Selecting Precision Motors for Smart Defense Applications

Today’s leading defense contractors rely on key manufacturers to provide critical control and actuation solutions to achieve superiority through the deployment of various smart munitions. Finding a precision motor solution that meets the demanding performance requirements of a guided missile or Electro-Optical/Infrared (EO/IR) imaging system cuts across several performance factors, including mechanical, electrical, and environmental. Each smart defense application has unique challenges to find the optimal precision motor solution. This white paper will focus on three specific applications and the distinctive requirements that influence motor selection.

SMART DEFENSE APPLICATION CHALLENGES

Smart defense applications refer to the use of advanced technologies and intelligent systems to enhance defense capabilities and improve military operations. Miniature motors play a crucial role in various smart defense applications.

Fin Control Actuation Systems

A missile fin control actuation system (CAS) precisely directs the position of control surfaces (fins, canards) based on the inputs from the missile guidance system to execute the flight trajectory of the missile. The motor requirements for a CAS typically include:

•	Environment - Missiles are subject to an extreme temperature profile and must withstand high levels of shock and vibration. Operating temperature requirements extend from a low of -55°C to and can exceed 100°C.
•	Mechanical - Unique and highly custom designs are required to meet the space and weight demands of the overall system.
•	Electromagnetic - High efficiency and dynamic response are critical in order to react to the constant flight path updates from the guidance system.
•	Quality - Precise control of the flight surfaces determines whether a missile will strike its intended target. The motor must be equipped to handle long storage periods (20+ years) and perform reliably when placed in service.

Wing Deployment Systems

A wing deployment system (WDS) is used to increase the tactical range of guided munitions like glide bombs. The WDS allows the wings to move from a compact orientation when stored to an extended position; this maximizes glide time and distance during flight. The motor requirements for wing deployment systems are:

•	Environment - Similar to the CAS, the motors are subject to an extreme temperature profile and must withstand high levels of shock and vibration. Operating temperature requirements extend from a low of -55°C and can exceed 85°C.
•	Electromagnetic - Successful deployment of the wings determines whether the missile will strike its intended target. As with the CAS application, the motor must be equipped to handle long storage periods (20+ years) and perform reliably when placed into service.
•	Quality - Precise control of the flight surfaces determines whether a missile will strike its intended target. The motor must be equipped to handle long storage periods (20+ years) and perform reliably when placed in service.

Electro-Optical/Infrared Systems (EO/IR)

EO/IR devices are imaging systems used in seeker heads that combine both visible and heat signature sensors to provide precise navigation and targeting regardless of light and weather conditions. They are used across air, land, and sea vehicles, as well as on guided munitions, to identify, acquire, and lock onto targets. The motor requirements for EO/IR devices usually involve:

•	Environment - The azimuth, elevation, and zoom axis motors must all work in concert to provide both smooth and highly responsive motion regardless of the environment.
•	Mechanical - The limited space and low weight requirements of a guided missile or bomb necessitate that the motor is as powerful and torque dense as possible.
•	Electromagnetic - Even if the overall duration of use is short, the limited onboard power budget requires a high-efficiency motor to minimize power draw from the vehicle power system.
•	Quality - The information derived from the imaging sensors is a crucial element to complete the desired mission. The motor must be robust and reliable enough to perform as needed, even after extended storage periods.

MOTOR TECHNOLOGIES FOR SMART DEFENSE APPLICATIONS

The demanding challenges of most Defense applications require robust and reliable designs that can handle extreme environments. Smart defense applications add additional performance and efficiency parameters that must be optimized to meet tight space requirements. A variety of DC motor technologies with different form factors are ideal candidates for smart defense applications, including Brush DC coreless, BLDC Cylindrical (slotted and slotless), and BLDC flat (slotted).

Brush DC Coreless

A brush DC coreless design consists of a rotor, made in a coil arrangement fixed to a shaft, and a stator with fixed magnets. Since the coil arrangement is not constructed from iron laminations, the motor is considered ironless. Ironless designs have significant advantages since typical iron core losses are eliminated. The motors are commutated using precious metal or graphite brushes that lower the contact resistance and friction, as well as simplify the control electronics.

BLDC Cylindrical

BLDC cylindrical motors utilize a stationary coil with a rotating permanent magnet. The coil windings, which are part of the stator, can be energized and electrically commutated. This eliminates the need for a commutator and brush system.

BLDC Slotted Flat

BLDC slotted flat designs also insert the coils into lamination slots; however, they differ from their cylindrical counterparts in that they utilize an outer rotor configuration.

MOTOR SELECTION CRITERIA FOR SMART DEFENSE APPLICATIONS

There are numerous tradeoffs between the different motor technologies based on the specific criteria of each smart defense application. Certain requirements may lean towards one technology or the other, while combinations of requirements may demand tradeoffs. Criteria prioritization will help develop the optimal motor selection. Key motion parameters to account for include:

