DC Servomotors Selection GuideDC Servomotors Selection GuideDC Servomotors Selection Guide

Image Credit: Torque Systems | Moog Components Group | SDP/SI

 

DC servomotors are DC motors that incorporate encoders and are used with controllers for providing feedback and closed-loop control. Specifically, servomotors provide precise motion and position control, accommodating complex motion patterns and profiles more readily than other types of motors. Often servomotors have better bearings or higher tolerance designs than traditional DC motors. Some designs also use higher voltages in order to achieve greater torque. DC servomotors are commonly used in robotics, automation, CNC machinery, and other applications requiring versatility and a high level of precision. Low performance servomotors are also used by hobbyists, but these devices typically have a simple controller built-in (whereas in industrial servomotors, the controller is separate).

  

Servomotor vs. Stepper Motor

Stepper motors and servomotors both belong to a class of motors designed for precise position control. Stepper motors are brushless DC motors that move a rotor in small angular steps or fractions of steps, while providing fine motion control like servomotors. The advantage of servomotors over stepper motors is the incorporation of constant positional feedback, which results in higher torque at higher speeds. Servomotors also draw less current than stepper motors, resulting in higher efficiency and lower heat production. However, stepper motor systems are simpler and typically less expensive than servomotor systems, especially for smaller-sized applications. Because of this, stepper motors are often preferred for cost-conscious, low to medium speed control applications; servomotors are the best option for high-speed and high-volume requirements.

 

Types of DC Servomotors

The distinguishing factor of a DC servomotor amongst all DC motors is the additional encoder and controller components. The motor itself can be any type of DC motor. 

 

Construction

DC motors types vary mainly by construction, and include:

  • Permanent magnet
  • Shunt wound
  • Disc armature
  • Coreless or slotless

For an overview of these different types of DC motors and their performance, visit the DC Motors Selection Guide on GlobalSpec.

 

Commutation

DC motors can also differ based on commutation, the means by which current direction is reversed to allow rotors to rotate continuously.

  • Brushed DC motors use metal or graphite contacts called brushes which reverse current direction via direct contact with the rotor commutator. Brushed motors are cheaper and simpler, but require maintenance and replacement of the brushes over time.
  • Brushless DC motors rely on internal noncontact sensing devices and external electronics to control motor commutation. Brushless DC motors are quieter, more reliable, and maintenance free, but are more expensive than brushed motors due to the electronics used.

Gearing

DC motors can also incorporate gearing to adjust speed and torque output and reduce design complexity. There are a number of different gear assemblies that can be used:

  • Spur
  • Planetary
  • Worm
  • Harmonic

Specific advantages that gearmotors offer to a servo application include:

  • Operation of the motor over its optimum speed range
  • Minimizing motor size by multiplying torque
  • Minimizing reflected inertia for maximum acceleration
  • Providing maximum torsional stiffness

A more complete overview of gearhead selection can be found in the Gearmotors Selection Guide on GlobalSpec.

 

Specifications

 

Performance Specifications

DC servomotors share many performance specifications that are applicable to all types of DC motors. To properly size a motor, these specifications must be matched according to the load requirements of the application.

  • Shaft speed (RPM) defines the speed at which the shaft rotates, expressed in rotations per minute (RPM). Typically, the speed provided by the manufacturer is the no-load speed of the output shaft, or the speed at which the motor's output torque is zero.
  • Terminal voltage refers to the design voltage of the DC motor. Essentially the voltage determines the speed of a DC motor, and speed is controlled by raising or lowering the voltage supplied to the motor.
  • Torque is the rotational force generated by the motor shaft. The torque required for the motor is determined by the speed-torque characteristics of the various loads experienced in the target application.
    • Starting torque - The torque required when starting the motor, which is typically higher than the continuous torque.
    • Continuous torque - The output torque capability of the motor under constant running conditions.

Selection Tip: Servomotors should have enough speed and torque capability for the application, with a 20-30% margin between the load requirements and motor ratings to ensure reliability. Exceeding these margins by too much reduces cost effectiveness.

 

More information on these specifications and how to determine them can be found on the How to Select DC Motors page on GlobalSpec.

 

Inertia Ratio

Inertia defines how difficult it is to change the rotating velocity of an object (in the case of the motor, the motor shaft). Inertia increases with increasing mass. Servomotors are thus designed lightweight in order to be able to stop, start, and reverse direction very fast. The inertia ratio, another specification important to sizing servo motor systems, is defined as the proportion of load inertia (the inertia of the system load reflected in the motor shaft) to motor inertia (the inertia of the motor shaft itself).

 

There are a lot of variables which affect inertia matching (balancing the inertia ratio), and there is no rule of thumb for an ideal. All other factors being equal, a lower inertia ratio correlates to better performance. However, an excessively low inertia ratio can indicate an oversized, expensive, and energy intensive motor with little performance increase. If inertia ratio is the limiting sizing factor, it's important to fully understand the application's performance requirements before ruling out a higher ratio.

