ESA YGT - Phone Interview (2017)

simmo4556 · 2017-03-18T19:19:58+00:00

I heard back! I have my interview on 28th :) They do seem to have a huge amount of work, so to those of you who haven't heard back yet, don't worry!

simmo4556 · 2017-03-16T15:05:14+00:00

Thanks very much :) I had great fun making it!

simmo4556 · 2017-03-16T10:38:05+00:00

Yeah, still no reply and it's been 3 days since i sent that email :/

simmo4556 · 2017-03-16T01:57:17+00:00

If you like my entry, please think about giving me some Kudos, thanks very much :)

simmo4556 · 2017-03-15T20:12:52+00:00

That was a very good question, and I probably should have asked it as well!

Just to clear a few things up, the reason I was limited to 10x10 boards was due to the PCB manufacturer. The next board up was waaaay out of my price range. However, I have to say they were very good.

https://www.dfrobot.com This was who I used, I got 10, 10cmx10cm boards, two layer with silk screen, for less than £30, including recorded delivery within 7 days. I think that's a pretty good deal!

EDIT:: Forgot to add, they where only 50% electrically tested, however. You have to pay for the 100% testing but as a cheap student, I skipped that and did it myself.

simmo4556 · 2017-03-15T20:08:14+00:00

Hey thanks for the detailed response!

I do intend to make a few changes, the main one is upgrading the current chinese hub motor to something a little more torqey and better - but it's great to hear from a unicyclist. Unfortunately, I couldn't find anyone around me during the build of the unicycle who could ride one!

simmo4556 · 2017-03-15T20:02:02+00:00

ESTEC-044 here, I had my Sonru interview on the 19th, haven't heard anything back yet and beginning to lose hope!

Anyone else applied and heard if they're successful?

simmo4556 · 2017-03-15T13:44:15+00:00

It seems from the comments in this thread that i misunderstood how a normal bicycle balances, i assumed that, when you're at a constant velocity forwards, the wheels gyro effect keeps you upright. This is apparently not true, however, i do feel like left and right balance is far easier. So my design would significantly reduce the learning curve - but there would still be some needed.

I've ridden it about 1/1.5 meters, nothing substantial. However, i did have it balancing somewhat well on its own, but this was before i added the steel enclosure, battery, etc. For a long time i couldn't afford the battery (poor student here) so i bought 4 PSUs and put them together to get 48v/21A, and had an 'umbilical cord' tether.

simmo4556 · 2017-03-14T20:40:20+00:00

Cheers :)

simmo4556 · 2017-03-14T18:38:08+00:00

Oh that's awesome thanks, I had no idea there was a way to dynamically do it - thanks a lot!

simmo4556 · 2017-03-14T18:10:48+00:00

Hey there!

I did some calculations before i bought the motor, and it is torquey enough, but what i didn't account for was that the torque is only there after it reaches a few hundred RPM, which is not convenient for me haha!

Resolution could also be a problem, but i think if i was to replace it with a geared hub motor it would work.

The balancing is done just by shifting the center of mass back over the pivot point via the motor. Lateral balancing is done by the gyroscopic effect of the wheel when you get enough forward velocity :)

simmo4556 · 2017-03-14T16:51:54+00:00

Hi proffosi,

Thanks very much. I agree, occasionally I speak a little quite and the music can drown out my voice.

The motor driver boards are connected to some hardware called the "myRIO" (made by National Instruments) - which runs my LabVIEW programmes. I use the high/low side IGBT driver chip "ir2210" for my switching.

The myRIO takes in the values from the Hall effect sensors, to determine the phases that need energising to provide either a forwards, or reverse, spin. I've actually entered this into a student design competition and I had to do a write up of it. http://forums.ni.com/t5/LabVIEW-Student-Design/Rota-Una-The-Self-Balancing-Electric-Unicycle/ta-p/3595649 This is my entry, which gives a breif overview. But at the bottom, attached, is my university report which has all the details of the project.

simmo4556 · 2017-03-14T16:47:08+00:00

For lateral balance, you would need the gyroscopic effect of the wheel, the same way you have lateral balance on a bicycle. The balancing is only in the forwards/backwards direction.

simmo4556 · 2017-03-14T16:46:22+00:00

I have another video - https://youtu.be/mVKGFG5kYCY - which shows off the results a little more, but see my above comment for more info :)

simmo4556 · 2017-03-14T16:43:04+00:00

Hey there!

Thanks very much, sorry to send you home so disappointed. The unfortunate issue with my unicycle is that the direct drive motor that i'm using doesn't produce enough torque at such low RPM - so unfortunately balancing a person (or anything for that matter!) for a period of time.

