Wireless or Wired Sensors?

As we go by the name Zensor, people often see us as a 'Sensor' company, but a bit quirky. A bit quirky we are, but a Sensor company we're not. We do of course make use of sensors to monitor critical machines and assets. But for us sensors are a tool, not the goal. Our goal is to make you feel Zen when we're monitoring your assets! We focus on what can be done with the data, where the sensors are a mere means to create the required data. Of course we make sure that, when we use sensors, we use the right sensors for the job.

Whenever people we meet start a conversation about sensors the big 'wireless' topic almost inevitably comes up... What is best, a wireless sensor, or a wired one? Guess what: no clear answer! As with many things in life: it all depends. In general the big advantage of wireless sensors is that they're fast to deploy and don't require meters of cable being pulled, an often labour-intensive process. On the other hand wired sensors have the upper hand when a high data density is required or time syncing is essential. Looking at both options in more detail:

Wireless sensors:

• In an industrial setting the biggest advantage of wireless sensors is that they're easy to install. This advantage is mainly related to the fact that one doesn't neet to pull cables between individual sensors and a data hub or acquisition system. Pulling of cables comes with 2 cost factors: the mere cost of the multiple meters of copper, but also the cost of the associated working hours for the people doing the actual works.
• Wireless sensors are available for a number of years. In the early days the majority of these sensors had network-related issues: sensors couldn't find the base station or stopped transmitting data after a period of perfect functioning: interference problems. Also 'poor network' often lead to rapidly depleting batteries. It would be unfair to say that these problems are gone, but today there's a series of reliable solutions available on the parket that do not suffer any more from these issues. A small sidenote: not all wireless sensor suppliers offer every type of sensor of course, so not every type of data one could need today can be supplied using reliable wireless solutions.
• When network issues are encountered, this leads to rapidly depleting batteries. Als when lots of dat needs to be transferred or data needs to be transferred frequently the battery charge of a sensor may go down quickly. In these cases battery replacements may be required. In the best case this is associated to the cost of buying new batteries and the man-hours to perform the replacements. In the worst case the batteries used are proprietary to the sensor supplier, resulting in high costs or a complicated (thus often expensive) procedure to be followed.
• In general wireless sensors aren't very 'context aware'. In practice this means that, some exceptions aside, wireless sensors available on the market today don't know about the parameters or data measured by other sensors installed on the same component or a neighbouring component. As such they're only able to take 'decisions' based on what they measure themselves. From experience we know that deciding if a situation is 'bad' or 'good' often depends on context. In this respect wireless sensors in general are still a bit behind.
• A common trait of wireless sensors is that they don't transmit all the data they record all of the time. Data transmission is the most energy-intensive function a wireless sensor performs. Therefore most often one of 2 strategies is followed: or the sensor transmits a data package at regular, discrete, time intervals or transmission of data takes place once a pre-defined condition is met. Combinations of both strategies are encountered as well, but in general one has to select on beforehand the data transmission strategy of a wireless sensor.
• Properties of data transmission and data acquisition are important parameters to be considered when applying a wireless sensor. Most high-quality suppliers these days however allow optimising or changing these settings from remote as well, either directly on the sensor, either through a gateway or hub. As such the data stream can be optimised should in-use conditions learn the user that conditions are not as expected or that the choice initially made wasn't the optimal one.
• An important aspect to take into account is that the term 'wireless' covers a large group of technologies, all of them with their own implications and limitations. One can encounter wireless transmission based on Wifi, LoRa, Sigfox, nbIOT, mesh networks, 3G/4G/5G... On a regular basis even new technologies appear. When using multiple sensors to monitor a certain installation the compatibility between the wireless principle the sensors rely on is a relevant factor to take into account as well.

