Designing Sensors for Harsh Environments

Tom Kevan

As sensors rise to prominence with the growing footprint of the Internet of Things (IoT), a close look at the sensor nodes populating the digital landscape reveals boards and chips packed with supporting computing, data storage and communications units. Couple this digital overpopulation with growing deployments of nodes in harsh environments, and you begin to appreciate the scale of the challenges facing design engineers.

Key among these challenges: temperature extremes and electromagnetic interference, which can corrupt data and even trigger system failure. The good news is that designers can counter these factors. Following are some best practices.

Taking the Heat

Harsh environments run the gamut of temperature extremes, from sweltering heat to sub-zero temperatures. Both conditions pose serious threats to a design’s efficacy. Low temperatures can diminish the efficiency of electronic components, reducing system reliability by changing values or creating timing errors. High temperatures, on the other hand, can trigger various issues.

One such condition, called “thermal runaway,” comes into play when temperature spikes cause the semiconductor to release energy. This increases system temperature and can ultimately lead to system failure. Engineers can prevent thermal runaway by incorporating current-limiting protection. Here, the designer has several options, which include using thermal fuses, circuit breakers or positive temperature coefficient current limiters.

Problems associated with thermal cycles also include issues arising from differences in thermal-expansion coefficients. A variety of materials make up electronic circuitry, the substrate on which it’s built and the encapsulation materials connecting the various components. Each material has its own thermal-expansion coefficient. Temperature changes cause the materials to expand and contract at different rates, compromising system interconnections. Compounding the situation, problematic interactions also occur between different materials at high temperatures.

Engineers can avoid these problems by choosing components certified for high-temperature operations and by matching thermal-expansion coefficients of materials in their designs.

Minimizing the Effects of EMI

Another factor to consider when designing devices and systems for harsh environments is electromagnetic interference (EMI). This phenomenon is produced by an external source—such as power switching circuits and RF devices—that affects electrical circuits via electromagnetic induction, conduction or electrostatic coupling. EMI can be conducted physically or radiated via the air.

Designers can mitigate the effects of EMI by adopting a few well-established practices. These include grounding all electronic equipment and shielding electronics and cables with conductive or magnetic materials to prevent incoming or outgoing electromagnetic frequency (EMF) emissions. An example of this can be seen in smartphones, where a metallic shield protects electronics from emissions from its cellular transmitter/receiver.

EMI abounds in application areas like industrial environments, far outpacing its presence in consumer and in-home electronics. One reason for this is that industrial applications tend to use more electrical devices with higher voltages and currents.

Another tool in the designer’s arsenal are EMI filters. These passive devices suppress conducted interference found on a signal or power lines. Most systems and devices contain EMI filters, either as separate units or embedded systems. These components include line filters, capacitors and inductors.

Aside from these specific solutions, engineers should follow a few general rules of thumb:

• make establishing EMI protection an upfront process, where you consider what measures to use when designing the circuit;
• position any section that can be exposed to EMI as far as possible from sensitive circuitry; and
• block interference as close to the source as possible.

Further Complications

Although effective techniques have been developed to address temperature extremes, EMI and other harsh environment challenges, the components required to implement these strategies are sometimes unavailable. The sad fact is that the market has a limited pool of products. As the IoT takes shape and harsh-environment applications become more prevalent, increased demand may cause vendors to remedy this situation.