4-Bar Walking Linkage Optimizer
Posted by Wade
The below interactive simulator allows you to quickly check how changing the linkage's bar lengths affect the path it draws as the crank is rotated.
The below interactive simulator allows you to quickly check how changing the linkage's bar lengths affect the path it draws as the crank is rotated.
|
A Basic 4-Bar Walking Linkage with a Crank and Rocker
|
Here is the a video of the simulator in action:
Jeep2003's awesome mechanical ostrich uses a similar 4-bar linkage:
|
And the below "Spot Micro" uses the linkage dimensions shown in the image to the right, scaled so that the crank length (bar b1) equals 3 LEGO holes. A building tutorial is on our LEGO Spot Micro page. |
|
Spot Micro's linkage has a fairly consistent foot-speed, which makes it a good candidate for rotating the crank by driving the legs, as is done in the following treadmill video, but it doesn't step very high so it can only walk on flat ground.
Version 2 is a bit bigger to better accommodate a motor with RC control. It's still a work in progress.
Below are the linkage dimensions we used for Spot Micro Ver 2, scaled up by 10x, which is similar to the GIF at the top of this page but with shorter legs so that the front/rear feet don't collide when the crank is in the 12 o'clock position. (NOTE: we used our 6-bar linkage optimizer and set bar B5 to 0, and bar B7 to 20)
Linkage dimensions of Spot Micro Ver 2, using our 6-bar optimizer which doesn't use an input angle for B4 and instead uses bars B6 and B7 to make a lower triangle
|
Below is the interactive simulator. It allows you to simulate between 1 and 4 legs to make it easier to gauge the amount of foot contact at each corner of the robot. The sim is started by clicking the green flag. You can also run it on MIT's site where you can modify the Scratch code. After changing a bar's length, click somewhere other than a slider bar and then press the space bar to see the new linkage, and use your keyboard's left/right arrows to rotate the crank. Standard caveat: just because something looks good in a simulation does not necessarily mean it work well when built, at least not without some tinkering such as mitigating the tendency of this linkage's knee joint to hyperextend inward. |
|
A description of the algorithms, and how we used them to simulate linkages, is here.
When building a simulated linkage in LEGO, remember to use beams with one more hole than the bar's length. For example, a bar of length 8 requires a LEGO beam with 9 holes, because when determining the length of LEGO beams, the first hole is always counted as zero.