Vertex Metrology Stiction
Last Update: JeffMangum - 09 August 2012
Table of Contents:
The Solution (2012-06-18)
Klaus Wilmeroth, as a result of discussions with NRAO, notes the following and suggests a solution:
- ACU CPU2 Software V B1.2.7: This software applies the FS blanking time (4s) if the step size of any of the two axes is less than 1.6 deg on the sky.
- ACU CPU2 Software V B1.2.8: This software applies the FS blanking time (4s) if the step size in azimuth is less than 1.6 deg on the sky, i.e. <1.6°/cos(El). Steps in elevation never trigger the extended blanking time even if identified as Fast Switches. With 1.2.8 several problems were observed. Besides the fact that in some cases with Fast Switches in EL the standard 2s blanking time appeared to be too short, some other unresolved phenomenons were observed. This was never investigated further. The ACU was downgraded to version B1.2.7.
- In order to improve the behavior with blanking times and Fast Switching the following changes are suggested:
- movement is identified as FaSw when the total step size on the sky is less than 1.6 deg, where Total step size = sqrt [ (step_az/cos(el))^2 + step_El^2 ]
- If a movement is identified as Fast Switch 1 submode "Fast Switching" is to be reported 1 the FaSw control loop and profiler parameters are to be used 1 the FaSw accelerations are to be used 1 the extended FaSw blanking time (4s) is to be applied after the movement
- If a position step is not identified as FaSw it is considered a slew (or OTF PM/IM if applicable). The related parameters must be used. The standard acceleration and blanking times apply.
- A new parameter will be added to the ACU GUI: "upper El limit for Fast Switches" [deg]. If a position step begins or ends above this elevation position it is never considered a FaSw, even if criterion a) is met. Such a movement is handled as described in c).
- The ACU software covering the changes defined in 3) shall be based on A1.5.12/B1.2.7 (not B1.2.8!).
The Verification of the Solution on DV23
The ACU software fix noted above was installed at the beginning of pointing acceptance testing on DV23. As no metrology stiction episodes were noted in the offset pointing measurements made with this ACU software upgrade on DV23, it appears that this problem has been solved.