Wired:

• The fact one can continuously acquire data at high data rates is one of the main advantages of wired sensors. As energy is most often supplied by the same wires of leads through which the data is transferred, there's virtually no limitations in amounts of data being transferred, nor is the sample rate impacting the lifetime.
• A second advantage of wired sensors is the ability to record data simultaneously on multiple positions on a single asset or on multiple components in the same installation. Some of the wireless sensors on the market have a sort of time-syncing between sensors offered, but true simultaneous acquisition, essential for some situations, isn't available today in a robust way. Simultaneous acquisition is relevant to see how pressures, vibrations or shocks or electrical signals propagate through an installation or to perform cause-effect monitoring.
• As wired sensors do not rely on batteries they can operate for very long times without requiring battery replacement. Continuous acquisition for 20 years in a row isn't impossible.
• As, compared to their wireless companions, wired sensors already exist for many more years, the number of options with a 'proven' track record or lifetime is much higher. This isn't a result of wireless concepts being inferior, but purely related to the difference in age between both technologies. For a certain group of applications that proven reliability is however a relevant factor in the overall equation, providing an additional competitive edge for wired sensor types.
• In general one can say that the 'digitalisation' of the measured signal in case of a wired sensor is taking place in the acquisition device, where for wireless sensors this operation is performed in the sensor itself. Overall 'high quality' (high resolution in amplitude and time) digitalisation is a costly operation. Therefore one will encounter a higher number of 'high resolution' options when considering wired sensor types as opposed to the wireless counterparts. If a phenomenon or quantity is measured with a higher resolution this often translates in a more sensitive follow-up: deviations can be observed earlier on and/or more advanced correlations can be detected, leading to a higher quality for the overall follow-up.
• As discussed wireless sensors suffer from a number of inherent drawbacks: non-continuous data transmission and sometimes a lower resolution. Added to that it is seen that not all measurement principles are suited for use in wireless sensors. If for example a certain measurement requires a filament to be heated, a wireless sensor based on tis measuring principle will not have a long battery life as heating actions consume high amounts of energy. The same holds for sensing principles where an electrical transfer function is to be determined, requiring relevant amounts of power per measurement as well. This combined with the difference in age between wired vs. wireless sensors in general results in a situation where there's simply a lot more options available in the segment of wired sensors as opposed to that of wireless sensors. For some cases or measurement types a wireless options isn't available (yet).
• Not all positive news however. As wireless sensors may suffer from issues with the network, also wired sensors have a vulnerability w.r.t. data transfer. As often 'industrial' environments are monitored, this means there's actual physical activity going on. In a manufacturing setting this means people step on things or items get dropped. Tis may lead to physical damage to cables or leads. On itself nothing dramatic, but these situations require new cables being pulled or local repairs to be conducted, operations that come with a price tag as well.
• Issues with data transfer illustrate wel that in an advanced monitoring solution sensors cannot represent the full solution. In an advanced monitoring approach one should also monitor the data flow itself, regardless if it's supplied by wired or wireless sensors. Is the data coming in? If it's coming in, does it look like normal data coming from that type of sensor? All important aspects to be taken into account.
• Also wired sensors can be based on different (analog) measurement principles. One can encounter, amongst others, 4-20 mA, 0-10 V, IPC, Wheathstone bridge, Pt100 and many other types of measuring principles. Again measuring devices exist for all types of signals, but one needs to be aware of their existence and between the need for compatibility between acquisition hardware and the signal type.

At Zensor we have no preference at all regarding the overall wireless vs. wired question: Depending on the situation that presents itself we will use either of both types. We focus at what is required: why is this installation monitored? What types of situations do we want to predict? What sort of behaviour do we want to detect in an early stage? How early? What are other constraints one needs to take into account? Based on all these aspects the most optimal choice is made.

A few final takeaways:

• Some of the wireless sensors come with a bad name, but there's ample suppliers of reliable, high-quality systems we're happy to use. For every 'bad' example for a supplier of wireless sensors one can come up with, there's at least as many companies selling wired sensors that actually supply unreliable or unstable hardware.
• It is important to be aware that the above analysis is based on our experience to date. Of course technology evolves continuously: new solutions appear regularly, but also requirements evolve based on experience. As such, a few months from now the comparison between both types of data transmission can differ again. A few years from now the entire question may even become entirely obsolete, when wireless and wired sensors offer exactly the same value...
• In short: none of both options can be considered as being 'the best', one only can be very aware of what she or he needs and make sure to match the requirements with the most suited technological solution available.