So I've started evaluating microcontrollers for the "brain" of this system. I think I've said before that I'll be using a PIC chip.

The actual chip I'll use is still to be decided, but since I've got a few old chips laying about I'm going to start using these and plan to port code later.

The chip I'm going to start using it the PIC16F877.

Esentially it seems like a good starting point. the 40 pin chip that I have in my spares box has 8 Analog pins.

will be used for:
Current Sense
Voltage Sense
Temperature sense of the rectifier/H bridge heatsink

leaving 5 spare inputs for analog control of the settings.

(additionally leaving plenty of pins for driving an LCD, having display lights and acceptting inputs from button switches etc).

The settings the operator inputs to the machine won't be strictly analogue however. -more likely there will be step changes than true smooth analogue operation. for some settings I envisage up/down buttons would be a better.


I've spent some time trying to make sense of fuel maps for my car, (with the intent of tuning it), and I've more or less decided that's how I'd like this machine to work. -i.e sensors detect inputs, and given those inputs the machine is able to determine what fuel to put into the engine, how much boost pressure to apply with a variable turbo etc.

In this case however the machine will check the position of the switches, and detect the current flow at the output, continuously conparing what's happenning, and making adjustments to make the output make the operators wish.


I believe that it would be a neat idea to enable the use of maps to select waveforms etc on this machine, that would enable people to determin what the waveform should look like. (a basic map) and then an operator can "tweak" parameters with the machine settings. (amplitude/frequency)


Which leads me onto I guess the party piece of this machine.
There will be a few pins left for external communication. an RS232 (serial) port will allow for either new operation maps to be uploaded to the machine, or allow for the welder to be conputer controlled entierly.

Allowing this ability to upload new maps (or waveforms) to the device means that I'd expect that a person making this device was able to decide that they may like a specific waveformation and chose that.



Additionally the "maps" will be able to select a preference to ignore the pannel controls.

What this means is that if I'm performing a task that will be repetative rather than trying to remember where the dials are set or play around with them, I can create a map setting all parameters AND turn off the pannel controls within the maps.

Kind of how the "limp mode" map in a car is set such that no matter what you do it won't go over a given rev range. in this case, the welding current would be restricted,




The device will come with six essential maps,
Sine wave
Saw tooth wave
Modified sine
and square tooth
DC
DC pulsed

Most maps will essentially be 2 axis maps,

the current selected will form the Y axis of the map.
and time would form the X axis.

The actual current will be measured, and from this a delta value derived that would be used to adjust the output.

The output from the chip will be a simple number, then a DAC will convert this to an analog voltage, which will power the IGBT transistors (voltage controlled current device) which will operate within their linear region


The purpose of using maps will be to save processing time, for example, a sine calculation whilst not impossible on chip will take some time. Looking up a reference from a map/lookup table stored in memory will greatly improve the processing time, allowing more switching time.