One of the things I’ve been getting into recently is using Software Defined Radios (SDR) with cheap USB digital TV tuners that are available all over ebay, etc. These have been taking the radio community by storm as the chipset in them from RealTek can tune into a really wide band of frequencies. Usually from down around 20MHz, all the way up to nearly 2GHz.
Tracking the skies
One of the fun and cheap projects you can do is track all the aircraft that are flying around you. All commercial and some light aircraft are required to have an ADS-B transponder that sends out a radio signal on 1090MHz that contains their location, speed, etc. These signals can be received with one of these SDR dongles and decoded to display this information on your computer.
There is even a ready made piece of software called dump1090 (and many, many forks of it) that will also display this information on top of a google map.
The first thing you will need is a decent antenna for receiving these signals. The easiest to make is a quarter wave ground plane. In the simplest terms, this means that the main part of the antenna is a quarter of the wave length of the signal. The equation for this is:
f = c / λ
where f is the frequency, c is the speed of light in m/s, and λ is the wavelength. So to get the wavelength of 1090MHz, you rearrange the equation to:
λ = c / f
Putting the correct numbers into this means that a quarter of a wavelength is just 68.8mm. This makes for a pretty small antenna.
The ground plane is just four more wires at 90 degrees to each other, and bent down at a 45 degree angle.
All these wires are then soldered into a standard SO239 panel socket.
There are many sites that will give a much better walk through of how to make one and the details behind it.
Next, we need to look at the software. At this stage, if all you want to do is have a quick look for some fun, all you need is the antenna, usb stick, and the dump1090 software from one of the many forks (If found the dump1090-mutability one to be best). Download or clone this repo, and follow the compile instructions to give it a try. You should then be able to view the results either on screen or via a web browser.
The other option if you are going to go for a more permanent solution is to maybe use the data to improve a commercial sight such as FlightRadar24. This is what I chose to do, so I signed up for an account with them and followed the instructions to build a dedicated Raspberry Pi receiver that will automatically upload any information I receive to their systems, hence improving their reliability. This also gets you a full business account on there worth $500/year!
Lastly, if you’re doing a permanent solution, you need somewhere to put it. To save space and cost, I just used a Raspberry Pi with an decent sealed enclosure box.
I used some epoxy to glue the antenna on the end of some 20mm conduit that I had, with the cable passing down through it. Then drilled two holes in the box. One that I put a rubber grommet in to pass the power feed through, and the other to put an external antenna connector through.
Lastly, I used epoxy again to fasten some 20mm brackets to the box lid for the antenna to attach to. This was better than screwing, as fewer holes for moisture/bugs to get in through. Then I just mounted the whole thing on the side of my garage and passed a cable for power to it.
Ideally it would be placed higher, or at least the antenna would be, but that started to make things complicated. As it is, I can still get signals from planes nearly 200 nautical miles away.
Now I can access the Pi whilst I’m on my network and view all the planes around me. I also have a full business level account with FlightRadar24. Is there much point? Nope, not really apart from helping crowd source data for a company. Why’d I do it? Because I can and its fun!
I got a message a few weeks back to see if I was still looking for old laptop batteries for 18650 harvesting, and of course I said yes. The next day, I got one or two batteries dropped off at my house:
Wow. Its going to take some time to crack them all open, harvest the cells, and test them all properly. Of course, I’ve got two batteries for R2 now that seem to be in a good shape and last long enough for pretty much any event. So the question is, what do I do with all the cells I’m going to have once I’ve finally gone through all these crates.
I’ve a few projects in mind that will utilise a couple of cells each, including a standby battery for R2’s brain, but a rough calculation shows that once I’m through all these crates I’ll have approx 1200 cells of varying states. So far, I’m through about half a crate and the vast majority seem to be in a good condition and over 2000mAh capacity. That means I’ve got nearly 8kWh of energy storage! Even if I assume half the cells are dead (so far only about 2% seem dead), thats still 4kWh.
The first thing that jumps out for this amount of storage is a form of power wall. Now, I’m not going to do anything grid tied, that is just too much hassle, but doing something off grid for the garage is definitely doable. If I can perhaps do enough to run the computer and other electronics, plus indoor and outdoor lighting, then I will consider it a success.
