EV3 TrotBot, ver 3
Version 3 uses the same linkages, retractable toe, and frame as ver 2, but adds the stronger leg attachments described in this post which allow TrotBot to turn on high friction terrain without the leg's pins coming out.
Version 3 also improves a few other joints by adding a 3rd beam to sandwich the joints, making them symmetrical and reducing their friction/wear.
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 the toe version below to the gait of the non-toe version here.
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UPDATE: Here are some simplified instructions for building TrotBot's leg mechanism. This leg will work for all 8 legs of TrotBot. |
Simplified Instructions for Building TrotBot's Leg Mechanism
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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.
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. If you want to try steel axles, the most important ones to replace are the axles for the 4 inner legs so that the inner and outer legs remain 180 degrees out of phase with each other, as shown in the beginning of the version 1 instructions.
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
If you've never used Bricklink, an easy way to browse for parts is by going to a larger store. Here's a list of stores in the USA.
And below is an example search in a store for Technic beams.
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
If you've never used Bricklink, an easy way to browse for parts is by going to a larger store. Here's a list of stores in the USA.
And below is an example search in a store for Technic beams.
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LEGO Part List:
The part list can be downloaded here, or you can view the images below. If I made any mistakes please email me at [email protected] |
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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.
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 your legs collide with the frame or cranks when rotating, then rotate the legs by hand (by turning a large gear) and look carefully at your build to see where the collisions are occurring. Perhaps you are missing some of the bushings highlighted below, or perhaps your double length crank has knobs protruding out.
TrotBot's "Kinematic Chain"
TrotBot Ver 3
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 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 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.
You may have noticed the 9-hole diagonals above are not the correct length for the triangle's hypotenuse, which I describe in this post.
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.
Legs and Cranks
Legs and Cranks
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Below I show how I built the stronger version of TrotBot's legs, where the joints are sandwiched between two beams, allowing the legs to better handle sideways forces without the pins coming out. Update: The following instructions have 4 of the legs built in a left-facing orientation, and 4 in a right-facing orientation, but you can build all 8 legs in either orientation since it won't affect TrotBot's functionality. Here are some simplified instructions for building TrotBot's leg mechanism. This leg will work for all 8 legs of TrotBot. Alternatively, instructions for building both left and right-facing legs are below. |
Simplified Instructions for Building TrotBot's Leg Mechanism
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Hexapod TrotBot
I'll build the left-facing leg first. Make 4 of the following legs.
NOTE: The reason I added a second 11-hole beam to the top of the legs (above) is the previous, single beam versions allowed the top right joint to bend somewhat, and the joint was starting to wear. Below is a picture of this previous version. Grey dust had accumulated around the joint, indicating the frictionless pin was being worn , so rather than be cheap with parts I decided to build TrotBot properly with symmetrical joints.
Next, I add the front side of the lower leg, which I created with 3 beams to accommodate the retractable toe's linkage (added later).
NOTE: Previously I had used a hobby saw to make some 8-hole beams, and used these spare 8 hole beams in the remaining pictures of the above leg section. However, you don't need to cut beams to create the below 8-hole beam version of this leg section - the above version works fine with two 7- hole beams and one 3-hole beam. Sorry, I didn't catch this until after I uploaded the pics.
To be clear, below is the same leg section as above, but uses an 8-hole beam in the center.
To be clear, below is the same leg section as above, but uses an 8-hole beam in the center.
Next, I add the back portion of the lower leg as well as the beam for the heel's linkage.
Above is the saw I used, but if you don't want to cut LEGO beams to create 8-hole versions for the heel then below is Catweazel's solution for the heel's 8 hole beam:
Notice the above 11 hole red beam is not attached to the end of the black 3x5 L-shaped part. Instead, the red beam is attached one hole from the end of the black 3x5 L-shaped part!
If you don't want to cut a 7-hole beam, here's another option for making a 6-hole beam
Next, I add the retractable toe
This 3x5 L-shaped part is the key to sandwiching the leg joints, which prevents the pins from pulling out during turns.
Next, I add the inner crank, and then the middle (double length) crank. I show how I made the cranks a few pictures down. The axle length for all 8 legs is 7 holes.
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You will need to make a total of 8 single-length cranks, 4 to link the inner legs to motor's axles coming out of the frame, and 4 to link the outer legs to the outer frame. Here are the parts I used to make them. The cams are called "Technic, Liftarm 3 x 5 L-Shape with Quarter Ellipse Thin", and the pins are "Technic pins 1/2" (which were blue in my part list, but I don't think the color matters) |
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You will also need to make 4 double-length cranks that link the inner and outer leg pairs, which I build below. (the red parts are called "2m Cross Axle with Groove")
Notice that the double length crank (on top) steps away from the leg, which is critical to ensure the legs don't collide with the cranks. If instead the crank was flipped upside down, the leg parts would collide with it and lock the linkage.
Next I'll build the right-facing leg. You'll need to make 4 of them as well - unless you decided to simply make 8 of the left-facing legs.
Next, I build the front section of the lower legs, which I built from 3 beams to accommodate the retractable toe's linkage. The following pic (from the left-facing leg instructions) is how I recommend making them, and you do not need to cut 8 hole beams like I used 2 pictures down.
As I mentioned above you don't need to cut a beam to create an 8 hole beam like I used in the following pictures. Two 7-hole beams and one 3-hole beam are fine for making this leg section (as shown above).
Next, I add the retractable toe:
And then add the cranks in the same manner as the left-facing legs.
Next I show how to build the outer frames. You'll need to make 2, one for each side.
Next I show how to build the outer frames. You'll need to make 2, one for each side.
Notice there are no LEGO pins in the lower corners since the two motorized axles will pass thru these holes, as you can see below.
Hopefully the following pictures make it clear how to put the legs on the robot. As you will see, it's easier to slide the legs on by slightly rotating them on the metal rods (or rotate the rods).
The left and right sides should be mirror images of each other.
Make sure the front leg is in the same phase as it's rear leg - in other words, if the front crank is pointing at 12 o'clock, then so should the rear crank. NOTE: you may need to loosen the gear train to get the front in rear legs in the same phase. Also, make sure you attached the middle (double length) crank so that the inner and outer leg pairs are 180 degrees out of phase, and that the outer cranks return the outer axles to the center of rotation.
Below I flipped TrotBot over to show the brick. This mounting arrangement makes it pretty easy to pull out the pins from the sides of the brick to change batteries.
