1

Mechanical Sensors

• Introduction• Position Sensors

–Linear–Rotary–Capacitive

• Level Sensors–Thermal–Ultrasound

• Stress & Strain• Accelerometers• Piezoelectric Effect• Microphones• Pressure sensors• Flow sensors• Doppler Effect• Hall Effect

2

Breadth and Depth of this one subject

Sensors Volume 7

3

Manufacturers of Mechanical

Sensors

http://www.sensorsportal.com/HTML/SENSORS/MechSens_Manuf.htm

http://www.eurosensors.cz/techprogramme.html

4

Position sensors

• Potentiometric• Capacitive/Inductive• Variable Reluctance• Level

– mechanical– electrical– ultrasonic– pressure

Typical Applications: •Ignition timing •Power sensing •Valve position •Robotics control •Current sensing •Linear or rotary motion detection •Length measurement •Flow sensing •RPM sensing •Security systems

http://content.honeywell.com/sensing/prodinfo/solidstate/

5

Linear Transducers

5 kohms

6 to 48 inches extension can be purchased.

6

Longfellow applications

Printing press roll alignment for multicolored images: When you see a clean, clear, sharp, multicolored image in your newspaper,it may have been controlled by a three (3) inch Short Longfellow. Many printing rolls use Short Longfellows to provide the control signal for continuous rapid dynamic alignment.

Injection molding machine clamping, injection, and ejection control: This process is set up as closed loop feedback and monitored continuously. The location of the moving mold head, the auger feeding the plastic, and the ejector position are all monitored by continuous output potentiometer position sensors. The rugged LONGFELLOW and SHORT LONGFELLOW position sensors are used extensively on machines world wide.

7

Capacitive Sensors

Capacitive sensors and flexure stage with integrated capacitive sensors

http://www.polytecpi.com/capsens.htm

Ranges up to 300 µm with 0.1 nm resolution. Integrated Linearization system (ILS) provides 0.05% linearity. Features of •Highest resolution (<0.1 nm) of commercially available displacement sensors. •Measuring ranges of 15, 50, and 100 µm (45, 150, and 300 µm with slightly reduced accuracy). •Extremely high long term stability (<0.1 nm / 3 hours). •Up to 3 kHz bandwidth •Invar versions for highest temperature stability (5 X 10-6 / °K) •Stand-alone and modular electronics

8Measuring

Capacitance

d

AKC 0

K = dielectric constantε0 = permittivity = 8.85 pF/meterA = area of one plated = distance between plates

d A

Film-Foil Capacitor Contstruction

9

Measuring Capacitance

There are many ways to measure capacitance. One common way involves putting a current into the object (or a commercial capacitor), and seeing how long it takes to reach a certain voltage. The longer it takes to reach a particular voltage, the higher the capacitance.

Another way is to use a bridge circuit, which work a lot like a balance-beam for capacitance.

dttiC

v

dt

tdvCi

CC

CC

)(1

)(

CjZC

1

The Derivative of a function is its SLOPE

The Integral of a function is its AREA

10AC Bridge

http://www.tpub.com/neets/book16/66e.htm

Z1

Zz

Z2

ZS

Z1ZX=Z2Z3

S

X

SX

SS

XX

C

C

R

R

then

RRRR

ifandoutthisMultiply

CjRR

CjRR

2

1

21

21

____

)1

()1

(

Using the designations in the figure:

11

Turck Capacitive Sensorhttp://www.turck-usa.com/literature/index.htm

Applications•Liquid Level Control for both explosive and non-explosive materials.•Package Inspection for product content and/or fill level.•Wire-Break Detection for wire sizes down to .003".•Plastic Pellet Detection in a hopper for injection molding processes.•Grain or Food Products Level Detection; intrinsically safe models available.•Small Metal Parts Detection; greater sensing range than comparable inductive sensors.

12Turck Capac. Sensor

The active element is formed by two metallic electrodes positioned much like an “opened” capacitor (Figure 1). Electrodes A and B are placed in a feedback loop of a high freq. oscillator. When no target is present, the sensor’s capacitance is low & the oscillation amplitude is small. When a target approaches the face of the sensor, the capacitance increases. This results in increased amplitude of oscillation. The amplitude of oscillation is measured by a circuit that generates a signal to turn on or off the output (Figure 2).

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A capacitive level sensor

http://www.americanmagnetics.com/level/sensors.html

The capacitance-based liquid level sensor is typically manufactured of 3/8 inch OD stainless steel tubing and will operate with virtually all cryogenic liquids including nitrogen, carbon dioxide, liquified natural gas, argon, neon, and other cryogenics. Upon request, special assembly techniques can be applied for sensors required for liquid oxygen or hydrogen measurement.

