Need inspiration, motors placement on cable driven robotic leg

[sunangel46]

Ok, now I have a cable driven robotic leg to simulate a walking gait.
I have 6 motors to drive the cables to drive the leg.
As oppose to conventional robot, I can place my motors anywhere I want.

But I have a question, where is the best place to place the motors?
I.e.
1) To reduce the weight imposed to the legs(trunk of the robot? but where exactly should i put to get the optimize result, and how can I go about doing this?).
2) To create dynamical balance while the robotic leg is doing the walking gait.

I have no idea how to start on and has been thinking for quite sometime, but no breakthrough. (Can hardly find any research being done in this field either)
Or anyway I can reduce the number of motors to drive a 4 dof robotic legs?

I appreciate if I can get a link or something to work on this.
Thank you very much.

 

[dknguyen]

The best place to look to answer your questions is how your own body works. Sit, stand and walk and pay attention to how you move your own body weight and where your largest muscles are and your smallest muscles are.

From the large number of motors per leg I assume you are talking about a bipedal walker? You haven't told us if it has 1, 2, 4, 6, 8, or more legs. But assuming a bipedal walker, why 6 motors? I thought you only need 4 motors to crudely simulate a human leg? 2 for the hip, 1 for the knee, 1 for the ankle, and let's just forget about the fact the leg can about it's longitudinal axis in the pelvis.

For the most, part the the most ideal place is usually the trunk of the robot (or along the arm close to the trunk as possible). Arm and legs act like levers. The farther any weight is toward the tip, the more that torque is amplified by the lever effect will load down any and all motor responsible for lifting that segment and all segments supporting that segment. This applies very strongly to most arms and "horizontal" walking platforms like 4, 6, and 8 legged walkers. It's also true for bipedal walkers, but with these you want some more weight lower so that the center-of-gravity is lower which makes it easier to not topple over (not too much though since the lever effect will still load the motors down more). If you have more legs you don't have to worry so much about having a low center-of-gravity since you can be more easily statically balanced. Once you have 6 or more legs, it is possible to always be statically balanced throughout the entire gait.

Also, more weight lower down on the legs makes it harder to topple over in pretty much all cases. THis makes the robot more statically balanced. If you want dynamic balancing, more weight should be kept in the trunk. Dynamic balancing in this case means a system that is inherently/statically UNBALANCED but is actively kept stable by some a brain so that it can still react quickly when required. (ie. Wearing 30 lbs weights on each foot would make you very statically balanced so you could stand and not fall over with very little effort or thought. BUt you'd be hard pressed to walk, or react to anything quickly.)

The main reason why you don't see more motors so close to the trunk is that it requires a transmission system to sit along the length of the arm/leg which themselves weighs something and introducing additional complexity.

In short, best places for your two goals: Stick the motors (and as much weight in the arm/leg as possible) on to the trunk, or as close to the trunk as possible.