I've been thinking about creating an EV3 Strider, but to handle the increased weight and width Strider needs to be improved in a few ways, like by increasing the amount of foot-contact it has with the ground.
One way to increase foot-contact is to add four more legs. To check how this would smooth the gait I simulated one side of a 12-legged Strider, and if you watch the video below you'll see Strider bounce whenever the feet touching the ground switch. This bouncing shouldn't be much of a problem at LEGO scale, but it would be at large scale.
While a scaled-up Strider's linkage could be optimized for a smoother gait, it can also be smoothed by adding feet. As an example, in the second half of the video I added small triangular feet to the front legs, which act like heels and toes. These feet reduce the gait's bumpiness by about 1/3rd. However, the toes are more likely to catch on obstacles, which can cause the linkage to lock and gears to grind, so I wouldn't add them to an EV3 Strider. I only posted the sim to show the effect of these simple feet.
Any suggestions for Strider's feet?
Scott Anderson has taken TrotBot to a new level. In addition to using modern Maker tools and gear to create his TrotBot, Scott reduced its width by printing leg parts that link in-line, strengthened the upper leg parts by printing them as bend-resistant triangles, and added those cool hinged feet!
Scott's shares how he created his TrotBot here.
I rushed my Klann Ver 2 build and didn't build the outer frames in an optimal way:
The only thing keeping this frame's corners at right angles are the two 3x5 L-shaped LEGO parts. If this frame were put under a lot of force, like what would happen at a larger scale, the corners would be subjected to torque that could easily cause the rectangle to collapse. To avoid this diagonals need to be added, which will convert the rectangles into triangles and lock the corner's angles.
The challenge with walkers in LEGO is we often need diagonals for rectangles that define the linkage's parameters. In other words, these diagonals need to be the hypotenuses of right triangles. As you can see below, my Klann's upper support rods are 7 holes above the motor's axle, and the lower support rods are 2 holes below the motor's axle. Neither of these lengths work with a 3-4-5 or 6-7-8 right triangle with LEGO parts for hypotenuses. What can we do using the integer lengths of LEGO's beams?
NOTE: When determining the length of LEGO beams the first hole is always counted as 0! If you don't measure LEGO beam lengths in this way you won't be able to use the Pythagorean theorem to calculate which beams to use as hypotenuses.
Fortunately, with LEGO we don't have to be at precise integer numbers, and we can use hypotenuses that are close enough.
1. Hypotenuses for rectangles of height 7.
Plugging in a 90 degree angle with a side length of 7, plus a few other whole number sides into the Pythagorean theorem yields this near integer number triangle:
I used this triangle for Klann's inner frame where two 9 hole beams create hypotenuses of length 8:
I also used this triangle for TrotBot's frame:
2. Hypotenuses for rectangles of height 9.
We can also connect the top and bottom beams of Klann's frame with a hypotenuse:
Plugging a 90 degree angle with a side length of 9, plus a few other whole number sides into the Pythagorean theorem yields another near integer number triangle:
So, 13 hole LEGO beams can be used to lock Klann's outer frames into triangles, like this:
Here are a few other useful triangles for LEGO frames:
Also, the below yellow 5-3 bent beam can be used as a hypotenuse (the angles of the bend are noted in green).
Can you come up with any other methods?
The first result of that effort was TrotBot's heel linkage. As you can see below, TrotBot's heel strikes before the main foot, resulting in a smoother gait and lower power requirements (for an analogy of why bumpy gaits require more power, think how much harder it is to do lunges than it is to simply walk).
Another benefit of TrotBot's heel is it steps higher on the backside of the foot-path, allowing TrotBot's rear legs to step about as high as the front legs to avoid getting stuck astride obstacles, as can be seen in this heel-path simulation:
Without its heel TrotBot's rear legs would probably get stuck on a few of these 2x4s:
We've also played around with a few options for active toes that push down on the ground as the foot begins to lift. Here's an example of simple feet with toes.
Catweazel, AKA Michael Leefers, was kind enough to create computer-rendered instructions for building TrotBot in LEGO, and share them with us! Instead of using 3/16" aluminum rods to prevent TrotBot's inner frame from sagging, Catweazel cleverly added a Technic beam to connect the inner frame to the outer frame, which helps to prevent the plastic support axles from bowing. This is the same solution I was planning on using for the support rods of my large bamboo TrotBot - great stuff!
You can access or download Catweazel's instructions and efficiently purchase parts via his Rebrickable page. Thank you Mr. Leefers!
Over my break I ran some optimization code to improve Strider's lackluster footpath. Here's how Strider ver 2 looks:
And here's a sample of the LEGO version walking:
I kept this as an entry-level build with only one motor. Build instructions are here
When two linked bars are nearly parallel their connecting joint can easily flip orientations and cause the linkage to lock. This phenomena is known as a "Dead Point". Here's an example:
The below right picture is near the Dead Point, where two bars highlighted in red are nearly parallel:
Due to the force on the foot, the joint can "flip" as shown below, which causes the linkage to lock::
Linkages need to prevent joints from ‘flipping' at these Dead Points. Below are two methods I've used to prevent joints from flipping in my LEGO builds.
This solution for my Klann Ver 1 simply blocked the joint from flipping by the addition of a red LEGO beam:
My Strider build encounters the same problem as shown below:
This solution for my Strider build uses an additional linkage on the inside of the joint that allows the joint to bend toward the robot, but prevents bending away from the robot:
Klann's Spider was the first walker I ever built. I like it since it's robust and not too complicated (as mechanical walkers go). I'll try to get some build instructions posted over the holidays.
Walkers with multiple pairs of legs spaced out from the frame subject axles to more stress than what they are typically designed to handle. Furthermore, LEGO's plastic axles twist easily under torque, which can be especially problematic for walkers since such twisting can disrupt a walker's gait by delaying the leg's movement.
How much do LEGO axles twist?
The above experiment was run with LEGO's M-motor 8883, geared down in a 5:1 ratio - the same set up I used for TrotBot.
With some walkers the gait is smooth enough and the weight low enough that axle twisting doesn't harm the gait much. However, with heavy and wide walkers, like the Mindstorms TrotBot I just finished, axle twisting can be a problem. Fortunately, Brick Machine Shop makes stainless steel axles for LEGO:
These steel axles resist twisting and help to keep leg movement closer to the mechanism's designed movement. They also fit more tightly, so cranks won't come off axles while operating your walker - but this also means it can be difficult to insert these axles into parts. I usually use something like a flat piece of wood (Kapla block) to help press parts onto them, and needle nose pliers to take parts off.
Welcome to DIYWalkers! My name is Ben Vagle, I'm 17 years old and I've been building mechanical walkers for the past 4 years, at both LEGO-scale, and SUV-scale. I started this blog to share what I've learned, and to collaborate with you. Let's see if we can take walkers to the next level!