenum 5.17 100 1 mil 2 milsNickel 7.85 600 1 mil 2 mils
1030 Steel 14 400 1 mil 2 mils4130 Steel 65 450 1 mil 2 mils
Skin depth is the depth into the target material at which the current induced is ~36% of that at the surface.
Application Concerns
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Target size and shape § Diameter sufficient to engage entire
field produced by sensor
§ 1.5X to 2X sensor diameter for shielded sensors
§ 2.5X to 3X sensor diameter for unshielded sensors
§ Surface finish of 32 is sufficient for accurate measurements
§ Cylindrical targets (rotating shafts) OK if diameter is 8x probe tip
Application Concerns
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Environment
§ Changes in the sensor temperature cause changes in the coil resistance which changes the output
§ Most sensor are not suitable for pressure barriers, exception is the extreme environment sensor line
§ Fluids will not typically affect the sensor performance
§ Extreme vibration is not recommended without customization
§ Electro-magnetic interference (EMI) can affect performance
Application Concerns
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Range
Distance Inductance
Indu
ctan
ce
Distance
§ Proportional to coil diameter, typically 25% - 35%. Up to 50% with larger sensors
§ Standard published ranges are set to meet published performance specs
§ Longer (1.5X) or shorter (0.5X) calibrated ranges are possible, but typically with negative affects on linearity and stability
Application Concerns
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Mounting
§ A physically and thermally stable sensor mounting design is best
§ Eliminate cantilevers, ensure parallelism
§ Use low thermal expansion materials
§ Avoid side loading
§ Synchronize multiple sensors in close proximity
Application Concerns
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Speed § Reciprocating targets show a decrease in
amplitude as the target frequency approaches –3dB point.
§ Rotating targets show an increase in output as surface velocity limits are reached.
§ Analog systems typically offer 50KHz frequency response.
§ Can open up to >100KHz with decrease in resolution.
§ If target speed is slow, filter to lower frequency response and improve resolution.
Application Concerns
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Performance § Analog outputs 0-1VDC, 0-10VDC, +/-10VDC,
4-20mA
§ Typical resolution of analog bridge systems 0.01%
§ 0.001% is achievable with pulse width demodulated systems by sacrificing other specifications
§ Linearity specs use the least squares method, 0.5% to 1% typical
§ Thermal sensitivity 0.1% typical, 0.02% with temp comp cal
§ System accuracy is not specified 4 x 10-9 x bandwidth (inches)
0.01%FS
Application Concerns
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Typical error sources when applying inductive displacement sensors:
§ Electrical runout
§ Surface velocity
§ Nonlinearity
§ Thermal sensitivity
§ Cosine error
§ Cross axis motion
§ Inadequate target
Error Sources
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§ Only seen with ferrous (steel) targets
§ Caused by minor changes in conductivity/permeability in ferrous targets
§ Worse with small sensors and high oscillator frequencies
§ Reduce the effect by
§ Using larger diameter sensors
§ Averaging the output
§ Key phasor sensor and map the electrical runout, extract from run data
Electrical runout
Error Sources
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§ Dependent on sensor diameter and oscillator frequency, 50 oscillator cycles/coil window (sensor diameter)
§ As surface velocity reaches the limit, output will increase
Surface velocity
Calculating surface velocity….. SV = π x diameter (inches) x rpm / 60 Ex: 18-in diameter @ 500 rpm 3.1416 x 18 x 500 / 60 = 471 in/sec Minimum sensors diameter…. (SV (ips) / oscillator frequency Hz) / 0.02 Ex: (471 / 500,000) / 0.02 = 0.047-in diameter
Faster
Slower
RPM Past
S.V.L.
Increases
Decreases
Output VDC
Error Sources
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§ Output deviation from a least squares fit straight line
§ Inherent in nearly all sensors
§ Different curve with different electronics
Nonlinearity
Bridge Circuits: KD-2306, KDM-8200, Extreme
Colpitts Circuit: KD-2446
Phase Circuit: SMT-9700-9700
Error Sources
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§ Output deviation due to temperature changes in the sensor coil
§ Can be seen as zero and/or slope shift
§ Electronics have separate sensitivity
Thermal sensitivity
Zero Shift Slope Shift
Zero & Slope Shift
Error Sources
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§ Primarily due to displacement differences, based on pivot location
§ 1 to 2 degrees can be ignored; more should be addressed
§ Calibration in-situ (or mocked up) will minimize the error
Cosine error
A B
C D
B A
C D
Error Sources
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§ A concern when flat target diameter is not optimum.
§ 2.5X to 3X for unshielded
§ 1.5X to 2X for shielded sensors
§ A concern when cylindrical shaft diameter is not at lease 8X that of the sensor diameter.
