Wednesday, November 08, 2006

Sensors: Ultrasonic Phased Array

So, with the laser system falling through, I've been looking around at alternate distance measurements. Hence, ultrasonic. This isn't your typical sonar arrangement, though.

Your typical sonar system is composed of two ultrasonic transducers and associated electronics. One is used to transmit, one is used for recieve. Higher end units use one transducer (still figuring it out) and usually have closer range requirements. I'd like to build this one-transducer arrangement. Another issue/advantage to sonar is the wide detection spread. Some units can sense everything within a 90 degree cone, or so. Good if you're looking for ANYTHING, bad if you're looking for SOMETHING. Transducer frequency varies with size (smaller = tighter pattern). Some higher frequency ones (commonly 235KHz) have a much narrower cone, also.

I have a 25 KHz transducer, and might get my hands on a few 40KHz models. So, the pattern will be exceptionally wide. Cheap, though. So, how to counteract this? We turn to technology to do some cool effects?

Modern radar often times uses a phased array instead of mechanical means to slew the radar beam. A phased array is made up of many small radar units. These units have their signals carefully timed to generate specific phase relationships across the entire antenna. Due to the constructive/destructive nature of waves, this can be used to both combine the power of the antenna units and to focus the useful energy into a single beam. It also allows the unit to both be redundant and to electronically steer the beam path just by adjusting the phase from antenna to antenna.

Modern sonar also uses it, but in different ways. There may be a single sound source, but an array of recievers is used to detect the signal, allowing the arrival time and phase difference from each sensor to successfuly determine the direction and range (if active) of the target. This method is also used in modernday medical ultrasound to allow a 3D map to be generated from all the responses. We're looking at quite the DSP problem, though!

Considering I haven't found any hobbyist doing this, I thought it'd be nice to attempt to build a modular sonar array that can be configured to have any and all units be phase-related transmitters and recievers. I could control the whole thing via a CPLD, but my test unit will be based on a dsPIC probably.

The initial design will have single transducers (with associated filter and power electronics) placed on individual PCBs. Each PCB will have power, ground, digital in, digital out, analog in, analog out pins plus perhaps a serial port. The digital pins will allow a CPU to trigger and measure all recieved signals digitally, as most sonar units do. The analog pins will be there to let the user experiment with generating a cleaner analog frequency out and get the analog return. The transducers band-limit the signal, so the analog in may not be necessary. The analog out is more important as this could be coupled to a high speed ADC to allow more accurate phase measurements and to get certain signals out of the process (doppler shift, signal strength returns, etc). If some digitally programmed controls, like a digital pot, are used to control signal strength, a serial connection will also be supplied of the appropriate type. The individual boards have the advantage that you can reconfigure them to test out different transducer arrangements. They have the disadvantage of causing possible alignment difficulties that may result in bad results.

The CPU board would have headers for these boards. I could either generate the signals via PWM or output compare hardware. Returns would either go for input capture(CCP) or ADCs. Many dsPICs have a 500Ksps or 1Msps ADC with a 4 lane sample and hold system, allowing me to accurately measure 4 sensors at once. They also can have up to 8 PWMs and CCP units. In theory, this means that up to 4 analog signals or 8 digital signals could be processed at once. I think I'll start with 4 and go from there. I probably could do 8 analog signals at once at the speeds these ADCs work at without concern of being off too much (25Khz x8=200Khzx2=400Khz, so worst case is slightly better than the nyquist limit). Other architectures may be better for this (ARM people? AVR people?).

One idea for the mobile PC robot crowd is to use a 18F4550 PIC. These chips have a 200Ksps ADC and enough CCP units to possibly support a full 5 transducers. This data could be sent back on the USB bus for capture analysis inside a math program (I'm considering this myself for algorithm and hardware testing). After that, take what you've learned and put together a DSP algorithm for a dsPIC.

I think that this could result in quite exceptional sonar mapping with fairly low cost 1D, 2D, and 3D arrays.

Thoughts? Did I miss someone out there who's done this before?

Wednesday, November 01, 2006

Oops! The Laser Rangefinder

Why you need to research before buying parts...

I meant to build a laser rangefinder (see paper that drove this idea here).

I'd like to get away from having to frequency adjust the laser, so I run across an Erbium rod for $10. Even if it's too damaged to work, I'm not out much. Now, my research into Erbium is that it's used in eye-safe lasers with a wavelength of 1.5um. Wonderful. It's on order and shipped as I type this.

