SITCOMTN-176

M2Hexapod - Strut oscillation analysis

This is a draft.

By:

• Bruno C. Quint
• Nacho Sevilla

Abstract

There are several events and failure modes on M2Hex. Even after replacing one of the struts. This technote compiles the analysis focused on these issues.

Introduction

The M2 Hexapod has been experiencing seemingly random faults which require the system to be reset. In the worst cases, these obstruct regular observations during the night.

With the substitution of one problematic motor strut, the problems have mostly subsided, but there are occasional failures with unidentified causes. Some of the most salient cases during November and December 2025 are summarized in this technote. Despite this fact not being a stopper anymore, this report summarizes the state of M2Hex failures for future reference and as a starting point in case further occurrences of these problems need tackling.

Related tickets

• SITCOM-2045: Support investigation on M2Hex recurrent faults

• OBS-1103: M2 Hexapod - Multiple Low-Level Controller Faults

• OBS-1373: MTHexapod.2 faulted due to low level controller issues, nans in mount telemetries

• OBS-1400: M2 Hexpod fault when we move the TMA.

Test details and results

We have examined the period after the engineering shutdown in September-October 2025. from 24 October 2025 to 15 December 2025, where an intervention replaced one of the M2Hex struts. We have selected those events logged with error code 1 in MTHexapod.logevent_errorCode corresponding to CONTROLLER_FAULT. With this selection, we identify 20 faults in said period.

Of particular interest are those faults which accumulate on 13/11 (see https://rubinobs.atlassian.net/browse/OBS-1373) and 20/11, as they correspond to repeated occurrences within 1 hour during observations (as identified by the time of occurrence and comments in night logs).

We analyzed the currents going through the strut motors (MTHexapod.electrical.motorCurrent) in the minutes prior to the failure, finding an erratic behavior, somewhat correlated between them. The telemetry corresponding to the events on November 13 are shown in the following plots, in each case showing the motor currents about one minute prior to the fault.

In this particular case, OBS-1373 indicates that simultaneous movements of O(1000 um) were requested in more than one direction, which is indicated as ‘not healthy’ for the hardware. A corrective measure on STRUT_MAX_ACCELERATION/VELOCITY was applied.

However, in 20/11, a similar case was found during a ‘TMA movement’ according to the night log (OBS-1400), with the appearance of a low frequency oscillation in motor 3, possibly due to a loose bolt. A similar fault as in 13/11 happened in the morning.

Other periods show occasional similar deviations of the currents. A follow up study would be to identify any unique characteristics for those deviations that happen prior to the faults. There is not a clear dominant frequency for the currents in this period prior the fault.

Conclusions

The current status of M2Hex triggers an error every few days, in the last two months it has interrupted observations twice. Prior each fault case, the currents going through the motors experienced large variations, a few seconds before the fault, with some signs of correlation.