Capacitive Sensors
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CONTENTS Capacitive Sensors General Information 62 Calibration 65 Miniature Caps 66 Mini Cap Discs 67 Sensor Amplifiers 68 DC Sensors 69 AC Sensors 71 DC Level 73 AC Level 74 High Temp Level 75 61 CAPACITIVE SENSORS GENERAL INFORMATION Application Repeat accuracy Material correction factor Capacitive proximity switches may be used to The repeat accuracy parameter describes the If the material of the object in question is not metal detect non-metallic and metallic materials. They maximum deviation from the sensing distance or water, the rated sensing distance (Sn) is can be utilised as contact or non-contact sensors when the object in question is approached several reduced. The reduction factors for the different depending upon the application and the sensing times. materials are given in the table below. medium concerned. Capacitive sensors may be used for controlling and monitoring machine Rated sensing distance (Sn) Material Thickness 3 r Reduction processes or as primary detectors for product The rated sensing distance is the usable sensing d/mm factor counting. Level indication for liquids or granular distance of a sensor when directed at a metal Steel ST-37 1.5 1.0 substances can be achieved either through the plate. In accordance with DIN VDE 0660 Part 208, Brass Ms 1.5 1.0 outer container or with direct contact with the it is a 1-mm-thick square metal plate, consisting of Water approx. 81 1.0 medium itself. We distinguish between two areas carbon steel Type FE 360 (definition to ISO Mikanit 132 2 4.5 0.44 of application for capacative sensors: 630:1980). The surface must be smoothed. (mica) 4 0.52 L 6 0.57 1. Sensor version flush mounted The size must be dimensioned so as to ensure that Sensors with a straight-line electrical field. UP 2 4.0 0.41 the edge length (m) of the square metal plate is equal Polyester, glass- 4 0.51 These units scan solids (e.g. wafers, components, to the diameter of the circle drawn on the active fibre-reinforced 6 0.54 PCB’s, hybrids, cartons, paper piles, bottles, surface or equal to three times the rated operating plastic blocks and plastic plates) at a distance, or Polyamide A 2 4.2 atm. humidity 0.34 distance Sn, depending on which is greater. (nylon 6.6) 4 0.45 liquids through a separating wall (glass or plastic 6 0.51 up to max. 4mm thick). If the plate is made from a different material, or has smaller dimensions, is unearthed, or exhibits a Polyamide B 3 5.3 atm. humidity 0.41 (Nylon 6) 6 0.48 different shape or surface quality, the rated 9 0.56 operating distances will be smaller. Melamin 2 7 0.53 Fabric-base laminate 4 0.62 Fig. 1.03 (HGW 2271) 6 0.66 Paper-base laminate 2 5 0.56 2. Sensor version non-flush mounted I ◗ (HP 2061) 4 0.62 Sensors with a spherical electrical field. These 6 0.68 units are designed to touch the product, bulk Polystyrene 2 2.5 DIN53483 0.24 goods or liquids involved (e.g. granulate, sugar, (PS) 4 0.31 flour, corn, sand, or oil and water) with their active 6 0.36 surface. Polycarbonate 2 2.92 DIN53484 0.26 (PC) 4 0.36 6 0.40 Fig. 1.06 Polymethyl- 5 2.9 DIN53483 0.39 methacrylate 10 0.45 Size correction factor (acrylic glass, PMMA) 15 0.47 Polyvinyl chloride 6 2.9 0.41 For objects which are not flat and are smaller in (PVC) 12 0.47 relation to the active sensor surface, the following Fig. 1.04 PVC foamed 3 1.5 - 2.5 0.22 sensing distances are obtained in dependence on 6 0.25 Sensing distance S the scaled object surface F/Fo, where Fo = sensor front surface (active surface), and This is the distance between the active sensor Fig 1.09 surface and the product being scanned at the F = front surface of the object being scanned. The moment of output-signal change. It depends on figures in the table below (Fig. 1.07) refer to flush shape, size and nature of the object concerned. sensors, and objects in the form of long thin rods. The diagram below (Fig. 1.08) shows in graphic form the generally valid particulars from the table Sensing (Fig. 1.07). distance Hysteresis Scaled object Sensing Øof object F in S in surface F/F0 distance S in % in mm mm2 mm 1.50 100 22.0 380.0 8.0 1.24 100 20.0 314.0 8.0 0.80 100 16.0 201.0 8.0 0.61 100 14.0 154.0 8.0 0.31 94.0 10.0 79.0 7.5 0.20 85.0 8.0 50.0 6.8 0.15 82.5 7.0 38.0 6.6 0.05 67.5 4.0 13.0 5.4 0.03 57.5 3.0 7.0 4.6 Fig. 1.07 Fig. 1.05 Fig. 1.08 Hysteresis The three right-hand columns of the table (Fig. Hysteresis is the distance differential between the 1.07) reflect the application example for an SK1-8- switch-on point (as the object approaches) and the M18-…-b sensor. switch-off point (as the object recedes again). 