This post is mainly contributed by the members of the electrical team of ARES robotics.

For designing a rover prototype, we have to select motors for applications like Robotic arm, wheels, etc.  This post aims to be a single guide containing all the information required for someone to choose an engine for an application. I’ll try not to cover motor constructional details, but rather their types, advantages, and application areas.

 

Type of DC electric motors used in robotic :

• Servo
• Stepper
• Brush-less DC
• Brushed DC
• Geared DC
• DC linear actuators

Servo motors

It is a self-contained electrical device that rotates parts of a machine with high efficiency and great precision. The output shaft of this motor can be moved to a particular angle. Following are the type servos:

• AC servo -:

  • They offer more torque per weight, efficiency, reliability, and reduced radio frequency noise.
  •  Applications of an AC motor mainly involve automation, robotics, CNC machinery, and other applications with a high level of precision and needful versatility.
  • Brushes are not within the motor, thus need less maintenance.
  • A feedback system controls and alerts the engine to the location of the rotor to initiate the sequence of current through the coils.
  • Feedback alignment is used within most AC servo motors to function properly with a controller or amplifier.
  • Inertia is lower on rotors
  • Sophisticated control system
    • Stator current magnitudes, frequencies, and phases require a coordinated control process.
    • Closed-loop communication system located with the controller/amplifier
    • Motor speed operates by a sinusoidal PWM, where the speed control is in the frequency of the PWM
  • Higher RPM with the ability to reach 6000 RPM on many servo motors

• DC servo-:

  

  •  DC servo motor generally has a separate DC source in the field of winding & armature winding.
  • Application areas include Air craft control systems, robotics, Machine tool, process control, etc.
  • Brushes are within the motor
    • Brushes wear and need replacement
    • Preventive maintenance recommended
  • Current moves through coils which sequences by the brushes and the commutator
  • Control is much simpler than AC
    • Field or current armature magnitudes are the only requirements for motor control
    • A duty-cycle-controlled PWM operates motor speed
  • Torque is controlled independently by a controlled flux, which allows the torque to maintain consistency while in operation.
  • Inertia may reach a higher level in some situations

• Positional rotation servo

  • It can turn only 180 degrees. Has barriers in gear to prevent motion beyond these limits.
  • The application includes RC car, toys, RC ships, etc.

• Continuous rotation servo

  • Can turn full 360 degrees in either direction.
  • they offer open-loop speed control instead of their usual closed-loop position control
  • They can be thought like normal dc motor but with position feedback.
  • A continuous rotation servo cannot be commanded to go to a specific position and stop; there is no way your controller can tell what position it is at any specific time.
  • (I don’t feel it should even be called a servo, it’s more of a variable speed bi-directional geared motor drive)

• Linear servo motor

  • The linear servo motor is also similar to the positional rotation servo motor is discussed above, but with an extra gear to alter the o/p from circular to back-and-forth.

Selection criteria of servo?

Does your application work on AC or DC, most robotics application works on DC? Does your application need a fast/medium output response? How much torque/power you want from the servo? Mostly, servos with quick response have very little torque. How many degrees of rotation is required, 150-180, or 360 degrees? How much accuracy in position is needed, allowable tolerance, etc? Any upper limit on Cost/Controller complexity?

Stepper motors

The stepper motor uses the theory of operation for magnets to make the motor shaft turn a precise distance when a pulse of electricity is provided. Every revolution of the stepper motor is divided into a discrete number of steps.

• The rotation angle of the motor is proportional to the input pulse.
• The motor has full torque at a standstill (if the windings are energized).
• Precise positioning and repeatability of movement since good stepper motors have an accuracy of 3 to 5% of a step and this error is non-cumulative from one step to the next.
• Excellent response to starting/stopping/reversing.
• Very reliable since there are no contact brushes in the motor. Therefore the life of the step motor is simply dependent on the life of the bearing.
• The stepper motor’s response to digital input pulses provides open-loop control, making the motor simpler and less costly to control.
• It is possible to achieve very low-speed synchronous rotation with a load that is directly coupled to the shaft.
• A wide range of rotational speeds can be realized as the speed is proportional to the frequency of the input pulses.
• Resonances can occur if not properly controlled.
• Not easy to operate at extremely high speeds.

Types of stepper motors on the basis of their working:

• variable reluctance

  • Rotor teeth are electromagnets with alternative coil windings. The rotation occurs when stator, and rotor coils are energized.

