VARIABLE INDUCTANCE TRANSDUCER These are based on a change in the magnetic characteristic of an electrical circuit in response to a measurand which may be displacement, velocity, acceleration, etc. 1. Self-generating type: Voltage is generated because of the relative motion between a conductor and a magnetic field. - Electromagnetic type - Electro-dynamic type - Eddy Current type 2. Passive type: Motion of an object results in the inductance of the coils of the transducer. - Variable reluctance - Mutual inductance - Differential transfer type
Electromagnetic Type Transducer When a plate of iron or other ferromagnetic material is moved w.r.t. the magnet, the flux field expands or collapses and a voltage is induced in the coil. Used for indication of angular speed. Speed can be measured when the pick-up is placed near the teeth of a rotating gear.
Electro-dynamic Type Transducer Coil moves within the field of magnet. The turns of the coil are perpendicular to the intersecting lines of force. When the coil moves it induces a voltage which at any moment is proportional to the velocity of the coil. Used in magnetic flow meters.
Eddy Current Type When a plate of nonferrous material is moved cutting magnetic flux lines, a voltage is induced in the coil.
Variable Reluctance Transducer Magnetic circuit reactance may be changed by affecting a change 1. In the air gap (reluctance type) or 2. In the amplitude/type of core material (permeance type) In variable reluctance type transducer, the change in inductance may be calibrated in terms of movement of armature. Used for measurement of dynamic quantities such as pressure, force, displacement, acceleration, angular position, etc.
In Variable Permeance type Transducer the inductance of coil is changed by varying the core material. When the coil on insulating tube is energized and the core enters the solenoid cell, the inductance of the coil increases in proportion to the amount of metal within the coil. Used for measurement of displacement, strain, force, etc.
Mutual Inductance Transducer A change in the position of armature by a mechanical input changes the air gap. This causes a change in output from coil Y, which may be used as measure of the displacement of mechanical input.
Linear Variable Differential Transformer (LVDT) Passive inductive transformer Used to measure force ( or weight, pressure, acceleration etc which depend on force. LVDT (Linear Motion Type) Ms. Amit Saxena (Asst. Prof. DCE Gurgaon)
Linear Variable Differential Transformer (LVDT) LVDT (Rotary Motion Type)
INDUCTIVE TRANSDUCERS In these transducers, the transduction mechanism is one where the self-inductance of a single coil or the mutual inductance between two coils is changed by a measurand. In general, the measurand could be a linear or rotary displacement, pressure, force, torque, vibration velocity, and acceleration. The inductance changes are brought about by the movement of a concentric magnetic core. The inductance of a single coil increases as the core is inserted in the coil and reaches a maximum value when it is centered on the coil length. Similarly, two separate coils L1 and L2 wound on the same bobbin can also be used as a displacement transducer. Any measurand that moves the core directly through a summing device will produce a change in the impedance of the coils that is proportional to the magnitude of the measurand. The coils can be used as the adjacent arms of an impedance bridge. Since L1 increases by the same amount that L2 decreases, or vice versa, the bridge output will be doubled. A variation of the inductive transducer, shown schematically in Fig. 5.6a, is known as the linear variable differential transformer (LVDT).This transducer consists of a primary coil L1, two interconnected coils L2, L3, and a common magnetic core.
The coils are wound on a hollow nonmagnetic glass-filled nylon tube and the core slides coaxially inside the tube. The excitation frequency for coil L1 ranges from 1 to 10 khz. Coils L2 and L3 are wound in phase opposition in a way that the voltages induced in them by coil L1 are 180 out of phase. Fig 5.6 Inductive transducers. (a) Linear variable differential transformer (LVDT) (b) LVDT accelerometer. Consequently, the voltage at terminals c-d is zero when the core is centered inside the tube between coils L2 and L3. When the core is moved away from the null position, the voltage at terminals c-d changes in amplitude and phase (polarity). This change, when brought about by a measurand, is proportional to the magnitude of the measurand.
LVDTs are available in linear stroke lengths of ±1 to ±300 mm and sensitivities of 1.7 to 250 mv/v/mm, depending on the stroke length. The same transduction mechanism is also used in a rotary variable differential transformer (RVDT) to measure angular displacements and torque. To achieve good linearity, the angle of rotation is limited to ±40. The LVDT can be used with Bourdon tubes, bellows, and proving rings to measure force and pressure and the leaf springs provide the restoring force. This is an open-loop accelerometer Source: http://mediatoget.blogspot.in/2012/05/inductive-transducers.html