Standard Target for Inductive Proximity Sensors

Size: px

Start display at page:

Download "Standard Target for Inductive Proximity Sensors"

Lucas Johns
6 years ago
Views:

1 Principles of Operation for Inductive Proximity Sensors Inductive proximity sensors are designed to operate by generating an electromagnetic field and detecting the eddy current losses generated when ferrous and nonferrous metal target objects enter the field. The sensor consists of a coil on a ferrite core, an oscillator, a trigger-signal level detector and an output circuit. As a metal object advances into the field, eddy currents are induced in the target. The result is a loss of energy and a smaller amplitude of oscillation. The detector circuit then recognizes a specific change in amplitude and generates a signal which will turn the solid-state output ON or OFF. A metal target approaching an inductive proximity sensor (above) absorbs energy generated by its oscillator. When the target is in close range, the energy drain stops the oscillator and changes the output state. Standard Target for Inductive Proximity Sensors The active face of an inductive proximity switch is the surface where a high-frequency electro-magnetic field emerges. A standard target is a mild steel, 1mm thick, square form with side lengths equal to the diameter of the active face or 3X the nominal switching distance, whichever is greater. Target Correction Factors for Inductive Proximity Sensors To determine the sensing distance for materials other than the standard mild steel, a correction factor is used. The composition of the target has a large effect on sensing distance of inductive proximity sensors. If a target constructed from one of the materials listed is used, multiply the nominal sensing distance by the correction factor listed in order to determine the nominal sensing distance for that target. Note that ferrous-selective sensors will not detect brass, aluminum or copper, while nonferrous selective sensors will not detect steel or ferrous-type stainless steels. The correction factors listed below can be used as a general guideline. Common materials and their specific correction factors are listed on each product specification page (Nominal Sensing Range) x (Correction Factor) = Sensing Range. The size and shape of the target may also affect the sensing distance. The following should be used as a general guideline when correcting for the size and shape of a target: Flat targets are preferable Rounded targets may reduce the sensing distance Nonferrous materials usually reduce the sensing distance for all-metal sensing models Targets smaller than the sensing face typically reduce the sensing distance Targets larger than the sensing face may increase the sensing distance Foils may increase the sensing distance Principles of Operation for Capacitive Proximity Sensors Capacitive proximity sensors are designed to operate by generating an electrostatic field and detecting changes in this field caused when a target approaches the sensing face. The sensor s internal workings consist of a capacitive probe, an oscillator, a signal rectifier, a filter circuit and an output circuit. In the absence of a target, the oscillator is inactive. As a target approaches, it raises the capacitance of the probe system. When the capacitance reaches a specified threshold, the oscillator is activated which triggers the output circuit to change between on and off. The capacitance of the probe system is determined by the target s size, dielectric constant and distance from the probe. The larger the size and dielectric constant of a target, the more it increases capacitance. The shorter the distance between target and probe, the more the target increases capacitance. Standard Target and Grounding for Capacitive Proximity Sensors The standard target for capacitive sensors is the same as for inductive proximity sensors. The target is grounded per IEC test standards. However, a target in a typical application does not need to be grounded to achieve reliable sensing. 2 9

