Small Yachts

I remember a quote from MMI some years back on the IOM "there's nothing wrong with it another metre wouldn't put right!". Well the days of the A-boat and 10-Raters are pretty much gone in most countries, and now there are a number of sub-1-metre boats or kits becoming popular. Some are popular in great numbers, like the Micro Magic (see my post from Feb 2007). Here are two home-built boats/boat-types I've been notified of lately:

This one is 750 mm long, comes from NZL, and is called a Racing Sparrow. The website looks exceptionally good with a lot of information, pictures, and downloadable plans. Designer Bryn Heveldt has also written a book, available on Amazon, about building the boat. There's a YouTube video here.

This is a Strathclyde 70 class (does the class have other 'homepages'?) boat, also around 700 mm long, planked in balsa by Peter Trimmer (AUS). It's actually a 70 % scaled down version of the Laerke IOM.

I wonder how these boats sail? I've sailed a MicroMagic a couple of times, so if anyone can comment and compare these two to a MicroMagic or an IOM I'd be interested. Do people really want the 1+ metre boats, or is racing just as fun with a smaller cheaper boat?

As before I would predict that any class that aims to be popular needs to have good availability of boats. There are a lot of MicroMagics and RC-Lasers around because you can buy them (more or less cheaply) over the counter at the hobby shop. It's just a shame that usually the designs and class rules of home-builders and accomplished radio-sailors (you know, the ones who still have their A's and 10R's in the basement) seldom meet with the needs/requirements of people who are able to mass-produce a boat. I think currently there is no commercially mass-produced kit or ready-made boat sold that fits in any international class (A, 10R, M, IOM) ? The Windstar probably comes close, but I understand it requires a lot of modifications before it's competitive with a standard IOM.

I've toyed with some ideas around IOM-production, but it's not likely that 2008 will see major progress (too much else to do...). If anyone has some news or ideas let me know! Now it looks like we will have MicroMagic sailing at both the Helsinki Boatshow (Feb 08) and the Helsinki Model Expo (Apr 08) - and I did sort of promise last year that I would turn up with a boat of my own in 2008...

Update: There's an article in the latest Model Yachting magazine about Jon Elmaleh's latest project the Twang IOM. It's going to be mass produced and will hopefully be available in the summer of 2008. No word on Jon's site yet...

Aggregating and Filtering Feeds

I'm trying to keep up with the ever increasing volume of scientific publications in my own and related fields. I've been using the Biophysical Journal's email based service for some time, but lately it has been very unreliable - often alerting me about supposedly 'new' papers that have been published in 1994 or so. Another way is to subscribe to the RSS/Atom feeds many journals provide, and I've been doing that also with Google reader, but it easily means wading through 100s of papers per week.

It's clear I need a better solution, something that first aggregates all the new papers into one big feed from the journals I am interested in, and then in a second step filters the big feed down to the few new papers that contain interesting keywords. Yahoo pipes could do that, but the LabVIEW-ish editor doesn't scale very well to a situation where you have 20+ feeds and 20+ keywords you are looking for. There's also google-mashup, but it isn't open for the public yet.

A complex solution would be to set up my own Planet, but it doesn't have web-based setup and administration so requires tinkering with config files etc. which I want to avoid.

So far I've only come up with this Thunderbird-based solution:

On the left I've subscribed to a number of journal feeds and put them in a folder of their own. On the right is a list of filters I am running. Each journal feed needs its own 'dummy filter' which does nothing but moves all the entries into the 'all papers' folder. Then I can run a filter of my own that looks for things in the subject or body of the paper. It's simple, ugly, but seems to work somehow.

Please tell me there is a simpler way to do this in Thunderbird! Or is there already a good web-based service like this around?

My requirements would be:

  • able to read and aggregate: RSS/Atom etc. (whatever the journals provide)
  • set up filters that look for keywords in any field or in only one field (title, author, abstract etc.)
  • output an RSS feed with all papers, and the filtered papers that I can read with Thunderbird or Google-reader.

So far I haven't found anything that would do this in a pain-free way. The aggregation part is handled by most web-based services, but there aren't many that allow searching/filtering and can provide the results as a separate feed.

