CP-8 Filter Changer Cassette Calibration

Written By:
Bruce Truax
388 Wedgewood Road
Southington, CT 06489
860-276-0450
btruax@home.com

10/4/00

 

 

Introduction

The purpose of calibration of the filter changer is to tell the filter changer control software the location of the filter slots for a particular filter cassette. There are two cassette descriptor files located on the instrument PC for each of the filter cassettes currently in use. One of the files contains the calibration information for cassette when used on the HRCam and the second contains the calibration information when the cassette is used on the WFCam. Each cassette calibration file contains a list of the stepper positions for each cassette slot. These positions must be correct to within a few (+/- 3 steps) in order for the filters to line up correctly with the filter shuttle insertion mechanism. This document describes how to determine these slot locations and enter them into the cassette calibration file.

There are two primary reasons why the filter changer cassette file would need to be calibrated. The first, and most common reason is to initially specify the location of the slots when the cassette file is first created. The second reason is that the home limit switch has been moved because the Filter Changer has been removed or partially disassembled, or the switch was damaged or disturbed. If you are calibrating a cassette for the first time, skip to the second entitled Cassette Calibration. If the limit switch has been moved then begin at the next Section entitled Limit Switch Adjustment.

Limit Switch Adjustment

Refer to Figure 1 which is a drawing of the cassette translation mechanism as viewed with the Filter Changer Box cover removed. The home limit switch is can be found on the right hand side of the figure. The home switch is a microswitch with a lever mechanism which is actuated by a roller on the end of the lever. As the cassette stage moves towards the home position (to the left)) the Limit Switch Cam strikes the roller and depresses the actuation lever. Figure 2 shows a close-up of the interaction between the cam and the actuation lever. Note that the actuation position should occur when the line between the corner of the cam and the roller axis is about 45 degrees. If it occurs after this point the repeatability will degrade. The lateral position of actuation must be adjusted such that the actuation occurs before the stage reaches its hard stop. Finally, if the stage does overrun the switch, the lever should not be depressed so much that it stresses or bends the lever arm.

FIGURE 1 - End view of Filter Changer

The position of the switch can be adjusted by loosening the two adjusting screws located just below the switch. Before any adjustment is made to the position of the switch, it is important to make sure that the two attachment screws that attach the limit switch mounting plate to the changer are tight.


FIGURE 2 - Switch Lever Actuation

Limit Switch Adjustment Procedure

  1. Turn on the instrument power at the main power control panel in the electronics room. Note that the main power must be turned on first. When the instrument power is turned on, the instrument PC will boot and automatically run the filter changer control software. After about 4-5 minutes the software will have booted and initialization will be complete.
  2. Go to the observing level and remove the filter changer box cover by removing the thumbscrews on the corners of the box.
  3. Locate the filter changer pendant control and cable. They should be located in one of the left hand drawers of the toolbox base.
  4. Connect the pendant cable to the "D" connector on the upper forward corner of the right hand power supply enclosure (when viewing from the end of the instrument). This pendant will allow you to move the cassette stage by using the JOG buttons. You will notice that there are two sets of jog buttons. If you press the correct one, the cassette stage will move. If you press the wrong one and you are on the HRCam side you may hear the ADC mechanism in the fork actuate. If you are on the WFCam side, nothing will happen.
  5. Because the instrument PC has been initialized the cassette should be approximately in the center of the travel range. Press the JOG- button and confirm that the stage moves to the right. Notice that initially, the stage will step slowly and after holding the button for about 5 seconds the stepping rate will increase.
  6. While holding the JOG- button, gently depress the roller on the Home Limit Switch. The stage should stop immediately. If it does not, then either the switch is damaged, the limit switch cable ii disconnected or the switch receiver in the controller is damaged. If one of these problems occur you need to contact Bruce Truax or Frank Scinicariello.
  7. Once you have confirmed that the limit switch works you should release the switch and allow the stage to continue to move in the JOG- direction until the cam is just ready to engage the roller. When you get to this point, release the JOG- button.
  8. Gently depress the limit switch fully and make sure that the roller is just below the lower flat of the cam. If not, loosen the two adjusting screws and move the switch up or down until the roller is just below flush relative to the flat part of the cam. Tighten the adjusting screws.
  9. Press the JOG- button again and let the stage run. When the switch is approximately in the position shown in Figure 2 it should actuate and the stage motion should stop. If it actuates too soon lower the switch slightly, if it actuates too late, raise the switch slightly. Note that if you raise the switch you must carefully check the full travel location to make sure that the lever arm will not be stressed.
  10. After adjusting the switch, press the JOG+ button and drive the stage out of the limit so the roller is not in contact with the cam.
  11. Press the JOG- button and wait until the limit switch actuates. If the actuation position is acceptable you can proceed to the Cassette Calibration step, otherwise return to step 9.

