Hi Ho,

I was wondering if anybody can help with a lil' problem i've been having. I decided to build a hex robot after seeing the fantastic Hexapods on this site, it's a wonderful hobby and up till now was going along pretty smoothly. However i'm having trouble controlling my servos correctly... My question is, Do different servos have varying degrees of accuracy, and does that mean that i'm going to have to buy some pretty expensive servos for this project? At the moment i'm using Ripmax SD200 servos but can't seem to find the right pulse width for the specific angles needed to move the legs properly.

I have developed and programmed a servo controller that lets me set PWM at 1uS intervals, meaning that i can set very specific pulse lengths. And i've worked out all the calculations for IK. So this is the last thing standing in they way before i can start experimenting with different gaits and all that fun stuff of a walking robot.

So are these Ripmax servos just not good enough for robotics like this, or am i missing something?

Thanks for reading.

-Marty

Hi Marty,

Well I have used many different types of servo over teh years, and some of the cheaper servo's have very poor linearity, which makes doing anything usefull with them very difficult. Having said that, my first hexapod V1 was built with SD200 servo's, so you should be able to get something reasonable working.

I presume you have some kind of calibration scheme for each leg joint?

Matt.

Hi Matt,

Thanks for the reply. It's good to know that it insn't impossible to use the SD200s, I know next to nothing about servos - Despite looking all over the place for info - so it is good to hear.

Yes i have got some sort of calibration scheme in place, actually i only just added it yesterday.

I tested each servo noting what pulse range they use for there full rotation (1900uS) and marked down what pulse length corrisponds to each 10 degree increment. Using that information i made something that'll send pulses a little more accurately. However i was a bit apprehensive before because the 10 degree increments seem to happen at random pulse lengths, and the centre position not where i would expect it to be - but was infact favouring one side of the pulse range.

It still can be upto a half degree off - as the pulse lengths that i marked down don't always hit the mark. But it is a lot better then before.

Thanks Again,

-Marty

Hi Marty

I'm have just added a post on my new forum which gives some clues to the problem with servo linearity, you can read it here: http://www.hexapodrobot.com/forum/index.php

I will also be closing this forum soon, so please visit and join the new one. :)

Matt.