Lately I've been working with a new hexapod project. This time I wanted to go a different way when it comes to design. I must say that all the talk and focus on the Mech warfare has inspired me too, therefore I choosed to try out a mech inspired body design. The legs are made of standard SES brackets and the only custom made parts are the two body panels and the spacers.
Part list (I'll add the SES parts and more details later):
The major reason for starting this project was that I wanted to make a hexapod with 4 DOF legs. The joint are called coxa, femur, tibia and tars. You may ask why I choosed to add an extra joint (the tars)? First of all this is more like an experiment and a proof of concept. A 3. link to the 2 link IK chain (femur + tibia) gives us an endless amount of IK solutions. So like a robot arm the tarsus act as an end-effector. The IK calculations are relative easy if the tarsus are defined to stay in a perpendicular position in relative to the ground. I'm using a variable called TarsToGroundAngle and as a reference I've set the perpendicular position to be TarsToGroundAngle = 0 deg (not 90 deg). So, if we wanted the tars to hold a constant perpendicular position to the ground the formula for finding the tarsus angle would simply be:
- 24 x 645 MG Hitec servos for all legs
- 2 x 85 MG Hitec servos for head (optional
- SSC32 V2 Servocontroller
- BB2 Botboard for BAP
- 2x Turnigy LiPo 6v regulator
- 1x 5400 mAh LiPo
- DIY RC remote (future plan to use XBEE)
- RC receiver
(The femur are calibrated with 0 deg in horisontal position and the tibia and tars are calibrated in a vertical position). This actual pose of the leg are illustrated here (screendump from PEP):
Tars = Femur - Tibia, or Tars = TarsToGroundAngle + Femur - Tibia
But using a constant TarsToGroundAngle wouldn't always be the best solution for a leg. I therefore made an algorithm/formula for calculating the variable TarsToGroundAngle. The input values are the feet positions (coordinates). Here are some pictures that demonstrates how the algorithm work: (pay attention to how the tarsus varies in relative to the ground).
I'll be the first to admit that adding the extra joint (tars) can be a vaste of weight, energy and cost. And its also a bit overkill when it comes to the IK part. But there are some advantages.. The leg are way more flexible and are able to walk over large obstacles without the need for extending the femur. Here is a picture of the SES based leg:
The maximum ground clearance is about 18 - 19 cm (about 7 - 7,4 inches). My idea is to use T-Hex for experimenting on terrain adaption. At this moment I've not added any feet sensors, future plans though.
Here are some pictures of the main body:
I'm using only one battery (LiPo) for both electronics and servos.
For those of you who are interested, here is a little tip of how I made the battery holder for T-Hex:
I used the little spring that are inside the Hitech plastic boxes:
Then I cut off the upper half of the spring clamp like this:
I made a custom spacer with a hole for holding the spring:
The spacer has a hole on each end, here are the spring mounted on the spacer:
And the spacers are mounted inside the body:
Battery holder with the battery in place:
Another view of how the spring holds the battery:
As an option I also wanted to make a little head for some distance sensors. I'm using the new SES microbrackets for the head, and the head are mounted inside the body. The only modification needed to do is to remove one spacer in the front of the body. The head are mounted using a L-bracket that are screwed onto the hub that holds the right front leg. I drilled out some extra holes for holding the bracket, not really necessary but made it a bit stronger since the thicknes of the L-bracket and the body plate made the screw a bit short.
Here are some pictures with the head mounted:
A closeup picture of the head:
Thats it for now. I'll post more info later.