To start the project, I got hold of a few small solar cells, one 25W, and a couple of 50W ones. The 25W one will be used for experimentation and testing theories out, and the two 50W panels will be mounted on the wall of the garage for a more permanent solution. I also purchased an EPSolar MPPT charge controller, to go with a couple of spare 12V SLA batteries I had spare from initial testing of R2. I went for this model as it has a serial out port on it that will allow me to tie it into my OpenHAB home automation system and graph things like battery charge, solar power production, and any load on the system.
Being able to graph those details will allow me to make an estimate of how much energy I can generate on a typical day, and from that calculate how much I can actually run off my system for a given amount of solar panels, and also work out just how many kWh of energy storage I need.
The charge controller however will only work with standard lead acid batteries, whilst I want to make use of the 18650 cells. To this end, I did a lot of reading and it seems that there are very few hobby level solar charge controllers that will work properly with lithium technologies. Some charge controllers can be made to work with them, but it is more of a bodge.
After much searching, I did find one chinese charge controller that said it worked with lithium batteries, and actually seemed to back that up in the details. One of the main things to look for is that it supports the typical CC/CV (constant current/constant voltage) charge methods that are required for all lithium cells. A few clicks, and it was on its way on a slow boat from china.
For now, I’ve got the EPSolar charge controller mounted on the wall of the garage, connected to the 100W of panels outside.
Next steps are to get some data logging from the serial port, probably using an ESP8266 based device, dumping the data into my MQTT server, which in turn will be monitored by OpenHAB to be dropped into an influxDB store for graphing with Grafana.
Along with this is the slow process of breaking open a lot of laptop batteries and harvesting the cells. Once I have enough for a decent sized test, I will be looking into various ways of mounting them and hopefully adding an individual fuse to each cell for safety. More research into BMS for making sure the battery is properly balanced is required too.
There are many options for a battery to power an astromech, from the tried and tested Sealed Lead-Acid, to the latest LiFePO4. This article will look at utilising the very common 18650 cells. These are used in power tools, laptops, even Tesla cars.
WARNING, this article will talk about opening old packs, harvesting their cells, soldering cells, spot welding cells, and lots of other things that could be quite dangerous.
Lithium cells of any type can heat up or burst into flames if mistreated. Only attempt the things in this article if you are entirely comfortable with any possible outcomes. Do other research, read other articles, the author accepts no responsibility for any injuries or death from the instructions given.
18650 refers to the size of the cells, 18mm x 65mm. They generally have a capacity between 1500 and 3500mAh. If you see anything saying 4000mAh or above, chances are its a scam. There are a lot of cells branded ultrafire that claim over 6000mAh capacity which is a total lie. Voltage ranges from 4.2v when full, to 3.2v when empty. These cells use Lithium-Ion technology, which is a lot safer than the Lithium Polymer that is used in many radio control devices. The drawbacks are that it has a much lower discharge rate. LiFePO4 are even safer, but are also more expensive. Li-Ion seems to be a middle ground, which is why it is used in so many places.
Generally, these cells are arranged in series/parallel to get the desired voltage and capacity. For example, a 24V battery is made of 6 cells in series. Extra capacity is added by putting more cells in parallel, so that if you use cells with 2500mAh capacity and want a 24V battery with 10Ah capacity, then you will use 4 rows of 6 cells, commonly written as 6s4p.
The current drain allowed on a battery is usually 1C, or 1*<capacity>, so in the same example 6s4p battery, you can have a maximum drain current of 10A. Doubling the battery up to be a 6s8p will give you 20Ah and a 20A potential drain. 1C is the safe limit using recycled cells. If you are using brand new cells then you may be able to get a higher current draw by checking the datasheet. For example a NCR18650B can draw 2C and a NCR18650PF can go up to 3C.
As mentioned above, 18650 cells are used in many places, and can generally be recycled. The best place I have found for second hand cells is from laptops or power tools. These battery packs can be cracked open and the cells removed. It is quite a labour intensive task, but saves a lot of money. You can pick up job lots of second hand cells from eBay, tho this is getting more expensive as more people are harvesting cells this way.