62 CAPACITIVE SENSORS CONNECTION Minisensors capacitive, Series SK and SKF: Proximity sensors capacitive and level sensors Analogue sensors capacitive, Series SK1-A: capacitive, Series SK1, DC: High temperature sensors capacitive Sensor-amplifiers capacitive, Series SK-HT: Series SV: Quick Disconnect Wiring Diagram for Y2-series, DC: (Male Receptacle End View of sensor) Series SNG-K: Proximity sensors capacitive and level sensors capacitive, Series SK1, AC: Power supply, control units, Series SNG: Power supply: Series SV-2VX: Timer: Series SV-X2L: Quick Disconnect Wiring Diagram for Y2-series, AC: (Male Receptacle End View of Sensor) Min-Max: Note: Plastic Units w/Receptacle (2-Pins) 63 CAPACITIVE SENSORS Core identification Parallel connection Output current The cores are colour-coded in order to prevent This is the maximum current with which the sensor connection errors. may be stressed in continuous operation at its DC-units: output. brown (br/bn) = + The minimum loading is the lowest switching blue (bl/bu) = - capacity for A.C. sensors, and is essential for black (sw/bk) = Output proper functioning. AC-units: Short-circuit protection and black (sw/bk) or brown (br/bn) = L1 overload protection blue (bl/bu) = N The sensors normally contain this protective green/yellow (gr/ge, gn/ye) = earth feature. In the event of overload or short circuit at the output, the output transistor is automatically Output stages switched off. As soon as the malfunction has been corrected, the output stage is reset to normal PNP output: functioning. The load is connected between output and ground. Polarity reversal protection The sensor electronics are protected against Short overl. possible polarity reversal or interchanging of the protect. connection wires. EMC protective circuit Appropriate measures in the supply voltage line Fig. 1.18 ensure that interference voltages are decoupled. Ambient temperature NPN output: This specifies the temperature range in which the Fig. 1.23 The load is connected between output and sensor may be operated without the housing being positive supply voltage. damaged or the sensor electronics failing. Series connection Temperature drift This states the amount by which the sensing distance changes in dependence on temperature. Switching function Short - N.O. contact: the switching output of the sensor overl. protect. is not switched through in its de-activated state. Fig. 1.19 AC: The sensor involved here is a two-wire model, where the load is connected in series with the Fig. 1.16 sensor. -N.C. contact: the switching output of the sensor is switched through in its de-activated state. Fig. 1.20 Fig. 1.17 Analogue: The load must be connected between Operating frequency output and ground. The sensor supplies an output The operating frequency is a succession of current proportional to the distance from the Fig. 1.24 object involved. This current produces a periodically repeated activation and de-activation corresponding voltage drop in the load resistor. of the sensors during one second. Measuring Residual ripple method in conformity with DIN 50010. This is the maximum permissible A.C. voltage which may be superimposed on the supply voltage without affecting the function of the sensor. Fig. 1.22 Fig. 1.14 64 CAPACITIVE SENSORS CALIBRATION Important: When calibrating capacitive sensors, So that our sensors operate reliably within their Example: the different material properties of the scanned technical specifications, they are set to a greater A granulate in a vessel is to be scanned by a non- product must be taken into consideration. switching gap than the rated switching gaps Sn flush mounted sensor of type SK1-30-M30-P-nb- Capacitive sensors are therefore equipped with specified in the catalogue. If the operator S. The sensor is mounted so that its active surface potentiometers which can be trimmed to adjust increases the switching gap to 4mm over ceramic (free zone at head as described in the catalogue) the sensitivity of the device. plate as described above, the sensor will be projects into the product in the vessel. As depicted operating in an impermissible range (Fig. 1.28). in Fig. 1.30, the sensor must be completely This may lead to the risk of faulty switching in the covered by the product before calibration can sensor due to temperature effects and voltage proceed. transients in the power source.