Position Sensors play an important role in our daily lives. A position sensor, as the name indicates, provides feedback on the position of the measurand by detecting the presence/absence of an object (or disturbance of an electric or magnetic field), and converting that physical parameter into an electrical output.
Inductive Proximity Sensor – An Introduction
Here, in this article, let us discuss about the Inductive Proximity Sensor/Switch which is the most frequently used position/proximity sensor in many applications. Inductive proximity sensors can be used to recognize any conducting metal target. Inductive proximity sensors make use of electromagnetic field that penetrates through the target. An inductive proximity sensot consists of an oscillator that generates a high frequency electromagnetic field. This field is radiates from the sensing face of the system. When this field contacts a conducting metal target, a small current is induced within the metal target. These currents will generate their own electromagnetic field that interferes with the field originating from the system. This causes a change in the amplitude of the oscillations of the signals from the system. The output voltage can be calibrated to this change. When the system is closer to the target, the more current reacts with the field originating from the system and the output is greater.
The material and size (also the thickness) of the target is an important factor for inductive proximity sensors. Generally there are two types of target materials for inductive sensors; ferrous and nonferrous. Ferrous materials (iron and most steel materials) are magnetic in nature while nonferrous materials (zinc, aluminum, copper and brass) are nonmagnetic. Note that some inductive sensors will work with both ferrous and nonferrous target materials while others will work only with one type of material, and most inductive proximity sensors are independent of the material in the gap between the probe and the target (they can be used in hostile environments where oil or other liquids may appear in the gap).
Since the output voltage (and current) of an inductive sensor is directly proportional to the distance between the sensor and the target (the voltage and current represent the absolute measured values corresponding to the distance), inductive proximity sensors are widely used in numerous applications.
Inductive Proximity Sensor – Inside Electronics
In an inductive proximity sensor, a magnetic field is produced by a precision coil at its heart. Whenever, this magnetic field is disturbed by the close proximity of a metal object (when the target -metal object enters in this field), an Eddy current will be generated that circulates within the target. Due to this, load ing will be caused on the sensor that decreases the amplitude of the electromagnetic field. If the metal is moved towards the sensor, then the eddy current will varies accordingly. The trigger/amplifier circuitry inside the sensor is used to monitor the amplitude of the oscillator, and at certain levels (predetermined levels) the circuitry switches on (or off) the sensor/switch output.
Here is the reference circuit diagram of a simple inductive proximity sensor, wired around the 8-pin proximity detector chip TDA0161. This monolithic integrated circuit (from STMicroelectronics) is designed for metallic object detection by sensing variations in high frequency Eddy current losses. Using an externally-tuned circuit, it act as an oscillator. The output signal level is altered by an approaching metallic object. The output signal is determined by supply current changes. Independent of supply voltage, this current is high or low, according to the presence or absence of a closely located metallic object.
As TDA0161 is now an obsolete device, the indicated circuit diagram can only be considered like a reference material. However, if you have one TDA0161 in your junk box, then try to build this simple inductive proximity sensor circuit. The circuit (works at 425KHz) promises a detection range of about 5mm.
Metal Proximity Detector Module – Ready To Go…
For basic microcontroller projects, it is possible to rig up your circuit with a pre-wired module like the one described here. At the heart of this module is an inductive oscillator circuit which monitors high frequency current loss in the coil. The circuit is designed for detecting metal objects upto 7 cm giving active-low output with audio and visual indications. The active-low (L) output can be given directly to a microcontroller for further processing tasks. In India, this module is available from: Sunrom Technologies (www.sunrom.com/p/metal-proximity-detecting-sensor)
The LMP91300 is a readily-available complete analog front end (AFE) optimized for use in industrial inductive proximity sensors. The LMP91300 detects the presence of a metal object based on the RP change of an LC oscillator, depending on the distance of the metal object. The LMP91300 can be used in 3-wire NPN/ PNP inductive sensors. Refer the datasheet of LMP91300 to get in touch with this dedicated chip.