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Capteurs de force, capteurs de couple sur mesure, amplificateurs pour pont de jauges

Force transducer with strain-gauge - Force measurement

A force sensor is a set made up of sensitive elements: strain gauges and a test body generally a metallic material, on which the strain gauges are glued.

Thus a force sensor can be assimilated to a transducer, that is to say that it allows via a linear relation to transform a mechanical physical quantity into an electrical physical quantity.
The strain gauge has a resistive impedance expressed in ohms (Ω).
Below, the relation between the relative variation of the resistance value of the gauge which is linked to the relative variation in length of the test body according to:
∆R / R = k.∆l / l

k is called the gauge factor, it is a constant.

The major technologies used for the production of strain gauges are: gauges with film screens on an insulating support, semiconductor gauges, thick-film gauges with screen printing inks and thin-film gauges by means of evaporation on a substrate.
The most used gauges and very far are the gauges with film screens made of Nickel-chromium, platinum-tungsten, constantan because one can modify via heat treatments the physical characteristics of the strain gauge for example its resistive coefficient in temperature ( CTR).

Why use a 4 strain gauge bridge to make a force transducer instead of a single strain gauge?
It is quite simply the precision obtained and the possibility of compensating for physical phenomena with the Wheatstone bridge.

Strain gauges have different profiles but the most commonly used have:
straight gauges and shear gauge, see profiles below:
 
 
 
Longitudinal strain gauge
Shearing strain gauge (wire grid at 45°)
Here below a visual exemple showing,a strain gauge under a strain : compression and tension.
 
 
When the force transducer is unloaded, the ohmic value that we have called for exemple R0 has a nominal ohmic value in Ω (ohm).
 
WHEN THE STRAIN GAUGE IS UNDER A COMPRESSION STRESS
When the force sensor is submitted to a compression stress, the strain gauge bonded on the metallic transducer has a decrease of the value due to the lenght diminution thatr we call -∆l .
The relative variation of the ohmic value of the strain gauge under compression can be written : -∆R / R0.
 
THE STRAIN GAUGE UNDER A TENSILE STRESS
 
When the sensor is submitted to tension, the strain gauge undergoes an elongation of the length that we will call + ∆l, this elongation causes a positive variation in resistance.
Thus, in first approximation the relative variation of the ohmic value of the gauge subjected to a traction is written: + ∆R / R0.
 
SENSITIVITY AND PRECISION OF A SENSOR
These characteristics known to sensor designers are essential since, taking into account what has been mentioned above, the gauge will have to be judiciously positioned in a deformable zone so as to have high sensitivity and consequently high precision.
The sensitivity is expressed in mV / V and the global precision of a sensor is in percentage: 0.05%, 0.1% ...
Below is a diagram of the Wheatstone bridge, the bridge wiring is typical in the realization of sensors with the addition of thermal compensation circuits.

The output signal measured between the output + _output- terminals gives a voltage in differential mode.
This voltage, once amplified, via assemblies with operational amplifiers, will give a signal in common mode (the conditioner for the gauge bridge): volt or Ampere and usable by acquisition systems or displays.
The different types of force sensors
 
Depending on the technical specifications: environment, volume, destination of the integrated sensor: the designs are different and the most commonly used models such as in weighing have given rise to standards.
Broadly speaking, there are: load washers, load cells, load buttons, dynamometers, binocular bending sensors ... / ...