I started testing last night after finishing initial motor and wheel sensor hookups to my development board. Couple of minor issues so far.
I’m using a pic based board from Modtronics. These are great and reasonably priced, along with the prototyping daughterboard. However I have since found that the onboard RS232 is not on the same pin as the PICAXE28X, so I had to wire in an additional programming circuit, no biggy. The other issues so far were the wheel sensors. From my initial design I had used 10K resistors to bias the output of the opto-transistor, this was not tested and when testing the logic probe was picking things up ok, but not the PICAXE, after much stuffing around and not being able to find my multimeter (it was still packed away from shifting house) I ended up prototyping the same circuit, still had some issues, so I went hunting for my multimeter and of course it was on the bottom box lol. Luckily thanks to my trusty multimeter I found that the sensor was just dropping down to 3.8v when interrupted, anyway the solution to the problem was a larger resistor, a 22K on the output, this tested AOK, now getting ttl levels when switched, I also noted that the distance of the sensor to the reflective object (paper in this case) was fairly critical. I modded the breadboard and then found one side was working fine, the other was not being seen. Checked and found that the sensor was slightly further away than the one that was working. So out with the hot glue gun and moved the sensor. Both side are now giving me a reading on the PICAXE.
Next I adapted some motor control routines on Hippys PICAXE site to test my motor circuit. First I tested without my motor control board plugged in, didnt’ want any smoke to escape, and all logic tested aok, then came big crunch of running motors live. I can gladly say that all tests worked A1 with no issues. Motor was tested though a range of 3 speeds in FWD, REV, Turn right FWD , Turn Left FWD, Turn Right Rev, Turn Left Rev, Spin Left and finally Spin Right. While Testing the FWD and REV, I checked the Pulses from the wheel encoders to see how much the same each drive was over 100 counts from the wheel encoders and it looks like the Right side is slightly stronger than the left, no real surprise here as no two motor/gearbox combinations are the same. I had thought about this a few days ago and knew that with any differential drive system I will encounter this type of problem.
Motor Control Algorithm
This then leads me onto my motor control algorithm. Over the last few days I have been researching PID Controller and their application to motor control for robots, as I want Buzzbot to drive in a straight line, also been looking at fuzzy logic, but I think will give that a miss at the moment. I have been thinking of using and adaptation of the PI Proportional controller from the book “Robots – Inspiration to Implementation” however so far I’m not 100% happy with the PWM and the way it is from the code I adapted, while it is very simple and works, I would really like to use the onboard PWMout routines, but as I mentioned in an earlier entry it is only limited to two PWM and to two physical pins on the PICAXE – Bummer!!!! I also thought I can use the old PWM command. This command requires it to be refreshed, much like what is happening in my test code, except with a agreater resolution, unlike the PWMOUT that runs in the background, perfect I thought!, but when I went to test it I found it’s only available on the 08 series…double Bummer!!!!! So now may have to look at teh PulseOut command.
While the existing configuration does give me three speeds, the lowest is time critical and I notice that when I add a debug routine the slowest speed does start pulsing, also I don’t think it will give me the fine control needed on each of the wheel speed controllers. I’m going to have to think about this a bit deeper.
All in all everything is working as expected, with a couple of minor hicups along the way. Biggest issue will be PWM resolution and time of my routines. I think I’ll need to come up with another way of doing what I want simply and without too much additional hardware if I want a closed loop control system. While I ponder on this I’ll test the IR Sensor circuits. These I’ll prototype first before I hardwire in. Once I finish these I can then start working on behaviour. I would like to look at using some sort of subsumption model, but will look at it deeper as I get closer.