Sensor selection
Instrumentation for Measurement
M.EM040
Pedro J.S.C.P. Sousa
ppsousa@fe.up.pt
Master in Mechanical Engineering
2025
Sensor selection
Main factors to consider
• Functionality : It must measurewhat is needed
• Resolution, Accuracy, and Precision : They must be compatible with the phenomenaunder analysis
•
We can’t choose a sensor with a resolution of 5 mm when we expect the monitored object to move 2 mm.
• Measurement Range: The measurementrange should include all possible values.
•
If we expect the pressure to vary between0 and 100 bar, we can’t select a transducer that can only reach 50 bar.
• Sensitivity and immunity to external factors: How dependenton other factors the measurementcan be.
•
If we have a load cell measuring tensile loads, how immune is it to torsion? Can we mount it in such a way that avoids torsion?
• Capability of being used in the target environment: The environmentwhere it is used can limit our choices
•
If we need to measure a force inside an oven, we need equipment that can withstand that temperature, and that is immune to, or that compensates for, any variations.
• Dynamic Response: How fast the signal changes.
•
•
For example, environmentaltemperaturechanges slowly, but the temperatureinside an industrial oven may change very rapidly.
As we have seen in the lab, a large PT 100 (with plenty of thermal inertia) will have a significantly slower responsethan a thermocouple.
• If any changes to the system are acceptable
•
If we have a 100 g accelerometer,we shouldn’t use it to measure the acceleration of a 200 g specimen.
• Calibration requirements
•
Is it necessary to calibrate each sensor individually? Are any two sensors interchangeable, or do I need to calibrate them individually?
• Cost and ease of use
•
•
Is it compatible with the available equipment? We may choose to buy another sensor that may be inferior, but that is compatible with the available hardware.
In this case, “cost” includes not only the sensor itself, but also any equipment that is required to acquire the data, as well as man-hours to get it operational. For example, 3D printers
usually use thermistors instead of PT 100s or thermocouples.
• Output type
•
Analog output: Voltage, Current. Digital output: PNP/NPN (detectors), IO-Link, I2C, SPI, UART . Depending on the Data Acquisition hardware, but also on the distance and noise level.
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Problem 1
a)
A recycling centre has a conveyor belt, where gathered plastic and metallic materials are
deposited. Present a sensing solution that enables the separation of these materials. Consider
that the materials are already inside cardboard boxes, and that each box is full and contains a
single type of material (either plastic or metal). Select the most appropriate sensing element,
explaining your reasoning.
b) To separate these boxes after detection, it is
important to have information regarding the
conveyor belt’s position and speed. Present a
sensing solution that enables the acquisition of this
data, ensuring a resolution of at worst 0.5 mm.
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Problem 1
c)
Consider that you want to measure the
conveyor position with a resolution of
0.5 mm, using a rotational incremental
encoder from the series on the right.
Which of these options would you
choose and why?
Phase Z is the index signal.
Phases A and B are the two quadrature tracks
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Problem 2
Which of the following NTC thermistors would you select to monitor the temperature of a water
bath whose temperaturefluctuates between7 and 15 °C? Why?
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Problem 3
An automated system is being developed to perform mechanical testing procedures on tires,
specifically pressurizing and depressurizing them cyclically. To that end, a method is required to
measure pneumatic pressure at each point in time, complemented by the monitoring of tire
expansion.
a) Considering that there are gauge, differential, and absolute pressure sensors, which type would
you select for this application? Why?
b) Since it is possible for the tire to burst during the test, consider that a magnetic proximity
detector will be employed to detect whether the access door is closed. What type of magnetic
detector would you use? Why?
c) Propose a solution to measure the tire expansion during the tests. Present a mechanical sketch
of the system and discuss the advantages and drawbacks of your solution.
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Problem 4
a)
An experimental mechanics lab aims to test the response of a new material to high-speed
impacts. For this purpose, a pneumatic gas gun is available, which will fire a projectile at the
specimen. Propose a sensing solution that enables the measurementof the projectile’s speed,
explaining your logic, and presentinga mechanical sketch of the way it would be installed.
b) In case a resolution of 1 km/h is needed when performing tests at 120 km/h, what should the
resolution of the employed equipment be?
Gas gun
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Projectile
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Problem 5
a)
In an aluminium extrusion line, there is a need to monitor the temperature of the material as
it exits the die, although contact with the part before it cools may cause damage. Propose a
sensing solution, explaining your choices, and present a sketch showing how it would be
installed.
b) The furnace that is used in this extrusion line must be kept between 650 and 660°C. In case
there are any problems with the heaters’ power supplies, the furnace temperature will fall 10
°C (approximately instantly). Propose a sensing solution and an appropriate sensing element
that can be used in this solution and that ensures that this fall is reflected in the output in
under 2 seconds (at 95%). Explain your choices and define the required resolution and time
constant of the sensor in your solution.
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Problem 5
• In real-world cases, temperature does not fall instantly. Something like “dropping
10 °C in 2s” would be more realistic. This could be solved as:
𝜏𝑦ሶ + 𝑦 = 660 −
𝑦 𝑡
10
𝑡,
2
𝑦 0 = 600
𝑡
−𝜏
= 660 − 5𝑡 + 5𝜏𝑒 + 5𝜏
Note: This is simply a real-world example. You will not be required to solve
differential equations in the exam.
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Problem 6 (2024/2025 Exam)
Consider a potentiometer-based joystick, such as the one shown in the figure.
a) Sketch a wiring diagram that includes all of the required connections that
make it possible to read the joystick’s position in both orthogonal directions.
Include all required equipment, such as voltmeters, potentiometers, and power
supplies.
b) Name at least two other technological solutions that could be used to
instrument a joystick. What are the advantages and drawbacks of those
solutions when compared to the potentiometric one?
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