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PUMPS, VALVES & ACTUATORS
INNOVATION IMPROVES PRECISION
Steve Sands, head of product management at Festo GB, explains how a recent
pneumatics innovation can improve precision and control without additional cost
C
ylinder sensor switches are mounted on the majority of pneumatic cylinders and provide sensor feedback. They may appear to be an automation commodity beyond advancement – but there is room for improvement.
Cylinder switches 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. These devices 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 close 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. This matching minimises the potential constructive/ destructive influence of adjacent actuators' magnetic fields, mounted in close proximity to one another, triggering or blocking the reliable sensing of adjacent sensors. However, setting these devices accurately ‘in the field’ has always been problematic. Mounting a cylinder sensor on the bench is straightforward enough. You slide the sensor switch into place from the end cap or drop it into the sensor slot from above and then adjust 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, the optimum switching position is mid-way within the sensing position hysteresis. The challenge once the actuator is installed into the machine is that you can’t tell precisely where the magnet inside the piston is in relation to the external barrel, and often you don’t know the end final positions of the actuator. This results either in the elastomer
Faster assembly times and more reliable cylinder switching are two advantages of the pneumatic innovation
Below: The SDBT-MSX programmable cylinder switch learns the required switching position simply by cycling the machine four times during set-up
cushioning being compressed beyond the ambient pressure position at the end of stroke, or the cylinder not reaching its end- position due to external stops. Consequences include wear and tear on the cylinder, noise and vibration.
So, 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. It becomes even more time-consuming on machines with multiple cylinders – and yet precise cylinder
positioning
can make a big difference to long- term reliability and
operating lifetime. A couple of years ago, Festo introduced the patented self-adjusting, end of stroke air cushioning system [PPS], which addressed part of this issue and reduced build times. Now the introduction of the latest cylinder sensor switch, the Festo SDBT-MSX, is set to create a corresponding saving for 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, which 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, either on the bench or in situ. 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 technology has 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 also 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 potential to use one cylinder sensor for all applications could deliver savings in inventory, warehousing and cost. And on that last point, the SDBT-MSX can be manufactured in-house by Festo at virtually the same cost as standard sensors. So, most users can buy them at virtually the same price they are currently paying for their standard reed switches or solid-state magneto-inductive cylinder switches. The SDBT-MSX demonstrates that even in pneumatics, a mature technology, innovation is still possible. Faster assembly times, more reliable switching, plus less time on installation – including contorting, going up ladders or lying under machines – will be welcome improvements for those involved in delivering control systems.
Festo
www.festo.co.uk
MARCH 2022 | PROCESS & CONTROL 27
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