Most people these days talk about using servo motors with Arduino microcontrollers in their robots but there are plenty of high end youtube robot builders like James Bruton who are using stepper motors and rotary encoders to position their motors.
So here we will try to talk about your various motor options be they brushed or brushless motors, stepper motors, dc or ac motors and the various sizes and types of servo motors.
Youtuber Jeremy Fielding gives amazing clear advice on choosing a type of motor to power a go kart and basically says to forget about ac motors like something found in a refridgerator or washer and try instead to get a dc motor or something that will more closely match what you are trying to do. https://www.youtube.com/watch?v=SrPHQh-M3pM&t=581s
Jeremy mentioned that you could get a cheap ac motor from an appliance but then you would have to buy an expensive power inverter to convert the power to dc to run the robot.
He talks about increasing your rpm but lowering your torque and the current and how you may burn out your wires if you don’t understand what your power requirements are..
In the old days or maybe 15 years ago, rc(remote control) enthusiasts and electronic toy users had simple motors in these things.
Simple dc motors are different than servos in certain ways.
Basically a servo motor is much more controllable than a simple motor.
Some say that a servo has these 3 parts …dc motor, circuit board and a potentiometer. But technically a servo has 4 main parts…a DC motor, a gearing set, a control circuit and a position sensor whereas a simple motor doesn’t have that.
A motor goes around and around continuously but servos are generally made to go back and forth only 180 degrees.
Now there are servos that just rotate 180 degrees and servos that rotate the full 360 degrees and of course both can go back and forth.
One could get into a discussion of open loop or closed loop systems and encoders and you can see some of this discussion on the Rozum website where they make collaborative robot arms and other amazing things. https://rozum.com/encoder-servo-motor/
The two main types of servos are controllable and continous which basically means that the controllable ones are made so you can control its position up to 180 degrees but the continuous ones just go round and round or backwards at whatever speed you want.
Another way to categorize servos is to say that there are also 2 types of servos, linear and rotational servos.
A linear servo motor will usually have an arm or piston that goes in and out which could be used to move a robot’s leg or arm.
A rotational servo will spin around and could also be used to move a robots arm but in a different way.
In the hobby world there are three main sizes of servos, the micro size for about $5, the standard size for around $45 and the giant size for around $150.
Then you can get servos that are extremely big like a basketball size and they are more for industrial use costing hundreds of dollars and some vary in their shape with some being lower profile to fit your arm or leg mounting bracket better.
Now usually a physically larger servo will be able to generate more turning power for your robot or more torque.
So one way to rate your servo is with the kg/cm rating. With consumer level servos the small ones are rated as perhaps 3kg/cm or 6kg/cm or 12/cm.
These numbers basically refer to the amount of weight the servo on an arm could lift on an arm that stretches out about a cm(centimeter).
Imagine how much harder it is to pick up a heavy weight if you must extend your whole arm out first and then lift it as opposed to having the same object right next to you and you have your arm bent.
On the Savoxusa.com servo site they mention what their most powerful servo is and it is the Power HD1218th with a coreless motor, metal gears and 2 ball bearings on the output shaft.
One of the builders at the DIY robot site inmoov.fr uses the Hitek 8A5BB servo for its arms.
James Bruton uses several different types of servos and stepper motors and several other types of motors. He has used a larger servo or a Jumbo servo called the Hitech HS 805 BB+ for his Robotx robot arm and his Ultron’s head. Hear his video here. https://www.youtube.com/watch?v=lo5Ldd5gcBw&t=279s
In this Youtube video James Bruton is using the Dynamixel smart servos by the Robotis company for a project. https://www.youtube.com/watch?v=tIob2_Rze80
Here in this ultra realistic moving head Youtube video by Waldo Mason you can see them using the Hitech Digital Servo HS 5125MG servos. https://www.waldomason.com/video
Now there are dc motors and ac motors and their are servos for DC power and servos for AC power.
You can quickly judge the quality of a servo by its manuacturer but then you may also check the quality by the build quality and its internals by the servo size and the spline count like C1.
The more expensive quality servos or motors will have metal gears as opposed to plastic and may even have more metal on the outside. Better servos also have bearings inside.
Some servos have attachment holes in different spots that may be more suited for the brackets you will be using for your robot. There are also low profile servos which are narrower or smaller and will fit in tighter spaces.
There are even double shaft servos which means that there are two spinning shafts on the front and back or each side of the servo.
Of course one could stick to purchasing well known servo brand names like Futaba and Hitech but there are so many new players in the market that it can be hard to choose especially if you are buying handfuls and you are on a budget.
Now remember that motors do not have gears built into them but servos do.
If the motor does not have extra gears attached to it then it will turn at a very fast speed and be unusable to whatever robot part you are attaching the motor to like a set of eyeballs or the robots arm and elbow.
So you add some gears to a servo which slows it down and increases its torque or its turning or lifting power.
The potentiometer has two signal lines….and sends out a pulse (PWM). The width of the pulse determines the angle or position of the servo.
A dc motor can act like a servo by adding a motor controller or a rotational encoder which allows the motors positions to be adjusted and worked with.
So we will explain it better later but on servos, the pulses are timed and there are three measurements that are monitored. A pulse duration of 1 milisecond indicates a value of zero and then there is a 1.5 and a 2.0 milisecond duration of a pulse which would be the full 180 degree of movement of the servo.
Now the practical use of a servo, is that the PWM (pulse width modulation) allows your software and in some cases the Arduino board to move that motor arm a little bit back and forth to allow some robotic device like an arm or finger or eyelid to move up and down or side to side.
There can be lower power DC servo motors and higher power AC servo motors.
There are about 5 specifications of a servo that you may need to know.
The better servos would have metal gears and maybe also metal casings instead of plastic and they would have a higher torque.
The cheap plastic gear servos that are used in experiments that don’t require much torque sell on sites like WISH for $4 each wherease a quality metal gear servo could cost $15 and upwards of $200.
Industrial servos will of course be made out of a quality metal or some very durable material and will have the appropriate amount of grease on it and may have bearings and it will be housed properly so air flow can cool it down so it won’t catch fire.
Here is a listing of some name brands of by hobby servos: Futaba, Hitek, Traxxas, Tactic, Spektrum
The name Futaba is very well known by anyone who has ever looked at the inside of a remote control car, boat or plane.
There are brushless and brushed motors. The brushless ones are considered better and require less maintenance.
Here is a video on Element 14 Youtube site where they make a giant servo on the cheap. Its good to see his explanation of the parts that go into making a servo. https://www.youtube.com/watch?v=0nhxqqtfhoc
(under construction Aug 25, 2021)e