Sensing Distance from Work Piece

Quality of product depends on the ability to detect the distance of the cutting tool from the Work Piece. The nature of this sensing problem changes depending on the cutting tool.

Ultrasound ranging may provide general-purpose sensing, independent of the specific tool type.

Plasma Torch

Here is a great article on controlling Torch to Work Distance. The quick story is that voltage between the torch and the work piece increases as the torch gets further away from the material. There is a discussion of building a z-controller here-- some of the content may be helpful. From that is a potential collaborator.

Documentation on torch height control in Linux CNC. Centers around use of the THC 300, which appears to no longer be available, and the CandCNC units. Note that CandCNC claims to only work with Mach3 and offers a $250 stand-alone torch height controller.

Experimental data points (Feb 2013, ChuckH): Hypertherm torch on Torchmate plasma cutter

• cutting 1/4" mild steel, 60A current setting, arc voltage approx 137V
• cutting 12ga mild steel, 40A current setting, arc voltage approx 125V
• approx 1 volt arc voltage change per 1/16 inch height change (very approximate guess).

Notes from Tom

Plasma Technology

There is a lot of information about torch tables using plasma torches, but I haven't seen much about those using acetylene torches.

The articles about plasma torches seem consistent:

• Torch height should be regulated to optimize cut pattern:
  • If too high or too low, the cut will have beveled edges
  • Most torch head damage results from head too low
• Initial torch height should be 150% - 200% of height after cutting has been initiated.
• Height can be sensed by voltage of plasma arc: Torch height is proportional to arc voltage. Open circuit voltage is around 300vdc, cutting voltage ranges from 80vdc to 200vdc, depending on operating current and cutting height.

Relevant articles:

CamSoft Torch Height Controller

Hypertherm Torch Height Controller

Oxy Torch Technology

Torch tables using flame technology abound as well. They are called "Oxygen Fuel" tables, as they can use fuels other than acetylene.

Capacitive Height Controllers induce a high frequency signal between a sensor ring and the subject steel and measure the resulting capacitance. This capacitance is inversely proportional to the distance between the sensor ring and the steel surface. The controller uses this measurement to generate signals to control a motor to adjust the head height.

Agelkom uses two rings to control head height for both plasma and oxy fuel torches:

• Plasma ring
  • Dimensions: 34 mm ID, 47 mm OD
  • Head height: 1.5 - 10 mm
• Fuel ring
  • Dimensions: 50 mm ID, 69 mm OD
  • Head height: 7 - 35 mm

Sensing rings must be electrically isolated from all conductive parts.

The Rutex controller employs two relays to drive a single motor to control the up/down positioning of the torch head as in the following diagram:

Data sheets on a controllers for controlling the head height on an oxygen fuel system:

Agelkom CAP04 Torch Height Controller

Rutex Oxy Torch Height Control

DIY, standalone sensor ideas

• Jcc3inc's torch height control (doesn't include initial height)
• ultrasonic distance sensing: if mounted some distance from the torch, an ultrasonic sensor controlled by an arduino running the z-axis motor might be a low-cost way to control torch height. One might provide an input signal from the CNC-controlling computer (or cheat, and use a photocell aimed at torch location) to set pierce height differently than ongoing cut height.
  • From the PING sensor datasheet:
    • For accuracy, temperature compensation should occur.
    • Maximum operating temperature of 70C.
    • Minimum distance 2 cm (sensor should be mounted higher than tip of torch)
  • [P.A.D.S], a bit more sophisticated than PING
  • [Maxbotix] ultrasound sensor
  • See also Ultrasound_Torch_Height_Controller
• Capacitive sensors have also been interfaced with arduinos and other microcontrollers.
• industrial-grade, extreme environment capacitive sensors are also available
• stepper motor control with an arduino
• servo motor control with an arduino
• integrate with torch table grbl