Wednesday, June 27, 2012

TOBL2: Fire Day

Today we had what I'm going to call a fire day. It's just like a snow day, except with fire...never thought that was an option but that's what happened. Fortunately, where I work was closed due to smoke/air quality and not because there was nothing left of the place. Going outside wasn't the best option and we don't have a TV to watch Spain vs. Portugal in the EURO, so I spent the whole day playing with TOBL instead!

One thing  you don't realize when you relocate for a significant amount of time, is all the little things you're used to having access to. For example, usually when I do any design work I like having paper around to make sure what's in my head can actually be drawn and physically embodied. Well, I don't have any paper here, but we do have some old pizza boxes lying around...

Here was the original idea, of bringing the belt to the motor

And here would be some servo mechanism to tension the belt

What's this, moving the motor to the belt. Is this opposite day? No, it's fire day...

Now whenever somebody says to do envelope calculations, I'm going to do them on a pizza box, so much more room! Anyways, a few minutes into my brainstorming session I realized that my original plan of bringing the belt to a fixed motor was going to be difficult. Initially I wanted to do something like this:

The second spring isn't shown, but if it were the servo would have a neutral point where the spring displacements are equal and thus F1 = F2. This way, the servo could be floating during driving (tension) and the spring (F2) could do all the work. Then, when you need to detension, the servo arm would rotate downwards. However, this gets complicated when trying to load the pin that the tensioner bearings are riding on. This pin would require a slot on both of the plates that sandwich the wheels, and if the spring forces on either side weren't perfectly balanced the pin would tilt and not tension very well.

At this point, I thought back to my real first idea, which was to have the motor on a pivot. I threw out that idea originally because I didn't like having a motor that's not rigid with the frame (and I still don't). Practically though, this is a lot simpler because a motor itself does not rotate like a belt does -you can't bolt anything to a belt. So, that's where my third pizza box drawing comes in, which looks like this in SolidWorks form:

Most of the drivetrain complete 

Better shot of the motor mount plate

View of the drivetrain from the inside where the electronics will eventually sit

This of course had it's own challenges. The primary difficulty was making a robust mounting system for the motor that would withstand the torque from the belt tension on a single pin! To that end, the motor is centered as much as possible so that it's entire mass isn't cantilevered. The plate that carries the motor will serve as one washer of a thrust bearing, the other will be the inner plate, and a matching thrust bearing on the other side. Thrust bearings can take a lot of lateral load, these ones are rated to about 26lbs and were the only ones on McMaster small enough for the task. This picture should clarify what that actually looks like:

In the middle of the frame you can see the darker grey thrust bearings sandwiching the inner plate. To get everything to fit and not interfere (still not entirely done with that) took a lot of geometry and trial and error. Probably took more time to dimension this plate than design all of the electronics.

Not including the servo or supports to keep the inner and outer plates together. 

There are several advantages to this design. Firstly, due to the way the motors are mounted there is more room in the middle section for the electronics and battery to sit. I'm still playing with the tray that will mount all of that but I'm going to do my best to make TOBL2 narrower -this would make driving a lot more controlled. Unlike the original design, I don't have to worry as much about the springs. As you can see above on the shots of the drivetrain in its entirety, there are holes galore on the motor mounting plate, both to accept some sort of turnbuckle from the servo and a spring(s) to maintain belt tension. Having all these mounting options will be important in finding the perfect spring displacement which keeps tension in normal and 60 degree mode, but also isn't too strong for the servo to de-clutch. Next step: figure out the supports which hold the two drivetrains together and carry the electronics. Maybe tomorrow will be a fire day too?

Sunday, June 17, 2012

Summer in Colorado: HardwareHardwareHardware

An embarrassing amount of time has passed since my last blog post. I don't resent that, because I think it just means my life is getting more interesting, so I have had less time to myself. Certainly though, I have missed my secret life as a rogue robot tinkerer, and all the creative stimulation that having projects to work on seems to bring. My six-wheeled friend, TOBL (soon to be TOBL2), has been waiting patiently, yet eagerly in his little box. He's finally about to get some attention!

Box O' TOBL has a pretty nice view. 

Yesterday, I moved to Colorado to spend the summer interning as a hardware engineer...sort of. My first day is Monday so we'll find out what work really entails then. As far as I can tell, I'll be beating some oscilloscopes to death and then when they fail, attempt to figure out why from a hardware perspective. I was told that you're either a hardware guy, and you'll absolutely fall in love with it, or the opposite. I'm hoping for the former.

In a way, I'll learn more about my various robotics projects this summer more so than ever before. Hardware is at the heart of each of them after all. It's also going to be tantalizing working somewhere with so many oscilloscopes, logic analyzers, a machine shop complete with CNC mills and a water jet, pick-an-place machines, clean room, the lot. Surely, I won't have access to any of these nice things for my personal projects, so it'll be a true test of character not to use my keycard to sneak in on weekends.

So, last time I had just got my motor controller boards back and soldered together. There were (and still are) a lot of loose ends to take of for MKI. In the original DigiKey order, I forgot to put in the resistors for the current detection circuit and the ones for the hard-short between the signal and power ground planes. The board has also yet to be hooked up to a scope (this is going to kill me), though this isn't as critical until I get around to the current detection stuff. It would also be nice to take a look at the pwm too, though.

All of the hardware! Arduino board, motor controller, motors, radio, servos, everything.

As for the rest of TOBL2, all of the pieces for the new drivetrain are in but I still don't have a frame. Most likely this will be a water jet piece(s) so that's something I can work on here. Since last time I got some new XBee radios. They're still Series 1 but this time I opted for the chip antenna, which has slightly worse range but should help keep the electronics a little more compact. The old ones I had worked great but were salvaged and had such old firmware that it couldn't be changed. This doesn't matter since they were already set to talk directly to each other. It's nice to have a second set so I can have multiple boards going at once. I updated the firmware and paired them so they're ready to go. I also modified the code so the servos trigger on a new button I added to the iPhone control layout. Though the way I did it would probably make CS people cry.

Well, that's not much of a progress post but all I got for now. Looking forward to summer of hardware and outdoorsy things!