PE2-4.3 Mechanical, Passive & Active Electrical Transducers

What this is and why it exists

Transducers divide cleanly in two, and that division is the organising idea of this topic.

An active transducer generates its own output from the quantity it measures. A passive one changes a property, and something else has to supply an excitation for that change to be read.

The vocabulary

• Active transducer — generates its own signal, needing no excitation.
• Passive transducer — modulates an externally supplied excitation.
• Displacement-to-pressure transducer — converts movement into a pressure signal.
• Seismic transducer — a mass on a spring, whose relative movement reports motion of the case.
• LVDT — a linear variable differential transformer, giving an output proportional to position.
• Resistive moisture transducer — one whose resistance changes with moisture content.
• Hall effect transducer — produces a voltage across a current-carrying material in a magnetic field.
• Piezoelectric transducer — produces charge when mechanically stressed.

The mental model

Start with the split. An active transducer is a small generator: apply the quantity and a signal appears, with no supply needed. A passive one is a component whose value changes. Apply the quantity and its resistance, capacitance or inductance moves, but nothing happens electrically until you excite it.

That single fact predicts a great deal. An active transducer needs no excitation and cannot usually measure a steady value. A passive one measures steady values happily and brings the noise and drift of its excitation source along with it.

The mechanical family is the oldest and it survives where electronics cannot go. A displacement-to-pressure transducer turns movement into a pressure signal, which can then be piped somewhere with no wires and no spark risk.

The seismic transducer is worth understanding once, because the principle appears everywhere afterwards. Mount a mass on a spring inside a case. Move the case and the mass lags behind, because of its inertia, so the mass moves relative to the case. Measure that relative movement and you have measured the motion of the case. Every accelerometer works this way, including the microscopic ones in the next topic.

The LVDT is the classic passive device and it earns its place for a mechanical reason rather than an electrical one. A coil is excited, and two more coils are wound so their outputs oppose. A movable core sets how much couples into each, so the difference gives position, and the sign gives direction. The important property is that nothing rubs. There is no sliding contact, so there is nothing to wear out. That is why it is used where a potentiometer would fail within months.

The resistive moisture transducer is in the topic for its limitations as much as anything. It is cheap and its resistance changes with moisture, and it drifts, ages and depends on temperature. That is worth knowing before trusting one.

The active devices come last. A Hall effect transducer produces a voltage across a current-carrying material placed in a magnetic field. The field deflects the moving carriers to one side. It has no moving parts and it is inside most position and current measurements you will meet. Measuring current without breaking the circuit is done this way.

The piezoelectric transducer produces charge when it is stressed. It is fast, it is sensitive and it generates its own signal. It also has a limit that defines the family, and the limit follows from the principle rather than from poor engineering. The output is a charge, and charge leaks away through any real resistance. So the device responds to a change in force and its output then decays. It cannot measure a steady force at all.

That is exactly the kind of limitation worth predicting from the principle rather than discovering in a laboratory. Ask what a device actually produces, and what happens to that quantity if you wait, and half the datasheet becomes obvious.

What you should now be able to explain or do

State the active and passive split and what each implies about excitation and steady values. Explain the seismic principle and connect it to accelerometers. Say why the LVDT is used where wear matters. Explain the Hall effect and one thing it makes possible. Say why a piezoelectric transducer cannot measure a static quantity, from its principle rather than its datasheet.

Check yourself