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Inductive Non-Contact Position/Displacement Sensing: Technology-Application-Options
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Inductive Non-Contact Position/Displacement Sensing:
Technology-Application-Options
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
Moderator Presenter
Leslie Langnau Design World
Dan Spohn Kaman Precision Products / Measuring
WELCOME Inductive Sensing Technology, Application Concerns, and Options
Non-contact, high precision, high resolution options: • Inductive • Laser • Capacitance
Linear Displacement Technologies
Linear Displacement Technologies
LVDTs25%
Encoders32%
Magnetostrictive9%
Potentiometers14%
Laser8%
Ultrasonic3%
Inductive6%
Capacitance3%
Conductive Target
Sensor
Cable
Oscillator
AC current Coil
EM field
AC “Eddy” current
Opposing EM field
Electronics
Linear Inductive Technology
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
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
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
Mounting
Performance
Range
Target
Speed Environment
Application Concerns
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
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
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
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
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
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
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
Typical error sources when applying inductive displacement sensors:
§ Electrical runout
§ Surface velocity
§ Nonlinearity
§ Thermal sensitivity
§ Cosine error
§ Cross axis motion
§ Inadequate target
Error Sources
§ 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
§ 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
§ 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
§ 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
§ 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
§ 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
§ 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
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
§ 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
§ 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
§ 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
§ 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
§ 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
§ 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
§ 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
§ 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
Engrave head feedback
§ Bridge circuit or phase circuit
§ Custom calibration, 8 mil offset, 5 mil range
§ Precise control of ink pocket depth
Example Application
Ammunition Primer Position
§ Multi-channel bridge circuit
§ Integrated automation
§ Go/No-Go detection of primer location in shell
Example Application
Thrust-bearing wedge measurement § Digital circuit
§ Highly customized
§ In-situ calibration
Example Application
§ Bridge Circuit
§ Customized open sensors
§ Positive and negative peaks on single output
Projectile velocity measurements
Example Application
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
Thank You q This webinar will be available at
designworldonline.com & email
q Tweet with hashtag #DWwebinar
q Connect with Design World
q Discuss this on EngineeringExchange.com