Let's Make Robots!

Advice on building a pan/tilt robot arm

 

Hello LMR folks,

I am in need of some advice over a robot project I plan to build. I have a small amount of experience with using an Arduino, soldering and general circuit making, but I've never used motors yet.

What I plan to do is to have a "robot arm" attached to the ceiling with a camera at the end of it. The arm need to be able to rotate the camera around and depending on some conditions tilt it at different angles.

I was thinking of driving the entire rig with an Arduino Uno and using a pan/tilt servo combination from DAGU (this one : http://robosavvy.com/store/product_info.php/products_id/1182 )

My main questions would be the following:

- Can the arduino control and power both servos ?

- Does it need any extra circuitry to do that ? (e.g. a motor controller, something to protect it from back current from the servo, etc...)

- Most importantly, would these servos be good enough to handle a camera that weights 250-300g ?

 

- Can the pan servo handle 360 degrees of rotation (it doesn't need to be continuous, i.e. moving 0 -> 360 -> 0 is fine) ?

Thank you in advance for all your answers

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- Can the arduino control and power both servos ?

Yes but it is bad practice. 2 servos may draw lots of current and make the arduino unstable. You should build a regulating circuit or use separate power for the servos to avoid problems. This can be as easy as hooking up 4 AAA batteries to the servos and only hooking up the signal line to arduino. 

- Does it need any extra circuitry to do that ? (e.g. a motor controller, something to protect it from back current from the servo, etc...)

Again depends on your chosen path. Avoid connecting the servos to the arduino as it may cause problems. If I was doing this I would build an external regulator circuit and connect different power supplies to the arduino and servos.

- Most importantly, would these servos be good enough to handle a camera that weights 250-300g ?

Standard hobby servos should be able to do that. Micro servos may not be able to. You can easily calculate if a servo can move something. Look up the stall torque. Torque is mass times distance. Now measure the weight of the camera and approximate the center of mass (usually around the center of the camera). Multiply the weight of the camera by the distance from the center of mass and the center of the servo horn. Convert your measurement to the units of the servo stall torque. Compare. If the torque caused by the camera about the servo shaft is less than the stall torque; the servo will be able to move that object.

For the Dagu example if your camera is 300g the farthest you can mount it from the servo horn is 5cm.

- Can the pan servo handle 360 degrees of rotation (it doesn't need to be continuous, i.e. moving 0 -> 360 -> 0 is fine) ?

No most servos are only 0 - 180 degrees. For 360 degrees you need more expensive Hitec ones.

It would require mechanical multiplication and would reduce resolution. The easiest way I can picture is two gears. A large one on the servo and one on the joint to be rotated with half the number of teeth the one on the servo has. One would hope the servo can manage a touch over 180 degrees so you could be sure to get the full 360 degrees rotation.