PE2-4.2 Sensors: Proximity, Motion, Force, Flow & Light

What this is and why it exists

There are a great many sensors and it is tempting to learn them as a table. That is the wrong way, because the table tells you what each one does and not what each one fails at.

Learn each by the physical principle it exploits. The principle predicts the failure modes, and the failure modes are what decide a real selection.

The vocabulary

• Proximity sensor — one detecting presence without contact.
• Inductive sensing — detecting a conductive target through the eddy currents it carries.
• Capacitive sensing — detecting a target through the change it makes to a capacitance.
• Tachogenerator — a small generator whose output voltage is proportional to speed.
• Optical encoder — a disc of marks counted or read as the shaft turns.
• Incremental encoder — one reporting change, so position is accumulated and lost on power failure.
• Absolute encoder — one whose pattern gives position directly at any moment.
• Strain gauge — a resistance that changes slightly as it is stretched.
• Bridge circuit — four elements arranged so a small change produces a readable difference.
• Tactile sensor — one responding to contact and contact pressure.
• Photodiode — a device producing current in proportion to the light falling on it.

The mental model

Take the two proximity sensors together, because the comparison is the lesson.

An inductive sensor generates an alternating field and detects the currents it induces in a nearby conductor. So it detects metal, and only metal. That sounds like a limitation and is often an advantage. It will not be triggered by a hand, by water or by a plastic guard.

A capacitive sensor detects a change in capacitance, and almost any material changes it. That is more general and it is less selective. It will detect a hand, a build-up of dust, and it drifts with humidity, because water changes capacitance readily. Same job, opposite failure modes, and the choice is decided by what the target is and what else is nearby.

For rotation there are two answers. A tachogenerator is a small generator whose output voltage is proportional to speed. It is simple, and it gives speed only. An optical encoder counts marks on a disc, so it gives speed and position, and that is why it dominates.

The encoder raises a distinction that recurs throughout motion control. An incremental encoder reports changes, so position is a running total held in software, and it is lost when power fails. An absolute encoder carries a pattern that reads out position directly, so it knows where it is the instant it is switched on. It costs more, and in a machine that must not lose its place it is worth it.

The strain gauge is the one to know properly, because it is underneath load cells, pressure sensors and torque measurement. Stretch a conductor and it becomes longer and thinner, so its resistance rises. Bond it to a part and it stretches with that part, so its resistance reports the strain.

The change is tiny, a small fraction of one per cent, so reading it needs care. The bridge arrangement is the answer. Four elements are connected so the output is the difference between two divided voltages. That difference is near zero at rest and moves when the gauge does. That gives a sensitive reading of a small change. It also has a second benefit that matters more in practice. A temperature change affects all four arms similarly, so its effect largely cancels. A gauge without a bridge would report temperature almost as strongly as force.

Flow and level sensors share a common idea. Both infer their quantity from something else. An ultrasonic flow sensor times a pulse travelling with the flow and against it, and the difference gives the velocity. A laser sensor measures the light scattered by particles moving with the fluid. Neither obstructs the pipe, which is why they exist. Ultrasonic level sensing times an echo from the surface, and capacitive level sensing reads how much of a probe is covered. Their failure modes differ usefully: foam absorbs and scatters an echo, while a coating left on a vessel wall confuses the capacitive method.

Photodiodes close the topic and are more general than they appear. Many of the sensors above are optical underneath. An encoder is a light source and a photodiode with a patterned disc between them. Recognising that saves learning the same device several times.

What you should now be able to explain or do

Choose between inductive and capacitive proximity sensing from the target material and the surroundings. Say what an encoder gives that a tachogenerator does not. Explain the difference between incremental and absolute encoders and when each is required. Explain how a strain gauge works and give two reasons for the bridge arrangement. Say what ultrasonic flow and level sensors each infer, and how each fails.

Check yourself