You have to force the two halves of the battery case apart, usually with a screwdriver or similar flat sharp object, and then separate the cells from the circuitry and cabling inside. Always wear heavy gloves, and take extra care when using a lot of force. Its easy to slip and damage yourself or the batteries. Also make sure to take care not to use the cells as a fulcrum as this will also damage the cell. Basically, be careful and take your time.
The drawback is that each cell is of unknown capacity and life, some cells may even be totally dead. They could already have been through a few thousand cycles. Each cell needs its capacity testing with a charger/tester such as the Opus BT-C3400. Of course, if you can ask friends and family for donations of old laptop batteries, you can save even more money. I managed to get a lot donated for free. Despite the drawbacks and amount of work required, you can end up with a battery for next to nothing that would cost a lot if you bought a ready made one. For example, I built a 24V 25Ah (approx) 6s11p for around £50 of cells, plus a few other bits.
The other option is to buy brand new cells in bulk. Either from Chinese sites such as aliexpress.com, or from other sites closer to home such as eu.nkon.nl. Chinese ones are generally a little cheaper, but you do have a long lead time and the risk they are counterfeit. A typical cell such as the NCR18650B (high capacity/average discharge rate) or NCR18650PF (medium capacity/high discharge rate) can be bought for approx £3 a cell.
As well as the actual cells, there are a couple of other essentials. These are cell spacers, which clip into various configurations to hold the cells in place, and allow air flow around them. You’ll also need nickel strip to connect all the cells together. Both of these items can be bought from aliexpress.com in bulk. If you are buying brand new batteries from NKON, they also sell nickel strips for a decent price when bought in batches of 10m.
Lastly, you’ll need battery connectors and a balance lead. The battery connector can be anything you wish, as long as it will take the current. The balance lead is a connector so you can make sure that all the series cells are at the same voltage. This is important so you don’t let one cell run down lower than the others, which will potentially damage the cell, and maybe the whole battery. You need one for the correct size of battery (eg, a 6s battery will need a 7 wire balance lead) which can be got again from aliexpress.com or ebay.
Constructing the battery
Once you have enough cells together, and all the other items, time for construction. The general process is:
Sort the batteries into parallel sets with the same total capacity. The idea is to have them well balanced before you even start. You can use a site such as repackr.com to help with that
Clip the cell spacers together in the required layout (eg 6×12 for a 6s12p), then lay the cells out. Each parallel set should be the same orientation (eg, negative to the top), but alternate them as you fill in the series set.
Once you have all the batteries in place, clip the top of the frame into place
Now its time to connect the parallel sets up. Using either a soldering iron, or a spot welder, connect strips along all the parallel sets. These are the ones that are all the same way up. What this does is create the capacity for battery pack. Be careful if soldering, don’t allow too much heat to build up on the cell, do it as quick as possible. You can get spot welders from aliexpress.com for around £200 that will do the job a lot better.
You can also get a device that will give you a full readout, just from plugging the balance connector in. They are only a few pounds from places like ebay. They will let you view the total voltage, each parallel set voltage, and also the max/min/dif between the cells.
For the initial charge you will need to use a decent balance charger, such as an imax B6. These are generally for lipo batteries, used in radio controlled quad copters or planes. The benefit of a charger like this is that it will balance the cells out and has lots of monitoring and protection built in. Follow the instructions in the charger manual closely.
Once charged, leave your pack for a while, even a month, testing the voltages periodically. If you have a dead cell, then it can manifest as one of the parallel sets slowly loosing charge. When this happens, you’ll have to dismantle the battery and retest all the cells.
If you have the time, you can also do a full discharge test with the charger on the battery to get an accurate reading of its capacity. This will take a long time if you’ve made a big battery, depending on the charger you use. If you aren’t overly bothered about an accurate capacity test, just run the battery in the droid (or whatever other use) and monitor the voltage. Don’t let the voltage go down below 3*<number in series> (eg, a 6s should never be let to dip below 18v). To prolong the life of the battery, don’t even let it go that far. Full charge/discharge cycles are the worst case for wear on them, and will shorten the lifespan. I recommend discharging it to around 40-50%, at least on the first try.
