axis_limits/hard
az_margin, el_margin, {limit_regions}
Specify the limits of the telescope axes.
/hard
Double az_margin
Double el_margin
listof AxisLimits regions_min
axis_limits { {-90.0, 11.5, 90.5}, {155.0, 7.0, 90.5}, {205.0, 11.5, 90.5}, {335.0, 0.0, 0.0} }Note that the above example sets the soft axis limits. Alternatively, if the
axis_limits
command
had been specified as axis_limits/hard
then it would
have set the hard limits. The meanings of soft and hard limits
are described below.
axis_limits
command can be used to tell the
control system either the soft axis limits, beyond which it must
not attempt to position the axes, or the emergency hard limits,
beyond which it should apply the brakes if the physical limit
switches fail.
The soft limits must be inside the limits that are enforced by limit switches, and they must be far enough within these limits, that the axis can be brought to a halt on crossing a soft limit, before triggering the corresponding physical limit switch.
The hard limits must be outside the limits that are enforced by the physical limit switches. Their purpose is to tell the control system when to abort a hardware mediated stow or service command, in the case that the limit switch that normally ends one of these operations, fails.
The total azimuth range of the telescope covers more than a turn of the axis. So the azimuth limits must be expressed as wrap-extended azimuths.
The axis_limits
command allows different elevation
limits to be specified for different azimuthal regions. To do
this, the axis_limits
command takes a list of limit
regions that are contiguous in azimuth. For each region in the
list, the starting azimuth of that region is specified, along
with the lower and upper elevation limit of that region. The
ending azimuth of each region is set by the starting azimuth of
the next region in the list. The final region must be terminated
by specifying an extra dummy region whose starting azimuth
denotes the most positive wrap-extended azimuth that the
telescope can reach, and whose arbitrary elevation limits are
simply discarded.
When the telescope is halted at the azimuth at which one region
ends and another starts, even the slightest wind or other
disturbance can cause the azimuth to move rapidly back and forth
between azimuths of the two neighboring limit regions. To prevent
this from causing rapid changes in the effective elevation
limits, which could then cause rapid changes in the clipped
target elevation and potentially destructive instability, a
hysteresis margin is used. This specifies the minimum azimuthal
distance that the azimuth must move into a new limit region,
before the control system concludes that the new region has been
definitively entered. This margin, whose size is one of the
arguments of the axis_limits
command, must be
greater than the maximum expected jitter in the azimuth, caused
by wind etc.
When the elevation is found to be at or beyond an elevation
limit, the azimuth drive is halted until the elevation has been
raised back inside the limit. Thus to allow continued azimuth
motion while the elevation is being constrained by an elevation
limit, the control system attempts to always keep the elevation
at least a specified margin width from either elevation
limit. This margin, whose size is one of the arguments of
the axis_limits
command, must be greater than the
maximum expected jitter in elevation, caused by wind etc, such
that these disturbances don't sporadically push the elevation
below the limit and briefly stop azimuth motion.