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Multiped

Still Under Development

My walking robot software.

Nature loves animals/insects with legs. Even birds have legs! Legs allow for easier navigation across rough terrain. The more legs you have, the more redundant and robust you are. If you are a centipede and you break a few legs, no big deal. However, if you are a human and break one leg, then walking (forget running) becomes extremely difficult.

In robotics you see many different types of walking robots. Common ones are: 2 legs, four legs, and 6 legs. There are advantages and disadvantages for each of these types of robots.

  • 2 legs
    • balancing and shifting the body's center of mass is critical to the robot not falling over
    • the 2 legs are generally need to be more powerful to lift the body mass
  • 4 legs:
    • there is an inherent stability with this configuration. You only need 3 legs (tripod) to keep the robot standing, leaving one leg to move freely as needed
    • each of the legs can be weaker than its 2 legged counter part
    • However, there is still a lot of motors, data lines, coordination complexity than if you only had 2 legs
  • 6 (or more) legs:
    • having more than 4 legs contains all of the same advantages/disadvantages of 4 legs
    • you also have more redundancy, essentially able to loose 2 legs and keep walking
    • there are also more types of gaits available to you, which is a fancy word for how you control your leg moves when walking, trotting, running, etc
    • You can even use some of your lets as arms to move objects around while still maintaining a stable tripod stance
    • However, more motors, more weight, more power consumption drives you to needing larger and more expensive batteries
    • at some point (maybe a centipede) more legs doesn't really add any advantage

Robot based on Robotis XL320 Servos:

Robot based on Robotis AX12 Servos

  • Not fully working yet

Required Software

This software requires pyservos to run. This software talks to both the XL-320 and AX-12A servos from Robotis.

Class Layout

Here is sort of the layout of the code:

      cmd      3d pts      DH angles        servo packet
robot --> gait -----> legs --------> engine -----------> servos
  • Robot(data):
    • Holds all of the other classes: Kinematics, Gait, Engine
    • Holds all sensors
    • Runs any logic/AI for the robot
  • Gait:
    • Gait calculates the foot positions for 1 cycle of a movement
      • All values are in (x,y,z) of units mm
    • command() - plans all feet through 1 gait cycle (say 15 steps)
  • Engine:
    • Engine takes the output from kinematics and translates it to servo space for each time stop and each leg in the cycle. It then sends the commands (in bulk) to move the servos each step
      • legs[4]
        • servos[3-4]
          • current angle
          • ID
          • angle offset (based on kinematics to align with DH reference frame)
  • Kinematics: Takes the 3d positions from gait and performs the inverse kinematics
    • leg parts: coxa, femur, tibia, [tarsus]
    • forward() - forward kinematics
    • inverse() - inverse kinematics

The example quadruped (in the examples folder), takes a dictionary (or you can use a json file). Currently it takes:

data = {
    "numLegs": 4,
    "numServosPerLeg": 4,
    "coxa":   [52, 150],
    "femur":  [90, 123],
    "tibia":  [89, 194],
    "tarsus": [90, 167],

    "sit": [80, 0, 1],
    "stand": [120, 0, -70],

    "servoType": 1,
    "bcm_pin": 11,
    "serialPort": "/dev/tty.usbserial-AL034G2K"
}

If you don't pass it a serial port, then it falls back to a simulated serial port which does nothing but is useful for testing.

Bulk Writing

Normally, you would send an individual command to each servo and get a response back from it. That creates a lot of back an forth communications. Instead, this library uses a bulk write to send commands to all servos at once with no reply. This results in smoother motion and greatly reduced data transmission.

