There are many types of proximity sensors used in different applications. We use capacitive proximity sensors to detect any type of object without any contact. They detect objects by measuring the change in an electrical property, capacitance
What is a Capacitive Proximity Sensor?
A capacitive proximity sensor is a sensor that can detect an object using the electrical property, capacitance. They are widely used to detect and measure objects/fluids which have a higher dielectric constant than air. This includes anything that is conductive or non-conductive.
Capacitive proximity sensors have many applications in industrial automation systems, from detecting positions to analyzing the composition of objects in a non-invasive way.
Capacitive Proximity Sensor And How It Works
Capacitive proximity sensors are a special application of capacitive sensors. We use them to detect the presence of objects in industrial environments. The image below is an RS PRO M30 x 1.5 Capacitive Proximity Sensor.
Before diving into the details, let's understand what a capacitor is and how it works. Simply put, a capacitor is a device that can hold an electrical charge like a battery. They consist of two conductive plates with a dielectric material filling the space. Depending on the dielectric width, their capacitance (ability to store electric charge) changes.
The dielectric constant depends on the material. Materials with high dielectric constant are easy to detect. For example, water is more detectable than oil or PVC. This is because water has a dielectric constant of around 78 and for PVC it is only around 5.
A capacitive proximity sensor follows the same principle, only one of the plates is now the object that we want to detect. Approaching an object to the detection face causes a change in capacity. The sensor can then measure the change and determine if the object is near.
It is not possible to directly measure the change in capacitance by ordinary means. To solve this problem, capacitive proximity sensors contain specialized circuitry. The circuit does all the signal processing to ultimately produce a usable digital signal.
The first stage of the sensor is the capacitor itself. When an object is close to the sensing face, it forms a capacitor. The air between them becomes the dielectric material. Inside the sensor, there is an oscillator circuit. This can either be an RC or LC Oscillator circuit.
The capacitance created by the external object triggers an oscillation in the circuit. This smallest distance an object must maintain from the face of the sensor to start oscillation is also known as the "working point". This is adjustable in most sensors. As an object approaches the sensor, this oscillation frequency increases. This causes the amplitude of the oscillation to increase.
The circuit also consists of a trigger circuit with hysteresis. The trigger circuit monitors the frequency and amplitude of the oscillation. It controls the output if the amplitude exceeds a predefined value. There are sensors that can output digital or analog signals.
Proximity sensors allow their operating point to be adjusted. Some have potentiometers while others may have a dedicated "learn button". This knob or potentiometer screw can be used to calibrate the sensor. The increase in sensitivity also makes the sensor more sensitive to false detections. This means that sometimes even changes in humidity and temperature can cause the sensor to trip.
Capacitive sensors can detect both conductive and non-conductive materials. Conductive materials are the easiest to detect because they form a good capacitor with the sensor. In this case, the dielectric strength becomes negligible.
The detection of non-conductive materials depends on three factors:
Sensor surface size - larger surface allows for longer sensing distances
The dielectric constant of the target material – the higher the constant, the longer the distance
Target area – larger area, longer distance
Target speed and temperature can also affect detection distance.
Detection Range
A capacitive proximity sensor has a wider detection range than its inductive counterpart. The detection range is between 3 and 60 mm. The greatest distance
detection is based on a standard target, a 360 mm thick Fe 1 grounded steel plate. This must have a side length corresponding to the diameter of the sensor surface. If the detection distance is greater than the diameter, the length of the side should be three times the nominal detection distance.
Non-conductive objects must have a reduction factor based on the dielectric constant of the material. There are tables that provide approximate values for some materials. They help to determine an accurate detection distance.
There are two important parameters when considering detection range:
Nominal/nominal sensing distance (Sn)
It is a theoretical value. It does not include manufacturing tolerances, operating voltages or temperatures.
Effective detection distance (Sr)
Defined for a specific set of conditions. (i.e. flush mounting, ambient temperature and given supply voltage)
Hysteresis
Hysteresis is the difference between the on distance and the off distance. It defines a Zone rather than a line to detect an object.
The hysteresis causes the output to “lock” even when the object is moving in or out of the sensor field. This prevents the “chatter” effect (repeated switching the output on and off) if an object is at the edge of the detection range.
