One step at a time

Happy New Year Everyone!!

I'm back from a short holiday down to the Melbourne Cricket Ground to watch the Boxing Day test. It was very relaxing, but now that I have returned I have lots of robot news to share!

Firstly, despite being unable to find wCK gears anywhere in the southern hemisphere, I have sort of fixed the broken knee servo. Thanks to an emergency surgical field graft, he can now walk fairly normally but has lost the use of his right arm. Apparently I'm not the only person who is having problems with the plastic gears. If you are concerned about damaging your own servos, I suggest you set them to the lowest torque setting. Note that 4 is the LOWEST torque setting, and 0 is the HIGHEST - the manual could probably be more forthcoming on this point.

Now that he's up and mobile again, I've resumed work on getting him to keep his balance while standing.

Whenever I start work on an ambitious project, I find that the only way to keep on track is set myself small targets along the way. I am always coming up with ideas that are easy enough to design, but take far longer to implement - and more than a few of them have fallen by the wayside because my eyes were bigger than my stomach.

So having small, short term achievements keeps me motivated to move on with the long term goal. A very simple method of controlling complex systems (particularly when computing power is limited) is called discrete state-space control. Rather than trying to model the system and predict how it will behave, the system is manually broken down into a discrete set of inputs and outputs. Then, using some very basic rules, the behaviour of the system can be described in very simple terms like: if A, then B.

In the case of the robot trying to maintain it's upright position, you can simplify things by firstly holding most of the joints rigid. Using the feedback information from the ankle servos (04 and 00), you can predict if the robot is tilting too far to the left or right. Then, depending on which state the robot is in, it can take the appropriate action to regain it's balance. Simple, no?



In the video, you can see the robot measuring the angular velocity of it's ankles, and detecting when it's balance is being compromised. It simply steps away from the force to maintain it's stability - just like a human would.

Right now, it only work for left and right - simply because the canned robobuilder animations for forwards and backwards consist of several steps, meaning I will have to write my own motions.

I should probably start putting together my own library of poses anyway, since I'm not too happy with the default ones. The walking and turning motions have a real problem on many surfaces - polished wooden floors are the best so far, but far from perfect.



Here you can see that the robot is having problems moving in the right direction with the stock SidewalkRight/Left actions, to the extend that he goes the wrong way! Because he was off centre to begin with, one foot is carrying most of his weight, so he pushes against the ground and towards my hand.

Code will be available soon, once I've cleaned it up a little and found a nice place to upload it. I think that robosavvy has personal file space for it's members, so I will see if I can make the files public.

It was only a matter of time...

It was inevitable, really.

While chasing a guinea pig under a table, I managed to cause enough vibrations for the robot to topple off and land face-first on the ground. Although this may sound both comical and adorable, I was not amused.

At first, everything seemed OK, but closer inspection revealed that one of the spanner shaped connectors had snapped, and more thorough testing revealed that one of the servos was making a grinding noise.

Disappointed, but not discouraged, I voided my warranty and opened it up to see what the problem was.


As it turns out, I had stripped some teeth of a plastic gear, which meant that the motor was spinning uselessly as the feedback loop was broken. Fortunatley - in one of the more pleasant customer experiences I have had - it turns out that the robosavvy people had included a bag of spare servo gears.

So fixing it was just a matter of unscrewing the case and feasting on the delicious goo inside. If a screwdriver was included in your kit, don't use it to open the servo cases! It's just a shade too big, and I ruined the tip by trying to get the screws out. Just one size smaller should do it, but be sure to push down hard against the screw so that you don't strip the head off.

Included in my kit was an instruction sheet (the backdrop of these photos), which explained how the wCK modules can be re-assembled. I don't know if this is because the expect them to need servicing at some time, or just because they had the CAD models lying around. Either way, there isn't much to the gear assembly, so don't be afraid to open one up if you are curious. After replacing the damaged gear, I simply slid the bearing back on to the drive shaft and put the case back on.
At this stage, I was worried that I might have not oriented the output shaft correctly, resulting in the servo position being slightly out of phase with the desired angle. However, the wCKs must use optical encoding or something to establish their position, so after putting the case back on it just worked straight away. [UPDATE - The servos actually use a potentiometer, but the shaft is designed to only fit correctly].

So there you go - a potentially disastrous situation was averted thanks to the inclusion of some spare parts. Since nothing lasts forever, I would recommend you order from robosavvy - their service so far has been exemplary.

[UPDATE - the official servo repair instructions are available]