Featured Parts: Axles

Lego axles, in addition to connecting pegs, will be a crucial element of any robot you choose to construct. Whether used for structural support, or connecting two wheels along a single axis, lego axle beams have hundreds of important applications.

Axles come in about a dozen different sizes, and they geometry (unsurprisingly) meshes well with other lego blocks. It's not easy to determine spacing rules for beams, because most of the time spacial distances are dictated by the axle itself!

In my post about differential drives, we detailed how you can connect wheels on two separate axles; however keeping them aligned in the same order, spinning independently. You can also use wheels on the same axle, so that it will spin by itself along with the attached wheels, however this construct has many limitations when it comes to application in advanced topics.

As mentioned above, the geometrical construction of axles coincide nicely with corresponding lego parts. Here's an example of a gear run through an axle, all held in place by a supporting beam block. I thought it illustrated the different relationships between the parts nicely:

Concept Vehicle: ATV

I've always had an interest in ATV's and there are numerous necessities I've found that are required to build a successful one:

- Power/Torque (any speed is just bonus)
- Flexible chassis (stiff vehicles get stuck and lose traction)
- Good suspension system (goes along with above; as suspension systems enable you to keep traction)

Trying to implement all these into a Lego robot, I came up with this:

This is a concept ATV I made out of Mindstorms, I followed the three guidelines I set for myself above, and I'll explain all of them in further detail here in this post.

1. The drive system- Power and torque are essential in an ATV, as the motor must be able to push the robot across any terrain. If you recall an earlier post, I demonstrated how you can get power through gear ratios. I utilized a worm gear meshed with a 24t gear to give me a ridiculous 1:24 gear ratio. That's 24x the power the motor is outputting.

Example of a worm gear              

I transferred the motors power into the worm gear, which turned the 24t in a gearbox that came standard with my RCX. So though I dealt with a low speed bot, the power that came with it was extremely helpful.

The gearbox 

2. A flexible chassis- I put this as a guideline for myself because I felt like without a certain degree of flexibility from your robot, Mindstorm bots can be prone to getting stuck on uneven terrain. Adding a degree of rotation on your chassis helps keep this from occurring.

To achieve this, I simply built the chassis in two separate parts; the drive system, and the suspension mount. This way I was able to lock the two together using two single beams, adding a degree of flexibility.

The rotation radius of the bot is illustrated in these two pictures of the chassis.






3. A suspension system- A good suspension system is NECESSARY for an ATV. Unfortunately these are a bit tricky to build out of Mindstorm parts. I found an interesting solution using the rubber bands found in many kits, and attached the front two wheels to independently attached mounted suspension beams, supported by said rubber bands. By inverting the system, you have a make-shift suspension.

Normal drive

There are other guidelines into making a successful ATV as well, however these should get you going for now.

Send me pictures of your own ATV's! ashwin.johri@gmail.com
Happy building guys.

Suspension flex

Building a Steering Drive

Steering is an essential component of building a Robot. What fun is a machine that can only go forwards and backwards? 

There are a lot of different ways to steer a robot, and one of the most common one is to build a rack and pinion steering drive. The steering drive is used by basically anything with a steering wheel, it's set up so that a gear on the end of the wheel (called a pinion) is meshed with a rack connected to the wheels. 

The rack is set up such that there are two degrees of motion, on either side of the set-up, and there's also a non-moving piece to give the wheels something to rotate around. Here's a good (non-lego) diagram of the system:

Now implementing the rack and pinion system into Lego form is tricky, but there are tons of unique solutions out there that I encourage you to take a look at. A quick google search of "Lego Steering Drive" will pull up some really cool results. 

In an effort to demonstrate on possible model for a Lego steering drive, I built this: 

The steering shaft is controlled by an independent motor, and by playing with the rack sizes, and wheel spacing, you can manipulate the turn radius as well. Building a solid chassis is important for steering drives, as they can quickly become heavy, and excessive moving parts can put stress on a robot. I locked mine with perpendicular blocks you can see laying across the underside, beneath the steering column. 

Here's a view on the pinion gear meshed with the rack, it's important to make sure the pinion has enough torque to turn the whole system. Locking gears, and jammed motors are a nightmare to builders.

Steering drives, while not an easy build for most people, really provide versatility to the bots you make. In the future, we'll explore some more steering systems, like the differential pivot wheel, and reversing motors. 

I'll leave you with a video displaying the turning radius and the piece movement involved with the execution of the drive:

Differential Drives

By far the most common drive system for Lego robots, the differential drive is probably the easiest to make. Utilizing two parallel drive wheels on either side of the robot, they can be powered separately, or use a single motor as well.

The diagram to the right shows what a basic differential drive looks like. Bevel gears placed in the central gear housing allow the axles to turn independently, enabling the robot to turn in place, and maneuver pretty easily.

The following chart is adapted from "Building Robots with Lego Mindstorms" by the Ferrari Brothers and displays the different behaviors of  a robot in accordance to its wheels (powered by the differential drive).

Using different combinations of speed and direction, the robot has the ability to cover any amount of space, and turns at any angle. All this in addition to being very easy to implement. 

I assembled an example of a standard drive, showing how you can use the bevel gears inside the housing to attach the axles, and connecting it to an RCX is as easy as sliding two beams through it. 

As always, I encourage you to send me your designs, and contact me with any questions/comments/concerns. Happy building!

The Skid Steer Drive

The Skid Steer drive is truly a classic drive system. Usually incorporating treads or 6 wheels, good examples of skid-steer drives in real life include excavators, tanks, and other utility construction equipment.

A common variation of the differential drive (more on that later), the skid-steer has:

-better grip on rough floors/terrains
-more torque and power, where friction uses up some of the motors power
-easily maneuverable, however programming a skid steer to go straight is a little tricky.

The one I built above uses two motors powered by the same RCX. The chassis is divided into three parts (seen here). Two different mounts for each independent tread, and a central mount for the RCX and motors.

The wider base provides stability, which is important for slow moving treads, and a low center of gravity keeps accidents like tipping, and slipping from occurring.

An example of a clutch gear.

Another important component of a skid-steer (and pretty much all drives) is to incorporate elements to keep your motor from jamming. If a tread or a wheel gets caught, but the motor keeps trying to apply power, there's a dangerous chance of a lot of broken parts. My design incorporates a clutch gear which provides a certain domain of resistance before the clutch keeps the motor spinning with no damage (regardless of the status of the wheels).

Finally, shifting gears, and increasing the ratio provides massive increases in power, sacrificing speed in the process. Because my motor is directly linked to the 8t, which turns the 24t, my gear ratio ends up like:

                                                                               8t : 24t = 1:3

Which basically means that my wheels are turning at 3x the power the motor is outputting.

Skid-steer drives are fun to build, and even more satisfying to watch in action, and I definitely recommend experimenting with a few of your own designs to get started. If you have any alternate designs or suggestions shoot me an email: ashwin.johri@gmail.com