Hysteresis is an independent parameter. This is a percentage of the nominal detection distance. For example, a sensor with a nominal sensing distance of 20mm may have a maximum hysteresis of 15%. This corresponds to approximately 3 mm of the detection range. This may vary from sensor to sensor, even between the same model.
Several factors can affect hysteresis:
Sensor ambient and local temperature
atmospheric pressure
Relative humidity
Mechanical stress of sensor housing
Sensitivity correlation - higher sensitivity, greater hysteresis
Capacitive Sensor Types
Capacitive sensors are used to detect many types of materials. This also includes fluid flow, fluid level, and even pressure. There are a variety of capacitive proximity sensors on the market:
Miniature Capacitive Sensors
Miniature capacitive sensors are available in cylindrical or wafer-style packages to save mounting space. These sensors do not have the signal processing circuit inside. A separate amplifier is used to process the signals.
Cylindrical capacitive sensors
These are larger than miniature sensors and can range in diameter from 6.5mm to 30mm. Their detection distances are adjustable.
Capacitive high temperature sensors
High temperature capacitive sensors are designed to withstand extreme temperatures and can even handle direct contact with high temperature objects/fluids.
Analog Capacitive Sensors
These find their applications in material selection, thickness monitoring and concentration control tasks. Analog sensors output a range of voltages/currents to help determine the type of object it is monitoring.
Capacitive Sensor Wiring Diagram
There are many sensor wiring diagrams used in the automation industry. We can classify sensors according to their type of supply voltage and type of output:
AC or DC power supply
Determines whether the sensors work with a 220V AC or 24V DC power supply
The type of output
Transistor output (3 wires)
Transistor output sensors can be NPN or PNP. For these two types, here are the NO (normally open) and NC (normally closed) output options. Some sensors may even support both. (NO+NC).
Relay output (2 wires or 3 wires)
2-wire and 3-wire AC sensors are always relay output type. DC sensors can be relay or transistor output type. Relay output sensors also have NO, NC and NO+NC options.
There are many types of proximity sensors used in different applications. We use capacitive proximity sensors to detect any type of object without any contact. They detect objects by measuring the change in an electrical property, capacitance
What is a Capacitive Proximity Sensor?
A capacitive proximity sensor is a sensor that can detect an object using the electrical property, capacitance. They are widely used to detect and measure objects/fluids which have a higher dielectric constant than air. This includes anything that is conductive or non-conductive.
Capacitive proximity sensors have many applications in industrial automation systems, from detecting positions to analyzing the composition of objects in a non-invasive way.
Capacitive Proximity Sensor And How It Works
Capacitive proximity sensors are a special application of capacitive sensors. We use them to detect the presence of objects in industrial environments. The image below is an RS PRO M30 x 1.5 Capacitive Proximity Sensor.
Before diving into the details, let's understand what a capacitor is and how it works. Simply put, a capacitor is a device that can hold an electrical charge like a battery. They consist of two conductive plates with a dielectric material filling the space. Depending on the dielectric width, their capacitance (ability to store electric charge) changes.
The dielectric constant depends on the material. Materials with high dielectric constant are easy to detect. For example, water is more detectable than oil or PVC. This is because water has a dielectric constant of around 78 and for PVC it is only around 5.
A capacitive proximity sensor follows the same principle, only one of the plates is now the object that we want to detect. Approaching an object to the detection face causes a change in capacity. The sensor can then measure the change and determine if the object is near.
It is not possible to directly measure the change in capacitance by ordinary means. To solve this problem, capacitive proximity sensors contain specialized circuitry. The circuit does all the signal processing to ultimately produce a usable digital signal.
The first stage of the sensor is the capacitor itself. When an object is close to the sensing face, it forms a capacitor. The air between them becomes the dielectric material. Inside the sensor, there is an oscillator circuit. This can either be an RC or LC Oscillator circuit.
The capacitance created by the external object triggers an oscillation in the circuit. This smallest distance an object must maintain from the face of the sensor to start oscillation is also known as the "working point". This is adjustable in most sensors. As an object approaches the sensor, this oscillation frequency increases. This causes the amplitude of the oscillation to increase.
