Sunday, November 21, 2021

Mojo3 - A Robotic Dog (#11) - Wrapping up the Design

Mojo 3 has been a good learning experience, but for now i will shortly be moving to a 5 bar design and more complete robot concepts.

Here is the latest video:


I really like the small compact design of Mojo3. The prototype fit nicely on my desk. It is easy to assemble and did not take long to print on the 3D Printer.  I intentionally made it smaller than Mojo2 in order to get it to work with small 9g servos.

Also in this design, the weight of the robot was carried from the legs directly to the chassis through a 8mm (skateboard) bearing. This is a significant improvement over Mojo2.

Finally, I took the 'compliance' part of the robot out of the design. These legs are driven strictly by the servos, and gears attached directly to the servos.  This approach has the advantage of providing the most direct torque to the leg components. 

Unfortunately the Knee arrangement with a gear and type of pinon/cam, does not work.  It was easy to calibrate. You could just lift the arching gear teeth and put them in the right locations.  However, once the robot was under load, it was prone to slip.  The video shows this very clearly.

Going Forward

I have started a 5 bar leg design.  This arrangement should be easier to get a full range of motion. The next design will incorporate Inverse Kinematics. I will try to write some meaningful blog entry on that.

Monday, November 8, 2021

Mojo4 - 5bar design (#1)

Time for A new Mojo Design!  Time to get back in to the wonderful world of Quadrupeds!

I am trying out a new design approach for a new Quadruped.  The 5-bar linkage.  

This approach has been successful in several quadruped designs found on the internet. (one of my favorite is by Oracid: video.) 

It has a full range of motion with only requiring only 2 servos per leg. in addition the leg linkages are supported by pin/hinges on the frame. This should reduce, but not eliminate the force exerted on the servo motors.

Linkages are just mechanical arrangements of bars/links/connectors and pivoting connections. Given different types of configurations, it is possible to get a wide range of applications. One of the most common linkage people have seen on old steam engines. Here, the steam exerts a linear force that is then converted to a rotational force to move the trains wheels.

For a 5 bar linkage, only two bars are connected to the servos. by changing the angle on just these two servos, one can get a wide range of motion.

By I.Elgamal - Own work, CC BY-SA 4.0

For my design, I will be connecting the top two pivot points together to create a 'diamond' of the linkages.  next, at the bottom, the bar is extended further out. Here is where the 'paw' will be connected.

5 Bar Linkage in a diamond with 'paw' configuration

I am hoping that this arrangement of parts should provide significantly more vertical lift in the leg. Mojo2 did not have significant lift and often shuffled.  Also, Mojo3 had more left control, but the servos were overpowered. This combination should provide both the needed strength and lift required.

To control the leg, the two servos will be connected in smaller 4 bar connection to the sides of the Diamond. The key in the design is to keep the servo horns parallel to the bars that they control. In operations, the servos themselves act virtually identical to the top diamonds sides.

Mojo4 Robot Leg design with control actuators

The linkage parts will be 3D printed (of course) and I will be using a small nail as the hinge.  This hinge arrangement is very similar to the track design I used on the Wild Weasel. (video)  Having a reliable design in the CAD and 3D printing makes building robots a lot easier. With this design I can remove a freshly printed part and assemble it with only basic smoothing and cleaning.

For my first rapid prototype testing, I will only have a test stand to validate the movement of the leg. I am re-using parts and the wiring from Mojo2.  I will reuse the Mojo2 servos, MG995 servos. These are 13kg servos. They are slightly heavy for this application at 65g.

Here is what the constructed leg mechanism looks like:

Mojo4 Robot Test Leg

Next Steps:
Now that the basic structure is printed and built for the 'test stand'. I will work on the software to move the leg. For this robot, and the first for Mojo, I will implement some form of Inverse Kinematics. With Inverse Kinematics, it will be possible to instruct the leg the exact position it should talke, and the IK will determine the correct angles to command to the servos.