5 Common Industrial Sensors You Should be Familiar with

Sensors

One type of feedback frequently needed by industrial-control systems is the position of one or more components of the operation being controlled. Sensors are devices used to provide information on the presence or absence of an object.

Types of Sensors

The following are the types of sensors used in industrial automation:

1. Photoelectric sensor

2. Inductive sensor

3. Capacitive sensor

4. Ultrasonic sensor

5. Temperature sensor

1. Photoelectric sensor - 

Photoelectric sensors detect objects, changes in surface conditions, and other items through a variety of optical properties.

A photoelectric sensor consists primarily of an emitter for emitting light and a receiver for receiving light. When emitted light is interrupted or reflected by the sensing object, it changes the amount of light that arrives at the receiver. The receiver detects this change and converts it to an electrical output. The light source for the majority of photoelectric sensors is infrared or visible light (generally red)

Base on operation, there are different types of photo electric sensor - 

- Through-beam sensors

In through-beam photoelectric sensing, also known as opposed mode, the transmitter and emitter are in separate housings. Light emitted from the transmitter is aimed directly at the receiver. When an object breaks the beam of light between the emitter and receiver, the receiver’s output changes state.

Through-Beam Sensor

Through-Beam Sensor

Through-beam sensing is the most efficient sensing mode which results in the longest sensing ranges and highest excess gain. This high gain enables through-beam sensors to be reliably used in foggy, dusty and dirty environments.

Applications:

• Monitoring of production and packing lines
• Product fill measurements through transparent containers
• Protecting dangerous areas for automatic doors

- Retro Reflective Sensor

In the retro-reflective mode, a light path is established between the sensor and a special reflector. A target is detected when it breaks the light beam. Typical retro-reflective sensors provide a longer sensing range when compared to diffused sensors. In some units a visible red LED is used which aids in the installation and alignment of the reflector. A corner cube reflector is employed to accurately return the light energy, on a parallel axis, into the receiver.

Retro-reflective sensor

Retro-reflective sensor

These reflection light beam switches are available with various types:

With polarization filter

A polarizing filter is utilized to eliminate false signals that may occur if a shiny target passes in front of the retro-reflective sensor. The filter enables the trouble-free detection of glossy and high reflective objects.

For clear object detection

The clear object detection sensor is a special retro-reflective mode photoelectric sensor that detects clear objects ordinary retro-reflective sensors may not. Through the use of a low hysteresis circuit, the sensor detects small changes in light typical when sensing clear objects. The clear object detector used polarized filters on both the transmitter and receiver of the sensor to reduce false responses caused by reflections from the target.

With foreground suppression

This foreground suppression sensor is a retro-reflective mode photoelectric that does not erroneously identify glossy targets as the reflector when they are within a certain distance. This mode is well-suited for detecting shrink-wrapped pallets, as a retro-reflective mode sensor may mistake the glossy covering for a reflector and not change state. Optical apertures in front of the transmitter and receiver elements in the sensor housing produce a zone for no erroneous detection of reflective, depolarized material.

Retro-reflective area sensor

With several transmitters and receivers in one housing forming a continuous wide or high detection area over the relevant sensing range.

- Diffuse Reflective Sensor

2 Inductive Sensor

An inductive sensor is a device that uses change in coil self-inductance or mutual inductive to respond to electric signal. Their operating principle is based on a coil and oscillator that creates an electromagnetic field in the close surroundings of the sensing surface. 

Inductive proximity sensors are used for non-contact detection of metallic object.

Inductive sensor

Inductive Sensor

3. Capacitive Sensor

Noncontact capacitive sensors work by measuring changes in an electrical property called capacitance. Capacitance describes how two conductive objects with a space between them respond to a voltage difference applied to them. When a voltage is applied to the conductors, an electric field is created between them causing positive and negative charges to collect on each object.
Like inductive sensors, capacitive sensors function without physical contact with the target material. 
Capacitive sensors allow for the detection of both, conductive and non-conductive materials, but are most commonly used for sensing materials such as plastics, liquids, powders and granular materials.

In applications utilizing capacitive technology the sensitivity of capacitive proximity switches is dependent on the material characteristics of the target object. This material characteristic of the metal, plastic or liquid object will influence the sensing range and therefore the output detection point of the capacitive sensor. 