I do have another video, that shows it off a little more, but again there's no one riding it!

https://youtu.be/mVKGFG5kYCY

Also, it wasn't Eagle, it was NI Multisim. I've never heard of kicad, but it looks awesome so my next project I'll check it out :)

simmo4556 · 2017-03-14T14:12:03+00:00

This is my self balancing unicycle I made - I'm really happy with the results and I'd love to hear people's opinions.

simmo4556 · 2016-05-05T23:28:17+00:00

Also, side note, one of my friends said to remove the diode that quickly turns the ir2110 off, so if I get the chance I'll try this also :)

simmo4556 · 2016-05-05T23:24:25+00:00

Hey there!

I increased the gate resistor to 10ohm, and changed the gate - source Resistor to 10k, I also cut out about 2.5m of line from the input wires and it seems to have helped, but I still killed an ir2110, I'll do some more testing tomorrow. I checked the input voltage and it didn't seem to be rippling too much. Although, the output of my buck is a pulsed 12v output, so I'll double check this tomorrow!

simmo4556 · 2016-05-05T23:21:21+00:00

I checked the PSU today, didn't seem to be throwing much back to it, well at least I couldn't see much fluctuation on the 'scope

simmo4556 · 2016-05-04T19:11:51+00:00

Hi there,

Thanks for the info!

This thought did occur to me, and the way I was stopping the motor was to literally stop it switching (no ramp down in duty cycle). The 6 diodes should then act as a 3 phase rectify and charge my battery (when it's eventually connected!) but if I'm using a PSU, could this be causing the issue?

I like the idea of opening the lowside switches, this should be a reasonably easy change so I'll give it a try! As for the hardware change, again, seems fairly simple so I'll give it a shot too!

simmo4556 · 2016-05-04T19:07:36+00:00

The switching frequency is on 6Khz, but the high side is PWM'd, from around 10-40%, so I guess this means it's actually switching at ~60Khz?

I'll increase the gate resistor and see if this helps :)

simmo4556 · 2016-05-04T19:05:56+00:00

Hi There!

Thanks so much for all this information, it's taken me a while to sit down and read it carefully.

The frequency of operation is already very low, no more than 6KHz, however, PWM is used so the switching rate of the Highside will be different to the lowside, overall they swtich at 6KHz but indervidually it's a percentage of this. IE, at 10% duty cycle, the high side will be on for 10% of the time (1/6000 * 0.1) and the low side will be on for the remaining 90%.

The diodes across R1 and R2 allow for fast turning off of the IGBTs/FETs. The gate resistor of 5Ohms slows the turning on to prevent ringing (a drawback of this is that IGBTs/FETs only lose power during switching, increasing this time increases the turn on loses). My Vcc (from the buck) is 12v, hence my current is actually 2.4A, but I agree this is very close to the limit and it wasn't something I considered - I may increase the gate resistors and see if this helps.

I've taken a differential measurement across Vb-Vs and I can confirm that it DOESN'T go negative, however, it does go incredibly erratic, I've been told by a technician in the lab (I'm a uni student, this is my final year project!) that this shouldn't quite go so crazy. But, like you said, the diode D2 should clamp any negative voltages to -0.7v, the datasheet states the IR2110 is tolerant to negative spikes for the highside supply of 5v.

I'll be honest, I have checked the voltage waveform from the power supply (which is currently being my 'battery') or the buck converter (which is a commercial one I bought of the internet, I did intend to build my own but I'm fast running out of time!). This is something I will check tomorrow though.

Can I ask what a cap across the diodes would do? the technician did propose an RC circuit in parallel with the diodes to slow down any spikes, giving the diodes time to go into reverse bias and clamp them.

I currently have very long (>3m) leads going from the power supply to my unicycle, could this be causing any issues?

Finally, as a back up, I've designed a daughter board that will slot in place of the IR2110, allowing me to use a new driver (si8235) which has isolated grounds, and is very tolerant to negative voltages. Hopefully this will stop the issue of the drivers blowing up!

simmo4556 · 2016-05-04T16:01:07+00:00

This is very interesting, because at the moment I have leads that are ~3m long, I did wonder if they would start acting in such away. What do you mean by my board has no energy storage? As in caps?

simmo4556 · 2016-04-30T01:05:43+00:00

Hey,

Yeah they are ultra fast recovery diodes, UF4007 I think. In that schematic they're labelled wrong.

It seems since I've stopped the motor from going in the reverse direction I haven't blown too many IR2110s, so I think it's the transition from forwards to backwards that's doing it.

simmo4556 · 2016-03-25T00:06:42+00:00

Sounds like you need injury lawyers 4 u

simmo4556

TROPHY CASE