Military and civilian aerospace products feature highly sophisticated electronic systems that are completely reliant on the robust transmission of electrical power and signals. To make the engineering challenge tougher, many of these systems service components that need to move relative to each other.

There are both common and high-tech instances of this challenge. Some examples would be transmitting signal and power between stator and rotor electro-optic sensors for a weapon system. Pan-tilt and mechanisms for video cameras or other sensors are another. On spacecraft, antenna pointing mechanisms, elevation and azimuth gimbals and other sensitive equipment must reliably transmit power and data in the harsh environments of space. Cristek has participated in the development of many major complex systems in various environments.

The gimbal must rotate in multiple axes and yet system- and subsystem-engineers must design the unit to ensure reliable transmission of electrical power and signals across these moving interfaces. And to make things even more challenging, military and aerospace applications must operate reliably even when subject to high temperatures, harsh vibration, noxious fumes and the vacuum of space.

There are several technical solutions for reliable power and signal transmission across a rotation interface. For example, if the interface is the continuously rotating azimuth axis of an electro-optic gimbal, then only a slip ring will do; but if defined maximum angle rotation is needed, other options present themselves. Examples of limited angle rotation applications are the elevation, pitch or yaw axes of a missile’s infrared (IR) sensor or a security camera that needs to perform a 360˚ plus rotation before moving back to a central point. For these applications, cable wraps and twist capsules come into their own.

The engineer’s final component choice hinges on more than just the appropriate technology. Reliability is paramount when failure of the rotation interface component can lead to a breakdown of a very high-value system. And the project engineer needs to be confident that the product supplier not only offers timely and reliable delivery, but also intimately understands the intended application. With a complete systems engineering approach, the supplier should be able to facilitate a comprehensive solution that considers application parameters, manufacturability, testing and continuous improvement.

Simply put, for high-end aerospace applications, the right supplier makes the difference between a successful end-product and a commercial dead end.

This article examines the challenges faced by engineers designing signal transmission across rotation interfaces and describes the importance of not only choosing the right technology but partnering with the right supplier.

Rotating joint interface solutions

While many technical parameters influence the choice of technology for a rotating joint interface, the primary design choice comes down to the degree of rotation required. For example, the continuously rotating azimuth axis of an electro-optic gimbal demands a different mechanical solution to the limited-rotation elevation, pitch or yaw axes of a missile’s IR sensor.

For continuous rotation, the only solution is the slip ring. While there are several alternative mechanisms, a basic slip ring comprises a discrete noble metal ring and corresponding sliding wiper (or brushes) for each channel — with the rings for each channel separated by an insulator — that conduct the power and signals from the moving rings to the stationary conductor. It is a robust and proven solution, but because the connection is not permanent, slip rings struggle to cope with signals in the gigahertz (GHz) frequency range. Other downsides are loss of signal integrity, large size (particularly for an interface with many channels), complexity, modes of failure, lifetime requirements and cost.

For limited angle rotation (typically between 365° and 720°) there are two proven options: cable wraps or twist capsules. Because these devices are based on fixed mechanical connectors, they support high-integrity, high-frequency, continuous power and signal transfer with minimal losses and low interference. Cable wraps or twist capsules look similar on the outside, but internally the cable wrap uses a bundle of cables to carry the power and data, while the twist capsule uses a flexible printed circuit tape (flex cable).

The design of a cable wrap solution should ensure that there is sufficient length of cable for the device to freely rotate to the maximum required angle before returning to the start point. The advantages of a cable wrap are that it allows the use of composite conductors (for example, twisted pairs, coax and shields), it is relatively inexpensive and supply lead times are relatively short. The downsides of the solution are a relatively high and inconsistent drive torque requirement, poor torque repeatability, and limited cycle lifetime due to cable flexing.

The twist capsule replaces a cable bundle with a flex cable to carry the system’s data and power. While other types are available (for example, a rolling twist capsule, which uses a U-shaped flex cable fixed to facing surfaces), a typical arrangement sees the flex cable wrapped around the capsule’s rotational axis and shaped like a clock spring (Figure 1). The device operates by winding and unwinding the flex cable wrapped around the central shaft. The shaft is typically supported by ball bearings (although bearing-free assemblies are available).

Compared with cable wraps, the use of flex cables in a twist capsule brings several advantages to the rotating interface, including:

• Compact construction and low weight.
• High flexibility and minimum torsional counterforces.
• Defined stiffness for specified movement conditions.
• Individual conductor geometry for impedance control, radiofrequency integrity and current load capability.
• High reliability and contact stability even at operational temperatures up to 150° C.

