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What does the future hold for hydraulic and pneumatic technologies?Steve Sands, product manager at Festo, challenges the myths that one media is more efficient than another.
Many commentators and vested interests claim that electric actuation, motion and control technologies are cheaper, more environmentally friendly, more energy efficient and easier to work with. But it is too simplistic to say that electric drives are better than pneumatics without considering the application and whether the technology is being applied correctly.
Consider energy efficiency: if either pneumatic or electrical drives are applied incorrectly they can be inefficient and waste energy. Only a direct comparison of two correctly dimensioned drives - one electric and one pneumatic, being operated effectively - can reveal the advantages and disadvantages of the technologies. It almost goes without saying that poor practices resulting in wastage should be eliminated through good knowledge and skills within the workplace.
An efficiency of 80-90% is generally attributed to electric motors, while an efficiency of approximately 10% is often quoted by electric drive manufacturers for compressed air. Based on these figures, it would be easy to draw a conclusion that pneumatics require 8 to 15 times more energy than the corresponding electric solution. However, even with the most basic analysis this clearly ignores the fact that the assumptions made for electric drives only apply to motors in continuous operation (for example, pumps, fans, etc) and not for cycling or inching operations, which are common in handling technology. Here the efficiency of electric components is far lower.
Each industrial application has its specific requirements with regards to technical criteria such as speed, acceleration, load capacity and force required, cycle time, holding duration, power to weight ratio, accuracy, control behaviour, torsional rigidity, efficiency or robustness. Both pneumatic and electric technologies have specific advantages and disadvantages, which is why it doesn't make sense to make bland statements on the benefits of a technology, when energy efficiency wholly depends on the task at hand. Taking some simple examples, testing and measurements reveal that for a simple motion task, an electric drive can be efficient. For a pressing or clamping operation, the level of process force and the duration of the operation decide which technology is more efficient. However, if the application requires a holding force, pneumatics is clearly at an advantage.
For example, comparing electric and pneumatic grippers shows how choosing the right solution depends on a clear definition of the task. If we consider the energy consumption during the gripping process, a pneumatic gripper will be superior to an electric gripper in applications involving long cycles and with just a small number of gripping operations.
A pneumatic gripper requires pressure only once to initiate continuous holding. No more pneumatic energy is consumed or required for the duration of the gripping operation. However, an electric gripper needs electricity for the entire duration of the gripping operation and can be more energy-efficient than a pneumatic gripper only if the application in question is composed of short cycles with many gripping operations.
For applications which require large holding forces for extended periods, pneumatics has a major advantage compared with electric drives which require constant electricity consumption to generate power.
Energy efficiency rules of thumb
The following rules of thumb can generally be applied: the shorter the stroke length, the greater the end position force and the longer the holding duration, the more efficient pneumatic technology is. In contrast, the greater the stroke length, the lower the end position force and the shorter the holding duration, the more efficient electric drive technology is.
It is important to bear in mind that a technology cannot be selected based on energy aspects alone; other properties including purchase cost, ruggedness, weight, the environment, installation costs and operating costs play an significant part and must be incorporated in the decision making process. A lighter-weight pneumatic gripper can reduce the moving mass of all the actuators throughout a handling system, dramatically reducing the overall energy requirements. It's clearly better to have an efficient overall system rather than focus on the efficiency of the single components.
Efficiency can only be achieved through understanding the application, technologies and the technical product data to optimise the solution. One way to ensure you are using the most appropriate solution is to utilise engineering software tools, such as Festo's sizing software, which helps with the correct dimensioning of systems. Software enables the selection, configuration and sizing of the control chain and accurately simulates the application in place of expensive reality tests.
In summary, there is a place for both pneumatic and electric drives as the most energy efficient selection is application dependent. There is no quick-fix solution for increasing energy efficiency of the system as it must be viewed in an overall context and parameters must often be looked at in detail to provide final data on potential efficiency, savings and economy. Festo has recently published a report entitled 'EnEffAH' which evaluates the energy efficiency in production in the drive and handling technology field, which is free to download from www.festo.co.uk/energyreport.
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