2 Shielded vs. Unshielded Capacitive Sensors Shielded capacitive proximity sensors are best suited for sensing low dielectric constant (difficult to sense) materials due to their highly concentrated electrostatic fields. This allows them to detect targets which unshielded sensors cannot. However, this also makes them more susceptible to false triggers due to the accumulation of dirt or moisture on the sensor face. The electrostatic field of an unshielded sensor is less concentrated than that of a shielded model. This makes them well suited for detecting high dielectric constant (easy to sense) materials or for differentiating between materials with high and low constants. For the right target materials, unshielded capacitive proximity sensors have longer sensing distances than shielded versions. Unshielded models are equipped with a compensation probe which allows the sensor to ignore mist, dust, small amounts of dirt and fine droplets of oil or water accumulating on the sensor. The compensation probe also makes the sensor resistant to variations in ambient humidity. Unshielded models are therefore a better choice for dusty and/or humid environments. Unshielded capacitive sensors are also more suitable than shielded types for use with plastic sensor wells, an accessory designed for liquid level applications. The well is mounted through a hole in a tank and the sensor is slipped into the well s receptacle. The sensor detects the liquid in the tank through the wall of the sensor well. This allows the well to serve both as a plug for the hole and a mount for the sensor. Target Correction Factors for Capacitive Proximity Sensors For a given target size, correction factors for capacitive sensors are determined by a property of the target material called the dielectric constant. Materials with higher dielectric constant values are easier to sense than those with lower values. A partial listing of dielectric constants for some typical industrial materials follows. For more information, refer to the CRC Handbook of Chemistry and Physics (CRC Press), the CRC Handbook of Tables for Applied Engineering Science (CRC Press), or other applicable sources. Dielectric Constants of Common Industrial Matierials Acetone 19.5 Acrylic Resin Air Alcohol 25.8 Ammonia Aniline 6.9 Aqueous Solutions Bakelite 3.6 Benzene 2.3 Carbon Dioxide Carbon Tetrachloride 2.2 Celluloid 3.0 Cement Powder 4.0 Cereal 3 5 Chlorine Liquid 2.0 Ebonite Epoxy Resin Ethanol 24 Ethylene Glycol 38.7 Fired Ash Flour Freon R22 & 502 (liquid) 6.11 Gasoline 2.2 Glass Glycerine 47 Marble Melamine Resin Mica Nitrobenzine 36 Nylon 4 5 Oil Saturated Paper 4.0 Paraffin Paper Perspex Petroleum Phenol Resin 4 12 Polyacetal Polyamide 5.0 Polyester Resin Polyethylene 2.3 Polypropylene Polystyrene 3.0 Polyvinyl Chloride Resin Porcelain Powdered Milk Press Board 2 5 Quartz Glass 3.7 Rubber Salt 6.0 Sand 3 5 Shellac Shell Lime 1.2 Silicon Varnish Soybean Oil Styrene Resin Sugar 3.0 Sulphur 3.4 Teflon 2.0 Toluene 2.3 Transformer Oil 2.2 Turpentine Oil 2.2 Urea Resin 5 8 Vaseline Water 80 Wood, Dry 2 7 Wood, Wet Principles of Operation for Ultrasonic Proximity Sensors Ultrasonic sensors detect objects by emitting bursts of high-frequency sound waves which reflect or echo from a target. These devices sense the distance to the target by measuring the time required for the echo to return and dividing that time value by the speed of sound. This allows these devices to detect objects of any shape and material that can sufficiently reflect an ultrasonic pulse. Analog models provide an output voltage proportional to the distance from the sensor face to the target, while digital/discrete output models change output state when this distance crosses a pre-set threshold. Because ultrasonic sensors depend on a reflected sound wave for proper operation, the correction factors and target requirements used for inductive proximity sensors do not apply. Refer to the Bulletin 873C product pages in this catalog for target considerations. Hysteresis (Differential Travel) The difference between the operate and the release points is called hysteresis or differential travel. The amount of target travel required for release after operation must be accounted for when selecting target and sensor locations. Hysteresis is needed to help prevent chattering (turning on and off rapidly) when the sensor is subjected to shock and vibration or when the target is stationary at the nominal sensing distance. Vibration amplitudes must be smaller than the hysteresis band to avoid chatter. 2 10

3 Switching Frequency The switching frequency is the maximum speed at which a sensor will deliver discrete individual pulses as the target enters and leaves the sensing field. This value is always dependent on target size, distance from sensing face, speed of target and switch type. This indicates the maximum possible number of switching operations per second. The measuring method for determining switching frequency with standard targets is specified by DIN EN Sn Ripple Ripple is the alternating voltage superimposed on the DC voltage (peak to peak) in %. For the operation of DC voltage switches, a filtered DC voltage with a ripple of 10% maximum is required (according to DIN 41755). Mounting Considerations for Weld Field Immune Proximities Reliable operation is dependent on the strength of the magnetic field and the distance between the current line and the sensor. Perpendicular Mounting to the Current Line Parallel Mounting to the Current Line Use the following chart or formulas to determine the spacing requirements between the current line and proximity sensor. Select a distance that falls within the safe zone. H = I 2 r B = H Gauss = 10* B where: I = welding current (in ka), H = field strength (in ka/m), B = flux (in mt), and r = distance between sensor and current carrying lines (in meters). Weld Field Immunity Safe Zone 2 11

4 Series Connected Sensors Sensors can be connected in series with a load. For proper operation, the load voltage must be less than or equal to the minimum supply voltage minus the voltage drops across the seriesconnected proximity sensors. + + Wiring Diagram for Series Connected Current Sink Sensors (NPN) + V DC Wiring Diagram for Series Connected Current Source Sensors (PNP) V DC + Parallel Connected Sensors Sensors can be connected in parallel to energize a load. To determine the maximum allowable number of sensors for an application, the sum of the maximum leakage current of the sensors connected in parallel must be less than the maximum OFF-state current of the load device. Note: Care should be taken when designing parallel proximity circuits. If too much leakage current flows into the load it may cause the solid state input to change state or a small relay not to drop out. Sensors connected in parallel do not provide a higher load current capability. + Wiring Diagram for Parallel Connected Current Sink Sensors (NPN) + V DC TTL Wiring Note: When using sourcing outputs, ground must be floating and cannot be common, or short circuit will result. Wiring Diagram for Series Connected AC Sensors L 1 V AC L 2 + Wiring Diagram for Parallel Connected Current Source Sensors (PNP) V DC + PLC Wiring For PLC wiring information for Inductive and Capacitive sensors, refer to publication , June Wiring Diagram for Parallel Connected L 1 AC Sensors L 2 V AC * Add R in series with sensor to maintain minimum voltage when sensor is switching. 2 12

