Clock projected onto the wall with cheap laser galvos
Two exciting developments today.
In comments on a previous log, [Stefan Kruger] reminded me that common, cheap, poorly designed 532nm laser pointers also emit a surprising amount of infrared light, which is a potential danger. He referenced this great paper from NIST, which features a laser pointer very similar to the one that donated my previous green laser.
After a bit of research, I ended up buying a 50mW 505nm laser, which does not have the same problem.
The driver board I received looks a little different, and it has both "TTL" and analog inputs.
I hooked up the "TTL" input to my PIC32 and it Pretty Much Just Worked. Here's a cross-in-a-square test pattern, which demonstrates the laser can be turned off between drawing the square and drawing the cross.
One change I made: I moved the laser control to a 5V tolerant pin since the input is marked "TTL" and it wouldn't be unreasonable to find 5V their at some point. However, the input pin seems to float at around 1.8V, and connecting the pin to ground turns the laser off. Therefore, I set the PIC's output to use an open drain.
I'm working on plans to mount all this hardware on the ceiling of my kitchen. I'm using Fusion 360 to model all the components so I can figure out how to place them and where all the holes need to go. It's taking a while, but I did make this blocky but dimensionally accurate laser galvo model.
Another thing that needs doing is to rewrite the PIC32's DAC output loop. It has two problems. First, it's too slow to output 20,000 points per second to the galvos. Second, there's a bug in there because it seems to stop working from time to time. A rewrite will fix both.
More progress: I now have a Python program, running on the RPi to send data to the PIC32 Frame Driver. It was about 500 lines of new C code, plus 100-odd lines of Python, but I can now dynamically update frames.
It projected this little thing onto my notebook.
As predicted, the communications protocol was tricky. I wrote a 4 page document to describe it. The main complication is that the PIC32+MP Harmony approach to the SPI chip select (aka CS, aka Slave Select aka SS) line doesn't conveniently allow it to be used to define the end of packet, which makes handling variable-sized packets difficult.
As a result, the protocol I ended up with has fixed size command packets, and one of the commands says 'hey! there's data coming and it has length N'. The PIC32 then goes into a special mode where it waits for exactly that much data.
Here's the state diagram from the PIC32's point of view:
Also, inspired by Ben Eater's recent video about CRCs, I use 16 bit CRCs on all requests and responses. I used pycrc to both calculate CRCs in Python and to generate the C code for the PIC32. The PIC32 actually has built in facilities for CRC calculation, but I'm not clear on how to use it in conjunction with the MP Harmony SPI drivers.
Speaking of MP Harmony, I now have a whole rant about MP Harmony which I might polish up and publish.
The Frame Driver is now Almost Finished Enough that I could start building the clock daemon that will run on the Raspberry Pi. However, I think my next task will be to integrate the new, 505nm 50mW laser that is due to be delivered mid-next week.
So far, my experiments have used the cat's battery powered laser pointer. Today, I disassembled a few laser pointers and powered laser modules directly from my bench power supply.
The green laser module was the brightest and looked the best. Also, I know it will be visible in the day time. According to the sticker that was on it, it is a "class III" and "<5mW", which I believe makes it a class 3R laser, which I'm happy with from a safety point of view.
However, it also draws between 250mA and 350mA, which is much more load than I want to put on the RPi's supply, especially since that load will be switched rapidly and repeatedly.
So, instead of using the output of the 3.3V LDO to supply the laser, I'll use a 12V wall adapter, along with an eBay buck converter module.
Progress! Here's a pattern generated by the PIC32, sent to the DAC, turned into twin differential signals by the op-amp and sent to the galvo:
It is a cross inside a square. However, I have not yet hooked up the FET to control the laser, so the laser doesn't turn off while moving between the ends of the cross and the bottom left of the square.
Apart from that, there are several noteworthy artifacts in this image:
I plan to address all of these problems later in the build, after I have a basic clock showing.
Here's a scope screenshot showing SPI data being sent to the DAC. The DAC output we're following is channel 3, the magenta line. The SPI command 0xBFFF causes to the output to transition from 0x000 to 0xFFF. The transition from min to max value takes about 6uS
Channel 2 (light blue) is the signal inverted and amplified, then channel 1 (yellow) inverts the signal again.
