Topic 1 different attributes that
characterize sensors
ETEC 6405
Sensors and transducers
• Analogue signal– this is a continuous signal.
• Sensors measure physical phenomenon. Some
physical processes are –
• Angular or linear position
• Acceleration
• Temperature
• Pressure
• Stress
• Light intensity
• Sound
Properties by which we characterize
sensors
• -Accuracy: maximum difference between the
indicated and the actual reading.
Maximum
error or
accuracy
Problem
A sensor with an accuracy of 10 mm has a position reading of 1.34 meters.
What is the maximum and minimum possible readings for this sensor based on
the sensors accuracy?
Resolution
• Resolution: used in systems that step through
readings. The smallest increment the sensor
can detect.
resolution
length
Measurements from A sensor span a distance of 3 meters in 50 increments. What is
the resolution?
• Repeatability: When a sensor measurement is
repeated and there are errors associated with the
measurement, we can use a standard deviation to
describe repeatability.
• Linearity: A linear relationship between the input
phenomena of the device relative to the output to
another device
• Precision: Considers accuracy, repeatability of the
device relative to another device
• Range: Natural limits of a sensor.
• Dynamic Response: frequency range for a sensor, i.e.
from 1KHz to 10KHz.
• Calibration: this is the relationship between the input
phenomena and the sensor output.
• Cost: sensor pricing, generally more precision equals
more cost.
• Environment: Factors which affect the sensor
performance i.e. humidity, temp, etc
Angular Displacement
• Potentiometers
• The wiper
moves across
the resistive
film, changing
the resistance
between V1 and
V2
• Potentiometers
are used as
voltage dividers
Encoder disk
• There are two types of encoder disks, relative
encoders and absolute encoders
Tacogenerator
• If ω is the angular velocity of the shaft, the
output voltage of the tachometer is given by
• where k is the gain constant of the
tachometer.
Variable reluctance tachometer
• When the magnetic moves past a stationary
pick-up coil, current is induced.
• For each rotation of the shaft there is a pulse
in the coil.
• This technique often requires some signal
• conditioning circuitry
Strain gauges
• Strain gauges measure stress-strain in a material by measuring
the resistance in a small piece of wire
• The resistance of a wire is a function of the length, width and thickness.
When the wire is stretched, these parameters will change.
• We relate the change in resistance to strain/stress
Wheatstone bridge
Temperature sensors
• Thermistors are used in low- to mediumtemperature applications, ranging from −50 ◦C
to about +200 ◦C.
• RTDs are used in medium-range temperature
measurements, ranging from −200 ◦C to+600 ◦C.
• thermocouples are best suited to very low and
very high temperature measurements. The
typical measuring range is from −270 ◦C to
+2600 ◦C.
• Integrated circuit temperature sensors are used
in low-temperature applications, ranging from
−40 ◦C to +125 ◦C.
The LM35DZ, manufactured by
National Semiconductors Inc. This is
a 3-pin analogue output sensor
which provides a linear output
voltage of 10 mV/◦C. The
temperature range is 0 ◦C to +100
◦C, with an accuracy of +/-1.5 ◦C.
Thermister
• Thermistors are non-linear devices, their
resistance will decrease with an increase in
temperature.
• They are constructed from metal oxide
semiconductors
Thermister instrumentation circuit
Thermocouple
• Thermocouples use a junction of dissimilar
• metals to generate a voltage proportional to
temperature
• The basic calculations for thermocouples
provides the measured voltage using a
reference temperature and a constant specific
to the device
Equation characterizing a
thermocouple
Thermocouple classes
An instrumentation rig
• Often the signal from the transducer needs to
be modified by a signal conditioner before it
enters the control system
Signal conditioning
• Signals from transducers are typically too small to be
read by a normal analogue input card or a MCU
• We often use signal conditioning to obtain a signal of
suitable size and format for the Analogue to digital
process
• Signal conditioners often contain amplifier circuits.
• There are many many different amplification circuits
that use operational amplifiers.
• Instrumentation amplifier circuits often have some
capacity to change the gain and offset.
• What is important is that you know what the common
ones are, how they are used and how to derive the
gain for the amplifier circuit
Definitions
• Gain – ratio that relates the input signal entering
the amplifier to the output signal leaving the
amplifier
• There are two types of gains
• Voltage gain
• 𝛽 = 𝑉𝑜𝑢𝑡 𝑉𝑖𝑛 – given as a dimensionless
constant
• Power gain (Pout/Pin) – given in decibels
G(dB)=10 log(Pout /(Pin))
or
G(dB)=20 log(Vout /(Vin))
Offset
• The offset in an amplifier circuit is obtained by
changing the value of a resistor. This adds a
linear value to the output of the amplifier
Inverting amplifier
• 𝑉𝑜𝑢𝑡 = −
𝑅𝑓
𝑅𝑖𝑛
𝑉𝑖𝑛 where the gain 𝛽 = −
𝑅𝑓
𝑅𝑖𝑛
Non inverting amplifier
• 𝑉𝑜𝑢𝑡 = 𝑉𝑖𝑛 1 +
𝑅2
𝑅1
Voltage follower
• 𝑉𝑜𝑢𝑡 = 𝑉𝑖𝑛
Summing amplifier
• The summing amplifier produces an inverted
output
Single ended signal amplifier
• Inverting amplifier with adjustable gain and
offset
Differential voltage amplifier
Under the condition that the Rf/R1 = Rg/R2, the output
expression becomes 𝑉𝑜𝑢𝑡 =𝛽(𝑉2 -𝑉1 ) where 𝛽~ 𝑅𝑓 𝑅1 is
the differential gain of the circuit
Alternative differential amplifier
• Differential amplifier with current converted to
voltage input
• The circuit below will convert a differential (double ended)
signal to a single ended signal.
• The two input op-amps are used as unity gain followers, to
create a high input impedance.
• The following amplifier amplifies the voltage difference
Comparator
• A comparator is constructed using an
operational amplifier.
• The comparators output is Bistable (Vs+ or Vs-).
• The output indicates which of the two inputs
has a higher voltage.