Force sensors measure force by detecting the displacement of an object when a force is applied. Common force sensing methods include strain gauges, piezoelectric transducers, and magnetostrictive transducers. Strain gauge transducers detect strain on a spring element caused by an applied force. Piezoelectric transducers generate an electric charge proportional to an applied force. Magnetostrictive transducers convert mechanical energy to magnetic energy or vice versa using a magnetostrictive material.
2. The basic principle behind the measurement of force is – when a force
is applied on an object, the object gets displaced. The amount of
displacement occurred can be calculated using the
various displacement transducers, and thus force measurement can be
done. Some of the methods for measuring force are given below:
• Strain Gauge Transducer
• Piezo-Electric Transducer
• Magnetostrictive Transducer
• Force Measurement using Pressure
• Other Force Measuring Systems
3. Strain Gauge Transducer
• Force transducers based on strain gauges have a so-called spring
element or loaded member where the forces to be measured are applied.
The spring element deforms and strain is produced on the surface. The task
of the spring element is therefore to convert the forces to be measured
into strain as reproducibly and linearly as possible. Many force transducer
properties are defined by the selection of the spring element material
and design. In general, four strain gauges are used, installed so that two
are stretched and two are compressed when force is applied.
• These four SG are connected in a Wheatstone bridge circuit. The complete
Wheatstone Bridge is excited with a stabilised DC supply and with
additional conditioning electronics, can be zeroed at the null point of
measurement. As stress is applied to the bonded strain gauge, a resistive
changes takes place and unbalances the Wheatstone Bridge.
4. This results in a signal output, related to the stress value. As the signal
value is small, (typically a few millivolts) the signal conditioning
electronics provides amplification to increase the signal level to 5 to 10
volts
5. • four gauges (full bridge), two gauges (half bridge), a single gauge (quarter bridge)
6. Piezoelectric Transducer
The main principle of a piezoelectric transducer is that a force, when applied
on the quartz crystal, produces electric charges on the crystal surface. The
rate of charge produced will be proportional to the rate of change of force
applied as input. As the charge produced is very small, a charge amplifier is
needed so as to produce an output voltage big enough to be measured.
If a force is applied on the pressure port, the same force will fall
on the force summing member. Thus a potential
difference will be generated on the crystal
due to its property. The voltage produced will
be proportional to the magnitude of the applied
force.
7. Applications
• Due to its excellent frequency response, it is normally used as an accelerometer, where
the output is in the order of (1-30) mV per gravity of acceleration.
• The device is usually designed for use as a pre-tensional bolt so that both tensional and
compression force measurements can be made.
• Can be used for measuring force, pressure and displacement in terms of voltage.
Advantages
• Very high frequency response.
• Self generating, so no need of external source.
• Simple to use as they have small dimensions and large measuring range.
• Barium titanate and quartz can be made in any desired shape and form. It also has a
large dielectric constant. The crystal axis is selectable by orienting the direction of
orientation.
Disadvantages
• It is not suitable for measurement in static condition.
• Since the device operates with the small electric charge, they need high impedance cable
for electrical interface.
• The output may vary according to the temperature variation of the crystal.
• The relative humidity rises above 85% or falls below 35%, its output will be affected. If so,
it has to be coated with wax or polymer material.
8. Magnetostrictive Transducer
Magnetostriction can be explained as the corresponding change in
length per unit length produced as a result of magnetization. The
material should be magnetostrictive in nature. This phenomenon is
known as Magnetostrictive Effect. The same effect can be reversed in
the sense that, if an external force is applied on a magnetostrictive
material, there will be a proportional change in the magnetic state of
the material. This reverse process is also called Villari Effect.
A magnetostriction transducer is a device that is used to convert
mechanical energy into magnetic energy and vice versa. Such a device
can be used as a sensor and also for actuation as
the transducer characteristics is very high due to the bi-directional
coupling between mechanical and magnetic states of the material.
9. This device can also be called as an electro-magneto mechanical device as the
electrical conversion to its appropriate mechanical energy is done by the device
itself. In other devices, this operation is carried out by passing a current into a wire
conductor so as to produce a magnetic field or measuring current induced by a
magnetic field to sense the magnetic field strength.
10. Force Measurement Using Pressure
1. Hydraulic Load Cell
As shown in the figure given below, the inside chamber of the device is
filled with oil which has a pre-load pressure. The force is applied on the
upper portion and this increases the pressure of the fluid inside the
chamber. This pressure change is measured using a
pressure transducer or is displayed on a pressure
gauge dial using a Bourdon Tube.
11. • When a pressure transducer is used for measuring the value, the load cell is
known to be very stiff. Even at a fully forced condition, it will only deflect
up to 0.05mm. Thus, this device is usually used for calculating forces whose
value lies between 500N and 200KiloN. The force monitoring device can be
placed at a distance far away from the device with the help of a fluid-filled
hose. Sometimes there will be need of multiple load cells. If so, a totalizer
unit has to be designed for the purpose.
• The biggest advantage of such a device is that it is completely mechanical.
There is no need of any electrical assistance for the device. They can also
be used for calculating both tensile and compressive forces. The error
percentage does not exceed more than 0.25% if the device is designed
correctly.
• The device will have to be calibrated according to the temperature in which
it is used as it is temperature sensitive.
12. 2. Pneumatic Load Cell
The working of a pneumatic load cell is almost same to that of a hydraulic load
cell. The force, whose value is to be measured, is applied on one side of a piston
and this is balanced by pneumatic pressure on the other side. The pressure thus
obtained will be equal to the input force applied. The value is measured using a
bourdon tube.
The pneumatic load cell has an inside chamber
which is closed with a cap. An air pressure is
built up inside the chamber until its value equals
the force on the cap. If the pressure is increased
further, the air inside the chamber will forcefully
open the cap and the process will continue until
both the pressures are equal. At this point, the
reading of the pressure in the chamber is taken using a pressure transducer and
it will be equal to the input force.
13. Other Force Measuring Systems
1. Elastic Devices
• The strain gauge can be replaced with a Linear Voltage Differential Transformer (LVDT) inside a
load cell to know the displacement of an elastic element. The device is best suitable for dynamic
measurements as it has good features like high resolution and hysteresis.
• Another device most suitable for the measurement of force in an elastic element is the capacitive
load cell. With the device, the displacement can be calculated by measuring the capacitance. The
sensor has two parallel plates with a small gap in between. According to the force applied on the
device, there will be a change in length of the spring member, which in turn changes the gap
distance between the plates and produces a proportional capacitance. This measured capacitance
value will be proportional to the force applied.
• Optical fibers can also be used to design an optical strain gauge to measure force. When a force is
applied on the force summing member, it causes a change in length of the optical fibers that are
bonded to the strain gauge. If the level of strain is different for two optical fiber strain gauges,
then the phase difference between the monochromatic beams that strike the optical gauges will
be proportional to the value of force applied.
• For obtaining a displacement value of high resolution, a device called interference-optical load
cell can be used. A Michelson Interferometer is used to measure the amount of force that has
caused the change in shape of the fork-shaped spring. The highest amount of elastic deformation
and along with it, the strain of the material need not be as large as in the case of the strain gauge
load cell. The spring is made of quartz with very little temperature dependence.
14. 2. Vibrating Elements
The principle of resonance is used in the force transducer of vibrating
elements. If a tuning fork load cell is used, the transducer will have two
parallel band plates connected at both ends and will be made to
vibrate in opposite directions. The change in resonance thus caused
will be proportional to the force applied. The transmission and
reception of the signals are carried by two piezo-electric elements kept
very close to the fork.
