aa88kk / hexastorm

migen FPGA core for controlling a laser scanner

Laserscanner

Implementation of a laserscanner on a FPGA. In a high-speed polygon scanner system, the laser is deflected by a rotating prism or reflective mirror. The position of the laser is determined via a sensor such as a photodiode.

Code is tested on the system shown in the image above, branded as Hexastorm. The bill of materials (BOM) and links to FreeCad and PCB designs can be found on Hackaday. The code took most inspiration from LDGraphy.

Install Notes

On Raspberry, install libatlas so latest Numpy, etc. can be installed via pip.

sudo apt update
sudo apt install libatlas3-base

Install required libraries

pip3 install -r requirements.txt

Make folder and install litex, this downloads and installs a lot!

mkdir ~/litex
cd ~/litex
wget https://raw.githubusercontent.com/enjoy-digital/litex/master/litex_setup.py
chmod +x litex_setup.py
./litex_setup.py init install --user

Install Hexastorm in develop mode so you can edit.

python3 setup.py develop --user

Install ice40 and yosys

apio install yosys
apio install ice40

Install icezprog

git clone https://github.com/cliffordwolf/icotools
cd ~/icotools/examples/icezero
make icezprog
mv icezprog ~/.local/bin

Parameters

The following parameters describe the system.

parameter	description
RPM	revolutions per minute of the rotor
Start%	fraction of period where scanline starts
End%	fraction of period where scanline stops
SPINUP_TIME	seconds system waits for the rotor to stabilize speed
STABLE_TIME	seconds system tries to determine position laser with photodiode
FACETS	number of polygon facets
DIRECTION	exposure direction, i.e. forward or backward
SINGLE_LINE	system exposes fixed pattern, i.e. line
SINGLE_FACET	only one of the facets is used

Using the above, the code determines the number of bits in a scanline. Via a serial port interface the user can push data to the scanner. A line is preceded with a command which can be SCAN or STOP. The data is stored on the chip in block ram. Once turned on, the scanner reads out this memory via First In First Out (FIFO).

Commands

command	reply
STATUS	retrieve the error state and state of the laser scanner. This is the default reply of each command.
START	enable the scanhead
STOP	stop the scanhead
MOTORTEST	enable the motor
LASERTEST	turn on the laser
LINETEST	turn on the laser and the motor
PHOTODIODETEST	turn on motor, laser and turn off if photodiode is triggered
WRITE_L	next byte will be stored in memory

Detailed description

Look at the test folders and individually tests on how to use the code. The whole scanhead can be simulated virtually. As such, a scanner is not needed.

Limitations

Only one of the 32 block rams is used.
The laser can be pulsed at 50 MHZ but data can only be streamed to the laser scanner at 25 megabits per second.
Parameters can not be changed on the fly. The binary has to be recompiled and uploaded to the scanhead. This is typically fast, i.e. seconds.
The current implentation is targeted at a system with one laser bundle
System is for writing to, i.e. exposing, a substrate. Reading should also be possible to enable optical coherence tomagraphy.
System has no link for LIDAR measurements, circuit can be found here.
The FPGA controls all the stepper motors. At the moment it is not possible to use GCODE or apply acceleration profiles.

Most of these implementation can be removed by improving the code. The current focus is on a proof of principle. Next step would be to create a copy of beagleg with a FPGA code parser. In a later stage, they might be merged.

Alignment procedure

An instruction video is shown below;

Other notes

Migen examples

Examples used to gain experience with migen.

OpenCV

For image operations, opencv is required.

pip3 install opencv-python

Also install the following dependencies

sudo apt install -y libopenjp2-7 libilmbase-dev libopenexr-dev libgstreamer1.0-dev ffmpeg

Camera

Two camera's have been tried; uEye camera and Arducam Global shutter ov2311. Currently, the ov2311 chip is used.

uEye camera

Disadvantages; the uEye is more expensive, drivers require an account and there is no good Python driver.
Advantages; the product is more mature.
On Ueye website select Ueye 2240 monochrome. Download and install the driver for linux, arm v7, as this is the raspberry pi platform. A python library for the camera is available in the source code. My version can be installed via uEyeCamera.

Arducam

Install my version of the Python libary ArducamPython.

SPI

In the current board, the SPI1-1 select pin is not routed to the correct pin on the Raspberry. In /boot/config.txt ensure you have the following

dtoverlay=spi0-1cs
dtoverlay=spi1-1cs,cs0_pin=7

There should not be dtparam=spi=on, somewhere. This would enable two chip selects for SPI0 and create a conflict with the pin select of SPI1. You can check the configuration via

ls /dev/spi*
sudo vcdbg log msg

Usefull links are 1 and 2.

I2C

In the current scanhead, I2C is used to set the power of the laser via a digipot. I2C can be enabled on the Raspberry Pi as follows.

i2cdetect -y 1

This will produce output, here 28 is the address of the I2C device.

     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:          -- -- -- -- -- -- -- -- -- -- -- -- -- 
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
20: -- -- -- -- -- -- -- -- 28 -- -- -- -- -- -- -- 
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
70: -- -- -- -- -- -- -- --

APIO

An alternative to apio is yowasp or compile from source. Apio is chosen as compilation takes a long time on a Raspberry. Yowasp comes with python support but only works on a X86 system. Yowasp takes a lot of time to run the first time. The FPGA toolchain can be build from source via ICE40.