Technical Guide Proximity Sensors
Overview
What Are Proximity Sensors?
"Proximity Sensor" refers to all sensors that perform non-contact detection, unlike limit switches which detect objects by physical contact. Proximity Sensors convert information about an object's movement or presence into an electrical signal. Three primary detection systems are used: those utilizing eddy currents generated in metallic objects by electromagnetic induction, those detecting changes in electrical capacity as an object approaches, and those using magnets and reed switches.
The Japanese Industrial Standards (JIS) define proximity sensors in JIS C 8201-5-2 (Low-voltage switch gear and control gear, Part 5: Control circuit devices and switching elements, Section 2: Proximity sensors), conforming to the IEC 60947-5-2 definition of non-contact position detection switches. JIS broadly names all sensors providing non-contact detection of nearby or generally present target objects as "proximity sensors," classifying them by type such as inductive, capacitive, ultrasonic, photoelectric, and magnetic. This Technical Guide covers inductive sensors for metallic objects, capacitive sensors for metallic or non-metallic objects, and sensors utilizing magnetic DC fields as Proximity Sensors.
Features
- Non-contact Detection: Proximity Sensors detect objects without touching them, preventing abrasion or damage. Unlike contact-based devices like limit switches, they detect object presence electrically.
- Longer Service Life: With semiconductor outputs (excluding magnet-based sensors), there are no physical contacts to wear out, leading to a longer service life.
- Resistance to Environment: Unlike optical methods, Proximity Sensors are suitable for wet or oily locations, with detection largely unaffected by dirt, oil, or water. Models with fluororesin cases offer excellent chemical resistance.
- High-Speed Response: Proximity Sensors offer faster response times compared to physical contact switches. Refer to the "Explanation of Terms" on page 3 for details on high-speed response.
- Wide Temperature Range: Proximity Sensors can operate within a wide temperature range, from -40°C to 200°C.
- Unaffected by Color: Proximity Sensors detect physical changes in an object, making them nearly immune to the object's surface color.
- Environmental Sensitivity: Unlike contact switches, Proximity Sensors are influenced by ambient temperatures, surrounding objects, and other sensors. Inductive and Capacitive Proximity Sensors can be affected by interaction with other sensors, requiring careful installation to prevent mutual interference (see page 8). Inductive sensors are also affected by surrounding metallic objects, while capacitive sensors are affected by all surrounding objects.
- Two-Wire Sensors: These sensors combine power and signal lines, reducing wiring by two-thirds compared to three-wire sensors. Incorrect wiring of only the power line can damage internal elements; always include a load (refer to page 6).
Operating Principles
Detection Principle of Inductive Proximity Sensors
Inductive Proximity Sensors detect magnetic loss caused by eddy currents generated on a conductive surface by an external magnetic field. An AC magnetic field from a detection coil detects changes in impedance due to eddy currents on a metallic object. Other methods include Aluminum-detecting Sensors (detecting frequency phase components) and All-metal Sensors (using a working coil to detect impedance changes). Pulse-response Sensors generate eddy currents in pulses and detect time changes via induced voltage.
Qualitative Explanation: The sensing object and sensor form a relationship akin to a transformer. The coupling condition is replaced by impedance changes due to eddy-current losses. These impedance changes can be conceptually viewed as resistance changes in series with the sensing object, aiding qualitative understanding.
Detection Principle of Capacitive Proximity Sensors
Capacitive Proximity Sensors detect changes in capacitance between the sensing object and the sensor. Capacitance varies with the object's size and distance. A typical capacitive sensor functions like a two-plate capacitor, detecting changes in capacity. One plate is the object (with an imaginary ground), and the other is the sensor's sensing surface. Detection depends on the object's dielectric constant, allowing detection of resin and water in addition to metals.
Detection Principle of Magnetic Proximity Sensors
A magnet operates the reed end of the switch. When the reed switch is turned ON, the sensor is activated.
