03-07-2012, 12:03 PM
ROBUST TRACKING CONTROLLER DESIGN WITH APPLICATION TO THE MOTION CONTROL OF AN X-Y FEED TABLE
A ROBUST TRACKING.pdf (Size: 4.18 MB / Downloads: 120)
ABSTRACT
This work is all about design of robust tracking controller of feed drive
for high-speed machining application. In order to increase the material
removing rate in HSM application, not only the spindle speed has to be
increased, but also the feed rate has to be increased. With the increase
feed rate, the robustness of the tracking performance of the feed drive has to be studied. Both traditional cascade PI/P servo motion controller and the modern sliding mode controller are studied.
Introduction
High Speed Machining
The idea of high-speed machining(HSM) originated as early as April 27,
1931, when the German company Friedrich Krupp A.G. was granted a
patent based on the metal cutting studies of Dr. Carl Salomon, who was
able to cut different materials of steel, bronze, copper and aluminium at
cutting speed of 440 m/min, 1600 m/min, 2840 m/min and 16500 m/min,
respectively [54]. Dr. Carl Salomon observed that the chip removal temperature
at the cutting edge starts to decrease when the cutting speed is
5-10 times higher than in conventional machining. Based on this observation,
the original definition of HSM is given as a machining process
with high cutting speed, at which the chip removal temperature starts to
decrease, and thus allows to improve the productivity in machining with
conventional tools [9].
The Machine Tool Structure
Moving from conventional machining to high-speed machining, the static
and dynamic behavior of the machine tool structure have to be reconsidered.
Static and dynamic stiffness have to be high enough so that the
structural deformation is kept small. However, metal structures usually
have low damping at the structure’s natural frequencies, which may deteriorate
the dynamic stiffness when the natural frequencies are excited. It
has been reported recently that the use of composites in the machine tool
structure can achieve the same stiffness while increasing the damping in
the structure [56]. As another solution, some damping actuators can be
used to actively damp the vibrations [12, 13].
High Speed Feed Drive
In general, the achievable control system performance depends on the
performance of sensors and actuators. Rotary and linear encoders for
the position measurement are now commonly used. They usually provide
accurate measurement within a wide bandwidth. However, a slow actuator
can not fully reject fast disturbances, regardless of the control design
method used, and HSM cannot benefit from high material removal rate
without high feed rate in the machining process. The feed drive motion
has to be fast and accurate.
Prospective Mechatronics Approach
Presently, machine tools are built for HSM, but suffer from many problems,
such as structural vibration, chattering, etc., when real high speed
machining processes are practised. To solve these problems, only local
solutions are provided.
When machine tool structures are excited by the cutting process, special
dampers are designed to actively control the excited vibration in order to
keep sufficient dynamic stiffness of the structure. Stability lobes [59] for
each machine tool/cutting system/workpiece combinations are measured
in order to avoid the chattering problem. In most of the time, the spindles
are running at its low speed range. Feed drive controller design becomes
complicated. In order to achieve a higher bandwidth, it has to be designed
taking the machine tool structure into account [72].
Motion Control Architecture
A dSPACE DS1102 DSP controller board is used to implement the
tracking controller for both axes. The controller board combines the high
computing performance of a TI TMS320C31 floating-point DSP with
a set of I/O modules frequently required in control systems. With its 2
incremental encoder interface and 4 12-bit D/A output interface configured
at ±10 V output voltage range, it is possible to read in the digital
encoder position signal of the X- and Y-axis of the feed table, and send
the control command to the motor amplifiers. The motion controller architecture
is presented in Figure 2.2. This structure conforms with the
requirement of an open controller architecture [see 50, 48], by separating
the different motion control functionalities into different levels. The current
controller and the power amplifier are implemented in the ETEL
drive, and the position and tracking controller are implemented with the
dSPACE DSP controller board.