Proximity Switches

The proximity switch uses a principle called eddy current killed oscillator, so it's sometimes called an ECKO switch. The proximity switch is a non-contact limit switch.

ill. 1 shows several examples of proximity switches. The inductive proximity switch detects the presence or absence of a target, which means that it can be used to tell when the switch comes into close position (proximity) of metal. The switch is an on/off sensor since it indicates the presence or absence of a target. Typical sensing distance for a proximity switch is 0.2 mm to 10 mm. The distance of 0.2 mm is about the thickness of a business card. The capacitive proximity switch can detect the presence of material such as plastic, wood, cardboard, paper, and nonferrous metals such as aluminum.

Above: ill. 1 Examples of proximity switches. The size of the barrel of the larger switch in this picture is about the size of a 50-cent piece, and the smallest one is about the thickness of a pencil.

ill. 2 shows a block diagram of the internal circuit of the proximity switch. From this diagram, notice that a power supply energizes the oscillator portion of the circuit. The oscillator produces magnetic flux lines that emanate out the end of the sensor. When the switch is not sensing anything in the field, the strength of the oscillator circuit is at its maximum. The integrator op amp detects the difference in the oscillator circuit and sends an output to the trigger, which activates the output part of the proximity switch. It does not matter if the metal is moved close to the switch or if the switch is moved close to the metal.

Above: ill. 2: Block diagram of an eddy current-killed oscillator (ECKO) proximity switch.

The proximity switch is available as a switch for ac or dc loads. ill. 3a shows the proximity switch wired to the coil of a motor starter. The proximity switch in this diagram is a two-wire switch, which means it's basically wired in series with the coil and the 110 volt ac power supply. This type of sensor uses the same two wires to get its power and to switch current on and off to the load. The proximity switch will have low impedance (resistance) like a closed switch when no target is present and it will have high impedance like an open switch when a target is sensed. ill. 3b shows the internal electronic circuit for an ac-type switch. From this diagram notice that the ac voltage passes through a bridge rectifier so that the oscillator portion of the switch can be supplied with dc voltage. An SCR is connected in series with the dc side of the bridge circuit. The gate of the SCR is connected to the oscillator, and when the proximity switch is not near metal, the oscillator will provide enough current to trigger the SCR. When current flows through the SCR, it allows ac current to flow through the ac side of the bridge circuit. When the SCR is not turned on, it will block dc current flow in the bridge, which will also stop ac current flow in the bridge. This causes the proximity circuit to look like an open switch.

Above: ill. 3: (a) A simple electrical diagram that shows a two-wire proximity switch controlling ac current to the coil of a motor starter. (b) An electronic diagram of the proximity switch. Notice that an SCR is used to interrupt current in the dc side of a bridge circuit.

The proximity switch is also available to sense nonmetallic targets. These types of switches use a variable capacitance circuit and anything in the target range that changes the capacitance will cause the switch to trip. The capacitive proximity switch can be used to indicate the presence or absence of plastic caps on bottles or other similar applications. ill. 4 shows several typical applications for proximity switches. The first application shows a proximity switch used to detect the caps on bottles as they come through a wash-down area. The proximity switch is very useful to detect the cap because it can sense the metal or plastic in the cap, and the proximity switch is a sealed-type switch. The second application shows a proximity switch detecting the level of milk in a cardboard carton. The capacitive proximity switch is good for this type of application. The third application shows two proximity switches used to determine the high and low levels of liquid in a tank. This type of sensor is good in applications where the liquid may foam, such as in the storage of beer or ale.

Above: ill. 4: (a) Application of a proximity switch that is used to check for bottle caps in a high-humidity location. (b) Example of a proximity switch used to detect the level of milk in a carton-filling application. (c) Example of a proximity switch used to determine the level of liquid in a tank.