When I think about the term Size, Weight, Power, and Cost, or SWaP-C, what usually comes to mind is aerospace and defense: reducing the weight of avionics subsystems to improve flight endurance; minimizing power consumption to eliminate extra batteries carried by ground forces; removing nonessential components to meet tight budgetary constraints. But isn’t decreasing size, power, and cost a primer for most embedded applications?

Take electromagnetic sensors for the Internet of Things (IoT), for example. Many sensors are deployed in ultra compact or mobile applications, making them extremely size and weight sensitive. Projections that the number of Internet-connected devices could reach 75 billion by 2020 also have huge implications on IoT power consumption, which along with trends toward “green computing” demand that individual components be as energy efficient as possible[1]. Similarly, the quantity of deployed sensors combined with commodity inflation in the rare earth magnetic materials market will keep unit price an ongoing concern[2].

Available sensor technologies

To date, many IoT-type applications have employed one of three electromagnetic sensing technologies to meet the aforementioned SWaP-C requirements: reed switches, Hall effect sensors, and Magneto Resistive (MR) sensors. And, while each of the three offers benefits, all three also have limitations:

• Reed switches have been a popular design choice over the years thanks to extremely low power draw and design flexibility for systems that require large air gaps. However, glass components and moving parts also introduce reliability and durability issues in harsh-environment applications (which for the purposes of this column I will consider a “Cost”).
• Hall effect sensors provide a more rugged alternative to reed relays, as they have no moving parts and are less sensitive to environmental conditions such as vibration, dust, and humidity. On the other hand, Hall effect technology is limited to sensing in perpendicular planes and often requires more magnetic material to achieve the same sensitivities as alternative technologies, which increases cost. They also typically carry a power overhead in the low microamp-range (µA-range) that is too high for many applications.
• Similar to Hall effect technology, MR sensors are solid state, making them well suited for rugged operation. They sense in parallel planes and have a wider detection area than Hall effect sensors, which offers increased sensitivity and greater design flexibility. Conversely, MR sensors have traditionally come at price and power premiums.

As you will note, selecting one sensing technology over another has been a game of tradeoffs wherein a designer will sacrifice power, performance, or price based on the application requirements. At other times, the power envelope of a given application – such as smart meters with battery lifecycles of 7-10 years that require sensor power draws under 500 nanoamps (nA) – make design decisions simpler. But advances in MR technology could be shifting these design rules.

Honeywell Sensing and Control recently released the industry’s first nanopower Anisotropic Magneto Resistive (AMR) sensor ICs with current draws as low as 310 nA (Figure 1). This is a significant development in sensor technology for a number of reasons, but principally because the durable AMR sensors can be leveraged in battery-powered applications at a price, power, and performance comparable to that of reed relays. In addition, the higher sensitivity of the omnipolar AMR sensors provides design flexibility over Hall effect sensors that reduce system cost.

SWaP-Cing the future

As mobile and connectivity requirements continue to “shrink” all aspects of system design, SWaP-C is becoming increasingly important across industries. As explained here, it is important to consider all aspects of SWaP-C in relation to each other and the application, as the smaller we get with applications like IoT sensors, modifying one aspect of a product can turn how you interpret SWaP-C entirely on its head.

1. Danova, T. “Morgan Stanley: 75 Billion Devices Will Be Connected To The Internet Of Things By 2020.” Business Insider, Inc.

2. Constantinides, S. “The Demand for Rare Earth Materials in Permanent Magnets.” Arnold Magnetic Technologies.