How to Select Capacitive Proximity Sensors

 

Capacitive Proximity SensorsCapacitive High Temperature Sensor Capacitive Proximity

 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

 

Sensor Operation

 

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).

 

 

 

 

Application

 

 

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

 

 

Selection Criteria

 

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.

Sensing Performance

  • 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. 

Sensitivity "V/mm"

 

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.

 

 

Body Type

 

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

 

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.

 

Features

 

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.  

Resources

  

Lion Precision - Capacitive Sensors Operation and Optimization

Automation Media.com - Capacitive Proximity Sensors Theory of Operation

Design World - The Search for a Better Proximity Sensor Starts Here

 


Related Products & Services

  • Eddy Current Proximity Sensors

    Eddy current proximity sensors and switches detect the proximity or presence of a target by sensing the magnetic fields generated by a reference coil.

  • Hall Effect Proximity Sensors

    Hall effect proximity sensors are used to detect the proximity, presence or absence of a magnetic object using a critical distance.

  • Inductive Proximity Sensors

    Inductive proximity sensors detect the presence or absence of an object using a critical distance.

  • Photoelectric Sensors

    Photoelectric sensors use emitters and receivers to detect the presence, absence, or distance of target objects.

  • Ultrasonic Proximity Sensors

    Ultrasonic proximity sensors use reflected or transmitted ultrasonic waves to detect the presence or absence of a target component.