After the first discharge, check the balance of the cells again. Ideally there should be little difference between them in a fully functional battery pack. If there is significant difference (IMHO, 0.1v between the highest and lowest voltage) then you may have a bad cell somewhere. Do another balanced charge and discharge cycle and see if the same cell has troubles. If it does, rip it apart and try again.
If the battery remains balanced, then you can actually use a none balance charger (cheaper, and usually higher current for rapid charging) for most charge cycles. Tho make sure it is balanced occasionally and no harm in doing a slow balanced charge once in a while.
Using 18650 cells gives you great flexibility in not only the size (voltage and Ah), but also the shape. This example has shown creating standard blocks, but with some creativity you can make a battery that follows a certain shape (ie, follows the outer curve of an R2 unit’s interior). If you want to make use of recycled cells, then this is a very cheap option to get some very high capacity batteries built. Even buying brand new cells will still save you a lot of money.
For example, I’m currently building a 6s12p pack using NCR18650B cells. I’m getting these for approx £3 a cell. That makes the total cost of cells £216, which gets me a 24V/40Ah capacity battery in a fairly small form factor that can give out nearly 80A (my droid barely pulls 10A at full speed!). I doubt I could fit enough SLA batteries in my droid to get that, and a similar capacity of LiFePO4 would set me back about £800. Even taking into account the cost of a spot welder (which can be used many times of course) its double the price.
One thing I haven’t covered in this article is a BMS. This is a Battery Management System makes sure nothing is going wrong with it, and will cut off the output when the battery gets too low. I’m still researching these myself, and will possibly mess with them on my next pack. IMHO, if you are keeping an eye on the battery voltage during use and doing periodic balance tests and charges, then a BMS is not necessary.
Also note that capacity of the cells will drop over time, depending on number of cycles, how deeply they were charged/discharged, and how rapidly they were discharged. Take care of the battery, and it will last longer, drain it constantly at high current and it will be dead within a few hundred cycles.
Ok, so remote means just a few meters away, either in the house or in the car. Somewhere warm anyway.
So, as mentioned in my previous post, I’d done a lot of research, and one of the things I came across was this video:
This is what I want to be able to do. I’ll never have the room for an actual observatory like this one, but I could at least automate a lot of the work. That’ll scratch at least two, maybe three, of my geek itches. Of course, everything has to be Linux based, and also as cheap as possible. With that, I decided on at least the following to start with:
AstroEQ – Definitely needed goto support on my EQ5 mount to start with.Fully made systems can be bought (minus steppers and mounting hardware), but I already had most of the parts laying around the place so decided to make it myself using an Arduino Mega
Indilib – This, running on a raspberry pi acts as a remote control server for anything that I wanted to add. All devices had to be supported, or easy enough for me to add with my limited programming skills.
Guide Scope – These are used to ‘lock’ onto a star and make sure that the telescope mount follows it precisely. Long exposures of up to even 30 minutes can then be achieved without too much difficulty. Initial plans are to try and use the Raspberry Pi camera (will try both standard and NoIR) versions.
Focuser – Last essential part for remote control is the ability to focus the telescope. This will use the DSLR attached and a stepper motor coupled to the focus knob. There are a couple of arduino based projects that emulate the MoonLite protocol, which is supported by indilib.
Once I’m happy with this lot (and I *will* blog my progress) and have some of my other projects finished (*cough* R2), then I want to take a look at a couple of other add ons such as:
Filter wheels – I can use kstars to take many photos with different filters in place, and also with a black filter I can automatically take dark frames for stacking images. (Dark frames are used to remove noise in the picture that is generated by the DSLR)
Auto lens cap – A simple servo driver to cover the telescope main lens. Not really necessary, but figured it would be a nice project.
I should be able to do all of this fairly easily. I already have most of the components necessary, and the software running on my workbench. One of the big issues I’ll need to work on is just how to mount it all to the scope and stop the cables getting tangled!
My next blog should be on building and configuring the AstroEQ.
