Background:

Originally, this safing sequence was written to protect the wax actuator power supplies from remaining on too long while performing door opening operations. It is possible to overheat and rupture a thin membrane in the actuator which would send contaminants into the telescope. However, somehow the watchpoint logic for the wax actuator current was modified from 0.75 amps to a value of 0.5 amps, which is the nominal value when opening the doors. It was decided to leave this watchpoint alone and simply turn off the actionpoint when opening the doors. At any other time besides the door opening operation, this action point will remain active to prevent any inadvertent enabling of the door actuators. Backup RTS commands will be used to protect the wax actuators from overheating when opening the doors, turning off all doors after two minutes.

RTS48: Wax actuator high temperature response

WATCHPOINTS sampled:

Instrument Door 1 Actuator (inside)

Instrument Door 2 Actuator (inside)

Instrument Door 3 Actuator (outside)

Wax Actuator Current

ACTIONPOINT logic:

If ( ITTETH12 > 50 degrees C OR

ITTETH13 > 50 degrees C OR

ITTEPR04 > 50 degrees C OR

Wax Actuator Current > 0.50 Amps (all for 2 or more consecutive samples) )

Execute WAX_TEMP

file: wax_temp.atf

00:00:00 /PSWAXACTPWR OFF

00:00:00 /ICPWD1PD

00:00:00 /ICPWD1SD

00:00:00 /ICPWD2PD

00:00:00 /ICPWD2SD

00:00:00 /ICPWD3PD

00:00:00 /ICPWD3SD

Key Telemetry:

ITTETH12 > 50 degrees C OR

ITTETH13 > 50 degrees C OR

ITTEPR04 > 50 degrees C OR

Wax Actuator Current > 0.50

Instrument: Wax Actuator High Temperature or OverCurrent Sensed

Value #1: The Actionpoint number that failed its limit.

Value #2: The RTS number that was requested.

Value #3: The number of consecutive failures that triggered this Actionpoint

Value #4: Unused.

Recovery Procedure:

Notify EOF of event and find out how the doors were inadvertently turned on. No additional FOT commanding action is necessary since it is desired to have the doors off at all times.

References:

TRACE C&DH FSW User’s Guide

http://tracedata.nascom.nasa.gov/~trace/cdhsw/trace/design/sc/pre_rts.htm