Explain the principle, construction and working of a Magnetostriction Oscillator.
This is based on the principle of magnetostriction effect.
⦁ A ferromagnetic rod MN is clamped in the middle. Two coils L1 and L2 are wound round its two halves.
⦁ The coil L1 is connected in the plate – cathode circuit and the coil L2 is connected in the grid cathode circuit of a triode valve oscillator.
⦁ The capacitor C has a variable capacitance .
⦁ As the circuit is switched on the plate current Ip starts flowing through coil L1 producing magnetic field B along the axis of the rod as shown:-
⦁ Now the rod is placed in a magnetic field along its length as a result of which its overall length will change by a small amount.
⦁ As the length changes the flux linked with L1 and L2 also changes giving rise to an induced emf , e= -dφ/dt across L2 .
⦁ The induced emf is fed to the grid-cathode circuit and the oscillator valve gives an amplified alternating voltages as output to the plate-cathode circuit causing an alternating plate current.
⦁ Due to the alternating current flowing through coil L1 the magnetic field becomes an alternating magnetic field giving rise to the vibration of the rod.
⦁ The frequency of mechanical vibration of the rod is given by
`f_(rod) = p/(2L) √Y/ρ`
Where p = mode of vibration
L = length of the rod.
Y = Young’s modulus of the material of the rod and, density of the material of the rod.
For first harmonic p =1 and the frequency of vibration becomes
`f(rod) = p/(2L) √Y/ρ`
The frequency of the triode valve oscillator is given by
`f_(osc) = 1/(2π√L_1 C)`
By varying the capacitance C of the variable capacitor the oscillator frequency , fosc can be adjusted to the mechanical frequency of the rod frod . In the case
frod = fosc
And resonance takes place.
⦁ At resonance the rod vibrates vigorously emitting ultrasonic waves from both its ends.
⦁ By adjusting the length of the rod and the capacitance of the variable capacitor ultrasonic waves of frequency up to 300KHz can be produced .