Hi, I’m designing a robot on a scaled up version of the U arm. It is 1m in length and has all motors in its base. Based on extensive research, my calculations tell me I need a 100 Nm motor. Where do I get such a motor (preferably servo motor)? Also using a gearbox, won’t it only increase the dynamic torque, holding torque would still remain the same, until I use a worm gear based mechanism in my gearbox?
@Shashwat Not aware of any RC servo motor which can provide that torque - you might need to consider an industrial servo motor. A gearbox theoretically increases “torque”, which would be both dynamic and static (holding). A worm gear locks the output in place (i.e. not back driven).
dear coleman benson
i am trying to make robot arm. your method found very helpful to calculate the torque for joints.
but plz can you suggest further details for material selection for robot arm making…
@sachin pati; Most hobby robot arms use RC or smart servo motors. Some use stepper motors, and a few use DC motors with encoders. The mechanical structure is really up to you. You can consider a building system where the actuators can easily connect to the mechanics, like the Lynxmotion Servo Erector Set.
Hi sir,plzz explain me moment of inertia of various shapes…how do i get?.
@anesh You can take a look at this link that explains how to calculate the moment of inertia for some common shapes.
Should we add the static torque and inertia torque to get the motor torque??
@anresh Ideally yes, to get a more accurate calculation, you would consider static friction and inertia, where inertia would have the greater effect.
Hello Sir,
Can you help me with torque calculations of Delta Robot
@ekta Ghonge Unfortunately that is beyond the scope of this tutorial. Knowing there is interest in delta robots, we may create a tutorial at a later date.
I want to use a servo motor in a automatic level crossing system. Servo has to lift a load of 2.5 kg rod. the load is 4 meter long and connected to the servo at one end only. it will rotate 90 degree in 10 seconds. how much torque will generate or what will be motor required power. it will be helpful if you show me how to solve this. Thanks in advance
@Aman Assuming the weight of the rod is roughly evenly distributed along its length, the actuator at one end would need to be able to provide 2.5Kg x 4m /2 = 5Kg-m, or 500Kg-cm just to hold its position. You will need more than that to accelerate it upwards. This is why most of these crossing bars have counterweights on the other end. If it’s for outdoor use, an RC servo motor would not be a good choice. Look into what actuators are used for existing tech.
Hi Coleman! Thank you for such a great tutorial. It was very helpful. I am designing a 6 DOF articulated robotic arm for my final year project. In this tutorial you mentioned the case in which weights are acting perpendicular to the shaft. Please tell me that how to calculate the torque for base motor, because weight of next links and motors is acting parallel to the shaft of the motor (axis of rotation). Also I want to know that how to decide the link lengths of the arm ?
@Sami Assuming the servo is oriented vertically, the load is minimal, and it really only needs to be able to overcome the inertia of the arm, which unfortunately is quite a bit harder to calculate if you’re in the design stage, so choose something around the same torque as what’s required at the elbow. Don’t rest the weight of the arm on the actuator directly. Regarding the link lengths, that’s really up to you and your application, keeping in mind that ~20" will set you back around $400 USD, a large part of which goes towards the price of the actuators. Keep in mind T=F*d, so a longer link means more torque is required by each joint.
Thanks a lot for reply. Sir! Are you sure that torque calculated for elbow will work for the base? What if I take the base torque equal to the torque calculated for the Shoulder? The address for the reference image is:
@Sami It’s just a quick estimate - without calculating the actual inertia / friction. Since it’s only one servo, choose something which should be stronger than needed (a bit overkill).
what is a pressure exerted by robot arm its lifting capacity.
@khant Pressure is a force exerted over an area. Not sure where that applies in most robotic arms. The lifting capacity is a measure of force (and most often torque at each joint).
How to calculate the torque required for motor for scara robot arm which is moving in horizontal plane and axis of motor is at Y-axis
@Praveen You can see if you can assume the arm is fairly rigid and there are no forces in z axis. As such, you need to take into consideration inertia, static and kinetic friction. We do not have the equations for these to provide, so additional research on your end is needed.