Sensor Selection: Making Sense of it All

By Jim Romeo

They can help record a toll fare when we drive over a bridge or roadway without ever slowing down; provide remote indications of system or equipment about to fail on a pipeline or power station thousands of miles away; and are in our cell phones, garage doors, automobiles and security systems. Sensors are an integral part of nearly every design nowadays—and the quest to select ones that provide the most value is always a challenge.

Different technologies are used in sensors. This chart shows where they are in their lifecycle (mature or young), and what the expectations are of their growth. Chart courtesy of Frost & Sullivan.

“Sensors are used in the most mundane to the most critical functions,” says Sudhir Sharma, electronics industry director at ANSYS, Burlington, MA. “Modern cars, for example, now include blind-spot monitors, which could be considered a safety critical feature.”

The best way to ensure robust field performance is to do regression simulations of models of the sensors in their operating environment, he notes. The simulations need to test for corner conditions, including variability in temperature and voltage.

Applications and Parameters

Sensor selection begins with an understanding of the sensor applications, and some basic parameters and requirements of the equipment and systems in which it will be integrated.

According to Rajender Thusu, a sensors expert and an analyst for Frost & Sullivan in Mountain View, CA, basic sensor selection is very simple: It should include a careful consideration of the type of parameter to be sensed, the level of sensitivity required, and the physical size of the sensor itself.

“As the sub-systems and systems in which the instrument/device has to work gets more complicated, the demands on sensor selection start becoming more stringent,” he says. “In a wired system, connectivity is another prime consideration. As the level of automation increases, the use of field bus protocols fitted onto the sensor package becomes another criterion, which further raises the issue of compatibility.”

Many of these protocols are open and the rest are proprietary, Thusu points out: “Hence, the selection of the right protocol and its compatibility are the other selection criteria for use of sensors in automated environments. In the case of the deployment of wireless systems, the set of selection criteria changes.”

Key characteristics of the sensor should always be evaluated, such as the trade-off values in size, weight, cost, reliability, accuracy, longevity and frequency response.

“Engineers may validate these parameters through the simulation of vendor-supplied models—when they exist,” says Sharma of ANSYS.

The environment in which the sensors will operate, both regularly and occasionally, must be factored into the selection process. But they must be balanced against the required reliability and available budget, Thusu points out.

“The measure of quality depends on the environment in which the sensor is likely to operate,” he states. “In cases where the environment is very harsh, which means high temperatures, is acidic or carcinogenic—then this factor determines the material of which the sensor is made.”

In normal conditions, Thusu says, a silicon-based sensor is used. But silicon carbide material can be used to manufacture sensors that offer high sensitivity under difficult working conditions and harsh environments. Recently, germanium material-based sensors have been developed, which offer better sensor quality, he says.

“Different sensors are at different stages of their product life cycle, but none of the sensors have yet reached maturity,” he adds. “Although price is an important factor, it is not the most important criteria for any purchase decisions in sensors.”

Consider the Value Chain

The original equipment manufacturer (OEM) has the ultimate responsibility to put all systems together, and look at the backward value chain of the sensor. The contribution of the sensor will contribute greatly to the overall system and product quality and functionality.

“In many cases, say in automotive, OEMs even share or fully fund R&D activities that are focused on product, function and application improvement or product innovation that the OEM requires and uses,” explains Thusu. “In other words, it is an integrated approach that is prevalent within the sensor market, where companies at various levels of the value chain work closely with one another.”

For example, General Electric has designed electrical generators driven by steam turbines to last 30-plus years. The formulas for power and design, while being continually improved, are also enhanced by the use of sensors to tell GE’s customers—and its customers’ stakeholders—many things. With the right sensor in the right place, measuring the right variable, accurate predictions can be made about the equipment, its performance, and predictable maintenance requirements.

But sensors may also help others far removed from the equipment determine things about the generator use, such as fuel costs, its relation with weather patterns, and how it reacts to changing load demand from the grid that it serves. Such More Information is critical, and is becoming increasingly important. Sensors are giving way to more reliability and relationships among suppliers, users and society.

“There are several factors that impact sensor quality, starting from materials to the ultimate end product,” explains Thusu. “At every level, the right type of components have to be implanted that have complete compatibility with the already existing components that have already been put within the sensor package. In terms of use, the selection of appropriate sensor is the most important component to be installed in the system.”

The Digital Side of Sensors

An ancillary, but important consideration when choosing sensors for design, is their incorporation in the network and control systems in which they will be used. This means that the electronics and accompanying software to support them will become increasingly important as smart phones come of age—and wireless sensors come to control many different facets of operations in our lives and that of our global industrial stratosphere. According to WinterGreen Research, the markets for wireless sensor networks were at $552.4 million in 2012, and are expected to reach $14.6 billion by 2019.

Sharma says sensors are going to be at the front end of the Internet of Things (IoT) revolution. To ensure that sensor data is interpreted correctly, he notes, engineers designing the printed circuit boards (PCB) need to perform signal integrity and power integrity (SI/PI) analysis.

“After all, if the power integrity or signal integrity is not assured, the best sensor data is rendered useless,” he adds. “Assuming that the sensor data is valid and correctly received, intelligent software is needed to ensure the proper next steps. Software engineers today use sophisticated, high-level modeling tools. For safety critical applications, software design tools may need to comply with safety standards, such as DO178C and ISO 26262.”

While their use may seem trivial or on the periphery of design, the overall demand for the many different categories of sensors and sensor technology is growing by leaps and bounds. The elements of sensor design and selection may have a not-so-trivial impact on larger components, equipment and systems that they help operate and control and the myriad of lives they will touch, improve and save in the years to come.

Jim Romeo is a freelance writer based in Chesapeake, VA. Send e-mail about this article to [email protected].

More Info:
ANSYS

Frost & Sullivan

WinterGreen Research