Building and Documentation

Details for how to build the robot and other information are in the docs folder in the git repo

Tools

This directory contains several tools for the robot:

  • get_leg_angles.py: prints out the joint angles for all 4 legs
$ ./get_leg_angles.py /dev/tty.usbserial-AL034G2K
Opened /dev/tty.usbserial-AL034G2K @ 1000000

Servos: 1 - 12
All angles are in degrees
         Leg 1 |         Leg 2 |         Leg 3 |         Leg 4 |
   ID | Angle  |   ID | Angle  |   ID | Angle  |   ID | Angle  |
-----------------------------------------------------------------
    1 | 149.56 |    4 | 149.56 |    7 | 149.56 |   10 | 149.56
    2 | 239.88 |    5 | 271.55 |    8 | 269.79 |   11 | 270.38
    3 |  99.41 |    6 | 100.29 |    9 | 100.00 |   12 |  99.41
-----------------------------------------------------------------
  • get_leg_info.py: prints out servo information for all 12 servos on the robot
$ ./get_leg_info.py /dev/tty.usbserial-AL034G2K
Opened /dev/tty.usbserial-AL034G2K @ 1000000

Servos: 1 - 12
--------------------------------------------------
Servo: 1          HW Error: 0
Position [deg]: 149.6  Load:   0.0% CCW
Voltage [V]  7.0     Temperature [F]:  80.6
--------------------------------------------------
Servo: 2          HW Error: 0
Position [deg]: 239.6  Load:   0.0% CCW
Voltage [V]  7.2     Temperature [F]:  80.6
--------------------------------------------------
Servo: 3          HW Error: 0
Position [deg]:  99.4  Load:   0.0% CCW
Voltage [V]  7.2     Temperature [F]:  82.4
--------------------------------------------------
Servo: 4          HW Error: 0
Position [deg]: 149.6  Load:   0.0% CCW
Voltage [V]  7.3     Temperature [F]:  80.6
--------------------------------------------------
Servo: 5          HW Error: 0
Position [deg]: 271.6  Load:   0.0% CCW
Voltage [V]  7.2     Temperature [F]:  80.6
--------------------------------------------------
Servo: 6          HW Error: 0
Position [deg]: 100.3  Load:   0.0% CCW
Voltage [V]  7.4     Temperature [F]:  82.4
--------------------------------------------------
Servo: 7          HW Error: 0
Position [deg]: 149.6  Load:   0.0% CCW
Voltage [V]  7.2     Temperature [F]:  80.6
--------------------------------------------------
Servo: 8          HW Error: 0
Position [deg]: 269.8  Load:   0.0% CCW
Voltage [V]  7.1     Temperature [F]:  78.8
--------------------------------------------------
Servo: 9          HW Error: 0
Position [deg]:  99.4  Load:   0.8% CCW
Voltage [V]  7.2     Temperature [F]:  82.4
--------------------------------------------------
Servo: 10          HW Error: 0
Position [deg]: 149.9  Load:   0.0% CCW
Voltage [V]  7.1     Temperature [F]:  80.6
--------------------------------------------------
Servo: 11          HW Error: 0
Position [deg]: 270.1  Load:   0.0% CCW
Voltage [V]  7.2     Temperature [F]:  80.6
--------------------------------------------------
Servo: 12          HW Error: 0
Position [deg]:  99.4  Load:   0.0% CCW
Voltage [V]  7.1     Temperature [F]:  84.2
--------------------------------------------------

History

RC Servos

This was the original version, shown above around Aug 2016. It used toy RC servos (9 g's), they didn't work that great. I think I lost the code, because I can't find it.

XL-320 Servos

work on this robot has stopped This worked OK, see the movie above. The servos were not very strong and the lego like rivets were kind of a pain to use. The rivets also didn't hold things together very tight so there was a lot of wiggle. Overall, I prefer nuts/bolts.

AX-12A Servos

this is the current robot I am focusing on All of the current code is designed for this robot. I sort of maintain backwards compatability with the XL-320, but I don't guarentee it.

ToDo

  • change more return values to tuples instead of lists for performance
  • do CM shifting
  • make generic, so I can do 6 or 8 legs
  • flush out the config file better, I have hard coded too many things

Change Log

Date Version Notes
2018-11-27 0.6.0 Reorg'd, moved robots to own repo
2017-12-25 0.5.0 Clean up and reorg, removed unnecessary libraries
2017-06-07 0.4.1 broke out into package and published to PyPi
2016-08-10 0.0.1 init

MIT Software License

Copyright (c) 2016 Kevin J. Walchko

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.