Cryogenic: of or relating to the production of very low temperatures

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Other Level Sensors

• Thermal Differential• Mechanical• Electrical• Ultrasonic• Pressure RTD’s

TempOfLiquid

TempOfGas

15

Thermal Differential Sensor

• Thermal point level switch for the detection of level in all vessels.

• Wide operating temperature range -100°F to +850°F.

• Switch on level change of .03 inch without concern for changing temperature, density, detective constant or chemical composition.

• Removable, plug-in electronics

• Free of all moving parts that can stick, coat or fail.

• Fast response time of .1 to 1 second on wetting, media dependent.

http://www.sag-automation.com/ls3100.html

Response Time: The time a systemor functional unit takes to react toa given input.

http://www.its.bldrdoc.gov/fs-1037/dir-031/_4584.htm

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Thiokol Propane fuel level sensors

Thiokol Propulsion's electronic propane fuel level sensors can be designed for a variety of tank configurations, either for an infinite variety of vehicles or stationary storage and dispensing tanks. Most of the issues facing the emerging alternate-fuel vehicle market revolve around driver convenience.

Fuel Level Sensor

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Ultrasonic Sensor

Applications

• Non-contact level measurement

• Tank level measurement

• Proximity/distance measuring

• Snow depth sensor

• Operates by transmitting a series of ultrasonic sound waves in a cone shaped pattern• Sound waves reflect from target back to sensor, which measures the time interval between transmitting and receiving the sound wave.• Calculated by the speed of sound, the time interval is converted to a distance measurement.• 18 programmable modes• Can be adjusted to meet the requirements of various monitoring sites• Programmable modes include sensitivity, calibration, signal averaging, and pulse control.

http://www.sutron.com/products/sensors/waterlevel/5600-0157.htm

18Ultrasonic Example

Range: Compact and Modular models as close as 5 cm (2 inches) and as far as 1128 cm (37 feet), other models vary.

Power Input: DC all models, AC input options available for Modular sensors

Beam Angle: Conical shape, 15 degrees total angle unless otherwise noted

Adjustment: All models are push-button adjustable for basic setup. Compact and Modular sensors are additionally PC configurable using SoftSpan™.

Update Rate: 20 Hz nominal, adjustable from 2 to 120 Hz on PC-configurable models Ultrasonic Frequency: 50 kHz

http://www.ultrasonicsensors.com/technolo.htm

19Ultrasonic Example

http://www.measure.demon.co.uk/Acoustics_Software/speed.html

Speed of sound at sea level 250 Cand 25% humidity: 346.71 M/sec

20 feet

sec03807.40sec

1051

1sec

105112

137.36

sec71.346

ftft

ft

in

ft

m

inm

20Stress and Strain

2

11

M

NPascal

L

L

http://fermi.bgsu.edu/~stoner/p201/shm/sld002.htm Force

Force0

02l

lRR

A

lR

GNDDMM

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Wheatstone Bridge Strain Measurement

http://www.tokyosokki.co.jp/e/product/strain_gauge/what_strain.html

The output voltage of a bridgecircuit is given as follows.

http://bits.me.berkeley.edu/beam/sg_1.html

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Load Cell Principles

• A load cell is a force transducer. This device converts force or weight into an electrical signal.• The strain gage is the heart of a load cell. A strain gage changes resistance when it is stressed. These gages are developed from an ultra-thin heat-treated metallic foil and are chemically bonded to a thin dielectric layer. "Gage patches" are then mounted to the strain element with specially formulated adhesives. •The precise positioning of the gage, the mounting procedure, and the materials used all have a measurable effect on overall performance of the load cell.

Multiple strain gages are connected to create the four legs of a Wheatstone-bridge configuration. When an input voltage is applied to the bridge, the output becomes a voltage proportional to the force on the cell. This output can be amplified and processed by conventional electrical instrumentation.

Force

23Load Cellhttp://www.massload.com/brochure_-_ml200.pdf

Capacities 250 lbs. to 15,000 lbs. Safe Overload 150 % Full Scale Output 3 mv/v + or - 0.25% Non- Linearity < 0.03 % FS Hysteresis < 0.02 % FS Non Repeatability < 0.01 % FS Thermal Sens. Shift 0.0008% of readingThermal Zero Shift 0.0015% FS/deg. F Bridge Resistance 350 ohms

