Feature Request: Linear Encoder / Scale Feedback for Closed-Loop CNC Control

[manisaranbala]

Hi Terje,

Thank you for developing ioSender — it’s a clean and powerful sender that pairs perfectly with grblHAL.

I would like to request support for a software-based closed-loop feedback system using linear scales / linear encoders.

Feature Request

• Read linear scale feedback for X, Y, and Z axes.
• Use this feedback in software to:
• Compare commanded position vs actual position
• Detect missed steps, backlash, or mechanical drift
• Apply real-time correction (closed-loop behavior), or raise alarms when error exceeds a configurable limit
• Closed-loop logic implemented in software, without requiring external servo drives.
• Optional per-axis enable/disable to maintain full backward compatibility with open-loop systems.

Motivation

Many CNC retrofits use stepper motors with external linear scales for high accuracy. A software-based closed-loop approach would:

• Convert traditional open-loop CNC machines into true closed-loop systems
• Improve positioning accuracy and reliability
• Reduce dependence on expensive servo/drive hardware
• Expand grblHAL’s suitability for industrial and precision CNC retrofits

Possible Implementation Direction

• Linear scale data could be acquired via available hardware interfaces and routed through AUX inputs.
• Feedback can be processed as:
• Error monitoring + alarms (initial stage)
• Closed-loop correction using PID or proportional correction (advanced stage)

Even partial support (feedback monitoring and error reporting) would be extremely valuable as a first step.

I understand the complexity involved and appreciate any guidance on feasibility or future plans. I’d be happy to help with testing or validation if needed.

Thank you for your time and for maintaining such an outstanding project.

Best regards,
Veeramani

[terjeio]

This cannot be implemented in ioSender, it has to be added in the controller.