Tag Archives: mBed

AVC – 2014 Report

The Sparkfun Autonomous Vehicle Competition this year was lots of fun, as usual. I ran a similar setup to last year, with the same chassis, mBed microcontroller, Magnetometer & GPS. The only difference was a new controller PCB with some extra features, and some changes in the code. The new control board has some incremental improvements from last year, with on-board battery monitor, better layout, built in Mux to multiplex RC and Autonomous control, and an on-board RS232-TLL converter (MAX3221).

AVC 2014 controller PCB v3.2

AVC 2014 controller PCB v3.2


AVC 2014 controller board v3.2

AVC 2014 controller board v3.2

The battery monitor uses a shift register to control the 8 LEDs, so I only need 4 IO lines to control 8 LEDs. The mux uses a 74S157D to multiplex the PWM lines from the RC receiver and mBed out to the RC car steering servo and speed controller. By changing the mux select line, I can control the RC car from the RC controller or the mBed, either forced with a jumper (RC_SEL) or by using channel 3 on my RC transmitter. To decode the channel 3 signal and give me a digital output I use a Pololu RC Switch with Digital Output.

The other useful addition was a 1F super capacitor, attached to the vBAT pin on the mBed. One problem from last year was that all my log files were dated “1/1/1970” as when the mBed boots the clock isn’t set, and I create the log file on boot. I set the clock once I have a GPS lock, with the time from the GPS, but until I have GPS lock, I don’t know what the time is. The mBed has a real time clock, I just need to keep it powered between power downs.

Supplying 3.3v to vBAT keeps the RTC running, even when the main power is off. Usually, you would use a small lithium coil cell, but I had just bought some super caps to play with and they seemed perfect. I used a diode and resistor from the 3.3v power rail to charge the super cap while the power is on and limit the charge current. It works perfectly and kept the clock running for days while the main power was discounted. Now, all my log files have the correct date and time, and once I get a good GPS lock, I reset the RTC, just in case it has drifted.

The bot was ready a couple of weeks before the competition, but as usual, I didn’t have time to do much testing, and my flight only got in to Boulder the night before the competition, so I couldn’t spend the day before at the course. The day of the competition arrived and I got there early to setup and do some last minute testing.

I was in group 7 of the Peloton class. Team : “Mostly Robots”. Robot : “Eleanor”.

There are 3 heats (3 attempts at the course). The are timed, with bonus points for navigating obstacles.

AVC Ground Course ((c) Sparkfun)

AVC Ground Course ((c) Sparkfun)

Heat 1

Heat 1 got off to a good start.  I’m always apprehensive as the bot approaches the first corner. Anyone can build a bot that just drives in a straight line and crashes in to the fence. Turning at the first corner autonomously is a good feeling ! 🙂 The bot was swerving badly on the straight, more than last year. Same old magnetometer issues from last year, but it turned perfectly on the 1st corner. Its programmed to avoid the barrels and zig-zag through them, which it did, and made the 2nd corner. However on the 3rd straight it started to wobble more and zig-zag quite badly instead of driving in a straight line. It made the 3rd corner, but then started to get a bit confused. It spun round in circles a couple of times, then crashed in to a bollard. Disappointing, but not bad for a first run.

AVC 2014 - Heat 1 GPS log

AVC 2014 – Heat 1 GPS log

Heat 2

The less said about heat 2 the better :-). It all went wrong, and straight off the start line the bot crashed in the kerb.

Heat 3

Heat 3 started well, the bot made the 1st corner, avoided the barrels and made the 2nd corner. But on the back straight got confused again and crashed in to the kerb.

AVC 2014 - Heat 3 GPS log

AVC 2014 – Heat 3 GPS log

In the end, a fun but disappointing day, as I know the bot can navigate autonomously, as it makes it round three quarters of the course, but the gremlins stopped me from completing all of the three heats.

I came 14th overall out of 25 in my class. Respectable, but I could have done better.

The problem is always the magnetometer. I need to find some time before the next event to work on fine tuning it and experimenting with some different positions to stop interference from the car’s motor and servos.

Roll on next year ! 🙂


AVC – All ready for AVC2014 next week

So, my bot is ready (ish) for this weekend. I have the new controller board installed. Only found 3 mistakes (and isolated ground, the wrong supply voltage on the RS232 header, and the SD card holder hasn’t soldered correctly.) everything else worked. I have fixed the first two, but I cant reflow the board again to fix the SD card holder, as I’ve already soldered all the plastic headers on. I can live without the SD card, I will use the internal flash disk on the mBed. It has a few restrictions, but I only store a waypoint file and a config file on it.