Something like this is already going on with 'virtual-journals' that aggregate papers across journals in one field (e.g. Virtual Journal of Biological Physics Research or Virtual Journal of Nanoscale Science & Technology). Papers get selected to these 'VJs' by their editors, but I'm thinking my aggregator+filter idea will be able to cover a broader range of journals and look for more specific search terms.

Mowing tactics

Moving forward with the CAM coding, the sensible thing would probably be to work on mundane things like 2D offset generation, a kd-tree for faster drop-cutter searches, and zigzag-paths from 2D outlines... There's again been some talk about open-source CAM on cnczone, but not much in terms of results or actual descriptions or implementations of toolpath algorithms.

Anyway, here's something more fun than the traditional computational geometry problems I referred to above. It's lawn-mowing tactics, or how do you program the circular robot to mow the red pixels while not cutting too many of them at a time. This is a slightly improved version of my earlier trial. This one considers a number of angles in all directions for each move. From these moves the ones that cut away a suitable amount of material are selected. Additionally I've introduced a cost function for changing direction, it should be easier for the cutter to continue traveling in approximately the same direction than to do abrupt turns. In spite of this, about half-way through the cutter reverses direction...This is obviously done with a bitmap representing the grass to be mowed, but I wonder if it would be better to try to do it more exactly: represent the boundaries of the grass with lines and arcs. A variable step-length also seems like a good idea, on long straight bits the cutter should be able to move in one go as far as it goes.

Microfluidics test

I've been playing around with a microfluidic channel, to be used with optical tweezers experiments later. There's clear fluid coming in from the left in the wide (ca 30um) channel, and I've colored the fluid from the top red and the fluid coming in from the bottom blue. The top and bottom channels are narrower, ca 10um.

This page should have 3 videos, but I put them on Jumpuct and they have disappeared - sorry!

Here all the channels are on at first, then the red channel is switched on/off two times.
Here's the same thing, but switching the blue channel on/off.
Here the clear channel is switched off, and the main channel fills with red/blue fluid.

It's interesting to follow the laminar flow at these very small Reynolds numbers - the fluids effectively don't mix at all (they do mix by diffusion, but very slowly) and there's a clear boundary between red and blue. At the end the clear channel is switched on again.I'm using pressurized air to drive the fluid flow, similar to a product from French company Fluigent (nice videos here and here). Their product sells for around the price of a small car, so I'm thinking I can come up with a DIY solution for slightly less. Switching is by solenoid valves that switch either high pressure or ambient pressure to the fluid bottles (2ml Eppendorfs). The pressures required are surprisingly small, here I'm using the smallest pressure my regulator will output, 5 psi, but I have a feeling this is too much... so I'll need a pressure regulator with fine control between 0 and 5 psi, any ideas?The other option is using gravity to drive fluid flow, 0.5m H2O is around 5 kPa or 0.7 psi which could be OK. The problem is you then have to switch the fluid lines directly. I tried this with solenoid pinch-valves, and the valves create huge pressure transients when switching off - completely flushing the channel with rapid flow. So the gravity driven solution requires valves that open and close very gently.

Spektrum DX6 antenna mod.

By popular demand, some notes on how I've placed the antenna of my Spektrum DX6 transmitter inside the case. I've been using the radio like this ever since I got it and provided that you hold up the radio more or less vertically and not hide behind large metal constructions or things like that the range is fine. The benefit of the internal antenna is that I don't have to worry about breaking it while sailing it or storing the protruding thing in the toolbox. When it rains it's nice to fit the whole transmitter into the rain-cover which doesn't have any (potentially leaking) holes (other than the two holes for my hands!). A plug for the antenna hole to prevent dirt etc. entering the Tx would probably be a good idea.

If someone has a feeling for what theoretically a 2-3 mm wall of plastic does to an RF signal at 2.4 GHz, let me know.

Here is the back cover and six screws that hold it removed along with the battery (I've put Deans connectors on the Tx battery to simplify charging). With a stock DX6 the antenna would be sticking out at the top and there would be a few extra pieces of black plastic supporting it. I remember I broke some of those black plastic parts when I disassembled the antenna - so proceed carefully if you think you want to go back to the stock configuration sometime. I didn't touch the electrical connection of the antenna at all, the thin grey coax that comes out of the antenna attaches to the RF PCB just like it does on the stock Tx.

