Table of Contents

Concepts

This page summarizes the core motion concepts used by Struckig.

Online trajectory generation

Struckig computes a valid next motion state every PLC cycle from:

  • current position, velocity, acceleration
  • target position, velocity, acceleration
  • kinematic limits (velocity, acceleration, jerk)

The function block is called cyclically and produces updated values (New* outputs, plus propagated Current* when enabled).

Control interfaces

Struckig supports different control interfaces depending on the machine behavior you need:

  • Position: full state-to-state motion planning
  • Velocity: velocity-profile-centric behavior, useful for controlled stopping or drive-level velocity control loops

In most point-to-point applications, start with Position.

Important behavior:

  • In Velocity interface, position limits are not the primary driver of trajectory generation.
  • Velocity interface is well suited for controlled stop logic and velocity-profile transitions.

Synchronization modes

For multi-axis trajectories, synchronization mode defines how axes coordinate:

  • TimeSync: all axes finish together in time
  • TimeIfNecessarySync: synchronize only when needed for target kinematics
  • Phase: all axes evolve with coordinated phase progression
  • None: each DoF is calculated independently

Phase is often preferred for XY path-following motion.

Motion order

  • Third-order (jerk-limited): set MaxJerk
  • Second-order (acceleration-limited): leave jerk unconstrained

Jerk-limited motion is typically smoother for mechanics and process quality.

Discretization

Struckig supports two discretization modes:

  • Continuous: trajectory duration can take any value
  • Discrete: synchronized duration is aligned to task-cycle multiples

For PLC command generation, Discrete often gives more predictable setpoint handover at cycle boundaries.

Propagation model

With EnableAutoPropagate := TRUE, each cycle automatically uses the newly generated state as the next cycle's current state. This is the default online operation mode.

Set it to FALSE only when you need manual propagation control for offline or custom stepping scenarios.

Manual propagation is done with PassOutputToInput().

Enabled DoFs and per-DoF overrides

Struckig supports per-axis control for complex machines:

  • Enabled[i] lets you disable specific DoFs in a calculation cycle
  • PerDofControlInterface[i] can override global control interface
  • PerDofSynchronization[i] can override global synchronization mode

If a per-DoF override is set to Undefined, the global setting is used.

States and error codes

At runtime, use both state and error methods:

  • State: Busy, Idle, or Error
  • ErrorCode(): machine-readable failure category
  • ErrorMessage(): human-readable reason

Frequent error classes include invalid input values and zero-limits conflicts.

Units and consistency

Struckig is unit-agnostic, but units must remain consistent per axis:

  • position: mm, deg, or similar
  • velocity: position unit per second
  • acceleration: position unit per second squared
  • jerk: position unit per second cubed

Mixing inconsistent units is one of the most common causes of unstable behavior.

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