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Target, commanded and actual

Three positions exist at every moment, and they differ.

  • The target is where you asked the robot to go: the end of the move.
  • The commanded position is where the planner says the robot should be right now, on the trajectory it computed from the profile, sent to the control board in this cycle.
  • The actual position is where the motors are, as the control board counts their steps: what rbt.angles() and the commander's display show.

Why commanded and actual differ

The board receives a speed and a position every 10 ms and runs the stepper at that speed until the next packet. Two things pull the actual position away from the commanded one:

  • The PC is not a real-time machine; the packets are 10 ms apart on average, not exactly, and a late packet lets the motor run a little too long or short.
  • A stepper cannot make the very small speed changes at the start and end of a smooth profile exactly; it rounds them.

If the board followed the speed alone, the errors would add up over a long move and the robot would miss the target by a few steps. So the planners use the go-to-position command (156), in which the board runs each joint at the average of the commanded speed and the speed that would close the position error within one cycle: the speed curve gives the smoothness, the position curve pulls the error back to zero every cycle, and the robot arrives exactly.

Commanded (blue) and actual (red) speed on a trapezoidal move; the robot spends a third of the time in each phase
Figure 1: Commanded (blue) and actual (red) speed on a trapezoidal move; the robot spends a third of the time in each phase.
Position with speed-only tracking and with speed-and-position tracking: the second reaches the target exactly
Figure 2: Position with speed-only tracking and with speed-and-position tracking: the second reaches the target exactly.
The same with a polynomial profile
Figure 3: The same with a polynomial profile.

The commander's RBTscript exposes the choice: a move with speed as its last argument tracks the speed curve only, which is smoother and quieter and may stop short of the target; without it, speed and position are tracked.

What the software reports

The commander shows the actual joint angles and the actual tool pose; the Python API's status stream carries the actual angles and speeds, plus executing_index and completed_index for where the queue is. "Actual" means "counted steps": a lost step moves the real arm away from all three without any of them noticing (Homing and mastering).

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