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.
Three
ways to
find the right factory automation or machine control sensor
Locating the right sensor for the application requires:
Top
six operating condition require
ments for sensor selection
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:
Six
more requirements for
factory automation sensor selection
Here are an additional six requirements for more specific considerations:
Eight
mu
st-know factory automation sensor
terms, sensor
technologies
, sensor selection tips
The most common types of sensors used in manufacturing, factory automation, and machine control are proximity sensors, position sensors, inductive sensors, flow sensors, optical sensors, and vision sensors. A signal converter is a critical related technology. See tips for sensor selection and other sensor technology insights for each.
P
rox sensor selection tip
s
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 sensor selection tips for factory automation
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 sensor selection tip
s
for factory automation
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.
P
ressure and vacuum sensors
selection advice for machine control
applications
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.
Flow
sensor
selectio
n tips for machine control
applications
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.
Optical
sensor-
sensor
types and sensor
option
, se
lection
tips
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
sensor
use and application
tips
Vision sensors can be used for bar code reading, counting, shape verification, and more. Machine vision sensors are a cost-effective use of vision system 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 vision sensor can verify, for example, the exact number of features present on a part.
A machine 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 machine 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.
helps with sensor
output
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.
Sandro Quintero, product marketing manager, electric automation, at Festo. Edited by Chris Vavra, production editor, Control Engineering, CFE Media, cvavra@cfemedia.com.
KEYWORDS: process sensors, discrete sensors
factory automation
application requires testing.What else should be considered when selecting a sensor for an application?
ONLINE extra
The New Products for Engineers database has multiple sensor categories. See www.controleng.com/NP4E.
For more information about sensors from Festo, see:
Sensors overview
Vision sensors
Integrated sensors and safety
Sensors background
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.
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In this post, we will look into various selection criteria for sensors. Finding the right sensor can be a tough call. Hopefully by the end of this post, we would have given you some pointers to think about and make the process a bit easy.
Definition: A device that detects the changes in electrical or physical or other quantities and thereby produces an output as an acknowledgment of change within the quantity is named as a Sensor. Generally, the sensor output is going to be electrical or optical signal.
Finding the right sensor can be a tough call. Hopefully by the end of this post, we would have given you some pointers to think about and make the process a bit easy.
High Level Selection Criteria for Sensors:A sensor is usually not good or bad on it’s own. It totally depends on the application. To give you an idea, a sensor with 10-bit resolution could be a good fit for an application whereas another one with 16-bit resolution could be overkill.
At a high level, the selection criteria for sensors involve two steps:
Narrowing down the search list of sensors (typically to 2-3). In this step, you consider all parameter and select a few sensors that suit your requirements.
Testing the sensors in an environment similar to the application setup, so that we can analyze the sensors accordingly.
Selection criteria for sensorsSo while comparing the Sensor, consider the following parameters:
1. Range:
Difference between Maximum and Minimum value which can be sensed by the sensor. What is the minimum value you need to sense? What is the maximum value you need to sense?
2. Resolution:
The smallest change which can be sensed by the sensor. High is good but not always. If it is too high, it would pickup even very minute fluctuations which would then require additional processing.
3. Sensitivity:
Ratio of change in output to a unit change in the input. Again, high is good, but too high could be a problem. Also, higher the sensitivity, more will be the cost in most cases.
4. Error:
Difference between the Measured Value and True Value. You want this value to be low. All sensors have a margin of error. Does you application allow you to have that margin of error?
5. Accuracy:
It is inversely proportional to Error, i.e. How close the sensor reading is to the True Value. (Should be high).
6. Precision:
Ability to give/reproduce accurate value repeatedly. If a sensor is giving different values for the same physical conditions, it is not a good choice.
7. Response Time:
Time lag between the Input and Output. (Should be Minimum)
8. Signal-to-noise Ratio:
Ratio between the magnitude of the signal and the noise at the output.
9. Calibration:
As sensors need frequent calibration, so it should be easy to calibrate.
10. Cost:
It shouldn’t be expensive.
11. Nature of Output:
Do we need Analog output or Digital output, it should be clear.
12. Environment:
It is one of the most important parameters because not all sensors can work in extreme conditions. Sensors can get affected due to the non-ideal conditions(like temperature, humidity, etc.) which may affect the output of the sensor.
13. Flexibility:
We check whether the sensor can adapt to changes in the product with a simple OTA.
14. Interfacing:
It should be compatible to use with a wide range of instruments.
Some sensors need an external power source to produce an output, so it important to provide the power source, so that additional errors aren’t introduced.
15. Size and Weight:
Sensors should be compact and lightweight.
Select a Sensor – Example:Lets see, how we will be using the above points in choosing the correct temperature sensor for our application.
Application to be used in: In a home setup, where I have to check the temperature, so we will be comparing LM35 and DHT11 here. Let’s have a quick comparison on the basis of the above points.
LM35 is an analog temperature sensor used to measure the temperature with an electrical output which is proportional to the temperature(in °C). The range of operation is -55°C to 150°C. As the scale factor is 0.01V/°C so if my temperature is 27°C, then the voltage obtained will be 0.27V. It doesn’t require external calibration and maintains an accuracy of ±0.4°C for room temperature and ±0.8°C over a range of 0°C to 100°C. It has Low Self-Heating(0.08°C), which doesn’t affect the reading.
DHT11 is a digital temperature and humidity sensor. You can learn more about DHT11 and DHT22 sensors and its working in the below mentioned post:
The range of operation is 0°C to 50°C with ±2°C accuracy for temperature sensing and 20 to 80% with 5% accuracy for humidity sensing. The sampling rate is 1Hz(i.e. Give one reading for every second).
Here as we can see, LM35 has a wider range of operations than DHT11, but as we are will be using it in a home setup, where our maximum temperature can go till 45°C, both of them can be used.
But for more accurate results, we can choose LM35. But we wish to even monitor humidity with temperature, then DHT11 is a better choice.
Kunal is a keen learner who is exploring the domain of Embedded Systems and IoT. He is passionate about learning new technologies and sharing it with whoever is interested.
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