Cassette Calibration

Calibration of a cassette involves determining the stage stepper counts for each of the filter slots. The correct position will align the slots in the cassette with the slot in the filter shuttle rail. This is done by removing all of the filters from the cassette and translating the stage to each filter position and recording the stepper value for that position. Once all of the positions have been determined the cassette file is updated with the new positions and then the alignment of the slots is verified. If the slots look properly aligned, the filters can be inserted and the operation of the filter changer is verified for each slot position. If one of the slot positions is incorrect, the corresponding filter may jam during insertion or retraction. If this occurs, the procedure must be repeated (at least for the offending slot(s)) until all filters can be inserted and retracted correctly.

Cassette Calibration Procedure

NOTE: This procedure will require two people and they should have two way radios to communicate during the procedure.

  1. If the filter changer pendant is connected to the power supply enclosure, disconnect it and put it away.
  2. Go to the electronics control room at the base of the pier. In the NEMA#4 enclosure there are two sets of three beige wires sitting on the bottom of the enclosure. The set labeled "Instrument PC" must be connected to the Monitor, Keyboard and Mouse which are located just to the right of the enclosure.
  3. Turn on the instrument power at the main power control panel in the electronics room. Note that the main power must be turned on first. When the instrument power is turned on, the instrument PC will boot and automatically run the filter changer control software. After about 4-5 minutes the software will have booted and initialization will be complete.
  4. Once the initialization is complete, the Instrument Controller software window should be present on the screen of the Instrument PC. Press the radio button for the desired instrument (the High Resolution camera is the one with the Red and Gold Dewar). Next from the View menu select the filter changer.
  5. Using Windows Explorer, navigate to the INSTRUMENT directory which is located at the root level of the C: drive. Locate the cassette file you wish to calibrate. If it does not exist you will need to create a new one. To create a new one take an existing file from the same instrument and copy it to a new file. Use the naming convention <HRCam or WFCam>C<n>.dat, where n is the number of the cassette. By copying the cassette file from an existing file the slot locations should be close to correct and the file format will be correct. A correctly formatted file is shown below:
    6. ser#	001		//Cassette Serial Number
rotator	NA1		//Specifies whether this file corresponds to NA1 or NA2
slot	1	0610	//Slot 1 Position	
slot	2	1950	//Slot 2 Position
slot	3	3510	//Slot 3 Position
slot	4	5070	//Slot 4 Position
slot	5	6630	//Slot 5 Position
slot	6	8190	//Slot 6 Position
slot	0	4290	//Load Position
  6. Record the coordinates listed in the third column for each of the slot positions as well as the load position.
  7. Go to the observing level and remove the filter changer box cover by removing the thumbscrews on the corners of the box.
  8. The installed cassette has a cover on the rear to protect the filters. This cover must be removed for this procedure. The cover can be removed by removing the two 2/56 Allen screws located on the top rear of the cassette.
  9. Remove the filter cassette currently loaded in the filter changer by loosening the Allen screw which is on the stage mechanism just to the right of the filter cassette (this will require a 3/16" ball end or "L" key). After loosening the screw, remove the cassette by carefully sliding the cassette towards you. If the cassette appears stuck, or if a filter starts to slide out as your remove the cassette, then it is not in the proper load position and you will need to translate it left or right until it comes out cleanly. This can be done either by using your index finger to rotate the motor coupling which is accessible at the right end of the stage or by temporarily plugging in the Filter Changer Pendant and using the JOG+ or JOG- button. NOTE: You will notice that there are two sets of jog buttons. If you press the correct one, the cassette stage will move. If you press the wrong one and you are on the HRCam side you may hear the ADC mechanism in the fork actuate. If you are on the WFCam side, nothing will happen. WARNING: When removing the cassette, be careful to hold it level and use one hand the rear of the cassette to keep the filters from sliding out!
  10. Once the cassette has been removed you can insert the new cassette. If you are calibrating the current cassette, you must remove the filters from the cassette before reinstalling. To remove the filters, simple press them out from the Dewar side of the cassette. Handle the filters with care and make sure that you note the order and orientation so that they are returned to the cassette in the same order and orientation. To insert the cassette slide it into the filter changer making sure that the dovetail mount slides underneath the two dovetail capture plates on the stage. Slide the cassette in until it is solidly seated against the stop. The rear of the cassette base plate should be flush with the mounting plate on the stage. Tighten the clamp screw securely.
  11. One of the two people should now go down to the electronics control room.
