# 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).
The plots show the 60 seconds of motion:

• 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 velocity.

• 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 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.

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.

`processing: x101/010506/doit.pro`
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