Unresponsive Antenna Watchdog Algorithms

When drive is applied to the antenna, the antenna usually accelerates. Failure of the antenna to accelerate can be because of one or more of the following reasons.

• Servo amplifier failure or servo amplifier turned off
• RCP8 drive output failure
• Drive cable failure
• Catastrophic gear failure of the antenna drive
• Obstacle impeding the antenna motion, such as a person, a ladder, or a stow pin inadvertently left in the antenna

With the exception of the servo amplifiers being turned off, any of these events warrants an antenna shutdown. However, if the antenna scans at its equilibrium velocity, the output drive does not cause the antenna to accelerate since it is just balancing against frictional losses. This must be taken into account to avoid false alarms.

The unresponsive antenna algorithm is based on a linear model of the antenna velocity, with a constant moment of inertia, and frictional losses that are proportional to velocity. Under this model, the expected change in velocity can be calculated by numerical integration. The expected change is then compared to the actual change in velocity.

The following AXIS setup parameters for this algorithm are defined as:

• Permissible Tach Prediction Error - 15 Tach units
• Maximum duration of such error - 2 seconds
• Moment of inertia - 4.00 Drive/Tach units

The moment of inertia is computed when the antenna accelerates and is shown in an alt display in the Control and Monitoring menu. You can then enter a representative value entered in the setup.

The algorithm performs a numerical integration, over the previous 2.5 seconds, to obtain the expected change in velocity (in Tach units.) If the difference between the expected velocity and the current velocity exceeds the Permissible Tach error for a period greater than the Maximum duration, the watchdog forces an antenna shutdown.