Space
Motor driver applications in space
By Tony Marini, power technologist, EPC Space A
s the outer reaches of the Earth’s atmosphere and space are opened to commercial development, motors will become increasingly important
to systems placed there for various functions. With the inevitability of manufacturing in space, motors – including their drivers – will take on even more functions! Of equal importance will be the motor drivers selected to drive those motors efficiently and reliably. As a tandem system, the motor and its driver will define both the mechanical and the electrical attributes of the system they comprise. A motor cannot operate efficiently and yield the expected performance without a corresponding high-performance motor driver. These two functions – efficiency and high performance – are inextricably linked, and this is especially so in space where the electrical attributes such as efficiency are of equal importance as the mechanical attributes of speed or positional accuracy of the motor.
Applications and types of motors used in space
Motors are used to convert electrical energy 16 October 2021
into work. This work might be rotational (turning a load at a variable or constant speed) or positional (moving a load to a precise position relative to some reference location). NASA/MSFC identified four types of motors that would serve some useful purpose in space; 1) AC induction motor, 2) brushed DC motor (BDC), 3) brushless DC motor (BLDC), and, 4) DC stepper motor. Another useful motor missing from this list is the linear motor or actuator, which can be thought of as a positionally-controlled solenoid. Applications for motors in space include those that will control and move large masses such as the type found in lifting equipment or conveyors, reaction or momentum wheel assemblies (R/MWA), which are used to control the altitude and position of a spacecraft in orbit spinning large masses; motors used to accelerate and launch a spacecraft from another spacecraft, the moon or another planet while in space; or in the future for mining equipment on the moon or another planet.
Motor drivers in space Regardless of the type of motor, applications requiring motion systems in space must deal
Components in Electronics
with environmental issues not found in terrestrial systems. The challenges in space also vary according to the physical location of the system, particularly regarding radiation. For example, satellites in Earth’s orbit must deal with radiation fields not found in stationary locations such as on the surfaces of the Moon or Mars. The types of radiation found in space fall into two classes: 1) cosmic radiation comprised of high-energy particles moving at relativistic speeds in the vacuum of space, and lower energy protons and electrons (from solar wind) trapped in the Earth’s magnetic field (magnetosphere) in the Van Allen radiation belts. Although sensitive electronics can be shielded from radiation, there is only so much shielding that can be accomplished, due to the mass of the shielding required. And, since mass requires energy in the form of propulsion to achieve Earth orbit or to escape Earth’s gravity, there is a cost factor to the radiation shielding that can be onboard a particular spacecraft. This limitation is highlighted for commercial space ventures where cost is an important factor, and where constellations of low-cost satellites are stationed in primarily low Earth
Figure 1: Satellite positioning.
orbits. This means that the electronics themselves must have some inherent radiation “hardness” or tolerance, such that they can survive both high energy bombardment and, longer-term, lower dose radiation exposures over the duration of their planned mission life. The type and magnitude of the radiation encountered in Earth’s orbit is also greatly influenced by the orbital altitude of the satellite. Satellites in low Earth orbit (LEO) have a different radiation hardness requirement than those in geosynchronous Earth orbit (GEO) satellites. Landers and missions to the Moon and other planets have another set of radiation requirements that they must tolerate. Finally, deep-space probes have yet another set of radiation requirements. The radiation profile over the life of the satellite or spacecraft must be determined and specified, and the proper components for each piece of the motion control system carefully chosen if the desired lifespan for the system is to be achieved. For a motion control system, the weakest link in the system in terms of radiation susceptibility is the electronics associated with the motor drivers. The motors
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