Classification
Selection by Detection Method
| Items Requiring Confirmation | Inductive Proximity Sensors | Capacitive Proximity Sensors | Magnetic Proximity Sensors |
|---|---|---|---|
| Sensing object | Metallic objects (iron, aluminum, brass, copper, etc.) | Metallic objects, resins, liquids, powders, etc. | Magnets |
| Electrical noise | Affected by positional relationship of power lines and signal lines, grounding of cabinet, etc. CE Marking (EC Directive compliance) Sensor covering material (metal, resin). Easily affected by noise when the cable is long. | Almost no effect. | |
| Power supply | DC, AC, AC/DC, DC with no polarity, etc. Connection method, power supply voltage. | Depends on the power supply, i.e., DC 2-wire models, DC 3-wire models, AC, etc. DC 2-wire models are effective for suppressing current consumption. | |
| Current consumption | |||
| Sensing distance | The sensing distance must be selected considering factors like temperature, sensing object, surrounding objects, and mounting distance between sensors. Refer to catalog specifications for set distance. Use a Separate Amplifier model for high precision sensing. | ||
| Ambient environment | Temperature or humidity, or existence of water, oils, chemicals etc. Confirm that the degree of protection matches the ambient environment. | ||
| Physical vibration, shock | Provide an extra margin in sensing distance for environments subject to vibration and shock. Refer to catalog values for tightening torque during assembly to prevent sensors from vibrating loose. | ||
| Assembly | Consider effects of tightening torque, sensor size, wiring steps, cable length, distance between sensors, and surrounding objects. Check for effects of surrounding metallic objects and mutual interference specifications. | ||
Explanation of Terms
Standard Sensing Object
A reference object made of specified materials with specified shape and dimensions, used for measuring basic performance.
Sensing Distance
The distance from a reference position (reference surface) to the measured operation (reset) when the standard sensing object is moved by a specified method.
Response Time
- t1: The interval from when the standard sensing object enters the sensing area and the sensor activates, to when the output turns ON.
- t2: The interval from when the standard sensing object leaves the sensing area, to when the sensor output turns OFF.
Response Frequency
The number of detection repetitions per second when the standard sensing object is repeatedly brought into proximity. Measured as 1 / (t1 + t2).
Set Distance
The distance from the reference surface allowing stable use, accounting for temperature and voltage effects. It is approximately 70% to 80% of the normal (rated) sensing distance.
Hysteresis (Differential Travel)
The difference between the distance at which the sensor operates and the distance at which it resets, relative to the standard sensing object's distance.
Shielded
Sensors with magnetic flux concentrated forward and sides covered by metal. They can be mounted by embedding into metal.
Unshielded
Sensors with magnetic flux spread widely forward, and sides not covered by metal. These are easily affected by surrounding metal objects, requiring careful mounting location selection.
Expressing the Sensing Distance
The reference position and direction of approach of the sensing object determine how sensing distance is measured.
Cylindrical/Rectangular Sensors
Perpendicular sensing distance: Measured from the reference surface when the standard sensing object approaches radially (perpendicular to the sensing surface).
Horizontal sensing distance and sensing area diagram: Measured from the reference axis when the standard sensing object moves parallel to the reference surface (sensing surface). This distance depends on the transit position and can be expressed as an operating point track.
Output Configuration
NPN transistor output: A general-use transistor output connectable directly to a Programmable Controller or Counter.
PNP transistor output: Primarily used in machines exported to Europe and other overseas destinations.
Non-polarity/non-contact output: A 2-wire AC output usable for both AC and DC sensors, eliminating polarity concerns.
DC 2-wire model considerations:
- Leakage current: A maximum of 0.8 mA flows to the load even when the output is OFF. Ensure the load does not operate with this current.
- Output residual voltage: When ON, voltage remains in the sensor, reducing voltage applied to the load. Ensure the load operates with this voltage.
Output Configuration States
- NO (normally open): The output switching element turns ON when an object is in the sensing area.
- NC (normally closed): The output switching element turns ON when no object is in the sensing area.
- NO/NC switchable: Output operation (NO or NC) can be selected via a switch or other means.
Interpreting Engineering Data
Sensing Area
Graphs show engineering data from moving the sensing object parallel to the sensor's sensing surface, useful for applications like positioning. For high precision, use a Separate Amplifier Proximity Sensor.
Sensing Distance vs. Display Characteristics
Graphs used with Separate Amplifier Proximity Sensors show values for Fine Positioning (FP) at specified distances. FP settings can be made at any desired distance, with a digital value of 1,500 often used as a reference (e.g., for E2C-EDA).
Effects of Sensing Object Size and Material
Graphs illustrate how sensing distance changes based on the size and material of the sensing object. This data is crucial when detecting various objects with the same sensor or confirming allowable detection leeway.
Leakage Current Characteristics
Related to transistors in the output section, leakage current flows even when the sensor is OFF. Higher voltage generally means larger leakage current. Care must be taken to select a load that does not operate from this leakage current, especially when replacing limit switches.
Residual Voltage Characteristics
Similar to leakage current, residual voltage occurs in transistor-based switches. It refers to a voltage level remaining in the switch when it's ON, affecting load operation. Similar precautions as for leakage current apply.
General Precautions
WARNING: These products are not for safety devices (e.g., presses) protecting human life. They are designed for sensing workpieces and workers where safety is not compromised.
Precautions for Safe Use
Always observe the following precautions for safety.
Wiring Considerations
Power Supply Voltage: Do not exceed the operating voltage range. Using a higher voltage or an AC power supply for a DC sensor may cause explosion or burning.
