Grating mechanisms position the diffracted spectrum on different parts of the detectors, keep the gratings stable in a certain position during the data acquisition and scan the spectral lines across the detector pixels. According to the already mentioned grating optical properties, in order to hold spectral focus the grating has to be both rotated and translated. For this reason the axis of rotation is not through the optical vertex of the grating. In Table III are the functional requirements specified for the mechanism.
The grating mechanism is illustrated in Figure 11 
.
The grating is mounted in its housing which is directly connected to
the rotation axis by two arms that accommodate flexural pivots. Flexural pivots have been introduced because the needed rotation is too little to be performed by a bearing system and furthermore
the absence of rotating components reduces lubrication
and contamination problems.
A voice coil actuator is inserted in the mechanism for the grating positioning. In this kind of motor there is no contact between the rotor and the stator and, according to its characteristics it is possible to use it for both coarse and fine movement. The control action is performed by two pairs of differential impedance transducers: this measuring system provides exceptional resolution, repeatability and nulling accuracy for detecting the aligned and centered position of the conductive target relative to a pair of non-contacting sensors. This is the way in which the system works: for differential measurement applications, two precisely matched sensors are positioned on opposite sides or ends of a target; as the target moves away from one sensor it moves toward the other an equal amount so that a differential, bipolar output is available.
These mechanisms include launch locks. These locks
introduce a constraint in the z direction perpendicular to the grating
surface and a limited clearance in the other two directions in such a way as to
limit the grating housing motion. The blocking action is obtained by two
shaped wheels mounted on a stepper motor axis. The rotation of 180
of this axis changes the position of these wheels and presents sectors
where the wheels work as stops. In this way, the gratings are locked for shipment and launch with the primary spectral lines positioned on the center of the detector arrays.