Rotating the axis relative to the outer housing of the twist capsule allows the end-product to rotate while “winding up” the clock spring. The minimum bend radius of the clock spring is large enough to keep the material stresses of repetitive bending low. As the interface rotates, the flex cable clock spring produces a small counter torque that increases linearly with rotation. For a well-made twist capsule, this torque is low enough and consistent enough that it causes no system-level challenges. Most designs incorporate symmetrical circuit tapes with identical torque values for clockwise and anti-clockwise rotation. Moreover, the reproducible and dependable torque characteristics of the twist capsule enable improved servo control on the driven axis, which is particularly important for precision pointing applications.

During twist capsule manufacture, care is taken to size the flex cable lengths to prevent the system from binding up at either end of travel as well as allow for shrinkage in the cable lengths due to decreases in temperature. Combined with the material properties of the flex cable, specified minimum bend radius and low torque operation, these precautions are the key to system longevity and ensure that the twist capsule can meet lifetime performance attributes required from the system without performance degradation

Table 1 summarizes the characteristics of slip rings, twist capsules and cable wraps.

Table 1. Comparison between custom application slip rings, twist capsules and cable wraps. Source: Hermetic Solutions Group

Choosing the right solution and supplier

Once the project engineer decides that a twist capsule is the best component for his or her limited angle rotation interface application, there are several other factors to consider when narrowing down the choice to a specific solution from a specific supplier. These factors include:

• How much current and voltage needs to be transferred across the interface?
• What types of signals need to be transmitted (for example, analog/digital, electrical/optical)?
• What is the maximum transfer frequency?
• What level of signal integrity does the application demand?
• How much space is there in the system for the twist capsule?
• What is the project budget?
• What is the torque profile of the application (for example, maximum torque, requirement for constant torque, maximum hysteresis torque)?
• What is the expected lifecycle?
• To what specifications and standards is the solution qualified?

While these engineering specifications must all be satisfied by the chosen solution, there is one more important consideration. Although twist capsules account for only a small part of the cost of the system into which they are integrated, they form a vital link. For example, an electro-optic gimbal sub-system for an aircraft might cost hundreds of thousands of dollars and in turn will form part of a system that costs millions of dollars. In contrast, the twist capsule will form just a small fraction of the overall system cost — yet its failure would put the entire multi-million-dollar system out of service.

This is why experienced project engineers look to field-proven suppliers to ensure the quality and reliability of the twist capsule. One such company is Anaheim, California-based Cristek. Founded in 1985, the company combines proven interconnect design, flex cable expertise, long-term manufacturing experience and superior customer service. But more than that, the company’s competitive advantages include its high level of engineering support, dynamic responsiveness to customer needs, connector expertise and space application qualification.

The company’s position as a leading manufacturer of specialty interconnect products and assemblies was strengthened by its acquisition by Hermetic Solutions Group in December 2020. Hermetic is a global provider of highly engineered performance and protection solutions for mission-critical microelectronics and has served the defense/aerospace industry for over 35 years.

A respected twist capsule manufacturer

Cristek has been designing and manufacturing twist capsules since 2008. The company offers a selection of commercial grade twist capsules based on modular designs that provide a wide range of options at reduced cost and with short lead times. The twist capsule designs use the company’s own military-specification connectors and flex circuits — many of these components are qualified for use in space programs — and each has passed key qualification tests. By ensuring its twist capsules are pre-qualified, Cristek frees the customer from the burden of expensive and time-consuming testing.

Cristek’s service extends beyond supplying a qualified and reliable product. For example, the company dedicates an engineering team to each customer’s project to not only guarantee the best solution but to also make sure that the solution is integrated into the sub-system and system without compromise. This service readily extends to customizing the solution design and modifying the system to ensure it works perfectly with the twist capsule.

Examples of the company’s focus on customer satisfaction include in-house precision cleaning and shipping fixture supply such that one client can now just remove the twist capsule from the fixture and fit straight into the system. Another example of Cristek’s customer service was replacing a soldered joint with a mechanical one such that if rework of the twist capsule was ever needed, it would be much easier and less expensive to achieve.

Beyond customer service, the company understands that solution reliability is imperative. To that end it employs an in-house testing regime that tests during development, tests again during qualification and tests for a final time before the unit leaves the factory. As a result of this intensive test regime, Cristek is yet to have a twist capsule field failure.

Finally, Cristek prides itself on prompt and accurate delivery of twist capsules. It has never held up a customer’s production line and in some cases has even enhanced customer productivity through regular on-time delivery.

Mission accomplished

While cable wraps suit some low-cost limited angle rotation interfaces, for high-reliability, long-life applications twist capsules are the only solution. The use of flex cables inside the twist capsule enables the solution to offer compact size, tight control over electrical characteristics, high-temperature and high-frequency capability, and low, consistent torque characteristics.

But the reliability of the product can only be guaranteed by working with the right partner; a company that has been in the business for many years, uses high-end components — especially the flex cables and connectors at the heart of the twist capsule — customizes the solution to perfectly suit the system, pre-qualifies the product and tests, tests, tests. That partner is Cristek. Visit their website for more information.