lecture 7 In The Name of Allah - Ali Karimpourkarimpor.profcms.um.ac.ir/imagesm/354/stories/Instrumentation/7... · In The Name of Allah Dr. Ali Karimpour Associate Professor Ferdowsi

lecture 7

Dr. Ali Karimpour, May 2017

Instrumentation

In The Name of Allah

Dr. Ali Karimpour

Associate Professor

Ferdowsi University of Mashhad

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Force Transducer

Topics to be covered

v Introduction

v Strain Gauge Load Cells

v Piezoelectric Load Cells

v Hydraulic Load Cells

v Pneumatic Load Cells

v Inductive and Reluctance-Based Load Cells

v Magnetoelastic Load Cells

v Fiber-Optic Load Cells

v Resonant Wire Load Cells

Lecture 7

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For many types of force measurement system

load cell ≈ force transducer

A dynamic force is one which varies with time.

Response of the force measurement system must be sufficient to be able to follow the changing force.

Introduction

A definition of frequency response is the highest sinusoidal frequency of applied force which the system can measure to a specified accuracy.

Fatigue life may be defined as the number of total full cycles of applied force which may be applied before the measurement uncertainty will be altered beyond specified limits.

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Introduction

Application of force to the elastic element Deflection

Deflection Secondary transducer converts it into an output

v Strain Gauge Load Cells v Piezoelectric Load Cells

v Hydraulic Load Cells v Pneumatic Load Cells

v Inductive and Capacitive-Based Load Cells v Magnetoelastic Load Cells

v Fiber-Optic Load Cells v Resonant Wire Load Cells

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Typical elastic elements and their usual rated capacities

Introduction

The aim of an elastic material:

• Linear relationship between the force applied and strain (output).• Low hysteresis and low creep in the working range. • High level of repeatability between force cycles.

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Introduction

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Strain Gauge Load Cells

• Semiconductor strain gauges. • Wire and foil strain gauges.

• Thin-film strain gauges. • Wire strain gauges.

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• The output from semiconductor gauges is non-linear with strain.

• Semiconductor gauges exhibit essentially no creep or hysteresis and

have an extremely long fatigue life.

• High temperature sensitivity of the semiconductor gauges.

• The output from a semiconductor gauge is very high compared to a wire or foil gauge.

• The gauge factor is a measure of the output for a given strain, and is typically 100-150 for a semiconductor and 2-4 for wire and foil.


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Thin-film strain gauges are produced by sputtering or evaporating thin

films of metals or alloys either directly onto the elastic element or onto

a small metallic substrate for subsequent attachment to the element.


The wire strain gauge was the original type of resistance strain gauge,

though now widely replaced by cheaper foil or thin film types.

The nominal resistance of the strain gauge:

• Wire gauges have resistances in the range of 60 Ω to 350 Ω.

• Foil and semiconductor gauges from 120 Ω to 5 kΩ.

• Thin film types around 10 kΩ.

Selection criteria may include size, self-heating and power requirements.

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Instrumentation used for strain gauge devices


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Force is exerted on certain Electrical chargecrystalline materials

Piezoelectric Load Cells

This devices are often known as quartz force transducers, or piezoelectric crystal will be used.

Electrical charge Charge amplifier

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• It is active sensing elements.


• No power supply is needed.

• Deformation to generate a signal is very small which has the

advantage of a high frequency response of the measuring system

without introducing geometric changes to the force measuring path.

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Packaged as a load washer A force of 10 kN deflects only 0.001 mm.

• High frequency response (up to 100 kHz).

• Very suitable for dynamic measurements(Not suitable for static).

• The measuring range is very wide.

• High overload (typically > 100 % of full-scale output).

• Operation over a wide temperature range and survive temperatures

of up to 350 ◦C.

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Three dimensional piezoelectric load cells.

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Measuring force through pressure

• Deflecting only about 0.05 mm under full force conditions.

Hydraulic Load Cells

• When used with a pressure transducer, hydraulic load cells are

inherently very stiff.

• Most devices fall in to the range of 500 N to 200 kN(Available

up to 5MN).

• The pressure gauge used to monitor the force can be located

several meters away from the device by the use of a special

fluid-filled hose.

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Measuring force through pressure

Hydraulic Load Cells

• No external power.

• They are inherently suitable for use in potentially explosive places.

• Uncertainties of around 0.25 % can be achieved with careful design

and favorable application conditions.

• Uncertainties for total systems are more realistically 0.5 % - 1 %.

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Pneumatic Load Cells

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Inductive and Capacitive-Based Load Cells

The magneto-elastic force transducer is based on two orthogonal

electromagnetic coils.

• Sturdy construction. • High signal level.

• Small internal resistance. • used in rough environments.

• The rated capacities lie in the range from 2 kN to 5 MN.

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Fiber-Optic Load Cells

An optical strain gauge can be formed in a manner similar to a wire

strain gauge by the use of optical fibers.

The deflection of the elastic Result in length changes

force-bearing member in the optical fibers.

Monochromatic light is used Phase difference between the to

feed two optical strain gauges two beams emerging.

• Not subject to EMI.

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Fiber-Optic Load Cells

The interference-optical load cell uses a high-resolution displacement

measuring method.

A fork-shaped spring is deformed by the force, the deformation being

in the region of 40 μm.

The change of the aperture of the fork is measured by a Michelson

interferometer

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Resonant Wire Load Cells

The vibrating-wire transducer consists of a taut ferromagnetic wire that is excited into transverse vibrations by a drive coil.

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Selection Criteria

• Capacity.

• Basic types and mode of operation.

• Method of mounting.

• Accuracy level.

• Approval requirements.

• Cost.

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Application

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Application

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u Dynamometry (Leg Force Measurement )

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Application

u Hand force measurement

measurement of the hand force after an injury of the hand

shows considerably smaller values (comparison between

injured and healthy hand)

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v Traffic Engineering

u Rail Monitoring

Rail

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Application

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v Process monitoring

u Pressing of pills in the Pharmaceutical Industry

Application

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Application

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Application

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“Guide to the Measurement of Force”

The Institute of Measurement and Control 87 Gower

Street London WC1E 6AF

Reference

Documents

lecture 7 In The Name of Allah - Ali Karimpourkarimpor.profcms.um.ac.ir/imagesm/354/stories/Instrumentation/7... · In The Name of Allah Dr. Ali Karimpour Associate Professor Ferdowsi