ADR-014: Z-Probe Selection (SuperPINDA vs BLTouch)

Status

Accepted

Context

The Amalgam requires a Z-probe for homing and bed-leveling. The choice depends heavily on the bed surface material:

Two Probe Technologies: 1. BLTouch: Physical Hall Effect probe with moving pin that touches bed 2. SuperPINDA: Inductive proximity sensor that detects metal without contact

Bed Surface Constraints: - Inductive Probes: Only work on conductive metal (magnetic spring steel sheets like MK52) - Physical Probes: Work on any surface (glass, spring steel, wood, carbon fiber, plain aluminum)

Engineering Philosophy: - “Tractor” Goal: Set-and-forget reliability with minimal maintenance - “Scavenger” Goal: Universal compatibility with repurposed beds

This creates a decision matrix based on both bed surface and reliability requirements.

Decision

We adopt a bed-surface-dependent probe strategy with SuperPINDA as Reference Spec and BLTouch as Scavenger Fallback.

Probe Options

Option A: SuperPINDA (Reference Spec) - Technology: Inductive proximity sensor (solid-state) - Bed Requirement: Magnetic spring steel sheet (MK52 or compatible) - Mounting: Fixed position on X-carriage or gantry - Wiring: 3 wires (VCC, Ground, Signal) - Reliability: Maximum - no moving parts, resin-encased, nearly indestructible - Cost: ~$30-40 AUD - Best For: Tier 3 Reference Spec builds with MK52 bed

Option B: BLTouch V3.1 (Scavenger Fallback) - Technology: Physical Hall Effect probe with deployable pin - Bed Requirement: Any surface (glass, carbon fiber, aluminum, spring steel) - Mounting: Fixed position on X-carriage or gantry - Wiring: 5 wires (VCC, Ground, Signal, Control Servo) - Reliability: Moderate - moving pin can snap or bend if it hits print/clip - Cost: ~$50-60 AUD (authentic), ~$15-25 AUD (3DTouch clone) - Best For: Tier 1-2 builds with scavenged beds (glass, etc.)

Technical Comparison

Feature SuperPINDA (Inductive) BLTouch (Physical)
Bed Surface Metal only (conductive) Universal (any material)
Accuracy Extremely high, repeatable Extremely high, but can drift if pin bent
Robustness Maximum (solid-state, no moving parts) Moderate (pin can snap/bend)
Complexity Simple (3 wires, always-on) Moderate (5 wires, deploy/retract logic)
Wear Items None Pin, springs, servo
Thermal Drift Minimal (modern SuperPINDA) None (mechanical contact)
Calibration Z-offset once Z-offset once, pin deployment verification

Why SuperPINDA for Reference Spec?

1. Solid-State Reliability - Zero moving parts to fail - Immune to “pin-snag” failures during high-speed travel - Resin-encased design is nearly indestructible - No deployment mechanism to jam or misfire

2. Thermal Stability - Older inductive probes drifted with bed temperature - PINDA V2 added thermistor to compensate (0.02mm adjustments) - SuperPINDA uses higher-quality components that don’t drift - Trigger height identical at 20°C or 60°C chamber temperature

3. “Set-and-Forget” Philosophy - Once calibrated, requires no ongoing maintenance - No pin to straighten or replace - No servo mechanism to lubricate or adjust - Perfect for 1000+ day maintenance intervals

4. Simplicity - 3-wire connection (vs 5 wires for BLTouch) - No deployment/retract logic in Klipper - Always-on proximity sensing - Lower failure surface area

Why BLTouch for Scavenger Builds?

