ADR-006: Z-Probe Selection (SuperPINDA vs BLTouch)

Status

Archived - Superseded by ADR-014: Z-Probe Selection

This ADR has been replaced by ADR-014, which provides more comprehensive coverage including BOM implications and implementation details. See 014-z-probe-selection.md.

Context

The Z-probe is the “eyes” of the Tractor’s Triple-Z system. While Klipper’s software intelligence handles the math, the probe provides the physical ground truth required for kinematic leveling and first-layer perfection.

Decision

We adopt a Bimodal Reference Strategy:

  1. Reference Spec (The Tractor Standard): SuperPINDA (Inductive). Chosen for its solid-state reliability and “set-and-forget” robustness.
  2. Sovereign Fallback (The Scavenger Standard): BLTouch v3.1 (Physical). Chosen for its ability to probe any surface, including salvaged glass or non-metallic beds.

🦾 The Case for SuperPINDA (Reference Spec)

  • Solid-State Reliability: With zero moving parts, it cannot snag on a print or fail to “deploy.” It is nearly indestructible.
  • Thermal Stability: Unlike the older PINDA v2, the SuperPINDA is temperature-compensated, ensuring the same trigger height at room temperature or 100°C.
  • Simplicity: Uses only 3 wires (VCC, GND, Signal) compared to the 5 wires required for a BLTouch.
  • Constraint: Requires a conductive metal bed (e.g., Spring Steel, Aluminum).

🛠️ The Case for BLTouch (Sovereign Fallback)

  • Universal Compatibility: The “Swiss Army Knife” of probes. It works on glass, ceramic, wood, or aluminum.
  • Scavenger Hero: Essential for Tier 1 builders using salvaged Ender 3 or i3 Mega glass beds.
  • Trade-off: Higher mechanical complexity. The moving pin can bend or break if it strikes a print or bed clip.

📊 Revised Decision Tree

graph TD
    A[Start: Choose Z-Probe] --> B{Do you have a Steel/Magnetic Bed?}
    B -- YES --> C[Reference Spec: SuperPINDA]
    B -- NO --> D{Using Glass or Non-Metal?}
    D -- YES --> E[Sovereign Fallback: BLTouch]
    D -- NO --> F[Manual Paper Method - Tier 1 Only]
    
    C --> G[Industrial Robustness: No Moving Parts]
    E --> H[Universal Compatibility: Probes Any Surface]

🏗️ Implementation Tweaks

1. Wizard Configuration (configure.py)

The build script should now ask the user: “What is your bed surface?”

  • Glass/Ceramic/Other: Forces the BLTouch configuration in printer.cfg.
  • Steel/Aluminum: Recommends the SuperPINDA but allows the BLTouch as an alternative.

2. Klipper Configuration Permutations

# The script will now output distinct blocks based on the choice:

# [PATH A: INDUCTIVE PROBE]
[probe]
pin: ^board:probe_pin
z_offset: 0.0  # Adjust for SuperPINDA

# [PATH B: BLTOUCH]
[bltouch]
sensor_pin: ^board:sensor_pin
control_pin: board:control_pin
z_offset: 1.5  # Adjust for BLTouch

3. Z-Max “Bed-Drop” Brake

Regardless of the probe chosen, the Z-Max physical microswitch at the bottom of the frame remains mandatory. The probe handles the surgery at the nozzle; the switch handles the safety at the floor.

Consequences

  • Benefits: Builders aren’t forced to buy a new steel sheet if they have glass beds, but are encouraged toward the PINDA for long-term reliability.
  • Documentation Effort: The assembly guide must now provide wiring diagrams for both 3-wire (PINDA) and 5-wire (BLTouch) sensors across all supported MCUs (SKIPR, Spider, etc.).

“A PINDA is a sensor for the machine; a BLTouch is a tool for the material. We provide the iron for the path you choose.”