How Digital MEMS Sensor Technology Revolutionizes What Can Go Where

One underappreciated benefit of MEMS technology is the ability it gives designers to partition systems in new ways to create much more flexible end products. In this article, we take a look at a common application for vacuum sensors — the portable vacuum gauge — to illustrate how digital MEMS sensor technology can help to create a much more capable product.

Analog vs. Digital

The traditional thermistor-based Pirani vacuum sensor is analog in nature. It consists of a thermistor which is connected to the system whose vacuum is to be measured. The raw voltage output from a thermistor-based vacuum sensor must be connected to a sophisticated electrical circuit for amplification, digitization, temperature compensation, and calibration. 

The digital alternative to thermistor-based vacuum sensors is based on a MEMS Pirani chip-level sensor that combines a micro-heater, a thermopile (i.e. thermocouples in series to greatly enhance sensitivity) and a measurement cavity into an area of silicon measuring 0.8 mm x 2 mm before packaging. This chip (Figure 1a) becomes in turn the sensing element of a MEMS Pirani vacuum transducer (Figure 1b) that integrates microcontroller-based measuring electronics and packs it all into a compact PCB assembly with a connector-terminated wire harness (Figure 1c). This assembly can in turn be packaged into a metal fitting that connects easily to the environment where vacuum needs to be measured (Figure 1d). The resulting micro-Pirani sensors offer better accuracy, repeatability, range, and power efficiency than thermistor-based Pirani sensors, while allowing designers to miniaturize the vacuum measurement application to a greater degree than ever before.

Click image to enlarge

Figure 1b. Pirani vacuum transducer PCB assembly

Click image to enlarge

Figure 1c. transducer + wire harness

Click image to enlarge

Figure 1d. transducer + wire harness packaged in metal fitting

A Choice of Integration Strategies 

Digital MEMS micro-Pirani vacuum sensors greatly simplify the transition away from analog gauges by offering a solution that handles amplification, digitization, and temperature compensation straight out of the box. And there are several other benefits. But why would designers want to transition away from analog in the first place?

First, traditional analog thermistor-based Pirani gauges have very high operating temperatures, which tends to shorten their lifetimes. Operating temperatures for digital micro-Pirani vacuum gauges are much lower, so their time to wear out is much longer.

Second, for repeatability, nothing beats MEMS technology, which originates in a state-of-the-art semiconductor CMOS fab. This means that digital micro-Pirani sensors offer a much higher level of interchangeability, which is one aspect of how they reduce maintenance costs.

Third, this digital technology is able to achieve the level of accuracy needed for handheld vacuum gauge applications at a much lower cost than thermistor-based sensors.

Fourth, because the MEMS sensor elements are so small, there is much more freedom and flexibility in the possibilities for end system design. The MEMS solution basically rewrites the rules about what can go where, what part of an end product can be easily maintained by an actual user, and what are the consequences of swapping out replacement parts.

Revolutionizing What Can Go Where

To understand what we’re talking about, please have a look at the two portable vacuum gauges below. On the left-hand side (Figure 2a) is a traditional analog-based vacuum gauge; on the right side is a prototype of a vacuum gauge using Posifa Technologies’ digital MEMS sensor (Figure 2b). 

Click image to enlarge

Figure 2a. analog-based vacuum gauge with permanently attached probe

Click image to enlarge

Figure 2b. digital MEMS-based vacuum gauge with replaceable probe

Both fit easily in the hand. The main difference between them is that the probe on the analog-based gauge is by necessity permanently attached to the user interface, i.e. the reader. It is also essentially dumb. It is simply a device that provides physical signal that corresponds to the vacuum level; usually it’s an uncalibrated, unamplified, undigitized voltage. The manufacturer may have done some trimming to improve uniformity, but the output is rather crude. Even if it were detachable and replaceable, moreover, you would be left with the problem of how to calibrate it. 

The micro-Pirani MEMS approach is quite different. Because the digital sensing element is so small and inexpensive to produce, it’s possible to co-locate it in the probe with the signal conditioning circuits and the digital microcontroller unit. In other words, the manufacturer can relatively inexpensively put a complete transducer into the probe, rather than just the sensing element. 

Bringing these “smart” elements of a vacuum gauge right into the probe has a number of implications. For one thing the process of swapping out a contaminated or damaged probe starts to make sense since there is no need to recalibrate the sensing element. It’s possible to guarantee interchangeability and the same accuracy from one sensing element to another since they’re calibrate digitally as part of the manufacturing process and the calibration data is stored in the on-board microcontroller. 

In the real world where these gauges are used, being able to easily swap out a contaminated or damaged probe can greatly reduce down-times for technicians. The benefits include but are not limited to improved brand loyalty, since now it’s significantly easier to repair a damaged gauge than to simply replace the gauge, and possibly with a competitor’s product. The reusable part of the gauge can then focus on optimization for the user interface (such as display and buttons), transmission, and IoT features.

It is possible to imagine a “smart probe” in a Pirani sensor based on analog technology, but the implementation would be impossibly expensive given the requirement for more complicated signal conditioning componentry, which would also likely be too big to fit into the probe’s normal form factor. It’s one of those things that would already exist if it were practical for it to exist. 

The benefits of being able to repartition vacuum measuring components are by no means limited to portable vacuum gauges. Take, for example, the vacuum sensors used in various industrial processes. Rather than being built on thermistor technology, these analog vacuum sensors consist of a metal wire (usually a gold plated tungsten or platinum filament) suspended in a tube which is connected to the system whose vacuum is to be measured. The raw voltage output from a filament-based vacuum sensor probe must be connected to a sophisticated electrical circuit for amplification, digitization, and calibration.

Now, if that probe is damaged or contaminated, the user needs to take it off and send it to a third party for service and recalibration. There’s no possibility of instant interchangeability with a backup probe. Moreover, this scenario is liable to happen more often with filament-based sensor solutions because the high temperatures at which they operate make them prone to contamination. But by replacing analog filament technology with digital MEMS technology, required maintenance of the vacuum sensor becomes less frequent, and when it is necessary, it happens much more quickly, literally plug-and-play, because there’s no need for a third party to recalibrate the gauge.

The same digital MEMS technology that enables replaceable vacuum sensor probes has the potential to transform many sensor applications across a wide range of markets since it can be used not only to measure vacuums but also air flow, liquid flow, and air velocity, and gas leaks. The point is not to think merely in terms of miniaturization of the sensing intelligence but also its placement and location within the application.

Posifa Technologies