Mechatronics (Sensors)

Praveen.R, Asst. Professor,
SNGCE, Kadayirrippu.

 Range and span
 Error
 Accuracy
 Repeatability & Precision
 Resolution
 Sensitivity
 Hysteresis Error
 Non Linearity Error
 Stability
 Dead Band
 Output Impedance

 It is the ability of the of a transducer to give
the same output when used to measure a
constant input over a period of time.
 Expressed in percentage of the full scale of
the transducer.
 Zero drift is the change in output to zero
input.
 Thermistors over passage of time fluctuate
over time.
 Close tolerance may not be sufficient for a
transducer but it should also have good
stability.

 Deadband of a transducer may be defined as
the range of input values for which there is
no output.
 For instance if a pressure sensor can sense
beyond a threshold value of say 4Pa and
above but not from 0-4 Pa then the deadband
of that sensor is 0-4 Pa.

 Electrical impedance is the measure of the
opposition that a circuit presents to the passage of a
current when a voltage is applied.
 Impedance extends the concept of resistance to AC
circuits, and possesses both magnitude and phase,
unlike resistance, which has only magnitude.
 When a circuit is driven with direct current (DC),
there is no distinction between impedance and
resistance.
 When a transducer is connected to an interfacing
circuitry parallel or series to it the impedance of the
circuit gets affected and will affect the behaviour of
the system.

 Whatever we discussed till now is all about static characteristics of
a transducer.
 Static characteristics of a transducer are those characteristics
when the output signal is in the steady state in response to an
input.
 Now let us consider some dynamic performance characteristics of
a transducer.
 Response time (time taken to rise to 95% of the actual value)
 Time constant (Inertia of the sensor, 63.2 % of the response time)
 Rise time (It is the time taken by a signal to change from a specified
low value to a specified high value. Typically, in analog electronics,
these values are 10% and 90% of the step height)
 Settling time (time taken to settle within 2% of the steady state value)

Time(s) 0 30 60 90 120 150 180
Temp. (˚C) 20 28 34 39 43 46 49
Time(s) 210 240 270 300 330 360 390
Temp. (˚C) 51 53 54 55 55 55 55
Steady state value =55˚C
95 % of 55˚C is 52.5˚C and so the response time about 228s
Time constant ≈ 60s and Settling time ≈270s
0
10
20
30
40
50
60
0 100 200 300 400 500
Temperature(˚C)
Time (s)
Temperature Vs. Time

 Non linearity error 0.1 % to 1%.
 Resistance range is in the order of 20 Ω to 200 kΩ.
 For conductive plastics non linearity error is about
0.05 %.
 Polyphenylene vinylene (PPV),
 Polyacetylene (PAC),
 Polyaniline (PANI)].
 Resistance range is in the order of 500 Ω to 80 kΩ.

 A strain gauge is a device used to measure the
strain of an object.
 Invented by Edward E. Simmons and Arthur C.
Ruge in 1938.
 The most common type of strain gauge consists
of an insulating flexible backing which supports a
metallic foil pattern.
 As the object is deformed, the foil is deformed,
causing its electrical resistance to change.
 This resistance change, usually measured using a
Wheatstone bridge, is related to the strain by
the quantity known as the gauge factor.

 Gauge factor (G)
 G ε = ∆R/R.
 Where ε is the strain developed,
R is the initial resistance and ∆R
is the change in resistance.
 For metal wires, G is 2.
 For p type semiconductors G is
about +100 and that for n type
it is about -100.
 Consider a strain gauge of 100Ω
resistance and let the stain
induced be 0.001 then assuming
a gauge factor of 2 the
corresponding change in
resistance will be,
2 x 0.001 x 100 = 0.2 Ω.

Limitation: sensitive to temperature
Linearity error : about 1% of FSD
Measurement range : 1 to 30mm

 Capacitance of a parallel plate capacitor is
given by,
 Capacitive sensors monitoring linear
displacement may be of the following type.
 Plate Separation Type
 Variable overlapping area Type
 Moving dielectric type

PlateSeparationType
 To avoid nonlinearity we use a push pull
arrangement

Moving Dielectric Type
Variable overlapping area Type
Proximity Sensor

 When the beam of
charge particle passes
through magnetic field,
it deviates from its
straight path, this
effect is known as Hall
effect.
 In order to know about
Hall effect, we consider
a beam of electrons
passing from a
conductive plate on
which magnetic field is
applied perpendicularly.

 Due to the presence
of magnetic field,
electrons deviate
from straight pass and
bend on a side
creating negative
charge on that side
and positive charge on
the other side.
 This uneven
distribution of
electron will result in
the formation of
electric field.

 This electric field will create a tilt equal to
the magnitude of magnetic field. This
relation can be shown as follows
 V = KHBI/t, where
 V is the transverse potential difference,
 KH is the Hall coefficient,
 t is the thickness of conductive plate,
 I is the current passing through the plate &
 B is the magnetic flux density perpendicular to
the plate.

 Hall effect sensors come in two different forms; linear
and threshold.
 In case of linear Hall effect sensor, output voltage varies
linearly with magnetic flux density and in case of
threshold Hall effect sensor, output voltage show us
abrupt drop in voltage at particular magnetic flux density.

 As this sensor works in the presence of magnetic
field, it is immune of environmental effects, it means
that it can be used in severe service conditions.
 We can use this sensor as position, displacement
and proximity sensors as well.
 The only thing we need to do is to attach small
permanent magnet to the object.

 By using Hall effect sensor we can measure
the level of fuel in fuel tanks.

This type of sensors
can be used for
detecting speed of a
gear.
The arrangement will
consist of a magnet
attached to the gear
and a reed sensor.

There are 2 types
of photoelectric
proximity sensors.
They are;
1) Obstruction type
2) Reflection type

These type of sensors consist of a wound coil around a core. When
then ends of the coil is close to a metal object its inductance
changes and this change can be measured by its effect of
impedance.
It can be used only for metal object and is best for ferrous metals.

Mechatronics (Sensors)

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