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Servomotor torque requirement for a quadruped robot


I'm planning to build a quadruped robot soon. My design is very similar to the design in the following link.


I'm not that good at mechanics. I like to calulate the torque required for each motor. Is it ok to use the formulas/procedure in the following link to design a quadruped shown in the above link?


In the above link, it says that the torque equation obtained cannot be used for making the bot move. How do I have to modify the equation?

Reply soon.

Thanks in advance.

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@ lumi and Bajdi thanks.. :)


I think my robot won't weigh 1Kg. It is going to carry Arduino UNO, servo controller and LiPO battery, which will all together weigh round 200gm. If the servo motors weigh 45gm each, then at some point of time, 2 legs will be on air, therefore the robot will weigh (200gm + 270gm = 470gm). If I use acrylic sheet for the base and Aluminium sheets for feet, then the total weight might add up to 600gm.

So which of the following servomotor do you suggest?

HiTec HS 311?

Well, it always depends what the purpose of the robot is. My Chopstick Junior is running with very cheap (i mean really cheapy) mini servos. It was only used for running on a display, exhibition or fair, got touched by children, bended, pulled and squeezed but the servos are still working. I agree, I got to change 3 of them since the plastic gear broke but that's all. They have enough torque to pull the thing forward, no question about that. For more joy and a longer life I also recommend metal gear servos.

When I come back from vacation I should have all the parts to built a hexapod with 3 DOF legs. I bought the Chinese hexapod kit that you can find on Ebay (without the servos). I will be using 18 MG996R servos, these have a torque of 10kg/cm and are really cheap on Ebay. I have done some testing and was quite surprised how little current the servos needed. This was of course with no load, the current drawn when walking will probably multiply: http://www.youtube.com/watch?v=tyLeuQJIWX4 

Thanks for the reply!

I think my bot will weigh around 1Kg. It is gonna have a microcontroller, servocontroller and batteries. I want my robot to walk on smooth surfaces and in slopes of inclination 20 degrees. Robot's legs will be approx 12cm.


I looked at servo's which gives a torque about 10Kg-cm. They seem to be cost around $30. 12 of them will cost around $360! I can't spend that much on servos.

If you want to build a 1Kg walker with 12cm long legs then you better bite the bullet and get those metal geared servos. If you get ones with plastic gears then they will break in the first week and then you will have to buy more servos. The metal geared servos work out cheaper in the long run.

If you can't afford that much then I suggest starting smaller and use miniature servos. You can build a robot like my little 3V7 robot for under $100.

How heavy is your robot going to be? How long are your legs? What maneuvers will your robot perform besides walking? The fact is, precise torque calculations are not that much help.

In any case for walking robots the servos have to work pretty hard and will come under a lot of strain during some maneuvers so you are better off just getting ball raced servos with metal gears and a torque of at least 12Kg/cm. That is about as strong as they get for standard sized analog servos. Digital servos can go a bit higher. Make sure your servos are rated for running off a 7.4V LiPo.

If for example your servos have 12Kg/cm torque and the robot weighs 1Kg then in theory the leg can be up to 12cm long and still have just enough strength to hold the robot in position. In practise I would de-rate the servo torque to 10Kg/cm as it will depend on the battery voltage.

In practise, for normal walking, the weight of the robot will be supported by half the legs while the other half are being raised to take a step. Depending what maneuvers you want to perform then you are probably better off making sure that any 2 legs can raise the weight of the robot.

When you build your robot then take into consideration the amount of power you will need. If you assume a worst case senario then it is possible for each servo to draw as much as 2A. This means your current could peak at about 16A! Under normal walking conditions it will probably be only about 1/3 of that. Make sure your battery, power switch and power wires can handle your peak current otherwise you will get significant voltage drop that can cause your processor to reset.