1. Universal Compatibility - Works on any bed surface (glass, carbon fiber, plain aluminum) - Only option for salvaged Ender 3, i3 Mega, Anet A8 beds - Allows flexibility in bed choice during Tier 1 builds - Doesn’t force MK52 bed purchase

2. Well-Understood Technology - Years of community documentation and troubleshooting - Wide availability of clones and spare parts - Proven track record in millions of printers

3. Bed-Surface Flexibility - User can experiment with different bed surfaces - No commitment to magnetic spring steel upfront - Universal “key” for uncertain scavenger inventory

Build Wizard Logic

Configuration Wizard → Z-Probe Selection
├─ Question: "What is your bed surface material?"
│  ├─ Glass / Carbon Fiber / Plain Aluminum
│  │  └─ Force: BLTouch (inductive cannot see)
│  ├─ Magnetic Spring Steel (MK52 or compatible)
│  │  └─ Recommend: SuperPINDA ★ Reference Spec
│  │     └─ Option: BLTouch (if preferred)
│  └─ Unknown / Not sure
│     └─ Default: BLTouch (universal fallback)

Bed-Probe Compatibility Matrix

Bed Surface SuperPINDA BLTouch Recommended
Magnetic Spring Steel (MK52) ✅ Works ✅ Works SuperPINDA ★
Plain Aluminum ✅ Works ✅ Works BLTouch (easier)
Glass ❌ Won’t detect ✅ Works BLTouch only
Carbon Fiber ❌ Won’t detect ✅ Works BLTouch only
PEI on Glass ❌ Won’t detect ✅ Works BLTouch only
BuildTak on Aluminum ❌ Won’t detect ✅ Works BLTouch only

Consequences

Benefits

  • Tiered Flexibility: Reference path (SuperPINDA) and Scavenger path (BLTouch) both supported
  • Bed-Surface Logic: Clear decision matrix based on bed material
  • Reliability Optimization: Reference spec maximizes solid-state reliability
  • Universal Fallback: BLTouch works with any scavenged bed
  • Cost Awareness: Clones available for low-budget builds

Trade-offs

  • SuperPINDA: Locked to metal beds, no flexibility in surface choice
  • BLTouch: Higher failure rate (moving parts), pin maintenance required
  • Cross-Compatibility: Cannot easily switch probes without changing bed
  • Clone Quality: BLTouch clones vary in reliability (3DTouch vs authentic)

What This Enables

  • Tier 3: SuperPINDA + MK52 bed = maximum reliability (reference spec)
  • Tier 1-2: BLTouch + scavenged glass bed = universal compatibility
  • All Tiers: Z-tilt calibration and mesh bed leveling (both probes support)

What This Replaces

  • Single-probe assumption (no longer viable for mixed bed inventory)
  • Glass-bed + inductive probe combinations (won’t work)
  • Universal probe recommendation (bed-surface dependent)

BOM Implications (Generic)

Tier 3: Reference Spec (SuperPINDA + MK52)

  • Parts needed:
    • 1x SuperPINDA probe (Prusa or high-quality clone)
    • 1x MK52 magnetic spring steel sheet (250x250mm)
    • 1x Magnetic build surface (under spring steel)
    • 1x Probe mounting bracket
    • 3x Female-to-female dupont cables (VCC, GND, Signal)
  • Cost implication: Low-Medium (~$50-70 AUD for probe + bed)
  • Bed compatibility: Metal only (spring steel required)
  • Reliability: Maximum (solid-state, no moving parts)
  • Maintenance: None (set-and-forget)

Tier 1-2: Scavenger Fallback (BLTouch + Glass)

  • Parts needed:
    • 1x BLTouch V3.1 (authentic) or 3DTouch clone
    • 1x Scavenged bed (glass, etc.)
    • 1x Probe mounting bracket
    • 5x Female-to-female dupont cables (VCC, GND, Signal, Control, Servo)
  • Cost implication: Low (~$15-60 AUD depending on authenticity)
  • Bed compatibility: Universal (any surface)
  • Reliability: Moderate (moving parts, potential pin failure)
  • Maintenance: Periodic pin inspection, occasional replacement

Hybrid Option: BLTouch on Metal Bed

  • Use Case: User has metal bed but prefers BLTouch familiarity
  • Cost implication: Same as BLTouch + metal bed (~$50-70 AUD)
  • Trade-off: Loses SuperPINDA reliability advantages
  • Recommended: Only if user has strong BLTouch experience or spare parts

