For robot wheel control : Brushless DC motor or Servo Motor?

Question

Simply, when to use brushless dc motor and when to use Servo Motor ?

what are the differences , specially when adding an encoder to the dc motor you can have the position and it will be similar to Servo Motor ?

Answer 1

Let's try and clear things up a little bit...

"RC Servo" motors

These can be found in radio controlled cars and airplanes to e.g. turn the wheel or actuate various controls in the airplane. They are pretty cheap brushed DC motors, with gears and some sort of crude feedback mechanism. They typically convert an input voltage into a position within a given range.

"Exploded Servo" by oomlout - Exploded Servo. Licensed under Creative Commons Attribution-Share Alike 2.0 via Wikimedia Commons

Brushless DC motors

Brushed motors rely on "brushes" for commutation to drive current through the rotating motor coils. In a brushless motor in contrast the coils are fixed and potentially some sort of feedback mechanism is used to control the current through those coils to rotate the motor. Brushless motors can also be run open loop (e.g. stepper motors). The DC is a bit of a misnomer as the motors themselves run on AC.

Brushless DC Servo aka "Servo Motor"

These would be typically found in applications such as industrial robots or CNC machines. They come in a very wide range of sizes and power. They are a combination of a BLDC motor with a rotary encoder, a hall effect sensor or both. They are meant to be used with a closed-loop controller. Pretty much any type of motor can be used in a closed loop servo but since those are more commonly used in that sort of application the term "servo motor" stuck.

"3417 09 Servogetriebe" by SEW-EURODRIVE GmbH & Co KG. Original uploader was Sewmecm at de.wikipedia - Transferred from de.wikipedia; transferred to Commons by User:Brackenheim using CommonsHelper.(Original text : SEW-EURODRIVE GmbH & Co KG). Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons

---

For driving a robot wheel you could consider:

• A DC brush motor with no rotor position feedback (cheapest).
• A DC brush motor with feedback and some sort of controller.
• A brushless DC motor with electronic speed control (such as the one you'd find in a quadcopter). These are quite different than the industrial BLDCs mentioned above and I won't go into the details. They can be had relatively cheaply due to the popularity of all these "toys" and are pretty high performance.
• A more industrial BLDC/encoder combination with a closed loop controller. Those would probably be the most expensive option.

Answer 2

It really depends on the economic details of the project you are working on. If it's a one time project with limited budget I would say a DC brush motor with or without position feedback would do the job. Sure you will need a controller. If you aim to get a consistently high mechanical work output, precision positioning, and compact size you might draw your attention to servo motors. Some of them already come with a gear and a controller integrated. If you still feel embarassed about making the right choice, you could find some hints in here.What you need to know about robot servos before purchase