Monday, August 27, 2018

Mojo (#13) - Knobby Knees

I am still investigating the best leg design for Mojo - The compliant quadruped walking robot.  It is clear from the videos that perhaps the legs are "too compliant". The springs are showing some compression, even as the robot stands. Also, in the configuration, the robot can not handle the weight of the battery without further compression. 
Mojo - The Compliant Quadruped Walking Robot - with sexy robot legs
I believe when the knee servo pulls the leg up, the robot is not gaining much more additional compression. In addition, when the leg is moving back in the gait, the leg also compresses.  So, it seems, I will need either stronger/tunable springs or perhaps a 'less compliant' compliant leg.

So, we iterate through a different design! The new design that will allow for the leg to be pulled back, with out compliance.  But, still have the ability to be compliant when the knee is flexed and absorb some ground shock.

Straight legs with "knee joint" - 3D printed
The new legs are straight, with a "knee cap", and the Serial Elastic Actuator (SEA) is located on the back. Unfortunately this design does not have much in aesthetic appeal. - They are ugly!!

Mojo - A Compliant Quadruped Walking Robot - now with "knobby knees"
Hmmnnn...  I am not sure if I like this tangent in the design. The knees perform better in the standing position. Walking is still very unstable. The new actuators do not have much clearance Unfortunately, this design experiment did not yield good results!  Such is prototyping.

Back to the Drawing Board!!

Sunday, August 19, 2018

Fail Lizard Lives!

The "Fail Lizard" is alive, and crawling around so be careful. no worries, it is 'Totally' not evil!  It just looks that way.  :)

From the video you can see that I have been working on some different robot parts like GRIPPERS!  I thought it would be interesting to see if I could create something that could move, with only two servos.  That is how Fail Lizard came about.

Unfortunately, there was a mis-hap with the polarity of the power source. Learning point, if you have your servo power cross wired with the ground, you will start to smell the smoke. once I found my mistake, I was able to quickly rebuild the Fail Lizard and take it for a crawl. Despite being thrown together very very quickly, it was able to 'Autonomously' crawl/drag its self around.  At the same time, gnashing it's gnarly (totally not evil) grippers.

Fail Lizard casting a long shadow
Fail Lizard is certainly a robo-hack, it was created for the primary purpose of creating a crawling gait using only two servos, one to 'twist' the back and the other to 'twist' its legs. This seems to work fairly well, the video segment had no tuning of the gait at all. AND the secondary purpose was to put (totally not evil) grippers on something!

Build your own Fail Lizard:
- 3D printed parts, including reused Mojo Chassis
- 3 servos: Grippers, Legs, back-twist
- Arduino Pro Mini, breadboard, jumpers, clothespins(!)
- Intentionally heavy USB power bank, zip tied
- all the love that only an inventor can give to an otherwise ugly beast! :)

I also learned that despite the smell of burning servos, the servos only had minor damage and with a little solder I was able to put them back together.

Smok'n Servo just needs the power lead re-soldered

So, actually the Fail Lizard resulted in some good experience in servo debugging, USB connector wire (why does it seem that they are always upside down?!?), dynamics of a crawlers, and some sweet new grippers!

Finally, a few new upgrades.  I have been watching tutorials on ShotCut, and was able to produce a little more professional video. Thanks MechaLion for the support! And thanks to Gunnar Olsen for providing some cool music for the Fail Lizard to crawl to.  :)

AND we now have the domain  !!

Friday, August 17, 2018

DARPA - Subterranean Challenge

Earlier this year DARPA (Defence Advanced Research Projects Agency) announced the next Robotics Challenge. This challenge will be underground! They are looking for robots to be capable of exploring and mapping underground structures.  The challenge will have three subdomains: Man made tunnel systems, Urban Underground, and Natural Cave Networks.

The previous Robot Challenges included the original grand challenge of self navigating robots and more recently general robots for human activities.  NASA also recently completed a Robotics Challenge, and will have a new one starting later this year.  NASA Space Robotics Challenge 2

Both DARPA and NASA will have a track that allows for Virtual competition.  The simulation platform of choice is Gazebo part of the Open Source Robotics Foundation managers of the Robot Operating System ROS.

