Inductive Non-Contact Position/Displacement Sensing:

Technology-Application-Options

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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 llangnau@wtwhmedia.com Phone: 216-860-5270 Twitter: @DW_3DPrinting

Dan Spohn Kaman Precision Products / Measuring dan.spohn@kaman.com Phone: 719-635-6957

Thank You q  This webinar will be available at

designworldonline.com & email

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