Proximity sensor
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Proximity sensors are sensors able to detect the presence of nearby objects without any physical contact. A proximity sensor often emits an electromagnetic or electrostatic field, or a beam of electromagnetic radiation (infrared, for instance), and looks for changes in the field or return signal. 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.
The maximum distance that this sensor can detect is defined "nominal range". Some sensors have adjustments of the nominal range or means to report a graduated detection distance.
Proximity sensors can have a high reliability and long functional life because of the absence of mechanical parts and lack of physical contact between sensor and the sensed object.
IEC 60947-5-2 defines the technical details of proximity sensors.
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[edit] Dealing with output from a proximity sensor
Conditioning the output of a proximity sensor is frequently difficult. Proximity sensor designers must confront linearity, hysteresis, excitation voltage instability, and voltage offset.
[edit] Types of sensors
- Capacitive
- magnetic
- inductive
- photocell (reflective)
- laser rangefinders
- sonar (typically active or passive)
- radar
- doppler effect (effect not a sensor)
- passive thermal infrared
- passive optical (such as CCDs)
- reflection of ionising radiation
[edit] Capacitive sensors
In capacitive proximity sensors, the sensed object changes the dielectric constant between two plates. A proximity sensor has a range , which is usually quoted relative to water. Because changes in capacitance take a relatively long time to detect, the upper switching range of a proximity sensor is about 50 Hz. The proximity sensor is 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.
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.
[edit] Inductive sensors
Inductive proximity switches take advantage of the physical effect of the change in the quality factor in a resonant circuit caused by eddy current losses in conductive materials. The detector circuit inside the sensor detects the losses and when these losses are over the set point, cause the sensor output to change state (if it is a digital output).