• permanent magnet

  • The rotor has no teeth, rather alternating south, and north poles. The rotation occurs when the stator coil is energized.
  • It has low resolution but better torque characteristics.
  • Usually, the most commonly used motor type.

• hybrid

  • It has a magnetized rotor teeth. Having teeth on the rotor increases and redirects magnetic flux flow from stator to rotor.
  • A bit more expensive than permanent magnetic type
  • Has better resolution + high torque.

Types of stepper on the basis of their step size:

• Full step

  • Single-phase – Only one rotor teeth coil is energized at a time.
  • Two-phase – Two adjacent rotor teeth coils are energized. Thus has better torque characteristic, but needs more current from the driver.
  • Full phase operation means if there are N number of rotor teeth, motor completes one revolution after N pulses, in other words, the motor resolution is 360/N

• Half step

  • It’s a combination of one phase and two-phases working. Thus, If there are N rotor teeth, the motor needs 2N pulses to complete a revolution, or resolution is 180/N.
  • Better resolution, but usually have less torque

• Micro step

  • Uses two 90-degree phases-shifted sine waves (produced via a DAC with M resolution) in adjacent rotor teeth, so that now each full step is broken down into M different steps. Thus, Now for completing one revolution, M*N pulses are needed! i.e resolution is 360/(M*N). Usually, M is 256 i.e., 8-bit DAC is used.
  • Used where very high resolution is needed, but again at the compensation of torque.

Type of different stepper configurations:

• Open-loop

  • No feedback is available. Thus, To keep track of rotor current position, a number of pulses needs to be counted.

• Closed-loop

  • Has additional circuitry for positional feedback. No need to count!!

Selection criteria of stepper motor?

How much accuracy do you want? If there is no specific need for accuracy, I’d say go for Permanent magnet, full step stepper motor. Then, if you want positional accuracy, better go for closed-loop configuration. It’s used widely for robotic arms(High torque + more resolution), electric cars (where more torque is needed, then speed).

Brushless-DC(BLDC) Motor

Coils are not located on the rotor. Instead, the rotor is a permanent magnet; the coils do not rotate but are instead fixed in place on the stator. Because the coils do not move, there is no need for brushes and a commutator. Typically three Hall sensors are used to detect the rotor position, and commutation is performed based on Hall sensor inputs.

Stepper motor is a type of BLDC motor with discrete steps. Here, I’m more concerned with its a continuous operation.

Advantages:

• Moderate torque, high speed, high durability, now operation noise (Best choice for drones, which needs high speed, low torque)

Disadvantages:

• Expensive + needs a specialized external controller

Brushed DC motors

It does not require a controller to reverse the polarity of the current in the rotor windings; rather it has mechanical split sleeves(commutator).

• Very cheap construction + no need of external controller, just a constant dc source.
• Ideal for extreme operating environments
• More operation noise, less efficient, less durability, low torque, moderate speed.
• Mainly used in toys and other less demanding applications.

Geared DC motors

An attempt to increase torque at the expense of speed involves the use of gear assembly with any standard DC motor. Speed can be reduced to theoretically any desired value, that too in a very economical way without any external closed-loop system.

• Due to gear assembly, it usually weighs more than other non-geared motors.
• Gear assembly sometimes can significantly decrease the life of the motor. If not handled properly, gear assembly might break.
• Proper oiling is needed for gears to operate smoothly.
• Attaching a very high load to the shaft might cause the gears to break down.

DC linear actuators

DC Linear actuator is an electrically operated device which helps push or pulls objects

ts. they usually have high power, good reliability and but no position control without external feedback closed loop.

 

 

 

 

Conclusion

There are three main factors other than budget while selecting motors: Speed, Torque, Positional accuracy. One usually has to figure out the balance between the above factors that satisfies the application.

References:

http://icrservices.com/2015/08/10/how-a-ac-servo-motor-differs-from-a-dc-servo-motor/
Servo Motor – Types and Working Principle

https://www.rs-online.com/designspark/stepper-motors-and-drives-what-is-full-step-half-step-and-microstepping
https://www.renesas.com/us/en/support/technical-resources/engineer-school/brushless-dc-motor-01-overview.html

Wikipedia,sparksfun, mouser

 

Tagged BLDC, motors, servo motor, stepper, TeamARES