Capacitive Sensors
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General
Presentation
Capacitive proximity sensors
Advantages
p No physical contact with the object to be detected. p High operating rates. p Solid state, no moving parts (service life of sensor independent to number of operating cycles). p Detection of any object irrespective of material or conductivity, for example : metals, minerals, wood, plastic, glass, cardboard, leather, ceramic, fluids, etc.
Principle of operation
A capacitive proximity sensor basically comprises an oscillator whose capacitor is formed by 2 electrodes placed in front of the sensor.
In open air (εr = 1), the capacitor capacitance is C0. εr is the dielectric constant, which depends on the object material. All materials where εr > 2 will be detected.
Electric field Electrode
Air εr = 1 C = C0
When an object, of any material, (εr > 2) passes the sensing face of the sensor, it modifies the coupling capacitance (C1). The capacitance variation (C1> C0) instigates the starting of the oscillator. This in turn causes the output driver to operate and provides an output signal.
Object
εr > 2
C = C1
1150 Ver2.00-EN.fm/2
Schneider Electric
General
Types of sensor
Capacitive proximity sensors
Sensors flush mountable in support
Metal case cylindrical sensors and plastic case block sensors. For detection of insulated materials (wood, plastic, cardboard, glass, etc.). These sensors are recommended when :
p Detection distances are comparatively small. p It is required that the sensors be flush mounted. p The application necessitates the detection of a non conductive material through a non conductive partition (for example : detection of glass in a cardboard box).
Contamination
Front face
Main electrode Compensation electrode Earth electrode (a) : compensation field (suppression of external contamination) (b) : main electric field
Sensors non flush mountable in support
Plastic case block sensors. For detection of conductive materials (metal, water, liquids, etc.). These sensors are recommended for :
p Detection of conductive object materials at considerable distances. p Detection of conductive materials through insulated partitions. p Detection of non conductive materials on or in front of earthed metal parts.
Earth
Front face
Main electrode
(a) : electric field
Schneider Electric
31150 Ver2.00-EN.fm/
General
Terminology
Capacitive proximity sensors
Nominal sensing distance Sn
Similar to that defined for inductive proximity sensors, the nominal sensing distance being calculated using a standard mild steel square target, 1 mm thick. The side dimension of the square plate is equal to that of the sensor sensing face.
Sensitivity of the sensor
Ø 18 or Ø 30 mm cylindrical type and 40 x 40 mm block type sensors incorporate a 20-turn sensitivity adjustment potentiometer. This enables the sensitivity of the sensor to be adjusted to suit the type of object to be detected. The sensors are factory preset for nominal sensitivity. Depending on the application, adjustment of the sensitivity could be necessary as follows : - increasing the sensitivity for objects which have a weak influence (low dielectric constant,εr), for example : paper, cardboard, glass, plastic, - decreasing the sensitivity for objects which have a strong influence (high dielectric constant,
εr), for example : metals, liquids.
Telemecanique capacitive proximity sensors incorporate compensation electrodes so as to eliminate the affects of varying environmental conditions (humidity, pollution). However, in the event of severe variations in the ambient conditions, do not increase the sensitivity of the sensor such that it is set to its maximum operating limits. An increase in sensitivity causes an increase in the switching hysteresis.
P : sensitivity adjustment potentiometer
1150 Ver2.00-EN.fm/4
Schneider Electric
General
Terminology
Capacitive proximity sensors
Operating distances
The operating distance of the sensor is related to the dielectric constant (εr) of the object material to be detected. The higher the value of εr, the easier the detection of the object will be. The working distance depends on the object material : Sa = Sn x Fc Sa = working distance, Sn = nominal sensing distance of the sensor, Fc = correction coefficient for the object material. Example : sensor XT1-M30PA372 used to detect a rubber object Sn = 10 mm, Fc = 0.3 Working distance Sa = 10 x 0.3 = 3 mm
The list below indicates the dielectric constant values for the most common object materials, together with their correction factors (Fc) for the nominal sensing distance of the sensor.
Schneider Electric
Material
εr
Fc
Material
εr
Fc
Acetone
20
0.8
Paper
2…4
0.2…0.3
Air
1
0
Paraffin
2…2.5
0.2
Alcohol
24
0.85
Petrol
2.2
0.2
Ammonia
15…25
0.75…0.85
Plexiglass
3.2
0.3
Cement (powder) 4
0.35
Polyester resin 2.8…8
0.2…0.6
Cereals
3…5
0.3…0.4
Polystyrene
3
0.3
Damp wood
10…30
0.7…0.9
Porcelain
5…7
0.4…0.5
Dry wood
2…7
0.2…0.6
Powdered milk 3.5...4
0.3...0.4
Ethylene glycol 38
0.95
Rubber
2.5…3
0.3
Epoxy resin
4
0.36
Salt
6
0.5
Flour
2.5…3
0.2…0.3
Sand
3...5
0.3...0.4
Glass
3...10
0.3...0.7
Sugar
3
0.3
Marble
6…7
0.5…0.6
Teflon
2
0.2
Mica
6…7
0.5…0.6
Vaseline
2...3
0.2...0.3
Nylon
4…5
0.3…0.4
Water
80
1
Oil
2.2
0.2
31150 Ver2.00-EN.fm/
General
Environment
Capacitive proximity sensors
Electromagnetic interference Telemecanique sensors undergo electromagnetic interference testing in accordance to the recommendations of standard IEC 947-5-2 (electrostatic discharges, radiated electromagnetic fields, fast transients, impulse voltages).
