# 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](https://docs.source-robotics.com/PAROL-docs/1.1/assets/trap_with_lines.webp){ width="700" }

![Position with speed-only tracking and with speed-and-position tracking: the second reaches the target exactly](https://docs.source-robotics.com/PAROL-docs/1.1/assets/trap_position_ex.webp){ width="700" }

![The same with a polynomial profile](https://docs.source-robotics.com/PAROL-docs/1.1/assets/poly_position.PNG){ width="700" }

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](https://docs.source-robotics.com/PAROL-docs/1.1/theory/homing-and-mastering/)).