A designer's primer on pneumatic sensors and switches

29 April, 2015

Pressure sensors: Pressure sensors on the other hand, are analog or digital-style devices that output a continuously varying value (e.g. voltage, current, resistance, I2C, etc.) that's proportional to the pressure they see on their input. There are several types of devices which can translate a mechanical pressure into a detectable electrical signal, including piezo-resistive sensors, whose performance, versatility and cost have made them the most commonly-used sensor for many applications.

Piezo-resistive sensors operate on the principle that certain semiconductor materials, such as silicon, change resistance with stress or strain. These piezo-resistive elements are implanted on a silicon chip that is attached to a mechanical sensing element (such as a diaphragm) or used as the sensing element. When the piezo-resistive elements are used as part of a bridge circuit (as with the wire filament strain gage sensor), an analog voltage signal is produced that is proportional to the applied pressure.

Like most sensors, piezo-resistive devices don't react in a linear manner to pressure stimulus. They also exhibit a tendency to drift over time, or in response to environmental conditions. Traditionally, drift and non-linearities were corrected by external means. Now however, many modern sensors contain integrated electronics that linearise the sensing element's raw output and convert it into one of several standard electrical voltage or current ranges. Many of these devices also provide some degree of stabilisation against temperature and time-related drift.

Weighing your options

There are some applications, especially those involving detection of an upper or lower pressure threshold, where it can be difficult to decide which device is most appropriate. This is most apparent in new products or radical updates of existing designs. It can appear to be less of an issue for compressors, pneumatic control systems and many other mature products and applications whose requirements are already well defined.

It is probably recommended to weigh your options for nearly every design project. Sometimes a new look at a mature design can yield fresh insights and unexpected improvements. Here are a few things to consider:

Advantages of pressure switches

• Simple mechanism offers low cost, enhanced reliability.

• Straightforward output simplifies interface.

• Provides consistent, reliable performance and enhanced repeatability.

• Available in ultra-compact form factors.

• Factory set threshold can be fixed or field-adjustable.

Disadvantages of pressure switches

• On/off signal only provides threshold alerts, no granularity or trend information available.

• Even the most durable mechanical switches eventually wear out.

• Switches may have some degree of contact bounce and/or hysteresis.

Advantages of pressure sensors

• Provide continuous real-time pressure information for safety and controls systems, enabling use of advanced algorithms and trend analysis.

• Available in a wide range of sensing technologies in order to match your design's accuracy and range requirements.