Detection Objectives and Sensor Types—Mechatronics Course for Mechanical Designers—The Role of Device in Machinery (Equipment)—“Detection”

What is Detection?

“Detection” refers to the process of activating a signal from a sensor when the sensor perceives a specific type of information (such as the presence or absence of a workpiece).
For example, an optical sensor detects the absence of a workpiece by detecting that the light from the input reaches the receiver without being blocked by an obstacle on the optical axis. If the sensor can’t detect the light at the receiver, an obstacle (the workpiece) is present.

Detection objectives and sensor types

This article describes four objectives of the detection processes used for testing. These objectives are “detecting the presence or absence of a workpiece”, “detecting the position of a workpiece”, “detecting the level of a liquid”, and “detection done by safety light curtains”.

1. Detecting the presence or absence of a workpiece

Detecting the presence or absence of a workpiece is the most basic and typical detection objective.
Typical items to be detected include wafers, glass substrates, metal objects and paper.
Special care is needed when detecting highly reflective items such as wafers, and transparent items such as glass.
The main sensor types used to detect the presence or absence of workpieces are “proximity sensors,” “micro photosensors” and “photoelectric sensors.”

	Proximity sensors	Micro photosensors	Photoelectric sensors
Features	Sensors that operate using magnetism Suitable for detecting magnetic metal objects Can detect metals such as stainless steel and aluminum, but sensing distance is short (about 0.5 mm). Sensing distance is limited—the object to detect should be within a distance of about 0.5 to 3 mm from the sensor.	Sensors that operate using light Two types (through-beam and reflective) High repeatability (an error of just a few microns) Wide range of applications; inexpensive Large variety of shapes to accommodate different mounting orientations	Sensors that operate using light Two types (through-beam and reflective) More expensive than micro photosensors Amplifiers are often tunable, making it easy to adjust the sensor to the optimum gain.
Uses	Detecting metal parts for machining or other workpieces mounted in jigs or tools	Checking the operation of stage sensors, moving parts, etc.	Detecting a wide range of different workpieces

2. Detecting workpiece positions (seating, etc.)

Along with detecting the presence or absence of a workpiece, the sensors below can also determine whether the workpiece is in the right position (properly seated).

	Photoelectric sensors	Laser sensors
Features	Sensors that operate using light Two types (through-beam and reflective) More expensive than micro photosensors Amplifiers are often tunable, making it easy to adjust the sensor to the optimum gain.	Sensors that operate using light Very high accuracy Expensive
Uses	Detecting a wide range of different workpieces	Applications such as length measurement

These sensors detect workpieces from the performance of the sensor itself.
But in some cases they can also achieve detection by operating a component within the seating mechanism and detecting that component.

3. Liquid level detection

Liquid level detection is a common requirement when working with liquids.
The sensors below are used to detect liquid levels. Note that factors such as different difficulty levels and accuracy requirements need to be considered when selecting the sensor to use.
The use of individual sensor features may sometimes also require careful checks of individual specifications and discussions with the manufacturer.

	Ultrasonic sensors	Photoelectric sensors	Float sensors
Features	Emit oscillating ultrasonic waves onto the liquid surface, and use the wave return time to measure the distance.	The sensor is mounted in intersection with the liquid surface, and detects the liquid level from the light blocked by the liquid surface.	Used while immersed in liquid. Have a float that detects the liquid level by moving up and down with the liquid. Include a sub-category known as magnetostrictive sensors.

4. Safety light curtains

Since safety is a key area of concern for machinery (equipment), operation safety measures are studied carefully.
Safety light curtains are a typical safety measure.
Safety light curtains detect area encroachments by detecting objects cutting across any of multiple optical axes.
They are installed in places where arms, fingers or other body parts entering the zone of operating machinery (equipment) is deemed dangerous.
Since these safety-related sensors play such an important role in saving lives and preventing injuries, the same sensor model may be listed in different categories

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