  12. On the Filter Changer window there are two text edit boxes used for specifying the configuration file names. One of them contains the name of the cassette configuration file and the other contains the name of the filter specification file. You must edit the name of the Cassette Configuration file to match the name of the file you created in step 5.
  13. Press the initialize changer button. The initialization process will take about 30 seconds and an alert box will pop up when the change is complete. The person on the observing level will note that the cassette stage will move to the right until it contacts the home limit switch at which time the stage should stop and reverse direction and drive to approximately the middle of the cassette. This should be the load position. If the cassette does not stop before the limit switch is fully depressed then you need to perform the limit switch adjustment procedure listed above.
  14. Once initialization is complete, press the radio button for filter 1 and then press the "Change Filter" button. The cassette will move to slot 1 (the leftmost slot) and the filtter shuttle will insert. You will get an error box telling you that the shuttle is inserted without a filter. Since the filters are not installed, this error is expected. Dismiss the box by pressing the OK button.
  15. The person at the observing level should illuminate the inside of the filter changer with a flashlight and sight down slot 1. The two sides of the slot should be well aligned with the two sides of the shuttle slot. If they are then proceed to step 18 Otherwise continue on to the next step.
  16. The person on the observing level must instruct the person in the control room to move the stage in either the + (left) or - (right) direction until the slots are properly aligned. It is recommended that motions of 5, 10 or 25 steps be used. Move the stage, the person in then control room must enter the number or steps in the top edit box in the filter changer window (make sure that you also enter the correct sign). Write this number down next to the slot 1 position you recorded earlier from the configuration file. When you are ready, press the MOVE button.
  17. Once the stage has completed the motion, the person at the observing level should once again check alignment. If necessary repeat step 16 until the alignment is correct. The person in the control room must record the magnitude and sign of each motion. When one slot is complete, these motions must be summed and then added to the original position to compute the new position.
  18. Press the radio button for the next filter and repeat steps 15-17. Continue this process until all 6 slots are properly aligned.
  19. Compute the new coordinates for slots 1-6. Compute the new load position by taking the midpoint between slots 3 and 4 and enter this as the slot 0 coordinate.
  20. Edit the Cassette configuration file by entering the new coordinates and save the file. You can either save the file over the old file or give it a new name. If you give it a new name, make sure this name is entered into the edit box in the filter changer window.
  21. Once the cassette configuration file has been edited and saved, press the "Initialize Changer" button once again. This will cause the software to read the new numbers and reinitialize the changer.
  22. Repeat steps 14 - 21 until all slots are correctly aligned. If the process does not converge then either: a) you are not properly recording the move coordinates, or b) the home limit switch is not repeatable and may need to be adjusted.
  23. The person in the control room should press the "Unload Cassette" button. This will cause the cassette to translate to the load position.
  24. The person on the observing level should loosen the cassette clamp screw and remove the cassette from the changer. If the load position is specified properly, the cassette should slide out smoothly.
  25. Insert the filters into the cassette. The filters must be inserted from the front of the cassette (the end with the small sliver latches at the bottom of the slots). The correct insertion orientation is with the slot on the bottom of the filter inserted last. When the filter is fully inserted you will hear a slight click as the latch engages. Be careful because the filters can slide out the rear of the cassette since the rear cover is not in place.
  26. If this is a new cassette of filters you will have to create a new Filter Description file with a name like HR_Set1.dat with the following format:
    27. //Note that no comments are allowed after the description
//The start of the description is delimited with a semicolon.
//    Slot Center wavelength	Bandwidth	Description
filter	0	0	0	;Load Position
filter	1	400	100	;U
filter	2	450	100	;B
filter	3	550	150	;V
filter	4	650	175	;R
filter	5	750	100	;I
filter	6	550	400	;Wide_V

    If a slot is empty, either comment out or leave it's entry blank. This will keep someone from accidentally selecting this position.

  27. Insert the loaded cassette into the changer. WARNING: When inserting the cassette, be careful to hold it level and use one hand the rear of the cassette to keep the filters from sliding out!
  28. Have the person in the control room press the "Initialize Changer" button (making sure that the correct Cassette descriptor file is entered in the text edit box).
  29. Once the changer is initialized use the radio buttons and the "Change Filter" button to step through all 6 filters sequentially and in random order. Each filter should insert and retract smoothly. The person in the control room should see a message confirming the correct insertion of each filter. If any of the filters does not insert or retract properly, its slot position will have to be recalibrated. Using the above procedure.