Understanding Industrial Automation Sensors: Types and Principles

1. Working Principle

Sensors are generally composed of four parts: sensitive elements, conversion elements, conversion circuits, and auxiliary power supplies.

1. The sensitive element directly senses the measured quantity and outputs a physical signal that has a definite relationship with the measured quantity;

2. The conversion element converts the physical signal output by the sensitive element into an electrical signal;

3. The conversion circuit is responsible for amplifying and modulating the electrical signal output by the conversion element;

4. The conversion element and conversion circuit generally also require auxiliary power for operation.

2. Human Senses and Sensors

3. Classification of Sensors

1. Classification of sensitive elements:

Physical types, based on physical effects such as force, heat, light, electricity, magnetism, and sound.

Chemical types, based on the principles of chemical reactions.

Biological types, based on molecular recognition functions such as enzymes, antibodies, and hormones.

Typically, they can be divided into ten categories based on their basic sensing functions: thermal sensitive elements, light sensitive elements, gas sensitive elements, force sensitive elements, magnetic sensitive elements, humidity sensitive elements, sound sensitive elements, radiation sensitive elements, color sensitive elements, and taste sensitive elements.

2. By purpose:

Pressure sensitive and force sensitive sensors, position sensors, liquid level sensors, energy consumption sensors, speed sensors, acceleration sensors, radiation sensors, thermal sensitive sensors.

3. By output signal:

Analog sensors: convert non-electrical quantities into analog electrical signals.

Digital sensors: convert non-electrical quantities into digital output signals (including direct and indirect conversion).

Switch sensors: when a measured signal reaches a specific threshold, the sensor outputs a set low or high signal accordingly.

4. By composition:

Basic type sensors: the most basic single conversion device.

Combination type sensors: sensors composed of different single conversion devices.

Application type sensors: sensors formed by combining basic type or combination type sensors with other mechanisms.

4. Proximity Sensors

Also known as proximity switches, these are position switches that can operate without direct mechanical contact with moving parts. When an object approaches the sensing surface of the switch to the action distance, the switch can operate without mechanical contact or applying any pressure, thus driving DC appliances or providing control instructions to a computer (PLC) device.

Proximity switches are a type of switch sensor (i.e., non-contact switches). They have characteristics of limit switches and micro switches while also possessing sensing capabilities. They are reliable in action, stable in performance, have fast frequency response, long service life, strong anti-interference ability, and are waterproof, shockproof, and corrosion-resistant.

Classification of proximity switches:

1) Inductive proximity switches

2) Capacitive proximity switches

3) Hall effect proximity switches

4) Reed switch proximity switches

4.1 Inductive Proximity Switch

Principle: Composed of an inductive coil, capacitor, and transistor to form an oscillator, generating an alternating magnetic field. When a metallic object approaches this magnetic field, eddy currents are generated within the metal object, causing the oscillation to stop. This change is amplified and processed to convert it into a transistor switch signal output.

Characteristics:

1. Good anti-interference performance, high switching frequency, greater than 200Hz.

2. Can only sense metals.

Used in various mechanical devices for position detection, counting signal pickup, etc.

4.2 Capacitive Proximity Switch

Principle: The measurement end of the capacitive proximity switch is one plate of a capacitor, and the other plate is the outer casing of the switch. When an object approaches the capacitive proximity switch, regardless of whether the object is conductive, its dielectric constant will differ from the original medium (air, water, oil, etc.), causing a change in capacitance, thus causing changes in the internal circuit parameters of the switch, allowing it to detect the presence of an object and control the switch’s on or off state.

▲ Schematic of cylindrical capacitive proximity switch structure

Characteristics:

Can detect not only metals but also plastic, glass, water, oil, and other substances.

Easy to be interfered with, pay attention to installation position.

Induction distance can be adjusted.

Frequency about 50Hz.

Applications:

Based on its characteristics, particularly suitable for detecting non-metallic objects, such as in the food and chemical industries. The capacitive proximity switch can detect any medium, including conductors, semiconductors, insulators, and even liquids and powdered materials.

