Building a battery for R2 with 18650 cells

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.

General theory

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.

Sources

Cells

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.

An Opus BT-C3100 18650 charger/tester

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.

Additional

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:

  1. 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
  2. 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.
    The start of a 6s12p pack. Can see the parallel sets run down the picture, with the series sets alternating across

    The start of a 6s12p pack. Can see the parallel sets run down the picture, with the series sets alternating across

     

  3. Once you have all the batteries in place, clip the top of the frame into place
    Here is a small 3s5p pack, ready for the nickel strips

    Here is a small 3s5p pack, ready for the nickel strips

     

  4. 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.

    Balance capacity checker

    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.

    Testing

    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.

    Conclusion/Notes

    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.

    Further research

    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.

     

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R2 ventures out

The day finally arrived. I was going to show off R2 to the public. Up until this point, he’d not been out of the garage and only had a handful of visitors (including a few local kids… ‘have you got an R2D2 in your garage?!). I was a little nervous.

Transport

I’d only arrived home at about midnight the night before after having left a convention early (unheard of! Miss the survivors photo! What?!), not to mention I’d been in the convention hotel for 10 days by that point. After contacting the organiser of Morecambe Comic Con, he had said I could get there early to avoid the crowds and find a safe spot for R2. This meant being up, ready, loaded, and in Morecambe by 9am! One slight issue, well many issues with that time, but the main one being I hadn’t actually tried to load R2 into my car at all yet. I knew from measurements that he wouldn’t fit in in three legged mode, which was one of the reasons I’d built his sled. I also knew that the sled made it nice and easy to get him stood up in two leg mode. So all that was left was to see if the sled would hold up and get him into the car.

Success, R2D2 is in the car!

Success!

He fitted. Was a two person job unfortunately, but I have a few ideas to make it easier, and hopefully turn it into a single person job for the next event.

Arrival

I managed to get to The Platform in Morecambe at about 8:55am, and hunted down the organiser. Thankfully Joy had followed me down, so helped me unload R2 infront of the Platform. Switching him from two leg mode to three leg isn’t too hard, 8 nuts to tighten on the shoulders, and some ankle locks to put in place on the feet. Unfortunately I had forgotten the spanner to tighten the nuts so had to get them as tight as possible by hand. Then came the fun of getting him in the building. The pavement surface outside was not conducive to operating an astromech. For something supposedly highly advanced, he doesn’t like to run on anything but a smooth surface. So, with a bit of lifting and a bit of dragging R2 entered the building. Thankfully once inside the floor was nice and smooth, perfect for R2 to have a wander.

R2D2 waiting for the crowds to turn up.As I brought him in and scouted out the place, I encountered a few handling problems. It appeared that I’d lost one of the shims from a foot which meant one of his drive wheels wasn’t getting the grip it should. This led to him veering off to the right all the time and at one point he encountered a stall and destroyed a lego figure! After apologising profusely, I found a corner to sit him in and waited for the crowds to appear.

Time for the fun

R2D2 found a nice place to stand.

Right about 10am, the doors opened for early access and I decided to move to the now closed fire escape. Up to this point, it had been open for people to bring their wares in for the stalls. Now it was a nice empty space to place R2.
Slowly the place started to fill up and R2 had his first visitors. The day started to just fly by after that. I just loved the reactions he was getting from people of all ages. It was so enjoyable to make people jump a little when he moved over to them, and I slowly got used to the controls and being able to give him a little personality. Up to this point, I’d only had a tiny space in my garage to move him, now I had a much larger area and could try a few different things. Kids seemed to love him, sometimes a little too much, but I’d already decided to work on the premise that if they managed to break something then I need to make it stronger. R2 should be a bit interactive. Tho there were times when I wished I had the cattle prod attachment on him. He got plenty of tugs on the HPs, and a few extra spins of the dome, and I managed not to trap any inquisitive fingers in the dome panels when they opened. One little girl however didn’t like him waving at her with a utility arm. I loved some of the cosplay at the event too, there were some fantastic costumes on show, including a friend of mine who came as Kylo Ren and hung around for pictures.

R2D2 playing nicely with Kylo

R2D2 playing nicely with Kylo

I tried to stay out of the way as much as possible as it breaks the magic if they see the wizard behind the curtain, or rather the guy holding the controller. When I was spotted, quite a few people were amused at the fact I was using a standard PS3 controller to operate him. A few people wanted to know how he worked, or more details about the build, which I was more than happy to supply. Possibly a bit too much information was given at times, but they were polite enough to keep smiling whilst I talked.

At one point, I was even interviewed by a reporter from the local newspaper. She asked a few questions and took a short video of R2 doing some spins, and uploaded it to their facebook page. The main site had an article released today, and I was mentioned and quoted in it.

I had a few friends come and visit me too, after all I had been talking about R2 so much over the last few years they wanted to actually come and see what I had been wittering on about. I’d like to apologise now for the nonstop talk of R2. Most who saw him agreed it was a worthwhile project!

Oh No, breakage!

