Here we go over the different types of force measurement, how strain gauges are used how to calibrate a system and more.

1. Phone Number: 1-800-550-0280
Contact Email: contact@tacunasystems.com
Website: https://tacunasystems.com/
Measuring Forces in the Force Shunt
Introduction
This article discusses the processes and techniques that can be used in
measuring forces in a force shunt. There is a need to understand the con-
cepts of force sensors and transducers and force measurements.
Force Sensors and Transducers
Force sensors are devices that respond to or detects physical force caused
by a physical load, weight or pressure. A force transducer is then a force
sensor that is able to transform this physical force into an output electrical
signal (voltage or current). Examples include piezoelectric transducers,
strain-gauge load cell, pneumatic and hydraulic pressure transducers.
Force transducers are also integrated into process control systems when
incorporated into an open-loop control system, calibration is necessary so
as to obtain accurate results. Calibration gives a characteristic curve which
is a plot of the transducer response (output signal) to the applied stimulus
(force). Once this is established for the transducer, then it can be repro-
duced even after installation.
These devices need to be interfaced with the machine producing the force
or a point of application of the force needs to be set up. Therefore, the
force measurement technique used will require a specialized type of de-
vice, thus leading to the concept of force measurement techniques.
Force Measurement

2. This can be categorized into two, which are direct force measurement and
force shunt measurement.


1. Direct force measurement: This requires that a large part of the
process force flows through the transducer or force measuring in-
strument. This simply means that the measuring instrument is in-
stalled directly in the force flux of a separate device.An example
is in the installation of a load cell, a direct force measurement will
require the use of mounting kits fixed through bolts, screws, and
nuts. Hence, the mounting kits must have flat surfaces and be
rigid.
2. Force Shunt Measurement: This concept which is the center of
this article can best be understood by defining the word shunt.
According to the Merriam Webster dictionary, Shunt refers to a
means or mechanism for turning or thrusting aside.
In electronics, a shunt is a device that creates a path of low resistance, so
as to allow or divert the flow of electric current through another point in the
circuit. Simply put, the electric current will flow through the shunt because it
has low resistance.
Therefore, a force shunt can be defined as a device that offers a low re-
sistance to the flow of force such that a small part of the process force flux
or a useful amount flows through it. In our case study, the force being con-
sidered is a mechanical force which can be created by physical load,
weight, and pressure.
As can be seen, a force shunt is different from an electrical shunt as the
bulk of electricity flows through a short circuit but the concept is still the
same. Hence, by measuring the force along the force shunt, the applied
force can be determined in both magnitude and direction.
Direct force measurement offers benefits of high accuracy but sometimes it
is of greater advantage to measure force in the force shunt, hence special
sensors needs to be designed precisely for these applications.
Force Measurement Methods

3. There are different methods used in the industry for determining force.
These methods include lever-balance methods, force-balance methods, hy-
draulic and pneumatic pressure measurement, an acceleration measure-
ment, the use of elastic elements etc.
Each of these methods has evolved over the years, however, the most
common ones are carried out based on the deformation of the structure in-
side the instrument that measures the forces. They involve the use of elas-
tic elements.
This article discusses two major closely related deformation methods and
they are the use of strain gauges and the use of strain gauge transducers.
The Use of Strain Gauges
Strain gauges can be installed on the machinery/object to directly measure
force. Its two categories are bonded and unbonded strain gauge. For the
purpose of measuring force in the force shunt, the bonded strain gauge is
the most appropriate.
Bonded strain gauges have practically no influence on the structure of the
object under test. They do not affect the stiffness and the dynamic behavior
of the test object as a whole. The figure bellows shows a full-bridge strain
gauge.
Figure 1. A Full-bridge Strain Gauge
The gauge to be used must be carefully selected in a way that it compen-
sates for parasitic effects of bending moments or torsion, compensate for
temperature shifts and other undesirable effects.

4. In the installation of the strain gauge on site where the object to be meas-
ured is situated, the gauge is carefully attached/bonded to its body. This
must be carefully and properly done so as to ensure that only the tensile
and/or compressive forces acts on it and not any bending moments.
Furthermore, the output of the strain gauge bridge as shown in figure 2 be-
low depends on the gauge factor of the strain gauge, the strain level as well
as the supply voltage.
Figure 2. A Quarter-bridge setup
This is quarter-bridge (a single strain gauge) setup and it is expressed
mathematically as follows.

5. The bridge output signal can be increased by a controlled weakening of the
area on the object’s body surface where the strain gauge is attached. How-
ever, this affects the object’s stiffness, dynamic behavior, and stability.
The Use of Strain-Gauge Transducers
Strain gauges transducers have the strain gauge as its underlying mecha-
nism along with other component parts such as the elastic element (also
called the structural member) and the housing unit.
The strain gauge[s] are fixed onto the elastic element so that this elastic el-
ement acts as a primary transducer that converts the force exerted by the
test object into the strain. Then the strain gauge[s] acts as a secondary
electrical transducer that converts the strain to changes in electrical re-
sistance.
On the transducer device, the strain in the area of the installed strain
gauges is bigger than the strain value between the two screwed connec-
tions or load application points. Figure 3 below illustrates this better.
This shows that the strain generated by the force applied to the transducer
is concentrated on the zone where the strain gauge is attached. The gauge
is mounted so that the long lengths of the conductor are aligned in the du-
ration of the force acting on the test object.
The approximate excessive increase in strain can be expressed mathemat-
ically as follows.