4.3 Hall Effect Proximity Switch

When a current-carrying metal or semiconductor sheet is placed vertically in a magnetic field, a potential difference will be generated at both ends of the sheet, a phenomenon known as the Hall effect. The potential difference at both ends is called the Hall voltage U.

This type of proximity switch must detect magnetic objects.

4.4 Reed Switch Proximity Switch

Reed switches, also known as magnetic switches, close when a magnetic object approaches, changing the state of the internal circuit of the switch, thereby detecting the presence of a magnetic object nearby and controlling the switch’s on or off state. Follow our WeChat account for more insights!

Suitable for position determination in pneumatic, hydraulic, cylinders, and pistons.

This type of proximity switch must detect magnetic objects.

4.5 Applications of Proximity Switches

5. Parameters of Proximity Switches

Detection distance: The maximum distance at which the sensor can detect an object.

Power range: The range of power voltage within which the sensor can operate normally.

Output current: The maximum current the sensor can provide to the load.

Output voltage drop: The voltage drop occupied by the sensor when outputting.

Response frequency: The maximum number of times the sensor can detect an object per second.

Hysteresis range: The distance between the point of detection of an object and the point where it is no longer detected, usually expressed as a percentage of the detection distance.

Consumption current: The current consumed by the sensor when there is no output.

Repeat accuracy: The difference in detection distances between multiple measurements by the sensor.

Short-circuit protection current: When the output load is short-circuited or too large, the sensor will enter a protection state when the output current reaches the short-circuit protection current.

5.1 Hysteresis Distance

If there is no hysteresis, the proximity action distance will result in repeated ON/OFF switching.

Generally, hysteresis distance is controlled within 5-15% of the detection distance.

6. Optical Sensors

6.1 Classification of Photoelectric Switches

1) Through-beam photoelectric switches

2) Diffuse reflection photoelectric switches

3) Reflective photoelectric switches

4) Fixed distance photoelectric switches

5) Color mark sensors

6.2 Through-beam Photoelectric Switch

Characteristics:

Long detection distance.

Both transmitter and receiver are required.

6.3 U-shaped (Slot-type) Photoelectric Switch

U-shaped photoelectric switches integrate the receiving and emitting functions into one unit.

Due to the integration of emission and reception, installation is very practical and convenient.

U-shaped (Slot-type) Photoelectric Switch Applications

6.4 Diffuse Reflection Photoelectric Switch

Diffuse reflection photoelectric switches are sensors that integrate both the emitter and receiver. When an object passes through the detection area, sufficient light emitted by the photoelectric switch is reflected back to the receiver, generating a switch signal.Follow our WeChat account for more insights!

Characteristics:

Short detection distance.

Only requires wiring at one location, no reflector is needed.

Has both emitter and receiver lenses.

6.5 Reflective Photoelectric Switch

Combines the emitter and receiver. The light emitted by the photoelectric switch is reflected back to the receiver by a dedicated reflector. When an object passes through and completely blocks the light, the photoelectric switch generates a detection switch signal.

Characteristics: Detection distance is less than that of through-beam type. Only requires wiring at one location, the reflector aligns conveniently with the light’s incidence direction.

6.6 Fixed Distance Photoelectric Switch

Characteristics:

Working principle is similar to that of diffuse reflection photoelectric switches, with the emitter and receiver integrated. The photoelectric switch will only operate when an object appears at the focal point.

6.7 Color Mark Sensor

Characteristics:

Used to distinguish colors, commonly used for detecting labels.

Works similarly to diffuse reflection type, requires wiring at one location, no reflector needed.

Working principle of color mark sensors:

Based on the emissivity difference of objects of different colors.

6.8 Fiber and Photoelectric Sensors

1. Photoelectric sensors: Use photoelectric components as detection elements. They first convert the measured changes into variations in light signals, and then further convert the light signals into electrical signals using photoelectric components. Photoelectric sensors generally consist of a light source, optical path, and photoelectric components.