Well maybe not that bad, most of my hobbies are pretty much related (electronics, computers, science), and a lot are things I’ve been interested in since I was a kid. Most recently, I’ve invested in a fairly decent telescope and mount to do some visual astronomy, but more for astrophotography. I want to take pretty pictures of things very far away! So after a lot of reading of various blogs and websites (Star Gazers Lounge forum is fantastic), and watching numerous youtube videos, I got a tripod for my camera and a couple of cheap lenses off eBay. That is all that is needed and you can get some half decent shots.
But it wasn’t enough. So I dove back into the forums and did even more research, and learnt a few important things.
Telescope – Numerous different types, mainly split into reflectors, refractors, and catadioptric. All have their benefits and downsides, but for doing astrophotography the telescope isn’t the most important item surprisingly.
Mount – This, for astrophotography, is the most important thing to get right.You need to have a solid mount for doing anything more than a few seconds exposure, and one with tracking in Right Ascension at least, to track the stars. And it really needs to be an equatorial mount to avoid rotation of the starfield as it rotates.
Eyepieces – You need eye pieces to view through a telescope, and the shorter the focal length, the greater the magnification. These are generally only used for visual astronomy, as cameras bypass the need.
Camera – Most DSLR cameras block out a large part of the infra red by design, but you can get them modified to remove this filter and get much more vibrant images. Its not a necessity, but definitely a nice to have.
Whilst learning all this, I had a thought in my head about some form of computer control (Linux based, of course) and actually stumbled upon a few projects to help with this. The first was AstroEQ which was an opensource ‘Goto’ system (select a star, and the telescope will automatically move to center on it) designed around an arduino. That was a perfect start for me, and I was pretty sure I could get it working from Linux. Thats when I discovered indilib!
Indilib is an open source system for controlling all sorts of astronomical instrumentation, not just goto mounts, but also things like auto focusers, digital camera, filter wheels, and other custom devices you may want. Even better, all this can be run from a Raspberry Pi as the control server and a laptop using the actual astronomy software. This would mean I could set it all up, and retreat to somewhere a little warmer to actually do my observations and photography. I’m sure this is against the amateur astronomers code or something, but damn it gets cold out there.
Along with indilib, there is kstars. This is a planetarium program written for the K Desktop Environment, and with EKOS plugin can control any indilib hardware. Not only that, it can schedule work and sequences, and help you plan your observations.
I’m going to (try to) write more blog posts chronicling my progress on getting all this set up, and some HowTo posts on using indilib on a raspberry pi, with kstars, and any custom hardware I make.
It seems that xmas and new year is when all the part runs start. At least that is what it feels like to me, for all the parts I want.There has been a sudden rush in ordering things for R2 which means I nearly have everything I need to get him put together and mobile. The one part I’m still missing is the outer ankles, which I am hoping will be on a run soon. The last few cosmetic pieces I need are due soon too, such as utility arms and LDP. Progress whilst waiting for these parts has not been too bad, but I do keep coming across problems to work around. I guess that is the fun part tho.
On the electronics front, I managed to (I think) blow up my amplifier. I still need to hook it all up again and test it. I’ve a feeling that the switch I’ve got for main power isn’t rated high enough for the current that is going through. I’ve also decided to change the layout of everything, and install an actual touch screen inside R2 for the Raspberry Pi. This will give me the ability to control certain aspects of the software, and also at a pinch I can plug a usb keyboard and mouse in to do onsite programming whilst away at a convention or such like. I’m also currently waiting a Raspberry Pi v3 which will give R2’s brain a bit of a boost. Overall design hasn’t changed much, it’ll still all be controlled via i2c, but will also have wifi and 3g internet connectivity, turning R2 into a wireless hotspot! I will have to see how much the aluminium body affects the signal, but can always put an external antenna somewhere.
I have more or less got the legs finished, and have done a test fit! Must say, they are looking rather good. All the parts slot nicely together and are pretty solid. Of course, I still have the problem of a lot of the screws and bolts being imperial (We’re part of the rebel alliance, don’t want any of that imperial rubbish!) rather than metric, so getting hold of replacements can be tough. This is more of a problem seeing as I’ve had some of these parts for quite a while and not only been moved around the office in the old house, but have moved to the new house and gone in and out of the garage, so some of the fastenings have been misplaced along the way.
