Purifier

Purifier is PreciCore's signal processing node. It sits between raw operator input and the control system, applying a Kalman filter to convert noisy, tremor-affected movement into stable, clinically precise commands.

Why filter input?

Physiological hand tremor in a surgeon's hands oscillates at 8–12 Hz. In the context of corneal surgery — where tolerances are measured in micrometres — even small tremor causes tip displacements that exceed safe operating margins.

Purifier doesn't just smooth the signal. It estimates the intended position of the operator's hand and publishes that estimate, not the raw measurement. The result is that the robotic arm responds to deliberate movements while ignoring involuntary oscillation.

Signal flow

input/raw  →  Kalman Filter  →  input/clean

Purifier subscribes to input/raw, processes each message through the filter, and immediately publishes the filtered result to input/clean. The latency introduced is one tick of the filter loop — typically under 1 ms.

How the Kalman filter works

The filter maintains two state vectors:

• Predicted state — where the operator's hand should be based on the previous position and a motion model
• Updated state — the predicted state corrected by the new measurement

At each step, the filter computes a Kalman gain that balances trust between the prediction and the measurement. When measurements are noisy (high measurement noise), the filter trusts the model more. When the model is uncertain (high process noise), it trusts measurements more.

Parameter	Effect of increasing	Effect of decreasing
process_noise	More responsive, less smoothing	More smoothing, more lag
measurement_noise	Heavier filtering, more lag	Less filtering, noisier output

Configuration

Purifier is configured via environment variables or a YAML config file:

purifier:
  process_noise: 0.01      # motion model uncertainty
  measurement_noise: 0.1   # sensor noise estimate
  initial_state_covariance: 1.0

Start with the defaults. If the arm feels sluggish, lower measurement_noise. If it feels jittery, raise it.

Subscribed / published topics

Direction	Topic	Type	Description
Subscribes	input/raw	ControlMsg or IMUMsg	Raw operator commands from any input node
Publishes	input/clean	ControlMsg	Filtered, tremor-reduced commands

Purifier accepts both ControlMsg (from keyboard/Xbox) and IMUMsg (from iPhone IMU) on input/raw. It normalises both into the same ControlMsg format before publishing.

Links

• GitHub

See also

• Input nodes — upstream sources of input/raw
• Kinematics Engine — downstream consumer of input/clean
• Glossary: Kalman Filter — conceptual explanation
• Glossary: Tremor — why filtering is necessary