6. This expression is based on ideal conditions with the assumption that the
zone around the application point is strain-free. However, in practical terms,
this is not entirely correct.
Furthermore, from the mathematical expression, it becomes clear that the
strain transducer’s sensitivity can be adjusted by means of the length ratio
of strain zone (the ideal strain zone is at the area where the strain gauge is
mounted) and the distance between the screw connections. This then
means that a very high sensitivity can be achieved theoretically.
Furthermore, strain transducers need additional integrated electronics so
that the raw output signal can be conditioned (filtered, isolated and ampli-
fied), so that calibration can easily be done. Temperature compensation
can also be performed digitally or by the use of self-temperature compen-
sating strain gauges or dummy gauges.
Parallel connections made easy
Tacuna systems strain transducers without inbuilt electronic circuits can be
easily connected together in parallel since each device has a high bridge
resistance of the same value.
This high bridge resistance will ensure that no excessively high amplifier
supply current is needed, hence a less costly amplifier device can be used.
A parallel connection between these transducers will enable strain effects
that need to suppressed be compensated for.
Consider an example

7. Where the press forces acting on a column are to be measured, the only
proportion of strain that is relevant is the one resulting from tensile/com-
pressive loading. Assuming this setup is such that two single point load
cells are connected in parallel and mounted on a column at the same
height, opposite from each other.
Now, when a bending load is applied to the setup, one transducer will ex-
perience a higher strain while the other experiences a lower strain. Hence,
only the tensile or compressive portion of the strain is measured and the
bending gets compensated for.
Tacuna system load cells are of different types and are not limited to
just single point and double-ended load cells. They can be mounted in a
force shunt setup by screws. Additional corrosion protection and coating
can then be applied to the whole setup.
The Relative Advantages and Disadvantages of these Two Methods
The advantages of using strain gauges for force shunt measurements
include:
 It does not occupy much space
 It is suitable for highly filigree structures with a small force ap-
plied to the gauge as the other method would create excessive
force shunts.
The disadvantages of using strain gauges for force shunt measure-
ments include


1.
 The installation process which requires bonding, wiring, protec-
tive coating etc. hence, causing increased installation time.
 Calibration of the force shunt is required also resulting in time
consumption.
The advantages of using strain transducers include.
 It offers for easy installation and it is rapidly deployable. This is
due to the fact that our load cells can be screwed readily onto
existing structures.

8.  The load cell output can easily be interfaced with electronics
amplifiers and conditioners. These electronic devices are read-
ily available to be purchased with the load cell.
 Temperature compensation has been accounted for and the
range can be seen in the load cell datasheet.
The disadvantages of using strain transducers include.
 Calibration in the force shunt is still required.
 Our calibration services are readily available and this ser-
vices can be purchased in conjunction with a load cell.
Calibration of the System
Calibration is the process of comparing a measuring instrument against an
authoritative reference for the same type of measurement. The force meas-
urement system needs to be calibrated before or after installation.
In calibrating a direct measurement system, it is advisable that no force
shunts should be presented and it should be noted that the characteristic
features of the instrument such as stiffness and dynamic behavior affects
the overall design and can require the use of very large structures for
measuring large force vectors.
Load cells can be calibrated with very high precision before mounting in the
field (also called formal calibration), however, for force shunt measurement,
calibration should be done directly on the object in the field right after instal-
lation is carried out.
Data sheets for products
Finally, a wide range of load cell product types and designs are available to
choose from for any of your applications. Each product has a data sheet
that shows its uncertainty, accuracy, sensitivity and other important specifi-
cations that are needed in designing a force measuring system in the force
shunt.
Zero load

9. The calibration procedure will involve the measurement of the force when
zero-load is applied on the shunt. Also, the output is measured when the
maximum rated force is applied to the shunt. The necessary adjustments
can then be made to the procedures until the output of the measurement
instrument matches the nominal value specified in its datasheet. When cali-
brating the force shunt, conditions such as temperature, alignment of load-
ing and humidity should be controlled as much as possible.
Calibration is highly important especially in the case of determining the
magnitude and direction of the force in the force shunt. This means it helps
to ensure that the instrument is providing an accurate indication of the ac-
tual force in the object being tested.
Instrument and systematic errors
Furthermore, instrument error or systematic errors caused by the inappro-
priate installation of the setup can easily be noticed and catered for before
the whole system is made fully operational. At the end of the calibration
procedures, a calibration certificate is provided that contains the docu-
mented readings that can then be referenced for other purposes and can
as well be presented to the client.
Conclusion
This article explains the various concepts of force measurements and the
useful methods based on our product line that is readily available for meas-
uring forces in the force shunt. The two methods discussed have very mi-
nute or close to no effects on the dynamic mechanical behavior of the
structure being monitored as a whole.
It also shows the superiority of strain-gauge based instrumentation technol-
ogy for very high accuracy measurements.
SOURCES

1.
 Instrumentation, Transducers and Interfacing, B.R Bannister and
D.G. Whitehead.
 A Guide to the Measurement of Force, Andy Hunt.
 Instrumentation Reference Book, Walt Boyes.
 The Force Measurement Glossary

10.  Force and Weight Measurement Resources
Phone Number: 1-800-550-0280
Contact Email: contact@tacunasystems.com
Website: https://tacunasystems.com/