Configure the azimuth overlap.

Arguments:

SwitchState turn1_state

The state that the hardware overlap switch bit has when the azimuth axis is on its first turn (0 or 1).

Double axis_start_az

A wrap-extended topocentric azimuth slightly negative of the most negative physical azimuth limit of the telescope. Note that this should be a bit more negative than the most negative azimuth limit applied with the axis_limits command.

Double switch_start_az

A wrap-extended topocentric azimuth slightly negative of the most negative edge of the small azimuthal region within which the overlap switch changes state. This must be more positive than the preceding axis_start_az argument.

Double switch_end_az

A wrap-extended topocentric azimuth slightly positive of the most positive clockwise edge of the small azimuthal region within which the overlap switch changes state. This must be more positive than the preceding switch_start_az argument.

Double axis_end_az

A wrap-extended topocentric azimuth slightly positive of the most positive physical azimuth limit of the telescope. This must be more positive than the preceding switch_end_az argument, and should be a bit more positive than the most positive azimuth limit applied with the axis_limits command.

Note that since the telescope can turn through more than 360 degrees, the control system employs a continuous "wrap-extended" azimuthal coordinate system that has a range of more than 360 degrees. The above arguments must be specified in these units, and each argument must contain a more positive value than the argument that it follows. In general, axis_start_az will be negative, and axis_end_az positive.

Example 1:

Consider the following example from the OVRO 40m telescope:

  azimuth_overlap on, -100, -5, 1, 340

The first argument of this command tells the control system that while the azimuth axis is on its first turn, the hardware bit that indicates the state of the overlap switch is 1.

The 2nd argument indicates that when the azimuth axis is turned in the anticlockwise direction (looking down from above), then the furthest anticlockwise that it can physically ever reach is a few degrees clockwise of -100 degrees in topocentric azimuth.

Similarly, the 5th argument indicates that whenever the azimuth axis is turned in the clockwise direction (looking down from above), then the furthest clockwise that it can physically ever reach is a few degrees anticlockwise of 340 degrees topocentric azimuth.

Note that the above two limits define an angular range of 440 degrees that, turning clockwise, starts from -100 degrees topocentric azimuth, passes through 0 degrees topocentric azimuth, and then continues clockwise for almost another full turn, to end at 340 degrees topocentric azimuth.

When turning the azimuth axis clockwise from its anticlockwise limit, the 3rd argument says that the overlap switch always remains in its first-turn state until a few degrees clockwise of -5.0 degrees topocentric azimuth.

Similarly, when turning the azimuth axis anticlockwise from its clockwise limit, the 4th argument says that the overlap switch always remains in its second-turn state until a few degrees anticlockwise of 1.0 degrees topocentric azimuth.

Note that the above two limits define a 6 degree region of azimuth within which the azimuth overlap switch changes state, starting from -5 degrees azimuth, and moving clockwise past 0 degrees azimuth to 1.0 degrees azimuth.

Context: For the sake of discussion, let motion towards more positive azimuths be refered to as clockwise motion. Also let the anticlockwise edge of any azimuthal region to be refered to as the start of that region, and its clockwise edge be refered to as the end of the region.

The azimuth axis can continuously rotate through a bit more than one turn. Thus, whereas topocentric azimuths run from 0 to 360 and then jump back to zero, the control system implements a continuous azimuthal coordinate system whose azimuths can go from negative angles to angles that exceed 360 degrees. These coordinates are refered to as wrap-extended azimuths.

Since the azimuth axis can rotate through a bit more than one turn, there is an overlap region of topocentric azimuth within which the value of the wrap-extended azimuth can have two different values, separated by 360 degrees, depending on which turn the azimuth axis is on. Within this region the state of the azimuth overlap switch indicates which turn the axis is on.

The first turn of the azimuth axis is defined to go from the start of the wrap-extended azimuth range to the start of the second turn. The second turn continues from there to the end of the wrap-extended azimuth range.

The start of the second turn is defined to be at the start of a small region called the overlap-switch transition region. This is the angular region within which the overlap-switch is expected to change state. The width of this region must be configured to be a bit larger than the maximum hysteresis in the position at which the overlap switch changes state. For the OVRO 40m telescope the overlap switch changes state between -2.5 and -1.8 degrees of topocentric azimuth. So in the example above, the transition region is configured above to run from -5 to 1 degrees.

The overlap-switch transition region must not extend so far clockwise of the start of the second turn, that it encroaches on the start of the first turn. Assuming that this requirement is met, then when the topocentric azimuth is within the transition region, the control system can conclude that the azimuth axis is unambiguously on its second turn, regardless of the state of the overlap switch.

Between the topocentric angles of the end of the overlap-switch transition region, and the start of the first turn, the azimuth axis is also unambiguously on its second turn, since the first turn can't turn that far anticlockwise. So if the overlap switch is found to be in its first-turn state within this range of topocentric azimuths, then an overlap-switch error is signaled.

Similarly, if the overlap switch is found to be in its second-turn state when the topocentric azimuth is clockwise of the end of the second turn but anticlockwise of the start of the second turn, then an overlap switch error is signaled.

Within the overlap-switch transition region, the overlap switch can legitimately be in either state, since the exact angle at which it changes state within this region is not known. So within this region, no checks are performed on the overlap switch.

In the overlap region between the topocentric angles of the start of the first turn, and the overlapping end of the second turn, the overlap-switch can also legitimately be in either state at startup, since the axis can be on either turn within this region. However after startup, for as long the azimuth remains within the overlap region, then the overlap switch state shouldn't change from one servo cycle to the next, since that would involve rotating 360 degrees in less than a second. So the control system checks for illegal changes of its state from one servo cycle to the next, and signals an overlap-switch error if this happens. In practice, it doesn't only do this when the azimuth is within the overlap region, but also whenever it is outside the overlap-switch transition region, since the switch is only expected to change state within that region.