Q: I am involved in the design of closed loop control systems and typically specify electromechanical drives. I have been told that new hydraulic systems can now provide clear advantages over traditional drives. Is this the case and what are their advantages?
A: Peter Buxton – fluid power manager
New developments in the form of robust, highly dynamic servo solenoid valves with high signal resolution, used in conjunction with modern CNC systems are now creating opportunities for hydraulics in this field. Indeed, when combined with closed loop control technology, hydraulic drives can provide solutions with clear advantages over electromechanical alternatives.
Hydraulic axes are superior to their traditional counterparts when it comes to reliability, servicing, noise and energy storage.
In terms of reliability over an operational life, an electromechanical drive depends heavily on the performance of the linear guides. An electrical drive with ball and screw spindle can also suffer problems as the system is sensitive to impact and the life of a ball and screw spindle is limited. A closed loop hydraulic system is very robust and insensitive to impact, providing overload protection through pressure relief valves. Cylinder seals and valve metering notches have a long life and the operator is warned of wear.
Service costs must also be taken into account. The repair and replacement of electromechanical drives (and the ball and screw system) is time consuming. With the hydraulic system, however, fault diagnosis, basic repair and replacement of valves and cylinders is relatively simple.
Hydraulic drives are also suited to high natural frequencies combined with small masses and strokes. For several years these systems have been used with success in machine tools such as rotary cycle machines, special purpose machines and multi spindle lathes. Over 5000 Bosch hydraulic feed axes with closed loop control are currently in use.
The guidance for the linear motions of machine slides and tables is effected with as little friction and as much precision as possible by means of hydrodynamic and hydrostatic slideways or via roller slideways. A certain degree of friction may be extremely useful in dampening vibrations. However, excessive friction, especially abrupt transition between static and sliding friction, has a negative effect on the control action result and the stability of the control loop
The cylinder represents the simplest form of linear motor and is easily integrated in the machine guidance system. In most cases it takes the form of a one-sided piston rod. Decisive criteria are the surface quality of the cylinder and rod, and the seal rings and guides (low friction, servo quality).
This is the actuator in the closed control loop and makes up the electro-hydraulic converter. The valve converts the electrical signals steplessly into a hydraulic flow rate. Its quality is defined by static and dynamic characteristics such as zero overlap, hysteresis, limit frequency, and so on.
The hydraulic system also provides energy storage, enabling sudden demand spikes to be smoothed out and providing a rapid traverse in a differential circuit. A reduction in the size of the installed power source is achieved, whereas in other systems, peak demands must be catered for when installing the power source.
With increasingly strict health and safety requirements in the workplace, a reduction in noise pollution is an important issue. All systems generate a level of noise, from either the linear guides and ball screws or servo motors. Hydraulics generate noise by the flow in the valve or from the power supply unit, but simple measures can be taken to reduce this level.
Q: I work for a US owned manufacturing company that encourages US specifications in fieldbus applications. In view of last year’s rejection of the international standard (IEC 6158), is international harmony ever likely to happen in this often overlooked area of manufacturing?
A: David Fray – PLC systems product manager
To answer your question, let us first give an update on the recent history of fieldbus systems. Fieldbus networks allow pieces of control equipment to work together, improve flexibility and reduce the amount of wiring needed in factory automation.
Bus systems, as they are called, are a key area of competitive advantage for many manufacturers and can give users the edge over their competitors. While many potential users of bus systems have been put off by the bewildering profusion of alternatives, choosing to ignore them could prove costly.
Back in October, the Fieldbus Foundation (backed by the US providers) lobbied for the adoption of a new standard, which was rejected by supporters of the European equivalent, EN 50170. This standard, which would have been withdrawn if the IEC bid had succeeded, is closely linked to several European protocols such as Profibus.
While the battle between Profibus and Fieldbus appears to have subsided for the time being, it may be new technology and not diplomacy that solves the problems of harmonisation.
The Ethernet has traditionally been too slow and unreliable to handle control networks, but in recent years has improved to the extent that many commentators believe it could shift manufacturers away from proprietary protocols. This development, along with the growing use of PC-based industrial controls could move the debate about fieldbus harmonisation onto a different level.
While these developments are taking place on a global level, it should be stressed that the issue for most companies isn’t `what system should we choose?’, but `why aren’t we using the technology available to bring about benefits now?’.
However, it’s important to consider that if you take the equipment from the same manufacturer who designs its equipment to interface using open not proprietary standards, it could avoid the need for a company to feel at risk investing in one system or another.