Now that not only pneumatics and hydraulics but also electromechanics are jockeying for position when it comes to motion control, how do we evaluate the best method for each application? Phil Burge, Communication Manager at SKF, looks at the pros and cons.
Predicting the future is a risky business and the past is littered with enough errors of judgement due to overconfident predictions. The rapid rise of electromechanical technologies, for example, with its many economic and environmental benefits, led many to predict the extinction of hydraulics – but this has not come to pass.
While electromechanical technology is indeed quieter, more efficient and less expensive to operate and maintain, it is foolish to write off its alternatives, each of which has its own strengths. Furthermore, hydraulic systems have recently made increasingly good use of electronic control, which emphasises the point that ruling out any of these technologies as obsolete is flawed and, to some extent, irrelevant.
However, it is fair to say that fluid power systems are hampered by a number of inherent disadvantages. Pneumatic systems, for example, are typically prone to leakages, while hydraulics can consume high levels of energy when idling. In contrast, electromechanical technology can offer a higher level of efficiency, because energy is only required during a movement and there is no need for pressure to be held up permanently while the system is in operation. Nevertheless, electromechanical technology is not ideal in every application, especially if high power densities, small retracted length or 100% duty cycles are required.
When planning a motion control system, the key decision is whether to choose hydraulic or electro-mechanical actuators. Cost is naturally an issue and this may tip the scales in favour of hydraulics, because an individual hydraulic cylinder costs less than that of an electric actuator. However, it is worth bearing in mind that ancillary costs (oil tanks, pumps, accumulators, filter systems, hoses) may still drive up the price of a hydraulic system to exceed that of an electromechanical equivalent. The fluid itself can also increase costs, especially if bio-degradable oil must be used to satisfy environmental requirements. In contrast, electromechanical actuators only require cables for power, signal transmission and FieldBus connection. It must be added, though, that ancillary costs for hydraulics are reduced if a number of actuators are concentrated in a small space inside a machine, which could tip the scales back in favour of a hydraulic solution.
Electromechanics offer a clear advantage when it comes to drive control. Whereas the electronic control for a hydraulic system must be specially designed for the particular requirements of fluid engineering, a wide range of highly cost-effective electronic control systems are available for electromechanics. Control equipment for electromechanics is compatible with stored program control systems (SPS) and bus systems. SPS programming offers reliably constant control from the moment the machinery is switched on to the moment it is deactivated, which is not the case with hydraulic drives. For example, when an injection moulding machine using a hydraulic drive is switched on, it first produces a run of defective parts, whereas a fully electric-operated injection moulding machine produces parts without defects almost from start-up.
Energy costs are central to the debate when choosing between fluid or electromechanical systems, since the electrical current required by a hydraulic system to generate high pressure must be constantly maintained. In contrast, electromechanical equipment can be cost-effectively adjusted to the requirements of each task and is only consumed for the duration of the operating cycle. Whatever the output required, the relevant amount of current is tapped. If the load increases or decreases, the current input is adapted correspondingly.
One area where hydraulic drives may still currently have the edge is in construction machinery. If, for example, the shovel of a hydraulic excavator strikes rock, the hydraulic fluid is compressed, thus preventing damage to the moving parts of the excavator. It must, however, be said that power sensors can switch off or reverse the drive if electromechanical equipment encounters the same problem.
This evidence shows that there are many issues to take into account when specifying a motion control system, and that careful consideration is essential when weighing up the options.
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