Actuator delivers benefits of electronics and pneumatics

When designing machines and factory automation, conventional wisdom dictates there is usually a trade-off between speed and accuracy, and force and size. Predominantly, electric and electronic actuators are used to deliver the speed and accuracy, while pneumatic/hydraulics actuators deliver the required forces. However, after working closely with a major European manufacturer of food and beverage packaging machines, SMAC has designed an actuator that is a hybrid of electronics and pneumatics to deliver the considerable benefits of both.

Limitations of pneumatics

Until now, both technologies had a number of advantages and disadvantages;

whilst pneumatic devices offer a relatively high force in a compact component, there are a number of significant drawbacks to be considered:

A high degree of accuracy and repeatability is difficult to obtain. True positional programming is not possible. Over the years pneumatics manufacturers have introduced a number of mechanical clamping arrangements, braking systems and other nefarious devices, but it is argued by SMAC that none has been truly successful. The automatic change of machines to new and different processes has not been possible without some form of manual intervention. Relative to electric and electronic devices, pneumatics offer a low number of cycles due to shock and wear during cycles. Extremely soft/delicate contact with components and workpieces is not possible due to the dynamic sealing arrangement that is inherent in such pneumatic devices. Compressed air, when being exhausted to atmosphere, makes a lot of noise! So much so that legislation has recently been introduced to ensure it is kept at acceptable and safe levels.

Compressed air and pneumatic controls are notoriously inefficient. While the components are relatively low-cost, the running costs are considerable and should not be underestimated. However, they do have significant advantages of international dimensional standardisation and also the wide availability of pneumatic components. They are truly ubiquitous and be can be sourced and purchased in any town anywhere in the world – 'off the shelf'.

Advantages of electric/electronic actuators

The major disadvantages with electric and electronic actuators when compared against pneumatic and hydraulic actuators are:

Price - The unit or component costs are around 40 per cent higher or more than a conventional pneumatic cylinder. However, prices of electric products are rapidly decreasing from the majority of manufacturers. The other major drawback, up until now, has been the relatively low degree of force generated. They are, however, extremely quiet and, in comparison, extremely energy efficient – which is a major point for consideration with the current high profile and emphasis on global warming, climate change and carbon footprints.

The SMAC solution

To overcome these disadvantages, SMAC has gone 'back to the future' with a reliable 'tried and trusted' idea that has been reinvented, updated and improved. 'Air over oil' and 'accumulator' systems have been used for many years. This is where a pneumatic and hydraulic cylinder (or two pneumatic cylinders) have been connected together - one actuator to deliver the movement or travel to the desired position, and the other actuator to deliver the required force. SMAC says it is now the first manufacturer to apply this idea to electronic actuators.

A small pneumatic actuator (around 20mm diameter and 10mm stroke) is attached to the rear of a SMAC electronic actuator. The front of the piston rod is connected to the rear piston rod of the electronic actuator - thus creating the hybrid. The electronic actuator can then be controlled as normal, delivering all the advantages and flexibility of speed, precision, accuracy, repeatability and long life. When - and only when - the electronic actuator is in the required position, air is applied to the small pneumatic cylinder. It is this function that then delivers the higher force required. Due to the fact that the air is only applied once the electronic actuator has reached the desired position, and then switched off when no longer required, there is virtually no wear to the pneumatic sealing arrangement. This enables an exceptionally long life cycle – at least that equal to the electronic actuator. As the pneumatic actuator is small in both diameter and stroke, the amount of compressed air used is minimal.

This innovative concept was recently used successfully to replace a number of 63mm diameter pneumatic cylinders with SMAC electronic moving coil actuators (MCAs) for an ultrasonic welding application on drink cartons. SMAC created the profile required using a high-speed approach, then the new 'Softland' function placed the component in position before the holding force was applied by the pneumatic actuator in order that the welding could take place.

Hybrid advantages

The advantages and improvements to the system were considerable. While using the 63mm diameter pneumatic cylinders on the conventional arrangement, external electrical sensors were required to detect the position. Due to the fact the SMAC actuator has inherent positional control and feedback, these external sensors were no longer required. This significantly reduced the amount of external wiring required. It also enabled the customer to increase the cycle time, as previously the system had to wait for confirmation from the external sensor that the end of the pneumatic cylinder stroke had been reached. With the SMAC concept, the next part of the cycle could commence immediately after the electronic actuator had moved off – thereby saving valuable milliseconds.

Further to this, the two biggest problems for the customer were also solved, the first one being noise. With a number of large-bore pneumatic cylinders performing a high number of cycles in an enclosed space, the noise was above that of the legal limit. Therefore the operators were required to wear ear protection. This is no longer required with the SMAC system. Secondly, due to the high number of cycles involved and the impact and 'shock' at the end stroke of the conventional pneumatic cylinder, the life of the actuators was low – they were replaced four times per year as part of a preventative maintenance routine. This is no longer required with the electric system, as the cylinders are good for many, many millions of cycles.

Conclusion

To briefly summarise, the SMAC 'Air over Electric' hybrid concept has taken the best from both electronic and pneumatic technologies and delivered them as a single package. It is no longer necessary to compromise the need for speed, position and accuracy with that of force. It also brings total and true programmability, exceptionally long life, low noise and a significant improvement in energy and efficiency in a highly cost-efficient package.