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I had a busy weekend. Mike Lynch fwded me a Halloween project for making eerie flickering candles out of LEDs. Steve Karg brought lots of surplus bright green LEDs to one of the meetings and this is a great application for th em. The outline of a Faux Candles project is available as a PDF file from: Parallax EFX.
I also worked on getting a wiper motor hooked up to a rope to make a moving figure. The 1 1/2" pulleys made it move slowly. I used a 4" clothesline pulley and it runs at a great speed to make a halloween spook. I need some way to make sure the pully is fed to the motor so it doesn't jump the track so often (about one time out of 3 at the moment) I used wires around the rope as a safety catch to make sure no one gets hurt.
So here is the planned Halloween setup:
This morning I gave a short presentation to the Belmont Hills "Booming Bears" First Lego League team on the topic of "Design" and showed them some sample drawings and some techniques. They have their playing field all assembled (except for the boat) and have built a motor operated arm for one of the challenges already! I made a poster of Keith Rowell's awesome little Knewt robot to illustrate the evolution of a design. I also dropped off some graph paper, photo paper, 2H pencils for drawing, and a shoebox full of extra legos. They were all studying the Lego ROBOBuilder software for programming their robot.
Last night's AHRC Robot Builder's night out was terriffic. There was a good crowd with enough experienced folks there to help out some of us less experienced folks. Not only that, but the Norcross High Georgia BEST team was working on their project, which was pretty cool (a simulation of repairing the Hubble with a teleoperated robot.) I was blown away by the differnt types of equipment they had (CNC Lathes and milling machines, plastic injection moulding, even a wind tunnel!)
My mission for the night was to work on a drive train for my larger robot. I want to build a standard 2 wheel differrential drive train, and need to some how connect my motor w/ a 5/16" threaded shaft to a wheel with a 1/2" bore - preferably by using some pulleys to create a belt drive that would slip if the wheel stalls. Some folks were more interested in seeing what was going on inside the wiper motor, so we pulled the cover off and peeked inside the coils.
I had 1/4" vacuum cleaner belts and some pulleys I bought at the hardware store. 2x plastic 2" pulleys for clotheslines and a double metal pulley 1 1/2" dia all with 1/4" bore. I had some 1/4" and 1/2" shafts as well. Luckily, Mike Lynch was there and helped out a lot. We used the chop saw to cut some axles from my 1/2" metal rod, and we were able to remove the 1/4" bearing from the metal pulleys by just pressing them out with a vise and some hex sockets, which left us with 1/2" pulleys! We used hex couplers on the 5/16" thread from the motor which also fits the 1/2" bore of the pulleys with the bearings removed.
The pulleys had no hubs, but we drilled and tapped into the middle of the pulley for a 6-32 set screw. I'm advised to use lots of epoxy on the hex coupler to pulley setup, and some lock-tite on the threads of the set screws and the motor threads that connect to the hex coupler.
Don't you hate it when someone asks you what your robot is going to do? In my case, I have no real idea what I want it to do. I will be thrilled if I can get it to move into a straight line and not run into a wall at high speed. I'll have to make a new webpage for my this larger robot and post it on my projects webpage soon. That is, as soon as I can figure out what this robot is going to "do".
These past two weeks I've worked on my linefollower project a bit more:
As far as making the rs232 line level converter: A word to the wise, check out Al Williams' web site AWC for a cheap and easy way out. But I was halfway done with my version before I found it. Since I wanted this to be fairly sturdy so I could re-use it for multiple robots, I made a schematic in Eagle and etched a PCB for it. My first board was a flop. Several of the traces had bridges because I didn't scrub all the paper off, and I accidentally mirrored the image to boot. The second board came out of the etching process much better after using a toothbrush to scrub the smaller bits of paper off. But I didn't align the board and the drawing very well for my 2 layer design. And I crossed RX and TX. And I didn't wire up the power pin. And, well, it was pretty much the worst pcb I've ever made, but it does work. Because I didn't plan ahead on putting it in a particular project box, it won't fit in a project box, so I just sprayed the bottom side with clear enamel to prevent shorts and am using it as-is.
Next on my plate for the linefollower is to get it to drive in a straight line. I can work on my motor controller logic (a PID loop, I guess) now that I have the encoders installed and a serial port for debugging.
