Hi guys, me and my friend have built an arduino controlled Quadruped "mantis" robot.
We are programmers so we have some problems with the electrical stuff, we used 12*(3/leg) TowerPro SG90 micro servos, they all share a common power supply (4*AA alkaline(5-6V)) and share the ground too(all ground and V are connected in parrallel), the arduino and the external power circuit share a common ground. the problem is that the servos cannot take any load, the whole thing is only some 2mm acryl and an arduino mega on top of that(not much weight at all). Every movement is working fine in the air, but it can't move or lift itself on the ground. I know that the SG-90 servos are weak ones, but are they that weak, or maybe they just don't get the right current? I know that question is asked dozens of time, but I'm kind of stuck with this problem.
Thanks for any help or suggestion.
Battery Performance
Not all batteries are created equal. I found the following online via Google.
http://www.batteryshowdown.com
For my lightweight hexapod with 12 SG90 servos I use a single 7.4V 1300mah Lipo. It supplies enough power to operate for about 15 minutes.
As for “weak” servos, the more appropriate question should be how much power does the robot need. Computing torque is not something that most people do in their sleep. You need to analyze the loads at each joint to determine the torque required. I would assume that the maximum torque required would be when only 3 legs are supporting the unit. I would also assume that the load is equal on all three. You would then need to do the calculations for only 2 joints. The forward/reverse motion hip motors also consume power. Account for acceleration at each motor. Calculate you needs and triple that for good measure.
You also might find that the motors consume more power than the specs imply.
Please post some project details including pictures.
First of all thank you for
First of all thank you for your reply.
Now I’m at work but I’ll upload pictures asap. I used Varta HIGH energy alkaline, I have also tried with 2packs of 4*AA alkalines in parallel, but nothing changed. The plan was to attach the batteries to the body of the robot, but now it is not attached, so there is only a little weight. Most of the time the problem occurs when more(about 8 servos)are moving at the same time, then they do some weird stuff, the whole thing looks like when a spider is dying, thats why I assume that the powering is the problem. I read some posts about the topic, and I assume the alkaline batteries simply cannot deliver the current that the servos need, or maybe the problem is that when some servos want to move in the same time, it may cause voltage or current spikes. May I aks you, if you used any regulator or something for your project? Should I use one? Maybe I should only buy a decent LiPo reachargable battery and nothing more? Thank you again, and I try to upload some pics when I get home 
hexapod power
You may notice that the newer batteries, particular LiPO’s, have Q ratings. This I think refers to the amount of current that can be delivered in a short time. Most model aircraft apparently like higher Q numbers. It may also cause welding if shorted.
The hexapod with SG90 servos is a kit from DFRobot and a “Spider” controller with onboard power regulators. My big hexapod is entirely home brew and it uses a 3A switching regulator for each leg (3 servos). I had the similar behavior as you during development. As general rule, I prefer to use separate batteries for the logic and motors as the noise spikes and voltage drops are difficult to debug.
With an earlier power scheme I have burned up power regulators while doing hexapod pushups. My “big” hex is much too heavy and has been sitting idle for over 2 years.
So this is how it looks like
So this is how it looks like now, I detached the bluetooth module and the sensor. The plastic pieces are handmade so don't blame it :D.
So, as you can see, all of the servo grounds are connected together and all of he servo VCC-s connected together via blackboard(I don't want to solder it, just after I can get it to work). The battery(see below) ground connected to the ground row of the servos and the positive to the positives.
And the battery, it is a simple 4*AA holder with 4AA VARTA hightec batteries in it.