Load short-circuiting: Do not short-circuit the load, as it may cause explosion or burning. Load short-circuit protection functions when power supply polarity is correct and voltage is within range, but may not protect if polarity is incorrect (especially for 2-wire sensors).
Incorrect Wiring: Ensure correct power supply polarity and wiring to prevent explosion or burning.
Connection without a Load: Connecting the power supply directly without a load can cause internal elements to explode or burn. Always connect a load.
Operating Environment
Do not use sensors in environments with explosive or combustible gases.
Precautions for Correct Use
Consider the following conditions for application, location, and relation to control equipment.
Model Selection
Sensing object and operating condition of Proximity Sensor: Check the relationship between the sensing object and the sensor. Consider object material, size, shape, plating, movement interval/speed/vibration, peripheral metal (material, distance, orientation), and sensing distance fluctuations/allowable error.
Electrical Conditions
Verify control system electrical conditions and sensor performance. Consider power supply type (DC, AC, DC+S3D2 Controller), load types (resistive, inductive, lamp), steady-state/inrush current, operating/reset voltage, leakage current, and residual load voltage.
Environmental Conditions
Proximity sensors offer better environmental tolerance than other types, but careful investigation is needed for harsh temperatures or special atmospheres. Water Resistance: Do not use in water, rain, or outdoors. While water-resistant, use a cover for direct water contact. Ambient Conditions: Avoid chemical vapors, strong alkalis/acids. At low temperatures, vinyl cables harden and may break if bent; avoid bending standard or robot cables.
Mounting Conditions
When deciding on mounting, consider mechanical restrictions, ease of maintenance, inspection, and sensor interference. This includes wiring methods, connection types, mounting procedures (brackets, bolts), and installation location.
Influence of External Electromagnetic Fields
DC magnetic fields (20 mT max.) can affect performance. Avoid applications with frequent DC electromagnet ON/OFF switching. Do not place transceivers near sensors or wiring.
Other Considerations
Factor in cost feasibility (price/delivery time) and lifespan (power-ON time/frequency of use).
Design Considerations
Sensing Object Material
Sensing distance varies significantly with sensing object material. Engineering data on material and size influence should be studied for adequate leeway. For non-magnetic metals like aluminum, sensing distance generally decreases. For thin non-magnetic metal coatings (≤0.01 mm), detection is possible; however, pulse-response models may vary. Extremely thin or non-conductive coatings (e.g., vacuum deposited film) prevent detection.
Thickness of Sensing Object
Ferrous metals (iron, nickel) require a thickness of 1 mm or greater. For non-magnetic metals, a sensing distance equivalent to magnetic bodies can be achieved with coatings ≤0.01 mm. Detection is not possible with extremely thin, non-conductive coatings.
Effect of Plating
Plating on a sensing object can alter sensing distance. A table provides typical reference values (as a percentage of non-plated sensing distance) for various platings (Zn, Cd, Ag, Cu, Ni, Cr) on steel and brass bases.
Mutual Interference
Mutual interference occurs when sensors affect each other's output due to magnetism or static capacitance, causing instability. To avoid this, alternate sensors with different frequencies. When mounting sensors with the same frequency in a line or face-to-face, maintain a minimum separation distance as specified in the Safety Precautions.
Power Reset Time
Sensors are ready for detection within 100 ms after power-on. If sensors and loads have separate power supplies, ensure the sensor power turns ON first.
Turning OFF the Power and Other Precautions
Turning OFF the Power
An output pulse may occur when power is turned OFF. Design the system to turn OFF the load or load line power first.
Influence of Surrounding Metal
Metal objects near the sensing surface can affect detection performance, increase apparent operating distance, degrade temperature characteristics, and cause reset failures. The distance 'm' between the metal surface and the sensor's sensing surface is particularly influential, often shortening sensing distance. Nut materials provided with sensors are for reference; changing them alters surrounding metal influence.
Power Transformers
Use insulated transformers for DC power supplies; avoid auto-transformers.
Precautions for AC 2-Wire/DC 2-Wire Sensors
Surge Protection: If devices causing large surges (motors, welders) are nearby, insert a surge absorber near the source.
Influence of Leakage Current: A small leakage current flows even when the sensor is OFF, potentially causing load reset failures. Verify that the residual voltage is below the load reset voltage (leakage current below load reset current).
Using an Electronic Device as the Load for an AC 2-Wire Sensor: Devices using AC half-wave rectification may cause unstable sensor operation. Avoid using proximity sensors to switch DC half-wave rectified devices; use a relay and check system stability.