Over a longer period, we can see the differential output on channels 1 and 2 following the DAC output on channel 3:
Notice that the op-amp output tends to drift toward 0V over the course of a few milliseconds. That's interesting.
One thing I learned: the output pattern is reasonably stable despite not having a separate 3.3V supply for the analog circuitry.
Things to do next:
Questions to answer some day:
It turns out that the galvos I bought are closed loop devices. When a signal is applied to them, the move to a new position, and they also give feedback about their position to their controller. This feedback "closes the loop".
Taking the rear cover off one of the galvos shows the position sensor. It consists of an LED, two photodiodes, and a plate attached to the galvanometer shaft. As the shaft rotates, it allows more or less light to shine on each photodiode, which provides the feedback.
I had been wondering why the galvo needed a six-wire connector. Now it seems obvious that two of the wires are for the galvanometer itself, and four of the wires are for the position sensor - V+, V- and two photodiode signals.
This complexity also explains why each galvo control board has 4 quad op-amps: the board is an analog computer, trying to calculate how to accelerate and deaccelerate in order to bring the mirror smoothly to its new position with no over or undershoot.
I learned about the galvo position sensor from zenodilodon's Youtube channel, in particular the video "Inside the Closed Loop Laser Beam Stearing Galvanometer" in which Zeno shows several galvanometers that look like bigger versions of my cheapy.
Yes, the production is a bit rough. Yes, those dead pixels are on Zeno's camera sensor, not on your screen. Yes, I too find the paint flecks on his hands distracting. Despite all that, it's a pleasure to watch a gifted professional dispensing the kind of helpful information and tips that can only come from experience. I am totally subscribed.
Another one of Zeno's videos, "Complete Build of a 4 Watt Analog Modulated ILDA Show Laser" was also helpful. I'm building a just-good-enough for home single color, 5mW , digital version of the professional RGB, 4 Watt, four laser analog rig that Zeno puts together here. Plenty of good tips. My build will be simpler and involve less swarf.
I spent the past few days soldering up the PIC32 MCU and getting it to work with the Raspberry Pi.
It's Going To Work. Hooray! There were a couple of tweaks to the schematic, one dead PIC32, and a couple of moments of utter confusion with Microchip's MPLAB Harmony, but we're now in a good place.
I did this first since the PIC32 is an unknown for me - hence something that might go wrong. If it wasn't going to work, I needed to know early.
As a proof of concept, I had the PIC32 use a timer to blink an LED in a pattern. It also acted an SPI slave, ready to accept data from the RPi that it would use to change the LED blinking pattern.
The picture to the right is a scan of my first planning sheet. While constructing it, I made a few changes, which you can see in the larger image, below.
One of the nice things about working on a Blueboard#01 is that one can almost always try a new configuration if something doesn't work. It also lends itself to constructing a circuit in stages.
The picture below is me getting excited about soldering up my first PIC32. Flux is on the footprint, and moving the chip into position.
The end result was fairly neat, with only a few solder blots. Most of the wiring is hidden on the back of the board.
And the final planning sheet:
The PIC32 sets up one timer peripheral and one SPI slave peripheral. It has a buffer containing a pattern.
On the RPi side, I used Python and spidev. Spidev appears to be installed by default, which is great. It's pretty straight forward to send a bunch of bytes to an SPI slave.
Here it is, all together. The laptop has a window open to the RPi, the RPi is wired to the PIC on the Blueboard#01, and the Blueboard#01 is being probed by the oscilloscope.
There are a lot of wires. I did this because the PIC32 is highly configurable, and I wanted to make sure I had the connections right, so I used 0.1" header pins pressed into the Blueboard#01, with dupont wires to make temporary connections. I'll solder in permanent connections, behind the board now that I know it works.
The oscilloscope is probing via a row of 0.1" header pins pushed into the Blueboard#01. With SPI running a 1 MHz clock, there were no problems at all. Channel 3 (magenta) is the clock:
It also works at 20MHz. This is suprising to me, given all the dupont wires going everywhere. At 20MHz, the waveforms are less well defined:
Following on from last posts' high level design, I drew up schematics. I like to keep each part of the schematic down to about one A4 page, so I can easily print it all.
These schematics represent a point in time. The will change as the project goes on. When I'm finished I'll upload them in full.
The voltage shifter takes the DAC_X and DAC_Y signals, and turns each into a differential pair suitable for input to the galvos.