Here's a close-up of the antenna. You can see a part of the old antenna-hinge around the grey coax to the left (a bit dangerous to cut it away with a knife or pliers since you risk damage to the delicate coax). I've taped the antenna upside down to the RF board. There are probably other places inside the case the antenna could fit as well, but this seems to work OK.

If anyone has done something similar do let me know! I'd be happy to post pictures here if you send them to me.

Spektrum: I hope you are taking notes, I expect your next radio to have an internal antenna!

Talking about DX6 modifications, I did order the voltage regulator for the improved runtime modification, but the runtime with 2700 mAh NiMH's is just fine so I haven't installed the improved regulator yet.

Update 2007Nov17:

Olle Martonen sent me this pic of his modified Spektrum DX7. He mounted the antenna horizontally behind the regulator/switch PCB. Also note the wooden plug in the antenna hole. Not much sailing done with this system yet, but range-checking on the ground indicates there should be no problems.

I also got some observations be email on RF issues from a mobile-phone perspective: A few mm of plastic will not attenuate the signal measurably. Conducting materials are worse, like some mobile phone shells that are covered with carbon-containing paint, or your fingers on the back side of the transmitter. My placement of the antenna close to the RF-box (the metal square), and the PCB (also metal-coated), is not optimal, and could lead to an attenuation of 3-5 dB. A distance of 2-3 cm to the conductive parts would be better, so I'll maybe look for other places inside the Tx where the antenna could fit (Olle's example above is a bit better since the antenna is farther away from the RF-box).

Update 2007Nov22:

Winston Mathews sent me this picture along with a description: "Here are our modified DX6 radios. 2200 mAh batteries, new voltage regulator, jib-trim potentiometer and now "internal" antennae (mounted horizontally). Range is unaffected. Thanks for the idea and your help. I would advise to install the voltage regulator. We can sail for two days without recharging. " Photo by Jack Wubble, owner of the open radio in the pic. Discussion on this is over at the EC12 discussion forum.

Update 2007 Nov 23:

Some text and images on modifying a Futaba 2.4 GHz radio on the EC12 website.

Stretching DNA

jumpcut movie:DNA stretching

Some promising results yesterday with trying to stretch DNA molecules. The molecule is attached between two microspheres, and we are actively moving the smaller sphere while the force acting on the bigger sphere is being measured. The image and video shows the view through a 100x microscope objective on the optical tweezers instrument I am building. Towards the end you can see the construct breaking in two stages, so that probably means there were two molecules of DNA between the beads and not one as intended. This is a control experiment and will hopefully set the stage for bigger and better things to come...

Spinning the DC Servos

Some good steps towards driving our cnc-mill with DC-servos taken today. I got the pico-systems servodrives wired correctly, the new 50 kHz PWM m5i20 configuration loaded onto the fpga, and updated my pyvcp test panel a bit. I'm using three 19" rack enclosures. The lower one has a 1.8 kVA transformer, the middle one houses the servodrives, and the top one has differential encoder cards for the motors and optoisolator interfaces to the m5i20.

One small setback was that the servodrives wanted the PWM in reverse polarity compared to what I had available. There's nothing in the m5i20 driver to reverse the polarity of the DAC output PWM. Fortunately the drives have optocoupler inputs so instead of GND-PWM I wired them in a PWM-Vcc configuration and it worked OK. I did an open-loop no load test (below) where I monitored the RPM while changing the DAC output. There's a bit of dead-band in the middle where nothing happens between DAC values of about -0.2 and +0.2. After that the curve is pretty linear up to +9.7 after which the PWM pulse becomes unacceptably short for the servodrive and at DAC=9.8 or above the motors just jump and stutter. So eventually with EMC and PID control I need to limit the DAC range to [-9.7 , +9.7].

Next is probably trying out closed-loop PID control, and after that I need to look at the E-stop chain, home switches, a relay for the flood coolant pump, and controlling the VFD/Spindle.