Unfortunately, there's two types of Erbium doped rods. One is Erbium:Glass, with a pinkish color. This does the 1.5um lasing. Then there's Erbium:YAG. More expensive, it does 2.9um (half the frequency). It's greenish. See the link above? Yep, it's greenish. Unless they have a different color, I think I was shot down before I even started. There goes my "simple laser pulse". I also ordered several spare supposedly laser optics and several displosable flash units. Guess that means I'll have $20 or so in hardware sitting in my parts boxes for a while.

Now, depending on if the lasing rod lucks out in damage, I COULD try and get the rest of the parts, but I suspect it'd be cheaper for me to track down an Er:Glass rod than to get the frequency doubling crystal (KTP or KTA perhaps? I need a good laser handbook).

HMD: Where to get parts, what's new, what's soon

I'm trying to head back to the land of the electronic for a while, with that, however...

I decided to poke my nose into head mounted displays. I've always wanted to have a small glasses or otherwise transparently mounted information display. Not all the time, but enough. With my service job, being able to display the readout of my DMM or oscilloscope directly into my vision would be absolutely fantastic.

So, what's available?

Let's start with the "near mortal" components.


Ignore the marketing images, they don't do this product justice. This is perhaps the finest binocular HMD available. $400 is a bit steep for a QVGA display headset, but don't let the Star Trek visor design fool you. Micro Optical focuses on minimum occlusion display technology. That, translated, means it obscures as little of your vision as possible. This bar design allows you to see a lot above and below the displays, and the rest is actually tinted transparent, so you can see around the displays, too. The result? From the reviews I've read, it means you can actually wear them and use them without feeling any more strain on your eyes or stomach than a standard desktop LCD.

Apparently the design is rugged and lasts 6 hours on 3 AAA's (and the special IPod version runs on a custom lithium-ion in the special ipod case). The optics are a single fused block, so no worries about alignment.


Picking up where the original 3D headsets left off, 3DVisor is trying to bring high quality HMDs to a wider population. Yes, you do get the same disorientation issues as all enclosed HMDs provide. However, these aren't QVGA. These aren't VGA. Yes, they're full SVGA and have a VGA connector. Normally units of this resolution are $1000 to $2000. They're now selling this for $550. Rumor has it that they're going to be releasing an XGA (1024x768) model in the near future. Given that they were originally selling the current version for $900, the price break signals that both the OLED technology is maturing and that the future XGA model may break the $1000 barrier. XGA HMDs have been the realm of the military and research institutions and still traditionally cost as much if not more than a new car. I can't wait to see.

Now, parts


I linked directly to their Cyberdisplay section. Kopin is behind many low cost HMD hardware designs. I wouldn't doubt many of their displays have shown up in the $200 bargain basement HMDs. I know their displays (black and white) have shown up in camera, kid's night vision, and currently kids nightvision on an R/C car.

Now, most of these were black and white components. While I love hacking, sometimes you have to start from scratch. And where does that get me? Kopin sells both the main components (LCD, backlight, driver chip) and prepackaged assemblies (monocle display, binocular displays) for QVGA and VGA. The QVGA monocle (no VGA version, though) interests me as a possible core component to building a HMD. No driver chip, so it's going to cost almost as much as the VGA binocular arrangement, but that's how things work. You get twice the product for half the price with HMDs.

And in the future?


I ran across Microvision a year or so ago. At that time they were making high end HMDs for the military and had one $5000 red Nomad monocle. Sounds like a lot. They have a secret. They're the only group that I've seen that has an analog control 2D MEMS micromirror. I've seen the TI DLP hardware, that's pretty much a digital system, it's either pointing here, or pointing there. Microvision had used this to build the Nomad, and it let them put up any resolution they wanted to at extremely high framerates. Cost of the laser component and driver were the limiting factors. Apparently they've gotten over that. They're now working on integrating a fully working RGB laser MEMS projector system into things like car HUDs and cell phones. It's only a matter of time before the projector becomes available then. If it doesn't need any complex optics to work, this may be the best option to project an on-glass transparent HUD for a mobile user. The IPM looks quite small and hopefully inexpensive. Who knows, you might be able to pick up an XGA full color display for $100 at your local electronics shop in a year or two.