The circuit also consists of a trigger circuit with hysteresis. The trigger circuit monitors the frequency and amplitude of the oscillation. It controls the output if the amplitude exceeds a predefined value. There are sensors that can output digital or analog signals.
Proximity sensors allow their operating point to be adjusted. Some have potentiometers while others may have a dedicated "learn button". This knob or potentiometer screw can be used to calibrate the sensor. The increase in sensitivity also makes the sensor more sensitive to false detections. This means that sometimes even changes in humidity and temperature can cause the sensor to trip.
Capacitive sensors can detect both conductive and non-conductive materials. Conductive materials are the easiest to detect because they form a good capacitor with the sensor. In this case, the dielectric strength becomes negligible.
The detection of non-conductive materials depends on three factors:
Sensor surface size - larger surface allows for longer sensing distances
The dielectric constant of the target material – the higher the constant, the longer the distance
Target area – larger area, longer distance
Target speed and temperature can also affect detection distance.
Detection Range
A capacitive proximity sensor has a wider detection range than its inductive counterpart. The detection range is between 3 and 60 mm. The greatest distance
detection is based on a standard target, a 360 mm thick Fe 1 grounded steel plate. This must have a side length corresponding to the diameter of the sensor surface. If the detection distance is greater than the diameter, the length of the side should be three times the nominal detection distance.
Non-conductive objects must have a reduction factor based on the dielectric constant of the material. There are tables that provide approximate values for some materials. They help to determine an accurate detection distance.
There are two important parameters when considering detection range:
Nominal/nominal sensing distance (Sn)
It is a theoretical value. It does not include manufacturing tolerances, operating voltages or temperatures.
Effective detection distance (Sr)
Defined for a specific set of conditions. (i.e. flush mounting, ambient temperature and given supply voltage)
Hysteresis
Hysteresis is the difference between the on distance and the off distance. It defines a Zone rather than a line to detect an object.
The hysteresis causes the output to “lock” even when the object is moving in or out of the sensor field. This prevents the “chatter” effect (repeated switching the output on and off) if an object is at the edge of the detection range.
Hysteresis is an independent parameter. This is a percentage of the nominal detection distance. For example, a sensor with a nominal sensing distance of 20mm may have a maximum hysteresis of 15%. This corresponds to approximately 3 mm of the detection range. This may vary from sensor to sensor, even between the same model.
Several factors can affect hysteresis:
Sensor ambient and local temperature
atmospheric pressure
Relative humidity
Mechanical stress of sensor housing
Sensitivity correlation - higher sensitivity, greater hysteresis
Capacitive Sensor Types
Capacitive sensors are used to detect many types of materials. This also includes fluid flow, fluid level, and even pressure. There are a variety of capacitive proximity sensors on the market:
Miniature Capacitive Sensors
Miniature capacitive sensors are available in cylindrical or wafer-style packages to save mounting space. These sensors do not have the signal processing circuit inside. A separate amplifier is used to process the signals.
Cylindrical capacitive sensors
These are larger than miniature sensors and can range in diameter from 6.5mm to 30mm. Their detection distances are adjustable.
Capacitive high temperature sensors
High temperature capacitive sensors are designed to withstand extreme temperatures and can even handle direct contact with high temperature objects/fluids.
Analog Capacitive Sensors
These find their applications in material selection, thickness monitoring and concentration control tasks. Analog sensors output a range of voltages/currents to help determine the type of object it is monitoring.
Capacitive Sensor Wiring Diagram
There are many sensor wiring diagrams used in the automation industry. We can classify sensors according to their type of supply voltage and type of output:
AC or DC power supply
Determines whether the sensors work with a 220V AC or 24V DC power supply
The type of output
Transistor output (3 wires)
Transistor output sensors can be NPN or PNP. For these two types, here are the NO (normally open) and NC (normally closed) output options. Some sensors may even support both. (NO+NC).
Relay output (2 wires or 3 wires)
2-wire and 3-wire AC sensors are always relay output type. DC sensors can be relay or transistor output type. Relay output sensors also have NO, NC and NO+NC options.
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