AVC 2014 Robot

AVC 2014 Robot

I’m running an updated version of last years code. But seeing as it worked last year, I’l stick with it and improve it slightly. Now I have a reliable xBee connection, the compass calibration process is much easier. No more cables. I will try this year to calibrate the compass, with the main motor running (albeit slowly) to ensure any interference from the motor is present in the calibration settings.

However, I’ve been so busy recently, that again I have not manage to complete a full outside test of the bot before getting on a plane for Boulder ! 😦 the first full test will be in the car park in the hotel the night before.

Not to self – find more time to tinker next year before the competition !

Wish me luck …

SMD PCB Stencils

Usually I build my printed circuit boards by hand. Once the boards are fabricated, I apply solder paste using the patented “Jon Toothpick Method”, then place the components by hand using tweezers. This works fine, as I usually only build 1 or 2 boards. But recently some of the projects I’ve been building have either lots of SMD devices (a recent board had 30+ 0603 resistors), or the devices I’m using are tin, with very small pitch legs (I use SSOP & TSSOP packages, which are very small).

So, I decided to try getting a solder stencil made to use during my next build. I use OSH Park for my PCBs and they recommended OSH Stencils. The stencils were very affordable. They cost about $10 each, and they are about 6.5 sq in. I think they are laser cut Kapton film which according to online review, is pretty durable.

The process of applying solder paste using a solder stencil is pretty simple:

  • Use a old PCB or a guide taped to a flat surface to stably locate the PCB
  • Place a PCB in the guide and line up the stencil with the pads on the PCB
  • Tape the stencil to the guide to locate it in place
  • squeeze a small bead of solder paste  across the film at one end and smear it across the film using an old credit card or squeegee.
  • The film shouldn’t have any left over solder paste on it after you make the pass across the board. The squeegee should wipe it off as it passes. Paste should only be in the holes in the film.
  • gently peel back the film off the board and the solder should be neatly applied to all your pads.
  • place components and reflow the solder in an oven.

Here are some pictures of the process.

AVC 2014 PCB


PCB Stencil

PCB Stencil

PCB Stencil

Align stencil with PCB

PCB Stencil

Tape stencil in to position

PCB Stencil

Wipe solder paste across board

PCB Stencil - Solder paste

Solder paste applied on the pads

PCB Stencil - Solder paste

Solder paste applied on the pads

PCB Stencil - After soldering

After components and soldering

PCB Stencil - After soldering

After components and soldering

I’m very pleased with the results. There is almost no solder spilled on the PCB to clean off, you get consistently the right amount of solder paste on each pad  and the result looks very professional and neat.

The process was easy and quick. I think I’ll be using stencils with all my future PCB builds. I just need a pick and place robot now … 🙂

AVC – Round 1 video with on board camera

This is a video of my first fully autonomous round 1 attempt. I made the first corner and avoided two barrels, but then ran in to the inside fence. I don’t have any footage from the other rounds as my camera gave up shortly afterwards.

AVC – Outdoor testing video compilation

This is a video of the first few outdoor test runs of my AVC robot. As you can see, I had a lot of magnetometer issues, but eventually, the last run does make it to the waypoint, albeit a rather scenic route. 🙂

AVC – Competition Day

The morning of the competition arrived. After getting up early so I could be at the course as it opened to spend the final hour tuning my way points, I released when I arrived at the venue, I’d left my wallet in the hotel, and had to drive back to the hotel and get it, taking all the time I would have had to practice, driving up and down the motorway in Boulder. 😦

AVC 2013 Ground Course ((c) Sparkfun)

AVC 2013 Ground Course ((c) Sparkfun)

AVC 2013 Waypoints v1

AVC 2013 Waypoints v1

The competition began. I was in heat 8, so I had a good 45 minutes to tinker before my first run. I mounted a GoPro and double checked my way pints on Google Maps. I was kind of pleased to see that almost no one in the first few heats made it past the first corner. In fact by heat 8, I think only 2 robots had made it round the course, out of about 30.

It was time for my first run. I was actually nervous !? Even thought I knew I wouldn’t win, I wanted to make it round the course, and not look like an idiot. Also I was the only Brit in a sea of Yanks … I need to represent my country ! 🙂

The countdown began, then Go! I pressed the big red button and my bot shot off ! To my surprise and great relief in the right direction !!! and to my amazement, it made the first corner and turned 90 degrees at exactly the right waypoint, on to the second straight. My heart was pounding as my bot swerved to the 2nd way point. I had 3 waypoints on the second straight to avoid the barrels. It swerved and missed the first barrel, I was actually amazed it was working !

