Key requirements for Servos for walker robots? What can I sacrifice?

Community General Discussions
1

Hello all.

 

I am in the midst of building a six leg walker bot. I am following a set of designs I found posted on here. Initially, it will be a 2 DOF setup, but the plan is to use it as a test bed, and expand out to 3DOF, add sensors, add increasing complex algorithms etc.

 

I would like to keep the costs as low as I can initially, but the flip side is that I don't want to cost cut to the point of introducing significant issues into the build, which then takes a lot of additional coding to manage.

 

The thing I am reseaching at the moment is servo selection for the legs. I was wondering if I could ask your advice?

 

In order to keep costs down, it would be good to purchase lower cost servos. However, these come with some issues:

1) Possibly higher current draw - some super cheap servos look like they are pulling 1amp in normal operation

2) Slow movement - 0.23ms makes for a cheap servo, but a slow one. I'm not sure whether this will fit in the with concept of a walker - maybe it'll end up more of a geriatric?

3) reduced load capacity - for example cheaper servos seem to sit around the 3kg/cm range, where as higher cost sit aroun the 6kg/cm range.

4) wider dead ranges

 

Like anything to do with an engineering project, it's all going to be about comprimise. I was wondering which of the above I can get away with, and which will lead to major issues with my build?

Are there commonly used servo models used in walkers?

 

Some model specifics:

Model size - approx 600mm long x 450mm wide

Weight - as yet unknown until built - which is a bit of an issue in terms of this question.

          In terms of construction, I will be using 4mm carbon/ply/carbon composite for all structural components in order to give good stiffness at a low weight. Lets me have the structural strength better than Aluminium at less than 1/2 the weight. Batteries will initially be 4.3mA/H Ni-Mh's weighting about 400g, but if I need to I can shift to LiPo to bring down weight, but that means canabalising my planes.

 

The servos will all be based in the body itself, not on the legs - so maybe that means I can play with mechanical advantage, and get away with a reduced Kg/cm rating?

 

My thinking at the moment for the walker is that, in terms of priority, it gos something like:

1) As small a dead zone as possible

2) current draw

3) speed

4) torque.

 

Am I on the right lines?

 

 

 

2

There’s two things that I

There’s two things that I came up with on my quest for a hexapod (which is frozen for a while):

1: Use the same servo’s for the same joints. So for the hipjoints, use 6 identical servo’s.
2: Don’t cheap out! You don’t need the top of the bill ultra-fast digital extreme torque servo’s, but don’t work with the ultra cheapo’s. From RC racing it was already apparent to me that you don’t need a 90 euro servo for steering, but a 20 euro servo will not do the job it is supposed to do. Or it will, for a short while and then burn out (I had a Towerpro MG-945 set my car on fire in the middle of a race, not nice with a full tank of nitro-methanol, I can tell you that!)

I haven’t built one myself so I have no idea about the deadzones and everything, that is to be seen for myself as well.