scope:

Description of Driving Event:

During functional test of the Qualification Model of the Mobile Transporter (MT) Linear Drive Unit (LDU), mounted on the Functional Test Fixture (FTF) a power surge followed by a facility power outage occurred. A sequence of events began that resulted in two incidents.

Incident #1: When power was restored, the Special Test Equipment (STE) console and data acquisition systems (computers) were restarted. For the STE, the operator selected the appropriate program to power up the fixture motors to warm them up, in preparation for the start of the Fixture Characterization Procedure. Unexpectedly the X-axis motor started, raising the carriage until the screw securing the load cell contacted the lower end of the motor ball screw, causing the flexible coupling to be sheared off. Investigating of the uncommanded movement revealed that several Test Console electrical/electronic components were defective, most likely as a result of the power surge. The fixture motor controller PMAC card, the X-axis motor servo amplifier and the Y-axis motor relay were removed and replaced. The flexible coupling was also removed and replaced.

Incident #2: Under the assumption that the motor was still disconnected from the X-axis ball screw, the system check-out was started. The motor drove the X-axis to the lower end of travel at which point the kill switch was activated to disable the unit. During the uncontrolled motion the LDU qualification unit bogie wheels contacted the lower flange of the test rail and became loaded (no damage found, only superficial scratch) but damage was incurred by the STE X-axis load cell and related hardware.

Root cause:

Incident #1:

Primary Cause: Failure to properly check out the electronic equipment following a known power surge/power outage. Contributing Causes:

1. The ball screw mechanism was not disconnected from the actuator (motor) to guard against inadvertent/unintended motion.

2. An Uninterruptible Power Supply was not installed.

Incident #2:

Primary Cause: The safety of personnel and hardware was jeopardized when two different groups worked on the hardware simultaneously and without proper coordination. Contributing Factor: It should have been confirmed first that the motor was still disconnected from the test fixture before the motor system checkout was performed .