According to Bosch Rexroth, a 'third revolution' is currently taking
place in hydraulics. Thanks to the combination of tried-and-tested
mechanics, rapid actuator technology and high-resolution sensors,
electrohydraulic motion axes with fieldbus capability attain the highest
levels of accuracy and dynamics. Recent developments now allow these axes
to be mixed in a 'plug-and-run' manner with electromechanical axes
The precision and dynamic performance of modern electrohydraulic motion
axes can be demonstrated by examples from the machine tool industry. In
the case of nibbling machines, these axes can move loads of 30 tonnes
with up to 1,500 double strokes per minute, in engrave mode even 4,000
double strokes per minute can be achieved. In aluminium diecasting
machines, electrohydraulic axes can decelerate from 600m/min to
standstill within 30 to 40ms. And depending on the position measuring
system, accuracies of 1um are now commonplace. But it is not only the
performance that has improved. These axes can be incorporated into modern
control architectures with considerable ease, along with all the
advantages of transparent data access.
Rexroth has developed software modules that automatically compensate for
the non-linearities of hydraulic systems. The algorithms run in the
background, following a one-off calibration, without any manual
intervention on the part of the operator, who simply enters the
parameters, such as position, speed or force via the control panel.
Whether the relevant axis is driven electromechanically or
electrohydraulically is of no interest to the operator, and there should
be no perceptible difference in the quality of performance of the axes,
whether they are electrically or hydraulically driven. To support this
concept, Rexroth has just launched its HNC100 electrohydraulic
controller, which is claimed to be the only device of its type in the
world that controls both electrical and hydraulic drives via a Sercos
interface. As far as machine manufacturers are concerned, this
development means that they can now easily integrate mixed drive
technology into their machine designs even without any in-depth knowledge
of hydraulics. The HNC100, which can control from one to 32 axes,
features position, force, speed and pressure controls, multi-axis
synchronisation and interpolation between two or more axes.
With the development of new temperature and vibration-resistant
microcontrollers, axis control functions can now be implemented directly
into fluid actuators, such as proportional servo valves or variable
displacement pumps. In Rexroth's new integrated axis controllers, IAC-P
and IAC-R, the closed loop control electronics are fully contained within
the valve. These compact, intelligent drive units are already being used
for feed axes in revolving CNC machine tools as well as in paper
manufacture. The IAC-P valve, for example, which has integrated
controller, sensors and fieldbus connectivity, is currently being used to
compensate for sag in large paper machine calender rollers. In closed
loop mode with an incremental measuring system, an IAC valve is capable
of +/-1um positioning accuracy.
Electrohydraulic axes can be easily integrated into standard commercial
PLC, motion and CNC controls. In the case of PLC applications, where only
parameters and start time are transmitted, sequential control takes place
in the drive electronics. In addition to standard commands for compiling
NC programs, the control electronics supports special NC commands that
are matched to the hydraulics. The necessary programs are compiled
offline and then loaded. This programming tool is available on the
Internet as a free download and updates can be loaded online. In addition
to this, Rexroth can supply application-specific functions to aid the
installation and commissioning of, for example, presses and injec