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How to Specify Rotary Encoders

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How to Specify the Right Encoder Connector

Encoder connectors are available in various termination options, shapes and sizes but are a critical part of the overall encoder specification process. Given that industrial equipment is generally designed with reliability in mind, the encoder connector should also be selected based on the application.

The major factors to consider when selecting encoder connectors are:

  • Water Ingress, Dust and Other Environmental Conditions
  • Space Constraints
  • Overall Equipment Budget
  • Ease of Installation and Use
  • Commonality or Ease of Sourcing
How to Select IP Ratings Free White Paper: Understanding Encoder IP Ratings Download Now

Encoder Termination Types

Encoders are available with the following three types of cable terminations: pigtail, terminal strips, and connectors. The termination type, and, if necessary, the connector type should be specified when the encoder is ordered to ensure compatibility with the cable.

Pigtail with Leads

pigtail-with-leads-encoder-connectors

In an encoder with a pigtail termination, the cable is wired directly and permanently into the encoder at the factory. The other end will terminate in flying leads for connection to the PLC/controller/drive. Pigtails are typically available in sealed and unsealed versions. Sealed versions have a cord-grip-style locking mechanism over the PVC cabling, which creates a water-resistant seal. Pigtails are available as wires coming out of the side of the encoder (radial location) or end of the encoder (axial location). The user also specifies the length of the cable.

Pigtails are low cost and offer easy installation. The trade-off is that they can be problematic for field replacement. If the wire fails, the encoder needs to be removed as well. If the encoder needs to be sent for service, the wire needs to be disconnected and sent with it.

Terminal Strips

terminal-strip-example

Terminal strips are at the opposite end of the termination spectrum. A termination strip is a linear array of screw terminals in which each wire is manually screwed in place. Terminal strips are very versatile. They are economical and easily interchangeable with other screw-type terminals. On the downside, they are time-consuming to use and provide ample opportunities for error. They become extremely wide for large numbers of wires.

Encoder Connectors

Cables can be terminated at the encoder end with connectors designed for attachment and removal. The pin side of the connector is installed on the encoder at the factory. The socket is attached to the cable, either at the factory or by the user. Because connectors enable the cabling to be attached and disconnected at will, both cable and encoder can be removed independent of one another.

Encoder Connector Types

A wide range of connectors is available to address different application needs. The pin side of the connector is affixed to the encoder, while the socket side is attached to the mating cable. Although customers can supply their own connectors and solder them on to flying leads, the process requires a certain amount of skill. It may be faster and easier to purchase pre-assembled mating cables, which meet IP ratings and have already passed quality inspection.

Encoder wiring selection and best practices are covered in detail here

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MS Encoder Connectors

MS-encoder-connector

Specified by MIL-DTL-5015, MS connectors are round, threaded, heavy-duty connectors designed for high reliability. They are very finely threaded, which ensures a secure connection that won’t vibrate loose. On the downside, connecting the cable can be time-consuming precisely because of that threading. The fine threads also can be easily damaged if mis-threaded.

The specification calls out cramped and soldered connectors; only soldered connectors with aluminum alloy bodies should be used with encoders, since they provide better environmental protection. MS connectors are available in two different styles: the standard MS connector (IP67) and the NEMA4 MS connector (IP66 equivalent).

Another drawback of MS connectors is that they have only one keyway. The design doesn’t provide a method for differentiating cables from one another, making it possible to plug a cable into the wrong termination.

Environmental Protection High
Installation Soldered
Ease of use Fine threading extends connection time
Commonality Widely available
Best Used For Applications with harsh environments and/or-high reliability requirements

 

M-profile connectors are round, threaded connectors designated by the ASME B1.13M-2005 "Metric Screw Threads: M Profile" standard. Each class is defined by the outer thread diameter of the pin side of the connector. Within each class, the standard defines multiple thread pitches. Diameter is important, as it controls the number of conductors, the level of environmental protection and noise immunity, and the overall footprint.

Within M-profile connectors, the three most commonly used classes are the M12, the M17, and the M23.

M12 Connectors

m12-encoder-connector

The M12 connector is a round, threaded, quick-lock connector that provides good protection for industrial automation application. The maximum external thread diameter is approximately 12 mm. The M12 specification calls out multiple coding corresponding to a variety of communications protocols, including various fieldbuses and types of industrial Ethernet

Environmental Protection High
Installation Soldered
Ease of Use Fast and easy, multiple coding prevents errors
Commonality Widely available
Best Used For Wet environments, temperature extremes, absolute encoders

M17 Connectors

M17 connectors are round, threaded designs with very small footprints. The maximum external thread diameter is approximately 17 mm; thread pitch varies. M17 connectors are lightweight, fast locking, and water resistant.

Environmental Protection High
Installation Soldered
Ease of Use Fast locking
Commonality Common
Best Used For Harsh environments requiring watertight seals

M23 Connectors

M23-encoder-connector

M23 connectors are circular threaded connectors that can run both power and signal in the same cable. The maximum external thread diameter is approximately 23 mm; thread pitch varies. Designed for ease of use, M23 connectors are most commonly used in servo applications to link drives and encoders.

Environmental Protection High
Installation Soldered
Ease of Use Fast locking
Commonality Common
Best Used For Connecting electrical drives and servomotors in industrial automation applications

 

Latching Encoder Connectors

latching-encoder-connectorjpg

Designed for ease-of-use, latching connectors connect to the wire with screw terminals, enabling them to be installed in the field with just a screwdriver. They are very versatile and widely used in industrial automation. Latching connectors offer only limited ingress protection but can operate at very high temperatures. On the downside, latching connectors tend to be bulky.

Environmental Protection Moderate
Installation Screw terminal
Ease of Use Easy to install
Commonality Common
Best Used For Applications that need rapid, simple installation, high temperature applications

 

Bayonet Encoder Connectors

Bayonet connectors are aluminum connectors that also conform to MIL-DTL-5015. Bayonet-style connectors are keyed in multiple locations, protecting against cables being plugged into the wrong connector termination. Once the socket is matched to the keyway of the pins, a bayonet connector only requires quarter turn to lock. These terminators also provide strain relief.

Environmental Protection High
Installation Soldered
Ease of Use Fast, only requires one quarter turn after plugs are mated
Commonality Very common
Best Used For General industrial automation applications

 

Sub D Encoder Connectors

sub-d-encoder-connector

Sub D connectors are PCB style connectors that are mounted to the board directly. They are typically used in integrated feedback devices such as servo motors in which the encoder is integrated into a larger housing and the encoder itself does not need a high level of environmental protection.

Environmental Protection Low
Installation PCB Mounted
Ease of Use Difficult, as the connector itself requires precise PCB mounting. Field replacement requires access to inside the encoder housing
Commonality Common
Best Used For Applications requiring compact, integrated feedback such as servomotors

 

Additional Resources:

Learn more about encoder wiring best practices here

Discover how to specify rotary encoders for high shock and vibration environments

See the effects of temperature & moisture on encoders and how to design for these environments
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