4. Ultrasonic Sensor

Ultrasonic sensor perform electrical measurement using sound waves.
An ultrasonic sensor emits a sound pulse in the ultrasonic range. This sound pulse propagates at the speed of sound through air (about 344 meters per second) until the sound pulse encounters an object. The sound pulse bounces off the object and is returned in reverse to the sensor where this "echo" is received. By measuring the time it takes for the sound pulse to travel from sensor to object and back to the sensor. 
The distance to the object can be calculated very accurately. This measuring principle is also called "Time of Flight", or transit time measurement.
The frequency of the sound partly determines the distance that sound can travel. The lower the frequency, the greater the distance. Just think of a music festival, the low tones of the music can still be heard from a great distance, while the high tones fade much faster.

Types of Ultrasonic Sensor

There are two main types of Ultrasonic Sensors;

Proximity Detection Ultrasonic Sensors - this type of ultrasonic sensor will alert the user when an object enters an area of detection. The detection is irrespective of the objects size, material or reflectivity. 

Ranging Measurement Ultrasonic Sensors - this type of ultrasonic sensor can give exact measurements on the distance of an object which is moving to and from the sensor itself. It does this using intermittent sound waves. 

Application of Ultrasonic Sensor

• Detecting transparent materials
• Detecting liquid
• Detecting color
• Presence detection
• Box sorting
• Tank level detection
• Detecting depth of an ocean
• Distance measurement

5. Temperature Sensor

Temperature sensors are devices that measures temperature reading using electrical signal.

A temperature sensing device could be a Thermistor, thermocouple or an RTD (Resistance Temperature Device).

Based on operation, there are two major type of temperature sensing device - 

1. Contact type temperature device - 

They measure the hotness or coldness of an object by being in contact with the object.

Examples - Thermocouples, Thermistors, RTD

2. Non Contact type temperature device - 

They measure the temperature of a medium without being in contact with the object. This can be done by utilizing the radiation of the heat source.

Examples - Optical pyrometer, IR sensor 

Thermocouple - 

A thermocouple is made up of two dissimilar metals, joined together at one end, that produce a voltage (expressed in millivolts) with a change in temperature. A cold junction is always kept at a specific (constant) temperature, while the other end, the hot junction, is the measuring junction.

The junction of the two metals, called the sensing junction, is connected to extension wires. Any two dissimilar metals may be used to make a thermocouple.

Thermocouple image - source: instrumart.com 

Of the infinite number of thermocouple combinations, the Instrument Society of America (ISA) recognizes 12. Most of these thermocouple types are known by a single-letter designation; the most common are J, K, T, and E. The compositions of thermocouples are international standards, but the color codes of their wires are different. For example, in the U.S. the negative lead is always red, while the rest of the world uses red to designate the positive lead. Often, the standard thermocouple types are referred to by their trade names. For example,

• Type K thermocouple has the color yellow, and uses chromel – alumel, which are the trade names of the Ni-Cr and Ni-Al wire alloys.
• Type J thermocouple has the color black, and uses iron and constantan as its component metals. (Constantan is an alloy of nickel and copper.)
• Type T thermocouple has the color blue, and uses copper and constantan as its component metals.
• Type S thermocouple uses Pt/Rh-Pt
• Type E thermocouple uses Ni/Cr-Con

• Type N thermocouple uses Ni/Cr/Si-Ni/Si

RTD

The operation of an RTD is such that - as the temperature of a metal increases, do does the resistance to the flow of electricity. An electrical current is passed through the sensor, the resistance element is used to measure the resistance of the current being passed through it. As the temperature of the resistance element increases the electrical resistance also increases. 

An RTD consists of a resistance element and insulated copper wires. The most common number of wires is 2; however some RTDs have 3 or 4 wires. The resistive element is the temperature sensing element of the RTD. It is usually platinum because as a material it is highly stable over time, it has a wide temperature range, it offers an almost linear relationship between temperature and resistance and it has a chemical inertness. Nickle or copper are also other popular choices of material for the resistive element. 

Optical Pyrometer

An optical Pyrometer tracks and measures the amount of heat that is radiated from an object.

The optical system makes the thermal radiation into a better focus and passes it to the detector. The output of the detector will be related to the input thermal radiation.

The biggest advantage of this device is that, unlike RTD and thermocouple, there is no direct contact between the pyrometer and the object whose temperature is to be measured.

Fluke optical pyrometer