# Strong non dc motor

Hi, i'm getting into robotics and was wondering if anyone knew what the strongest controllable servo is? that's compatible with arduino of course! I hope that it will be mildly strong because i'm trying to make an arm that can lift a few pounds as i'm very weak!

Hi, i'm getting into robotics and was wondering if anyone knew what the strongest controllable servo is? that's compatible with arduino of course! I hope that it will be mildly strong because i'm trying to make an arm that can lift a few pounds as i'm very weak!

Well you need to do some calcs: all the servo vendors quote the torque so it's easy to figure out if a particular servo will do the job.

(They normally quote a figure of torque as kg which is bullshit: they mean kg.cm actually, or if it says oz they mean oz.in; torque of course being the force x the arm.)

Hi, i’m getting into robotics and was wondering if anyone knew what the strongest controllable servo is?

People make large servos out of windshield wiper motors. The pix is of a future project to make a super strong servo.

You can get as strong as you want, or as strong as you can afford to pay.

The kilogram is not even a unit of force, let alone torque. Do you mean kgf-cm ? 40 kgf-cm is 4Nm approx on the surface of the Earth, and 0.8Nm on the moon!

Its worth learning to always work with SI units for things like torque as all the equations work without arbitrary conversion factors.

power = angular velocity x torque (watts, rad/s, Nm)

MarkT: The kilogram is not even a unit of force, let alone torque.

The unfortunate fact of the matter is though, that the bloody manufacturers quote torque as "kg".

The kilogram is not even a unit of force, let alone torque. Do you mean kgf-cm ?
40 kgf-cm is 4Nm approx on the surface of the Earth, and 0.8Nm on the moon!

Hmmm…, that sounds strange.

Quick intro to dimensional analysis. Physical quantities are related to mass, length and time (M, L, T).

For instance dimension(velocity) is L / T, dimension(acceleration) is L / T^2, dimension(mass) is M force = mass x acceleration so dimension(force) is M L / T^2

torque is force times off-axis distance, so dimension (torque) = M L^2 / T^2 but energy has the same dimensions as its force times distance.

So its better to realize that torque is acutally energy / rotation angle, if we add radians as a pseudo-dimension to represent this: dimension (torque) = M L^2 / T^2 / radian

So you can use units of joules/radian or newton-metres as SI units of torque, the former being actually more descriptive. Thus the relation

power = torque x angular velocity

In imperial units you have the problem that "pounds force" isn't physically meaningful without saying what the local gravitation field is - but informally people take it to mean that at the surface of the earth (which unfortunately varies from place to place a little, so its no good for precision work without actually defining a value for g).

Great thing about dimensional analysis is that you can figure out what to measure in an experiment, before you start. You can show that the period of a pendulum is independent of the mass and only the length matters. So if you do any pendulum experiments, you needn't measure the bob's mass.

Thoris,

Why NON DC? Do you require a closed loop system? There are some AC servos out there but they are pricey!!! Perhaps explain a little more on what you are doing with the arm. Speed, Length of arm, accuracy, actual mass that you are dealing with. I am sure you will get a good answer. one of the cheaper solutions are geared Stepper motors. There are steppers with encoders for a closed loop system. There is considerable support for steppers on this Forum.

Yes more detail please. For an arm I suggest you look at the human body model.

A powerful motor for the elbow would work but a servo driven screw (muscle) would be lighter and give more precise movement.

you might like to think about hydraulics as well

Thoris: Hi, i'm getting into robotics and was wondering if anyone knew what the strongest controllable servo is? that's compatible with arduino of course! I hope that it will be mildly strong because i'm trying to make an arm that can lift a few pounds as i'm very weak!

An arduino could control a super tankers rudder with the right shield.

Not trying to be facetious but a description of what you are trying to achieve would be useful as a large part of your project is likely to be mechanical.

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