Axii for describing motion of the GBT subreflector are illustrated in Figure 1. Xs and Ys lie in the antenna plane of symmetry, and arrows indicate positive direction of motion. Zs is perpendicular to Xs and Ys, and positive is toward the viewer. The servo system displays subreflector translations X, Y, Z corresponding to Xs, Ys, Zs directions. Tilts Yt and Zt are about the Ys and Zs axii. Tilt Xt is about the Xn axis, moving the beam with constant elevation. Tilt directions are according to the right-hand rule (right thumb pointed in positive direction along the axis then fingers indicate the direction of positive rotation).
Figure 1
The subreflector is positioned by six linear actuators; the commanded and indicated length of each can be displayed on the OCT or Cleo screens. Each actuator has software limits as listed in Table 1. Hardware limits are at least 0.3 in. outside the software limits. Reset of an activated hardware limit must be done manually at the subreflector. In addition to these linear limits, there are "delta limits" given in Table 2 restricting the difference in lengths of certain groups of actuators.
Without violating any of these limits, the subreflector can be positioned anywhere within a "box" defined by:
-9.49 < X < +9.49 in.
-23.63 < Y < +11.63 in.
-0.83 < Z < +0.83 in.
-1.5 < Xt < +1.5 degrees
-0.5 < Zt < +0.5 degrees
Note that it is possible to move the subreflector to certain regions outside this box without violating any of the actuator limits, but proper caution should be used.
|
Actuator |
Retracted Limit (in) |
Extended Limit (in) |
|
Y1 |
98.64 |
135.69 |
|
Y2 |
97.88 |
136.57 |
|
Y3 |
97.88 |
136.57 |
|
X1 |
114.24 |
144.85 |
|
X2 |
114.24 |
144.85 |
|
Z |
56.70 |
76.11 |
|
Actuators |
Delta Limits |
|
Y1, Y2, Y3 |
Warning at |Delta| = 2.6 "S/R A-B Delta Warning" Warning at |Delta| = 3.6 "S/R A-B Delta Limit" Disables at |Delta| = 5 ± 0.5 * "Delta A-B" |
|
X1, X2 |
Warning at |Delta| = 1.4 "S/R A-B Delta Warning" Warning at |Delta| = 2.5 "S/R A-B Delta Limit" Disables at |Delta| = 5 ± 0.5 * "Delta A-B" |
|
Y2, Z |
Warning at Y2-Z > 63 or Y2-Z < 38 "S/R A-B Delta Warning" Warning at Y2-Z > 64 or Y2-Z < 37 "S/R A-B Delta Limit" Disables at Y2-Z > 64.5 ± 0.5* or Y2-Z < 36.5 ± 0.5* "Delta A-B" |
* Requires manual reset at subreflector.
Prepared by: Roger Norrod, NRAO, Green Bank
Date: 29 JAN, 2001