Dr.Y.NARASIMHA MURTHY Ph.D [email protected]Sensors and Interfacing techniques Introduction Sensors are sophisticated devices which will detect and measure any non-electrical physical quantity .A Sensor converts the physical parameter (for example: temperature, blood pressure, humidity, speed, etc.) into a signal which can be measured electrically. Sensor is sometimes called a primary measuring element, which can be found simply as a mercury thermometer to measure the temperature. It may be embedded in the transducer to perform its function. That means the transducer consists of a primary element (sensor) plus a secondary element (signal conditioning circuit) that transforms the passive change or small voltage signal into active signal range that can be easily used in other chains of the control loop. So, we can write that Transducer = Sensor + Signal conditioning circuit The important characteristics of sensors are Accuracy :High accuracy is needed Environmental condition – The performance of the sensor should not depend on environmental conditions like temperature or humidity etc.. Wide Range –.Measurement limit of sensor . The range should be high. Calibration - Essential for most of the measuring devices as the readings changes with time 1
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humidity, speed, etc.) into a signal which can be measured electrically.
Sensor is sometimes called a primary measuring element, which can be found simply as a
mercury thermometer to measure the temperature. It may be embedded in the transducer to
perform its function. That means the transducer consists of a primary element (sensor) plus a
secondary element (signal conditioning circuit) that transforms the passive change or small
voltage signal into active signal range that can be easily used in other chains of the control loop.
So, we can write that
Transducer = Sensor + Signal conditioning circuit
The important characteristics of sensors are
Accuracy :High accuracy is needed Environmental condition – The performance of the sensor should not depend on
environmental conditions like temperature or humidity etc.. Wide Range –.Measurement limit of sensor . The range should be high. Calibration - Essential for most of the measuring devices as the readings changes with
time Resolution – Sensor must be able to detect even small changes in the input signal. Cost- Sensor should not be very expensive. Repeatability – The sensor must be able to reproduce the same output at similar
conditions. Linearity : The output of the sensor must be linear .
There are various types of sensors available , which measures various physical parameters.
Sensor type Physical Parameter
Temperature Temperature
Light Light / dark
Pressure Pressure or barometric pressure or blood pressure
Types of Sensors : Based on the principle of working the sensors are divided into
Resistor type Capacitor Type & Inductor type etc..
Resistor type Sensors: A sensor whose resistance changes with the input signal .i.e based on the
vatiation of the resistance of the sensor element , the physical parameter is detected.
For example , Themistor , Platinum resistance thermometer or Thermocouple etc.. are used to measure the temperature with respect to the variation in the resistance of the material. In the case of a thermistor the temperature is given by
R t = R0.e β (1/T - 1/T0
) Here ,β is the constant of the thermistor. Rt is the
resistance of the thermistor at a temperature t. The variation of resistance of a thermistor with
temperature is given by the following graph.
A strain gauge is a thin metal foil that changes resistance with applied strain. Strain gauges are
the preferred choice in stress analysis due to their small size and relatively low cost. Strain
gauges can measure strain levels from a few micro-strain (μe) to over 100,000 micro-strain.
The changes of resistance associated with strain gauges are small and present measurement
situation. A Wheatstone bridge arrangement is commonly used to measure the small changes
in resistance associated with strain gauges.
Capacitive type Sensors: These sensors work on the principle of variation of capacitance with
the external input signal.
The basic operation of capacitive type sensors can be seen from the familiar equation for aparallel-plate capacitor.
C = Kε0 A /d
Here K is the dielectric constant of the material, ε0 is the permittivity of the free space and d is
the distance between the parallel plates and A is the area of cross section.
The capacitive type sensors are used to detect the physical parameters like Humidity or pressure
or proximity etc..
Inductive type Sensors: These sensors are based on the principle that the inductance of a coil
varies with the change input signal.
For example, if a permeable core is inserted into an inductor the net inductance is increases.
The best example is the LVDT (Linear Variable Differential Transducer) which is used to
measure displacements accurately. LVDTs can measure displacements from a few microns to
several feet in a wide variety of environments.
Inductive proximity sensors use an electromagnetic field to detect the presence of metal objects.
Inductive proximity sensors are available in a variety of sizes and configurations to meet varying
applications. The sensor incorporates an electromagnetic coil which is used to detect the
presence of a conductive metal object. The sensor will ignore the presence of an object if it is not
metal.
Limitations of Sensors : The sensors have certain limitations based on their working and construction.
Some sensors require complex signal conditioning circuitry. The output of the certain ssensors is not linear and hence additional circuitry is needed. The response time of sensors is some times very high
Pressure Sensors convert absolute atmospheric pressure into a linear, proportional voltage, which
may be used in any meteorological application. Pressure sensors can be classified in terms of
pressure ranges they measure, temperature ranges of operation, and most importantly the type of
pressure they measure.In terms of pressure type, pressure sensors can be divided into five
categories.
Absolute pressure sensorThis sensor measures the pressure relative to perfect vacuum pressure (0 PSI or no
pressure). Atmospheric pressure, is 101.325 kPa (14.7 PSI) at sea level with reference to vacuum.
Gauge pressure sensorThis sensor is used in different applications because it can be calibrated to measure the
pressure relative to a given atmospheric pressure at a given location. A tire pressure gauge is an
example of gauge pressure indication. When the tire pressure gauge reads 0 PSI, there is really
14.7 PSI (atmospheric pressure) in the tire.
Vacuum pressure sensorThis sensor is used to measure pressure less than the atmospheric pressure at a given
location. This has the potential to cause some confusion as industry may refer to a vacuum
sensor as one which is referenced to either atmospheric pressure (i.e. measure Negative gauge
pressure) or relative to absolute vacuum.
Differential pressure sensorThis sensor measures the difference between two or more pressures introduced as inputs
to the sensing unit, for example, measuring the pressure drop across an oil filter. Differential pressure is also used to measure flow or level in pressurized vessels.
Sealed pressure sensorThis sensor is the same as the gauge pressure sensor except that it is previously calibrated
by manufacturers to measure pressure relative to sea level pressure.
The NPC-1220 is a solid state IC Pressure Sensor used to measure pressure.series of solid state
pressure sensors are designed to provide a cost effective solution for applications that require
calibrated performance over a wide temperature range. Packaged in a dual-in-line configuration,
the NPC-1220 series is intended for printed circuit board mounting. The NPC-1220 offers the
added advantage of superior temperature performance over the temperature compensated range
A1 = Area of pipe upstream from restrictionA2 = Flow area of pipe at restrictionp1 = pressure upstream from restrictionp2 = pressure at restrictionρ = density of fluidC = correction factor for energy losses
The differential pressure sensors
The SDPx108 series sensors are a high-performance sensors , specially designed for air flow
measurements.The SDPx108 sensor provides an analog output signal with a range of 0-4 V and
is fully calibrated and temperature compensated. In addition, it has a high resolution.The
SDPx108 sensor series achieves outstanding sensitivity and accuracy even at extremely low
differential pressures and exhibits very high stability and has zero point drift. Furthermore, the
SDPx108 series offers square-root output characteristics for a wider dynamic measurement
range. The faster response time - compared to the SDP1000 series - makes the differential
pressure sensor a perfect alternative for respiratory devices and time-critical process automation
applications. The simple differential pressure sensor used to measure flow rate is shown below.
It is a 3-pin CMOS IC, whose output is at the pin 3. It has internal temperature compensation, on