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Thread: Low Cost Hexapod

  1. #1

    Low Cost Hexapod

    I became interested in hexapods after seeing Matt Denton's MSR-01 and Zenta's A-pod and Phoenix. I'm studying engineering so I decided to put some of the maths to use and write my own hexapod control program. To help with testing this out I built a hexapod using just about the cheapest servos I could find (HXT900, which were $3.60 at the time, they're now $2.99).
    I'm redesigning this to solve some issues I had. On the original, the coxa joint did not have enough movement and the legs were too flexible, which was also because of the coxa joint - it pivoted only on the servo shafts without support on the other end of the servo.

    Now my coxa and femur servo's are joined by a coxa part cut from a 4mm plastic chopping board. The coxa has pivot opposite the coxa servo's shaft. The femur, tibia and body plates are cut from 4mm plywood.

    The servo's are controlled by an SD21 I2C controller. A Picaxe 18x takes commands over bluetooth serial from the program running on my laptop or phone and sends them to the SD21. This is proving a bit slow, which leads to jerky servo movement. I'm probably going to get a Pololu Maestro servo controller which is serially controlled and has some other cool features like using pins and inputs and running scripts and sequences. I just need to assemble the parts and update the leg parameters and servo directions in the program and hopefully I can get a video of it walking. So far this has cost me about £100 to build. Check: Publix Ad and Kroger ad on Weekly Circulars.

  2. #2
    Join Date
    Feb 2008
    Rep Power

    Re: Low Cost Hexapod

    Welcome! And I'm very excited to see your progress. I really like your low-cost approach. Since you are a student of mathematics, I highly recommend this book:

    [ame=""] Theory of Applied Robotics: Kinematics, Dynamics, and Control (9780387324753): Reza N. Jazar: Books[/ame]

    It's my reference book on all things kinematics. I haven't had a chance to check out Matt Denton's suggested reading, but I found this book to be quite complete.

    The thing that I realized throughout my research and writing my own IK is that for just basic motion, there's two pieces:

    1) IK Transformation of global coordinates (body) to local coordinates (each of the six limbs)

    2) Simple Trigonometry to translate those angles calculated from above into PW (pulse width).

    Your project reminds me of this biped I saw a couple of years ago, I think it was called Pinnochio or something to that effect. The frame was wooden. I've always love to see robotics with a wooden frame.

    Good luck with your build/project. Don't be afraid to ask about IK in the "Let's discuss Inverse Kinematics" thread that Andrew kindly stickied for me last year. We have A LOT of talented guys in this community...
    Sleep? You don't need sleep...


  3. #3

    Re: Low Cost Hexapod

    I've actually got the basics pretty much nailed down on the IK side of things. Most of the work was in getting the gait and rotation equations equations worked out, but they appear to be working now. I'm learning programming as I go along so managing that side of things is more tricky for me than the geometry / trigonometry.

    Things that are working;
    - Body rotations and movement
    - Walking in any direction and while turning
    - Easily adding gaits

    Things still to be done;
    - Telling legs to step to new foot coordinates eg. for changing pose and for switching gaits
    - Making legs objects in the program so they can be queried for current position and whether they're lifted or set down
    - Adding balancing so the body shifts to the 'centre of gravity' of the legs which are set down at that point

    Yes I did look through that thread. I'm not sure if the way I'm doing the rotation matrices is the 'right' way to do it, but it seems to work. I'm rotating the feet positions through the pitch, then roll, then yaw just because this is the X, Y ,Z sequence . It's easy enough to change (each one is just a function) and as long as each leg does the same thing then the point will still be stationery on the ground (i.e. the distances between the feet will be the same, and the origin of the body won't move).

  4. #4
    Join Date
    Dec 2007
    Whidbey Island, WA
    Rep Power

    Re: Low Cost Hexapod

    alright that does it...Post some video before my head explodes.


