Switching on to pneumatic innovations

06 February 2023

Pneumatic cylinder switches may appear to be an automation commodity beyond advancement – but there is room for improvement, explains Steve Sands.

We often assume that new technology is required to achieve improvements in speed, accuracy and operations, but applying innovative thinking to mature technologies can deliver real benefits too. Pneumatics can certainly be considered a mature technology. It began to be widely adopted in the automation sector in the 1960s. Solenoid valves followed in the 1970s, driven by the early generations of PLCs. 

Today, we are seeing the ongoing integration of electronics creating smart devices integrating software apps, data collection, network communications and embedded machine learning diagnostics for predictive maintenance, improved quality, and energy reduction.

Pneumatic products have become smaller, and more accurate over time, but it is not just physical features that can be improved upon when considering mature technologies. Human factors like usability, convenience and safety are tangible areas for development and improvement. Machine assembly and build times, on-site commissioning costs and maintenance times are also areas where an innovative approach can be applied, beyond the size, performance and cost aspects of the technology.

Take cylinder sensor switches as an example. These devices are mounted on the majority of pneumatic cylinders and provide sensor feedback. They are most frequently used when the cylinder reaches its advance or retract end-positions and, less frequently, in mid-position – for example, when a cylinder movement has cleared an overlap position.  

Cylinder switches are triggered by a ring magnet or magnetic strip contained within the piston inside the cylinder barrel. The magnetic flux reaches beyond the non-magnetic cylinder barrel and indicates its proximity. Clever design and CAE simulations enable the flux strength and shape to be optimised to match a manufacturer’s cylinders and sensors for reliable operation.  Matching is an important stage that minimises the potential for adjacent actuators' magnetic fields, mounted in close proximity to one another, to trigger or block the reliable sensing of adjacent sensors.

It may appear that these devices are so well understood and so ubiquitous that there is no room for improvement. However, they do still have some drawbacks.

Installation challenges
Mounting a cylinder sensor on the bench is straightforward enough. The sensor switch slides into place from the end cap or is dropped into the sensor slot from above. It is then a simple matter of adjusting its position to reflect the switching point required. This requires bringing the piston-rod/piston into the end position and then, with a battery or power supply connected to the sensor, moving it until the LED position indicator shows it is in the correct output position. For reliable repeatability, setting at the edges of the switching window should be avoided because it can give rise to problems later. The optimum switching position is mid-way within the sensing position hysteresis.

This is a quick and simple operation – until the actuator is installed into the machine. At this point it becomes impossible to know precisely where the magnet inside the piston is physically located in relation to the external barrel. Frequently it is also difficult to ascertain the end final positions of the actuator. The result is a device that does not work properly. Commonly, either the air pressure compresses the elastomer cushioning at the end of stroke beyond the ambient pressure position, or the cylinder doesn't quite get to its end-position due to external stops. Consequences of poor cushion setting include wear and tear on the cylinder, noise and vibration.

The workaround used by those with experience is to set up the switch approximately on the bench and then make the final setting by cycling the cylinder in-situ on the machine. Again, what is an easy job on the bench can be much harder in practice, particularly when access is required inside the machine guarding, at height or in difficult to reach locations. In these circumstances, accessing the sensor Allen key position, adjusting and observing the indicator LED can be problematic.

This final setting and adjustment of cylinder sensors, along with speed adjusting flow controls and cylinder end-of-stroke adjustable air cushioning, takes a considerable amount of time and may result in sub-optimum performance. However, final set-up can make a big difference to long-term reliability and operating lifetime. 

Festo introduced a self-adjusting end of stroke air cushioning system a couple of years ago, which partially addressed this issue and helped to cut build times – particularly on multi-cylinder machines. We have now taken this innovation further with the introduction of the latest cylinder sensor switch which brings wider benefits to all users.

The SDBT-MSX is the first auto-teach cylinder switch that learns the required switching position simply by cycling the machine four times during set-up. No in-situ adjustment is required – instead, the switch is dropped into place in the approximate required position. This is made even easier on the latest generation of Festo cylinders with an indicative sensor mark. The fitter simply sets the sensor up by aligning the cylinder and sensor marks. This can be done on the bench. When the cylinder is fitted into position on the machine and the sensor is wired in, it’s LEDs will indicate that the switching point is not yet saved. Cycling the machine, including this actuator, four times enables the self-teach function and the optimum switching position is learnt by the sensor and saved.

This auto-teach cylinder is suitable for all compatible T-slot cylinder grooves. It utilises a 20mm long hall sensor array and integrated electronics. This technology offers several other benefits if users want to go beyond the basic, standard operation. These include the ability to manually teach-in a position using the integrated capacitive teach button and the ability to re-assign the switching operation from PNP to NPN, and from Normally Open to Normally Closed operation. Even the switching window can be adjusted from the pre-set 2mm to any length up to 15mm if the application requires it. The benefits in terms of purchasing, stores and maintenance are clear – one cylinder sensor for all applications.

Steve Sands is head of product management at Festo GB.

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