Liteplacer vacuum sensor

LitePlacer Vacuum Sensor

Small analog vacuum-sensor board for a pick-and-place machine (LitePlacer-style, Smoothieboard v1.1 + OpenPnP 2.6). It measures the vacuum at the nozzle so OpenPnP can check that a part was picked (Part On) and released (Part Off).

• Sensor: NXP/Freescale MPXV6115VC6U (−115…0 kPa, 5 V, ratiometric)
• Buffer: Microchip MCP6001UT-I/OT rail-to-rail op-amp powered from the Smoothie's 3.3 V, so the output can never exceed the ADC range
• Output: 0.2…3.05 V into a Smoothieboard thermistor input (P0.23 / T0), read with M105
• Board: 56 × 20 mm, 2 layers, 1.6 mm, M5 hole to mount it at the nozzle holder, 2 × M3 holes

Full design notes, calculations, bring-up, Smoothieware and OpenPnP configuration are in vacuum-sensor.md.

PCB

Top	Bottom

The 3D renders show only the footprint pads for U1 (sensor) and J1 (terminal block), because those parts have no 3D model in the project.

Schematic

Also available as SVG and PDF.

How it works

MPXV6115V (5 V)  ──  R3 51k / R1 100k divider (×0.662)  ──  MCP6001U follower (3.3 V)  ──  R2 100 Ω  ──  Smoothie P0.23
4.6 V at atmosphere      + C3 33 pF, C4 22 nF (~210 Hz low-pass)                                          (T0 thermistor input)
0.2 V at full vacuum

• The sensor output reaches 4.6 V (up to ~4.8 V worst case), so the divider brings it down to ~3.05 V.
• The follower drives the Smoothie input with low impedance, so the board's 4.7 kΩ thermistor pull-up doesn't load the divider.
• Because the op-amp runs from the Smoothie's own 3.3 V, the ADC pin can't be over-driven.
• R2 isolates the op-amp output from the capacitance on the Smoothie input (prevents oscillation).

Lower reading = more vacuum.

Connector J1

⚠ The Smoothie's VBB / 3.3V / GND connector also carries VBB (12–24 V motor supply). A wire on the wrong pin destroys the op-amp and probably the LPC1769. Check with a meter before powering up.

Bill of materials

Reference designators here follow the KiCad design. vacuum-sensor.md uses the numbering from the original hand-wired prototype (there R1 = 51k, R2 = 100k, R3 = 100 Ω).

Expected output

Sensor supply 5.0 V, reading from Smoothieware ad8495 channel (reading = V / 0.005):

* The Smoothie's 4.7 kΩ pull-up and R2 add a small linear offset; OpenPnP only compares readings, so it doesn't matter.

Firmware and OpenPnP (short version)

Smoothieware config:

temperature_control.vacuum.enable          true
temperature_control.vacuum.sensor          ad8495
temperature_control.vacuum.ad8495_pin      0.23
temperature_control.vacuum.ad8495_offset   0
temperature_control.vacuum.designator      V
temperature_control.vacuum.heater_pin      nc
temperature_control.vacuum.max_temp        1000      # must be above 660, or Smoothie HALTs

OpenPnP actuator (Double): read command M105, read regex .*V:(?<Value>-?\d+\.?\d*).*, set as the nozzle's Vacuum Sense Actuator.

The air line needs a restriction between the constantly running pump and the nozzle/sensor tee, otherwise the reading is the same with the tip open or blocked. See §8b of vacuum-sensor.md for measured values and starting part-detection thresholds.

Files

Regenerating the images

kicad-cli pcb render -o images/pcb-top.png    --side top    --width 1600 --height 640 --quality high --zoom 1.9 liteplacer_vacuum_sensor.kicad_pcb
kicad-cli pcb render -o images/pcb-bottom.png --side bottom --width 1600 --height 640 --quality high --zoom 1.9 liteplacer_vacuum_sensor.kicad_pcb
kicad-cli pcb render -o images/pcb-iso.png    --side top --rotate '-40,0,30' --perspective --width 1600 --height 1000 --quality high --zoom 1.1 liteplacer_vacuum_sensor.kicad_pcb
kicad-cli sch export svg -o images --exclude-drawing-sheet liteplacer_vacuum_sensor.kicad_sch && mv images/liteplacer_vacuum_sensor.svg images/schematic.svg
kicad-cli sch export pdf -o images/schematic.pdf liteplacer_vacuum_sensor.kicad_sch