Design Discussion   Motor   The Focus Mechanism is driven by a 2-phase driven, bipolar, permanent magnet, redundantly wound, 30°  stepper motor. The motor possesses a detent torque which prevents backdriving of the mechanism during launch. Integral to the motor is a 100:1 gear head which enhances mechanical advantage and  helps meet the strict Kepler resolution requirements. The detent torque combined with the large gear ratio  eliminates the need for a launch lock in the mechanism.   The Focus Mechanism is open-loop controlled by commanding the motor to move a specified number of  steps. During actuation a ‘commanded step’ from the control electronics moves the motor rotor four 30°  steps. This equates to 0.38 μm of axial movement at the primary mirror. The ‘commanded steps’ are used  to start and stop the motor rotor in the same phase which increases step count accuracy. The resulting  position of the primary mirror is verified with the LVDT sensors. 

Transfer Gears

Both the 40-tooth pinion gear and 80-tooth spur gear are made from custom 455 stainless steel heat  treated to condition H1050 to give adequate strength margins. Other than the special material callout, the  gears are standard off-the-shelf parts. There is a small amount of backlash in the system due to the transfer gears and 100:1 motor gear head which results is a maximum vertical motion of 0.43 μm at the  primary mirror interface shelf. This is well below the 1.25 μm repeatability requirement.   Ball Screw

The ball screw has an 8-mm diameter and 2-mm pitch and is preloaded with oversized balls to eliminate  backlash. The balls, shaft, nut and deflectors are fabricated from 440C passivated stainless steel. The  ball screw was sized to result in positive stress margins. 

Hard Stops   The upper and lower hard stops are non-jamming and use a radial face-to-face design which minimizes  stress on the ball screw. Figure 10 shows a close up view of the mechanism model with the lower hard  stop engaged. The hard stops were fabricated from Titanium and designed to insure that the maximum torque delivered by the gear motor did not over stress them. The hard stops were positioned onto the ball  screw during the build up of the focus mechanism so that they stop movement at approximately ±890 μm,  well beyond the required ±762 μm range requirement. After the hard stops were correctly positioned, the  ball screw was removed from the assem bly and the hard stops were match drilled to it. The threads of the  ball screw were completely covered to prevent contamination during this operation. The ball screw and hard stops were then reassembled into the mechanism after the match drilling operation. 

Wishbone Bracket

The wishbone bracket attaches the ball nut to the lever arm. It contains two flexures that extend down on  both sides of the ball nut. The wishbone flexures are designed such that they are compliant enough to  bend without binding when the Ball Screw rotates and t hey translate rotary motion to linear motion. At the  same time, the wishbone flexures are stiff enough to prevent ‘wind- up’ when the ball screw rotates. If the  wishbone flexures are too complia nt, the bracket will rotate along with the ball screw and then at an  unexpected time it will ‘unwind’ causing unwanted movement and unfavorably affecting the step size and repeatability of the system. Figure 11 shows the titanium wishbone bracket and the tension joint  attachment to the lever arm.  365