Fluid power systems – pneumatics and hydraulics – are subject to the longest stay of execution in the whole of manufacturing. Ever since electromechanical systems came to the fore, the imminent death of fluid power has been predicted – but has still not happened.
Nor is it likely to. Pneumatics and hydraulics were well established before electromechanical systems were first introduced, and a combination of familiarity, cost effectiveness and reliability has helped them keep their place in automation. Pneumatic actuators in particular have been used in the pharmaceutical industry for many years. However, it is true to say that electromechanical systems have gradually replaced pneumatics – not just in pharmaceuticals – and are increasingly specified as the preferred method of automation.
That said, equipment manufacturers have worked hard to resolve some of the more common drawbacks of pneumatic systems: the latest compressors use variable speed drives and heat recovery systems, for example. Suppliers have also focused on reducing air losses throughout pneumatic circuits and improved component designs to enhance functionality at the point of use.
As technology evolves it is tempting for automation specifiers either to resist any change or make a total switch to a new system. Ignoring an alternative technology – or, conversely, installing a new system that is wrong for the business – can have serious implications. For this reason, an informed perspective that considers the merits of each application is a better way forward.
Spoilt for choice
Choosing a motion control system – whether it is a simple component upgrade or modification for a filling unit, or a complete packaging machine build – requires a holistic approach that considers the fundamental system requirements and looks at the most appropriate methods of motion technology.
In the past, buyers were often presented with a stark choice of one technology or the other. Fluid power systems were well-understood but had their limitations. Electromechanical systems, on the other hand, were hailed as the future – but their merits were often over-sold, and they were not always suitable replacements. Today, companies recognise that it is not a question of one technology or another, but of using the most appropriate technology – or more likely a combination of them – to achieve the task in hand.
The questions that need to be asked will vary from project to project, but generally start with a definition of what needs to be achieved. For example: how many products must be processed per hour? Or it might be more specific: what is the speed or precision with which a filling head or sealing unit has to be moved and positioned? These types of questions – and the subsequent answers – inform the final system choice.
Cost factors
If a machine is already set up with a compressed air feed, it may be better to add further pneumatic devices – despite them typically offering lower levels of control or energy efficiency than an electromechanical system. For higher loads, the air losses and lack of precision of pneumatic actuators will make an electromechanical actuator and controller a more sensible choice. This is especially true if actuation systems have to move through different temperature zones, as the compressibility of air is temperature dependent.
For larger or higher load applications, a number of pneumatic actuators could be replaced by a single electromechanical device. Although the unit cost of the latter will be higher, the total system cost may well be lower.
An individual pneumatic cylinder usually costs less than an electric actuator. However, adding the cost of supplementary items – compressors, reservoirs, air preparation and system uptime to maintain constant pressure – means that the total may exceed that of an electromechanical equivalent. This cost can be reduced through careful system design and planning. By concentrating a number of actuators in a small machine space, for example, the system cost per actuator can be minimised – which in turn cuts the total cost of ancillaries.
Other generic factors to consider will include reliability, ease of construction, operating life and the environment in which the equipment is to be used. Both pneumatic and electromechanical systems offer high reliability and long service life, and can withstand dust, shock, vibration, and extreme temperatures. Both technologies are generally easy to assemble – with standard modular components making it easy to create complex motion – while adjustments and troubleshooting are easily achieved by any competent engineer.
One important point is this: treat headline performance figures quoted by suppliers of motion control equipment with a degree of caution. Instead, look at factors such as required speed, load and cycle rates. A device with a high maximum cycle speed or duty load, for example, may not meet the needs of functionality, reliability, operating life and total cost of ownership.
Added accuracy
Positional accuracy and repeatability is critical for many production and packaging systems. Pneumatic devices, while widely used, are constrained by the simple fact that air is a compressible medium. This compressibility depends on a number of factors, including variations in supply pressure, system leaks and changes in ambient temperature.
In practice, these variables are easily controlled by introducing proportional valves and regulators. However, it can be difficult to position pneumatic actuators repeatedly and precisely at multiple stop-start positions along the stroke length. In this case, an electromechanical actuator and controller can be a better choice, as this configuration can achieve a positional accuracy and repeatability that is typically better than 10 microns.
Break away forces in pneumatic cylinders – also known as stiction – must be considered in applications such as pressurised filling systems – where low break away force and steady actuation pressure is essential to prevent product foaming. Modern pneumatic cylinders can overcome this by using piston seals. However, an electromechanical actuator will offer greater benefit in certain applications: it is easier to use fewer components to create a slow, controlled motion from the initial moment of actuation – with gradually increasing force to minimise cycle time.
Electromechanical solutions are also preferred if high levels of applied force are needed and can apply loads approaching those achievable through hydraulics (up to around 180kN). Pneumatics, in comparison, only delivers around 30kN.
Speed and control
If speed of operation is a factor, then the compressibility of air can be an advantage. When compressed air energy is released it expands dramatically. This allows cylinders and other devices to be operated extremely rapidly. Electromechanical actuators offer similar functionality, with both technologies being capable of operating speeds that are considerably greater than 1.0m/sec.
The benefits are clearer when it comes to control – with electromechanical systems having distinct advantages. The use of standard, off the shelf control devices can deliver accurate motion, with instant and precise speed, force and position, to enhance the flexibility and repeatability of production processes.
While pneumatics also use similar low cost control technologies and open access protocols, in critical applications there can be brief – but important – time lags or response times between origination of a control signal and precise moment of actuation, due to the compressibility of air. This can result in inconsistencies in system response and delayed system start-up.
A further advantage of electromechanical technology is its inherently low noise. Mechanical compressor noise and air line leaks make compressed air systems generally noisy: compressors must often be isolated in separate areas of the factory to minimise the effects of noise, vibration and heat. This increases cost if setting up a new system – but is less important if motion control devices are being added or upgraded, and a compressor system already exists.
Energy efficiency
Electromechanical systems are more efficient in converting electricity to linear movement. Only 10% of power is lost at the motor and 10% for conversion, leaving 80% of input power available at the point of use. Furthermore, energy does not have to be stored for subsequent use and is only consumed when it is needed, at the level required for each operation and with an instant adjustment to changes in demand.
By contrast, a standard pneumatics system is wasteful. Where a compressor, filters, valves, regulators, dryer, receiver and pipes are all required before air can reach the point of actuation, energy losses are common: 10% at the motor, 45% for compression, 6% for air preparation and 8% in conversion. As a consequence, only around 30% of the input electricity is converted into useable energy. If leakages are factored into the equation, efficiency can fall to as low as 6%.
According to figures from the Carbon Trust, a typical compressed air system converts only 10% of the energy supplied to the compressor into useable energy at the point of use – because, even when idling, a compressor can use 20-70% of its full power load. These figures are supported by research from the University of Kassel in Germany, which found that – size for size – pneumatic systems consume over 10 times more energy than their electromechanical counterparts. Electromechanical systems only use energy when work is being carried out, such as when an actuator is in motion.
The risk of contamination is also a major factor to consider in areas where pharmaceutical products are being processed. As in other industries that require such high levels of cleanliness, hydraulics are normally unacceptable – but oil-free compressed air systems can be used, as can electromechanical devices that are tightly sealed to enable easy cleaning.
The argument between electromechanical and fluid power is far from over. Improvements to pneumatics, combined with a growing confidence in – and understanding of – electromechanical systems means that engineers have a far wider choice than ever before for pharmaceutical production and processing applications. Based on different but not necessarily competing technologies, these solutions open new design opportunities for machine automation and control.
