I’m building quadrupeds now,
I’m building quadrupeds now, but I still have some very fresh experiences that I can share with you.
1. The servos will depend a lot on the size of your robot. Remember, that the servo torque is given in kgcm (should be newton metre, properly, but this is a simplification). That means, that your servo can lift and hold a 1kg weight on a 1cm long arm. However, if you make the arm 2cm long, it will only be able to hold 0.5kg on it. So if your robot’s leg is 20cm in total, a 10kgcm servo will be able to lift/hold 0.5kg of the robot. Fortunately, you will probably have three legs on the ground at any time (for statically stable gaits), so your robot can weight up to 1.5kg then, at least in theory (you have to leave some margin for inertia). On the other hand, the servos and the battery are probably the heaviest parts of your robot. That means that using smaller servos and smaller battery will let you… use smaller servos and smaller battery. The dependencies are nonlinear, though, and it’s not clear where the sweet spot is.
There are several other things to consider when choosing the servo. Price is obviously important, as is the quality. You can have plastic gears, partial metal gears or full metal gears (often they will just write “metal gears” and not tell you that only the last gear in the gearbox is metal). The servos can have bearings (single or double) which can be important if you are going to apply side force on them. The servos that are specially for robotics often have a nub on the back of them, so that you can mount them properly on both sides – but you can glue something or make a proper mount for them with it too. They also have speed (which I think is not so important for you, unless you want to make a jumping spider) and accuracy – so called “dead angle” – basically how large the movement has to be to not be ignored by the servo as noise and actually performed. Lastly, there are things that the manufacturers don’t tell you, like the actual range of your servo, or the fact that the shaft on it is really wobbly, etc.
One more thing, you can use weaker servos at a cost of smaller range of movements if you make them move the legs through a lever, like this hexapod does, but that complicates the inverse kinematics calculations.
2. Two servos per leg are enough to have a simple walker (but then again, look at Penny, a hexapod build with three servos total, or the crawlies with a single engine that you can see in toy shops), but it would be very hard to have precise control over the posture and where each leg goes. You will also most likely have the legs slip a little on the floor – possibly scratching it. With 3 degrees of freedom per leg, you have full control over the position of the end of each leg. Some robots even go as far as adding a fourth degree of freedom – to be able to control the position and orientation of the foot! But that’s 24 servos we are talking about here – a considerable weight, power drain and amount of money. Since a hexapod can walk without having to balance with its body (unlike a quadruped), you should be fine with two servos per leg.
3. Power is a big problem with walking robots. Not only they have to carry their own weight, but also the weight of the battery, if you insert a larger one. And they have to expend power just to stand still, unless you come with some smart mechanical solution, like springs, counterweights or latches. When buying the batteries, pay attention not only to their voltage and capacity, but also to their rating – make sure they can really give you enough ampers. I had to change batteries twice in my robot because of that (the second time when I upgraded the servos). You will want to have a voltage regulator at least for the electronics, possibly also for the servos. Powering the servos with higher voltage than they are made for will make them move faster, but also more jittery, they can start to oscilate and overheat. They will also drain the battery faster this way. Personally I went with a 3S (~12V) LiPo battery, and an UBEC giving me steady 5V. Plus a large capacitor for any spikes. Going down from higher voltage also means that you are drawing less ampers from the battery, at least if you use a switching regulator, not a linear one. With LiPos the regulator is important, because they change their voltage a lot dependnig on how full they are.
4. There are many approaches. I even saw that people add an additional cable to the servo’s pot, so that they are able to tell the position of the leg – then they can record postures. But I think that inverse kinematics is the most interesting approach, giving you a lot of things you can do with it. But yeah, 3 degrees of freedom. It’s less expensive with quadrupeds, I guess. Whatever you choose, be prepared to spend countless hours debugging and fine-tuning your gaits.
