Proximity Sensor Basics: Types and Functionality

Proximity sensors essentially convert the movement of a person or object into an electrical signal without requiring any physical contact. The Japanese standard JIS C 8201, part-5, Section-2 defines proximity sensors. Based on their functionality, there are three main types of detection systems.

Types of Proximity Sensors

Proximity sensors fall into three main categories: inductive, capacitive, and magnetic.

• Inductive Type: Detects metallic objects such as aluminum, iron, copper, brass, and more. In this type, the coupling condition of a transformer is effectively replaced by an impedance change. This impedance change is caused by eddy current losses and is similar to adding resistance in series with the object being sensed.

• Capacitive Type: Detects both metallic and non-metallic objects, including liquids (water), resins, powders, etc. The sensor measures the change in capacitance between itself and the object. The capacitance value depends on both the distance to and the size of the object.

• Magnetic Type: Detects the presence of magnets. The reed end of a switch is triggered by a magnet. The proximity sensor is activated when the reed switch closes.

How to Make an Inductive Proximity Sensor

The image above depicts an inductive proximity sensor, which operates based on the principle of inductance. This type of sensor is relatively straightforward to build.

Four basic components are needed:

1. Coil: Forms the core of the sensor and generates a magnetic field.
2. Oscillator: Produces a fluctuating magnetic field around the coil. The coil is positioned near the object to be detected.
3. Detection Circuitry: Monitors the strength of the oscillator.
4. Output Circuitry: Triggers an output signal when the oscillator strength falls below a certain threshold.

The oscillator creates a fluctuating magnetic field around the coil. When a metallic object enters this field, eddy currents are induced within the object. These eddy currents reduce the sensor’s own oscillating field. The detection circuit monitors the strength of the oscillator and triggers the output when it goes below a predefined threshold.

Siemens offers a proximity sensor, such as the Simatic-PXI920, that can measure the distance to an object within 0-3mm at a frequency of 250Hz. This model has an M12 frame size and an IP67 degree of protection. It produces an output voltage within the range of 0-10V, proportional to the distance of the object from the sensor.