Today, Wuxi Rihuan Sensor Technology Co., Ltd. will introduce to you that there is no need to worry about the selection of proximity switches. Just remember these four words: material, distance, ring, and output.
Proximity switches are one of the most commonly used sensors in industrial automation. However, when facing the diverse requirements of different materials, different distances, and different environments in the field, many people are at a loss when choosing the model. In fact, it's not that complicated. Just remember four words: material, distance, environment, and output - follow these four steps to determine the model, and you won't be confused when choosing.
Step 1: Material - What material to detect?
Proximity switches are divided into inductive and capacitive types. The choice depends on what material the object being measured is made of.
Inductive proximity switch: Only detects metallic objects. An alternating electromagnetic field is generated internally. When a metal object approaches, eddy currents are produced, triggering the switch to act. However, the detection performance varies greatly for different metals - with iron as the reference (attenuation coefficient 1), stainless steel is approximately 0.85, brass is approximately 0.50, aluminum is approximately 0.40, and copper is approximately 0.20. That is to say, for a switch with a nominal detection distance of 4mm, when detecting aluminum, the actual detection distance is only about 1.6mm.
Capacitive proximity switch: It can detect any object with a change in dielectric constant - metals, non-metals, plastics, liquids, and particles are all included. It is suitable for applications such as liquid level control, material level monitoring, and non-metallic object detection.
Selection rule: Use inductance to measure metals; use capacitance to measure non-metals, liquids, and particles.
Step 2: Distance - How far is the detection distance required?
The detection distance refers to the maximum distance between the sensing surface of the proximity switch and the object being measured, within which the switch can reliably trigger its action.
Three key figures need to be distinguished:
Rated detection distance (Sn): The distance measured under standard conditions (standard detection object, rated voltage, normal temperature), which is the figure written in the product manual.
Actual working distance: In actual on-site usage, factors such as voltage fluctuations, temperature changes, and installation errors need to be taken into account. Generally, the reliable working distance is set at 70% to 80% of the nominal distance.
Effective action distance: Affected by the material of the object being tested - the detection distance for iron is the longest, while for copper and aluminum, it will significantly decrease.
The installation method also directly affects the distance:
Shielded (parallel) type: The sensing head is surrounded by a metal cover, which can be embedded in a metal installation surface. The magnetic flux is concentrated in the front, but the detection distance is relatively short.
Unshielded (non-level) type: There is no metal enclosure around the sensing head, which enables a wider magnetic flux range and a longer detection distance. However, it cannot be installed by burying it in metal; otherwise, the surrounding metal would interfere with the detection.
Step 3: Loop - How is the working environment?
The environmental conditions determine whether the proximity switch can operate stably for a long time. When selecting the model, the following points should be considered:
Temperature
The normal operating temperature range of ordinary proximity switches is generally -20℃ to +70℃. If the temperature exceeds 70℃, problems such as sensitivity drift of the internal circuit board and encapsulation adhesive, accelerated aging of components, and softening of the encapsulation adhesive will occur. In this case, a high-temperature type (up to 110℃ to 150℃) must be selected.
2. Protection Grade (IP)
IP65: Dust-proof and water-resistant, suitable for general industrial environments.
IP67: Dust-proof, capable of short-term immersion in water, suitable for humid and washing environments.
IP68: Capable of being submerged for a long time, suitable for underwater or extremely humid environments.
IP69K: Resistant to high-temperature and high-pressure water flushing, suitable for food processing and frequent cleaning scenarios.
3. Electromagnetic Interference
Devices such as frequency converters and large motors can generate strong electromagnetic interference, which may cause signal glitches. Solution: Use shielded cables, install magnetic ring filters, or select anti-interference products.
4. Explosion-proof Requirements
In flammable and explosive areas such as chemical plants, oil and gas facilities, and dust-contaminated sites, intrinsically safe or flameproof proximity switches must be used. Ordinary switches may act as ignition sources.
Step 4: Output - What output type should be selected?
The output type of the proximity switch directly determines whether it can "communicate" with your control system.
1. NPN vs PNP
NPN (low level valid) PNP (high level valid)
When an object is detected, the output terminal is connected to 0V and outputs a low level The output terminal is connected to the positive pole and outputs a high level
Load connection method The load is connected between the positive pole and the signal line The load is connected between the signal line and 0V
Applicable to PLC SINK type input port (low level valid) SOURCE type input port (high level valid)
NPN and PNP have no relation to normally open/normally closed - NPN can be either normally open or normally closed, and the same is true for PNP. The selection key is the PLC input port type. Mixed connection will result in the signal not being recognized or even burning out the module.
2. Two-wire System vs Three-wire System
Two-wire system: The power line and the signal line are shared, making the wiring process simple. It is suitable for scenarios with long wiring distances and numerous points.
Three-wire system: The power supply positive, power supply negative, and signal line are separate. NPN/PNP options are available. It is the most widely used.
3. Response Frequency
If used for high-speed counting or rotational detection, attention must be paid to the response frequency. The response frequency of the ordinary model is approximately 1 kHz, while the high-frequency models can reach 5 kHz or even 10 kHz. Selection principle: Response frequency ≥ Actual detection frequency × 2 (leave sufficient margin).
Four-step summary
Step Keywords Core Question
1 Material Is it a metal or a non-metal? → Inductive or Capacitive
2 Distance How far is it? Is the installation space suitable for shielding or non-shielding?
3 Environment What are the temperature, humidity, interference, and explosion-proof requirements?
4 Transmission Is the PLC NPN or PNP? Does it require high-speed response?
By going through these four steps, the model will naturally be determined.
Rihuan Sensor offers a full range of inductive and capacitive proximity switches, including NPN/PNP, two-wire/three-wire, and common/high-temperature specifications, suitable for various materials, distances, and environmental requirements. Welcome to follow Rihuan Sensor to obtain professional selection support.
