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'Resulting from constant current applied to motor.

Fig. 11-48. Typical Operating Rate Profiles and Derived Accelerations. The acceleration I calculated by dividing speed by the time increment

Fig. 11-48. Typical Operating Rate Profiles and Derived Accelerations. The acceleration I calculated by dividing speed by the time increment

Output Torque

Fig. 11-49. Derivation of Mechanism Stall Torque. Linear extrapolation of operating points establishes stall torque and no-load speed.

Fig. 11-50. Actuator Characteristics Based on Stall Torque Requirements. Empirical data based on wide range of aerospace mechanisms.

Although weight, power, and volume are usually the three major spacecraft system parameters, we must not severely constrain the mechanism's weight The mechanism design should be robust to withstand stall torques and to maintain its structural stiffness over a wide range of temperatures. The mechanism is not a major power consumer. Low-cyclic mechanisms operate only a few times in the mission. High-cyclic mechanisms draw high currents during the acceleration phase of the duty cycle, a phase that is generally 10% of its operating life. Volume constraints will dictate die design process. Also, requirements for mechanical and electrical interfaces will influence the mechanism's volume and structure. The mechanism will also produce its own requirements for torques or forces, operating rates, structural stiffness, operating life and histogram (torque/cycle matrix), and environments. The mechanism must withstand the launch and derived vibration tests, which will influence the strength and stiffness requirements. The mechanism must operate in orbit, where the thermal-vacuum environment will influence the selection of materials, lubricants, and coatings. It will also create thermally induced loads caused by difference in coefficients of thermal expansion of selected structural materials.

For more information on space mechanisms, see Conley [1998], Sarafin [1995], and Mil-A-83577 [1988].

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