GE BrightSpeed: collimator blades position erratically
What happened
The collimator did not move and position reliably, and the first suspicion in the case was a failing encoder. The reported trouble pointed instead toward the stepper drive side.
What was checked
- Power the unit up and watch the drive shafts turn to see where the motion stumbles or stops.
- Swap the suspect parts between collimator channels A and B to localize the fault.
- Inspect the standoffs that mount the control board on the stepper motor; loosened standoffs can break board contact.
Cause
Reported in the case: on a similar unit the engineers found the stepper motor driver at fault, localized by swapping parts between channels. They also named a second probable cause — degraded contact between the control board and the stepper motor through its mounting standoffs, which lets the motor skip steps.
The cause is probable and was not fully confirmed.
What was done
The fix reported in the case: replace the stepper motor driver, or, where the fault is board contact, re-seat the board and carefully crimp the standoffs so they grip it firmly again. The outcome was reported as probable, not confirmed on every unit.
Safety
Isolate the scanner from mains power and allow stored energy to discharge before touching the drive, and because the collimator sets the X-ray beam width, complete the manufacturer's radiation and electrical safety checks before returning the unit to service.
Lesson: Before blaming the encoder on a stepper-driven collimator, swap parts between channels and check the board mounting standoffs, since a weak driver or loose board contact can mimic an encoder fault.
Covered in the course: CT Service — Level 2: Fault-Finding →
Written from an anonymised case shared by service engineers. It is not a manufacturer instruction and grants no authorisation. Independent note; names are trademarks of their owners.