28 March, 2024

A designer's primer on pneumatic sensors and switches

29 April, 2015

Leslie Neill, EMEA senior product marketing manager, Honeywell Sensing and Control, outlines what he believes design engineers need to know when specifying pressure switches and sensors in pneumatic actuation and control systems.


Pressure switches and pressure sensors are the unsung heroes of pneumatic actuation and control systems. Even though there are only a handful of basic types of sensors and switches, the selection process isn't always as easy as it might seem. Each switch type is available in a near-infinite combination of packages, ports, pressure ranges, electrical outputs and connector styles which can make selecting the best match for a particular job a challenging, time-consuming task.

Pneumatic systems

Whether they're helping to control a high-speed manufacturing system or monitoring the various inlet and outlet pressures within an air compressor to determine its performance and efficiency, pressure-sensing devices play a critical role in every pneumatic (and hydraulic) application.

Industrial machines and manufacturing systems rely on pneumatic switches and sensors for the information that enables them to move precisely and apply the exact amount of force required for a particular task. In large heating and air conditioning systems, they are part of the pneumatic signaling network that connects the building's thermostats and control elements with the fans, burners, pumps and compressors that do the work. Pressure switches and sensors will also be found providing control and safety functions in the braking systems of trains, trucks, aircraft, and other transportation systems.

While all the pressure sensing devices highlighted in these examples employ a nearly-identical set of operating principles, their sensing ranges, performance requirements and operating conditions vary as widely as the pressure-sensing solutions available to meet a particular need.

Pressure switches versus pressure sensors

Pressure sensing devices fall into two broad categories: pressure switches; and pressure sensors.

Pressure switches: Pressure switches are relatively simple devices that indicate whether the pressure they are sensing is either above or below a predetermined threshold. Their output is a change in the state of an ‘on/off’ switch or a two-state electrical signal.

A typical pressure switch consists of a contact that's driven by a diaphragm or piston that is pushed in one direction by the gas or fluid being sensed and a bias spring that pushes in the opposite direction. The switch's contact changes state when the pressure on the switch's inlet side rises above the pressure exerted by the bias spring on the other side. Conversely, the switch returns to its original state once the inlet pressure falls below a predetermined threshold. Due to a phenomenon known as hysteresis, that threshold may or may not be the same as its actuation level. In some cases, hysteresis can be used to one's advantage when attempting to stabilize pneumatic systems.

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.

• Wide choice of output formats and ranges.

• Modern sensors can include signal conditioning electronics to provide highly accurate linearisation and compensation.

Disadvantages of pressure sensors

• Generally higher cost than a switch which meet equivalent environmental and reliability specifications.

• Analog outputs can be more susceptible to radiated emissions and other electrical noise than on/off signal.

• Integration of pressure sensor would mean the control system needs to be able to accept continuous output.

Matching your application

When your design does indeed require a pressure sensor, the trick to selecting the right device is to only buy the performance and capabilities you need, and avoid paying for the ones you don’t. In practice, that's not always as simple as it sounds, but the list of selection considerations below should make things easier. Once you've selected the criteria that are relevant to your application, add the values that will meet your requirements.

In terms of the Pressure range, for example, what are the maximum and minimum pressures you expect to see in your application? Do you need to expand the range requirements above and below your formal requirements to cover unanticipated conditions? And what are the units of measurement you'll use (psi, bar, inches of water, etc.)?

How accurate does the pressure measurement need to be? What is the maximum Total Error Band (TEB) your application can tolerate? Should the sensor output its readings as a voltage, current, or resistance? What's the output range you need from the sensor and what units is it expressed in?

In terms of its electrical and EMI protection, what are the levels of the worst extraneous system voltages the sensor could be exposed to and what is maximum level of electromagnetic interference (EMI) you'll need your application to operate reliably in? What are the highest levels of EMI and extraneous voltage your application's sensors must be able to survive without damage?

Depending on where your device is being used, what is the required operating temperature range? What levels of shock and vibration must the sensor be able to tolerate? What is the ingress protection that a sensor is required to meet to work in application conditions?

And when it comes to media compatibility, what's the temperature range of the media being measured and does it have corrosive properties?

Packaging options

The other big consideration is a practical one: the packaging and mounting options. The environmental, media compatibility and mechanical issues you've already considered will provide plenty of guidance on the packaging requirements for a stand-alone sensor. But for some applications it may be worthwhile to consider a board-mounted sensor that's co-located with the application's other electronics. Board-mounted pressure sensors can provide space-saving solutions with specialised port options that give the sensor access to the gas or fluid being monitored while it resides within the electronics module's relatively benign environment.

Lastly, designers need to consider the requirements of the mechanical and electrical interface, and electrical connectors: does the application have any specific mounting holes, brackets or other hard points the sensor needs to attach to? If so, what holes, flanges or other features does the sensor housing need to have to mount where it's needed? What types of pressure input fittings and electrical connectors will the sensor need?

You’ll also need to ask yourself a few basic questions: Will the operating environment pose any additional challenges in relation to vibration, rapid temperature cycling, or corrosive vapours etc.; what's the type of pressure you need to measure (i.e. gage, absolute or differential); and what are the application's size and price constraints?

www.sensing.honeywell.com




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