Assembly Techniques - A variety of techniques
can be used during assembly to improve performance. Repeatability has
been shown to increase significantly by switching from a basic installation
procedure to a refined procedure.
The basic installation procedure consists of:
- Gross pre-alignment of coupling surfaces
- Direct application of preload using full torque on preload bolts
- Dry bolts and contact surfaces
The refined procedure includes:
- Gross pre-alignment of coupling surface followed by minor adjustments
of alignment to allow for optimal, non-damaging contact between surfaces.
- Stepped application of preload (ie. 10% - 50% - 100%) in a proper,
consistent pattern. Stepped application controls initial contact of
the interface and reduces the initial static friction in the bolts.
Torque pattern should be designed to prevent asymmetric bending across
the interface and remain consistent through each bolt application.
- Lubricated bolts and contact surfaces to reduce frictional nonrepeatability
on interfaces. Grease is suggested for bolts while spray lubricants
or coatings are suggested for critical interface surfaces to prevent
the introduction of unknown particles.
Calibration - A variety of different calibration
methods can be used to remove the errors caused by many of the disturbance
actions. A basic calibration consists of measurements of the various
coupling features combined with intelligent storage of correction parameters
based on the measurements. Direct measurement of the contact points
on the halves of a kinematic interface can greatly reduce the effect
of tolerance errors on mounting accuracy, with the residual interchangeability
error based only on the error of the measurement procedure. By estimating
the total mounting accuracy of a kinematic coupling as the sum of the
measured repeatability and the simulated interchangeability, interface
manufacturing tolerances and the complexity of the calibration process
can be chosen to satisfy the accuracy requirement at minimum cost. While
past laboratory measurements of kinematic couplings have shown micron-level
repeatability at relatively small scales, in a test application to industrial
robot base and wrist mountings measured interface repeatability is approximately
equal to simulated interchangeability. The interface transformation
has the potential to become a universal kinematic handshake between
kinematically coupled objects, and could enable a conceptually new interface-centric
calibration process for modular machines, whereby:
- Interface halves are pre-assembled and encoded with their coupling
calibration information, relative to their centroidal coordinate frames.
- These calibrated interface halves are attached to machine modules
and the modules are calibrated by mounting the assembly to a reference
mating interface half. The coupling parameters of the reference
interface are known; hence a calibration Tinterface
- When the machine modules are brought to the production installation
site, the production Tinterface is calculated
from the coupling parameters of both production interfaces. A correction
is applied to the machine module calibration for the difference
between the calibration Tinterface and the
production Tinterface. This would allow the
machine to be more accurately programmed off-line.
In production, by making the contact surface measurements ahead of
time, calculation of Tinterface would be a step
of the machine calibration routine. Ideally, the software would take
the measurement values for the components, calculate the interface HTM,
and apply it to the global serial chain of transformations for the machine
kinematics. The pre-measured placements of the contacts could be written
to an identification tag on the interface, or the interface serial number
could serve as a database key to the calibration data.
Thesis for more information on calibration of coupling interfaces.
Iteration to a new design - If acceptable
performance is not acheived using the desired coupling type, further
iteration of the various design parameters should be performed. A small
change in one parameter, such as a surface coating to reduce friction,
may sufficiently improve performance.