View Full Version : Quadruped Design problems

06-18-2010, 06:06 PM
Hi everyone. I just completed my first robot build - a quadruped. I knew what I wanted from the start, and I built exactly what I had planned. The problem is that when I finally got everything together and started programming, it's having problems standing and moving.

First, let me share some details of the build and then I'll propose some of the changes I want to make.

The bot is a 3 DOF quadruped powered by Hitec 332HDs. I chose the 322HDs because they were cheap and I'm a poor college student. I realize that replacing them would probably fix my problem, but I can't afford it.


The chassis is a Radio Shack project box, which, in hindsight, is probably way too heavy.

Here's a close-up of the leg design:


I think that decreasing the weight of the chassis (material/design suggestions appreciated) is probably my best bet. If that doesn't work, I could stack the leg servos to make the leg shorter, and shorten the dowel rod as well.

Any other suggestions?


06-18-2010, 06:57 PM
Hey cshepp,

You've created 2 levers that are causing the servos to work hard. The first is the top leg servo, where the dowel pushes up against the bracket causing stress on the servo. The second is the combination of the dowell, upper leg servo, and lower servo body acting as a lever against the lower servo gears. Since these two servos are operating within the same plane, they are actually just duplicating one another.

You might consider removing the last servo, rotating the second one 180 degs. and just mount a small stub of dowel on the bottom of the servo. You would get the same basic movement, eliminated 4 servos, and taken most of the stress off of the leg servo since the pressure is now pushing up instead of acting like a lever.

Just a thought... I ran into some similar problems with my Mech "Bheka". There are some pictures in my gallery that show my leg evolution. I'm still trying for 3 DOF, but have moved to much stronger servos.

Good Luck

06-18-2010, 07:19 PM
Welcome to the TRC!

Your bot looks like it's pretty light already, I don't see many places where you could remove much weight (possibly drilling holes in the case to remove some material, but those project boxes aren't *that* heavy to begin with). The HS322 is a pretty weak servo for a walker -- and those look like some pretty long legs, I definitely think you're going to want to shorten the dowel part, and maybe change the mounting on the two servos in the femur to reduce it's length. For reference, if you cut the length of the legs in half, you cut the torque requirement in half, so it could be a very big gain.

If that still doesn't get the torque requirement down enough, you might look into an indirect drive.

BTW, what are you using to control that bot, I see what looks like a Mini-USB connector?


06-18-2010, 08:12 PM
With the servos you have you are going to have to shorten the last two leg segments by a considerable amount. You could considering mounting the tibia and femur servos body to body to shorten the femur along with shortening the tibia segment's wooden dowel.

You may also want to look into how you are powering the robot's logic and servos. The servos could be drawing enough power to reset whatever logic you have on the bot.

06-19-2010, 06:06 PM
Thanks everyone for your advice. I think I'm really going to enjoy being a part of this forum. It seems like a really nice community.

I think I'm going to try going down to 2 DOF legs first, and if that works, maybe later I can try for 3 DOF again. I'm going out of town this week, but I'll have plenty of time to do some calculations with the hope of thinking this 2nd design through. I have a 20 hour car ride to visit my Grandparents in Missouri, so I'll have plenty of time to crunch some numbers.

I'll update you all when I return.

Thanks again for your help.


p.s. - lnxfergy, I'm using an Arduino.

06-24-2010, 07:28 PM
Wood pegs as feet might slip, you could probobly use somthing rubber on the end. ;) Hope that helps.

Actually, if he ends up going to 2DOF at point, slippery feet are a good thing (since there is an inevitable amount of grinding through a arc on the ground otherwise). In the videos from Mech Warfare 2009, Issy has crazy red feet, this was a very slick red tape that I had used to cut down his friction with the floor to slow down how fast his servos were overheating.