  5. #5

    Re: Low Cost Hexapod

    All in good time . For now here's some more pics. The legs are mounted up so you can see roughly how it's going to look. I still have to centre all the servos though. I've got a graphical (Windows form) way of doing it, it could use some improvement but I have to figure out how to read and write text files properly . . .
    Assembled leg;

    And legs mounted;

  6. #6
    Join Date
    Jan 2008
    Montreal, Canada
    Rep Power

    Re: Low Cost Hexapod

    Looks really nice!

    I too can't wait to see how it moves. Seriously, nice job! For the money you spent it looks REALLY nice. Now hopefully it will walk as nicely

  7. #7

    Re: Low Cost Hexapod

    Ok, I got it walking today. I still need to get the wiring sorted out as it barely fits in the body and I need to extend another three servo cables. I also haven't thought about where the battery is going to fit yet. Anyway, the servo's seem up to the task. It can do a tripod gait carrying it's battery without much fuss. Carrying the smartphone (doing the IK) is too much for it when doing the tripod gait, but it's fine with a gait where only one leg lifts at a time (is this the ripple gait? Not too sure). I haven't tried gaits where two legs lift at a time yet.
    Unfortunately, sending servo values to the Picaxe over serial, and then to the SD21 over I2C is a bit slow, which means the movement is a bit jerky. Also, unless I want to slow communication down further, I can only specify positions to 6 µs which exacerbates the jerkiness. (This is using a mode on the SD21 which lets you use just one byte to set servo position, rather than two.)
    I'm ordering a Pololu Maestro 24 to hopefully get around these issues. Anyone want an SD21?

  8. #8

    Re: Low Cost Hexapod

    Quick update:
    The Pololu Maestro 24 turned up in the post today (or yesterday; had to pay VAT on it though ). It's a lot more compact than the SD21!

    This should save a bit of space in the body. I might just be able to squeeze the 3S 650mAh LiPo I have in there too, but I've already started on a mounting place elsewhere so I'll probably finish that anyway.
    I've done a few bug fixes in the code, mostly to do with walking at variable speeds. I still want to make some big improvements, but for now I'll just get the the servo controller incorporated and see what improvement that gives me. It uses the same protocol as the motor driver I used in my rover bot so it shouldn't be too difficult.

  9. #9

    Re: Low Cost Hexapod

    Well the weather has been too nice to do much robotics recently. Perfect for r/c helicopter flying though . Anyway I have made some progress since my last update.

    - Altered the code for the Pololu Maestro

    - Set the servo limits in the Maestro and re-tuned the gait to take advantage

    - Implemented reading the Maestros analog inputs over the Bluetooth connection

    - Used this to measure battery voltage and the analog range output of a Maxbotics EZ-1

    - Started modifying the code to use more object orientated features which should make it easier to add more capabilities

    - Added a mode where the hexapod copies the attitude of the phone (using accelerometer on the phone side and IK rotations on the hexapod).

    The Maestro is a lot better than the SD21. It can be updated a lot more frequently so movements are much smoother and it has a good Windows control/setup program which allows you to centre the servos, set the servo limits and create scripts and poses. By turning up the limits I can get almost 180º out of the HXT900s.
    I have also been talking to Jan at Pololu and at my suggestion he has started implementing a group move feature similar to that of the SSC32. While the Maestro can be told to move individual servos at different speeds, moving servos across different angles in the same time period saves calculating and sending speeds to the Maestro. I think Xan's Phoenix code uses this to smooth movements between positions calculated by IK.

    I've found the legs are a little too flexible to do fast walking, so I'm thinking of adding a second femur part to each leg. To do this, I'm adding a pivot point to the HXT900s (opposite from the output shaft) by drilling a hole in the back case and glueing an M2 nut over the hole (inside the case) for a bolt to thread into. There is a small plastic moulding in the case in just the right position to make drilling a precise hole easier. Here's some pics of the servo mod and the bot in its current state.

  10. #10
    zoomkat Guest

    Re: Low Cost Hexapod

    A lot of servos don't have room inside for hardware attachments, so I've mounted on the outside using hot glue. The link below has some pix of cheap/easy support ideas.

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