Wednesday, June 7, 2017

Indoors Navigation Robot ("Driving Drawers") Post 1

[Foreword: you may notice the fonts are mismatched in this post. Or maybe you noticed that the images in all the posts in this blog, when viewed on a larger screen (as opposed to a small mobile device screen), are placed haphazardly within the body of the post. This is because Blogger leaves much to be desired when it comes to the WYSIWYG post editor. In part because WYS ("What you see") is not always WYG ("what you get") in the context of Blogger's post editor, but also because there are very limited options for aesthetic image layouts in the body of a post. I am planning a Jekyll based webpage for this blog in the general future that will fix these problems. Until then, this is a cheap and easy way to make sure I keep a blog at all.]

I had an idea for a chest of drawers that would drive to my location in a space (assumes no stairs).

Here's the first part of my progress on that project.

Physical Build

I started by dissecting my collection of Roombas and taking the wheels from two different models and hot gluing them, along with some support material, to the base of a plastic chest of drawers.

I purchased a Raspberry Pi 3 and a PiCam, and while I waited I put together a 3D pan-tilt design from Thingiverse. I'll provide a link (link to thing), but keep in mind I do not recommend this design. Unless your 3D printer is very precise, you'll have to do quite a bit of filing and sanding to get it to go together. The pan servo will absorb all the impact whenever it hits something, and will fail (mine did). If you can find a ring of the right thickness and diameter to stick in between the orange disc and blue plate in the photo, the problem is mitigated (I used a bearing I found in a bin--way overkill as it isn't even acting as a bearing--but a quicker solution than cutting my own ring on the lathe).

Not entirely certain of the layout I wanted, I just taped everything in place with masking tape. The battery is stored underneath, in the topmost drawer of the robot. It's a 30,000 mAh battery I bought for use with my smart phone. It has a port that will source 2.5A, which is needed by the Raspberry Pi. I paid about $75 for this model; you should be able to find comparable batteries by other brands if that one is not available (sometimes, when an item is out of stock, a few vendors will offer it for an inflated price, so beware. The price on Amazon for this model was briefly $399 before dropping to $69.99 again). 
I pulled the Arduino Mega out of a project I'm not working on at the moment, though it is of course overkill for this application. I wasn't sure how many sensors and actuators I wanted on board, so this 54 I/O pin Arduino allows for quite a bit of room to grow the project. The Raspberry Pi 3 itself only has one PWM enabled pin available to me, so the Arduino is convenient for handling all the low level stuff. It talks to the Raspberry Pi over the USB. The micro servos are powered from the Arduino Mega which are in turned powered off the Raspberry Pi. The micro servos stall current is low enough for this to be possible with the Arduino Mega.

 The Roomba wheel motors are safe at 12 volts (the Roomba battery voltage), so I put another battery in the system just for them. The battery is a 3 cell Lithium Polymer battery, which measures in at roughly 11.1 volts when the battery needs to be recharged and 12.6V when the battery is fully charged. The motor drivers are L298 chips on those red-colored Sparkfun breakout boards, with the heatsinks mounted to them.

So at this point the robot was driving, but only in a straight line. Turns would drag at least one wheel and make a terrible noise. Only very slight turns worked. This was fairly predictable, but trying to make it work anyway was very much in keeping with my glue-and-tape, then iterate style of prototyping. Jesse helped me put together a steering mechanism in a very short amount of time. It worked, so that evening I took the robot out for an inaugural journey around Sector67, using it as a sort of telepresence robot as I controlled it from my desk.

Then I broke the steering mechanism gearmotor by switching it back and forth too fast when I got stuck in a corner. The gear before the final output shaft broke into a bunch of tiny pieces.

I replaced it with the gearmotor in the image above on the right that has the partly blue casing. Now that I had a working robot again, it was time to work on the high level path planning and code. I'll put that in the next post.

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