Maximum Excitation: 15V DC

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Position, Velocity, &

Acceleration

elerationangularaccdt

d

ocityangularveldt

d

itionangularpos

onacceleratidt

xda

velocitydt

dxv

positionx

2

2

2

2

dt

dvdt

dv

tdt

tdv

ttv

F

)0(

)0cos()0(

)cos()(

)sin()(

2

25

Integral Relations

onacceleratia

adttav

vdttvx

)0()(

)0()(

xvtVtdttVolt

vVtdttVolt

accelttVolt

)0()0()sin(1

)(

)0()cos(1

)(

.)sin()(

2

Newton’s LawF=Ma

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Racing Accelerometer

When it comes to being first over the finish line, Entran's MotorSport Racing Accelerometers have been there time and again. Entran's more than 20 years of experience in Formula I racing have produced an accelerometer series that has been custom tailored to the needs of Auto Racing Teams and can withstand the rigors and tough environment of motorsports. With integral overrange stops to survive high vibration levels, onboard EMI/RFI filtering for a clean signal, high level viscous damping to eliminate resonance, all in a compact size, the EGRH provides a winning performance for numerous measurements throughout the vehicle. The EGRH is part of Entran's large family of MotorSport Racing Sensors for F1, GTS, ALMS, CART, IRL and NASCAR.

http://www.entran.com/egr.htm

27Piezoelectric Effect

The Piezoelectric effect is an effect in which energy is converted between mechanical and electrical forms. It was discovered in the 1880's by the Curie brothers. Specifically, when a pressure (piezo means pressure in Greek) is applied to a polarized crystal, the resulting mechanical deformation results in an electrical charge. Piezoelectric microphones serve as a good example of this phenomenon. Microphones turn an acoustical pressure into a voltage. Alternatively, when an electrical charge is applied to a polarized crystal, the crystal undergoes a mechanical deformation which can in turn create an acoustical pressure. An example of this can be seen in piezoelectric speakers which are the cause of those annoying system beeps that are all too common in today's computers. http://ccrma-www.stanford.edu/CCRMA/Courses/252/sensors/node7.html

http://www.imagesco.com/articles/piezo/piezo01.html

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Microphone Types

• Carbon: Carbon microphones are made by encasing lightly packed carbon granules in an enclosure. Electrical contacts are placed on opposite sides of the enclosure. When an acoustical pressure is exterted on the carbon granules, the granules are pressed closer together which decreases the measured resistance. This is a very low quality acoustic transducer, but is still used in telephone handsets.

• Capacitor (condenser): Capacitor microphones are made by forming a capacitor between a stationary metal plate, and a light metallic diaphragm. When an acoustical pressure impinges on the diaphragm, the diaphragm moves and causes the distance between it and the stationary plate to change. This changes the capacitance of the device. To measure the capacitance, one must apply a current. When this is done, the change in capacitance will result in a change in the voltage measured across the device.

• Electret and Piezoelectric: Electret microphones are capacitor microphones which use an electret material between the plates of the capacitor. Electrets are materials with a permanent polarization, and hence surface charge. Many high quality, low cost electret microphones are available currently. As discussed previously, piezoelectric crystals are crystalline structures which are similar to electrets in that they exhibit a permanent polarization of the individual cells. It is possible to use piezo sensors as microphones as well. Since they are in the form of a thin film, they are very useful if one is interested in detecting surface vibrations of an object.

• Magnetic (moving coil): Moving coil, or dynamic microphones are based upon the principle of magnetic induction. When an electrical conductor is moved through an electric field, a voltage is produced. This voltage is proportional to the velocity of the conductor. A moving coil microphone is made by attaching a coil of wire to a light diaphragm which moves in response to acoustical pressure. The coil of wire is immersed in a magnetic field, hence the movement of the coil in the magnetic field will create a voltage which is proportional to the acoustical pressure.

http://ccrma-www.stanford.edu/CCRMA/Courses/252/sensors/node6.html

29Pressure Sensors

• Pressure sensors contain sensing elements that consist of four piezoresistors buried in the face of a thin, chemically-etched silicon diaphragm. A pressure change causes the diaphragm to flex, inducing a stress or strain in the diaphragm and the buried resistors. The resistor values change in proportion to the stress applied and produce an electrical output. We offer three pressure sensor measurement types—absolute, differential and gage—including vacuum gage and bidirectional types.

• Pressure ranges from 0.5 in H2O up to 30 psi.

Typical Applications Medical equipment: respiration, dialysis, infusion pump HVAC, data storage, and gas chromatography equipment Process controls Industrial machinery Pumps Robotics Off-road applications

http://content.honeywell.com/sensing/prodinfo/pressure/

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Flow

http://www.automationsensors.com/frames/indexFL.html

31Doppler Effecthttp://hyperphysics.phy-astr.gsu.edu/hbase/sound/radar.html

32Doppler Calculations

Source Approaching

Source Receding sources

returned

sources

returned

source

fvv

vf

fvv

vf

v

wavelength

velocityf

http://hyperphysics.phy-astr.gsu.edu/hbase/sound/dopp2.html#c1

33Hall Effect

34

Linear Hall Effect Sensor

Programmable Linear Hall Effect Sensor

Applicationscontactless potentiometers

rotary position measurementlinear position detection

magnetic field and current measurement

35

SummaryIntroduction

Position Sensors Level SensorsAccelerometer Piezoelectric Microphones

Pressure Flow

Doppler Effect Hall Effect