I turned again and aimed to miss the second barrel … but then it didn’t straighten up and started to veer in to the centre of the course … within a few seconds it had hit the boundary fencing, and toppled over. Disaster! I was out in the first round mid-way up the second straight.

Not bad all things considered. Seeing as it was the first time all that code had run at the same time, completely autonomously. Although I was disappointed, I was actually quite chuffed it had made the first corner and hit the first three waypoints. My code was working !
During the beak between rounds I debugged the code and analysed the log from the first run. The bot was working, but was veering to the left, so I adjusted the 3rd waypoint to pull the bot further in to the centre of the course.

The second round. Go! I hit the button and my bot shoots off in the right direction again, but where it had neatly turned 90 degree at the first way point on the first round, this time, it span in circles at the first way point, driving in endless circles until it hit a hay bail.

In theory that shouldn’t be possible. The code should have dropped out of the loop when the waypoint was close, plus the circle was also tight, i.e. full lock. There must be something else wrong.

I spent lunch time debugging the logs and analysing the data. To my horror, the magnetometer was completely out of whack. It would reading north to be 90 degree from where it should be. And in various walking tests didn’t even change readings as I rotated the bot. My magnetometer reading were the plague of this project.

I decided to run another calibration over lunch, it takes a while, as I have to change some connections as I can’t do it reliably over xBee, so I run the calibration over a direct serial connection.

It took about 30 mins to complete the calibration and the bot was reading north again. I had about 20 minutes left of lunch to test on the course before the final heat. I pressed the button for a final practice and the bot shot off in the right direction, first waypoint hit, second way point hit, barrel avoided. Third way point hit, this was the furthest I’d ever got on the course. The back straight was perfect. I purposely avoided the hoop, I wanted to get round, and wasn’t so bothered about points. On the third straight, it actually hit the ramp and completed a jump, without toppling. It hit the final corner turned perfectly and flew over the finish line ! I had completed the course perfectly ! Shame it was over lunch in a practice session and no one saw ! 😦 I do have some video from the GoPro to prove it through !
I decide not to touch ANYTHING and wait for the final round. More and more bots were completing the course. Each run they were getting better. It was now or never.

Finally the third round was here. Nate (The Owner of SparkFun!) was my referee for the final round. No pressure then !?

The referee counted down 5,4,3,2,1 – Go ! I paused slightly to let the faster bots shoot off before me, to try and minimise any potential collisions. Previous rounds had a number of collisions right off the start line, where out of control bots took smaller bots out completely. I saw one completely smashed to pieces. My bot shot off in the right direction … turned perfectly on the first courser … weaved perfectly in through the barrels, detoured slightly wide at the second corner, my heart pounded as it headed for the fence, but with a meter to go, it turned sharply and headed off past the hoop. It started to oscillate on the back straight – shame I never got the PID controller tuned I thought – the bot headed straight for the cameraman on the 3rd corner, again with a meter to spare turned sharply 90 degrees and headed down the final straight … missing the jump (shame. It made it perfectly in the practice, and I could have got some bonus points). The curb of the final corner loomed, the bot shimmied perfectly round it, turn sharply on the final straight and headed for the finish line. I held my breath as the bot bounced over the finish line perfectly, running over the SparkFun GoPro that was balanced on the start/finish line capturing the action … 2 or 3 meters after the finish line it ground to a halt. I was so happy ! I think I actually kissed it (which someone caught on camera) 3 months of late nights and constant problems culminated on a perfect run on the final round. I was elated. I’ll post a video …

What’s next ?

I want to finish the bot off, even though the competition is over. Not only to have something ready if I manage to find a way to enter again next year, but also to understand what’s wrong. I still don’t fully understand why the magnetometer is so shonky. It’s not the device, I’ve tried 3 different devices, and they all do the same strange things.

At the competition, nearly all the other bots has their magnetometer in the car, on the main PCB, not floating on a broom handle above it like mine. I need to understand what my issues are.

I want to finish the PID tuning and increase the speed. The RC chassis is very fast, and I’m running at about 1/10th speed. If I can remove the broom handle, the bot won’t topple over so easily and can turn faster. I also want to include a proportional speed control so the bot accelerates fast if it’s far away and on a straight heading, then brakes to make the turn as it gets close to the waypoint.

I also want to finish the obstacle avoidance, so if I do head for a barrel, I can veer away from it.

