Active vs Passive sensing: Is one better than the other?
The original article published on 15.11.2021, we updated it on 02.08.2023 with new information for better overview of the topic.
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, neither do they contribute anything to provide measurements, whereas in the case of active sensors it is quite the opposite – these require a source of power to operate and act alongside some sort of transducers that will create a stimul to be detected.
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
To better understand these sensors, we will go through some types, operating principles and general use cases of these.
Generally, active sensors emit energy (i.e LIDAR, RADAR, InfraRed. etc.), and the distance of reflection from that energy (the point where the energy returns) are essentially the appropriate measurements. 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.
The common issue with these is that there is almost always a need for an extra el. circuit to create the necessary energy.
Active sensors 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, 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 vacuum.
- Gauge pressure: these sensors on the other hand provide pressure difference between the measured, and ambient pressures.
- 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 an 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.
The more famous example 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 magnetic field sensing devices.
Some passive components are capacitors, inductors, antennas, diodes, etc., however, we would like to showcase the MicroWire sensors, which are the more novel types of sensors within this characteristic.
The MicroWire sensors are a magnetic, passive element, which are extremely sensitive to Pressure, Temperature and Magnetic field directly, and many other parameters indirectly (electric current, torque, flow, bending, etc.)
These devices 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 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.
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