Based on my experiments with the signal generator, I don't need the full galvo deflection range, so I decided to go for an +/-8V differential range rather than the full, standard +/-10V range.
Generating the signal is two step process:
In step 1, we map the DAC's range somewhat upside down, by mapping 0V to 4V rather than -4V. The reason for this is that the op-amp configuration is more straight forward this way. Since we are always going to need both the positive and negative signal, it doesn't matter at all which we calculate first.
I used the site earmark.net to understand the required amp configuration and calculate resistor values. Bruce Carter has been extraordinarily generous in providing this resource.
[Parenthetic note: the DAC's actual range is 0.01 to 2.048V, but using 0-2V significantly simplifies the selection of values. I'm using 1% resistors, so the error from this simplification is much less than the accumulated error from other sources.]
The PIC32 has some requirements, handily documented in section 2 of its datasheet. At least we don't need an external clock crystal. The Raspberry Pi comes with all its own support circuitry and the schematic just shows the Frame Driver's connections to the RPi's I/O header.
In addition to the usual bypass caps, the PIC32 requires:
There are also a couple of buttons and LEDs for debugging purposes.
(Looking at the schematic, I can see I'll likely want to add a reset push button, too. )
These are devices the PIC32 sends commands to, in order to drive the galvos and laser.
We power the DAC from a special 3V3 supply (3V3Ref) to help ensure the stability of its internal voltage reference. 3V3Ref is also shared by the voltage shifter.
The DMN2046U N-channel MOSFET controls power to the laser. Because the FET has ~300pF of capacitance, there's a 100Ω resistor inline on the LASER_FET output pin to reduce maximum output current to a peak of about 33mA - which is still twice the "maximum allowed" of 15mA. (Interesting discussion about gate resistors on Stack Exchange.)
In previous projects, I've been bitten hard by noisy power supplies. This is likely overkill, but I'll start from here and then maybe experiment with removing some of it.
I am using an 7808 and 7908 to generate +/-8V from the galvo's +/-15V supply. The values of capacitors were taken from the datasheets, but I don't think they're particularly critical. The +/-8V is used to power the op-amps. While the op-amps can happily use +/-15V, its not clear to me that the power supply would be clean enough, especially when the galvos are under high load.
3V3Ref is generated from the RPI's 5V supply. It's purpose is to be a clean 3V3 supply for the DAC and to be used as a reference by the op amps.
Next steps: soldering!
The purpose of the Frame Driver is to receive a "frame" of data from the Raspberry Pi and then output it until it receives a new frame. The Frame Driver will guide the laser beam around a path over the course of 1/30th or 1/50th of a second, turning the beam on and off as appropriate, and then do it, over and over again.
There are several steps between receiving a frame from the Pi, and outputting voltages to the Laser Galvos.
I've decided to use a PIC32MX170F256B-50 microcontroller. There were several reasons:
I chose MCP4822 dual 12-bit DAC. It has an internal 2.048 voltage reference and has an SPI interface.
In this post on laserpointerforums.com, the consensus seems to be that an 8-bit DAC is not sufficient, but that a 12-bit DAC is plenty. 12 bits gives 4096 steps, which would be somewhere between 0.2 and 0.4mm per step on my kitchen wall, which seems sufficient, especially given that the laser beam is 1-2mm mm wide. Beyond 12 bits, things start to get quite pricey, and there's little additional advantage in having 0.05mm precision over 0.2mm precision at normal view distances.
For the cheaper DACs, there are two common standards for loading data: I²C and SPI. Generally, SPI is a faster protocol, both in terms of physical link speed and in having a lower protocol overhead. The MCP4822 can transfer data with a 20MHz clock and I am expecting to be able to use at least 5MHz.
Outputting two values from the MCP4822 DAC requires two 16 bit SPI transfers and then setting the LDAC pin low. At 5MHz, this will take (16 * 0.2 * 2 + a bit)μs ~= 8μs, which is fast enough. The DAC requires a "typical" 4.5μs to change its output signal from one level to another, anyway.
To ensure that the internal voltage reference is stable, a very stable power supply is required.
