Proximity Sensors

Proximity Sensors - Inductive and Capacitive types

Proximity sensors use several detecting techniques like inductive, capacitive and magnetic.

A Proximity sensor can detect objects without physical contact. A proximity sensor often emits an electromagnetic field or beam and look for changes in the field. The object being sensed is often referred to as the proximity sensor's target.

Different proximity sensor targets demand different sensors. For example, a capacitive or photoelectric sensor might be suitable for a plastic target; an inductive proximity sensor requires a metal target.

Proximity sensing is the ability of a robot to tell when it is near an object, or when something is near it. This sense keeps a robot from running into things. It can also be used to measure the distance from a robot to some object. If the robot is approaching a light-reflecting object, its microprocessor senses that the reflected beam is getting stronger. The robot can then steer clear of the object. This method of proximity sensing won't work for black objects, or for things like windows or mirrors approached at a sharp angle.

Inductive Proximity Sensors

Inductive proximity sensors are widely used in the modern high speed process control environment for the detection, positioning and counting of ferrous and non-ferrous metal objects.

Due to the method of construction and superior performance of inductive sensors, they are increasingly used to replace the traditional limit switch, thus upgrading speed and reliability of existing machinery.

Inductive proximity sensors respond to ferrous and non - ferrous metal objects. They will also detect metal through a layer of non - metal material. An inductive sensor consists of an oscillator circuit and an output circuit including a switching device (that is transistor or thyristor), all housed in a resin encapsulated body. An essential part of the oscillator circuit is the inductance coil creating a magnetic field in front of the sensing face. When the magnetic field is disturbed, the output circuit responds by either closing the output switch or by opening the output switch.

Capacitive Proximity Sensors

A capacitive proximity sensor uses a radio-frequency oscillator, a frequency detector, and a metal plate connected into the oscillator circuit. The oscillator is designed so that a change in the capacitance of the plate, with respect to the environment, causes the frequency to change. This change is sensed by the frequency detector, which sends a signal to the apparatus. In this way, a robot can avoid bumping into things. Objects that conduct electricity to some extent, such as house wiring, animals, cars, or refrigerators, are sensed more easily by capacitive transducers than are things that do not conduct, like wood-frame beds and dry masonry walls.

Some typical applications for capacitive sensors are:

1. Level control of non-conductive liquids (oil, alcohol, fuel).
2. Level control of granular substances (flour, wheat, sugar).
3. Sensing substances through a protective layer (eg . glass).

The fact that capacitive sensors respond to most substances, necessitates some care during the installation, adjustment and long term operation of the sensor. The sensitivity of capacitive sensors is affected by the moisture content and the density of the substance to be sensed. Deposits of excessive dust and dirt on or around the sensing face of the sensor, cause erratic response and hence the sensor may require periodic cleaning if used in a polluting environment.

Capacitive sensors respond to any substance with a high dielectric constant (water, oil, fuel, sugar, paper) without necessarily making physical contact. They are less suitable for polystyrene and similar low density substances. Operation is based on an internal oscillator with two capacitive plateelectrodes, tuned to respond when a substance approaches the sensing face. When the target is sensed, the output switch will either close to activate a load for a normally open option or the switch will open to de-activate the load for a normally closed option. The LED will illuminate when the output switch closes.

Proximity sensors are often found in
 bulk-handling machines,
 level detectors, and
 package detection.

One advantage of capacitive proximity sensors is that they are unaffected by dust or opaque containers, allowing them to replace optical devices.

A typical capacitive proximity sensor has a 10-mm sensing range and is 30 mm in diameter.

The proximity sensor incorporates a potentiometer to allow fine tuning of the sensing range and can repetitively detect objects within 0.01 mm of the set point. Switching frequency is 10 Hz, and operating temperature range is -14 to 158°F.

Conditioning the output of a proximity sensor has always been difficult. Proximity sensor designers must confront linearity, hysteresis, excitation voltage instability, and voltage offset.

A proximity sensor that measures current flow between the sensing electrode and the target provides readouts in appropriate engineering units. Usually, one side of the voltage source or oscillator connects to the sensing electrode, and the other side connects through a current-measuring circuit to the target, which generally is a metal part at earth or ground potential.

Probes used with a capacitive proximity sensor have either a flat disc or rectangular sensing element surrounded by a guard electrode that provides electrical isolation between the proximity sensor and its housing. The guard also ensures that the lines of electrostatic field emanating from the probe are parallel and perpendicular to the surface of the proximity sensor.

Capacitance proximity sensor systems can make measurements in 100 μsec with resolutions to 10-7 in. (0.001 micron). Probe diameters range from a few thousandths of an inch to several feet for corresponding measurements ranging from thousandths of an inch to several feet.

Custom Search