Question

The features of push-pull self-inductance transducer are

• A. Immune to effects of external magnetic field and reduced mechanical input impedance
• B. Immune to effects of external magnetic field and increased mechanical input impedance
• C. Susceptible to effects of external magnetic field and reduced mechanical input impedance
• D. Susceptible to effects of external magnetic field and increased mechanical input impedance

Hint:

Differential push-pull transducers utilize symmetrical cancellation to cancel out external stray magnetic fields (common-mode noise) while balancing electromagnetic forces on the core, increasing mechanical input impedance.

Answer 1

The Correct Option is B

Concept: An inductive displacement transducer operates by changing its self-inductance in response to the movement of a ferromagnetic core. A push-pull configuration uses two identical coils positioned symmetrically around a central movable core. When the core shifts, the inductance of one coil increases ($L_1 = L + \Delta L$) while the inductance of the second coil decreases by a matching amount ($L_2 = L - \Delta L$).

Step 1: Analyzing external magnetic field immunity.
In a differential push-pull circuit configuration (such as when connected to an AC bridge network), the output signal depends on the difference between the two inductances ($L_1 - L_2$). If an external stray magnetic field introduces noise into the environment, it induces an equal electromagnetic interference (EMI) error voltage in both adjacent, symmetrically wound coils. When the differential subtraction is calculated ($V_{\text{out}} \propto \Delta L$), this common-mode noise cancels out. This makes the push-pull transducer highly immune to external magnetic fields.

Step 2: Evaluating mechanical input impedance.
Mechanical input impedance refers to the ratio of the applied mechanical force to the resulting velocity of the moving sensor element. In a single-coil inductive setup, the magnetic field exerts a continuous, one-directional electromagnetic pull on the core, requiring a larger external force to overcome this attraction and move it. In a symmetrical push-pull configuration, both coils exert equal and opposite electromagnetic forces on the central core when it is near the equilibrium position. These forces balance out, minimizing the net magnetic drag on the core. Consequently, less external mechanical force is required to displace the target assembly, which corresponds to an increased mechanical input impedance (meaning the sensor behaves as a lighter, more responsive load that minimizes mechanical loading errors on the moving target). This validates Option (B).