EV3 TrotBot, ver 2
UPDATE: TrotBot's legs in the new EV3 instructions (ver 3) are stronger and better able to handle the weight of the EV3 brick, so I replaced ver 2's legs with ver 3's. Version 3 uses use the stronger leg attachments described in this post which allows TrotBot to turn on high friction terrain without the leg's pins coming out.
Why add toes?
Adding toes smooths TrotBot's gait, reducing the power required to carry the heavy EV3 brick, which I show in my weight-bearing experiment. You can compare it to the gait of the non-toe version here.
Let's build it!
Overview
Linkage: TrotBot uses the linkage developed by Team TrotBot
Build Difficulty: Hard. These instructions cover building TrotBot's ver 2 legs with retractable toes, as well as the mounting of the EV3 brick on the underside of the frame. Mounting the brick on the frame's underside reduces TrotBot's width which improves how it turns, and it lowers the center of gravity which improves how it handles in general. After building the legs+toes and the frame you will need to refer to TrotBot's main instructions for details on how to finish and mount the legs, as well as to build and mount the crank/axle system.
NOTE: the retractable toes reduce the torque required to drive TrotBot's legs, so the steel axles I used in my EV3 version 1 are not as necessary, and LEGO's plastic axles can be used. However, the plastic axles do twist somewhat, making the leg action more jerky when the axles un-twist as resistance drops, like a wound spring being released.
Plastic LEGO axle permanently twisted by a LEGO XL motor
Trobot's Characteristics:
1. 8 Legs, or 4 "horses" with a gait that mimics a galloping horse
2. When building TrotBot with the EV3 brick make sure to add its "heels" to smooth its gait and lower the power requirements.
3. Steerable? Yes, by driving one side forward and the other in reverse, like a tank.
Parts Used: In addition to the Technic beams, pins, gears, etc. this build requires a programmable EV3 brick, and 2 EV3 Medium Servo Motor (45503). Also, to better bear the robot's weight I used 3/16" OD brass tubes for the leg's long connections to the frame (aluminum rods are fine too) rather than LEGO's plastic axles.
For batteries I recommend using Lithium Ion AA batteries as they are lighter, last longer, and will improve walking performance.
I purchased the plastic LEGO parts from Bricklink
I purchased the additional EV3 Medium Servo Motor (45503) from Amazon
I purchased the metal support rods from my local hardware store
Trouble Shooting a Finished TrotBot
A finished TrotBot's legs should be easy to rotate by hand with the motors disengaged. If you feel resistance, then look carefully at the legs to see if any look asymmetrical or skewed. Also, an easy mistake to make is to put the axles thru the wrong hole of the 3x5 cam part below. If the legs aren't working properly, check your cranks.
Trobot's Characteristics:
1. 8 Legs, or 4 "horses" with a gait that mimics a galloping horse
2. When building TrotBot with the EV3 brick make sure to add its "heels" to smooth its gait and lower the power requirements.
3. Steerable? Yes, by driving one side forward and the other in reverse, like a tank.
Parts Used: In addition to the Technic beams, pins, gears, etc. this build requires a programmable EV3 brick, and 2 EV3 Medium Servo Motor (45503). Also, to better bear the robot's weight I used 3/16" OD brass tubes for the leg's long connections to the frame (aluminum rods are fine too) rather than LEGO's plastic axles.
For batteries I recommend using Lithium Ion AA batteries as they are lighter, last longer, and will improve walking performance.
I purchased the plastic LEGO parts from Bricklink
I purchased the additional EV3 Medium Servo Motor (45503) from Amazon
I purchased the metal support rods from my local hardware store
Trouble Shooting a Finished TrotBot
A finished TrotBot's legs should be easy to rotate by hand with the motors disengaged. If you feel resistance, then look carefully at the legs to see if any look asymmetrical or skewed. Also, an easy mistake to make is to put the axles thru the wrong hole of the 3x5 cam part below. If the legs aren't working properly, check your cranks.
If leg parts aren't connected to parts in the adjacent plane the linkage can be twisted, which will cause the leg joints to bind. If your legs don't rotate easily, make sure your legs aren't twisted and that each part is in its correct plane as pictured below.
Hexapod TrotBot uses the same, 4-plane legs as EV3 TrotBot Ver 3
How to Build
General Comments:
1. Walkers stress frames more than wheeled vehicles so strive to utilize triangles that resist bending forces.
2. Be sure to use frictionless pins for movable joints!
3. Take care to space each bar properly, with the bar's joint on the correct side, and with the use of bushings on axles as necessary. If the leg parts collide the linkage can lock up causing gears to grind, and you will find it is much easier to get your leg spacing correct before assembling the entire walker than it is to fix the legs in a fully assembled walker!
How to Build
General Comments:
1. Walkers stress frames more than wheeled vehicles so strive to utilize triangles that resist bending forces.
2. Be sure to use frictionless pins for movable joints!
3. Take care to space each bar properly, with the bar's joint on the correct side, and with the use of bushings on axles as necessary. If the leg parts collide the linkage can lock up causing gears to grind, and you will find it is much easier to get your leg spacing correct before assembling the entire walker than it is to fix the legs in a fully assembled walker!
Because the EV3 brick will be mounted beneath the frame it will not provide much structural support, so we'll start by building a strong, triangle-based inner frame. You will need to make 2 of the following frames, one for each side of the robot.
The above 5-hole yellow beam is there to show the axle length is 5 LEGO holes
Build a second version of the above frame, and then connect them with the 3/16" metal tubes, cut to 18" in length. Be sure to file down the cut ends so they slide easily thru the LEGO parts.
I mounted the 7 hole crossing beams on the underside to provide a handy way to pick up TrotBot.
Notice the brick's LED screen and buttons face toward the ground, so you simply flip TrotBot over to turn it on.
To build the legs and cranks, scroll down to that section of Mindstorms TrotBot's, ver 3 instructions.