Manual Control

I saw recantha’s post pointing people at Brian Corteil’s post on how to control your robot and was surprised to see that the method I used for Metabot last year wasn’t listed.  So the obvious thing to do is to document it here, right? 🙂

Our method:

  • Connect a cheap USB wifi dongle to the Pi
  • Set the Pi up as a WiFi access point
  • Connect a laptop to the Pi’s access point
  • Connect an xbox 360 joypad to the laptop
  • Write the robot’s controlling python script – it listens for TCP on a port
  • The laptop runs a very simple pygame script which listens for joystick inputs:
    • These come into the script from the Pygame library as a dictionary.
    • The script then converts this dictionary to JSON (using the python json library)
    • and sends the JSON over TCP to the IP address/port that the robot is listening on
    • The robot then converts the JSON back into a dictionary and reads the joystick values out of it.

Note that if you want an easy control method, I do NOT recommend our one – it can be a pain to set up the wifi access point and the method you use depends on the chipset of the WiFi dongle you have.

Instead, I recommend the bluetooth/wiimote methodthis tutorial explains it better than I can and with code examples too.  My experience with bluetooth dongles and wiimotes is that Genuine Nintendo wiimotes are required – I tried using a cheap knockoff wiimote and it wouldn’t pair with the dongle, but my genuine ones worked fine.


Stepper drivers

We received some stepper drivers on Monday – TB6600 based boards.  These are 4A bipolar stepper drivers and are very cheap on ebay (£7 per motor).  The downside is this sort of ebay driver is often a bit rubbish – this thread documents some of the bad things.  The worst problems:

  • powering down the logic side of the circuit but leaving the motor supply connected = burnt out driver.  
  • output current is often less than it should be (due to shut-down circuitry)
  • circuit uses the a pin on the TB6600 to provide 5V to the supporting circuitry – but that pin is for a decoupling capacitor – it isn’t meant to provide power to anything else

So last night we decided to find out exactly what might be wrong with our drivers by tracing the schematic.  This way we can work out a method to avoid the worst problems and (hopefully) get reasonable performance out of our motors!

Here’s the schematic we came up with for our particular TB6600 driver – YMMV…

If you’re reading this because you’re interested in building your own robot, and you don’t fancy mucking about with reverse engineering cheap drivers from ebay, do yourself a favour and build/buy decent drivers – e.g.

It begins

Earlier this week, John and I had a design evening.  A few things were accomplished:

I brought in a THB6064 stepper driver (from my CNC machine) and we plugged that in to the stepper.  John had written an interrupt based arduino stepper driving program (complete with acceleration control!) and we tried that out.  We proved to ourselves that the motors we’ve chosen can indeed get to the maximum speed we’re after, but we’re still not sure about what acceleration they’ll give us.  We learned that smoothly accelerating steppers is important and that once they start to slip, you have to drop the step rate right down to re-capture the rotor.  More to do here we think…

We had a discussion about how we wanted the robot to look (which has a bearing on how the chassis should be constructed).  Likely answer (nothing is set in stone) is that we’ll have flat side panels with a nice profile cut out of the top instead of the flat plate we had last year.  This should mean we can make the robot a little prettier and possibly lighter too.  The downside is that its harder to make accurately, but if we use my CNC to carve out the side panels, we should be OK.

Material choice was also discussed – wood is nice and cheap, but heavy and needs painting. Metal is probably overkill, but we’ll certainly use it for anywhere we need to reinforce.  After a bit of googling, I discovered PVC foamboard – this is a plastic foam sheet with machining properties similar to wood, but less dense.  It can also be heat formed, so curves are possible. We’ve got a sample on order from ebay 🙂

Finally, we played skittles with the set we bought which was linked from the event page.  This gave us a clue how hard the ball will need to hit the skittles – and now I’d be very surprised if simply pushing the ball to the skittles was enough to get a strike.  Looks like we’ll need to design a mechanism to accelerate the ball…  Looks like Leo White has been doing the same thing 🙂


We got selected!

PiWars 2015 was so popular that they couldn’t accommodate all the teams. However, we were one of the lucky few to be chosen, so we look forward to seeing you there in December.

Now we’ve just got to build a robot 🙂

Here’s a shot of the motor test chassis (a very fancy name for a plywood sheet!):Metabot2 Motor test chassis

We’ll be using this to determine if the motors have enough grunt for the performance we want.  Initial indications are that the drivers (little polulu stepper drivers) get scarily hot when driving the current that the motors need for best performance – so we’ll need to figure something out or they’ll die.  Either a bigger heatsink for these drivers or bigger drivers.  Sadly, this also means we can’t yet tell if these motors are going to be up to the job either.  Maybe we can borrow some big drivers from somewhere?