Trying to hire R2D2 as an Auror

Trying to hire R2D2 as an Auror

There was an issue with the main dome drive, which I gave up trying to fix towards the end of the day. Thankfully noone seemed to notice that he wasn’t turning his head much. I was pleasantly surprised by the battery, which after a full day of entertaining was only just down to below half capacity. Also, taking him back to the car over the rough surface outside shook a few parts off him finally. I knew something would end up falling off, but they held on all day at least!

R2D2 gets the ladies...

R2D2 gets the ladies…

Conclusion

Already hoping to be at another event on Star Wars day, a May the Fourth event at Southport Vue cinema. Just need to get his MOT and my driving test, so he can be an official Builders Club droid. I’ll should have the door panels finished for the next event too.

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I set a deadline!

This year has been a bit busy so far, and in February I realised I only had something like three free weekends to get R2 ready for his first outing, Morecambe Comic Con, a deadline I was determined to keep. Between conventions, work trips, and more conventions (including two on back to back weekends), I knew I had a lot of work to do in a short time. Thankfully, with a bit of organisation and a few late nights I finally managed to get him to a showable state. Not quite to the level I wanted, but close enough.

I utilised Github’s issue and project management tools to help organise myself, putting issues in as todo items, as well as logging things that I found were wrong as I went along. This actually helped quite a bit, and I’m going to endeavour to keep using it. I slowly managed to close off some of the items, and R2 was getting more and more complete. I managed to get the electronics and code to a level where it was stable and he wasn’t too fast to react. Had a few dicey moments when direction changes at high speed made him teeter on the edge of doing a faceplant.

And more bits were added, I got his skirt installed finally, after having bought it nearly a year ago. This however involved some fairly major dismantling of R2, which in turn meant I had to finally get the sled finished for him. Overall, I’m quite pleased with the sled, and it allowed me to lay him down gently and take his legs off to get into the base of the frame.

With the skirt all painted and in place, it was time to test the electronics with his new battery. Up to this point I’d been using a couple of SLA batteries, but these were heavy and didn’t fit in properly. Not to mention they were very low capacity. Over the last year I had been collecting old laptop batteries from various sources, and stripping them down to get the 18650 cells out of them. Once I’d tested the cells and selected the decent ones, I made a new battery pack (6s11p) which should provide 24V, with about 22Ah of capacity. A bit of metal folding and riveting, and R2 also had a battery box.

A quick reassembly, and he was back on all three wheels, ready for the final touches. However, time was getting very short indeed by this point. I had one weekend and a few evenings to get the panels on the doors, and to sort out a few other annoying little details. I’d had the idea of using sheet steel for the doors, but had trouble putting the correct curve into the metal to make it sit nicely in the door areas. If I can get this right, then the doors can be attached easily with magnets to the hinge areas, letting me remove them so I can still get the skin off if needs be. Without the right curve tho, this just didn’t work and looked rather poor. Unfortunately, with no time left I simple hot glued them in place, so that at least there was something there. There wasn’t even enough time to paint one of the panels, so was left bare. 

On my last evening to work on him, I made the decision to change the dome drive mechanism. I’d got a new, more powerful motor, but this gave me troubles with the friction drive in that the rubber ring was coming off. I did have a dome gear set, and decided to try that out (once I’d found it. The garage is a bit untidy). Turned out this was a bad idea. The motor gear is only a thin piece of aluminium, and you have to get it exactly in line with the equally thin main gear. With more time (and the correct cad file) I’ll laser cut a much thicker motor gear out of acrylic. This will make it both easier to mesh the two gears, and also a little quieter hopefully.

So, at about 1am I called him finished and went to bed. I was due to set off to a convention the next day in London. To make it even more complicated, there was another convention the weekend after in the same hotel. Rather than drive home, just to drive back again a couple of days later, I opted to stay down in London. I would’ve got another couple of evenings to do more work, but in the end I decided it was the better option.

All I had left to do was get to the convention

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Slow but steady, or not…

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.

IMG_0298_CR2_embeddedI 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.

IMG_0304_CR2_embedded IMG_0309_CR2_embedded IMG_0489_CR2_embedded
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.

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More power!

11934535_10156061505195316_3190924556943319116_oIts 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:

Powerswitch 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.

Fingers crossed!

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R2D2 is all skin and bones

I recently got a nice parcel turn up at my door; my COM8B R2D2 frame. COM8 frames are one of the standard frames that are used by the R2 Builders club, with a number of different variations. I got the B, or budget, frame which is just a bare bones with nothing fancy to it.

A parcel is here Mmmm, packing peanuts! Shiny Aluminium

 

 

 

 

Assembly

The frame is really easy to assemble, as it is well machined and comes with some easy to follow instructions. It took less than an hour to put it all together, and at the end of it I had a nice looking frame. It also comes with handy mounting points for electronics. Some of the other features include a nice bracket for the dome motor, spring loaded to ensure a decent connection between the drive wheel or cog, and the dome. I’m just waiting for the charge bay panel to come through.

