There are four basic forces in nature:
The weakest of the four is the gravitational force. It is also
the easiest to observe, because it acts on all matter and it
is always attractive, while having an infinite range. Its
attraction decreases with distance, but is always
measurable. Therefore, positional "equilibrium" of a body
can only be achieved when gravitational pull is balanced
by another force, such as the upward force exerted on our
feet by the earth's surface.
Link-Type Load Cell
The diagram above represents what might happen if a strip of metal were fitted with four gauges.An downward bend stretches the gauges on the top and compresses those on the bottom.A load cell may contain several similar strain gauges elements.
Beam-Type Load Cell
The strain gauges are bonded on the flat upper and lower sectionsof the load cell at points of maximum strain. This load cell type is used for low capacities and performs with good linearity. Its disadvantage is that it must be loaded correctly to obtainconsistent results
A is the cross-sectional area
E is the modulus of elasticity
v is Poissin’s ratio of the material
Sg is a gauge factor
VO= k PVS
Therefore Force P is measured in terms of Voltage out put as VO
Ring-Type Load Cell
Where E is modulus of Elasticity of material;
K for given ring is constant = 1.79 R3/(Ewt3)
Out put Volts = Vo = S Vs and S is sensitivity of LVDT
Forces that cause extended objects to rotate are associated with torques. Mathematically, the torque on a particle is defined as the cross-product:
is the particle's position vector relative
to a pivot
is the force acting on the particle.
A circular shaft with four strain gages mounted on two perpendicular helices that are diametrically opposite one another as shown below. Gage 1 & 3 are mounted on right hand helix, sense a positive; while 2 &4 are mounted on left hand helix giving negative sense. The shearing stress in the circular shaft is related to the applied torque T by the equation:
D is diameter of the shaft
J polar moment of inertia
xzis shearing stress
T is Torque applied
E is Modulus of Elasticity
Sg is sensitivity of Strain Gage
Vo is output voltage, and
Vs is supplied voltage to Bridge
Torque is measured by either sensing the actual shaft deflection caused by a twisting force, or by detecting the effects of this deflection. The surface of a shaft under torque will experience compression and tension.
To measure torque, strain gage elements usually are mounted in pairs on the shaft, one gauge measuring the increase in length (in the direction in which the surface is under tension), the other measuring the decrease in length in the other direction.
A strain gage can be installed directly on a shaft. Because the shaft is rotating, the torque sensor can be connected to its power source and signal conditioning electronics via a slip ring.
The strain gage also can be connected via a transformer, eliminating the need for high maintenance slip rings.
The excitation voltage for the strain gage is inductively coupled, and the strain gage output is converted to a modulated pulse frequency.
Force or Torque Measurement