Can Sensors Detect Presence of Objects Without Any Contact?
Proximity sensor is a type of sensor which is capable to detect the presence of objects without any contact. These sensors help to detect the presence of objects using electromagnetic fields, light and sound by looking for the changes in the field of the return
signal. Here the object that has to be sensed is called as Target. Different proximity sensors detect different targets. In case of capacitive proximity sensors, it is used to detect a plastic target while in the case of inductive sensors it detects a metal target. Maximum distance that the proximity sensors can detect is defined as the Range (R). The main advantage of the sensor is that they have long life and better reliability because of the absence of moving parts. (IEC) 60947-5-2, International Electrotechnical Commission, defines the technical details of proximity sensors. Proximity sensors with very short range are often called as a Touch Switch.
Major Types of Proximity Sensors
There are different types of proximity sensors which are chosen depending upon the applications. Some of the main proximity sensors are:
1. Inductive Sensors
Inductive proximity sensors detect ferrous targets. They operate under the principle of inductance. Inductance is the phenomenon where a fluctuating current induces an emf in a target object. The components in the inductive sensors are ferrite core with coils, oscillator, Schmitt trigger and amplifier.
Oscillator creates a magnetic field that radiates from the ferrite core and coil array at the sensing area. When the target enters the magnetic field, eddy current will be induced on the surface of the metal. This changes the reluctance of the magnetic circuit and causes reduction in the oscillation amplitude. As more metal enters the magnetic field, oscillation amplitude reduces, and collapses. Schmitt trigger responds to the amplitude changes and adjusts the sensor output. When the target moves from the range, oscillation again starts, and the Schmitt trigger returns the sensor to its previous output. If the inductive sensor has a normally open configuration, the output is an on signal when the target enters the sensing range. With normally closed, its output is an off signal with the target present. Output of the inductive sensor is read by control unit and converted into usable information. Normal speed ranges from 10 to 20 Hz in AC or 500 Hz to 5 kHz in DC for the inductive sensor.
2. Capacitive Sensors
Capacitive sensors are similar to inductive sensors. The main difference is that capacitive sensors produce an electrostatic field instead of an electromagnetic field.
These sensors are used to sense metal and non – metal targets. Sensing surface of the capacitive sensor is formed by two concentrically shaped metal electrodes of capacitor. Capacitive proximity sensors commonly depend upon the dielectric constant of the target. If the dielectric constant of the target is larger, then it is easy to detect. When the object comes near the sensing surface, it will enter into the electrostatic field and changes the capacitance of the oscillator circuit. As a result the oscillator will be oscillating and the trigger circuit used will be reading the oscillator amplitude and when it reaches a particular level the sensor output changes. Now when the target moves away from the sensor, the amplitude of the oscillator decreases and the sensor output returns to the orginal state.
Applications for capacitive proximity sensors are:
- Level control of granular substances,
- Level control of non-conductive liquids.
- Sensing substances through a protective layer.
3. Ultrasonic Proximity Sensors
Ultrasonic Proximity Sensors consists of a transducer which helps to send and receive the sound signals. When the target comes to the beam, sound gets reflected back to the switch. Due to this, the output circuit will energize or de-energize. Usually in an ultrasonic proximity sensor, a piezoelectric ceramic disk is attached to the sensor surface. This disk can transmit and receive high frequency pulses. When a high voltage is applied to the disk, high frequency sound waves will be produced. Duration of the pulse reflected is evaluated by the transducer. When the target enters the range, output of the switch changes and when the target leaves the range, output returns to the orginal state.
4. Photoelectric Sensors
Photoelectric Sensors are proximity sensors that can detect targets within a range of 10 meters. They are commonly used to detect metallic and non-metallic targets. These sensors work in different modes, they are:
- Thru-Beam: Here the detector and the emitter are in two separate units. Firstly, the emitter will be sending a beam of light which will be detected by the detector. The target is detected when they pass between the emitter and the detector.
- Diffuse Reflective: In this, emitter and the detector are not separated. Here when the target comes in the range, light is reflected from the target and detected by the detector.
- Retro-Reflective: Here the emitter and the reflector are in the separate units. Light from the emitter is reflected by the retro-reflector and detected by the detector. When the target passes between the reflector and sensor the beam will not reflect back to the detector. To solve this, a polarizing filter is being used. With the filter, light reflected from retro-reflector is detected by the detector.
Proximity Sensor Comparison
The table below shows the comparison between the proximity sensors based on the technology, range, applications and target materials used.