hesela.devPosition error signal (PES)
In an HDD servo, the position error signal represents the difference between the head's measured position and its target position.
During track following, servo patterns provide a radial offset relative to the selected track. One illustrated encoding compares servo-burst amplitudes; that example is not a requirement for all drives. [1]
The controller uses position error to adjust actuator drive. Seeking moves toward another track, settling reduces the residual motion, and following corrects small positioning deviations during access. [2]
Position error is a control quantity, not a count of damaged sectors. Residual error during feedback operation alone does not establish mechanical damage; head-to-disk clearance is a different physical quantity. [1] [4]
The reference also matters: a servo-written center can differ from an ideal circle because of written-in repeatable runout. A small measured PES does not alone establish an ideal circular trajectory. [3]
Relations
Citations
- Honda: Temperature Dependent Robust Control of Hard Disk Drives (2016), sections 2.2-2.4
- Ali / Seagate: Servo wedge skipping during track seeking, US10706882B1 (2020), Figs. 2-4
- Wu and Tomizuka: Novel Schemes for Repeatable Runout Compensation Using Adaptive Feedforward Cancellation, CML report 06012, sections I and IV
- Hesela: HDD mechanical faults, evidence and diagnostic limits
Sources & caveats
- This definition concerns HDD positioning, not all uses of PES in other fields. The cited sources supply no universal diagnostic threshold or conversion from a SMART counter to displacement.