 

Feedback Devices

Servomotors can incorporate either encoders or resolvers as feedback devices to sense angular shaft speed, direction, and position.

 

Encoders

Encoders are low weight devices, meaning they contribute significantly less revolving inertia. This makes them more suitable for high acceleration/deceleration applications. They are also immune to electrical noise. Two types of encoders include incremental encoders and absolute encoders.

  • Incremental encoders are the most prevalent devices used on servomotors today, offering a standard interface that can be used on any servo drive. They are typically the lowest cost and most versatile, and are used when retaining absolute position is not necessary. They may be designed as optical encoders which use a light as the sensing/measuring mechanism or magnetic encoders which use a magnetized rotor and magnetoresistive sensor for feedback generation. Magnetic encoders are more environmentally durable than optical encoders.
  • Absolute encoders retain position information even when powered off. They also have no need for homing sensors or "homing" the axis when powered on. They are also less prone to the effects of EMI. Absolute encoders may be designed as single-turn or multi-turn. Single-turn encoders are only capable of retaining position within one revolution, while multi-turn encoders can keep track of the number of revolutions.

Resolvers

Resolvers are rotating transformers. They are much more rugged than encoders and are made of tough materials that can withstand harsh environments and high temperature over time. Resolvers are also less sensitive than encoders to shock and vibration.

 

 

Resolver

Encoder

Angle measurement

Absolute

Absolute or incremental

Absolute resolution

16 bits

13 bits

Incremental resolution

N/A

10,000 lines/revolution

Accuracy (arc minutes)

4 to 40

.25 to 6

Electronic interface

R/D converter

Direct

Noise immunity

Sensitive

Best

Output signal

Analog

Analog or digital

Construction materials

Robust

Fragile

Weight

Heavy

Lighter

Inertia

High

Low

Longevity

Very High

High

Shock/vibration

Rugged

Limited

Temperature

+150°C

+100°C

Contamination

Immune

Vulnerable

Implementation

Complex

Simple

Interchangeability

Limited

Vast

Retrofit/upgrade

Fixed

Open

Cost

Higher

Less

Table Credit: Donald E. Barnett - optoresolver.com

 

Other feedback devices include tachometers which sense only rotational speed (rpm). Modern tachometers are actually rotary incremental encoders which indicate the position, speed, and direction of rotation.

 

Features

Servo motors often require modifications to meet the requirements of different applications, typically in the form of different options and features.

  • Environmental sealing provides protection from dust, water, and other forms of environmental contamination.
  • Fail-safe brakes, mechanical or electronic, are often built in to servomotors to prevent overrotation.
  • Fan cooling can be incorporated to provide temperature control and cooling for larger servomotors.
  • Shaft, shape, and mounting modifications are made in order to properly fit/attach the motor to the system.

Environment and Housing Factors

In certain applications it may be important to consider the environmental criteria and ratings of the motor.

  • Operating temperature(s) specifies the rated temperature or range of temperatures that the motor is designed to operate at. 
  • Shock rating is the maximum shock the motor can withstand, commonly expressed in g's as a multiple of the earth's standard acceleration due to gravity.
  • Vibration rating is the maximum vibration the motor can withstand and still meet operating specifications. Vibration is expressed in the same units as shock, based on acceleration.
  • Enclosure ratings are used to rate the adequacy of motor enclosures to protect from various environmental conditions.
    • Dust-proof motors protect against dust infiltration with features such as total enclosure and labyrinth seals for shafts.
    • Drip-proof motors contain ventilation openings to prevent drops of liquid at any angle up to 15 degrees from entering the motor.
    • Explosion proof motors have totally enclosed housings that are constructed to withstand internal explosion, and prevent the ignition of surrounding gases and vapors in the event of an explosion.
    • Totally enclosed motors have an enclosure that prevents the free exchange of air between the inside and the outside of the enclosure.
    • Water-proof - There are several degrees of waterproofing applicable to motors and they are reflected in the IP rating for the motor:
      • IPx1 - Protection against vertically falling drops of water (drip-proof).
      • IPx2 - Protection against direct sprays of water up to 15 degrees from vertical.
      • IPx3 - Protection against direct sprays of water up to 60 degrees from vertical.
      • IPx4 - Protection against water sprayed from all directions.
      • IPx5 - Protected against low pressure jets of water from all directions.
      • IPx6 - Protected against high pressure jets of water from all directions.
      • IPx7 - Protected against the effects of immersion up to 1 meter.
      • IPx8 - Protected against long periods of immersion under pressure.

Some motors may also be rated for use in special or extreme environments such as clean rooms, vacuum chambers, and cryogenic temperatures.

 

References

 

Feedback Options - Parker Hannifin Corporation

 

Selection of Servo Motors and Drives (pdf) - Drivetech, Inc.

 

Servo Motor Sizing Concepts - Design News

 

Stepper or Servo (pdf) - Techno Inc.

 

Stepper vs. Servo Motors - CNC Router Source

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