This purple robot hovers in place until Trace gets close to it. When alerted to Trace's presence, it hovers at a distance above trace, firing both its turrets rapidly at him at about 4 shots each per second. It has a purple vertical sensor eye, two turrets and a segmented tail hanging under it. It has a tendency to get stuck in corners, therefore a good strategy is to attempt to trap it and use weapons that can go around or through terrain to damage it while blocking it's turret fire which does a whopping 500 damage to Trace if he doesn't have a coat upgrade.
When corrupted, the Purple Sentry moves at 1/2 its normal speed. It will appear in a similar spot on screen in an adjacent room when Trace goes through a door. This is integral to reach Power Node Fragments 11 & 12 as well as Note 06 in Mar-Uru. Demonstration video here.