Electric versus pneumatic actuation: the energy implications
09 June 2013
Olsen Engineering purports that electromechanical actuators are some ten times more energy efficient than pneumatic actuators and over four times more efficient than hydraulic actuators.
This stark claim is supported by a technical white paper that can be downloaded from Olsen's website. The white paper explains why the energy consumed by an actuator can have a significant impact on the lifetime energy costs of an application and how to specify an actuator correctly, citing research carried out at the University of Kassel in Germany.
Energy consumption comparisons from a series of experiments revealed the possible savings that can be achieved by choosing the right actuator, including the cost of implementing or retrofitting an existing system. Piers Olsen explains:
“The research shows that pneumatic actuators consume 10.3 times more energy than electromechanical actuators, and 4.4 times that of hydraulic actuators, not taking into account air leaks, which could add another 50 percent to the energy cost.
“This is borne out by our own experience in the UK of a margarine manufacturer which operates five 110kW compressors on site. The manufacturer concluded that one of those compressors is there purely to maintain air pressure because of all the leaks in the several kilometres of air lines around the plant. Therefore 110kW at 10p per kW hour multiplied by 24 hours a day; 365 days a year means a waste of over £95,000 a year.
“It is difficult to quantify the total equivalent hole diameter for the number of holes in an air line because often you don’t know where they are. Diagnostics personnel have to carry listening devices to detect hisses in the pipework and establish where the leaks are in order to seal them. Because the temperature of the air lines always fluctuates, condensation forms and leaks occur. You can never find all these leaks and even if you do fix them, they come back somewhere else.”
Olsen estimates that total air leaks will be in the region of the equivalent of a 6mm diameter hole per km, costing £20,000 a year in wasted energy. Citing another study - in this case the University of Pittsburgh’s 2003 study on continuous automotive welding guns - the company argues that the energy cost of operating a servo-electric actuator was a tenth of the cost of using pneumatic actuation at £470 instead of £4,700 a year. Piers Olsen again:
“We also know that users in power stations and other process industries like chemicals, that are using modulating and regulating control valves for fluid flow through pipes pay one tenth of the energy cost by utilising electric instead of pneumatic actuation.
“Pneumatic systems use air even when actuator motion is not required, while electric actuators only use energy when it is needed. The longer the dwell cycle between moves, the higher the energy savings. Furthermore, air-lines and seals wear and split due to contamination and ageing, and so need constant maintenance and replacement, often requiring many man hours and headaches to repair the kilometres of hoses.
“Crucially, it is very difficult to get repeatable constant air pressure, because cylinders running on the same compressor are always switching on and off around the factory. In contrast, electric cylinders always give repeatable results. This is critical in plastic sealing applications, for example, to avoid scrap.
Although the initial cost of an electric actuator is maybe three to five times more than a pneumatic actuator, if sized correctly the extra capital cost is guaranteed to be paid back within twelve to eighteen months.”
Olsen argues that electric linear motion is a clean technology, and hence one that is preferred by those industries operating hygienic processes, such as food, water, medical device manufacture and pharmaceuticals.
He also claims that some sectors of industry – in particular, the aerospace and defence sectors – have plans in place either to roll out, or convert to, all-electric actuation(where it is appropriate to do so) for existing and all new plant development programmes.
Air systems are difficult to control in terms of positioning and force due to seal stiction and overshoot. Furthermore, health and safety regulations state that machines should operate at noise levels of no more than 85dBA (in the case of an operator standing at a distance of one metre from the machine).
Electric actuators are much quieter than their pneumatic counterparts, and so the task of keeping factory noise at satisfactory levels is that much easier to achieve; moreover, you will have a happier workforce and your insurance premiums will be lower, adds Olsen.
Injection moulding is an interesting topic where choice of actuation technology is concerned. Hydraulic rams were likely as not the motion control devices of choice for most of the 20th century, but today, when it comes to creating the precise, rapid motions that are required by modern plastic inject moulding processes, electro-mechanical actuators with integrated servo motors are becoming the technology of choice.
Mould making is about creating voids. Moulds are generally male/female assemblies where the male forms the concave surface of the finished part. Volumetrics, gate design and temperature control are key elements, and mould makers use a host of tools to fill a mould quickly, after which time the mould has to be cycled. The speed with which the core can be moved in and out is critical.
Traditional hydraulic cylinders are fast, but in most applications, an electric actuator can enhance cycle rates significantly by allowing related operations to start sooner. This is because the actuator is part of a controller feedback loop, enabling the core’s precise position to be monitored and controlled in real time. This benefit is enhanced by the greater acceleration and deceleration rates and higher maximum speeds achievable with electric actuators.
Back in 1999, Milacron published a study that discussed the design rationale behind its all-electric injection moulding machines, listing the advantages of digital electro-mechanical actuators over the hydraulic motion control systems that the industry had traditionally used.
These advantages included accuracy and repeatability – as Piers Olsen points out, hydraulic fluid can heat up, hoses can expand, and valves can stick. As these components do not figure in the electric actuator design, digitally controlled electro-mechanical drives are not only more accurate over the long term, but they are also faster. The vast majority of new injection moulding machines are now sold as ‘all electric’, with many existing machines being retrofitted to gain all the benefits.
“Digital control permits operators to set tighter over/under tolerances,” Olsen explains. “Milacron’s customers reported a 50-90 percent power reduction compared with the consumption of machines equipped with hydraulic systems. Not only is the initial cost of oil purchase removed, but so is the ongoing cost for monitoring, disposal, slip hazard, clean-up and all the ensuing environmental implications.
“By completely eliminating the possibility of oil leaks, Milacron found that electric actuation became the obvious choice for medical, electronic and clean room applications. Furthermore, by replacing hydraulics with electro-mechanical actuation, noise levels on Milacron machines dropped by more than 30 percent.”
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