Assembled R2D2 frame

The next step was to work on the skins. I got these at the same time as I got the dome, and its been sitting gathering dust all this time. So far I’ve removed a few of the panels from the inner skins, but still have a lot more to cut out. Some of the seams are very thin, too thin for a hacksaw, so I have to be very careful with the cutting out. I may have to resort to the dremel to do these parts, but I’m loathe to do that as it could end up being very messy. There are some panels to remove from the front inner skin that aren’t pre-scored much like some of the inner dome was, so these need to be marked out and cut.

Skin test fit

For now I have drilled some of the mounting holes to attach the inner skin to the frame to give it a test fit. There are small 2mm gaps where the skins don’t meet, but from looking at other photos this is fairly standard and once the legs are on they hopefully won’t be noticed. I may end up filling them in a bit.

Test fit of front skins, both layers. Lots to cut out on both layers.

Test fit of inner rear skin. Need to cut out the rear door.

I’m just waiting for a nice day one weekend to finish the dome and do some cutting on the skins. Just a couple of holes to cut in dome, and a whole load of filing and sanding to make things fit properly. I have the hinges now so can start glueing those in and making the flaps work.

Slow progress, but it is at least progress

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Slicing my inner dome!

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).

IMG_0182To 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 disc grave yard. They really don't last long, and some of the shattered. The larger ones broke in the center so were unusable.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.

All the pie panels done now, along with the top HP. The metal on the spun domes is a fair bit thicker at the top. Just needs a load of filing and sanding to make smooth and safe.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.

Finally got round to cutting some holes into R2's dome. He now has lights! :)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.

 

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Inside Out

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.R2D2_Electronics_Block_Diagram

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.

PDUPower 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. 🙂

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Braaaaaaains!

So, whilst waiting for parts to turn up and paint to dry, I thought I’d make a start on the controls for R2. Simplest would be to just have the drive wheels, dome, and a couple of other bits controlled from a standard hobby RC controller. I’ve got a eight channel one and a few spare receivers so it would’ve been trivial to do that. However, I wanted something a bit more fancy. I’m wanting to have the panels on the dome to be controllable, as well as various controls for the main body panels, arms, fire extinguisher, etc.

My thoughts for this is to use a Raspberry Pi, along with a bunch of peripherals on an i2c bus. Adafruit do a nice i2c 16 channel servo board which a couple of should give enough channels to control everything I want. I will be putting one of these in the dome for all the various flaps, and another in the body for speed controllers and door mechanisms. Using a Pi will give me lots of power for doing controlling the servos and playing sounds, and hopefully I won’t need a real time OS. Future plans will involve voice recognition and maybe even facial recognition!

I’m also wanting to do this as modularly as possible, so to this end my first bit of programming was a daemon to run that listens for commands and sends signals over i2c to the servo controllers. My thinking behind this is that I can write many different interfaces for controls, from a simple web/PHP interface for testing and such like, to another interface to translate bluetooth signals from a PS3 controller. I’ll also be able to write a scripting system to do preset motions.

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I’ve put the code for my R2 Control software up on github to keep track of changes and such like. If anyone else feels the urge to use it, or even contribute, they are more than welcome to. I’m no programmer, so I may be going down the wrong path for all this, but its a learning experience. The code seems to work pretty well for now, but still needs to be tested in R2. Theres still a couple of things I need to add before I start doing any of the other programs. Currently the servos will move as fast as possible to their new position so I need to add a loop in there to allow different speeds. I also need to add support for multiple i2c addresses. I should have these things coded in fairly soon hopefully, and once my hinges for the dome turn up I can start installing it all and testing.

 

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Polishing the dome

The R2 dome that I got actually consists of 2 domes, one to fit inside the other. The outer one comes pre laser cut to make things easier, with all the various panels held on by a small tab of metal. A quick run with a file or fine hacksaw makes them a doddle to remove. Both domes are produced by a method called metal spinning, which means they have ‘spin lines’ running around them. This doesn’t matter for the blank inner dome as it is mostly hidden, but the outer dome needs the lines polishing out of it to get the correct look. This is a lot of work with wet and dry paper, going from a pretty coarse grit, up to some really fine paper.

Starting with the 240 grit and a large bottle of water, giving the dome a good sand down will remove the spin lines and make the surface rough, but even. Moving up through the different grit ratings slowly (very slowly) smooths the surface over until you get to the 3000 grit. The side product of this is silver hands!

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It is all a slow and laborious process, but the end results are worth it. Whilst doing this work, the panels removed earlier needed painting, so a load of paper was laid out and the painting begun. I’ve been using Halfords car spray paints, starting with a grey primer to prep the surface, then a coat of Ford crystal blue, followed by Citroen poseidon blue, and finally a coat of laquer to protect it. It seems the trick is to do light coats, with a little bit of sanding in between with something like a 600 grit paper.

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The end results look really good. I applied a little too thick in a few places, but its all a learning experience for me. I’ve just got a little more polishing to do on the dome, and I need to mask off most of it to let me spray the ring around the bottom of it blue. The next stage is to start cutting the inner dome to allow the panels to open and things to come out. I’m a little apprehensive about doing it, but once I’ve started it it will be fine. But for now, a taster of how it will look:

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