Mask exchange sequence details

In this section we describe one of the developed mask exchange sequences in more detail.

We use the ’Storage to FPU’ sequence in this example. Figure 3.5 and 3.7 show the corresponding sequence tree and 3.8 shows this sequence in six snapshots (see caption for details).

SWe start out when there is no mask inside the FPU and all masks are in their respective slot in the mask cabinet. In the first step the user/observer selects a mask number and

’FPU’ as the desired end position from the user GUI and commits the new instrument configuration.

The instrument manager service now calls the sequence server of the MOS unit, passing the two parameters (i.e. mask number and desired location) that the user provided. The sequence server starts to execute the sequence ’Storage-to-FPU’ since no mask is in use.

The first step in the sequence is the pre-condition check. Amongst diverse other items it is checked if the MHU-translator unit is initialized and if the FPU is empty. After the successful pre-condition check, the actual sequence starts to run.

’Storage-to-FPU’ is divided in two big sub-sequences, ’MOS - storage to turnout’ (cf.

figure 3.5) and ’MOS - turnout to FPU’ (cf. figure 3.7).

The first sub-sequence of ’MOS - storage to turnout’ is ’Translator to mask’. It is called to move the robot to the selected mask using the translation drive. Being a sequence itself,

3.4. Main function sequences

this sequence has a pre- (and post-) condition. This pre-condition checks if the motion is allowed at all and if e.g., the robot head is in the correct rotation position to move along the cabinet. Subsequently it powers up the motor, opens the lock and moves the robot to the mask. The movement itself is an atomic transition ’move motor to absolute’

that drives the translator stepper motor the calculated number of steps that are needed to reach the mask’s absolute position which was read from a lookup table. After that, the stepper motor is turned to ball screw angle that is associated with the mask, ’move motor to absolute angle’.

After the motion has finished, we check in the post-condition if the measured steps (using the incremental angular encoder) correspond to the commanded steps, i.e. the motion was ok without the motion being blocked. If a small deviation is present (like that the target translator ballscrew angle is off), we check if the value is within in the set toler-ance interval. If it is not, an auto correction is called and the position checked again.

This scheme of checking pre- and postconditions applies to all the next sub-sequences.

For simplicity we omit the description of every condition check hereafter. If the mask position has been reached and the post-condition is fulfilled, the next sub sequence is called.

While the retainer is still closed and the masks are locked, the retainer index shaft now rotates to the angle corresponding to the selected mask (’retainer - select mask’).

Next, the rotation head holding current is activated (’motor power on’) so the rotation head does not move by accident (or because e.g. the gravity vector changes) and the grabber grabs the mask (’MHU picker arms - grab mask’).

The grabber-motion is carried out according to a motor-step-lookup table that holds an array of motor steps and is run ’until state reached’ (the state here being ’the limit switch is hit’). The first array entry holds a number of motor steps which is normally sufficient to reach the limit switch. The next entries are fewer motor steps (motion correction steps).

These entries are only executed if the motion is not complete after the steps from the first entry have been executed.

The motion stops as soon as the state has been reached (the limit switch is hit) wether or not all array entries have been used. In case the limit switch has not been reached by the time all the lookup table entries (i.e ) have been executed, the sequence aborts.

The error would then cascade to the top level and the whole Storage-to-FPU sequence would abort. The strain gauges are read to check if the mask was properly grabbed (post condition). In our example, the grabber has successfully grabbed and thus the sequence continues.

The retainer drive shaft now rotates to ’unlocked’ position (’retainer - unlock mask(s)’).

This moves the selected mask’s locking arm backwards, unlocking the mask and leaving all other masks still locked. At this point the mask is held by the grabber but otherwise free and can be moved.

Figure 3.5.: First part of the Storage-to-FPU Sequence until ’Turnout’. The rest of the sequence from ’Turnout to ’FPU is shown in figure 3.7

Next the mask is rotated (’MHU rotator - storage to transport’) from the storage angle out of the storage cabinet into an extraction test position only 2 degrees away to check if the rotator motion is ok, and after that the rotator moves on to the transport position (cf.

figure 3.6). The holding current is still on for the rotator as we need to make sure that the mask stays in this position when we move along the cabinet in the next sub sequence.

3.4. Main function sequences

Figure 3.6.: The MHU takes a mask frame out of the cabinet. On the right: The corre-sponding arm of the retainer is retracted, all others stay locked.

The MHU moves now towards the FPU (’MHU translator - to open position’) and it stops at the FPU’s ’open’ position. With this step, the first big subsequence (’MOS - storage to turnout’, cf. figure 3.5) is complete and we jump to the second big subsequence ’MOS -turnout to FPU’ (cf. figure 3.7).

At first we check the pre-condition of ’MOS - turnout to FPU’, i.e. we test if the FPU is ready to receive the mask (and starting an auto correction if it is not). After that the MHU rotator rotates the MHU-head with the grabbed mask to the FPU rotator angle (’MHU rotator - transport to FPU’, ’move motor to absolute angle’). Magnets inside the mask frame activate the reed contacts insider the FPU which indicate the presence of a mask inside the FPU in open position once the rotation has been successfully completed.

The MHU translator now moves the robot (and thus the grabbed mask) forward in the direction of the focal plane into the ’hold’ position (’MHU translator - to hold position’).

When the mask has reached the ’hold’ position and the mask is in light contact with two spring loaded pads located to the left and right of each centering pin, the reed contacts switch to ’off’ and indicate that the mask is in hold position.

Now the two FPU clamp arms move until their spring loaded tips hold down the mask from the other side (’FPU - open to hold’). If the motion is completed successfully, micro switches are now actuated and indicate that the FPU arms are ’hold’ position.

Then the MHU grabber releases the mask (’MHU picker arms - release mask’). The grabber moves ’until state reached’ just reversing the grabbing motion as described above.

The holding current of the rotator is switched off (’Motor power off’). Finally, the FPU clamps close completely (’FPU - hold to locked’), pushing the spring-loaded pads com-pletely into the FPU base plate. The reed-contacts and clamp-arm limit switches are checked to see if the mask is in its final locked position.

Upon successful completion the MOS subsystem signals the high level software that the motion has completed successfully. The user/observer can now use the mask as desired.

Putting the mask back to its storage slot follows a very similar sequence in reverse, pre-and post conditions are sightly different.

Figure 3.8 shows six snapshots from the Storage-to-FPU sequence.

Figure 3.7.: Second part of the Storage-to-FPU Sequence from ’Turnout’ to ’FPU’. The beginning of the sequence from ’Storage’ to ’Turnout’ is shown in figure 3.5

3.4. Main function sequences

Figure 3.8.: Storage-to-FPU in 6 Snapshots:

a) The grabber (left arrow) is grabbing the mask, the mask’s retainer arm (right arrow) is still locking the mask. b) The rotator rotates the mask out of the cabinet, the retainer arm is open (arrow) c) The MHU robot moves in arrow direction along the cabinet towards the FPU on the left. Along the way the mask is held up in ’transport’ position d) The rotator rotates the mask into the FPU (arrow). e) The mask is nearing its FPU position, the FPU clamp arm drive is in ’open’ position (arrow). f) The mask is in its final place in the focal plane. The clamp arm drive is in ’locked’ position (arrow).