UNIT – 5 GENERAL INSTRUMENTATION

General Instrumentation System and its Components

• Instrumentation is a collective term for measuring instruments that are used for indicating, measuring and recording physical quantities such as flow, temperature, level, distance, angle, or pressure.

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• Instrumentation can refer to devices as simple as direct-reading thermometers, or as complex as multi-sensor components of industrial control systems.

• A transducer is a device that converts one type of energy to another.
• The input transducer is called the sensor.
• The output transducer is called the actuator.

Transducer

Basic requirements of a transducers

The main function of a transducer is to respond only for the measurement under specified limits for which it is designed.

• RUGGEDNESS (Capability of withstanding overload)
• LINEARITY (input – output characteristics should be linear)
• REPEATABILITY (should reproduce same output signal when the same input signal is applied again and again)
• HIGH OUTPUT SIGNAL QUALITY (quality of output signal should be good)
• HIGH RELIABILITY & STABILITY
• GOOD DYNAMIC RESPONSE (output should be faithful to input when taken as a function of time)
• NO HYSTERESIS (should not give any hysteresis during measurement)
• RESIDUAL DEFORMATION (should be no deformation on removal of local after long period of application)

CLASSIFICATON OF TRANSDUCERS

Basis - Application – method of energy conversion – nature of the output signal

(An inverse transducer is a device which that converts an electrical quantity into a non-electrical quantity)

• Active Transducers: The output energy of Active Transducers is supplied entirely by its input signal.
• Energy obtained from physical phenomenon being measured
• Passive Transducers: Have an auxiliary source of power. This power source is necessary for the operation of passive transducers.

Electrical Transducer - Output Electrical Energy

Voltage, current, frequency, signals based on resistive, capacitive, inductive effect

Displacement Transducer – Mechanical force into displacement and then to electrical parameters

Capacitive, oscillation, potentiometric, photoelectric etc.,

Analog to Digital transducers

Primary Transducer – The transducer consists the mechanical as well as the electrical devices. The mechanical devices of the transducer change the physical input quantities into a mechanical signal.

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Secondary Transducer – The secondary transducer converts the mechanical signal into an electrical signal. The magnitude of the output signal depends on the input mechanical signal.

Capacitive Transducer

• The capacitive transducer is used for measuring the displacement, pressure and other physical quantities.
• The capacitive transducer works on the principle of variable capacitances due to overlapping of plates, change in distance between the plates and dielectric constant

Principle

• The capacitive transducer contains two parallel metal plates. These plates are separated by the dielectric medium which is either air, material, gas or liquid.
• The input quantity causes the change of the capacitance which is directly measured by the capacitive transducer.

• The capacitive transducer uses the following three effects.
• Variation in capacitance of transducer is because of the overlapping of capacitor plates.
• The change in capacitance is because of the change in distances between the plates.
• The capacitance changes because of dielectric constant.

Capacitive Transducer

• used as a pressure transducer
• measurement of the force
• measurement of both the linear and angular displacement
• measurement of very small distance
• humidity in gases is measured through the capacitive transducer.
• The accuracy of the transducer depends on the variation of temperature to the high level

Applications

Inductive Transducer

• An inductive transducer (electromechanical) is an electrical device used to convert physical motion into inductance.
• Two kinds of inductive transducers available such as simple inductance & two-coil mutual inductance

• Force applied is directly proportional to the change in air gaps, thereby changing the reluctance of the magnetic circuit.
• Inductance measured is proportional to the force applied

Applications

• Proximity sensors to measure position, touchpads, dynamic motion, etc
• Used for detecting the kind of metal, to find miss lost parts otherwise counts the objects
• Detecting the movement of the apparatus which include belt conveyor and bucket elevator etc..

What is Linear Displacement Measurement?

• Linear displacement is movement in one direction along a single axis.
• A position or linear displacement sensor is a device whose output signal represents the distance an object has traveled from a reference point. 

Linear Variable Differential Transformers (LVDTs)

Linear Variable Differential Transformers (LVDTs)

• LVDTs operate on the principle of a transformer.

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• LVDT consists of a coil assembly and a core.

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• The coil assembly is typically mounted to a stationary form, while the core is secured to the object whose position is being measured.

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• The coil assembly consists of three coils of wire wound on the hollow form.

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• A core of permeable material can slide freely through the center of the form. The inner coil is the primary, which is excited by an AC source as shown. Magnetic flux produced by the primary is coupled to the two secondary coils, inducing an AC voltage in each coil. 

• The main advantage of the LVDT transducer over other types of displacement transducer is the high degree of robustness. Because there is no physical contact across the sensing element, there is no wear in the sensing element.

• Because the device relies on the coupling of magnetic flux, an LVDT can have infinite resolution. Therefore the smallest fraction of movement can be detected by suitable signal conditioning hardware, and the resolution of the transducer is solely determined by the resolution of the data acquisition system.