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We are making a robotic arm using Arduino Uno board in which we are not able to decide which kind of motor to use.

Which motor will be better if we want to lift weight up to 1kg using gripper at the ending shaft of arm? Specification of our design is as given below:

  • 4 DOF ARM
  • Movable vehicle as base weight of 1kg.
  • Individual length of link 20cm, 20cm, 10cm and after that gripper of 8cm length.

Which kind of motor should we use? DC or Servo?

  • So you want to lift 1kg at 60cm of distance? Well, the required torque will be 60 kg·cm. This is pretty high for servo motors. I suggest you to look for stepper motors, or if you have money to spend look at "real" servomotors (but they require a driver), not rc servos. And keep in mind that you have to have a torque around 100 kg·cm to lift that. And don't forget to add the other motors weight and the arm weight too... – frarugi87 Mar 30 '16 at 12:47
  • You might fit a DC motor with gears to give it a lot of torque. But you would then need a potentiometer to measure the angles. – Paul Mar 30 '16 at 12:49
  • Mind that you'll also have to calculate in the weight of the motors/gripper/structure... – Paul Mar 30 '16 at 12:51
  • @frarugi87 stepper motors are going to weigh a lot more than geared RC type servomotors, though they have less backlash. Generally speaking steppers don't belong on mobile platforms. The poster may need actual industrial (rather than hobby) type servos with low backlash reduction - not an inexpensive requirement! – Chris Stratton Mar 31 '16 at 3:34
  • @ChrisStratton yes but.. I never saw rc type servos with rated torques of more than 30 kg·cm; the base motor needs to have a torque of at least 100 kg·cm. I think that this is quite impossible, even with the best rc motors (and keep in mind that usual servos - the 10€ ones - have rated torques of less than 10 kg·cm) – frarugi87 Mar 31 '16 at 10:05
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So you have a total length of your arm of 58cm. The center of mass may be further past the end of the gripper so I am allowing 5cm for that. Total length of 63cm.

Holding torque required just for the load:

Force = mass x acceleration 
Torque = Force x radius
Torque = 1kg x 9.8 x 63cm    [in N.cm]
       = 617 N.cm

This is a lot of torque and we haven't factored in the weight of this robot arm.

Some motor datasheets/specifcations:

Whether you go DC servo motors or stepper motors, you will need to use decent motor drivers hooked up to your Arduino to drive the current needed by the motors.

You will need at least 4 motors for your 4 DOF. Lets assume each gearbox-motor combo weighs 700g. On top of that lets assume that the frame weighs 1kg. Lets assume the center of mass is exactly half way along the arm (ie. 31.5 cm)

Torque = (4 x 0.7 + 1) x 9.8 x 31.5
       = 1173 N.cm

The gripper needs to be decent to hold 1 Kg - so lets assume that it weighs 600 grams. Lets assume the center of mass is exactly in the middle of the gripper (ie. radius = 54 cm)

Torque = 0.6 x 9.8 x 54
       = 318 N.cm

So now lets add the total torque required:

Torque = 617 + 1173 + 318
       = 2108 N.cm
  or 211 kg.cm  

The motor closest to the base will need to be the strongest as it needs to provide the most torque. The motors need less torque the further away from the base they are.

If you were to use stepper motors, you will need gearbox assemblies to increase the output torque. It is even possible to use 2 stepper motors driven in unity to provide double the torque.

If you were to use a stepper motor you will need a motor like this: http://www.phidgets.com/products.php?category=23&product_id=3334_0

Alternatively, servo motor like this could be used: http://oceancontrols.com.au/MOT-280.html

Either DC servo motors or stepper motors (with gearboxes) would be OK for this robot arm. I would go with DC servo motors because they are slightly lighter and can handle the torque without needing a gearbox. However the DC servo motor is about 3 times the price of the stepper/gearbox combo.

  • You have the stepper vs. servo consideration backwards - geared servos have far more torque for their own mass than steppers, which are quite heavy and inefficient devices. Really the only advantage of steppers (even over unreduced servomotors) is simplicity of electronics, but that should not really be as much of an issue in this day and age as it was decades ago when they were invented. The continued use of steppers on relatively low torque 3d printers kinds of distort the popular perception away from the more general engineering reality that they are quite low performance. – Chris Stratton Mar 31 '16 at 3:37
  • @ChrisStratton you are quite right. I was only thinking of the hobby/RC servo motors. I have adjusted my answer to consider industrial servo motors. – sa_leinad Apr 1 '16 at 14:07
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