ECE132 - REPORT - Force Measurement

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



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



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.



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



Hooke·s Law

Elastic Stress (S), Shear Stress () and Elastic Strain (e or )

Uniaxial

Tension

Uniaxial

Compression

Simple

Shear



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



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.



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).



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)



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



Force Sensors

Hydraulic

Load CellPneumatic

Load Cell



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




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.




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



Force Sensors


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.



Force Sensors


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.


1. Resistive Method

2. Inductive Method

3. Piezotransistor Method

4. Multicomponent Dynamometers

Using Quartz Crystals As Sensing Elements



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.



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




Force Sensing Resistor




Construction of a Force Sensing Resistor



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.



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



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.

ECE132 - REPORT - Force Measurement

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