I also decided to get one of the nice new hydro formed domes that are available. I was never too keen on the existing one that I had, and the new domes come with the mounting ring to fasten it to the body which meant one less thing I had to fabricate. A lot of the tutorials on the forums are geared around these domes too. Not only did I get a new dome, but I figured whilst I was doing that, I’d also upgrade all the things to go in the dome. This meant getting the ultimate hinges, aluminium holoprojectors, aluminium logic surrounds, aluminium eye, and even the fancy PSI holders. All this together gives me a pretty much top of the range dome for R2. It also means I can do a quick and dirty rebuild of the old dome at some point and create a different astromech.
Which brings me more or less up to date. The dome is nearly finished, I just need to put a few final touches to it and tidy up the cabling inside. I even got the dome servos all hooked up and took a short video. I need to replace the arms on the servos with something a bit longer to get a bit more throw on them, but overall I’m pretty pleased.
Next step is to get to work on the body. I’ve got some of the ultimate hinges and have some installed already. Just need to fit the servos to them. I also need to trim down my data panel to fit it into the breadpan, but that shouldn’t be too much trouble. The one part I’m having issues with is the charge bay breadpan, it just doesn’t want to fit in properly. I may have to resort to some pretty hefty modifications on it.
I’ve given myself a deadline of June to get him mobile, but that depends on when part runs happen. Fingers crossed the outer ankle run starts soon.
Its been a long time since an update, but we moved house at the start of the year and things have been hectic. At least, thats my excuse and I’m sticking to it! I have been making progress with R2 in the last couple of months, doing a lot of work on his brain for starters, and painting various parts.
Code wise, there has been a couple of fairly drastic rewrites since my last update. The interface is a REST API, which sends commands to various modules as before. I’ve added a scripting module now, so that scripts or loops can be initiated such as random sounds, or a dance routine. The servo module had to have a major rewrite too as I discovered that I could only control one servo at once and had to wait for that to finish before another command could be sent. That wasn’t much good! I’ve also written the first of the actual controller interfaces (not counting a simple web one for testing), R2 can now be controlled from a PS3 controller. Button combos are read in from a csv file to trigger certain effects or scripts. Lastly, R2 now has a voice, and can play any mp3 stored in a directory, including selecting random ones from a list of types. Next step is to get either the Pi or the A la mode Arduino to control the speed controllers. I don’t want to run them off the Adafruit i2c servo controller for safety, I’d rather drive them directly and have some form of watchdog to make sure R2 doesn’t go on a rampage. All the code is still available on GitHub under my user, dpoulson
The PDU also needed a rethink, not least of all because of the amount of current it needed. The setup now has feeds directly to the speed controllers, with relays on the output from them to the motors so I can break the circuit if needs be. These relays will automatically turn off if the battery is disconnected so that any pushing of R2 will not feedback into the speed controllers and fry them. The relays will also be controlled from GPIO pins on the Raspberry Pi so I can disconnect them via an API call. I’ll also have an input for a kill switch that will have to be permanently on if any of the motors are to be powered, possibly using a transmitter in a replica droid caller or hilt of a light saber. I’ve a base idea for the new relay controls:
The relays I’ve found are Omron G4A-1EA, which have the benefit of the switched load being on spade connectors on the top, rather than through PCB traces, which when I did the calculations would need to be massive to support the potential current running through them. This allows me to make a simple PCB with the controller circuit, and hook the 24V battery up to it to power the coils. If the battery is removed, the coils turn off and the circuits are broken. No fried speed controllers.
The 24V connection will probably go through the fuse box I’ve installed, with a hefty fuse. The makers of the speed controllers don’t actually recommend a fuse but I’ve seen a few comments saying a 60+A fuse can’t be a bad idea, just in case!
The battery will connect directly to the center contacts of a DPDT switch, with the fuse box on one side, and the charger connection on the other. This will allow charging the batteries without taking them out of the droid. Not sure if this is best practice or not, needs more research. Currently they are just a pair of 12V SLA batteries that I had, connected in series to give the full 24V.
I’m hoping to get some time either this weekend or next, to hook up the motors, speed controllers, and battery, to test them out and get an idea of potential current draw. They’ll be controlled with a standard RC transmitter/receiver for now. If I can get the legs onto R2 he may even be drivable by xmas.
