Wind blows azimuth arm with brakes on.
06 may, 2001
On 6 may 2001 the azimuth was parked at az=214.4
degrees and the dome was at 19 degrees za with motors off and the brakes
on. At 7:40:49 the azimuth started moving do to the wind. The wind was
directly out of the south and the maximum velocity measured on jon hagen's
wind monitor was 40 mph. The azimuth continued moving till az=280 degrees.
It reached velocities of up to 1.84 degrees/second (about 5 times slew
speed). The maximum acceleration was .14 deg/sec^2 (about 40% above the
acceleration limit imposed by the computer). The operators began moving
the dome downhill at azimuth 258 degrees. The dome was at 18 degrees by
the time the azimuth stopped moving. The azimuth rail had been greased
the previous week.
The wind was out of the south and was relatively constant (it
wasn't just gusts). If we assume that the wind was constant and that the
two sides of the azimuth arm (dome,ch) present equal wind loads, then the
net force on the azimuth is the differential wind load presented by the
carriage house and the dome. The maximum force during this time would
be at azimuth = 270 degrees and gregorian = 19 degrees za (largest moment
arm). The relative force on the azimuth would be proportional to:
cos(270-az)*sin(gr za)/sin(gr at 19 Degrees).
show the 60 seconds of motion:
The cable were in no danger of breaking since the azimuth was on wrap 1.
If the dome had made it around to 360 degrees then it would have oscillated
about this position. If the azimuth had been on wrap 2 this would have
broken the cables if it got to 720 degrees.
Figure 1 plots azimuth position versus time in black. The red shows the
dome position during this time (the top red line is 19 degrees and the
bottom one is 18 degrees). The blue line shows the azimuth velocity during
the motion (the dashed blue line is the slew rate : .4 degrees/second).
The green line is the azimuth acceleration (the dashed green line is the
maximum legal acceleration of .1 degrees/sec^2). You can see that the azimuth
started slowing down at azimuth of 240 degrees.
Figure 2 plots the azimuth acceleration versus azimuth position (in black).
The dashed black lines are the maximum negative and positive accelerations
allowed (by the computer). The blue line is a scaled version of the azimuth
The azimuth wheels have flanges that ride down over the rails. These
flanges occasionally rub against the rails creating added friction.
On 26 apr01 an azimuth spin was done at .4 degrees/second with the dome
at 18 degrees. The red line is the sum of the 8 motor currents/torques
(in some arbitrary linear units). It shows the work the motors did to move
the azimuth at constant velocity.
The lower plot is a blowup of the upper one. The azimuth
started accelerating at az=214.4 and then it started to decelerate
at az=215. This corresponds the the large spike in the motor currents around
az=215. There is a valley in the motor currents around az=229 degrees.
Around this region the azimuth started to accelerate again. It looks like
the flange rubbing on the rails was controlling the azimuth acceleration
(along with the wind).
Figure 3 plots the azimuth acceleration versus the computed relative force
on the azimuth (assuming constant wind velocity). The green and red lines
are the data points where the dome started moving down hill. Moving left
to right on the plot is while the azimuth approached az=270 degrees. As
the dome starts to move down hill the deceleration actually slows a bit.
This corresponds to the low motor currents needed around 248 degrees. It
then picks up around az=252 when the flange resistance increases.
The azimuth stopped moving probably because of the variation of the
wind and the resistance of the wheel flanges on the rails. Bringing the
dome down 2 degrees made little difference.
During high winds we should probably park the dome at low zenith angle.
It appears that it is more important to park the azimuth in a location
of high flange/rail resistance than low zenith angle.
The grease applied to the rails benefits the wear of the wheels/rails
but it does cause slippage on the rails and increases the danger of the
azimuth getting blown by the wind.