How to select a Servomotor for your application

Here is a little background on what a servo motor is.
A servomotor is an electric drive with a feedback mechanism. The feedback loop allows the motor input current to be adjusted automatically to properly position the servo shaft when the sensors tell the controller that it’s time to move the shaft or that the motor is not performing as it should.


For example, say you were driving a car up a hill. You as the driver are the Controller. The speedometer is the sensor and the motor is (of course) the motor. You adjust the power to the motor based on the feedback you receive from the sensor, pushing harder on the gas pedal to maintain your speed up the hill.
Servomotors are available as either AC or DC. The main things that have to be considered when making a servomotor from any electric drive are:

• They must be able to operate at a wide range of speeds without overheating
• They have to be able to hold torque on a load at zero speed
• They have to be able to operate at low speeds for a long time

Servomotors are generally considered to have the following advantages:

• Wide range of speeds
• Hold torque at zero speed
• Operate at low speeds for long time

While servo motors can deliver excellent performance and high speed in a small size, the additional controls in the feedback mechanism make them cost more than stepper motors [insert link]. Another challenge you may face in selecting a servomotor is tuning the motor to ensure that it is performing optimally for your application.
When you are selecting your servomotor, you’ll need to consider:

• What shaft speed you’ll need. Manufacturers identify their shaft speeds as the no-load speed at the rated terminal voltage.
• The terminal voltage
• The continuous current you have available, which is the maximum rated current that can be supplied to the motor without overheating
• The continuous torque you will need in constant running conditions
• The continuous output power, which is the mechanical power your application requires.
• Of course, you’ll also need to consider your physical space in terms of shape, diameter and housing length

Even if you carefully specify your motor, the tuning stage may take you a while. You may have to purchase a few samples in different sizes and test them before placing a larger order. And working with the engineering teams at a motor manufacturer may be the right way to go if you have a specialized application.
There are lots of types of servomotors. Some of the most common types and their applications include:
- Permanent magnet and shunt wound motors which provide constant speed with varying load, so they are a good fit for machine tools, fans and blowers.
- Series wound motors provide high starting torque, so they are good for constant loads such as in heavy industrial applications.
- Compound wound motors provide a heavy starting torque and are typically used where adjustable speed is not required, such as in elevators and hoists.
That’s a high-level outline on selecting a servomotor for your application. For more details, watch some of the related videos in this section.

from : http://www.engineering.com

Newton’s Third Law: Action and Reaction

Having established that a force—the action of another body—was necessary to cause a body to change its state of motion, Newton made one further crucial observation: such forces always arise as a mutual interaction of two bodies, and the other body also feels the force, but in the opposite direction.

To every action there is always opposed an equal and opposite reaction: or the mutual actions of two bodies upon each other are always equal, and directed to contrary parts.

Newton goes on:

Whatever draws or presses another is as much drawn or pressed by that other. If you press a stone with your finger, the finger is also pressed by the stone. If a horse draws a stone tied to a rope, the horse (if I may so say) will be equally drawn back towards the stone: for the distended rope, by the same endeavour to relax or unbend itself, will draw the horse as much towards the stone, as it does the stone towards the horse, and will obstruct the progress of the one as much as it advances that of the other. If a body impinge upon another, and by its force change the motion of the other, that body also (because of the equality of the mutual pressure) will undergo an equal change, in its own motion, towards the contrary part. The changes made by these actions are equal, not in the velocities but in the motions of bodies; that is to say, if the bodies are not hindered by any other impediments. For, because the motions are equally changed, the changes of the velocities made towards contrary parts are reciprocally proportional to the bodies. This law takes place also in attractions.

The rocket's action is to push down on the ground with the force of its powerful engines, and the reaction is that the ground pushes the rocket upwards with an equal force. UP, UP, and AWAY!   All this maybe sounds kind of obvious. Anyone who’s had a dog on a leash, especially a big dog, is well aware that tension in a rope pulls both ways. If you push against a wall, the wall is pushing you back. If that’s difficult to visualize, imagine what would happen if the wall suddenly evaporated. Newton’s insight here, his realization that every acting force has a reacting force, and that acceleration of a body only occurs when an external force acts on it, was one of the big forward steps in our understanding of how the Universe works.

Newton’s Second Law 2: Acceleration of a Body is Proportional to Force

Newton’s next assertion, based on much experiment and observation, is that, for a given body, the acceleration produced is proportional to the strength of the external force, so doubling the external force will cause the body to pick up speed twice as fast.


The alteration of motion is ever proportional to the motive force impressed; and is made in the direction of the right line in which that force is impressed.

Mass and Weight
To return to the concept of mass, it is really just a measure of the amount of stuff. For a uniform material, such as water, or a uniform solid, the mass is the volume multiplied by the density—the density being defined as the mass of a unit of volume, so water, for example, has a density of one gram per cubic centimeter, or sixty-two pounds per cubic foot.
Hence, from Galileo’s discovery of the uniform acceleration of all falling bodies, we conclude that the weight of a body, which is the gravitational attraction it feels towards the earth, is directly proportional to its mass, the amount of stuff it’s made of.
 

The Unit of Force
All the statements above about force, mass and acceleration are statements about proportionality. We have said that for a body being accelerated by a force acting on it the acceleration is proportional to the (total) external force acting on the body, and, for a given force, inversely proportional to the mass of the body.
If we denote the force, mass and acceleration by F, m and a respectively (bearing in mind that really F and a are vectors pointing in the same direction) we could write this:
                                                               F is proportional to m.a
To make any progress in applying Newton’s Laws in a real situation, we need to choose some unit for measuring forces. We have already chosen units for mass (the kilogram) and acceleration (meters per second per second). The most natural way to define our unit of force is:
 
The unit of force is that force which causes a unit mass (one kilogram) to accelerate with unit acceleration (one meter per second per second).

This unit of force is named, appropriately, the newton.

If we now agree to measure forces in newtons, the statement of proportionality above can be written as a simple equation:

                                                  F = ma

FORCE = MASS times ACCELERATION

which is the usual statement of Newton’s Second Law.
If a mass is now observed to accelerate, it is a trivial matter to find the total force acting on it. The force will be in the direction of the acceleration, and its magnitude will be the product of the mass and acceleration, measured in newtons. For example, a 3 kilogram falling body, accelerating downwards at 10 meters per second per second, is being acted on by a force ma equal to 30 newtons, which is, of course, its weight.

This is an example of how Newton's Second Law works:

Mike's car, which weighs 1,000 kg, is out of gas. Mike is trying to push the car to a gas station, and he makes the car go 0.05 m/s/s. Using Newton's Second Law, you can compute how much force Mike is applying to the car.

Answer = 50 newtons

Newton’s First Law: no Force, no Change in Motion

Newton’s First Law: no Force, no Change in Motion

Law 1

Every body perseveres in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed thereon.

He immediately adds, tying this in precisely with Galileo’s work:

Projectiles persevere in their motions, so far as they are not retarded by the resistance of the air, or impelled downwards by the force of gravity.

Notice that here “persevere in their motions” must mean in steady speed straight line motions, because he is adding the gravitational acceleration on to this.

This is sometimes called “The Law of Inertia”: in the absence of an external force, a body in motion will continue to move at constant speed and direction, that is, at constant velocity.

So any acceleration, or change in speed (or direction of motion), of a body signals that it is being acted on by some force.

Let's study the "skater" to understand this a little better.

This law is the same reason why you should always wear your seatbelt.