Active vs Passive sensing: Is one better than the other?

Published: 15.11.2021

The original article was published on 15.11.2021, we updated it on 02.08.2023 and on 22.07.2024 with new information for a better overview and explanation of the topic.

Sensors Overview

Current technology progress seems to be unstoppable and never-ending. Technology companies progressively develop more sophisticated devices, while deep-tech startups bring completely novel sectors to the end users. All of these smart devices contain numerous sensors which make them smart.

Sensors are all around us, and society relies on them. Just imagine when was the last time you turned on/off wipers in your car manually? A rain sensor in your vehicle will take care of the job, your comfort and your safety. Similar examples can be found in all aspects of our lives. There is a wide pallet of different sensors measuring voltage, current, resistance, wind speed, acceleration, rain presence, magnetic field, sound, and more. These can be divided into various groups according to their properties for example analog/digital output sensors, qualitative/quantitative sensors, voltage/current output sensors, long-term/sacrificial sensors or active/passive sensors. We will delve into the critical differences between active and passive sensors to point out their main advantages and disadvantages with some examples.

Defining Active and passive sensors

First, to simply define the difference between active and passive sensing, we need to clarify the general rule that applies to the sensors of both types: passive sensors do not need an external power connection, whereas in the case of active sensors it is quite the opposite – these require a source of power to operate.

Both active and passive sensors are widely used in remote sensing technologies, these are also famous for surviving unusually harsh environments. Sensors can also be used in harsh environments and places that are inaccessible to people.

Active Sensing

Active sensors are the most common in the sensor family. They are characterized by continuous power consumption in order to perform measurements.

To better understand these sensors, we will review their types, operating principles, and general use cases.

One of the working principles is when active sensors emit energy (i.e LIDAR, RADAR, InfraRed. etc.), and the distance of reflection from that energy source (the point where the energy returns) is essentially the appropriate measurement. Not all passive sensors create resistor change, however, there are also the ones that have active components generating voltage, frequency changes, voltage changes, pulse width modulation, etc.

Examples of Active Sensors:

Hall effect sensors;
Accelerometers;
Ultrasonic Transducers;
Compass Sensors.

Other typical representatives could be RFID chips, optical sensors like cameras, acoustic sensors, or aforementioned rain sensors. Thanks to the external powering of the sensor, the whole measurement set-up can be more sophisticated and provide high-quality results with the best resolution. On the other hand, the reason for their most interesting properties is also the biggest drawback of active sensors. External powering usually implies high power consumption, as without it, active sensors are unable to react to any external stimuli that are being measured. The common issue with these is that there is almost always a need for an extra el. circuit to create the necessary energy. Complex electronic powering systems can also cause electromagnetic noise in the measured signal and results.

Active sensors are commonly used in manufacturing and networking environments (monitoring of data centers, industrial machinery, etc.). Examples of technologies that are based on active sensors are for example LiDAR, X-RAYs and others, however one of the more interesting and widely used, however not as celebrated one would be the Air Pressure Sensors.

These are practically pressure sensors that generate electrical signals which are proportional to the pressure. Some of the more common types are gauge, vacuum and atmospheric pressure sensors.

These measure different variables in terms of the pressure sensing:

Absolute (zero) pressure: Also known as absolute vacuum. This is essentially a negative gauge pressure. The sensors of this variable measure the pressure in comparison with a vacuum.
Gauge pressure: these sensors on the other hand provide pressure difference between the measured, and ambient pressure.
Differential pressure: is a bit unlike gauge or absolute pressures, however, it can basically be defined as the difference between two pressures
Atmospheric pressure: is produced by the atmosphere surrounding the earth (up to altitude ~ 480km) Its value at sea level is 101325 Pa.

Is LIDAR Active or Passive?

Light Detection And Ranging (LiDAR) is an active remote sensing technology that measures ground height. Essentially what is measured is a time for a laser pulse to bounce back to a detector.

Passive Sensing

Similarly to active sensing, passive sensing shall be examined through the capacity of the sensors. Identification of the types of these sensors have to come from the essential qualities they possess, which is that these sensors are designed to react to natural emissions produced in the vicinity of their environments.

This means that passive sensors need no additional powering as these do not create a special field of energy, but rather respond to changes of physical quantities within the spectrum of energy around them that is preexisting or created independent from them.

