Let's Make Robots!

Problem with hexapod

Hi, I'm new here and I'm not so used to forums so if I'll make mistakes posting, writing etc please forgive me and help me to find the way! :)

I also have to say that I'm not English so forgive me for my English or if I can't get something!

So... Let's talk about my problem..

I have built a 2DOF hexapod (should I post some photos, right?) using Arduino, 12 parallax std servos( Parallax Std Servo ) and building the body by myself. So, putting them in parallel I should give them at least around 1700mA and 6V.. Right? Because now I am no more shure of anything!.. In fact giving them 5A and 6V my poor creature walks, but dragging its "back" on the floor!.. I tried to recharge my "huge" battery and it made some steps as it should do normally but after that it turned again to drag its last 2 legs.. It seems that it has no force enough into this legs to hold the wheight..

I should explain better what I mean with "drag" with a video so I'll do it tomorrow! Meanwhile please suggest me something about what I said before! Thank you beforehand!

Famous quote: "LMR users, you're my only hope!"

[UPDATE]: Attached video so you can see it working! (Sorry for quality, I don't have time to make a better video <=D )

[UPDATE]: Attached second video about the problem I descibed. Note that the rear servos are unable to hold the wheight while the others have no problem!..

[UPDATE]: After a slimming therapy (from ~1.7kg to 1.380 kg, without arduino and battery) I am proud to present you the new design of my creature that allows it to walk autonomously with the battery on bard too!!! Watch the video!! =))) (it's better to download it and watching it on locale)

So the problem must be the power or the torque! (but I am inclined to think that the torque is enough..) Now I have to make it turn around!

Video0029.avi293.63 KB
CIMG4335.avi1.93 MB
CIMG43410.avi1.33 MB

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assuming you have all 12 servos working at the same time:

from the datasheet you have

"Maximum current draw: 140 +/- 50 mA at 6 VDC when operating in no load conditions"

 worst case cenario, with no load

 ( 140 + 50 ) * 12 = 2280mA

best case

( 140 - 50 ) * 12 = 1080mA

but this is ALLWAYS with no load

with load you can go way over the 5A


Video/photos will really help, as the physical layout of your legs has a significant impact on how much load the servos are taking during each step.

Unless your hex is very light weight, those servos aren't particularly strong.  I'm working on mine with Hitec HS-485HBs for the "knee" joints which take all the weight, and those have twice the torque.  Even with that it struggles, although it is a bit of a heavy beast (~3lbs). I'm using a 3700mAh battery with a theoretical max drain of 30A.

im not sure about the prob, but could you post your robot it sounds awesome

Thanks a lot for your advices and observations!! =)

@goncaloc: your last sentencesounds me dramatic in fact as I said before my battery is already huge ad I cant get how could it carry all its wheight!

@pixelseventy2: in the datasheet is written that each servo has a 43.1 oz-in torque.. but how many kg-m are? I repeat.. I am no more shure of anything.. =( consider that my creature's wheight is around 1.5 kg without battery..

Ok here are the photos! Click to enlarge!

Well, the metal parts are handmade so they are not precise as a machine could do.. Now I want to cut other material in order to reduce wheight. The bottom part where board and battery should stay misses.

If you need more photos just ask!




Left side:

Uploaded with ImageShack.us


It is only dramatic if you are making all the 12 servos move at the same time with load.

I'm almost sure you don't need them to move all at the same time

Maybe is just a question of optimizing your walking gate


Hmmm.. My servos have to hold the wheight tree at time.. and the other 3 have to perform a half ellipse to reach the front position.. take a look at the video I posted right now to figure its moving!

if you do decide its a servo weakness, try hobby city the have some cheap servos with more torque than your ones 

here is some for $6http://www.hobbycity.com/hobbyking/store/uh_viewItem.asp?idProduct=3743&Product_Name=HXT_6.9kg_/_39.2g_/_.16sec_Twin_bearing_servo

Thanks for the link! I hope the problem can be solved changing something else less expensive.. =(  now when I'll have time I'll cut some exceeding material to reduce wheight.. but I'm pretty shure that however it will not hold the wheight of the battery....

Looking much better now :D