Semiconductor Strain Gages

woundcallousSemiconductor

Nov 1, 2013 (4 years and 6 days ago)

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Semiconductor
Strain Gages

Brandon Withers

ECE 5320 Mechatronics

Assignment #1

Outline


Major Applications


Strain Theory


Strain Gage Factors


Unbonded and Bonded Strain Gages


Semiconductor Strain Gages


Principle of Measurement and Measurement
Circuits


Strain Gage Specifications for the HBM SLB
-
700A/06



Reference List


W. Tompkins, J. Webster,
Interfacing Sensors To The IBM PC.
Englewood Cliffs,
NJ.:Prentice Hall Inc., 1988.


National Instruments, (2004, Feb. 21). “Measuring Strain with Strain Gauges”
(2004) [Online]. Available:
http://zone.ni.com/devzone/conceptd.nsf/webmain/C83E9B93DE714DB0862568
6600704DB1?OpenDocument


Omega Engineering, (2004, Feb. 21). “The Strain Gage” (2003) [Online].
Available:
http://www.omega.com/literature/transactions/volume3/strain.html


HBM, (2004, Feb. 21). “SLB
-
700A/06


Strain Transducer” (2003) [Online].
Available:
http://www.ae.utexas.edu/courses/ase389/midterm/larry/sircxsar.htm



To Explore Further


Strain Gage Tutorials


--

http://www.omega.com/techref/strain
-
gage.html


--

http://www.vishay.com/company/brands/measurements
-

group/guide/indexes/tn_index.htm


Manufacturers of Strain Gages and Accessories


--

http://www.hbm.com/en/


--

http://www.smdsensors.com/


--

http://www.omega.com


--

http://www.vishay.com


Useful Site With Many Strain Gage Links


--

http://users.telenet.be/educypedia/electronics/sensorsdis.htm



Major Applications


Force, Torque and Pressure Transducers


Load Cells in Commercial Scales, Tank and
Vessel Weighing


Bathroom and Kitchen Scales


Stress Analysis Tests


Measurements of deflection angles of control
surfaces (helicopter blades, wing flaps,
elevators, etc.)

Strain Theory


Strain is the amount of deformation of a body
due to an applied force or in other words strain
is the fractional change in length shown in the
figure above. Strain can be positive (tensile) or
negative (compressive).

“Measuring Strain with Strain Gauges”

Strain Theory (cont.)


For a wire of cross
-
sectional area A, resistivity
ρ
, and length L the
resistance is given by

When the wire is stretched, the cross
-
sectional area A is reduced, which
causes the total wire resistance to increase. In addition, since the lattice
structure is altered by the strain, the resistivity of the material may also
change, and this, in general, causes the resistance to increase further.
Both effects are included in the following equation:

= fractional resistance change

= Poisson’s ratio

= fractional change in length

= fractional change in resistivity

Strain Theory (cont.)


To provide a means of comparing the performance of
various gage materials, the gage factor, or strain
sensitivity, of a gage is defined as

Higher gage factors are generally more desirable
because the higher the gage factor the higher the
resolution of the strain gage.

Strain Gage Factors

Semiconductor materials such as Silicon and
Germanium are used for strain gages
because of their high gage factors.

“The Mechatronics Handbook”

Unbonded and Bonded Strain
Gages


The unbonded strain gage
consists of a wire stretched
between two points in an
insulating medium such as air.
Four gages are normally
connected in a Wheatstone bridge
circuit and arranged so that two
gages are lengthened and two
shortened by the displacement.

A bonded strain
-
gage element,
consisting of a metallic wire, etched
foil, vacuum
-
deposited film, or
semiconductor bar, is cemented to the
strained surface.

“The Mechatronics Handbook”

Semiconductor Strain Gages


Strain
-
gage technology advanced in the 1960s with the introduction of the
semiconductor strain
-
gage elements


Silicon gages are formed from single
-
crystal silicon whose orientation and
doping are the most important design parameters. The gage factor depends
on the resistivity (determined by the doping) and the crystal orientation.


