Author: Caoimhe

Designing a sensor for wireless condition monitoring is a complex balancing act between meeting performance requirements whilst adding the most value for the end user. The choice of technology becomes very critical when trying to optimise certain aspects of the design. Form over function, or vice versa is just one of the design directions that needs to be established very early in development. Thus, choosing an appropriate power source is one of the earliest decisions that needs to be made.  

Here, we will focus on the implementation of the battery. Batteries are available in all shapes and sizes, different chemistries, non-chargeable, re-chargeable, etc. However, the form factor of the power source will largely dictate the overall dimensions of the sensor. Many wireless sensors coming to market have very large batteries in order to meet the longevity requirements that are needed by customers to trend their equipment performance over many maintenance cycles. If a more power-hungry technology is used to perform measurements and serve the wireless communications, then the battery consumption will be substantial. However, a larger battery can have many knock-on effects. A larger battery adds additional weight and influences the centre of gravity, which most likely increases the size of the magnet or mounting bracket needed to fasten or secure the sensor. It is difficult to describe how important and impactful this component selection is on the outcome of the product design.

Vibration analysts are calling out for smaller, compact sensors which means a tight balancing act between battery size and battery life. Most customers and product users are happy to concede a replaceable battery design, as a way of lengthening product service. However, this can often make ingress protection more challenging, as the design must allow easy access to change the battery but still maintain a high level of robustness. Having a small, lightweight, low centre of gravity sensor with a compressed mounting footprint, will allow the sensor to fit into tighter and more “ideal” surface mounting locations. This is very important when measuring vibration and temperature close to the source of the vibration.

Kappa X uses a ½ AA battery, which is one the smallest power cells on the market, yet it boasts a 4 year battery life (standard configuration). The battery cell is also replaceable, which often makes ingress protection more challenging, however, Kappa X is fully waterproof (validated IP69K). The in-house knowledge and expertise of true low-power electronics at Sensoteq enables Kappa X to have a small 25mm diameter mounting footprint and overall compact size. Having this small, lightweight, low centre of gravity sensor with a compressed mounting footprint, allows for Kappa X to fit into tighter and more “ideal” surface mounting locations. This is very important when measuring vibration and temperature in close proximity to the source of the vibration. To summarise, a high performing sensor, that you cannot mount correctly, has diminishing benefits in terms of machine health analysis. Ultimately, measurement performance and determination of early failure are very closely tied when it comes to adding the most value for the end user.

Kappa X makes no compromises when it comes to the designed, developed and implemented technologies. It uses a unique communications protocol capable of providing a reliable transmission over long-range in complex and challenging industrial environments. The sensing technology within Kappa X has been researched in collaboration with industry experts to deliver a market leading product.

By David Procter

In the world of vibration analysis frequency is everything.  Frequencies are generated by many sources within a machine. The running speed of the machine is one example frequency; this is the velocity of the rotating element which in many cases is a drive shaft connected to an application (like a fan or a pump). Each frequency will have a magnitude associated with it; this is the total amount of energy of that specific frequency.  The amount of energy of any given frequency can tell us a lot about what is going on within a machine.  When we use a vibration sensor to measure a machine, this is what we are detecting, frequencies and magnitudes of those frequencies.

The maximum frequency that a sensor can measure is called the Fmax.  This value lets the user know what type of faults they can detect on a machine. It is typically listed in hertz (Hz).  The higher the Fmax value, the greater number of faults that a sensor will detect, but it will also allow for earlier indication of potential faults, like a bearing failure. Some ISO standards will reference an Fmax of 1kHz – whilst taking a reading up to 1kHz is suitable for most acceptance testing, it will not highlight even the most basic of bearing faults.  A minimum of 2.5kHz Fmax is recommended for good coverage.

Recently, a new sensor has entered the market that boasts an Fmax up to 10kHz. This Fmax gives great coverage for a wide variety of faults, and will accurately inform users of issues on complex equipment like gearboxes that have many higher fault frequencies. In addition, a higher frequency will give earlier indication of bearing faults. Bearing failures will typically start within the subsurface of the metal with a very small amount of energy being emitted to the sensing element, thus having a sensitive measurement device with the right Fmax is critical to understanding the health of your machine early, prior to failure impacting any processes or causing downtime.

By David Procter

Let’s discuss for a moment; how can we design a decent, standalone sensor which can be put on the vibrating equipment, stay there and report reliably the measurements.

Let’s look at very frequent set of requirements:

• It is going to withstand (and measure) potentially very high g forces in excess of 50g
• It is going to withstand (and report) temperature over 60°C (140°F)
• The place is hardly accessible (not good for frequent human inspection)

Additionally, we would like the sensor to measure very precisely the equipment RPM (may be very slow rotating equipment) but at the same time detect very high frequency events around 10kHz (hitting the ‘hard-spots’). This is all becoming possible nowadays in the battery powered equipment but there are some limitations in the way. We can understand that there is practical limit for handling the sensor which does not have to be neither very big nor too small. There is obvious realisation that a lightweight sensor is easy to attach, and it is more likely that a smaller object can fit easier within the space provided.

