I am designing a remote controlled robot which will have a base with three wheels, two of them will be simple wheels at the back of the base and the third will be a ball wheel at the front. It will have a robotic arm which will have a gripper to hold objects up to 1kg. I have designed the arm like this

enter image description here

What I want to ask is how to calculate the length of the arm, the base of the robot, the torque and also which motor to use. Please suggest to me if there is a better solution for designing the robot.I am a robot enthusiast and I am designing a robot for the first time.

  • 1
    $\begingroup$ You might find some helpful information in this other thread: robotics.stackexchange.com/questions/284/… $\endgroup$
    – Ben
    Commented Mar 22, 2014 at 11:30
  • $\begingroup$ Can i use a wipper motor for a jont of robotic arm.? $\endgroup$
    – Roy
    Commented Dec 23, 2017 at 7:08
  • $\begingroup$ I did a similar simulation using SolidWorks: youtube.com/watch?v=crJXUlzJ918 $\endgroup$
    – LCarvalho
    Commented Feb 19, 2018 at 20:02

3 Answers 3


Every motor has a torque described in kgcm units. 1 kgcm means that the motor can lift 1kg weight on a distance of 1cm from the axis of rotation, or 0.5kg at the distance of 2cm and so on. Using that knowledge and the fact that you are lifting 1kg we can say that arm's length * 1kg must be less than the maximal torque of the motor. The stronger the motor - the longer the arm can be. How strong motor you'll use depends on how much money you'll spend and what length you need for your design.

As for the base of the robot, you need to measure where the center of the mass of the robot will be when the robot is holding the object and make sure that it is somewhere between the two red lines on the image (that is, between the wheels). If the center of mass goes outside of these boundaries, your robot will flip over. If your robot is accelerating, then not only the gravity can flip it but also the force due to acceleration (F=m*a). To check if your robot will flip, add the vectors of gravity force and force due to acceleration, position the beginning of the vector at the center of the mass and check if the vector passes between the wheels. If it doesn't, the robot will flip over. You can make sure that doesn't happen by keeping your center of mass low (that is, keeping your base heavy), keeping the wheels as separated as possible and not having a too long arm.

I hope this helped you as your question is a bit vague.


center of mass



Please check the above link for calculating torque.

  • $\begingroup$ Link only answers are seldom helpful. It would be useful if you can explain WHY this link would be useful. $\endgroup$
    – Andrew
    Commented Nov 16, 2014 at 19:42

You could go into a lot of detail on what the optimal design would be by taking into account the weights of the joints and where their centers of mass are, how fast the robot arm would be moving, what positions the arm will be in (which effects the center of mass of the entire arm), etc.

However, if you are just an enthusiast and want to make an arm that works, I would suggest just over designing the arm. Make your joints out of a light but strong material (PVC or some kind of plastic would probably work), get large enough servos that they will definitely be able to hold the weight of the arm and the object you are trying to pick up, and then make the base of the arm wide and heavy so that the arm is unlikely to fall over. You can probably work out some of these values by "going with your gut" and then adding a little more power, little less arm weight, and a little more base weight than what your gut tells you.


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