Training — Control Systems

Servo Tuning Strategy

Advanced Time: 25 min Type: Technique Focus: Controls / Motion
After this module: Loop-by-loop servo commissioning from mechanical validation through current, velocity, and position loop tuning — including resonance detection, notch filters, and feedforward.
Prerequisites: Control Loop Architectures

Purpose

Servo tuning is the process of adjusting control loop gains so a servo axis tracks its commanded position, velocity, or torque accurately and stably under real operating conditions. Autotune provides a starting point, but it assumes ideal mechanical conditions and validates against no-load behavior. Most servo problems encountered in practice — oscillation under load, resonance, instability on direction change — are not solvable by adjusting gains alone; they require mechanical understanding and loop-level diagnostics.

Control Architecture

A servo drive implements nested control loops:

Position loop (outer)
    ↓
Velocity loop (middle)
    ↓
Current / Torque loop (inner)

The critical rule: inner loops must be stable and well-tuned before outer loops are commissioned. A poorly tuned current loop makes the velocity loop unstable. A poorly tuned velocity loop makes the position loop unstable. You cannot compensate for inner loop problems by adjusting outer loop gains.

Step 1 — Mechanical Validation

Tuning cannot fix mechanical problems. Validate the mechanical system before touching gains.

Check What to look for
Backlash Free play before resistance when reversing by hand; spike in following error on direction change
Coupling Loose set screws, worn flexible coupling, cracked spider insert
Alignment Angular or parallel shaft misalignment — causes vibration and bearing wear
Stiffness Behavior that is stable in free motion but oscillates under load — low structural stiffness
Inertia ratio Load-to-motor inertia ratio; rule of thumb is ≤ 5:1 for standard tuning; higher ratios require feedforward
Bearing friction Binding or stick-slip under low-speed moves

A symptom that only appears under load (squeal, oscillation, instability) is a mechanical or resonance problem until proven otherwise — not a tuning problem.

Step 2 — Current Loop

The current loop controls motor torque. It runs at the highest bandwidth (typically 1–5 kHz) and is usually pre-tuned by the drive manufacturer.

Validation (not tuning from scratch):

  1. Disable velocity and position loops
  2. Apply a step current command (10–20% rated)
  3. Observe: fast rise, minimal overshoot, no oscillation
Symptom Likely cause
Slow rise Kp too low
High-frequency oscillation Kp too high or motor parameters wrong
Low-frequency drift Ki too high
Noisy signal Encoder grounding or shielding issue

The current loop bandwidth must be significantly higher than the velocity loop bandwidth (10× or more). This separation is what allows the outer loops to function.

Step 3 — Velocity Loop

The velocity loop controls speed and compensates for load inertia and friction.

Tuning procedure:

  1. Disable position loop; keep current loop active
  2. Command velocity steps at representative speeds
  3. Increase Kp until response is fast with slight oscillation beginning
  4. Back off ~20%
  5. Add Ki to remove steady-state error — add slowly; too much Ki causes low-frequency hunting

Monitor both velocity response and the current signal simultaneously. If velocity oscillates while current spikes sharply, the cause is mechanical resonance, not gain instability.

Symptom Cause
Sluggish response Kp too low
Overshoot then oscillation Kp too high
Hunting around setpoint Ki too high
Instability only on direction change Backlash
Instability only under load Stiffness / resonance

Step 4 — Position Loop

The position loop controls final position accuracy.

Tuning procedure:

  1. Keep velocity loop stable first
  2. Increase Kp (position gain) until following error is acceptable without oscillation
  3. Add velocity feedforward (Kvff) to reduce following error during motion — this improves tracking without increasing instability risk
  4. Add acceleration feedforward (Kaff) for demanding profiles (high acceleration, press applications)

Following error is the key metric: the difference between commanded and actual position during motion. High following error means the loop is lagging; oscillation around the target means gains are too aggressive.

Step 5 — Validate Under Real Load

This is where most commissioning fails. Always validate:

Systems with variable loads (press applications, clamping, cutting) may require gain scheduling: different gain sets for different operating modes (travel vs. loaded contact).

Resonance Detection and Notch Filters

Mechanical systems have natural frequencies. If control loop gains excite a natural frequency, the result is resonance: a high-frequency oscillation or squeal that cannot be resolved by reducing gains globally.

Detection:

Notch filter:

Key Tuning Metrics

Metric Definition
Rise time Time from step command to first crossing of target
Settling time Time until response stays within tolerance band
Overshoot % How far above target the response goes
Following error Command − actual position during motion
Gain margin How much more gain the loop could take before instability
Phase margin Stability reserve in degrees — target ≥ 45°

Engineering Takeaways

Trust Boundary — Engineering Judgment Required

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