Tonight I'm off to the AHRC Robot builder's night out. I've got my eye on making a drivetrain for the wiper motors for building some kind of larger size bot.
My new linefollower project is coming along nicely. Now the family is back, but I've got my project board w/ an ATMEGA162 MCU and HBridge wired up which is controlling both motors now.
I put the robot on the floor for a little test run (no wheel encoders yet.) Overall I was pretty pleased with the flat out speed of the bot and the ability to control speed of the motors w/ locked anti-phase PWM. But I can see that there are some issues:
The batteries are mounted toward the back of the bot. I probably need to move the over the front wheels to help with differential drive. I may need to replace the wheels with wheels of larger diameter (Currently about 1 1/2" in diameter, maybe go up to 2 1/2".)
The family is out of town, so I am working on robots while not out winning bread. To build a big bot with the wiper motors I'm going to need some more complex mechanics - starting with a coupler for the motor to wheels. I was going to use pulleys, but can't find anything remotely suitable at the local hardware store, so I might just have to make something myself. I can get access to a machine shop at the next AHRC RBNO, or at least some access to some good advice.
I was out and about this weekend and stopped in Hobby Town and picked up a pair of Tamiya gear boxes. Then it hit me - I could quickly build a little robot. So, I decided to go for another stab at a line follower. There was some discussion on the AHRC mailing list regarding locked anti-phase PWM which I put into action.
I've got my ATTINY26 based hbridge to work with PWM after nutsing around with it for a while. Don't ask me what the PWM rate is. I just kept changing the Timer 1 prescaler value until things work right (too fast, and the motor just makes a whining noise.) Now I'll try to get I2C communication and the encoder wheel working.
I need another chip to be the I2C master to communicate with the hbridges. I purchased a Wright Hobbies DevBoard-M32 for just USD$25. Nathaniel from AHRC brought one of the ATMEGA8 versions of this board to a meeting a few months ago and I really liked it. The board came quickly and I soldered it all up and tested it out in a little over an hour. Eddy Wright was very helpful too. It has a pinout for each pin of the ATMEGA32 and some extra pins for VCC, +5V and Ground for perephrial stuff. I highly recommend it.
Its my birthday today! Its been about a year since I did major work on my website. I did some housekeeping this morning and worked out a few kinks.
I haven't been doing nothing for all these moths.
- I finished my mini sumo mods and competed again. I won against my competitor from 14 February, but then I went up against delta force and lost. He just scooped me up and pushed me out of the way. I need more weight. I'm only at 350 grams. Something was going wrong with my eye sensors too.
- Built a digital thermometer out of an Atmel microprocessor and some nice freebie high intensity Green LEDs.
- I tried my HBridge again to try to control my 12V wiper motors, this time following a schematic gleaned from the Internet. I got it working for almost 30 minutes before it self destructed. No sparks, but some smoke from one of the MOSFET's
- I went and bought a current regulated power supply at Fry's the next day. That has helped a lot...
- I found that there was a followup schematic to the first one. I created a PCB for it and am now debugging it. I'll update the hbridge page on my website one of these days.
My mini sumo bot is all working now. I had my first Mini Sumo contest on 5 Feb 2005... and I lost big time. The competitor was another MarkIII kit, but when the two robots went head to head, his just pushed mine back and hardly slowed down.
I noticed he had Alkaline cells (1.5 volts) and I was using NiMH cells (1.2 volts), so I think that might have been the big difference.
I ordered the more powerful servos from Junun.org and also got the nice sticky tires. I'm going to add 2 more AA cells to my power supply too.
An issue I'm struggling with now is that I'm a little bit over on the regulation width. I've used my dremmel a bit on the servo and wheel to shave of a bit. I may have to cut off some of the fancy new tires to get them within the 10 cm width. That would be a shame - I want as much traction as possible!
I translated the C sample code for the MarkIII to be compatible with the CCS C compiler. It seems like I fiddled endlessly with getting the servo centering right. Reaming out will a bigger drill bit helped, but did not solve the problem. Then, someone tipped me off that if I shorten the potentiometer shaft on the servo it might help. That fixed the problem!
I broke down and read the CCS C Compiler manual. Now I think I can make the HEX files work with the default boot loader, but first, I want to get back to beating that inanimate block of wood before the AHRC meeting on Saturday.
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