•	Torque Density. DC motors provide very high torque in compact designs with the incorporation of high-energy magnets that generate high flux density coupled with a coreless coil configuration. Depending on the commutation type, a DC motor can provide up to 3-5 times the maximum continuous torque transiently. Both BLDC slotless and slotted motor technologies are designed to provide high torque and provide up to 10 times the maximum continuous torque without magnetic saturation. In comparison to cylindrical motors, brushless flat outer rotor motors can provide very high torque density with a lower form factor that is convenient for small space environments.
•	Friction Losses. No load or static friction losses are generated through the motor bearings and brush configuration in brushed DC motors. Brushed motors minimize friction with low-friction brush and commutator materials that provide smooth operation. Careful consideration of the motor bearings provides minimal losses in precision DC motors designed for smart defense applications.
•	Iron Losses. DC motors with iron cores are affected by eddy current losses at higher speeds, which add to motor losses. Ironless motors, on the other hand, have no iron losses and tend to be favored for higher-speed applications. Slotted and slotless BLDC motors with iron cores can be designed to minimize iron losses through magnetic design and strategic material selection.
•	Maximum Speed. Typical small DC motors are well suited to operate continuously at speeds of 10,000 RPM and even higher for short durations; the flat BLDC design with an outer rotor is also designed to run at 10,000 RPM. Both slotted and slotless BLDC cylindrical designs can reach speeds in excess of 40,000 RPM with high reliability. Motors that operate at these high speeds have bearing systems specifically designed to meet this requirement and are balanced for minimal vibration. To further improve high-speed performance, the motors can be balanced to a higher level.
•	Cogging Torque. Cogging torque is the result of the rotor’s preferred magnet position in relation to the stator lamination teeth. Coreless technology eliminates the iron laminations, which results in zero cogging torque in brush DC motors. Similarly, BLDC slotless motors do not have cogging torque due to the coreless design. Slotted motors inherently have some level of cogging torque that can be minimized with either a skewed alignment of the core laminations or the right combination of the number of poles and slots/teeth.
•	Motor Life. The main factor that limits brushed DC motor life is brush and commutator wear. Contingent on the working cycle and motor size, precious metals and/or carbon-graphite brushes can minimize wear and extend motor life by thousands of hours. BLDC motors are commutated electronically and are only limited by the life of the bearings.
•	Inertia. For applications that require low rotor inertia, the BLDC slotted motor is the best technology choice due to the relatively small diameter of the rotor. Conversely, the BLDC slotted flat design with the outside rotating configuration will have higher inertia due to the larger rotor design.
•	Robustness. One of the most critical requirements for smart defense applications is the ability of motors to withstand extreme environmental conditions. The BLDC slotted motor is best suited to handle harsh environments, such as high shock and vibration, high humidity, and salty conditions, due to its very robust stator and rotor designs. BLDC slotless and DC coreless motors can also be used in applications that must withstand the requirements of MIL-STD-810. The outer rotors of BLDC flat motors can be designed to meet Defense requirements; however, they are by nature less robust than inner rotor motors.

IDEAL MINIATURE MOTORS FOR SMART DEFENSE APPLICATIONS

Dependent on design priorities, BLDC slotted, slotless, and flat designs are all appropriate motion solutions for fin control actuation systems. Their high-power density and small package size allow a reduction in the weight of the actuation system while providing a high dynamic response to inputs from the navigation system to ensure excellent flight control. With wing deployment applications, both brush DC and BLDC technologies are an ideal fit. For simple motion systems, small DC motors are the most cost-effective solutions to meet all application requirements, which include performance, size, and extended life. For more complex requirements, a fully customized BLDC motor is likely the best solution. EO/IR applications typically utilize small brush DC and BLDC slotless motors. The high-power density of this motor technology helps to minimize the overall system package size. Brush DC motors also help to reduce the footprint, as the motor only needs simple control electronics to operate.

SPECIFIC DEFENSE REQUIREMENTS IMPACTING MOTOR SELECTION

Defense applications typically have numerous requirements and specifications that go above and beyond commercially available products; these unique features must be considered in the motor selection process. Most manufacturers can customize a motor to meet the most common environmental specification (STD-MIL-810), as well as other customer or application-specific performance requirements.

For high shock and vibration requirements, a selection of specific components (bearings, feedback devices, etc.) that can tolerate the requirements is a must. Additional manufacturing processes, such as potting windings or using laser welding for housing assembly, can help satisfy these requirements.

Temperature customizations to meet the -55°C to 180°C range involve the choice of low outgassing and extreme temperature lubrication for bearing and gearboxes. Custom electronics can be designed as well by using properly rated components or alternative temperature control options.

Long-term storage requirements dictate the need for special bearing materials and lubricants to avoid corrosion and the drying of bearing lubrication. For brush DC motors, special commutations materials and finishes may be required. High humidity and salt fog environments require corrosion-resistant materials for all metallic components and special PCB coatings for electronic components to prevent corrosion and absorption.

SUMMARY

Selection of the right precision motor for smart defense applications can be challenging, but manufacturers like Portescap, with more than 40 years of experience in many Aerospace and Defense-related applications, can help narrow the choices to obtain the optimal solution. These are just a few examples of smart defense applications that leverage advanced technologies to enhance defense capabilities. As technology continues to evolve, the scope and impact of miniature motors utilized in smart defense applications is likely to expand, enabling militaries to be more agile, adaptive, and effective in modern warfare scenarios.

WHY PORTESCAP

Portescap is a key supplier to the Aerospace and Defense market, with more than 40 years of experience in developing and deploying miniature DC motors for critical defense applications. Portescap’s engineer-to-engineer (E2E) collaborative design approach provides customers with a way to fast-track an optimized motor solution, whether as a COTS or fully customized solution. Multiple suitable motor technologies are readily available at manufacturing locations throughout the world.

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