So, finally got some time on a weekend with nice weather to get outside with the inner dome and my Dremel. Whilst the dome set I got had a laser cut outer dome, the inner one was totally uncut. The aluminium domes come as an inner and outer to allow you to have the indent around each of the panels, and also lets you have a nice lip if you have the panels opening, which is something I want. Of course, this means lots of holes to cut. Any panels that are going to open have to be cut out, as well as all the holes for the lights and holo projectors (HP).
To start off with, all the holes were marked using the laser cut outer dome as a template. HP and light holes were to be the same size as the outer ones, but the opening panels need more of a lip. A tip I learnt from reading other builder’s logs was to use a large washer, and to put the marker pen in the center, then roll it around the edge of the opening. This gives you pretty much a perfect size for the lip. Next comes the scary part.
Cutting the dome! These things aren’t exactly cheap, and even worse they’re quite hard to come by, having to wait for runs to be done of them by the guys in the states. But, its got to be done so I got the Dremel and a whole bunch of fibreglass reinforced cutting discs for it. I needed a lot as the wear down extremely quickly. I went through over two dozen of them just on the dome. Thankfully I got plenty.
I found the easiest way to do it neatly was to do the large part of the cutting with the dremel, at least enough to make a cut a few cm long so that I could get the hacksaw into the hole. The hacksaw made a much cleaner cut with more control. The Dremel had a habit of biting in and running off a bit, which made some of the fine cuts a bit difficult. By far the hardest bits to do were the circles for which I ended up making a load of straight cuts through the center to form a star pattern, then gradually cut each of the prongs off. Once I had the main parts cut I attached all the cuts with a large file to take it to the lines I had drawn.
It took a good few hours to get through everything, but it was worth it. The holes are still a little rough and still need some sanding down with wet and dry. Also the circular holes all need to be made a little bigger. They were originally marked up to be the same size as the outer dome holes, but ideally they need to be at least a few mm bigger, especially the HPs. Another tip that I’ve found on the net is using a glass wine bottle to help with the sanding of the circular holes. Wrapping some wet and dry around the neck, you can sand in a circular motion to get an even hole.
Still left to do are the rear PSI holes, in both the inner and outer domes. It is the one outer hole that wasn’t pre-cut, so I need to be extra careful with the outer part. The current run of laser cut domes are a lot nicer, with the inner and outer ones laser cut and all just about ready to just polish and paint. Also, I think I’ve been fairly lucky with this dome, as a lot of people report having to cut the inner dome in half to get the inner and outer to fit together properly.
Once the last PSI holes are cut I’ll be ready to bind the two domes together permanently, which is another scary one way step. I’m making sure I’ve as much done with them separate as possible to avoid damaging the outer dome with a slip of the Dremel. I still couldn’t resist having a test fit of the two domes and inserting some of the lighting. It looks pretty good.
Next main steps once its all bonded is to install the main radar eye which I’m hoping to bolt on to make it removable, and then I have to source a load of hinges which seem to be either very expensive, or hard to find.
All in all, a good weekend of work. I would’ve like to do more but due to losing an hour due to the clocks changing, an early morning call from work, and Mother’s Day, I didn’t have much time on the Sunday to do much. Fingers crossed for nice weather again next week to finish off the Dremel work outside. I might also make a start on the skins too.
So, I seem to be building R2 in the reverse of how most people build their versions. Whilst I started with the dome due to finding a good deal, I’ve spent most of my time working on his internals and very little on the actual physical droid. Since my last post back in August regarding R2’s brain, I’ve done a lot of work on how everything will tie together to do the control. My current working idea is to have an i2c bus running throughout as R2’s central nervous system both sending out commands, and receiving feedback.
The main control is still going to be a Raspberry Pi as this gives me much more range to do some interesting things later such as voice recognition, as well as letting me experiment with lots of different ways to actually control R2. I’m still thinking of using a PS3 controller as input, but also thinking of using a wii nunchuck is possible as a much smaller one to control simple operations.
