lead
to
changes in the temperature profile in the machine.
This may cause unequal thermal expansion rates in a
thermal loop, due
to
different thermal time constants
of
machine components, (see section
5.5).
Hence
Donaldson strongly recommends, as a principle in his
publication about Machine Tools [Donaldson, 19801 to
take the heat out at the source. Wetzels experienced a
thermo-mechanical stability problem with an integrated
heat source. After having removed this source thermal
drift was reduced with one order [Wetzels, 19981.
Internal heat disturbances have responses that can be
middled and compensated partially (See section
6).
However environmental temperature variations can only
be responsive and not predictive, because the input is
unknown [de Bra, 19981.
3.4
Structural
loop
According
to
[ANSI, 19921 a structural loop is defined as
'an assembly of mechanical components, which maintain
relative position between specified objects. A typical pair
of specified objects is a cutting
tool
and a workpiece: the
structural loop includes the spindle shaft, the bearings and
housing, the slide ways and frame, the drives, and the tool
and work-holding fixtures'. All mechanical components
and joints in the propagation path from the drive
to
the
point of reaction, e.g. the end-effector (cutting
tool
or
probe) or the centre of gravity, must have a high stiffness
to
avoid deformations under changing load. The design of
a machine or an instrument contains one or more
structural loops.
Essential in recognising good structural loop design is the
split-up in series and parallel paths. Along a series path
stiffness should not change
to
abruptly. Improvement of
the series path
is
possible by stiffening the most compliant
part preferably by 'transferring' material from more rigid
parts. Parallel path improvement is in contrary done by
improving the stiffest part further, preferably -for a system
with equal mass- at the cost of more compliant parallel
paths.
Due
to
physical limitations, the measuring system of a
closed loop system is unavoidably located at a certain
distance from the end effector. In addition
to
good
structural loop design, the path between measuring
system and end-effector has to be as stiff as possible to
minimise deviations, e.g. by minimising the length of this
path, called the 'measuring circle' [Kunzmann, 19961.
3.5
Metrology frame
A metrology frame is a reference frame for displacement-
measurements, independent
of
the machine base, i.e. the
external forces upon the metrology system must be
constant [Bryan, 1979bl. DeBra suggests to see the
metrology frame as an example of a broader principle, i.e.
the principle of 'separate functions' [DeBra, 19981. In fact
the routes for force and position information are
disconnected, an idea which is present as well in the
design of the rotary table of fig. 5.8 [Philips, 19941.
In
[Teague, 1989-1 9971 historical applications of the
'metrology frame' are discussed,
to
overcome the problem
of deformation of machine parts. The first example of a
metrology frame found
so
far is on the Rogers-Bond
Universal Comparator [Rogers, 18831. More recent
examples are found at NlST in measuring machines by
Hocken et al, at NPL in interferometric time standard
comparators, at LLNL
in
the 'Ultimat' CMM [Bryan,
1979b], the 84" SPDTM [Bryan, 1979al and the LODTM
[Donaldson,
19801, and at Cranfield Precision in the
Nanocentre by McKeown et al (see fig. 3.8) and
Wills-
Moren [Wills-Moren, 19821 and [Wills-Moren, 19891.
hydrostatic
-r
workhead spindle
\
Metrology frame
Gal
es
attached
to
z-carriage
Fig 3.8: Nanocentre, using a Zerodup metrology frame
for complying with Abbe in two axes and direct
measurement of horizontal straightness error
motion.
reague, 1989-19971 advises to make the metrology
frame as small as possible,
to
minimise environmental
influences. [Bryan, 1979bl suggests either
to
build the
metrology frame
in
zerodur or
to
use temperature control
(e.g. an oil shower). Furthermore the plane of supports
of
the metrology frame should coincide with the bending
neutral axis of the machine base.
3.6
Drive
offset
By combining sound mechanical design with closed loop
control, enhanced operating speed, accuracy and
flexibility of movement can be achieved. Typical examples
are the Compact Disc player, the waferstepper, advanced
CNC milling and turning machines and fast component
mounting (FCM) machines. In the development of servo-
controlled positioning devices it is essential
to
consider
the place where the actuator force is loading the slide to
supply the (resulting) force generated by inertia,
tool-
or
measuring forces, friction, etc. As stated in one of the
'Eleven Principles' [McKeown, 1986, 1987, 19971, drives
should be placed to operate through the axes of reaction.
If this is
not possible, the deviation from the axis of
reaction -called drive offset- induces moments on the
machine slides. The effect of resulting rotations of a slide
on
it's
controllability is minimised, if both the drive- and
measurement-axis are at the same side of the centre of
rotation [Rankers, 19971.
3.7
Force compensation
3.7.1
Weight compensation
In many 3D-CMM's vertical rams are used. In order
to
avoid loading the vertical slide's driving system with the
(constant) weight of the ram, force compensation might be
applied, thereby eliminating undesired heat dissipation in
the motor. Constant forces can be obtained in many ways,
e.g. using additional weights, which is unfavourable from
a dynamic point of view. Alternatives are magnetic fields,
cylinders under pressure or vacuum, or 'constant rate
springs', e.g. tensators [Tensator, 19971, [Rosielle, 19981.
Depending on the stroke of the slide and the
specifications on admissible force variations, one type of
562