Sensors of all types serve to direct process flow within a facility by sending crucial information to a controller. Much like receptors with the human body’s nervous system, these sensors provide feedback via electrical signals, which the controller (or brain in this analogy) use to make decisions or trigger specific actions.
Today we’ll be taking a look at just one of these types of sensors, commonly referred to as the proximity sensor.
Proximity sensors are capable of recognizing the presence of an object without any physical contact. This article breaks down the different types of proximity sensors and common features, plus highlights some of the common applications in automation.
Inductive Proximity Sensors
Inductive proximity sensors are one of the most common types of proximity sensors you can find. They operate by generating a magnetic field via an inductor (hence the name).
Metallic objects entering the magnetic field will disrupt it, which the sensor can detect to change logic states. As a result, inductive proximity sensors are completely unaffected by non-metallic substances like dirt or moisture, making them an effective choice for harsh environments.
This also enables them to detect metallic objects through non-metallic ones, like plastic or cardboard.
Capacitive Proximity Sensors
The second type of proximity sensor is known as a capacitive sensor. These sensors are similar to inductive ones but use a capacitor to generate an electric field.
The main advantage of these capacitive sensors vs inductive sensors is the ability to detect non-metallic objects. This property gives them an obvious edge in safety applications, where a foreign object could be any type of material.
However, this makes capacitive sensors more prone to environmental conditions, such as ambient moisture or temperature. Even air contaminants could have an adverse effect on their operation.
Ultrasonic Proximity Sensors
Unlike the previous two types of sensors discussed, ultrasonic proximity sensors do not rely on an electric field of any type. Rather, they operate by emitting very high-frequency waves (far outside the audible range for humans) and measuring the echo returned.
You can think of them as a type of sonar device, or at least working on the same principle. The frequency of the ultrasonic waves can vary depending on the desired application. Because of their mode of operation, ultrasonic sensors are prone to background noise and can also be affected by temperature.
Photoelectric Proximity Sensors
The final type of sensor we’ll cover is the photoelectric sensor. As the name suggests, this sensor uses light to detect objects. They are also commonly referred to as photo eyes.
There are several different modes of operation for photo eyes. However, all of these modes are similar in that they are immune to background moisture, temperature, and sound, but they are vulnerable to ambient lighting.
Common Specifications
Most sensors are configured to detect objects within a certain range of distances, known as the sensing range. For many sensors, the sensing range does not necessarily “start” at zero distance; rather, they are configured to best detect objects a certain distance away from the sensor head. This enables sensors to be mounted behind a barrier. Sensors like these are often defined as embeddable or shielded. It’s worth mentioning that embeddable/shielded sensors often have a shorter sensing distance compared to their unshielded counterparts.
Another common specification is output current. Output current is determined by the wire configuration, with many sensors using either two or three wires. Two-wire sensors are used when a direct connection to an output device is required. These systems may be standalone, with no controller and connected to the same power source as the output device. As such, they have capacity for high current levels.
In comparison, three-wire sensors are used when interfacing with a PLC, whose digital logic switching run at lower current standards. It is common for three-wire sensors to use a standard connection like an M8 cordset interface.
Speaking of sensor output, their mode of operation may be something to consider. Most sensors are either normally open (NO) or normally closed (NC) with some able to be configured for either mode. Regarding which one you should use, it really comes down to how the system is configured.
Say you have a sensor positioned across a conveyor belt, and you’d like for it to trigger a LED whenever an object on the belt passes through. A NO sensor would make sense, as it would energize the LED only when it actively detects something.
Conversely, if you were using a sensor to monitor liquid levels in a tank and wanted to trigger an alarm if the levels got too low, a NC sensor would be preferable, at it would energize the alarm upon failing to detect the liquid.
Relevant Applications for Proximity Sensors
With all of this in mind, what are the relevant applications for proximity sensors? Here are some examples:
- Can be used on conveyor lines or for liquid level measurement.
- For safety, as they can detect if a foreign object enters a restricted zone or if a safety door is opened.
- Inventory systems to automatically determine if something is removed.
- They’re reliable for applications that operate in harsh environments.
- Can be used as alternatives to more traditional limit switches.
- In ultrasound imaging, which is a procedure that uses a specialized type of sensor designed to detect objects within an organic body.
No matter the field, there is almost certainly some application for contactless sensing provided by proximity sensors. MISUMI offers a wide range of proximity sensor types from trusted brands.
