Automation Sensor Basics: A Crash Course in Sensor Technologies
Published on : Friday 03-12-2021
Knowing the basics of how sensors work and what to choose will help you be a more informed and valuable member of the automation team, says Will Healy III.
For those new to factory automation, the sheer number of technologies and things to know can be completely overwhelming. And new technologies, from robots to modern conveyors to assembly systems, are being introduced every day. Those people in a new job or a new assignment can find themselves working with technology they may have seen but never needed to know well. Or, possibly your role in manufacturing is not connected to technology, purchasing, or management, and you'd like to have some basic understanding to work better with the technical people in your company. Understanding some technical fundamentals will allow you to contribute faster to your next project and talk intelligently with your colleagues.
A machine works like the human body. You have five senses (touch, taste, sight, smell and hearing), and with those senses, your body receives inputs from the world. When you touch something hot, your senses send a signal up your arm, through your nerves to the brain, and the brain decides what to do. For example, it may choose for you to pull your arm back and scream—the result of signals going from your brain, down your spine to your muscles, and your muscles reacting. A machine works in a very similar way. The machine has sensors that provide inputs from the production and cables or networks that collect those inputs and send them to the brain of the machine. The brain of a machine is called the controller or the Programmable Logic Controller (PLC) or the Programmable Automation Controller (PAC). The sensors collect inputs and send them to the controller, where actions are determined based on programming, and outputs are sent to the actuators, which act as the muscles of the machine. These can include servos, motors, lights, pneumatics, or hydraulics.
Automation sensor electrical basics
Most automation sensors run on 24 Volts Direct Current (VDC). Every electrical circuit needs to be a closed loop from the source of the power (positive or 24VDC) to the common (negative or 0VDC). The most common automation sensors have three wires: two wires for power (plus and minus) and one wire for the signal of the sensor. The signal of the sensor comes in a few different combinations: the output type, the output direction, and the switching logic.
Output type
Sensors generate three types of outputs: discrete, analog and digital. A discrete output type is either on or off, like a light switch. Analog output types are like a dimmer or slider switch in a room to control the brightness level of the lights. And the third type, the digital switch, is like a smart switch for smart homes where additional functions can be controlled, such as timing of the lights, and data is obtained, like the light’s operating hours. Where discrete sensing is used most often is in applications where the sensor is being used to detect the presence of an object or machine linkage. With analog or digital, the application is typically for measurement or positioning where a specific numerical value is needed relative to the physical position, temperature, or pressure.
Output direction
The output direction is important as we need to know which way to close the electrical loop. Depending on the type of discrete sensor used, it can provide either 24VDC or 0VDC on its signal wire to the load when it detects an object. This is called Sourcing (PNP) or Sinking (NPN). The sensor load can be many different things like a relay or a PLC input or an indicator light or an I/O port on a network device. With a PNP sensor, a positive signal is provided from the sensor to the load. With an NPN sensor, a negative signal is provided from the sensor to the load. An easy way to remember this is P is for positive and PNP starts with P, while NPN starts with N so it brings negative to the load. PNP is used mostly commonly today in many parts of the world.
Switching logic
The switching logic of the sensor needs to be known. When an object is detected, does the sensor output turn on or does it turn off? This is called normally open versus normally closed. In a normally open situation, the sensor does not send power to the load until the target is present. Then the sensor closes the contact and sends power to the signal wire. Normally closed is always sending the signal until the target is present and then the signal stops. Normally open is the most common logic used with sensors.
The 5 most common automation sensing technologies
While there is a large variety of sensor principles and technologies available in discrete industrial production, the most common automation sensors are inductive, photoelectric, capacitive, ultrasonic and magnetic field sensors.