Thermal influences It is advisable to remain within the values stated on the characteristics pages, so as to avoid sensing distance drift and incorrect operation of the sensor. Chemical agents To ensure a long service life it is essential that any chemicals coming into contact with the case of the sensor are non corrosive.
Mechanical shocks The sensors are tested in accordance to the standard IEC 68-2-27, 50 gn, duration 11 ms. Vibration The sensors are tested in accordance to the standard IEC-68-2-6, amplitude ± 2 mm, F = 10 to 55 Hz, 25 gn at 55 Hz.
Earthing Earthing of an object that has a high conductivity increases the sensing distance.
Mounting precautions
In order to avoid mutual interference between sensors, it is recommended that the minimum mounting distances given for “Setting-up” are adhered to.
Flush mountable cylindrical models are suitable for applications where the head of the sensor needs to be flush with the support surface.
Flush mountable model
Non flush mountable cylindrical models require an area of free space around the head of the sensor.
Non flush mountable model
Additional information related to outputs
- Terminology. - Details and wiring constraints for 2-wire and 3-wire type sensors. - Connecting sensors in series or parallel. Similar to that detailed on the corresponding pages relating to inductive proximity sensors.
1150 Ver2.00-EN.fm/6
Schneider Electric
General
Application example : “Bottle filling”
Capacitive proximity sensors
Bottle arrival
Bottles are fed on a conveyor belt for filling. The sensors 1 (for insulated object materials) and 2 (for conductive object materials) are in the unoperated state.
Bottle filling
As soon as the bottle enters the detection zone of sensor 1, the filling operation commences. Sensor 2 remains in the unoperated state.
Filling complete
Sensor 2 detects that the required level has been reached and stops further filling.
Schneider Electric
31150 Ver2.00-EN.fm/
General
Presentation
Capacitive proximity sensors
Advantages
p No physical contact with the object to be detected. p High operating rates. p Solid state, no moving parts (service life of sensor independent to number of operating cycles). p Detection of any object irrespective of material or conductivity, for example : metals, minerals, wood, plastic, glass, cardboard, leather, ceramic, fluids, etc.
Principle of operation
A capacitive proximity sensor basically comprises an oscillator whose capacitor is formed by 2 electrodes placed in front of the sensor.
In open air (εr = 1), the capacitor capacitance is C0. εr is the dielectric constant, which depends on the object material. All materials where εr > 2 will be detected.
Electric field Electrode
Air εr = 1 C = C0
When an object, of any material, (εr > 2) passes the sensing face of the sensor, it modifies the coupling capacitance (C1). The capacitance variation (C1> C0) instigates the starting of the oscillator. This in turn causes the output driver to operate and provides an output signal.
Object
εr > 2
C = C1
1150 Ver2.00-EN.fm/2
Schneider Electric
General
Types of sensor
Capacitive proximity sensors
Sensors flush mountable in support
Metal case cylindrical sensors and plastic case block sensors. For detection of insulated materials (wood, plastic, cardboard, glass, etc.). These sensors are recommended when :
p Detection distances are comparatively small. p It is required that the sensors be flush mounted. p The application necessitates the detection of a non conductive material through a non conductive partition (for example : detection of glass in a cardboard box).
Contamination
Front face
Main electrode Compensation electrode Earth electrode (a) : compensation field (suppression of external contamination) (b) : main electric field
Sensors non flush mountable in support
Plastic case block sensors. For detection of conductive materials (metal, water, liquids, etc.). These sensors are recommended for :
p Detection of conductive object materials at considerable distances. p Detection of conductive materials through insulated partitions. p Detection of non conductive materials on or in front of earthed metal parts.
Earth
Front face
Main electrode
(a) : electric field
Schneider Electric
31150 Ver2.00-EN.fm/
General
Terminology
Capacitive proximity sensors
Nominal sensing distance Sn
Similar to that defined for inductive proximity sensors, the nominal sensing distance being calculated using a standard mild steel square target, 1 mm thick. The side dimension of the square plate is equal to that of the sensor sensing face.
