The basics of DSP for use in intelligent sensor applications: Part 3

Creed Huddleston

July 6, 2010

Creed Huddleston

How to Analyze a Sensor Signal Application
When analyzing a specific sensor signal-processing application, designers need to understand the following aspects of the system:

1. the physical property to be measured,
2. the relationship between the physical property being measured and the corresponding parameter value to be reported,
3. the expected frequency spectrum of the signal of interest and of any noise sources in the environment,
4. the physical characteristics of the operating environment,
5. any error conditions that may arise and the proper technique for handling them,
6. calibration requirements,
7. user and/or system interface requirements, and
8. maintenance requirements.

Often, the natures of the physical parameter being measured and of the operating environment will help guide the designer in the selection of appropriate signal-pro-cessing capabilities to include in the sensor system.

For instance, if one is measuring the temperature of a large metallic mass heated by a relatively small heating element, its safe to assume that the frequency content of the signal of interest is minimal since the temperature can change only gradually.

This means that the sensor can employ heavy filtering of the input to reduce noise. In contrast, a temperature sensor monitoring a small device being heated by a laser must be capable of reacting to intense changes in temperature that can occur very quickly.

In such a situation, noise filtering must be lighter and other processing may be required to address noise that gets through the initial filters.

Its also critical to understand the relationship between the physical property being measured and the corresponding parameter being reported to the user or to the rest of the system.

Does the reported parameter vary linearly with the physical property (as is the case with RTD temperature sensors), or does it have a nonlinear relationship (as do many thermocouples)?

If the relationship is nonlinear, is it possible to segment the relationship into piecewise linear regions to simplify computation? A poor or incorrect understanding of the relationship between physical property and reported parameter can render a sensor system useless.

Finally, designers must always consider that sensor systems are going into the real world, where problems are guaranteed to arise at the worst times. The system must be designed to detect common errors, and the more robust its error detection and handling scheme, the better.

The loss of a sensor on the production floor may stop production for an entire line, so any features that allow quick troubleshooting and easy repair are greatly appreciated by end users.

Of even more importance than maintenance, however, is the ability of the sensor system to detect dangerous conditions that may lead to unsafe operation unless corrected. Sensors that operate in a fail-safe environment must be designed with rigorous attention to fault detection, reporting, and correction.