The final component in the Frame Driver is the voltage shifter which takes two single-ended 0-2.048V signals and amplifies them to the +/-10V differential signals (maximum magnitude of any given line is +/- 5V) expected by the Galvo controller boards. It will be composed of 4 op amps, in a single package. I chose the TL084, because cfavreau used it successfully in his Open Laser Show DAC. It's also the same part used on the Galvo driver boards.
Finally here is how I plan to power each of the components:
|Component||What it Needs||How it Will Get it|
|PIC32||3.3V, < 100mA.||Pull from RPi 3.3V pins|
|MPC4822||3.3V, a few mA, low ripple||Dedicated 3.3V regulator running from from RPi 5V pins|
|TL084||+ and - 8V supplies, <20mA||Dedicated regulators running from Galvo +/-15V supply lines.|
In which I wire up the laser galvos in minimal way, validate some concepts and learn some new things.
Here's what I did. At each step I was careful to test to make sure that no smoke came out and that everything behaved as expected.
At that last step, the galvos moved! I had not been expecting them to move, but it seems that their off position is at one end of their range.
At this point, I used Blu-Tack to hold everything down, just to make sure nothing got lost in the tangle of wires. These are the grey blobs you can see in the photos above.
I then stole the cat's spare laser pointer, Blu-Tacked it in place and lo! there was a dot.
I have a dual-channel signal generator. Using a Blue Board #01 I soldered up a couple of connections. Each driver board is fed by three wires: ground, V+ and V-, with V+ and V- carrying a differential signal of up to 10V. I tied each driver board's V- signal to ground, then piped in the signal to V+.
The resulting patterns look different on camera than they do in real life, but are interesting nonetheless. The shapes are correct even if the colors are not.
Here are some things I learned:
I think this is going to work!
I bought the cheapest laser galvo mirror set that I could buy, that also had reasonable shipping. I ended up with this one.
The set comes with the twin galvanometers, mounted on a metal block, two driver boards and a power supply. It cost about 100AUD on eBay (listing). Several of the sets on sale also offer a "show card", which will run laser displays from an SD Card. I wanted to build the display logic myself, so I didn't buy a show card.
The specs from the listing are:
-Input voltage：15V -Peak current: 1A -Rated current：0.5A -Operating temperature：0~50℃ -Rated scanning angle：± 20 ° -Maximum scanning angle: ± 30 ° -Drive plate size:76mm × 48mm × 26mm -Wavelength:380nm-700nm -Analog signal input impedance: 200K ± 1% Ω (differential input) -Analog position input range: ± 5 V. -Scanning speed:20Kpps -Lens reflectivity:> 98% @ 45 ° incidence (coverage wavelength: 380nm-700nm）
Important to note here:
Given the amount of power consumed, I think it's probably a good idea to aim for smaller angles of defletion and slower scan rates.
The supply came with two 3 pin connectors pre-attached to the output screw terminals. I confirmed with the multimeter that FG is connected to the chassis, while N & L are not. The G terminal is not connected to FG or to N (or L!).
Label on the power supply says it supplies 1A at 15V, which nicely matches the peak current draw spec.
There's quite a difference between input power rating (0.5A@100V = 50W) and output rating(1.0A@15V + 0.5A @ 15V = 22.5W). I suppose this means that the supply gets hot.
This is all about what I expected.
As noted above, the galvanometers are firmly aligned in a metal block. Under the block are screw holes which are obviously useful for securing the block.
Hmm... looking at the photo, that mirror seems a little dusty. I'll need to clean them. The galvanometer PCBs each have the galvanometer, the connector, and nothing else:
I'm not entirely sure why the connectors have 6 terminals each, but based on the cables, I'm guessing the galvos give some kind of feedback to the driver board.
These boards are responsible for amplifying a high impedance, 10V differential signal into a low impedance 15V signal.
I've marked the connectors with their apparent function. There's signal input, two power connectors and an 8 pin output connector. The picture at the top of the page shows the cables that run between the 8 pin output connector on the driver board and the 6 pin connector on the galvo. The conductors run in two bundles.
The trimpots along the bottom each have two-letter markings on the underside of the board. I've put these markings in the picture. Based on this Aliexpress listing for a similar product, I'm guessing the meanings are:
All of them appear to be locked with a small amount of glue, except Input Scale. I won't touch any of them.
There is a 2x3 header area with two jumpers on it, next to the input. No idea what that is for.
Similarly, there's a 2 pin connector in between the trim pots that I have no idea about.