Usually, when you need to sense something in a project, the answers are straightforward. Want to sense air temperature? There’s a sensor for that. Particulate content in the air? There’s a sensor for that, too. Someone sneaking up on you? Get yourself some passive infrared sensors (PIRs) and maybe a smart camera just to be sure.

But sometimes you can be sneaky instead, saving the cost of a sensor by using alternative techniques. Perhaps there’s a way to use the hardware you already have to determine what you need. Maybe you can use statistical methods to calculate the quantity you’re looking for from other measurements.

Today, we’ll examine a great example of a “pseudo-sensor” build in an existing commercial device, and examine how these techniques are often put to good use in industry.

Case Study

When they were introduced in 2009, Coca-Cola Freestyle dispensers were a step change in the way soft drinks were dispensed. Suddenly, you weren’t limited to five or six choices on the soda fountain. You could instead sample virtually the entire Coca Cola range, all on one machine! If you’re a big soda head, this was a very rad thing. If you were a maintenance tech for Coca Cola, though, you probably saw the machine differently — not as some godly fount of soda, but as a machine to be troubleshooted, repaired, and improved. Over time, it became obvious that the Freestyle unit had a high rate of Flow Control Module (FCM) replacements in the field. And yet, 50% of the FCMs returned to Coca Cola weren’t faulty. There was something strange going on.

The problem, as revealed in a presentation from the company, was that the Freestyle machine didn’t have a dedicated pressure sensor in the fluid line. If a machine had an FCM fault or a pressure loss, it would present much the same way. Thus, techs would often swap out a perfectly good FCM when the problem was actually elsewhere. The solution was obvious: there needed to be a way to sense pressure in the system, so techs could determine if an FCM was faulty or if the problem was a lack of pressure upstream.

To address this, an engineer might have specified an off-the-shelf pressure sensor, figured out how to retrofit it to the machine, and rolled them out in the wild. Instead, Coca-Cola developed an innovative (and presumably cheaper) solution: a  pressure pseudo-sensor, largely using equipment already on the machine.

The pseudo pressure sensor operates by analyzing the relationship between electrical and mechanical work within the FCM. Basically, the FCM is a valve that opens to allow the flow of fluid through the machine. Thus, the pseudo-sensor monitors the current at which the valve starts to move, a value that correlates with the pressure inside the system. As pressure increases, a characteristic V-shaped drop in current is observed; this pattern shifts as pressure changes, allowing the system to estimate the pressure based on the observed current.

To create the pseudo-sensor, a whole lot of data was collected from the Freestyle hardware. Over 5,000 drink pours were performed with a number of FCM modules, at pressures from 1 to 140 pounds per square inch (PSI) at 5 PSI intervals. The data collected during testing was then fed into MATLAB and Simulink in order to create a mathematical model. The aim was to link the peak size of the current feedback voltage dip measured by the current sensor, and link that to pressure. Sadly, a good reliable correlation was hard to come by.

More work ensued, which tied pressure to multiple timing and voltage features on the curve. These were fed into a multi-variable regression that spat out a monstrous model that calculated pressure from six features and 26 terms. It was messy, but far more accurate, and it did the job.

From there, it was a simple matter of deploying the model that measured FCM current and spat our pressure measurements. It was loaded on an ARM Cortex M microcontroller and put through 3,300 tests over 10 different FCMs and two different Freestyle controller boards. The model predicted the correct pressure within a bound of +/- 10 PSI a full 85% of the time.

Admittedly, that would be rubbish for a proper pressure sensor. However, for a simple pseudo-sensor that’s mostly just used to see if there’s pressure in the system? It’s pretty darn good. The pseudo-sensor software has since been deployed on Freestyle machines in the field, with work ongoing to further develop the system’s diagnostics using this new tool.

Other Examples

The simple fact is that you can often get by with indirect measurement techniques if you’re constrained by things like cost, complexity, or practicality. We’ve seen other work along these very lines before. Back in 2022, we covered the work of Brian Wyld, who wanted to measure the level of a body of water. Pressure and direct surface-level sensors were impractical, so he got creative. He built a rotating arm with a float on one side, and threw on a microcontroller board with an accelerometer included. The accelerometer readings were enough to allow him to figure out the angle of the float, and in turn, mathematically derive the water level as desired via simple geometry!