Another point is not so obvious:

The smaller is the sensor and any of its electrical components, the more likely it can withstand very high g forces.

The tiny battery of small sensor dictates focus on low power and efficient RF link.

The lower is the frequency of operation, the greater is the RF link over the distance. Sub GHz ISM (Industrial, Scientific and Medical) bands are available to use for such applications E.g. 868MHz, 433MHz, 315MHz. These frequencies have wavelengths of 34.5cm, 70cm & 95cm and are close to the frequency range already used for some of the M2M communications and Smart Metering (‘450 MHz band’). Availability of any lower frequency would be nice but not offering advantage for the tiny sensors, as there is practical limit of the electrically shortened antenna efficiency.

As for vibration sensors, any use of the modem in licensed ‘450 MHz band’ is currently prohibitive, not just because of the equipment cost but in large part due to higher energy usage in data exchange. It is obvious that reporting of the vibration sensor every several minutes is much more frequent than Smart Meter (every several hours) and vibration sensor needs to remain very energy efficient. More importantly, the modems require high current pulses, which in case of small sensor battery must be buffered with massive capacitors, and these are not designed for high g forces experienced by the vibration sensors.

These details are subject to change and most likely in the future we would be able to use tailored 5G/6G solutions more suitable for the sensor needs. As for now, with current mobile networks deployments, there is much focus on bandwidth and GHz frequency bands suitable for the mobile entertainment. Human centred wireless does look for the highest possible data capacity and lowest bi-directional link latency in human-to-human or human-to-equipment interactions. Ideally vibration sensors are required to report the readings reliably but with the lowest energy per data used.

This indicates the main reason for the bespoke solutions like using the unlicensed ISM bands.

In ‘450MHz band’, the achievable distance is doubling that of 900MHz (frequency reduced by factor of two offers significant 6dB advantage). This allows maintaining a more reliable connection or reducing number of gateways by factor of 2 – 4 when aiming to achieve similar area coverage. The advantage of lower frequency band is also clearly visible when comparing very strong attenuation of higher frequency signals passing through thick structures like the concrete walls.

Graph showing the Decibel attenuation level across Frequency (from Phillips Laboratories “Measurement of RF Propagation into Concrete Structures…”)

On the small sensor’s side, one thing is the most likely to remain the same in the future: staying with low frequency bands for the sensor’s communications, in order to save the battery life and being able to maximise link budget with low transmit power of the sensor (or to minimise number of dedicated gateways to receive the sensors data in the area covered), whilst offering a robust communications link in an industrial environment.

By Thomasz Kawala

There are many things to consider when designing any kind of sensing product, but the main considerations are generally to do with measuring a clean, precise, and isolated signal. Many techniques are employed to minimise noise, filtering, amplification, etc, to allow the different kinds of sensing element signals to be processed. However, if you are trying to measure vibration specifically, and you design an assembled structure that contains various levels of self-resonance, then some of your electronic efforts to measure a “clean” signal may be compromised.

Any object with a mass will have a primary excitation frequency that will cause a resonating response. This will occur close to the natural component frequencies with potentially multiple orders and with additional harmonics at higher frequencies. When you assemble various components together, each component will behave both individually, and also, as part of a system. The most dramatic example of self- resonance causing catastrophic failure is arguably the well documented Tacoma Narrows Bridge collapse, where excessive oscillating deflections were introduced by 40-mile-per-hour winds exciting the suspended structure. When designing a vibration sensor, the effect of self-resonance may be considered less dramatic. However, it can mean that misleading peaks at certain frequencies in the measured vibration data plots, will be visible to the end user.

Ideally, if you can perform a frequency sweep, and then measure the frequency responses where high amplitude measurements occur, then it can be determined if there are any self-resonances within the actual measurement range of the sensor. Simulations can be carried out using modal frequency analysis software. This software is becoming more readily accessible and is provided by many of the main 3D CAD design packages as either a free or upgrade add-on. Be careful with simplifications that you make to your CAD geometry and the material properties provided or other inputs for the simulation, including dynamic parameters that may influence the results.

There are many 1.0 to 2.5kHz sensors on the market that may be manipulated to push those modal frequencies above the measurement range of the sensor. Just like mounting an engine on spring mounts with addition or reduction of mass, make sure you utilise techniques to change the frequency response of your system. Increasing stiffness, adding relieving cut-outs, changing the bracing/fastening, adding isolating features, encapsulation, understanding the individual and overall centre of gravities, changing or adding materials, etc, are just some of the things that you can try to learn more about the system responses.

The market now has more affordable 5.0 to 10.0kHz plus, sensors being developed. As a result, it might be almost impossible to try to shift all individual modal frequencies above the now wider measurement range, through design. With that in mind, the more important considerations will need to be more closely tied to the application, and less about eliminating them completely. Understanding the frequencies generated by the different types of equipment you are planning to monitor, will help you to decide which design manipulations you need to adopt to avoid self-induced noise.

By Gordon Maguire