8/6/2019 ECE132 - REPORT - Force Measurement
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TOPICS COVERED
Force: Brief Review
General Considerations in Force Measurement
Hooke·s Law
Fundamental Methods of Force
Measurement
Force Sensors
Strain Gage Load Cell
Piezoelectric Methods
Capacitive Force Transducer
Force Sensing Resistors
Magnetoresistive Force Sensors
Magnetoelastic Force Sensors
Torsional Balances
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FORCE
vector quantity
push or pull
an action that will cause an acceleration or acertain reaction of a body
Types of Forces
1. Contact Forces
2. Action-at-a-distance Forces
CONTACT ACTION-AT-A-DISTANCE
Frictional Force Gravitational Force
Tension Force Electrical
Normal Force Magnetic Force
Air Resistance Force
Applied Force
Spring Force
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General Considerations
If the forces acting on a body do not produce anyacceleration, they must form a system of forces in
equilibrium.
Two classification of Forces:
1. Internal Forces
2. External Forces
If a body is supported by other bodies while subject to the
action of forces, deformations and/or displacements will
be produced at the points of support or contact.
In considering a body at a definite section, it is evident
that all the internal forces act in pairs, the two forces beingequal and opposite, whereas the external forces act
singly.
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Hooke·s Law
´When a metal is loaded in uniaxial tension, uniaxial
compression, or simple shear, it will behave elastically until
a critical value of normal stressor shear stress is reached,and then it will deform plastically.µ
UniaxialTension
UniaxialCompression
SimpleShear
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Hooke·s Law
Elastic Stress (S), Shear Stress () and Elastic Strain (e or )
Uniaxial
Tension
Uniaxial
Compression
Simple
Shear
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Hooke·s Law
Poisson·s Ratio (v)
The ratio of transverse to direct strain in tension or compression.
For metals: 0.25 < v < 0.33
Stress- Strain Relationship (elastic region)
S = E*e (Tension or Compression)
= G (Simple Shear)
where:
E = Young·s Modulus of Elasticity
G = Shear·s Modulus of Elasticity
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Hooke·s Law
Basic Methods of Force Measurement
1. Balancing the unknown force against a standard
mass through a system of levers.
2. Measuring the acceleration of a known mass.
3. Equalizing it to a magnetic force generated by the
interaction of a current-carrying coil and a magnet.
4. Distributing the force on a specific area to generate
pressure, and then measuring the pressure.
5. Converting the applied force into the deformation ofan elastic element.
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Hooke·s Law
FORCE MEASUREMENT IN SENSOR DESIGN
The basics of sensor design can be resolved into twoproblems:
1. Primary geometric, or physical constraints, governed
by the application of the force sensor device.
2. The means by which the force can be converted into
a workable signal form (such as electronic signals or
graduated displacements).
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Force Sensors
A transducer that converts an input mechanical
force into electrical output signal
Force Transducers
Useful for understanding the response of different
mechanical systems.
Some types of force sensors are based on
measuring a deflection caused by the force.
(Force-deflection relationship thru Hooke·s Law)
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Force Sensors
LOAD CELLS
a transducer which converts force into a measurable
electrical output
Comprised generally of a rigid outer structure, some
medium that is used for measuring the applied force, andthe measuring gage.
Used for sensing large, static or slowly varying forces with
little deflection and are a relatively accurate means ofsensing forces.
Types: Hydraulic Load Cell
Pneumatic Load Cell
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Force Sensors
Hydraulic
Load CellPneumatic
Load Cell
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Force SensorsStrain Gage Load Cell
Consists of a structure that elastically deforms when
subjected to a force and a strain gage network that
produces an electrical signal proportional to thisdeformation.
Strain Gage - a sensor whose resistance varies with
applied force; It converts force, pressure, tension,
weight, etc., into a change in electrical resistancewhich can then be measured.
Example: Beam-type and Ring-type Load Cell
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Force SensorsStrain Gage Load Cell
BEAM-TYPE LOAD CELL
commonly employed for measuring low-level loads
Uses a simple cantilever beam with four strain gagesas the elastic member (sensor) for the load cell. The
gages are wired into a Wheatstone bridge.
The range and sensitivity of a beam-type load cell
depends on cross-section of the beam, the point ofload application, and the fatigue strength of the
beam material.
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Force SensorsStrain Gage Load Cell
RING-TYPE LOAD CELL
Used to measure axial forces onshafts and spindles where it is not
possible to fit a normal Load cell.
Elastic Element: Proving Ring
Sensor:
- Strain Gages
- LVDT
(Linear Variable-Differenetial
Trnasformer)
Elastic element with
Strain-gage sensors
Elastic element with
LVDT sensors
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Force Sensors
Piezoelectric Methods
Piezoelectric Devices
Used for sensors in buildings and bridges, crystals in
watches and as a key component in synthetic skin.
Piezoelectric Materials
Have the property of developing an electric charge
on their surface when mechanical stress is exertedon them
Piezoelectric Effect
This effect states that when asymmetrical, elastic
crystals are deformed by a force, an electrical
potential will be developed within the distorted
crystal lattice.
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Force Sensors
Piezoelectric Methods
The range of forces that can be measured using
piezoelectric transducers are from 1 to 200 kN
and at a ratio of 2 x10^5.
Piezoelectric Methods
1. Resistive Method
2. Inductive Method
3. Piezotransistor Method
4. Multicomponent Dynamometers
Using Quartz Crystals As Sensing Elements
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Force SensorsCapacitive Force Transducer
A transducer that uses capacitance variation to
measure force.
The force is directed onto a membrane whose
elastic deflection is detected by a capacitance
variation, then electronic circuit converts the these
variations into dc-voltage variations.
Advantages:
1. Extremely small force application requirement
in moving of one of plate relative to the other.
2. Stability and the sensitivity of the sensor is notinfluenced by pressure or temperature of the
environment.
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Force SensorsForce Sensing Resistors (Conductive Polymers)
A polymer thick film (PTF) device which exhibits a
decrease in resistance with an increase in the force
applied to the active surface.
Ideal for measuring forces without disturbing the
dynamics of a test.
Can be used to measure both static and dynamicforces.
FSRs should not be used for accurate measurements
of force because sensor parts may exhibit 15% to
25% variation in resistance between each other
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Force SensorsForce Sensing Resistors (Conductive Polymers)
Force Sensing Resistor
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Force SensorsForce Sensing Resistors (Conductive Polymers)
Construction of a Force Sensing Resistor
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Force SensorsMagnetoresistive Force Sensors
Make use of the magnetoresistive effect, the
property of a current-carrying magnetic material to
change its resistivity in the presence of an external
magnetic field.
The principle of magnetoresistive force sensors isbased on the fact that metals, when cooled to low
temperatures, show a change of resistivity when
subjected to an applied magnetic field.
These devices are severely limited because of their
high sensitivity to ambient temperature changes.
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Force SensorsMagnetoelastic Force Sensors
Operate based on the Joule effect; that is, a
ferromagnetic material is dimensionally altered
when subjected to a magnetic field.
Advantage:
Have a high frequency response (on the order
of 20 kHz).
Disadvantages:
1. Excessive stress and aging that may cause
permanent changes
2. Zero drift and sensitivity changes due to
temperature sensitivity3. Hysteresis Error
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Force SensorsTorsional Balances
Balancing devices that utilize the deflection of a
spring to determine forces.
Equal arm scale force measuring devices which
comprised of horizontal steel bands instead of pivots
and bearings.
The principle of operation is based on forceapplication on one of the arms that will deflect the
torsional spring (within its design limits) in proportion
to the applied force.
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