DC Electrics - Switches — Page 29, Lesson 31

I want to walk you through a type of switch that’s far more advanced than the simple mechanical ones you might already know — the proximity switch. This is a sensor that detects the presence of an object without any physical contact, and it’s widely used in aircraft systems, especially in the landing gear. Let’s start with the specific example given in the book: a typical undercarriage system uses proximity switches. Each proximity switch consists of three components: 1. A printed circuit card, which is located inside what’s called the landing gear accessory unit. That’s the electronic brain that processes the signal. 2. A sensor, which is mounted on the appropriate landing gear structure — so it’s physically positioned where it needs to detect something. 3. An actuator, also called a target, for each sensor. This actuator is located adjacent to its sensor — meaning it’s placed right next to it, but not touching it. Now, here’s the key point: the proximity sensor is a hermetically sealed unit. That means it’s completely sealed against moisture, dust, and air. It is actuated by the presence of the actuator or target — but it is not touched by it. Because there’s no physical contact, the proximity switch is unaffected by atmospheric conditions like ice, rain, or dirt, and it’s described as highly reliable. The book then introduces two main types of proximity detectors: capacitive and magnetic. Let’s start with the capacitive type. In this sensor, detection is made by a capacitor undergoing a capacitance change owing to the proximity of material. In plain terms: a capacitor normally stores a certain amount of electrical charge. When a material — any material — comes close to it, that changes the capacitance, and the sensor detects that change. The capacitive proximity detector is described as an extremely versatile device because it can detect all materials, both liquid and solid. That means it’s not limited to metal. It can detect the presence of a ferrous target (one containing iron) or a non-ferrous target (like aluminium or plastic). It can also be used to detect high or low liquid levels in a hydraulic or fuel system. So if you need to know whether a fuel tank is full or empty, a capacitive sensor can do that. Now let’s move to the magnetic type. The book explains the basic principle: a coil situated in a magnetic field will have an electromotive force (EMF) induced in it if the magnetic flux changes. The magnitude of the induced EMF depends on the rate at which the flux is changed. That’s Faraday’s law of induction, but you don’t need to know the name — just the principle: if the magnetic field around a coil changes, it generates a voltage. In its simplest form, a coil is wound around a bar magnet, and one pole of the magnet is located close to a ferrous object — that is, an object made of iron or steel. If that ferrous object moves, the magnetic flux in the magnet changes, and an EMF is induced in the coil. If a number of ferrous objects move past the magnet one after another, a train of pulses is induced in the coil — meaning you get a series of voltage spikes, one for each object passing by. The book then tells us how this is most commonly used in aircraft: magnetic detectors are used in conjunction with mild steel gear wheels. Each tooth in the gear wheel is, in effect, a ferrous object. The detector is located radially — meaning it points toward the centre of the wheel — and close to the periphery of the wheel. As the gear wheel rotates, each tooth passes the detector, and the output has a frequency equal to the frequency of passage of the teeth past the detector. So if you know how many teeth the gear has, you can measure the rotational speed of that gear — this is how many engine and wheel speed sensors work. Now let’s move on to time switches. A time switch, also called a time relay, can be initiated either electrically or mechanically to activate a circuit after a specific time interval has occurred. The book gives a practical example: the auxiliary power unit (APU) air intake door closes 30 seconds after the APU has shut down. So the time switch delays the closing of that door by exactly 30 seconds. Finally, we have centrifugal switches. These can be set to activate or de-activate a circuit as the RPM — revolutions per minute — of a device increases or decreases. The example given is a starter motor cut-out switch. On many aircraft engines, the starter motor must be disengaged once the engine is running under its own power. A centrifugal switch senses the RPM and opens the starter circuit when the RPM reaches a certain value, cutting out the starter automatically. That covers the proximity switch in its capacitive and magnetic forms, the time switch, and the centrifugal switch — all important types of switches you’ll encounter in aircraft DC electrical systems.