Cross axis motion
Error Sources
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§ Poor electrical conductivity
§ Less than nominal diameter
§ Plated with a different material
§ Not continuous (segmented or porous)
Inadequate targets result in less sensitivity, less resolution
If unavoidable, tune and calibrate with the actual target material
Inadequate target
Error Sources
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Inductive displacement sensors can be customized. Many standard options are available:
§ Cable length
§ Oscillator frequency
§ Temperature compensation calibration
§ Special calibration
§ Microseal treatment
§ Synchronization
§ Log amp bypass
Standard Options
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§ Higher oscillator frequency = shorter cables Lower oscillator frequency = longer cables
§ Larger sensors = longer cables Smaller sensors = shorter cables
§ 1MHz oscillator 30ft max
§ 500kHz oscillator 50ft max
§ Longer cables give more thermal sensitivity
§ Longer cables are more susceptible to cable motion noise
§ Shorter cables give better overall performance
Cable length
Impedance is a function of:
ü Inductance – L ü Capacitance – C ü Resistance – R
Longer
Shorter
Cable Length
More
Less
-Noise -Thermal
Standard Options
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§ Certain sensors operate best at lower or higher frequencies.
§ Increasing oscillator frequency improves surface velocity limits.
§ Lower oscillator frequencies increases skin depth.
§ Lower oscillator frequencies allow longer cable lengths.
§ Higher oscillator frequencies decreases skin depth.
§ Changing oscillator frequency can influence thermal sensitivity.
Oscillator frequency
Typical: • 500 KHz • 1 MHz
Optional: • 2 MHz, 250 KHz.
Higher
Lower
Oscillator Frequency
Thinner
Thicker
Target Thickness
Standard Options
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§ Standard option for KD-2306, KDM-8200
§ Standard with Extreme Environment systems
§ Trade off with linearity with the SMT-9700
§ Reduces thermal sensitivity by ~ 1 order of magnitude
§ Standard temperature compensation is over 100°F range, upper limit <150°F
§ Options, >100°F range, >150°F upper limit
Temperature Compensation Calibration
Standard Options
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§ Non-standard ranges — .5X to 1.5X
§ SMT-9700, KD-5100, DIT-5200 — very short ranges possible (± 25 micron)
§ Non-standard target material — 304SS, Titanium, Beryllium, etc.
§ 6061 aluminum nonferrous systems, 4130 steel for ferrous systems
§ Special fixturing
§ Customer supplied special targets, shape, plating
§ Bipolar outputs
§ High gain outputs
Special Calibration
Standard Options
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§ Epoxy dip
§ Coats sensor face, wicks into pores and micro cracks, crevices
§ Inhibits absorption of moisture into sensor body
§ NOT waterproofing
§ Recommended for applications that get washed down or intermittently sprayed with fluids
Microseal treatment
Standard Options
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§ Oscillator from one channel excites all sensors that are synchronized
§ Prevents beat note interference when two sensors are mounted close enough that their fields interact
§ Standard with the KDM-8200 when installed in a rack or NEMA enclosure
§ Auto synchronization for the KD-2306
§ Not available with KD-2446
Synchronization
Standard Options
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§ When extremely short range calibrations are required of linearized systems, the log amp is bypassed, because over such a short range, the sensor is inherently linear
§ Available on bridge circuits
§ Not available on colpitts circuits
§ Not required for differential or phase circuits
Log amp bypass
Distance Inductance
Indu
ctan
ce
Distance
Standard Options
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§ Complete application specific custom solutions
§ Highly flexible, PUR jacketed, hard-line, in-line spices
§ Sensor body — Thread pitch, no threads, body length, custom housing
§ Cables
§ Electronics
§ Calibration
§ OEM/Private label
§ Packaging, board only
§ Event capture vs. displacement
Customizations & Specials
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Engrave head feedback
§ Bridge circuit or phase circuit
§ Custom calibration, 8 mil offset, 5 mil range
§ Precise control of ink pocket depth
Example Application
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Ammunition Primer Position
§ Multi-channel bridge circuit
§ Integrated automation
§ Go/No-Go detection of primer location in shell
Example Application
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Thrust-bearing wedge measurement § Digital circuit
§ Highly customized
§ In-situ calibration
Example Application
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§ Bridge Circuit
§ Customized open sensors
§ Positive and negative peaks on single output
Projectile velocity measurements
Example Application
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Questions? Leslie Langnau Design World [email protected] Phone: 216-860-5270 Twitter: @DW_3DPrinting
Dan Spohn Kaman Precision Products / Measuring [email protected] Phone: 719-635-6957
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Thank You q This webinar will be available at
designworldonline.com & email
q Tweet with hashtag #DWwebinar
q Connect with Design World
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