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Plasma/Acetylene Cutter

Ramps 1.4 Heater circuit to be used for oxygen solenoid. Connector is a 6 terminal, screw terminal block. Solenoid voltage must be same as stepper voltage supplied to ramps board. Typically 12 or 24 volts DC. Media:180px-RAMPS1-3 6pos.JPG

Stepper motors are 425 4 volt 2.8 amp. This exceeds the 1-1.5A capability of the ramps Pololu drivers. Decision has been made to use the Powerlolu board at [1] Open source schematic seems ok(Eagle format). Board file seems unfinished. Powerlolu drive capability is 10A. Media:Powerlolu_web_preview_featured.jpg The board is commercially available at [2] This board does not directly attach to the Ramps board, but uses 4 wires to attach to one of the sockets that the Pololu board plugs into on the ramps board also commercially available. Note #1 - This is not very mechanically or electrically stable. Note #2 - The Powerlolu board is current limited but not fused. The power supply to the board should have an inline fuse no greater than 10A. To set the driver current limit see [3]. Stepper motor connects to Powerlolu via two screw terminal blocks.

Use a seperate Arduino Mega for the height control system. Requires a seperate Ramps & Powerlolu board for the z-axis stepper. Requires a custom software sketch in the Arduino to connect Ramps to the capacitive height sensor. Requires some pushbuttons for jog up/down and engage/disengage height feedback. With some custom g or m codes and a serial line between the two Arduinos the attached pc could jog the z-axis and engage/disengage feedback instead.

Note - Changing from an ultrasonic to a capacitive height sensor relieves the timing constraints considerably. I now think a single Arduino system will work.

Capacitive Height Sensor

A suitable height sensor would be the Omron E2K-C available from Mouser for about $167. Interface is open collector npn transistor. (wire) Alternatively Analog Devices have a suitable capacitive sensor chip AD7747 for $11. This would require a fairly simple pcb to be a height sensor. A 2x2cm. sensor area at a distance of 0.5cm gives 0.7pF scaling linearly with height. This is well within the range of the AD7747 which has a noise level of less than 100 aF and a maximum sense range of 8 pf. Would require a more complex sketch than the E2K-C but still fairly easy. The interface is I2C which is compatible with the Arduino.

Both of these sensors will require the z-axis controller to back off vertically then go down until the sensor (sees) the metal to establish a calibration point. Since th E2K-C is strictly on/off, the height following is going to be more noisy or oscillatory (hysteris unspecified) than the AD7747 which returns actual capacitance values which enables close tracking. Both of these sensors wiil requre shielding for a plasma cutter and its induced field. Omron also have an electric field based sensor good for iron and aluminum in the $450 range. All sensors require the metal object to be grounded and sense different distances if the ground is removed.

Welder Plasma Cutter Block Diagram Media:welder-plasma-cutter1.png