Dithering and Apparent Dithering: A Study of Apparent Thermostat Anomalies on Flight Missions

摘要

Multiple instances of apparent rapid cycling of thermostats from Mars Exploration Rover (MER), Mars Science Laboratory (MSL) Curiosity, to the Soil Moisture Active Passive (SMAP) observatory are analyzed to see how they differ from classic dithering that leads to failure. Additional analysis suggests that only two of the five cases involved an actual thermostat anomaly. Two cases involved implementation of the thermostat into the flight system (outside of the thermostat) that led to apparent rapid cycling, but in which both the thermostat and the thermal design were fine. The other three cases involve dithering in which the thermostat appears to continue working for many cycles. This paper reviews these five instances, explains the signature of a dithering thermostat, the cause of the apparent signatures, the probable causes of dithering in thermostats, and the discusses the probable root cause in the SMAP thermostat failure during system testing.