Universal Requirements (All Options)

  • Z-Max Switch: Required regardless of probe choice (see ADR-013)
  • Mounting Bracket: 3D-printed bracket to secure probe to X-carriage
  • Klipper Configuration: [probe] or [bltouch] section based on choice
  • Z-Offset Calibration: Required for both probes (probe to nozzle distance)

Implementation Notes

SuperPINDA Klipper Configuration

[probe]
pin: ^PC2    # Example pin, adjust for board
x_offset: -10
y_offset: 20
z_offset: 1.6   # Calibrated via PROBE_CALIBRATE
samples: 3
sample_retract_dist: 2.0
speed: 5.0

[bed_mesh]
mesh_min: 30,30
mesh_max: 220,220
probe_count: 3,3

BLTouch Klipper Configuration

[bltouch]
sensor_pin: ^PC2
control_pin: PA2
x_offset: -10
y_offset: 20
z_offset: 1.6
stow_on_each_sample: False
probe_with_touch_mode: True

[safe_z_home]
home_xy_position: 150,150
speed: 50
z_hop: 10

Probe Calibration Procedure

# Home all axes
G28

# Calibrate Z-offset
PROBE_CALIBRATE

# Test repeatability
PROBE_ACCURACY

# (BLTouch only) Test pin deployment
BLTOUCH_DEBUG COMMAND=pin_down
BLTOUCH_DEBUG COMMAND=pin_up

Mounting Considerations

SuperPINDA: - Mount ~5mm above bed surface when deployed - Ensure bed is flat (spring steel on magnetic surface) - No need for retraction mechanism - Secure firmly (no vibration or movement)

BLTouch: - Mount so pin deploys ~2-3mm above bed - Test pin deployment before first print - Verify pin retraction doesn’t collide with bed clips - Check for pin bending after print failures

Troubleshooting

SuperPINDA: - Issue: Probe doesn’t trigger - Check bed is metal (spring steel) - Verify probe is within 4mm of bed surface - Test with multimeter (check signal pin toggles) - Issue: Inconsistent readings - Clean bed surface (metal debris can affect inductive) - Check wiring for loose connections - Verify probe mounting is secure

BLTouch: - Issue: Pin doesn’t deploy - Check servo control wiring - Verify [bltouch] section in config - Test pin manually with BLTOUCH_DEBUG commands - Issue: Pin stuck in deployed position - Emergency retract: BLTOUCH_DEBUG COMMAND=reset - Check for debris or bent pin - May need to disassemble and clean

Safety Considerations

  • BLTouch Pin Breakage: Can occur if nozzle crashes into bed or print curls up
  • SuperPINDA Distance: Too far from bed = won’t trigger; too close = premature trigger
  • Wiring: Use shielded cables to prevent EMI interference
  • Hotend Temperature: BLTouch pin can deform near hot nozzle (ensure proper mounting)

References

  • docs/reference/ai-conversations/pinda-v-bltouch.md: Complete probe comparison
  • docs/adr/013-drivers-endstops.md: Driver and endstop strategy
  • docs/adr/005-triple-z.md: Triple-Z kinematic leveling
  • Klipper Probe Guide: Probe Bed Calibrate
  • Klipper BLTouch Guide: BLTouch
  • Prusa SuperPINDA Documentation: Prusa Knowledge Base

Evolution Notes

This ADR establishes SuperPINDA as the Reference Spec probe for maximum reliability. Future probe technologies will be evaluated against: solid-state reliability, bed-surface compatibility, thermal stability, and cost. BLTouch remains the universal fallback for scavenger builds with non-metal beds.

Emerging Technologies: - LVDT sensors (high-precision, expensive) - Capacitive sensors (work on some non-metal surfaces, experimental) - Optical sensors (complex, not widely adopted)

Current consensus favors inductive (SuperPINDA) for metal beds and physical (BLTouch) for universal compatibility.