5 Shielded vs. Unshielded Inductive Sensors Shielded Sensor Unshielded Sensor Shielded construction includes a metal band which surrounds the ferrite core and coil arrangement. Unshielded sensors do not have this metal band. Spacing Between Shielded Sensors (Flush-Mountable) and Nearby Metal Surfaces Shielded proximity sensors allow the electro-magnetic field to be concentrated to the front of the sensor face. Shielded construction allows the proximity to be mounted flush in surrounding metal without causing a false trigger. Cylindrical Style Limit Switch Style (871L and 872L) Limit Switch Style (802PR) * 802PR LB or 802PR XB can be mounted side by side. d = diameter or width of active sensing face Sn = nominal sensing distance 2 13

6 Spacing Between Shielded Sensors (Flush-Mountable) and Nearby Metal Surfaces (continued) Cube Style (871P VersaCube) Miniature Flat Pack Style (871FM) d = diameter or width of active sensing face Sn = nominal sensing distance 2 14

7 Spacing Between Unshielded Sensors (Nonflush-Mountable) and Nearby Metal Surfaces Longer sensing distances can be obtained by using an unshielded sensor. Unshielded proximity sensors require a metal-free zone around the sensing face. Metal immediately opposite the sensing face should be no closer than 3 times the rated nominal sensing distance of the sensor. Cylindrical Style Limit Switch Style (871L, 872L, and 875L) Limit Switch Style (802PR) d for capacitive sensors if mounted in plastic. 3d (12, 18mm models) or 1.5d (30, 34mm models) if mounted in metal. For capacitive sensors, 3d at medium sensitivity to 8d at maximum sensitivity. 8d for capacitive sensors. d for capacitive sensors. d = diameter or width of active sensing face Sn = nominal sensing distance 2 15

8 Spacing Between Unshielded Sensors (Nonflush-Mountable) and Nearby Metal Surfaces (continued) Cube Style (871P VersaCube) Miniature Flat Pack Style (871FM) d = diameter or width of active sensing face Sn = nominal sensing distance 2 16

9 Applications Machine Tools Plating Line Plating Line ÂÂÂÂÂ ÂÂÂÂÂÂ ÂÂÂÂÂÂ ÂÂÂÂÂÂÂÂ ÂÂÂÂÂÂÂÂ ÂÂÂÂÂ Grinding Machines Wood Industry Conveyor Belts Petroleum Industry Valve Position 2 17

10 Applications Ë Inductive proximity sensor used to detect a foil seasoning bag inside of a cardboard container. Ferrous selective inductive proximity sensor used to sort ferrous and nonferrous can tops. Food Industry Stainless Steel Sheet Welder Food Processing Printing 2 18

11 Applications On Line Parts Sorting Railroad Yard Position Sensing Coolant Resistant Sensing Up and Downslope Control of Continous Tube Welder Nut Placement on Transformer Closed Barrier Indicator Detect Presence of Bushing in Piston 2 19

12 Applications Control Presence of Mild Steel Bars in Grate Welding 871Z 871P 871Z 871Z 871Z Elevator Positioning Allen-Bradley produces rail guide inductive proximity sensors for the positioning of elevator cars. These sensors offer increased accuracy and longer life when compared to typical mechanical switches. They are a cost effective solution for lowering your repair costs and down-time. Contact your local Allen-Bradley salesperson for a proximity tailored to your requirements! 2 20

13 Applications Material Handling Printing Level Detection Ultrasonic proximity sensor used to indicate when paper supply is almost exhausted. ÎÎÎ ÎÎÎÎÎ ÎÎÎÎÎ ÎÎÎÎÎ Granular Fill ÎÎÎÎÎ ÎÎÎ ÎÎÎ Liquid Level Detection ÏÏÏÏ ÏÏÏÏÏ ÏÏÏÏÏ Liquid ÏÏÏÏÏ ÏÏÏ ÏÏÏ 2 21

14 Applications Top 23 Reasons to Use the 871TM DC and long barrel AC/DC models only. 2 22

Contents R4 1. Capacitive Proximity Sensors

Contents R4 1. Capacitive Proximity Sensors Contents General Information Quick Selection Guide...................... page R4 2 Technical Definitions and Terminology........ page R4 3 Introduction............................... page R4 5 Products