How to Select Cables for Servo ApplicationsDerek Lee, Motion Product Engineer, Yaskawa America, Inc.

Whether buying pre-assembled cables or building your own, be sure to meet servo system manufacturer specifications.

When machine and robot builder OEMs use servo systems, they have to carefully consider the cabling that connects the servo motor to the servo amplifier. This article covers critical design points in the cable selection process, allowing OEMs to pick the right cable for each application.

Choosing Connectors

Whether an OEM chooses a preassembled cable or designs one from scratch, the connector is a good starting point. Servo system manufacturers typically offer two different exit types (Figure 1). One type is a straight connector, which makes a perpendicular exit out of the side of the motor. The other kind is a right-angle connector, making a perpendicular exit out of the side of the motor, and then bending 90 degrees to run parallel to the motor.

Figure 1: Two connector exit types: Straight and Right-Angle

The most important factors to consider when choosing between these two connector types are the dimensions of the machine or robot where the motor is to be installed (so the connector will not interfere with other parts of the machine), and the routing of the cable that will be coming out of the connector.

For straight connectors, there is only one way to route the cable — perpendicular to the motor. For right-angle connectors, there can be 4 directions to choose from, so the location of any cable track or special required routing should be considered.

The connectors should be of the recommended type from the manufacturer of the servo system, and should be selected to conform to published connection ratings such as ingress protection (IP).

Selecting Cables

The two most important features to decide upon for cable selection are flexibility and shielding.

Servo systems by nature allow precise repeatable motion, and as such are often used in applications requiring cables that can be repeatedly flexed over millions of cycles, such as robotic arms (Figure 2). For this reason, the flex rating of a flexible raw cable used for servo system applications should be in the millions of cycles.

Figure 2: Flexing applications such as robotic arms demand millions of repeatable motion on the cable.

Regarding shielded cables, the two typical applications that benefit most from shielding are those that generate a lot of noise (such as arc-welding), and those that cannot afford noise (such as shape-cutting). For these applications, a shield on all cables (power, encoder, brake), including digital communication cables, may be recommended according to the servo system manufacturer's electromagnetic compatibility installation guidelines.

The purpose of a shield on cables that carry power is to prevent noise from being emitted from the cable; the purpose of a shield on cables that carry signals is to prevent noise from entering the cable. Servo system manufacturers will typically offer shielded cables for the power, encoder, and brake.

Because of the wide variety of applications that servo systems can be used in, preassembled cables offered by servo system manufacturers may combine the features of the shield and flexibility into one cable selection.

Yaskawa has done this in its line of Premium cables, where the cable has both high-flex and a shield (Figure 3). Yaskawa also offers a line of value cables, where the cable is a fixed installation low-flex type without a shield.

Figure 3: Cable with a braided shield vs. cable without any shield

If a servo system manufacturer offers a preassembled cable, it will typically stand behind the integrity of the entire servo installation if any issues should arise.

Building Cables

Some OEMs prefer to build their own cables, either because they can't find a cable of the correct length from the servo system manufacturer, or because they have certain design criteria of their own. Typically, custom cables are built from connectors and raw cable.

Some servo system manufacturers, like Yaskawa, can provide either a finished cable or parts for customers preferring to build their own cables. For OEMs who wish to roll their own, Yaskawa and other manufacturers can provide the raw cable, and part numbers or kits for the connectors.

If designing cables from scratch, not only do the flex and shield need to be considered, but also conformance to the servo system manufacturer's specifications.

If the cable is for carrying signals — such as the encoder or digital communications — the servo system manufacturer may provide suitable raw cable. If an OEM would like to purchase raw cable from a third party manufacturer, the servo system manufacturer may be able to provide specifications to which the raw cable must conform.

Signal cables can be sensitive and may not perform properly when using non-conforming cables, so care must be taken. On the other hand, power cables are much less sensitive and can generally be specified using basic design parameters.

Depending on the installation requirements for the cable, the common characteristics to consider for choosing an off-the shelf raw cable are:

Conformity to standards (UL, CE, RoHS) Resistance ratings (oil, flame, mechanical, UV, temperature) Flexibility rating (rolling flex, torsional flex, back-and-forth flex, number of cycles) Shield coverage (braided shields are typically 85 to 90% coverage) Electrical characteristics (max operating voltage, rated current carrying capacity) Physical characteristics (outer diameter, jacket color, stiffness of the cable – i.e., PUR tends to be stiff where PVC tends to be limp) Purchase details (minimum purchase quantity, price breaks at specific quantities)

Whether choosing preassembled cables from the servo system manufacturer or choosing to design the cables from scratch, the design points above are important to consider, and will help in determining final cable selection.