Proximity Sensors: Comparing Inductive, Capacitive, Photoelectric, and Ultrasonic Sensors
Smartphones know to dim the screen and disable touch interaction before it comes into contact with a user’s ear— and they’re only capable thanks to a photoelectric proximity sensor. Proximity devices can to infer the presence of a variety of objects and materials without direct contact. The most common proximity sensors are inductive, capacitive, photoelectric, and ultrasonic.
1. Inductive sensors
Inductive sensing operates via a solid-state control system. An oscillator circuit emits a magnetic field from a ferrite core and coil array. When the systems emitted frequency encounters a metal object, it shifts and causes the oscillation amplitude to drop. If this occurs, it signals the presence of the target object.
- Detects ferrous and non-ferrous materials such as: Iron, Steel, and Copper
- Low detection range of 4 - 40mm
- Typically rated by frequency
- AC: 10 - 20 Hz
- DC: 500 Hz - 5 kHz
2. Capacitive sensors
Relatively similar to inductive sensors, capacitive sensors emit an electrostatic field instead of a magnetic field. Two metal plates serve as a sensing surface, while an oscillating circuit forms an electrostatic field within the sensor. When it encounters a target object, the electrostatic field is disturbed, and changes the capacitance of the proximity detector.
- Detects conductive and non-conductive materials, unlike inductive detectors.
- Slower due to metal plates
- Ideal for detection within powder, liquid, or granulate
- Detection range of 3 - 60mm
- AC: 10 - 50Hz
3. Photoelectric sensors
They’re versatile sensors used in a variety of industries. There are three common photoelectric sensor configurations that operate similarly to one another but are suited to different environments.
Through-beam sensing uses two separately housed light emissions faced at one another. A beam of light is created when the light from the emitter and through-beam combine. Detection is triggered when an object intercepts the beam. Common with garage door operations and conveyor belt monitors.
- Effective among airborne contaminants
- Meet long range requirements of 25m and beyond
Costly, difficult to assemble
Retro-reflective sensing is similar to through-beam because it uses two beam sensors, however, it houses both the emitter and receiver in one location. A reflector is placed opposite, and reflects the emitter beam back to the receiver. Any break or disturbance in the beam signals the detector.
- Meets similar long-range requirements
- Sensitive to reflective items in their surroundings
Diffuse sensing works similar to retro-reflective, but instead of a reflector, the target object acts as the indicator. The device emits an infrared light or laser, creating a diffusion of light in a detection zone, which is then reflected off of the target object. Reflected light is then sent back to the receiver, where output is determined.
- Simple to install
- Can be adjusted to a specific target area
- Sensitive to reflective, and/or textured surfaces
4. Ultrasonic Sensors
Though these devices employ sound waves as their detector, the configurations are the same as in photoelectric detection. Emission of sound waves follows the process of either through-beam, retro-reflective, or diffuse sensing to identify a target object. However, they utilize a sound transducer to emit sonic pulses, rather than light. A receiver gathers the reflected signal and triggers an output.
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