2. Fiber sensors: Transmit light from a light source through optical fibers to a modulator, where the measured parameters interact with the light entering the modulation area, causing changes in the optical properties of the light (such as intensity, wavelength, frequency, phase, polarization, etc.), referred to as modulated signal light. This light is then sent to a light detector through the fiber, and after demodulation, the measured parameters are obtained.

3. In fact, fiber sensors should be considered a type of photoelectric sensor. Generally, fiber sensors have higher precision than ordinary photoelectric sensors. Ordinary photoelectric sensors refer to sensors that emit and receive light directly, and due to light diffusion and other reasons, the amount of received light cannot be precisely controlled, leading to a lack of measurement precision. In contrast, fiber sensors transmit light through fiber, enhancing the concentration of the light beam, making it easier to determine the amount of received light, thus achieving higher measurement precision.

7. Installation of Photoelectric Switches

1. Photoelectric switches can be installed according to the site environment. The measurement accuracy of the product is greatly related to the appropriateness of the installation.

2. Light will diverge when emitted, meaning there is a pointing angle; the detector or receiver will receive light within a certain range, so attention should be paid to this indicator during installation to avoid measurement errors.

3. The light emitted by the diffuse reflection photoelectric switch needs to be sufficiently reflected back to the receiver by the surface of the detected object, so the detection distance and the reflectivity of the surface of the detected object will determine the intensity of light received by the receiver. A rough surface will reflect less light than a smooth surface. Care should be taken regarding the reflectivity of the surface of the measured object during installation.

4. The application environment of photoelectric switches is also an important condition affecting their long-term stability and reliability. When the photoelectric switch operates at its maximum detection distance, the optical lens may become contaminated by the environment or even corroded by strong acidic substances, which can reduce its performance characteristics and reliability. A simple solution in applications is to derate the maximum detection distance (Sn) of the photoelectric switch to ensure optimal working distance.

8. Wiring Methods for Sensors

8.1 Two-wire Output

Characteristics: Simple wiring, the customer’s load can be connected to either of the two output wires.

AC type does not distinguish between positive and negative, while DC brown wire connects to positive and blue wire connects to negative.

8.2 Three-wire PNP Output (Normally Open)

Characteristics: More stable and reliable compared to two-wire operation.

8.3 Three-wire PNP Output (Normally Closed)

8.4 Three-wire NPN Output (Normally Open)

8.5 Three-wire NPN Output (Normally Closed)

8.6 Four-wire NPN Output (One Normally Open and One Normally Closed)

Characteristics: Multifunctional output, flexible wiring, can connect two loads simultaneously or connect any one load.

9. Precautions for Using Photoelectric Switches

1. When using reflective photoelectric sensors, the surface and size of the detected object affect both the detection distance and the action area.

2. Sensitivity is lower when detecting small objects compared to larger ones, resulting in shorter detection distances.

3. The higher the reflectivity of the object’s surface, the higher the detection sensitivity and the greater the detection distance.

4. The minimum size of the detected object for reflective photoelectric sensors is determined by the diameter of the lens.

5. Reflective photoelectric sensors are most suitable for detecting concave and convex surfaces.

6. Prevent interference between photoelectric switches.

7. High-voltage lines and power lines should be routed separately from photoelectric sensors to avoid induced interference causing malfunctions.

8. The power supply voltage should be used within the specified range.

9. Avoid using in areas with excessive dust, corrosive gases, water, oil, or direct splashes of agents, or outdoors in direct sunlight.

10. Use within the specified environmental temperature range.

11. Ensure stable installation, avoiding any looseness or misalignment.

12. Do not damage the emitting or receiving components during the installation of photoelectric sensors.

『Article and images sourced from the internet, copyright belongs to the original author. If there is an infringement, please contact for removal.』

Source: Huasheng Qilian Training Center

Editor:Shi Haijiang

Responsible Editor:Zhu Jinfeng

Review:Chang Haibo

Understanding Industrial Automation Sensors: Types and Principles

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