Passive sensors can be considered simpler than active since they do not require external power to respond to the changing external parameters. An anemometer (wind speed sensor) is an interesting example of a passive sensor, which can generate energy thanks to its propeller rotation during strong wind. Therefore, it can send information about the wind speed in order to protect, for example, the retractable roof during thunderstorms. Other examples include the ordinary thermometer based on liquid expansion, a bimetallic thermometer, or a more sophisticated sensor based on RVmagnetics MicroWire technology. In the case of passive sensors, you do not need any external power to perform the measurement, the power is necessary only for the reading of the measured property.

The more famous example of a passive sensor would be a digital camera sensor that receives the light on it(as an image pickup device) and encodes a digital file(photo or video), rather than the older, film cameras that had to go through the process chemically.

Passive sensors are famous for their responsiveness to vibrations, light, radiation, heat, magnetic field, etc. Some examples of the technologies and sensors used as passive sensing systems are thermal, photography, electric field, seismic, chemical and magnetic field sensing devices.

Some passive electrical components are capacitors, inductors, antennas, diodes, etc., however, we would like to showcase the MicroWire sensors, which are the more novel type of sensors within this characteristic.

The MicroWire sensors are a magnetic, passive element, which are extremely sensitive to Mechanical Stress, Temperature and Magnetic field directly, and many other parameters indirectly (electric current, torque, flow, bending, etc.)

MicroWires provide the measurements in real-time through a magnetic field that is induced within their vicinity. The resolution is high as can be, and due to the special size, flexibility and compatibility, these can be placed into a variety of materials and thus, find use in a variety of industries that require passive sensing.

Passive sensors like MicroWires are suitable for invasive predictive measurements. For example, a thermometer inside the aquarium tank can provide information about the temperature without any powering cables. Microwires can provide data about temperature, mechanical stresses and more, from the inside of a measured object, as it can be embedded without causing material integrity disruption. The only powered element represents the data acquisition device (reader) for the data gathering.

Passive Sensors vs Active Sensors: which one should replace the other?

It is common knowledge that when choosing a sensor, be it active or passive, the main aspects to consider are the fit to the necessary technical capacity, the cost of integration and maintenance (this includes the potential necessity to replace the whole system).

Currently, there is no one sensor that can cover all the needs from all the various industries be it in robotics, composite monitoring or rechargeable battery maintenance monitoring systems – there are different needs that cannot be met with either passive or active sensors, as a combination of both may be necessary, or depending on the exact use case, active or passive may prove more useful sensing method.

The proper selection of a suitable sensor, or combination of sensors can significantly affect the performance of the setup. Before the selection, it is crucial to consider various aspects of requirements, which should be met, such as the “trivial” ability to connect/read the output of the used sensor. Sometimes it might be advantageous to employ both types, active and passive sensors and benefit from their combined advantages. However, if the size is your biggest headache or you do not have space to lead cables to the sensor, RVmagnetics MicroWire is the right choice for you.

If we were to compare the sensors to human biology a MicroWire sensor could simulate our touch receptors located in the skin (detecting pressure and temperature with high accuracy), and an image detection system like a photosensitive capacitor, it would quickly become clear that the existence of one not only don't harm the other, but the absence of one can make general obstacles for the overall system. Thus a conclusion, active and passive sensing have their special use within the array of applications special to them, and even within the types of these sensors there are some that serve multiple purposes (MicroWire sensor), yet an existence of another type of sensor may be necessary for an assembly of a perfect function.

Difference Between Active sensor and Passive Sensor

Active sensor emits energy, transmits energy or drives a signal and detects and measures the reflected signal. As a result, external power source is needed for active sensors in order to detect and measure.

On the other hand, passive sensor only detects energy supplied from an energy source (natural radiation or emission) and does not require any additional energy source to produce output signal. The majority of passive systems used for remote sensing work on the visible, near-infrared, near-ultra-infrared, and microwave parts of the electro-magnetic spectrum.

Conclusion

All in all, it is generally difficult to make a blank statement crowning passive sensors as universally better than active ones. Depending on the specific scenario, where a passive sensor may have advantages over an active sensor is not going to describe the effectiveness of the measurement even halfway. As the more important questions, which come earlier are, for example

what to measure
what is the environment
what are the expected accuracies
what are the expected resolutions
what are the expected sensitivity levels
what are the space/size/weight limitations, and eventually
what is the budget