G100 Pulse Width Modulation and Control
The laser head has a DB-25 connector that allows the user to change power settings and monitor the condition of the system based on six output signals. A custom circuit board was designed to interface between the laser and a Mesa Electronics M5I20 board. The custom PCB fetches the status of the laser to M5I20 board and also receives data from the M5I20 to generate pulse width modulation (PWM) signals for the laser. There are other ways to get PWM signals to the laser but it was useful to have an independent source generating PWM signal - it simplifies testing and running of the system.
A block diagram of the system
Laser control board design
The custom circuit board is composed of a DB-25 connector which connects to line driver circuitry, and an M5i20 IDC connector which connects to an on board AVR AT90s8515 microprocessor. The board connects by a ribbon cable to the M5i20 board and sits was originally designed to sit in the computer's PCI slot. The PCI slot approach did not seem to work well, either because the computer just wasnt happy with occupying a lot of slots, or maybe due to my design.
The board was designed to monitor conditions of various pins to throw a firmware exception when the laser was failing; and it grabs bits from the M5i20 to generate PWM signals. The microprocessor firmware may be a bit confusing in that it converts the 6 bits of data to the the acceptable working range of pulse-widths for the laser, which is 5% to 60% duty cycle. Other than that its pretty straightforward. The circuitry was generated in Labcenter's Proteus, and a modeled PCB was sent off to Pool. Almost all parts were ordered from Mouser. I like their selection and records tracking better than other vendors.
Motion control software
The control software for my system uses EMC2. EMC2 is open source, runs on Ubuntu linux, comes with .iso files that you can load onto a CD and can be booted directly from disk. It also has tons of reasonable quality documentation.
NOTE: The rest of the comments here are not going to make much sense unless you read up on EMC2 first.
EMC2 is quite flexible and has several interfaces including the one that I use called Axis. Axis has a lot of great features including the ability to customize panels. This shows how my version of Axis is configured:
This page has lots of examples of the widgets that can be used to make the custom panels. What's also amazing about the system is that you can link the widgets to every signal inside the system. In my case I connected the LED lights shown on the panel to digital inputs that show the status of the laser. There's a dial on the interface that displays the laser power settings, and there's radio buttons that lets me turn on the appliances like the motor power supply and ventilation.
http://www.linuxcnc.org/docs/2.2/html/hal_pyvcp.htmlDriving the M5i20 from EMC
This required some digging on the wiki for debugging using lspci, to get to use the Halvcp Test Panel to see the computer drive the M5i20 card.
To simplify my thinking about connecting to the M5i20 I made a [pinconnections.htm table] describing the links between the ports on the AT90s8515 chip, the DB-25 connector of the laser and the 50 pin IDC connector of the M5i20. This was very useful when I got into linking the pins of the EMC M5i20 halpins to the rest of the system. As usual, the EMC team has put together really extensive documentation. Armed with the driver for the M5i20 I was able to code up just about everything. Basic tricks for debugging the hal involve starting up emc with my current .ini file, and running:
halcmd show pin
and...halcmd show sig
Frequently I would perform
halcmd newsig test_sig1 bit
and...halcmd linksp test_sig1 pin
My .hal file contains set of calls:
newsig PWM1-bit bit
newsig PWM2-bit bit
newsig PWM4-bit bit
newsig PWM8-bit bit
newsig PWM16-bit bit
newsig PWM32-bit bit
among other which will be signals that are linked to the interface.
Linking pyvcp to the hal
Driving the hal from pyvcp. As the documentation for Virtual Control Panels shows, a nice interface can be built using widgets like buttons, LEDs and meters which can be laid out in a vcp.xml file. In this case LEDs were used to display the status of the laser, a button fires the laser manually, and a meter displays the power settings of the laser.
For example, this .xml code was used to make a meter.
<halpin>"power-meter"</halpin> <text>"Power"</text> <size >200</size> <min_>0</min_> <max_>100</max_> </meter>
And calls like this will set the halpin "power-meter" to 5 in my .hal file.
setp pyvcp.power-meter 5
Some signals were created above that correspond to PWM bits, calls like this will link each signal to the M5i20 board, and an LED in the vcp:
net PWM1-bit => m5i20.0.out-08 pyvcp.led-21
Example of vcp.hal file
upload:laser_vcp.hal
Example of vcp.xml file
upload:laser_vcp.xml
As the documentation notes its possible to load your vcp.xml specification into axis directly, by putting the line:
PYVCP = laser_vcp.xml
into the [DISPLAY] section of the .ini file.
Then also place:
POSTGUI_HALFILE = laser_vcp.hal
in the [HAL] section of the .ini file.
Linking halpins to g-code
EMC provisions for a very interesting system that allows calls to executables to made directly from G-code. This created a straightfoward mechanism to set the power of my laser during machining. Commands can be named from M100 to M199, and the Mnnn programs must be stored in the directory described by PROGRAM_PREFIX in the [DISPLAY] section of the .ini file. In my case, I created a perl executable program with the following listing:
#!/usr/bin/perl
use strict;
my $input = $ARGV[0];
$input =~ s/P//;
$input = 0 if ($input < 0);
$input = 99 if ($input > 99);
my $cmd = "halcmd setp pyvcp.power-meter $input";
system $cmd;
my $val = int($input * (64 / 100));
my $bit;
foreach $bit (1, 2, 4, 8, 16, 32) {
my $pwm = $val & $bit;
my $n = 0;
$n = 1 if ($pwm != 0);
$cmd = "halcmd sets PWM" . $bit . "-bit $n";
system $cmd;
}
The arguement passed to the program specifies the power setting which ranges from 1-99. The program normalizes this value to 64, and performs bitwise AND operations on the value. Non-zero values resulting from the bitwise AND are sent out as bit signals of 1, zero values of the bitwise AND result in zeros being sent.
Hopefully you can tell from the code in the perl program how it makes the call:
halcmd sets PWM1-bit 1, etc...
which set the signals of the hal to the appropriate value. The M5i20 bits are picked up by my custom circuit board, and those signals are converted to PWM values.
When EMC launches the gcode parameters are passed to the Mnnn
command using P and Q values as arguments at runtime. Thus, the
call:
M102 P50
passes the string "50" to the perl program as the $ARGV[0] variable. This value is used to set power meter in the vcp to 50%, and the appropriate bit settings are sent to the laser.