Countermeasures for Leakage Current (Examples)
AC 2-Wire Sensors: Connect a bleeder resistor to bypass leakage current, ensuring the load current is less than the load reset current. Formulas are provided to calculate bleeder resistance and allowable power based on supply voltage (Vs) and load current (I).
DC 2-Wire Sensors: Connect a bleeder resistor to bypass leakage current. Formulas are provided to calculate bleeder resistance and allowable power based on supply voltage (Vs), leakage current (iR), and load reset current (iOFFR).
Loads with Large Inrush Current and Mounting
Loads with Large Inrush Current
Loads like lamps or motors that cause large inrush currents can weaken or damage the switching element. Use a relay in such situations.
Mounting the Sensor
Avoid tapping the sensor with a hammer or subjecting it to excessive shock, as this can weaken water resistance and damage the sensor. When securing with bolts, observe allowable tightening torque. Some models require toothed washers.
Mounting/Removing Using DIN Track (Example for E2CY)
Mounting: Insert the sensor's front into the special Mounting Bracket or DIN Track, then press the rear into place.
Removing: Press the Amplifier Unit in one direction and lift the fiber plug in another for easy removal without a screwdriver.
Set Distance
Sensing distance can vary with temperature and voltage fluctuations. It is recommended to base installation on the set distance.
Wiring Considerations and Examples
AND/OR Connections for Proximity Sensors
DC 2-Wire:
- AND (series connection): Formulas are provided to determine the maximum number of connected sensors (N) based on supply voltage (Vs), residual output voltage (VR), and sensor current consumption (i). Note potential issues with indicator lights and error pulses if power supply voltage/current is not adequately supplied to individual sensors.
- OR (parallel connection): Formulas are provided to determine the maximum number of connected sensors (N) based on load reset current and leakage current (i). Specific models (TL-NY, TL-MY, E2K-MY) cannot be used in series; relays are required.
AC 2-Wire:
- AND (series connection): Formulas are provided for calculating load voltage (VL) when ON, which depends on Vs, residual output voltage, and the number of sensors. VL must exceed the load operating voltage. A limit of three sensors is noted for AND circuits.
- OR (parallel connection): Generally not possible for OR circuits. Parallel connections are possible if A and B do not operate simultaneously and the load does not need to be held. However, leakage current increases, potentially causing reset failures. If simultaneous operation and load holding are required, parallel connections are not possible; use a relay.
DC 3-Wire:
- AND (series connection): Formulas are provided for calculating the upper limit of control output and operating load voltage, considering sensor current consumption (i), load current (iL), and residual output voltage (VR). Note potential for erroneous pulses when Sensor B operates, affecting Sensor A, especially with high-response loads.
- OR (parallel connection): A minimum of three OR connections is possible for current output sensors; model dependency exists for more.
Note: AND/OR connections may lead to erroneous pulses or leakage current issues; verify functionality.
Extending Cable Length
Built-in Amplifier Sensors can have cables extended up to 200 m. Refer to individual product precautions for Separate Amplifier Sensors.
Bending the Cable
Recommended bend radius is at least 3 times the outer diameter for standard cables, and 5 times for coaxial and shielded cables.
Cable Tensile Strength
A table specifies maximum tensile strength for different cable diameters (e.g., 30 N max for < 4 mm, 50 N max for ≥ 4 mm). Do not subject shielded or coaxial cables to tension.
Separating High-voltage Lines
Use metal conduits to separate high-voltage lines from proximity sensor cables to prevent malfunction or damage.
Example of Connection with S3D2 Sensor Controller
DC 2-Wire Sensors: Operation can be reversed using the signal input switch on the S3D2. Diagrams show connections to the S3D2 controller.
Connecting to a Relay Load: Diagrams show connecting a DC 2-Wire sensor to a relay load, noting residual voltage (3V for general, 5V for E2E-XD-M1J-T) and the need to check relay operating voltage.
DC 3-Wire Sensors: Operation can be reversed using the signal input switch on the S3D2. Diagrams show connections to the S3D2 controller.
Operating Environment and Maintenance
Operating Environment
Water Resistance: Do not use the sensor in water, rain, or outdoors.
Ambient Conditions: To maintain reliability, use sensors only within the specified temperature range and not outdoors. While water-resistant, a cover is recommended for direct water contact. Avoid environments with chemical vapors, especially strong alkalis or acids. At low temperatures, cables can harden and break if bent; avoid bending standard or robot cables.
Maintenance and Inspection
Perform periodic inspections for long-term stable operation, including checking for shifting/loosening/deformation of mounting, wiring issues, metal powder accumulation, abnormal temperatures, or indicator flashing.
Disassembly and Repair
Do not attempt to disassemble or repair the product under any circumstances.
Quick Failure Check
Use the E39-VA Handy Checker for convenient failure checks and operation verification.
