Let's Make Robots!

Servo Motors; Difference between a $5 motor and a $150 motor

Hi, it's me again. 

I recently tried making a robot with DC motors with bad results because the speed of the motors were different. I want to buy servo motors instead of DC motors, but I'm also confused about which one to buy. There is one that is $5, and one over $100. What's the difference between them? Does it use different commands? 

BTW, I am using Arduino.

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Instead of buying servos, you should try to work with the motors you already have. Bird also suggests the way it can be done.
However, for your knowledge, the difference between a $5 and $100 servo will usually be in the current it draws and the torque it provides. Torque is usually given by the kg/cm reading which isn't the torque it provides but will provide the torque value if you multiply it with acceleration due to gravity (torque = force/distance roughly and force = mass in kg *acceleration against which we do work usually gravity). Also a $100 servo will draw more current usually. For a general idea, look here- My tiny servo wiki
The differences that bird mentions are due to manufacturing. No two motors are same (you know that by experience you've had with your motors. Also, no two gear sets will be exactly same. Then there is a lot of difference which can be on the circuitry. As for which servos to buy, it depends on the job to be done and load you want to lift. Having a servo with higher torque will be preferable but you can, depending on your job, go for cheaper less torque servos as well.
When it comes to choosing between normal and continuous rotation servos, you should choose continuous ones only of you are trying to use it instead of motors for wheels. Even if you buy normal servos, later on you can modify them to become continuous rotation servos (search lmr for it). Continuous rotation servos are actually factory modified version of the normal servos.

Generally the higher priced servos are as you said high torque versions but the overall build quality is better because they are made for high end stuff (which doesn't mean their indestructible!). Also the higher priced ones will have metal gears insted of plastic to provide more torque and usually have brushless motors in them because they generally have more power and are less susceptible to wearing out.

why don't you learn to work with what you have?

Changing the speeds of the motors would be as easy as introducing a PWM signal on the ENABLE lines of the motor drivers.

If you are dead set on using servos, don't forget that they too are simply gearhead motors with extra circuitry to make them controlled by a single wire. That means that they will also likely have different rotational speeds due to manufacturing differences. I would not suggest spending $100 on a servo that will either be converted to continuous rotation or even be continuous rotation to start with. $5, $10, or even, $15 would be about as much as I would spend on something that I was just learning to use.