Sensitivity of the sensor
Ø 18 or Ø 30 mm cylindrical type and 40 x 40 mm block type sensors incorporate a 20-turn sensitivity adjustment potentiometer. This enables the sensitivity of the sensor to be adjusted to suit the type of object to be detected. The sensors are factory preset for nominal sensitivity. Depending on the application, adjustment of the sensitivity could be necessary as follows : - increasing the sensitivity for objects which have a weak influence (low dielectric constant,εr), for example : paper, cardboard, glass, plastic, - decreasing the sensitivity for objects which have a strong influence (high dielectric constant,
εr), for example : metals, liquids.
Telemecanique capacitive proximity sensors incorporate compensation electrodes so as to eliminate the affects of varying environmental conditions (humidity, pollution). However, in the event of severe variations in the ambient conditions, do not increase the sensitivity of the sensor such that it is set to its maximum operating limits. An increase in sensitivity causes an increase in the switching hysteresis.
P : sensitivity adjustment potentiometer
1150 Ver2.00-EN.fm/4
Schneider Electric
General
Terminology
Capacitive proximity sensors
Operating distances
The operating distance of the sensor is related to the dielectric constant (εr) of the object material to be detected. The higher the value of εr, the easier the detection of the object will be. The working distance depends on the object material : Sa = Sn x Fc Sa = working distance, Sn = nominal sensing distance of the sensor, Fc = correction coefficient for the object material. Example : sensor XT1-M30PA372 used to detect a rubber object Sn = 10 mm, Fc = 0.3 Working distance Sa = 10 x 0.3 = 3 mm
The list below indicates the dielectric constant values for the most common object materials, together with their correction factors (Fc) for the nominal sensing distance of the sensor.
Schneider Electric
Material
εr
Fc
Material
εr
Fc
Acetone
20
0.8
Paper
2…4
0.2…0.3
Air
1
0
Paraffin
2…2.5
0.2
Alcohol
24
0.85
Petrol
2.2
0.2
Ammonia
15…25
0.75…0.85
Plexiglass
3.2
0.3
Cement (powder) 4
0.35
Polyester resin 2.8…8
0.2…0.6
Cereals
3…5
0.3…0.4
Polystyrene
3
0.3
Damp wood
10…30
0.7…0.9
Porcelain
5…7
0.4…0.5
Dry wood
2…7
0.2…0.6
Powdered milk 3.5...4
0.3...0.4
Ethylene glycol 38
0.95
Rubber
2.5…3
0.3
Epoxy resin
4
0.36
Salt
6
0.5
Flour
2.5…3
0.2…0.3
Sand
3...5
0.3...0.4
Glass
3...10
0.3...0.7
Sugar
3
0.3
Marble
6…7
0.5…0.6
Teflon
2
0.2
Mica
6…7
0.5…0.6
Vaseline
2...3
0.2...0.3
Nylon
4…5
0.3…0.4
Water
80
1
Oil
2.2
0.2
31150 Ver2.00-EN.fm/
General
Environment
Capacitive proximity sensors
Electromagnetic interference Telemecanique sensors undergo electromagnetic interference testing in accordance to the recommendations of standard IEC 947-5-2 (electrostatic discharges, radiated electromagnetic fields, fast transients, impulse voltages).
Thermal influences It is advisable to remain within the values stated on the characteristics pages, so as to avoid sensing distance drift and incorrect operation of the sensor. Chemical agents To ensure a long service life it is essential that any chemicals coming into contact with the case of the sensor are non corrosive.
Mechanical shocks The sensors are tested in accordance to the standard IEC 68-2-27, 50 gn, duration 11 ms. Vibration The sensors are tested in accordance to the standard IEC-68-2-6, amplitude ± 2 mm, F = 10 to 55 Hz, 25 gn at 55 Hz.
Earthing Earthing of an object that has a high conductivity increases the sensing distance.
Mounting precautions
In order to avoid mutual interference between sensors, it is recommended that the minimum mounting distances given for “Setting-up” are adhered to.
Flush mountable cylindrical models are suitable for applications where the head of the sensor needs to be flush with the support surface.
Flush mountable model
Non flush mountable cylindrical models require an area of free space around the head of the sensor.
Non flush mountable model
Additional information related to outputs
- Terminology. - Details and wiring constraints for 2-wire and 3-wire type sensors. - Connecting sensors in series or parallel. Similar to that detailed on the corresponding pages relating to inductive proximity sensors.
1150 Ver2.00-EN.fm/6
Schneider Electric
General
Application example : “Bottle filling”
Capacitive proximity sensors
Bottle arrival
Bottles are fed on a conveyor belt for filling. The sensors 1 (for insulated object materials) and 2 (for conductive object materials) are in the unoperated state.
Bottle filling
As soon as the bottle enters the detection zone of sensor 1, the filling operation commences. Sensor 2 remains in the unoperated state.
Filling complete
Sensor 2 detects that the required level has been reached and stops further filling.
Schneider Electric
31150 Ver2.00-EN.fm/
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