Photoelectric sensor
With sensing ranges from 25mm to 60 metres or more, photoelectric sensors are the most popular automation sensor. They use light to detect an object either by breaking the beam between the emitter and receiver or by the object reflecting the emitter light back to the receiver. These sensors are highly versatile for detecting many types of materials but may have issues with transparent objects. Different light sources like visible red light, laser or infrared can be used to solve traditional, precision, or dirty environment applications. Through time-of-flight calculations these sensors are often used for positioning and measurement applications with high resolution capabilities.
Inductive sensor
One of the most established technologies in automation, inductive sensors create an eddy current or electromagnetic field in front of the sensing face that is triggered by the presence of ferrous metal or steel. The size of the sensor has a direct impact on the sensing distance, typically a few millimetres up to 50 millimetres. They are commonly used to detect part presence, verify machine position, or error proof a process. The sensing distance is reduced by the type of metal, the size of the object relative to the sensing face, and/or the target’s direction of approach.
Capacitive sensor
Like inductive sensors, capacitive sensors create a field in front of the sensing face. This field, however, relies on the capacitance of the object or density to detect the target. Capacitive sensors are typically used to detect liquids (in non-metallic containers), plastics, wood or metals. They commonly have sensing ranges shorter than 25 millimetres. Capacitive sensors have a special ability to see through non-conductive walls or barriers to detect material on the other side.
Ultrasonic sensor
Ultrasonic sensors use high frequency (silent) sound waves and a diaphragm to detect the presence or position of an object. They emit sound waves that hit the target and are reflected to the sensor. Ultrasonic sensors are great at detecting most targets but struggle with thin films or materials that absorb sound waves. With sensing ranges from 25mm to 6 metres or more, ultrasonic sensors have a special ability to bounce sound waves to get the desired detection position. And when looking for a measurement, ultrasonic sensors provide reliable analog or digital data for the application.
Pneumatic cylinder sensor
Pneumatic cylinder sensors utilise a magnet mounted on the piston of the cylinder to detect the extended or retracted position of the pneumatic cylinder. These sensors mount on the outside of the cylinder body in a T-slot or C-slot and are sometimes called reed switches or Hall Effect sensors. Newer solid-state technologies, specifically those utilising AMR (Anisotropic Magneto-Resistive) or GMR (Giant Magneto-Resistive) principles, provide more precision and reliability for the closed-loop feedback to the controller. While most often found on pneumatic cylinders, these sensors are also utilised on pneumatic grippers for robotic end effectors.
How to select an automation sensor
When selecting sensor technology for an application, there are five important questions to ask.
1. What is the target? What is the target material? What is the surface finish?
2. What is the target position? Is it repeatable? How close can I get? What is the direction of movement? How is it presented to the sensor?
3. What space is available for the sensor? Is there room to mount a sensor? How much room? Can I change anything? What shape will fit in the space available?
4. What is the environment? What hostilities exist? Vibration? Impact? Weld spatter? Fluids?
5. What is the connection? Wires or connector? Where does the input go? Is 24VDC nearby?
When making a selection, it is helpful to utilise the many online sensor selector tools that help guide choices and make selecting a part easier. If there are doubts that the sensor will work in the application, find a local sensor expert to discuss it and always test before relying on it for production.
Sensors are an important part of the central nervous system of the machine, bringing data from the process to the controller. Knowing the basics of how sensors work and what to choose will help you be a more informed and valuable member of the automation team.
Will Healy III has been with Balluff Worldwide for over 15 years and is currently the Marketing Manager for the Americas Region. He is enthusiastic about manufacturing, workforce development, smart factory technologies and raising the status of manufacturing in the community. A Purdue University mechanical engineer who loves to share his passion for automation, Will is a board member for the Advanced Manufacturing Industry Partnership (AMIP) in Cincinnati and is active with multiple university advisory boards and associations. He brings energy to dry subjects and speaks from personal experience about the industrial revolution, managing culture change in organisations, bridging the manufacturing skills-gap, and creating value through automation.
Connect with Will on LinkedIn, YouTube, Twitter or Reddit with the handle @WillAutomate.
