Tips on sensor selection
At the beginning of my engineering career, a sales engineer from a sensor company came to our plant, thumped a substantial and well-worn sample case down on our conference room table, flipped the case open to expose dozens of neatly packed sensors and said, "Let’s test your part."
He knew what he was talking about. Engineers must test the sensor with the part. Locating the right sensor for the application requires:
- Narrowing the search to a short list of sensors
- Ordering samples, and
- Testing the sensor with the part, the actuator, or the machine under conditions similar to where the sensor will be installed.
When identifying the short list of sensors to sample, make sure the set—based on the manufacturer’s data sheet—meets the basic operating conditions of the application. Here is my list of the top six operating condition requirements:
- Temperature range
- Protection class
- Voltage range
- Discrete or analog output
- Answering the question: Will it be beneficial to be able to change parameters? If the answer is yes, then an IO-Link enabled sensor should be considered.
Here are an additional six requirements for more specific considerations:
- Response speed
- Sensing range
- Repetition accuracy
- Electrical connection
- Mounting type
- Answering the question: Is on-sensor visual display required?
The following are the most common types of sensors used in manufacturing with tips and insights for each.
A proximity sensor detects the presence of nearby objects without physical contact. Presence sensors are discrete output devices. Typically, a magnetic proximity sensor is used to detect when an actuator reaches a specific position by sensing a magnet located in the actuator.
It is not a good idea to purchase actuators from one company and magnetic proximity sensors from another. While the sensor manufacturer may say the sensor is compatible with X, Y, and Z actuators, the reality is variations in magnets and mounting positions can cause sensing issues. For example, the sensor may activate when the magnet is not in the correct position or it may not activate at all. If the manufacturer of the actuator offers a matched proximity sensor, it should be the first-choice sensor.
Transistor-based proximity sensors have no moving parts and long service lives. Reed-based proximity sensors use a mechanical contact and have shorter service lives and cost less than transistor models. Reed sensors are best applied in high-temperature applications and applications where ac power supply is needed.
Position sensors have analog outputs indicating the position of the actuator based on the position of the magnet on that actuator. Position sensors provide flexibility from a control standpoint. The control engineer can determine a range of set points to conform to component variations. Since these position sensors are based on magnets, like proximity sensors, it’s a good idea to purchase the sensor and actuator from the same manufacturer if possible. Position sensors can be acquired with IO-Link functionality, which also can simplify control and parameterization.
Inductive proximity sensors utilize Faraday’s law of induction to indicate presence of an object or an analog output position. The most critical aspect of selecting an inductive sensor is determining what type of metal the sensor is detecting because that determines sensing distances. Nonferrous metals can reduce the sensing range by more than 50% compared to ferrous metals. Sensor manufacturer data sheets should provide the necessary information for sample selection.
Pressure and vacuum sensors
Make sure the pressure or vacuum sensor will accommodate the pressure range required as measured in pounds per square inch for imperial measurement and Bar for metric. Specify the form factor most suitable for the allotted space. Consider whether machine mounted sensors should have indicator lights or a display screen as an aid for operations personnel. If changing setpoints quickly is necessary, investigate IO-Link enabled pressure and vacuum sensors.
Like pressure and vacuum sensors, flow sensors are specified by flow range, size, and setpoint variability. They can be ordered with on sensor display options. Flow sensors can be specified for relatively low flow rates for one area of the machine and for whole machine applications.
The most common optical sensor options are photoelectric—diffuse, reflective, and through beam. Laser sensors and fiber-optic sensing units also fall under the optical sensor category. Photoelectric sensors are mostly presence sensors.
Photoelectric sensors detect the presence of an object via reflected light or an interrupted beam of light. These sensors are among the most applied sensors in manufacturing due to their low cost, versatility, and reliability.
Diffuse photoelectric sensors do not require a reflector. They are used for sensing the presence of nearby objects and are inexpensive sensors.
Through beam offers the longest sensing range and is installed at two points with an emitter unit and receiver unit. Garage door safety sensors are through beam sensors. Presence is indicated when the beam is interrupted. One interesting variate of the through beam is the fork light sensor that features an emitter and receiver in one compact unit. Fork light sensors are used for sensing the presence and absence of small parts.
Reflective photoelectric sensors have a sensor and a reflector and are used for mid-distance presence sensing. For accuracy and cost, they sit midway between diffuse and through beam.
Fiber-optic sensing units are used for presence and distance sensing. Parameters on these versatile sensors can be adjusted to detect various colors, backgrounds, and distance ranges.
Laser sensors are used for long distance presence sensing and are the most accurate in short distance measurement applications.
Vision sensors can be used for bar code reading, counting, shape verification, and more. Vision sensors are a cost-effective use of vision where camera systems would be too costly and complex. Vision sensor bar code reading can be used for tracking individual components and applying the processes identified for that component. In terms of counting, the sensor can verify, for example, the exact number of features present on a part.
A vision sensor can ascertain whether a specified curve or other shape has been achieved. Since these sensors are dealing with light, it is vital to test the sensor in as close to the operating environment in terms of ambient light and background reflectivity as possible. In most applications, it is recommended to place the vision sensor in an enclosure to isolate it from external sources of light. It is a good idea to enlist the aid of a vision sensor manufacturer in sensor testing. Make sure the right fieldbus is specified.
The signal converter changes the analog output signal from a sensor into switching points on the signal converter, another option is to convert to IO-Link process data.
KEYWORDS: process sensors, discrete sensors
- Locating the right sensor for the application requires testing.
- Sensor selection criteria include temperature, size, protection class, and whether the sensor requires a discrete or analog input.
- Also consider repetition accuracy, response speed, and sensing range.
What else should be considered when selecting a sensor for an application?
The New Products for Engineers database has multiple sensor categories. See www.controleng.com/NP4E.
For more information about sensors from Festo, see:
ABOUT THE AUTHOR
Sandro Quintero, product marketing manager, electric automation at Festo, has worked with customers across industries such as medical, food processing, mining, automotive, and most recently electronics and assembly. This has allowed him to acquire skills in areas such as sales, product support, and project management. He earned a bachelor’s degree in Mechatronics Engineering from the Universidad Autónoma de Ciudad Juárez, Mexico and an MBA from the University of Texas at El Paso.