Thursday 29 December 2016

PiWars 2017 - Autonomous challenges

Its the autonomous challenges at Pi Wars that separate the robots from the radio controlled cars, requiring one or more sensors on the robot to collect information about the real world, process it and make decisions on how to react in order to successfully complete the task at hand.

To add to the complexity you are competing against a whole raft of other robots, so you have to balance between going slow and steady or quick and fast. Do you concentrate on completing the course without any penalties, or risk pushing things to get a good time?

Of course the first step is working out how to approach each of the challenges and get a working, reliable system up and running, before trying to push things to the limits!

Line following

A returning event for Pi Wars 2017, the robot needs to follow a black line on the ground, following it around a twisty course as many times as possible within the time limit provided.

My attempts at line following in the last Pi Wars didn't go so well... In testing the night before the Arduino controlling the line sensor started triggering the watchdog and restarting, clearing the calibration data and causing the robot to lose the line, so on the day I only got about a quarter of the way around the course. So this time around I want to take an approach that uses just Raspberry Pis, partly to try and avoid this issue in future, and partly due to the fact it is a Raspberry Pi based event! 

The first approach would to re-use the line sensor from last time, connecting it directly to a Raspberry Pi instead of an Arduino (something I've successfully done before using the pigpio library). Due to Raspbian not being a realtime OS it can take a lot of CPU time to ensure no data is lost, so for this approach I may have to use a dedicated Raspberry Pi to do the sampling, and send the data to the 'master' Raspberry Pi for processing.

An alternate approach is to use the Raspberry Pi camera to do a spot of image processing to determine where the line is, and where it is going. I've not done image processing since University, so this would require lots of investigation, research and learning. As I like using these events to learn new things, I'll definitely be giving this approach some serous consideration.

Straight line speed test

Another returning event, the robot has to drive in a straight line as fast as possible along a 7.28m long trough. For Pi Wars 2017, however, the straight line speed test has been revised to be autonomous only, whereas previously I've only attempted this when under manual control (Attempts to use the compass on the Sense HAT having proven unreliable!).

A few approaches spring to mind for this event... Utilising feedback from motor encoders to try and ensure the robot is maintaining a straight line, using a sensor to ensure the robot stays a set distance away from one wall (probably reusing the range sensor from last Pi Wars) or utilising the Raspberry Pi camera to try and detect where the walls of the trough are and ensuring the robot stays in the middle.

Using the range sensor sounds the most feasible, but it would be nice to get something working with the camera.

Minimal maze

Another new event for Pi Wars 2017, the robot has to navigate its way through a maze, without touching the walls, to reach the exit. The walls will be of various colours, providing information the robot could potentially use to determine where in the maze it is.

Initial thoughts on this challenge are that I could use a variation of the speed test solution, using a range sensor to keep the robot following the left or right wall, combined with a second sensor on the front to work out when a corner is coming up. Alternatively I could use a single range sensor and start out following the 'right' wall, then when the right wall turns blue (A camera being used to determine this) switch over to following the left wall. Hopefully avoiding hitting the outer wall as the direction changes!

The maze needs to be completed twice, with the times combined for the final result, so can we be a little sneaky and use the first run through to 'map' out the maze? Then use this map on the second run to zoom through it? You'd need to get the starting point spot on for the second run, but it certainly sounds feasible!

So those are my thoughts on the autonomous challenges... Do they sound good, bad, very bad? Is there enough time left to implement all of these?


Wednesday 28 December 2016

PiWars 2017 - Manual challenges.

Its been a busy few months since I was accepted as a competitor for PiWars 2017, unfortunately very little of that time has been spent on PiWars itself. Work has been especially busy, culminating in a sudden business trip to Taiwan where I had access to shops with all the connectors, cables and components I'd need for a robot, but with no idea what I needed yet!

So in this post I've decided to jot down my ideas for the various manual challenges, as most of them have similar requirements, and its considerably easier to manually control a robot compared to telling it to deal with things itself.

My general approach will be similar to last PiWars, a four wheeled vehicle with tank style driving, controlled via a joystick, but with some improvements. Whilst I had the option on my previous robot to change the motors and wheels it ended up being too fiddly to do on the day and led to my robot being defeated by the 'humps' at the start of the obstacle course due to still being on small wheels. So this is an aspect I want to review and improve upon this time.

Obstacle course

As the exact details of the obstacle course won't be known until the day itself I have to make some assumptions about what will be required based on previous events. I would expect certain parts of the PiWars 2015 course to be reused (especially the rotating table) with a few new elements added. As such I'll be needing a setup with good ground clearance and fine controls. A high top speed won't help much here, with the marble obstacle from last year requiring the robot to drive slowly in order not to dislodge any and occur a time penalty.

All the above is subject to change of course, the marbles may be swapped out with a gravel pit and I'll be wanting chunkier wheels to cope with that.

Pi Noon

Having gotten to the final of Pi Noon last time (with a lucky win or two I must admit) there's probably not much to change here. I'm tempted by the mecanum wheels that Triangula was sporting last year, but I would need quite a lot of practise to be good at using them, something I've never ended up having much time for in previous events!


Skittles was a little hit and miss last time, with my robot knocking down more skittles during the practice run than it did in the official attempts. Due to time issues I ended up just pushing the ball for PiWars 2015 which, unsurprisingly, proved to be very unreliable.

Possibly approaches to it this year could be a motorised launching system (a spinning wheel or two to get the ball up to speed), a catapult type system (pulling back and releasing an elastic band), a spring loaded system or maybe something that picks up the ball and rolls it. In theory anything that can propel the ball forwards in a straight line should work out okay for skittles, whereas the next challenge potentially requires a finer degree of control.

Slightly Deranged Golf

A new challenge for PiWars 2017 is golf, and apparently a slightly deranged variant of the game. As such its a bit of an unknown, with only the details of the challenge to base things on. The game is just a single hole (No 18 hole courses here!) and the robot is only allowed to push or hit the ball to get it around the obstacles. What those obstacles are we don't know (sand pits?) and whether its better to hit or push the ball won't be known until the day itself.

The traditional approach would favour having a mechanism that can 'hit' (or propel) the ball forwards at different speeds, allowing the robot to progress through the course, changing directions to move around obstacles and eventually reach the hole. However this does sound like quite a complicated approach, as each time you need to judge how far to hit the ball, chase it with the robot, line up the next shot etc.

A simpler approach maybe to push the ball the entire way, potentially lowering a cup over the top of it to ensure the ball doesn't get away from the robot, leaving you to just drive to the hole and drop it in. This, of course, assumes the course is flat and there is enough space between obstacles for the robot to drive through. If not the ball could escape and you'd be stuck trying to capture it again.

So that's my current thoughts on the manual challenges, next up will be the autonomous ones!