Inductive Non-Contact Position/Displacement Sensing: Technology-Application-Options

1. Inductive Non-Contact
Position/Displacement Sensing:
Technology-Application-
Options

2. Before We Start
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

3. Moderator Presenter
Leslie Langnau
Design World
Dan Spohn
Kaman Precision Products / Measuring

4. WELCOME
Inductive Sensing Technology,
Application Concerns,
and Options

5. Linear Displacement Technologies
Non-contact, high precision, high resolution
options: • Inductive • Laser • Capacitance
Linear Displacement Technologies
LVDTs
25%
Encoders
32%
Magnetostrictive
9%
Potentiometers
14%
Laser
8%
Ultrasonic
3%
Inductive
6%
Capacitance
3%

6. Conductive
Target
Sensor
Cable
Oscillator
AC
Coil current
EM field
AC “Eddy”
current
Opposing
EM field
Electronics
Linear Inductive Technology

7. 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

8. 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

9. 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)

10. Application Concerns
Performance
Mounting
Target
Range
Speed
Environment

11. 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 mils
Beryllium 4.3 1 12 mils 17 mils
Brass 7.4 1 16 mils 23 mils
Copper 1.7 1 9 mils 13 mils
Gold 2.35 1 9 mils 13 mils
Graphite 1050 1 192 mils 272 mils
Inconel 127 1 67 mils 95 mils
Silver 1.59 1 7 mils 11 mils
Titanium 113 1 63 mils 89 mils
Tungsten 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 mils
Carbon Steel 17.5 213 2 mils 3 mils
Chrome Steel 29 144 3 mils 4 mils
Cobalt 6.24 250 1 mil 2 mils
Cast Iron 65 5000 1 mil 2 mils
Molybdenum 5.17 100 1 mil 2 mils
Nickel 7.85 600 1 mil 2 mils
1030 Steel 14 400 1 mil 2 mils
4130 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

12. 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

13. 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

14. Application Concerns
Range
Inductance
Inductance Distance
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

15. 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

16. 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.

17. Performance
§ Analog outp uts 0-1VDC, 0-10VDC, +/-10VDC,
4-20mA
§ Typical resolution of analog bridge systems
0.01%
0.01%FS
§ 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)
Application Concerns

18. Error Sources
Typical error sources when applying inductive
displacement sensors:
§ Electrical runout
§ Surface velocity
§ Nonlinearity
§ Thermal sensitivity
§ Cosine error
§ Cross axis motion
§ Inadequate target

19. Error Sources
Electrical runout
§ 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

20. Surface velocity
§ Dependent on sensor diameter and oscillator frequency, 50 oscillator cycles/
coil window (sensor diameter)
§ As surface velocity reaches the limit, output will increase
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
RPM
Past
S.V.L.
Slower
Increases
Output
VDC
Decreases
Error Sources

21. Nonlinearity
§ Output deviation from a least squares fit straight line
§ Inherent in nearly all sensors
§ Different curve with different electronics
Bridge Circuits: KD-2306, KDM-8200,
Extreme
Colpitts Circuit: KD-2446
Phase Circuit: SMT-9700-9700
Error Sources

22. Thermal sensitivity
§ Output deviation due to temperature changes in the sensor coil
§ Can be seen as zero and/or slope shift
§ Electronics have separate sensitivity
Zero Shift
Slope Shift
Zero & Slope Shift
Error Sources

23. Cosine error
§ 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
B
A
C B
A
C D
Error Sources

24. Error Sources
Cross axis motion
§ 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.

25. Error Sources
Inadequate target
§ 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

26. Standard Options
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

27. Cable length
§ 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
Impedance is
a function of:
ü Inductance – L
ü Capacitance – C
ü Resistance – R
Longer
Cable
Length
Shorter
More
-Noise
-Thermal
Less
Standard Options

28. Oscillator frequency
§ 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.
Typical:
• 500 KHz
• 1 MHz
Optional:
• 2 MHz, 250 KHz.
Higher
Oscillator
Frequency
Lower
Thinner
Target
Thickness
Thicker
Standard Options

29. Standard Options
Temperature Compensation Calibration
§ 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

30. Standard Options
Special Calibration
§ 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

31. Standard Options
Microseal treatment
§ 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

32. Standard Options
Synchronization
§ 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

33. Log amp bypass
§ 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
Distance
Inductance
Inductance
Distance
Standard Options

34. Customizations & Specials
§ Cables
§ Electronics
§ Calibration
§ OEM/Private label
§ Packaging, board only
§ Event capture vs. displacement
§ Complete application specific custom solutions
§ Highly flexible, PUR jacketed, hard-line, in-line spices
§ Sensor body — Thread pitch, no threads, body length, custom housing

35. Example Application
Engrave head feedback
§ Bridge circuit or phase circuit
§ Custom calibration, 8 mil
offset, 5 mil range
§ Precise control of ink pocket
depth

36. Example Application
Ammunition Primer Position
§ Multi-channel bridge circuit
§ Integrated automation
§ Go/No-Go detection of primer
location in shell

37. Example Application
Thrust-bearing wedge measurement
§ Digital circuit
§ Highly customized
§ In-situ calibration

38. Example Application
Projectile velocity measurements
§ Bridge Circuit
§ Customized open sensors
§ Positive and negative
peaks on single output

39. Questions?
Leslie Langnau
Design World
llangnau@wtwhmedia.com
Phone: 216-860-5270
Twitter: @DW_3DPrinting
Dan Spohn
Kaman Precision Products / Measuring
dan.spohn@kaman.com
Phone: 719-635-6957

40. Thank You
q This webinar will be available at
designworldonline.com & email
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