Robotics gurus and engineers: what is the disadvantage of shifting weight to the torso in a hexapod robot?
I’m thinking about designing a hexapod robot, mostly as a mental exercise, but perhaps one day as an actual build.
I’d like to build one large enough to handle stairs and so it will be significantly larger than the toy versions available on the market. I’ve notice that on the toy versions there is a servo at each the 3 joints of the hexapod leg. This makes perfect sense for simplicity sake.
This does add substantial weight to the leg segments (especially with larger motors and 3d printed cycloidal drives), which puts extra strain on the femur axis since it has to raise the weight of the femur, tibia and the tibia’s actuator. My idea was to move the servo for the tibia joint next to the femur and use a timing belt to actuate the tibia. Essentially all 3 motors would be stacked around each other in a cluster near the coxa joint. I would be taking inspiration from this quadruped’s leg design. This means much less torque is required to actuate an individual leg, but also that the torso becomes significantly heavier.
My thinking is that this additional weight at the torso will be distributed across 3 other legs (at minimum), since all hexapod gaits require at least 3 feet contacting the ground at any given time. I came across an old thread somewhere on the internet where shifting the weight from the legs to the torso in the design was considered a negative of the design by several people without much additional discussion. That made me realize that I’m probably not understanding something important about the weight distribution. Would an ideal hexapod robot design not have light legs and a heavier torso, all things being equal? What am I missing/not understanding?
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Well, not an engineer or anything, but the higher the centre of mass, the easier it is for the whole machine to topple over, especially when it goes up stairs.
Or, think of a car going through a corner. The higher the centre of mass, the slower it has to go through the corner, in order to not flip over.
@ragingloli That’s an interesting point about elevating the center of mass that I hadn’t considered. It may be a factor, though hexapod robots are usually very stable due to their minimum of 3 points of contact with the ground and “squatty” geometry. I would expect when crawling up/down stairs the gait would have 4 points of contact and be fairly stable. I could be wrong though.
A semi truck may have 10 wheels on the ground but a strong wind can still blow it over. The higher the mass the greater the distance needs to be between the ground contacts for the vehicle/robot to be really stable on inclines and/or turns.
If you make the torso low like an actual spider it will be stable. There is a reason nature makes spiders that way. Playing with robotics has never been easier. It used to be difficult to program and power them. No so much now. The most basic electro-mechanical and programming skills are all that’s required.
Off the top of my head, the main disadvantage of moving power sources (motors) away from where you’re applying power is the complication, efficiency, and reliability of whatever drive train mechanism you chose. One of the reason you find motors directly on the parts that are being moved is the simplicity of this arrangement.
I’ve been thinking about this more and I realized that I’m thinking about it wrong. Since I’d be moving 6 motors to the torso, but there’s 3 legs on the ground that would effectively double the weight of the motors/gearboxes as additional load on the torso in a 3-legged gait. I might be able to offset this somewhat by adding a spring to keep the legs in tension when they’re on the ground, at the cost of additional force required to extend them.
@dabbler great points about efficiency/complexity. There’s also additional costs by requiring more components (though hobby timing belts and pulleys aren’t too expensive. One of the things that makes hexapods so tricky is that there are 18 joints, so even cheap components like bearings start to add up quickly.
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