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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.
Automated test systems have to interact and measure things in the real world. Data on size, distance, strength, weight, pressure, temperature, color, surface finish, and more may be required to perform the test and analyze the results. Manufacturers use the terms “transducer” and “sensor” for devices that can convert various stimuli into electrical signals that can be read by the control system. Proper selection and use of sensors are one of the key factors that will determine how well a system functions.
For a given type of measurement to be made, there are often many different methods of measurement and types of sensors available. For example, for temperature measurement, there are resistive thermal devices, semiconductor chips, thermocouples, and non-contact infrared sensors just to name a few types. Even within those classes of temperature sensors, there are many choices to be made. With all of these choices, how is a designer to select the appropriate sensors when building a system?
Though the details of evaluating various instrumentation sensors are different depending on the type of measurement, each decision will be based on the same basic types of analysis. Obviously, it is important to gain familiarity with the various sensor options and tradeoffs between them before trying to make an evaluation. Many sensor manufacturers will provide some basic information about their products that can be used to gain some insights into various options for making measurements. However, it is important to make sure to research other options as sometimes that data will not discuss methods not productized by that manufacturer.
Six key items to consider when selecting a sensor will be highlighted here, but bear in mind that there may be other considerations or options for specific requirements. A company with experienced engineers that are not tied to a specific company’s products, such as Duotech Services, can provide invaluable support in evaluating requirements and selecting and implementing an appropriate instrumentation system.
1. Accuracy & Precision – These two terms do not mean the same thing, though they are often related. Accuracy has to do with how close the sensor reading is to the true value while Precision refers to the ability of the sensor to detect small changes. (As an example, a temperature sensor that measures boiling water at 97.53°C has high precision but low accuracy.) Both the accuracy and precision of a given instrumentation system must be appropriate for the requirements of the system. Too high of precision can give a false impression that the reading is also accurate or can result in the system detecting noise rather than the actual desired data. A sensor with more accuracy than necessary will be more expensive and more difficult to use properly than one more appropriate to the measurement required. Additionally, both accuracy and precision are affected by errors incurred throughout the system. Transducer error, wiring, signal conditioners, and the gauges or converters used to read the value each add their own errors into the system that must be understood in order to select the appropriate sensors.
2. Environment – The selection of the proper sensor requires a good understanding of the environment in which the instrument will be operated. Many sensors can be affected by the non-ideal conditions of a production floor (such as temperature variation, vibration, humidity, chemicals, etc.) It is important to take the environment into account when selecting the sensor and its packaging, mounting, and other options.
3. Excitation – Many transducers require power to produce an output signal and it is important to provide a power source that will not introduce additional errors.
4. Signal Conditioning – Unfortunately, the world is full of non-ideal realities in sensors. Electrical noise is always present, often more so on production floors, and can cause erroneous readings. Signal conditioners and other protection circuits can provide some protection from these effects before conversion. Sometimes these are useful, but other times it is possible or preferred to process the signals after conversion, so the use of conditioners must be evaluated during the instrumentation design process.
5. Conversion – In modern systems, it is often preferred that the instrumentation system provides digital data (rather than analog gauges or chart recorders). The analog to digital converters must be evaluated and matched appropriately to the sensors or errors can be introduced, or money wasted by overpaying for precision in one that is not present in the other. Make sure to properly handle ratiometric and non-ratiometric sensors by properly matching with converters that are the same.
6. Processing – Even if signal conditioning is performed, the sensor and conversion process is full of various sources of error. Some of these errors are linear (consistent effect across the measurement range), while others are non-linear. There are various methods and algorithms that can be used to compensate for these errors or to extract the best possible signal from the system.
The data ultimately must be displayed or used by the system and may be stored for later analysis. Whatever is done with the data, remember that a test system can only perform as well as the data that it is provided, so appropriate analysis must be done when selecting and implementing the instrumentation.
Duotech Services has extensive experience in using sensors from many different manufacturers for various types of measurement. This experience allows thorough analysis, design, and implementation of instrumentation systems that meet requirements while keeping acquisition and operational costs as low as possible.
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