How to Select Capacitive Proximity Sensors
Image Credit: AutomationDirect |Baumer
Capacitive proximity sensors use the face or surface of the sensor as one plate of a capacitor, and the surface of a conductive or dielectric target object as the other. The capacitance varies inversely with the distance between capacitor plates in this arrangement, and a certain value can be set to trigger target detection.
Capacitive Proximity Analog Sensor Experimenting. Video Credit: grahmaustin
Capacitive proximity sensors measure the proximity of conductive as well as non-conductive objects with high resolution. Analogous to the function of an inductive proximity sensor, where a magnetic field is generated, a capacitive proximity sensor generates an electrostatic field at the face of the sensor. The sensor face is surrounded by a guard with equal potential so that the electric field will be focused towards the target. The capacitance varies inversely with gap distance and directly with the dielectric medium as well as the surface area of the target. Target size and dielectric medium are important design criteria to take into consideration.
Image Credit: Lion Precision
The dielectric constant of ambient air is relatively constant and does not affect the sensor.
The dielectric constant will change with the intrusion of water (humidity) or solid objects (dust, dirt, or other material).
Capacitive proximity sensors can be tuned to detect a wide range of powder, liquid or solid materials. It is also possible to detect the presence or absence of materials within a barrier or package when the dielectric of the material is greater than the barrier or packaging. Capacitive sensors can also be used as a limit switch, for level detection, object detection and for counting purposes.
Object Detection Behind a Barrier. Image Credit: Design World
When searching for capacitive proximity sensors, sensing performance and sensitivity are the most important parameters to consider. Other considerations include switch performance, body type, electrical connections, and various features.
- Rated operating distance is the critical distance at which switching takes place. It is important to choose a sensor that will operate in the required sensing range. This could be dictated by process requirements and mounting option.
- Repeatability is the distance within which the sensor repeatably switches. It is a measure of precision. Depending on the application, precision could be an important design criterion when selecting a sensor.
The operating distance and repeatability of a switched output are dependent on the sensitivity of the device. Capacitance is converted to an analog voltage and the sensitivity is defined by the voltage differential per mm of target travel. The switched output is tuned to change state at a predetermined voltage, defining a critical distance.
Electrical Switch Performance
Depending on the power options available, power requirements may be a key specification when selecting a sensor.
The device can be powered by either an AC or DC power source.
Load configurations are important parameters to consider. Capacitive proximity sensors may switch an AC load or a DC load. DC load configurations can be NPN or PNP.
NPN is a transistor output that switches the common or negative voltage to the load; load connected between sensor output and positive voltage supply.
PNP is a transistor output that switches the positive voltage to the load; load connected between sensor output and voltage supply common or negative.
Wire configurations are 2-wire, 3-wire NPN, 3-wire PNP, 4-wire NPN and 4-wire PNP.
2-Wire DC circuitry typically includes a diode bridge in series with the sensor, allowing functionality either as NPN or PNP in a DC circuit.
Acceptable values of leakage current must be evaluated in this type of circuit
Switch types can be normally open (NO) or normally closed (NC).
Image Credit: Fargo Controls, Inc.
If the switch is a NAMUR type switch or a specialized switch for switching a resistive load, it will require an external amplifier.
Capacitive proximity sensors are typically housed in either a barrel or rectangular housing.
Threaded barrel and rectangular housing. Image Credit: AutonicsOnline.com | ifm efector inc.
A barrel body style is cylindrical in shape. The barrel of the sensor is typically threaded so that with the help of two lock-nuts the sensor can easily be adjusted closer or further from the target as necessary.
- A rectangular or block body style is a one piece rectangular or block shaped sensor.
Electrical connections for capacitive proximity sensors can be fixed cable, connector(s), and terminals.
A fixed cable is an integral part of the sensor and often includes "bare" stripped leads.
A sensor with connectors has an integral connector for attaching into an existing system.
A sensor with terminals has the ability to screw or clamp down.
There are a number of features that may be important to consider when selecting capacitive proximity sensors.
- Field adjustable sensors are required when adjustments need to be made while the proximity sensor is in use. Potentiometers are commonly used to control the electric field strength or detection range.
Self-teaching proximity sensors can be auto-calibrated to trigger the switch at a specific target location.
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