Types of Proximity Sensors used in Industrial Control

Almost every automated manufacturing operation has sensors that ensure that the system is working correctly.

Examples of Sensors that are used in industrial control are:

Non-Contact Presence Sensors (Proximity Sensors)

Contact sensors are often avoided in automated systems because wherever parts touch there is wear and a potential for eventual failure of the sensor. Automated systems are increasingly being designed with non-contact sensors. The three most common types of non-contact sensors in use today are:

• Inductive proximity sensor
• Capacitive proximity sensor
• Optical proximity sensor

The above sensors are actually transducers, but they include control circuitry that allows them to be used as switches. The circuitry changes an internal switch when the transducer output reaches a certain value.

Hall Effect Limit Switch

The inductive Sensor

This is the most widely used non-contact sensor due to its small size, robustness, and low-cost. This type of sensor can only detect the presence of electrically conductive materials.

The DC power supplied is used to generate AC in an internal coil, which in turn causes an alternating magnetic field. If no conductive materials are near the face of the sensor, the only impedance to the internal AC is due to the inductance of the coil. If however, a conductive material enters the changing magnetic field, eddy currents are generated in that conductive material, and there is a resultant increase in the impedance to the AC in the proximity sensor. A current sensor, also built into the proximity sensor detects when there is a drop in the internal AC current due to increased impedance. The current controls a switch providing the output.

Inductive Sensor

 

Capacitive proximity Sensors

These sensors sense the target objects due to the target’s ability to be electrically charged. This works both on conductors and non-conductors.

Inside the sensor is a circuit that uses the supplied DC power to generate AC, to measure the current in the internal AC circuit, and to switch the output circuit when the amount of AC current changes. Unlike the inductive sensor, the AC does not drive a coil, but instead tries to charge a capacitor. The AC can move current into and out of this plate only if there is another plate nearby that can hold the opposite charge. The target being sensed acts as the other plate.

Capacitive Proximity Sensor

If this object is near enough to the face of the capacitive sensor to be affected by the charge in the sensor’s internal capacitor plate, it will respond by becoming oppositely charged near the sensor, and the sensor will then be able to move significant into and out of its internal plate.

Optical Proximity Sensors

These are widely used in automated systems because they have been available longer and some can fit into small locations. They are commonly known as light beam sensors of the thru-beam type or of the retro-reflective type. A complete optical proximity sensor includes a light source, and a sensor that detects the light. The light source is supplied because it is usually critical that the light be tailored for the light sensor system. The light source generates a light of a particular frequency which is able to be detected by the light sensor in use. Infra-red light is used in most optical sensors. To make the light sensing system more foolproof, most optical proximity sensor light sources pulse the infra-red light on and off at a fixed frequency. The light sensor circuit is designed so that light that is not pulsing at this frequency is rejected.

Optical Proximity Sensors

The light sensor is a semiconductor device such as a photo diode which generates a small current when light energy strikes it or more commonly a photo transistor or a photodarlington that allows current to flow if light strikes it.

You can also read: Use of Smart Sensors in Industrial Measurement and Control Systems

Some of the manufacturers of proximity sensors include:

• Siemens
• Omron

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