While both linear and rotary servo motors use high-performance servo cables to transmit energy and provide closer feedback signals, the basic nature of the movements they employ makes different demands on the design, routing and durability of the cable. With linear or rotary servo motors, OEM engineers can avoid common specification errors that cause early wear, signal noise or expensive downtime in motion control systems by understanding the differences between linear motors and rotary servo motors.
This guide presents a comparison of the two applications from a cable point-of-view, highlighting the mechanical stresses, routing considerations, EMC factors and best practice for specifications that a typical machine builder will face in the real world.
Linear vs Rotary Servo Motors – Why Cable Requirements Change
Unlike mechanical servos that convert motion to other forms, such as ballscrews or belts, linear servo motors generate force straight along a track. The torque produced by rotary servos is usually transfered to linear motion via mechanical components. Although both cables are powered and both are feedback cables, they are quite different in terms of physical layout, travel distance, and dynamic stress.
Such differences directly affect cable flex life, tolerance to cable torsion, cable shielding and connector strategies, particularly for high duty cycle automation equipment.
Common Servo Cable Roles in Both Motor Types
Power and Feedback Functions Are Similar
In either system, the servo cables usually contain several pairs of conductors for low-voltage feedback signals, such as from an encoder or resolver, as well as a few power conductors to provide power to the motor. They coordinate or connect with specific feedback cables to provide precise position, speed and torque control. The overall layout of the electrical architecture is comparable for all types of motors.
Both Can Be Static or Dynamic
Either static or dynamic motion can occur in the cables for either type of motor. The selection always takes into account the duty cycle, bend radius and environmental exposure of the motor, not just the motor classification.
The differences in physical layout and routing.The differences in physical layout and routing.
Physical Layout and Routing Differences
Rotary Motors: Compact, Localized Cable Routing
In most cases, rotary servo motors are installed on brackets which have cables coming out from one connector. Usually the routing will have to include short bends close to the motor and some short travel in the axis following cable carriers. This configuration can be combined with other services and can be easily relieved.
Linear Motors: Long Tracks and Distributed Cable Paths
Linear servo motors have a moving primary (forcer) which moves along a fixed magnetic track. Cables need to carry long straight runs and are frequently required to be in close proximity to strong magnetic fields. During routing, it is important to pay close attention to the continuous support during the entire stroke to minimize sagging and excessive vibration exposure.
Mechanical Stress Differences on Servo Cables
Rotary Axes – Bend and Torsion Around a Compact Motor
This is because repeated bending is typically included in Rotary applications in the carrier whilst there is also some degree of localized torsion near the motor connector as the axis rotates or articulates. Twisting resistance: cables are designed to resist twisting without causing damage to shields or conductors.
Linear Axes – Long-Stroke Flexing and Vibration
Linear direct-drive systems provide high acceleration and long travels. Cables experience millions of bending cycles over long strokes, in addition to steady vibrations and possible exposure to strong magnets in iron core cable designs.
Selecting Flex-Rated Cables According to Motion Profile
Match cable construction to the dominant motion: continuous-flex for long-stroke linear axes or torsion-resistant designs with rotary motion focusing stress. The most important factors are travel length, travel speed and acceleration, rather than the type of motor.
Feedback and EMC Considerations in Linear vs Rotary Systems
Feedback Cables in Linear Motor Systems
The high-resolution feedback over longer distances is needed for long-stroke linear axes. Cables must be able to keep the signals intact in the presence of magnetic fields and mechanical noise. Good shielding and duly distancing from power lines are essential.
Feedback Cables in Rotary Servo Motors
Rotary encoders are frequently installed in a small motor casing, which increases the distance between the motor and the encoders (feedback runs). Although protection against EMI is still significant, most routing distances are more comfortable to deal with.
EMC Shielding and Routing Differences
Both applications require armoured servos to reduce electromagnetic interference. There may be additional considerations for linear installations to route through magnetic tracks and rotary installations to ensure compact separation between carriers and entry points into the cabinet. In either, good shield grounding practices (as recommended by the drive manufacturer) aid in maintaining performance.
Connector and Interface Differences
Rotary Servo Motor Connectors
Standardised round or rectangular connectors are provided on the motor housing for power, brake and feedback. Local flex and torsion must be allowed to occur at these interfaces where the cable assemblies are going to fit.
Linear Motor Cable Interfaces
Connectors can be connected to the moving forcer, junction boxes or distributed points on the track. Flexible saddles or intermediate support frequently are used in design for reliable long travel performance, with shield continuity.
Specification Differences OEM Engineers Should Capture
Motion and Routing Inputs
- The type of motor (linear or rotary)
- The length of travel, speed and acceleration
- Cable carrier, free hanging
- A bending radius and torsion exposure are required.A bend radius and torsion exposure is required.
- The vibrations and shocks are being measured.
Electrical and Feedback Needs
- The voltage and current required for the power source.Power supply requirements or supply voltage and current.
- The feedback protocol comprises the encoder, resolver and digital components.
- Protection level and EMC environment cover protection from the elements and EMC environment.
- The maximum length of cable for signals.
Environment and Protection
- Contact with coolants, oils, chips or chemicals.
- The ambient temperature and humidity range from
- Requires special jackets (PUR, TPE, etc.)
Practical Summary for OEM Cable Strategy
Use cable assemblies designed specifically for the rotary servo motor axes that have mechanical conversion for compact routing, localized bend/torsion and standard connectors. If using direct drive linear servo motors, consider cables with long stroke flex performance, strong EMC shielding, and mechanical support on the track.
If possible, use multi-purpose cable families that are suitable for both applications and make certain that there are clear differences in the specific cables used to ensure maintenance teams can operate without confusion. The right specification upfront, using real motion profiles, lowers life cycle costs and machine reliability.
Whether you are using a servo or motor cable in a rotary or linear application, in your robotics, CNC or automation system, at Hulk Electric we are engineers who customize cables to meet these specific requirements. Submit your axis parameters and we can help you choose the best or tailor the best solution for long term performance.