All in all and very pleasant way to spend a weekend. I got to talk to lot of interesting people with a similar passion, I got to spend the weekend in 90 degree sunshine and I got to indulge my passion for robotics and spend the weekend geeking out … 🙂

Thanks to everyone at SparkFun for running the event and organising it so well.

Roll on AVC 2014 … 🙂

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AVC – First test run

The mBed versions of the AVC PCBs arrived. I built the first version, with surface mount components in my oven at home. It works first time. It turns out I made one mistake on the PCB, using the wrong ratio SMD resistors with the battery divider in to the mBed, so I had to take them off and modify it with standard through hole resistors, as I couldn’t hand solder the 0602 surface mount resistors. I made the modification for the SPI connection for the IMU, and swapped a couple of sensor to different pins to give me SPI and Serial in the right places.

AVC 2013 mBed Custom PCB

AVC 2013 mBed Custom PCB

AVC 2013 mBed Custom PCB

AVC 2013 mBed Custom PCB

AVC 2013 mBed Custom PCB

AVC 2013 mBed Custom PCB

I mounted the circuit in a Tupperware box on the car to keep it water tight.

My first outside test of the complete system was very exciting, but a little disappointing. Not only did I manage to run my wife over with the car, giving her a nasty bruise, but also the car seemed to like to take the scenic route to the way point, rather than a more direct route. All down to magnetometer issues again. I was still getting erroneous and unstable readings. In fact this plagued me from start to finish on this project, and still hasn’t gone away.

The only solution I have found to improving the magnetometer readings is to re-run the magnetometer re-calibration process regularly (before each run). The numbers always come out slightly different?

The first end to end test of the system in my garden was one week before the competition. I still hadn’t written a lot of the other functions in the code. Sections of the code like; how to tell if I had reached a waypoint, loading way points from SD, obstacle avoidance using the sonar, and tuning the PID parameters. I also hadn’t actually mapped my waypoints for the route I was going to take through the course.

The PID tuning would have to wait. I was getting pretty good steering results just with the “P” terms. This would mean I was in danger of oscillations, but that was the least of my worries. I could hardly drive in a straight line at the moment …

I didn’t have time to write any sonar obstacle avoidance code. I had a plan to read the distance data from the sonar and if an object got within 2 metres, add an offset in to the steering calculation to veer slightly left or right to miss the object. However, I hadn’t quite worked out how to tell if I had “passed” it to remove the offset, as I only had a forward facing sonar. The other option was to just stop the bot if an object appeared less than 20cm in front of the bot, stop, turn a few degrees, go forwards, then continue under GPS control… I’ll get round to adding this later …

I arrived in the US for a conference about a week before the competition and spent the evenings in the hotel writing the missing code. I wrote a basic file reader to load way points and basic config data from file, plus as my xBee worked for about 2m at best, I wrote a simple logging function, which meant I had some debug data after each test run to investigate. My waypoint algorithm was relatively simple;

  1. Calculate the heading and distance to the next way point
  2. Use the PID control to steer on that heading.
  3. If the distance to the way point was less than 2m meters, we’re very close. At this point these it a danger the bot will drive in circles, as the turning radius is greater that the heading it needs to hit the way point exactly. So, once we get close, I no change the steering to steer to the waypoint, instead, continue on the heading I was on just before we got close. You can assume the heading I was on would hit the waypoint pretty close. So, if I stay on that heading, and monitor the GPS vector to the waypoint, as I pass the way point I should see the GPS vector increase to 90 degrees as I pass by the waypoint. i.e. the vector will go from straight ahead, to 10, 20, 30, 45, eventually 90 degrees. At this point I’m within 2m meters and its 45 degrees or 90 degrees to my left or right, so, that’s a pretty good assumption of hitting the way point, so I assume I’ve passed it, and move on to the next way point.
  4. Once the bot has visited all the way point it has loaded, it stops. Course complete.
Way Point Diagram

Way Point Diagram

I didn’t have time to write the obstacle avoidance code or tune the PID in time for the competition.

The day before the competition I spent the afternoon at the location planning the course and testing the code. I rewrote my magnetometer code again, this time using my own simple trigonometry rather than the library function. Plus I debugged the “hitting the way point” logic. By sunset, I’d navigated to a series of way points for the first time, but still careering off course occasionally. In addition I suddenly remembered that I hadn’t compensated for magnetic vs. true north, so added some Magnetic deviation code. This link has a great calculator for magnetic variance. It shows that Colorado has a deviation of about 8 degree, which would have been a big issue if I hadn’t remembered to compensate for it.

Other posts in this series :