Wednesday, April 29, 2020

Mojo3 - A Robotic Dog (#9) - Ready to Program the Step Sequence

Mojo3 is a 3D printed Compliant Quadruped Robot Dog

The 3D printing is done for this build. I am currently working on updating the Arduino software to control the robot leg sequence.  I need to add all the electronics and batteries as well.  More to come!

Mojo3 - Robotic Dog - ready for electronics

Mojo3 compared to Mojo2

Sunday, April 19, 2020

Mojo3 - A Robotic Dog (#8) - The New Alternative Leg Design

Mojo3 is a 3D printed Compliant Quadruped Robot Dog

The results of the last test (and blog post) were clear, the SEA design is not working as I had hoped. I maintain my believe that the compliancy in the SEA design is needed (superior) to a static approach. However, this current design will not work with the relatively weak pull of the 9g servos. At least not in the current configuration.

In my desire to move the prototype along, I have considered some other approaches. In my Mojo3 #6 blog post, I was considering using a hinged approach similar to a 4 bar mechanism. With such an approach this would allow for the servo to directly lift the 'arm' off the ground. This would provide the movement needed (ability to lift the foot when moving the leg forward), however, a different thought came to mind as I was adding the servo gear to the hip joint. 

Why not use a servo gear to raise the 'arm'?

Mojo3 - Robot Dog - Alternative leg approach (OpenSCAD)

The gear driven servo would provide small additional torque (without the servo horn) and less backlash. It will also be significantly easier to tune, being able to reset the location of the gear. Also, the Inverse Kinematics would be much easier to calculate. The downside is that I would lose the compliancy in the lifting mechanism. Compliancy would need to be moved to another location on the leg. (perhaps the 'arm' or foot). Note, the above picture uses "shorty" arm, just a very quick sketch to see if the concept was plausible.

Mojo3 - Old and New leg designs - "shorty" on the right side.

Testing the new design

Mojo3 Robot Dog - Test for Kinematic path

The new design proved to work well. It was easy to adjust and tune. Most important, I was able to get around 12mm of upward movement in the Kinematics.  With experimentation, I learned that increasing the delay or slowing down the motion, provided a very different Kinematic path. The servos had more time to get to their set points.

Mojo3 - impact of timing differences in the Kinematics

Putting all this design work together

I am pleased with the current design. This approach will be faster to get to a walking quadruped. The gear design will allow the robot to be adjusted much easier. I will need to have some compliant mechanism added to it in the future.  The new robot frame should look something like this:

Mojo3 - A Quadruped Robot Dog (OpenSCAD - exploded component view)

Tuesday, April 14, 2020

Mojo3 - A Robotic Dog (#7) - OK, One More Test!

Mojo3 is a Compliant Quadruped Robot Dog

Ok, maybe I should read my own blog posts. I believe I was supposed to do a new design. Instead I started the Milli - th WildWorm, but that is a different post.  Before I jump off to a new design, maybe just ONE MORE TEST?  I build the hip servo mechanism so that now we can see the actual distance traveled by the robot's leg.

Mojo3 - Leg Design with Gear Driven Hip and Bearing
The new hip design will be just a gear driven leg unit, and a gear on the servo - as the CAD picture suggests. This design should work, as the weight of the servo is now directly transferred to the Chassis through the bearing. This should remove the considerable load from the small 9g servos.

Mojo3 - Kinematic Motion Test

Here the test is set up, the leg is moving repeated through a single step. There is a 60 degree sweep of the hip section, and a 40 degree pull on the leg segment.  The results are (blurry) below.  In this configuration, the leg is only able to lift about 4mm on the return swing. (I will include the video in the future on my YouTube channel)

Mojo3 - Step Kinematic Motion Trace - Just Not Enough!

Only a 4mm lift on the return of the leg to the forward position.  I do not believe this is going to be enough to overcome the dynamic (tilting) nature of the gate.  Time to consider another design!

on the up side.  A shout out to Oracid1 on YouTube and his nice Inverse Kinematics Arduino code. It is a bit cleaner solution than the one I wrote for Mojo2. I have adopted some of the 'clean code' concepts for Mojo3.

Thursday, April 9, 2020

Milli (#8) - Rethinking the Motor and Spendel

Milli is a bio-inspired robot that uses a single actuator to create standing wave motion.

Where to go next on the design of the WildWorm drive and Milli??

As expected, the recycled printer motor that I am using (9-12v) powered by a 18v rechargeable drill battery, is too much for the little robot. (duh!)  When testing, the rotational speed of the motor is too much for the directly driven helix spindle.  After some testing this has lead to the helix base to be burnt out by the motor spendel.

Milli's WildWorm drive - burnt out helix mount separated from the motor.

What is needed is a gear reduction, to reduce the rotational speed, and increase the torque of the helix.  This is very similar to the gear reduction used for the same motors on the Wild Weasel track.  Here you can see the video of the gear reduction built into the track:

But Where to put this on the rather simple chassis of Milli?

Milli - Motor Mount

Actually, there is plenty of room.  With the next iteration of design, I will:

  • Add a bearing to the chassis, creating the mount for the helix.  this should reduce any load from the wildworm drive on the motor itself.
  • A shaft will connect the helix, through the bearing to a similar sized 3D printed gear.
  • The Gear train will consist of a small gear directly on the motor.  This will drive a large gear with coaxial small gear. this small gear will drive the helix gear.  (A quick estimate would have 18mm diameter to 40mm diameter gears. times 2 ~ 4.8:1 gear ratio)
  • If another gear set is needed, it can be added relatively easily.

Milli - Chassis with Recycled Printer Motor

Unfortunately, this is all conceptual at the moment.  It is not even in the CAD system just yet.  However - the thoughts have given me some new insight on Mojo3 - and there will be a new design blog post out shortly.

Stay Healthy!