{
  "slug": "position-error-signal",
  "term": "Position error signal (PES)",
  "aliases": [
    "PES"
  ],
  "definition": "In an HDD servo, the position error signal represents the difference between the head's measured position and its target position.",
  "note": "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. [servo-honda]\n\nThe 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. [servo-patent]\n\nPosition 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. [servo-honda] [mechanics]\n\nThe 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. [servo-rro]",
  "status": "shipping",
  "advantages": [],
  "limitations": [],
  "supersedes": [],
  "supersededBy": [],
  "firstSeen": null,
  "lastSeen": null,
  "updated": "2026-10-25",
  "related": [
    "hdd",
    "seek-time",
    "rotational-vibration",
    "embedded-servo"
  ],
  "citations": [
    {
      "id": "servo-honda",
      "title": "Honda: Temperature Dependent Robust Control of Hard Disk Drives (2016), sections 2.2-2.4",
      "url": "https://dept.aem.umn.edu/~SeilerControl/Thesis/2016/Honda_16PhD_TemperatureDependentRobustControlOfHDDs.pdf",
      "accessed": "2026-10-25"
    },
    {
      "id": "servo-patent",
      "title": "Ali / Seagate: Servo wedge skipping during track seeking, US10706882B1 (2020), Figs. 2-4",
      "url": "https://patents.google.com/patent/US10706882B1",
      "accessed": "2026-10-25"
    },
    {
      "id": "servo-rro",
      "title": "Wu and Tomizuka: Novel Schemes for Repeatable Runout Compensation Using Adaptive Feedforward Cancellation, CML report 06012, sections I and IV",
      "url": "https://cml.berkeley.edu/wp-content/uploads/blue-reports/06012.pdf",
      "accessed": "2026-10-25"
    },
    {
      "id": "mechanics",
      "title": "Hesela: HDD mechanical faults, evidence and diagnostic limits",
      "url": "https://hesela.com/analyses/hdd-mechanical-failure-points/#servo-positioning",
      "accessed": "2026-10-25"
    }
  ],
  "provenance": [
    "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."
  ]
}