Robotic Wire and Cable Selection Tips

It would be nice to go online and simply purchase robotic wire or cable for your robotic application. The truth is, selecting the right wire for robotics isn't that easy. Robotic movements vary and have different levels of demand on the robotic wire. Movement includes repetitive flexing, torsion/twisting and bending at multiple points. Off the shelf wire and cable normally isn't manufactured to endure these types of robotic movement. Robotic wire and has to be manufactured based on the type of movement and use. Construction and material selection are key factors in preventing robot failure. Robotic cable designs begin with material selection. Tell your custom cable manufacturer all about your robotic application and include:

• The type of movement is required of the wire or cable, number of cycles.
• The environment which it will be operating in. Include temperature, contact with fluids or moisture and abrasion resistance.
• Other requirements such as, voltage, EMI, flammability and any special needs.

The more your custom cable manufacturer knows about your application the better your cable design will be.

Material choices depend on the robotic requirements and begin with the conductors. Conductors (except solid core) are made up of multiple strands of fine wire, the more strands in the construction the more flexible and resistant to flex damage the conductor will be. The use of high strand count alloy conductors is recommended for high flex cycles and torsion. Alloy materials are stronger than standard copper conductors and can withstand repetitive flexing and vibration. High strand count conductors are available in several types of configurations), your cable manufacturer can recommend the right one for you.

Insulation and jacket materials also play an important role in the Robotic wire or cable's performance. Selection will depend on the surrounding environment. Silicone insulation and jackets are ideal for chemical resistance, high temperatures and super flexible cable designs. Silicone is not as resistant to abrasion as other materials. Understanding the material properties will help narrow down the choices.

High Voltage Wire and Cable: What you need to know before you buy

High Voltage (HV) wire (also referred as leadwire) and cable are used in a variety of applications such as, ignition systems, instruments, AC and DC power transmission in just about every industry.

The wire and cable construction and material are determined by three main factors:

• Voltage- To determine the thickness of the insulation

• Current capacity- To determine the cross-section size of the conductor(s)

• Environment- Temperature, chemical, UV exposure and mechanical impact to determine the cable construction and material of the outer jacket.

High voltage wire has two factors which require special attention. The first is the type of insulation needed to prevent arcs to nearby components, wiring or people. The other is the insulation diameter necessary to reduce corona loss. Reduction of corona is important because a common high voltage problem for insulation is damage to the insulation caused by corona discharge resulting in pinholes. With all high voltage applications, it is important that the insulation of the cable not deteriorate due to the high voltage stress.

Popular insulation materials for hookup type wire are polyethylene, PTFE, FEP, PFA, and silicone. Insulation such as PTFE is less bulky than silicone and is preferred where smaller sizes are required. If a flexible wire is required, PTFE is not a good choice. Silicone insulation is a common choice for high voltage applications especially when co-extruded. Silicone insulated wire is an extremely flexible wire, even at extreme temperatures, excellent electrical properties and is resistant to most chemicals and moisture to name a few.



Why co-extrusion?

Corona resistant wire (HVCR) is typically constructed with a conductor surrounded by a semiconducting sheath, which is then surrounded by the insulation. The semiconducting sheath effectively increases the diameter of the wire, reducing the tendency for corona discharge. Co-extruded wire and cable are common for many high voltage applications for it's ability to effectively reduce corona. Co-extruded wire is the same construction with the semi-conductive layer and outer jacket are extruded simultaneously. The co-extrusion process prevents air and gaps between the conductor and insulating layers thus reducing if not eliminating corona.

Custom wire and cable manufacturers can assist you with material choices and cable construction based on your high voltage requirements. Having the right cable from the start will save time and money down the line.

Flexible cables or high flex?

Robotic and automation applications require specialized cables that can handle repetitive flexing and movement. Design engineers must consider using custom cables designed to withstand flexing to prevent product failure and down time down the road. Robotic wire applications don't always require extremely flexible wire and cable, withstanding hi-flex environment is usually the main concern.

Flexible cables or high flex? These two terms are often misunderstood. Flexible can mean the limpness of a wire or cable and is used by some to describe high flex. The term high flex wire and cable is usually used to describe repetitive flexing. Can you have both? Yes you may. Ultra flexible wire designs use high strand count conductors for higher flexibility as does high flex wire. High flex wire normally will use alloys for the conductor material while maintaining the high strand count. High strand count conductors are made up of many fine strands of wire, the higher the number of strands the more flexible and resistant to breaking the wire will be. Depending on the wire gauge, strand counts can be in the thousands. Typical off the shelf wire is available in solid, 7 or 19 strands. These stranding configurations are better suited for static type of applications. When a wire has fewer strands, the strands will fatigue and begin to break under repetitive bending, vibration and flexing situations. As the strands break, conductivity will be compromised and may result in damage to the wire insulation. Conductor stranding is available in a variety of configurations.

For best results for high flex robotic cables need to be designed with high strand count conductors preferably using an alloy for added strength. Insulation can vary depending on the application. When designing the insulation and jacket, the environment must be considered. A few considerations are:

• Chemical resistance
• UV resistance
• Abrasion resistance

For super flexible cables, silicone insulation and/or jacket is a great choice. Silicone is resistant to chemicals and one of the most flexible of material choices. The downside to silicone is abrasion resistance. Other flexible wire insulation and jacket materials choices include TPE, polyurethane and Calmont's Superflex which is a special flexible PVC to name a few. Cable construction and shielding options also play a role in flexible wire and cable. Consulting a custom cable manufacturer and discussing the application, environment and use of the robotic cables prior to product design is recommended. Making the right cable design choices from the start will prevent down time and product failure in the future.

Medical cables and UL certification.

There are times where medical cable designs requiring super flexible wire and the available UL styles don't match the design requirements. This happens often. UL sets the standards for different categories of wire and cable. The standards include wall thickness, material, voltage rating, flammability, type of conductor and other factors. What these standards do not take into account are the requirements for individual medical devices for ultra flexible wire or cable, high flex wire applications and sterilization techniques. Custom wire and cable manufacturers can manufacture the same construction to meet the UL specification while making improvements on performance for example using higher strand conductors, alloys for added flexibility and strength or insulation and jacket material improvements.

Recently, we received design request for a flexible flat medical wire for a two conductor cable with multiple lumen, the customer required a UL style for the wires but not the lumen. We often receive requests for medical cable designs requiring UL certified conductors but not all the components in the cable.

The reality is that UL certification for individual components becomes null and void when used in medical devices. If the device needs to be UL certified then the entire device needs certification not just the components individually. Custom cables can be manufactured to perform better than a UL cable. Some examples of improvements are:

• More flexible construction and materials
• Withstand sterilization and chemicals
• Hi-Flex and vibration resistant
• Conductor sizes outside the UL style

There are many UL styles for wire available and finding a close match to your needs is possible however, finding a close match is not the same as having a cable to meet or exceed your requirements. Wire can be designed to meet voltage, flame and other requirements and doesn't have to be UL certified to do so. When designing your wire and cable it is important to look at your individual product requirements and design the wire to meet those needs.

If your medical device is required to be UL certified, then design the wire and cable to meet your product's needs instead of designing your device around available UL styles. In the end the entire device will need to be certified.