The Pi will be linked via i2c to the various modules such as the servo controllers mentioned in my last post, with one in the dome and one in the body, and also to the lighting systems with Arduinos programmed to receive the signals to trigger various effects. I’m using BHD‘s Arduino code for the TeeCees lights in the dome at the moment, with just minor changes to accept the i2c signals. I may write something at a later date to do more dynamic light displays such as free form text messages to scroll across the RLD, but for now this is more than adequate.
Communication between the spinning dome and body will be through a 6 wire slip ring connector. 2 wires will be enough for the data signal, and then I will pair up the others to provide the power. I’ll probably have to go for two separate 5V supplies to the dome, one for electronics and one for servos as there will more than likely be a lot of noise coming over the servo power lines as they move.
Power for all the electronics will come from a simple USB battery pack, which in turn will be plugged into the power distribution board I have designed. The PDU will take in a raw input from the sealed lead acid battery (or batteries) and produce clean 5v and 12v outputs, as well as a raw output direct to the speed controllers. The PDU also incorporates a few other features such as connectors for volt/amp meters that will be behind a panel on the front of R2, a voltage divider to allow the charge bay lights to function as a crude charge display for the batteries, and also a relay cut off for recharging R2. The last means that I can safely plug R2 into the charger (via an XLR connector), which will pull power going to the rest of the circuits. Lastly, there is the main power switch to kill power from the battery. There is a diode across the switch however which should allow any charge coming from the speed controllers to go back into the battery. This is a feature of the speed controllers to allow regenerative breaking.
The clean 12v will be used to power the audio amplifier. What is R2 without a few beeps and whistles?
I’m just waiting on the PCBs to come back from OSHPark, so I can try them out. Hopefully I managed to get most of it right and I haven’t seriously miscalculated the current draw from the batteries. I don’t want any tracks melting off the board!
Code wise, I’ve done quite a drastic rewrite of the controlling software to make it much more object oriented. Each different module (servos, audio, lights, etc.) is a module with a command keyword associated with it. This means adding new modules (LCD screen, extra lighting, drinks dispenser…) should be easy and just a case of creating a config file and possibly a class file if its a new type of module. All the code is available at github, along with the schematics and board diagrams of the PDU. The PDU is also available to get direct from OSHPark.
Fingers crossed I may be getting a few parts to build the actual droid with soon, including the feet, which means I now have to figure out a drive system for him. Mechanics isn’t exactly my strong suit, so should be interesting. 🙂
Ok, so along with my R2, a side project is something that will hopefully fit inside of R2 and make him even more popular. To reprise his role in Return of the Jedi he will be serving drinks, and not just glasses from a fancy tray bolted to his shoulders. He will be able to mix a drink for you and dispense it into your glass.
The idea is, to have a bunch of reservoirs in a caddy inside R2’s main body, with pipes to an automatic arm that will open a door and raise, allowing you to put your drink under it. My design so far allows for five bottles, with peristaltic pumps from adafruit (via a UK reseller, Phenoptix), some L293D motor drivers, a TLC5940 PWM driver, and an Arduino Pro mini. I still have a long way to go on the design, but I have managed to the main module constructed and the pumps installed. I’ve also got a first revision of the circuit schematic worked out, along with a PCB layout that I’ve ordered from OSHPark.
The main unit is made from a series of laser cut acrylic sheets, with a central threaded rod as the main shaft. The bottle tops still need holes for the tubing to go into, and the lids will be glued to the acrylic. Refilling will be done by removing the bottom plate and unscrewing the bottles. I also need to figure out ideas to check for the fluid level so I get notified when a bottle is nearly empty.
The main control for selecting the drinks will be handled by a Raspberry Pi, talking over i2c to the arduino to control the motors. This will allow me to do an embedded web server for selecting the drinks, depending on what options are available. R2 will also have most of his communication done over i2c which will allow the drinks dispenser tie into that and control the door and dispensing arm. Other future ideas are also having a few spare bottles and QR codes as labels on them so that I can automatically scan them in so that the Pi knows what drinks are available rather than having to key the data in manually.
This is my first attempt at doing an actual useful PCB. Probably many errors and will need another couple of revisions, but it is a start. The Schematics and other files can be found on github here: