RTC
®
5 PC Interface Board
Rev. 1.9 e
8 Advanced Functions for Scan Head and Laser Control
199
innovators for industry
8.11 Automatic Self-Calibration
8.11.1 Using for Drift Compensation
Long-term repeatability is very important in many
applications, e.g. for rapid prototyping in which the
processing operation can span several hours. For such
laser applications, the galvanometer scanner’s long-
term drift and temperature drift, which manifest as a
shift (offset drift) and increase or decrease in the size
(gain drift) of the working image field, can exceed the
allowable tolerances.
In such applications, it’s helpful to start up the appli-
cation only after the scanners have reached their
operating temperature. In addition, the magnitudes
of environmental fluctuations (e.g. operating
temperature changes to which the scan system is
exposed) should be kept as small as possible and the
scan system preferably operated with a constant
load.
For higher long-term repeatability requirements,
SCANLAB scan systems can be (optionally for
apertures
10 mm) equipped with an additional
internal sensor system for automatic self-calibration
(ASC sensor system, Home-In sensors). Together with
associated commands furnished by the RTC
®
5, this
reference system makes it possible to calibrate the
position detectors of the galvanometer scanners at
any desired time. Thus, the effects of gain and offset
drift can be reliably compensated and positioning
accuracy is maintainable over long periods of time.
Remaining long-term drift effects are the same order
of magnitude as short-term repeatability.
8.11.2 How it Works
The
command starts a measurement
routine for determining the exact control values for
reference positions (Home-In positions) defined by
the internal sensor system. For this purpose, the
RTC
®
5 drives the galvanometer scanners to the
appropriate positions within the scanning range.
For drift
measurement
, the routine should be
executed
• when setting up the equipment (to determine
reference values for the Home-In positions) and
• during an application’s execution at appropriate
time intervals (to determine if and how the
Home-In positions have changed).
To
compensate
the drift, appropriate gain and offset
values can be calculated and set separately for each
galvanometer scanner based on the determined devi-
ations between the current Home-In position and the
reference value. All subsequent vector or arc
commands for any points within the image field will
then be corrected (transformed) in accordance with
these new gain and offset values.
The RTC
®
5 provides commands for automatic self-
calibration as well as customer-specific calibration.
For automatic self-calibration, the RTC
®
5 automati-
cally determines and sets the gain and offset values.
Notes
• Prior to performing a measurement routine, you
Command
= 4) to check if the
attached scan system in fact has an internal
sensor system for automatic self-calibration
(Home-In sensors) and if the sensor system is
functioning properly. This ASC hardware check
also occurs automatically via
(
Command
= 0) and sometimes for
(
Command
= 1 and 3) (also see
command description and
).
• During execution of the measurement routine
determining the Home-In positions, the laser
should be switched off and no other commands
will be transferred to the scan system.
• For Gain/Offset Correction:
Control values corrected via the applied image
field correction table (*.CT5) are multiplied by the
currently set gain value and then additively
modified by the offset value. The final positions
output to the galvanometer scanners are
automatically clipped to the allowable range.