It is natural to think of the use cases and value an underground mapping robot capability could provide.  The recent search and rescue of the 12 boys in Thailand come to mind. Also, many science fiction movies show the future tech of a cave mapping robot, such as seen in the movie Prometheus.

It will be interesting to see the innovation that is spurred from these competitions.  Perhaps they will could find value in a Totally Not Evil Robot Army!

Friday, August 10, 2018

Mojo (#12) - Servo Up!

Mojo is my compliant quadruped robot, that 'almost' walks. Currently it is just a staggering, stumbling, and falling over compliant quadruped! :)

Over the past few weeks, I have been contemplating enhancements and improvements to make the robot more stable and capable of sustained gait.

Servo up!

The biggest change I will make will be 'leveling up' the servos. I started my Mojo project with simple plastic 9g micro servos - SG90.  These servos actually work pretty well for small projects. The robot has 8 servos, and in the past three months, I have only replaced 2 of them. This included my initial burn in phase where my controller code was making the robot do extreme splits!

Replacing micro servo with a standard servo
I will be replacing the small plastic SG90 with MG995 servo. There are considerable differences in size. The larger servo will have a stall voltage at 4.8v of at 11kg-cm, and 15kg-cm at 6v. This is considerably better than the SG90 which is around 2kg-cm.  The price of the servos is about 2.5 times more expensive.

To replace such a core component, it will require changes to the Chassis.  I will start enhancing the design for the larger, stronger servos. The placement is good, but will require a larger space. I will attempt to canter the legs in this process to try to improve the stability in the process. The current build has a high center of gravity, making it easy to topple over. a wider spread of the feet should reduce this.

I will continue to drive the servos with a PCA 9685 PWM driver board.  If I can drive the servos with more than 5v power, then I will get a higher performance.  That will require a new power source, perhaps time to consider hobby power packs.

Sunday, August 5, 2018

Robot Fail Week!

Nobody said inventing would be easy!

Go home robot, you're drunk!

This week has been mostly errors in the 'trial and error' school of invention.  But, alas! that is part of the process.

Mojo did get a new chassis and spent some time moving the components from the previous version to the upgrade.  Important features include:
  • additional separation of the knee and hip servos
  • roll bars and front/rear bumpers
  • more room for wiring and 9V battery
  • "learnings" from the previous design/print - 7cm spans worked with better design
  • room for future proto board, LEDs, Buttons, and Switches
He is a bit longer, I had some concern about effects to the gait.  but, the servo separation will allow for longer gaits.

Mojo - New Chassis, improved SEA, same Compliant Quadruped
Currently, It is still going through variations on the gait sequence.  The new Lateral Sequence is promising. I increased the gait length and speed up the swing, however this is had a negative impact to stability. I will need to rework the gait, but I think the fast swing is important. Might also be worth trying a very very slow gait to start.

Smokin' Servos!

The Fail of the Week was the secret "Lizard" project. I was kicking around ideas about a very basic crawler, something with two 'legs' that moved and drags the bot along. it would have just 2 degrees of freedom. In addition I added a pincher to the front, just because it was around. the pincher, takes away from the Lizard motif, but its cool.

Cool or not, I worked on a Minimal Viable 'Prototype', with servos stuck together and reused the old mojo chassis. I wried up a 5v, 2A power bank, a Arduino Pro Mini and the servos. A few lines of Arduino sketch, and I thought I was ready to go. Unfortunately, I got no movement from the servos. I checked the wiring, all good, code up loading, check, code running, check. Odd smell? check! When I picked up the bot, I noticed that the servos were 'Fire Dog Hot' and smelling cooked!  I am pretty sure they are toast now.  This I did not expect. Everything was 'safe', more debugging needed!!

Fail Lizard!

Back to the drawing board for the 'Fail Lizard' - might as well make it a new beasty!
The down side of mechatronics, is that you typically burn out boards, servos, and sensors - in addition to hanging code.  Fail Lizard brings my servo bone pile up to 5 servos!