Bonded semiconductor gages are made by slicing sections from specially
processed silicon crystals and are available in both n and p types. The high
gage factor is accompanied by high
-
temperature sensitivity, nonlinearity,
and mounting difficulties.


Diffused semiconductor gages utilize the diffusion process employed in
integrated
-
circuit manufacture. This type of construction may allow lower
manufacturing costs in some designs, since a large number of devices can
be made on a single silicon wafer. The deviation from linearity is
approximately 1%

Ideal Strain Gage vs. Diffused
Strain Gage

Ideal Strain Gage Properties



Small in size and mass


Low in cost


Accurate and repeatable


Easily attached


Perfect output signal


Infinite pressure range


Highly sensitive to strain but
insensitive to ambient or
process temperature variations

Diffused Strain Gage Properties




Small in size and mass



Low in cost



Accurate and repeatable



Tricky to attach



High output signal



Wide pressure range



Highly sensitive to strain but limited


to moderate
-
temperature


applications and requires


temperature compensation


Principle of Measurement

Mechanical loading produces a change of length in the
measurement object, which is conveyed to the strain
gauge. Because there is a change in length, the electrical
resistance of the applied strain gauge also changes in
proportion to the strain. If there is excitation voltage, the
circuit supplies an output signal proportional to the change
in resistance and therefore also proportional to the change
in length. A carrier frequency or DC amplifier suitable for
strain gauges enables measurement signal evaluation to
continue.


Measurement Circuits

(a)
Common Wheatstone bridge circuit, null
when R
1
/R
4
=R
2
/R
3

(b)
All resistances equal but one is variable by
a factor, (1+x), where x is a fractional
deviation around zero, as a function of
strain. Sufficiently linear for small values of
x.

(c)
Output doubles if two identical variable
elements can be used

(d)
Two variable resistors increase while two
decrease. Commonly used with two
identical two
-
element strain gages attached
to opposite faces of a thin beam to
measure bending. The output is four times
the output for a single
-
element bridge, and
it is linear with x.

(e)
Uses a zero
-
centered potentiometer to
constitute two adjacent arms.

(f)
Op amp forces the bridge to be balanced. It
has good linearity and very low output
impedance, thus making the output
measurement easier and more accurate.

“The Mechatronics Handbook”

Strain Gage Specifications

HBM SLB
-
700A/06

Key Features


For Monitoring strain in statically and
dynamically loaded units such as
cranes, presses and roll stands


Simply bolted into place


Stainless steel


Protected from harmful environmental
effects


Inexpensive


Strain gauge full bridge

“SLB
-
700A/06


Strain Transducer”

HBM SLB
-
700A/06
Specifications

Type

SLB

700A/06

Nominal strain

µ
m/m

500

Nominal sensitivity

mV/V

1.5
"
0.15

Restoring force

N

approx. 3110

Zero point deviation

%

5

Temperature effect on the sensitivity, per 10 K

%

0.2

Temperature effect on the zero point, per 10 K

%

0.5

Compensated temperature range

°
C


10...+40

Input resistance



> 1000

Output resistance



1000
"
3

Isolation resistance

GΩ

5

Nominal range of the excitation voltage

V

2...15

Max. operating strain

%

150

Breaking strain

%

300

Nominal displacement

mm

approx. 0.038

Cable length

m

6

Cable diameter

mm

approx. 3

Weight (complete with 6 m cable)

kg

0.16

Degree of protection acc. to EN 60 529

IP65

“SLB
-
700A/06


Strain Transducer”

HBM SLB
-
700A/06
Specifications

“SLB
-
700A/06


Strain Transducer”

HBM SLB
-
700A/06

Mounting

The SLB700A strain transducer is attached to the measurement object by

means of four normal M6 hexagon socket screws (e.g. DIN 912). We

recommend screws of resistance class 12.9, which should be tightened in a

sequence of diagonal opposites, using a tightening torque of 16 N

m.

Alternatively, use screws of resistance class 8.8 and a tightening torque of

8 N

m. The strain transducer must not be mounted in the central, offset area

and it must be installed free from distortion.


“SLB
-
700A/06


Strain Transducer”