PE2-4.3 Mechanical, Passive & Active Electrical Transducers
You can classify transducers and describe displacement-to-pressure and seismic displacement transducers, the LVDT and resistive moisture transducer, and Hall effect and piezoelectric transducers.
The active-versus-passive distinction is the organising idea: an active transducer generates its own output signal, a passive one modulates an externally supplied excitation. The LVDT is the classic passive device and earns its place because it measures displacement with no sliding contact, so it does not wear out. Piezoelectric transducers are active, fast, and fundamentally unable to measure a static quantity — the charge leaks away — which is exactly the sort of limitation worth predicting from the principle rather than discovering in a lab.
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Definition and classification of transducers
The parallel classification for transducers, following the sensor one. Read the two together rather than separately.
Mechanical transducers: displacement-to-pressure
Turning movement into pressure, which is where the mechanical family starts. It is the oldest technology in the subject and still used where electronics cannot go.
Seismic displacement transducers
Measuring motion using a mass on a spring, which is the principle behind accelerometers everywhere. It reappears at micro scale in the next topic.
Passive electrical transducers: the LVDT
The standard displacement transducer, giving an output proportional to position with no contact. Its infinite mechanical life is why it is used in demanding places.
Resistive moisture transducer
Measuring moisture through resistance, which is cheap and drifts. Its limitations are the useful part.
Active electrical transducers: the Hall effect transducer
Producing a voltage from a magnetic field, with no moving parts. It is the sensor inside most position and current measurements you will meet.
Piezoelectric transducers and their inability to measure static quantities
Producing charge from force, and the property that defines the family's limits: the charge leaks, so it cannot measure a steady quantity. That constraint is the item worth remembering.
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