Radio Gear

A stop at a local hobby shop brought me to this radio. The Spektrum DX5e has just enough features to allow me to control this thing remotely to test the chassis. It's a digital 2.4 GHz radio, but without any of the fancy features that would have driven the cost way up. The Sabertooth motor controller I mentioned several posts back will connect nicely to the included receiver. In addition, the Sabertooth has a BEC which will power the receiver off my main batteries. One more thing checked off my shopping list!

Frame Progress

A busy summer meant that not much got done, however we were able to get some of the frame assembled. The ends are attached to the beams that hold the motors (that are mounted now). I used some angle aluminum to join the pieces instead of welding. It feels like it will be a very sturdy frame! Some sheet aluminum covering the bottom of the main area will stiffen things up further.
The wheels we purchased are probably going to be too small, and difficult to attach a sprocket to. I saw this option which would really simplify the drive train construction. By combining a sprocket and wheel, it removes one challenging bit of construction. I wish I had my own lathe! Still, $200 for wheels is a little more than I wanted to spend. I also wanted to see if I could run with a live axle (where the axle is mounted on bearings and the wheel and sprocket get mounted directly to the shaft). For example, using a sprocket like this and a flange bearing like this, the robot could run a live axle.
I'm itching to order something new... drive train... electronics... hmm.

A Decision on Power

OK, so we jumped the gun, and didn't work on the frame. Instead we did some looking for batteries. I ended up looking at the PowerSonic PS-1270, which seemed like it met our Amp/hr needs, and with 2 of them, we get the 24 volts to power the motors. I just wasn't sure if they could be mounted on their side. The reason I need to know this is that the frame is 3 inches high and these batteries are less than 3 inches wide, which means I could fit them within the frame of the robot. Ryan wanted to buy them right away (of course!), but I said we ought to call PowerSonic to ask. A nice guy answered and said that running them sideways would be no problem, just don't mount them upside down. The cool thing is that he said the PS-1290 was the same size, but in a 9AH, instead of 7! Instant increase in power density is a good thing! I thanked the guy and started hunting for them at the cheapest price. I found them for $23 each at BatteriesASAP.com So, they are ordered and on their way. Now, to find a decent 24 volt charger for these AGM battries.
Oh, anyone know a good source of sprockets and chains? I looked at a bike repair shop, but those sprockets would all require me to machine a hub adapter, since the center holes were far to large for the motor.

Frame Mock-up

Since the last post, a couple of things have happened. We ripped and cut that aluminum tube into the 1x3 channel. The wheels were ordered and came in just a couple of days ago. So, we just had to lay things out on the floor to see how everything fits! There's a box under the motors so they can be positioned properly over the inside frame members. It looks like there will be plenty of room, which is good news!
The next steps are;

• trim the frame members to fit together
• weld the frame (but leave we'll bolt the pieces on outside the wheels, for easy servicing)
• drill the holes to mount the motors
• get axles for the wheels
• find some mounting hardware for the axles
• research and acquire the gears for the motors and wheels
• get the right length of chain to connect the motors and wheels

Once we get all of that done, we can start worrying about electrical items, such as batteries, motor controller, and radio-control hardware. I'm sure we'll be tempted to just apply power to the motors to see the thing move!

Designing the Frame

Been thinking about the frame size a lot lately. We thought about using an aluminum I-beam that is 3 inches high. The motor would mount nicely in the middle section and the wheels being 5 inches, would stick out 1 inch above and blow the frame. Sounds pretty good.
Shopping for I-beams, we found that the options were limited and we couldn't find a 3 inch that didn't have a pretty huge dimension for the horizontal parts. So, we visited a local store that has a customer counter where they sell odd bits left over from customer orders. We found a 3 inch square aluminum tube, 6 feet long. So, if we rip this, we'll end up with some 3x1 channel which will work out just fine!
To the right, you'll see a frame that is 21" x 24". The motors are drawn in to scale as are the wheels. Now, I need to get that tube ripped and make detailed drawings of the frame pieces. I'd like to weld as much of it as possible so that it will be strong. I might still bolt on the side pieces that cover the wheel mechanics.

The Motors are Here!

They've arrived right on schedule! I think they'll do nicely! We've been eyeing some wheels also. They have an aluminum hub and polyurethane "tire" and some nice roller bearings in the hub. I think we'll be able to attach a gear to these via some metal spacers attached to the hub.
With these wheels (at 5 inches diameter) and these motors, we can go 33 mph with a 2:1 reduction. But, Ryan thinks we should shoot for a slower top speed. So a 3:1 reduction yields 22 mph.
I did some research on motor drivers that integrate with a R/C receiver. I found this one, "Sabertooth dual 10A motor driver for R/C". It can more than handle the power needs of these motors, though I really don't know what they will draw at full load. We've also been talking about how to construct a frame. We've been talking about aluminum I-beams. Not sure where to get these yet. We'll do some more research and report what we found. Also need a source for gears and chain, but I'm sure we won't have any problem finding those!

In the Beginning

Ryan and I have been building with LEGOs for years. Robots came naturally because seeing your creation move under its own power is way cooler than having something static. Recently, we went to see a FIRST Robotics regional competition. While there were many fully functional robots there that had interesting solutions to the task, there was also a rookie team there. Their robot merely drove around the course, but that did score points for the team! The inspiration here is that any project has to start somewhere. We've decided to make a driving base that is both sturdy and fast. Our inspiration comes from both the FIRST robots and from Battlebots.
To get started, we needed to understand some design requirements. Initially, I wanted to decide on a top speed. I drove the car at 30 MPH and asked Ryan if that was a good speed for a robot. He thought 20 might be fast enough. I suspect he is the wiser of us! Last week, I worked out some calculations that started with wheel diameter and derived motor speed given different speeds and reduction ratios. The results is this spreadsheet, which does the later, but also allows you to calculate the top speed based on wheel diameter, reduction ratio and top motor RPM.
We started with a 6 inch wheel (seems like a good size) and started shopping the surpluscenter.com web site. We found a nice motor that has a top RPM of 4500 and draws about 1.5 amps at 24 volts. I figure that two of these ought to provide enough power for a nice differential drive platform. The motors should arrive today.. yea!! parts!!

Welcome

Since I'm just starting out with this blog, I think it would be a good idea to tell what its going to be about. My son, Ryan, is 8 and really likes building things. I'm trying to show him some real projects that demonstrate real world principals. We've been building robots with LEGO Mindstorms for a little while now and going over some basic circuits knowledge. The idea of building a battlebot type project gets both of us excited, so we'll be using this blog to post the project status.