We’ve also seen how this can go wrong. For example, capacitive sensors are often suggested for measuring soil moisture levels. The idea is that by measuring the capacitance of the soil, you can measure how much water content there is. The only problem is that moisture isn’t the only thing that changes the capacitance of the soil.

For these indirect techniques to work well, what you’re measuring needs to have a fairly direct correlation with what you’re trying to find out. Hence why Wyld’s float was a success — because the float angle is directly relevant to the water level. Similarly, in Coca-Cola’s case, pressure was what determined the change in the current curve of the Freestyle FCM. If the curve also changed significantly with ambient temperature or some other factor, it wouldn’t be possible to measure it and get out a reliable pressure value.

Ultimately, pseudo-sensors can be a useful tool to have in your engineering toolkit. They can let you achieve surprising feats with some mathematical insight and basic equipment. Just make sure there’s a strong basis for what you’re doing so you don’t end up with junk outputs that cause you more harm than good.

Magnetic levitation has not quite revolutionized the world of transit the way some of us might have hoped. It has, however, proven useful to [mrdiytechmagic], who has put the technology to grand use in making his levitating snail lamp.

The build is actually relatively complicated compared to some levitating toys you might have seen before. It uses a number of coils to produce a magnetic field to levitate the 3D printed plastic snail which contains the lighting element itself.

The actively controlled levitation base uses a magnetic sensor to detect the changing field as the snail moves above it. It then varies the current going to the various coils to keep the snail balanced and in place. Power is transmitted with a further larger coil, much as in a wireless phone charger. This is picked up by a circuit in the snail, and used to power the LEDs inside.

It might not have been our first choice, but having seen it in action, we can’t deny a levitating 3D printed snail is pretty impressive. If you’d prefer something slightly more befitting such a high-tech looking presentation, perhaps a hovering SpaceX Starship would be more your speed.

Monorails aren’t just the core reason why The Simpsons remains on air after thirty-six seasons, twenty-six of which are unredeemable garbage. They’re also an interesting example of oddball rail travel which has never really caught on beyond the odd gadgetbahn project here and there. [Hyperspace Pirate] recently decided to investigate the most interesting kind of monorail of all—the gyro stabilized type—on a small scale for our viewing pleasure.

The idea of a gyro-stabilized monorail is to use active stability systems to allow a train to balance on a single very thin rail. The benefits of this are questionable; one ends up with an incredibly expensive and complex rail vehicle that must always run perfectly or else it will tip over. However, it is charming to watch in action.

[Hyperspace Pirate] explains how the monorail vehicle uses control moment gyroscopes to keep itself upright. The video also explains the more common concept of reaction wheels so the two systems can be contrasted and compared. It all culminates in a wonderful practical demonstration with a small 3D printed version of a 20th-century gyro monorail running on a 24″ track.

If you’re studying mechanical engineering this is a great project to pore over to see theoretical principles put into obvious practice. Video after the break.

The loom has been a transformative invention throughout history, shaping the textile industry from simple hand looms to complex, fully automated machines. Now, thanks to advancements in 3D printing, this age-old craft is being revitalized by modern makers. One such creator, [Fraens], has recently designed a unique 3D-printed table loom with eight shafts, offering a simpler yet innovative approach to weaving. This project is a fresh take on traditional looms, blending centuries of design knowledge with contemporary technology.

[Fraens], a longtime enthusiast of looms, has spent considerable time studying the countless designs that have evolved over more than 200 years. Drawing inspiration from these, he has crafted a more accessible version—a table loom that can be operated using levers to control the warp threads. Unlike larger, more complex looms, this 3D-printed model allows users to experiment with various weaving patterns easily, using different colors and sequences to create beautiful, intricate designs. [Fraens] provides guidance on how to adapt patterns meant for larger looms to this compact, lever-operated version on his website and in a detailed video tutorial.

This project is perfect for anyone interested in weaving or DIY technology. [Fraens]’ 3D-printed loom offers a new way to explore textile creation, making it both approachable and rewarding. To see this innovative loom in action and learn how to build your own, check out the video below.