Electric drives are precise and flexible – ideal when the forces need to be developed quickly and positions need to be approached precisely. For these types of application, electric drives are invariably the first choice. Accuracy in the region of 0.001mm, constant speed, good controllability, defined travel profiles and rigidity under load are qualities that are difficult to ignore. On the downside, they need to be cooled, particularly when moving heavy loads.
Large loads, on the other hand, are generally not a problem for pneumatic drives. Furthermore, they are inexpensive, unaffected by difficult environmental factors such as dust, oil, water or cleaning agents, they are long-lasting and easy both to install and operate. Used with appropriate sensors and valve technology, pneumatic drives can also be positioned. However the precision of positioning is at least a couple of orders of magnitude less than that of electric drives, at around 0.1mm for the best systems.
Electrics versus pneumatics?
It is important that the user chooses the correct drive technology for the application, and that the selected components do their job reliably, regardless of whether they are electric, pneumatic or a combination of both. If you do go the hybrid route, it can be advantageous if you are able to obtain all the components from a single source, as they are likely to be mechanically compatible. In this case, we’re not talking ‘electrics versus pneumatics’ so much as ‘electrics and pneumatics’ – modular mechatronic systems that use all-pneumatic or all-electric handling axes or a mixture of the two.
While pneumatics is certainly at the core of the company’s business, in recent years, Festo has added considerably to its portfolio of electrically driven mechanical handling axes, and this combination of offerings opens up many possibilities for customers seeking to reduce their systems design and construction effort. For example, take Festo’s electromechanical handling axis component, the HME, which offers a repetition accuracy of ±0.015mm - an ideal choice for joining work to close tolerances.
The German machine builder, Manfred Merklinger Werkzeug & Maschinenbau GmbH, has integrated this axis into a machine in which tiny springs are fitted to a miniature motor, an application that makes good use of the HME’s accuracy and speed. Precision machines like this can be designed at modest cost, using components like the HME, with its backlash-free ball-bearing guide and high-precision linear motor drive. The axis combines the linear motor with a displacement encoder, a reference sensor and a precision heavy-duty guide, all of which are integrated into a single space-saving package.
With appropriate sensors and valve technology, pneumatics can deliver closed-loop drive technology. Servo-pneumatics, as an alternative to the familiar types of pneumatic and electric drives, permits movement of a pneumatic cylinder with closed-loop speed and position control. With regard to dynamic positioning, Festo insists that servo-pneumatic drives are every bit as good as electric drives. A typical system consists of a pneumatic cylinder with an integrated displacement encoder, a proportional valve in place of a simple switching valve, and a position controller such as the Festo SPC200.
The friendly robotic milking machine
A milking machine built by the Dutch dairy equipment specialist, Lely Industries NV (www.lely.com) makes full use of servo-pneumatics. The idea behind this milking robot is that the cows themselves decide whether to be milked or not. If they do wish to be milked, they enter a box in which the milking robot is able, by means of a transponder, to detect the cow’s specific data such as weight and size. It can also detect whether the cow has already been milked a short time before. The robot arm, a mechanism with three pneumatic cylinders, operates flexibly thanks to the use of servo-pneumatics, thus ensuring that the cow is not injured. A laser sensor also monitors the movement of the cow to make sure that the robot arm locates its udders precisely without injury.
Festo’s modular handling system allows users to exploit the individual strengths of matched electric and servo-pneumatic drives. The development trend is more and more towards combination drives in which pneumatics and electrics work hand in hand.
Festo believes that the drive of the future will be a hybrid drive in which both pneumatic and electric drives are integrated into a cylinder and work in parallel on the same handling task. This hybrid drive would combine the advantages of pneumatics – robustness, durability, low cost plus easy installation and operation – with the accuracy and dynamic performance of electric drives. Together, the two technologies could create a high-performance drive that operates quickly and with high precision in confined spaces while transporting large loads.