I am trying to build a 2-axis robot arm with pan and tilt mechanism. The gripper/holder will hold an object weighing 300 grams. The total weight of the arm including the motors will be around 2 kg. I have decided to use 180 degree servo motors. The maximum arm reach will be 340 mm. what I want to ask is:

  1. What kind of servos (analog/digital) will be suitable to support the total weight (2 kg) and the object weight (300 g)?
  2. How do I calculate the required torque?
  3. How many servos should I use to make sure that my arm doesn't flip over?

Please suggest me if there is a better approach to designing the robot. I am fairly new to electronics and this is the first time I am building a robot. Thanks in advance.

  • 2
    $\begingroup$ Can you give us a drawing of what the arm will look like? $\endgroup$
    – Chuck
    Commented Aug 9, 2015 at 18:13
  • $\begingroup$ @Chuck: I am responsible for the electrical part of the project. The mechanical guys are responsible for making the CAD models and drawings. The arm will look like a basic pan and tilt mount but instead of a camera, a phone will be held in the gripper. Unless I work out the type, weight and power requirements of the servo, the mech guys can't proceed to creating the models. Please ignore my third question. We will be using 2 servos, one in the base to pan and the other in the elbow to tilt. $\endgroup$
    – Abinaya
    Commented Aug 10, 2015 at 4:46
  • 1
    $\begingroup$ @Chuck asked for a drawing. Restating that you have a "pan and tilt mechanism" isn't exactly a drawing. If you provided an image, it'd be much easier to understand what you have in mind. Please keep in mind that there is usually a cyclic dependency of mechanics and electronics, that is, you need to know roughly how the mechanical parts look like to estimate the required torque (etc.) and the torque in turn is necessary to calculate the strength (stiffness) (etc.) of the mechanical parts. You have to iterate a few times and talk back and forth to design the system. $\endgroup$ Commented Aug 10, 2015 at 11:21

1 Answer 1


Torque is pretty easy to calculate. A 300 gram (2.94 Newton) mass at the end of a 340 mm (0.34 meter) arm is 1 Nm. (2.94 * 0.34). If the arm weighs 2 kg (19.61 N), and its center of mass is half it's length, (0.17 m), that adds another 3.33 Nm. So a total of 4.33 Nm. But this doesn't take into account the joint's rotational acceleration which you didn't specify.

You should check out this arm torque calculator. The robot arm tutorial on that same site also has lots of helpful info.

I'm not sure what you mean about the arm flipping over. But to ensure that a rigid body does not tip, simply sum the torques about the edge of the supporting convex hull.


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