Solving 4 challenges to stepper motors for space flight

The harsh environment of vacuum and microgravity presents a challenge to components, demanding capabilities not commonly encountered on Earth. This is especially true for motion control systems where movement is their primary function. Motors require considerable power to operate, can generate excessive heat, introduce unwanted vibrations, and create contaminants.

Combating these issues comes at a cost. Higher power requirements need larger power systems generating more heat, requiring larger cooling systems. This can lead to unwanted vibrations, requiring more robust damping systems, and contaminants can cause havoc on instruments and other on-board components. Added complexity can translate to component or system failures, unacceptable in space applications. It’s important to work with a company experienced with space flight requirements, such as Lin Engineering, with hybrid stepper motors designed to work in harsh environments and the inherent problems of motion control in outer space. The motors are assembled in accordance with AS9100 standards, tracing each component’s origin to maintain strict control from manufacturing to the final product.

To address the challenges:

In space applications, power comes at a premium – every watt wasted by a non-optimized system costs precious resources. Optimizing for power consumption includes customizing motor windings to deliver peak dynamic torque at the desired operating speed. This requires proper integration of high-precision components, such as low-inertia, high-efficiency rotors. Depending on the application, engineers tailor each motor to deliver the necessary performance while accounting for power constraints in the system. Using proprietary, proven algorithms, Lin Engineering optimizes torque and speed, noise reduction, heat generation or loss, and power optimization.

Two critical temperature concerns affecting hybrid stepper motors in space are temperature range and the amount of heat generated. Satellites and other spacecraft operate in extreme temperatures, requiring rugged designs for externally mounted systems.

For example, heat affects the strength of magnets embedded in the rotor. As heat increases, motor performance decreases. A solution – permanent magnets. Constructed from rare-earth samarium-cobalt or neodymium alloys, they provide greater magnetic power at higher and lower temperatures.

Heat also impacts the life of bearings used in motors, shortening the lifespan of the whole system. Using bearings with grease that can withstand temperatures from -80°C to 200°C, including dry-lube or no-lube is necessary. High-temperature, non-outgassing bearings also can be designed.

Excessive heat generated by the motor can be a concern, because in a vacuum environment, there is no atmosphere to dissipate heat from the motor or vehicle. On Earth, air conducts heat generated away from the vessel, but in space, dissipating heat requires other methods that often add unwanted weight, increased mass, and unnecessary build complexity into the craft or satellite. Further, heat generated by a stepper motor can affect nearby instruments and components, especially in insulated areas. To reduce heat, optimize the winding of the stepper motor. Incorporating conductive pathways with thermally conductive materials provides heat dissipation between the insulator (glue) and motor end bells, assisting temperature management.

Launching a spacecraft into orbit is violent, as components are exposed to high-amplitude vibration, low-amplitude vibration, and shock from several directions. In addition, stepper motors generate vibrations during normal operations.

Optimizing motor windings minimizes the resonance frequency that develops at specific operating speeds. Using components machined to high concentricity and dimensional accuracy helps ensure rotors or shafts do not introduce unwanted vibrations into the system.

In space, vibrations must be avoided because they can affect on-board sensors and instruments; low-level oscillations can affect measurement sensors and imaging device quality. Since the craft or satellite is in space, where no energy can be transferred, dampening vibrations is challenging. Every stepper motor designed for space requires material structural integrity to handle expected forces without altering dimensional accuracy or mechanical integrity.

On the microscopic level, gases and liquids are trapped inside of paints, coatings, greases, and materials. In the vacuum of space, these trapped gases inside the motor can expand or condense, and liquids evaporate, introducing unwanted contaminants into the environment. If these contaminants settle on imaging sensors or measurement instruments, they can reduce overall performance or render them useless.

Motors can be designed to minimize outgassing, when all stepper motor components – end caps, stator, rotor, screws – are manufactured from low-outgassing materials. Most components on the motors are metal alloys, which, without paint, are rated for vacuum use. Plastic materials should be non-outgassing polyamide or nylon. Sealed bearings filled with low-outgassing lubricants are also available. All components are thoroughly cleaned and vacuum baked, then each stepper motor is assembled in a cleanroom and vacuum sealed to prevent contamination. For corrosion protection, use specialty vacuum- and space-compatible coatings for ferrous components or aluminum.

Ensuring the longevity of components that go into space is a factor of the time and effort put into proper design of components and their final assembly using the latest materials, manufactured in a cleanroom facility. Space flight and satellite applications require a keen sense of the problems associated with their operating environment. Lin Engineering, which has provided custom motors for various applications, has the expertise to design a stepper motor for any harsh environment, including deep space.

Lin Engineering Inc. http://www.linengineering.com

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