Tiedown limit switch and encoder positioning


Limit switch positions/encoder reference:

Trying to drive find the end of jack travel for final limit:

    When setting the final limit, you need to know the end of the jack travel. We then set the final limit .7 to .8 inches inside the jack travel. You can find the end of the jack travel  with the pcu (if the limit switches are bypassed). If the pcu is not working, it can be done with the vxWorks software if the following is done:

Laser measurement:

    We used a laser to measure a reference value for the tiedowns. The setup is:

after jack replaced

after jack replaced.
before jack change (32kips)
after jack change (12kips)
before jack changed (10kips)
after jack changed. (10kips)

Current td hardware limits (encoder inches)
td/date jack
sw final limit  prelimit tapeMeas  inch / bar
(when enc=13")

(23.290,1.150) 23apr12
(23.36,1.2) 04feb09
(23.28,0.86) 14aug06
(23.28,0.86) 17mar06
(23.21,1.19) 04aug03
(22.28  ,1.809) 28aug17
(22.278,1.780) 23apr12
(22.92,1.81) 04feb09
(22.461,1.668) 14aug06 lost .3"!!
(22.69,1.57) 17mar06
(22.62,1.9 ) 04aug03
after jack replacement
28 1/8 (enc=12")23apr12
26 19/16 04feb09
26 3/8" 18mar05


(xxxxxx,1.547) 01sep17. stopped pos
(23.317,1.487) 24sep12   
(23.19,1.37) 13may04
(22.80,1.997) 25aug17        
(22.78,2.03)  24sep12
( 22.82,2.01) 13may04
see 170825 for new jack 
bar=11.01 "

(23.21,1.19) 30jul03
(22.498,1.907) 01sep17 (lite on)
(22.5,1.9) 30jul03
after jack replacement
25 15/16 18mar05

    There is a pre and final hardware limit switch for the upper limit and the lower limit on each jack. The actions of the limts are:

    We have  measured the location of the jack at the limit positions using the encoder value and a tape measure (see td manual log section for the reference positions). This has also been done at some interim jack positions. If you lose the encoder position (an encoder fails and the jack moves before a new encoder is installed), then these measurements can be used to calibrate the new encoder. This can be done by:
  1. move to a limit position (when the limit switch triggers) and then adjust the encoder to match the last encoder measurement at this position.
  2. Measure the jack position with a tape measure and then interpolate what the encoder position should by using the encoder reading/tape measuremets and the two limits
  3. Move the jack until the tape measure agrees with a miscLocation measurement and then set the encoder position accordingly.
    Using the limit switches is a bit tricky since when they trigger depends on how fast you drive into the limit (you should do it slowly). If you do have to recalibrate an encoder, it is a good idea to remeasure all of the positions (limits,tape, misc) so you can see how close you got to the old position.

    Whenever we survey from AO9, we use the current encoder values to determine how to move the tiedowns. So by definition, after an ao9 survey, the tiedown positions are now correct. This is not quiet true since we have been using the focus curves to determine the focus rather than the ao9 survey value.

The table below has the measured values of the position using encoder and tape measure. Encoder values 24 inches is when the jack is fully retracted (high tension). 0 encoder inches is when the jack if fully extended (low tension).The cols in the table below are:

Note: The computer displays call the limits at 24 encoder inches (high tension) the down limits while the limts at 0 encoder inches are called the upLimits.

Bar used to set encoder position:

    An aluminum bar was made in an L shape. It goes from on top of  the center bolt of the jack mount  (on top of cement block)  to the top of the yoke. When the jack yoke is touching the bar, the encoder value is recorded. This lets us change jacks (which are different lengths) and maintain the correct encoder position.

11.01 inces
bar on center bolt.
td12 -23apr12
11.692 inches
bar on center bolt, north side (see photos below)

Measured positions for limits/miscPos using encoder and tape measurements.
date td hardware limits (encoder inches) Limits Tape(hi,lo) MiscLocation 
encoder, tape
replaced jack. (more info)

jack replaced. Used laser to position jack (before,after) (more info)
prelimits were sent to fire at the same encoder position as 24sept12

adjusted  limits1091111 downlim
1047891 24"
jacklim24= +24.. encCnts:1091111    24"= 1047891

installed new jack.Used L bar for encoder.

jacklim0=.3134 (no tension)
jacklim24=24+ (little tension)
finalLim(23.290,1.150) these changed a bit each time we drove in.
New jack installed. Used bar to transfer old encoder value to new jack
bar on top of center bolt (north side of jack)
encoder=11.692 (old reading) when bar touches top of yoke
encoder=11.2643 (new reading before encoder adjusted. Difference .428 inches.Could be jack height or encoder moved.

12",28 1/8
jackLim0=.253    measured by running into end of jack throw (with tension in the cables).
jackLim24=23.998 and still had 1/4 inch before jacklim
finalLim12:(23.36,1.2) preLim(22.92,1.81) full speed motion.

limit switces were way off since the aluminum plate holding the limit switches was mounted incorrectly when the jack was replaced 09jan09 . Readjusted the aluminum plate to give the settings  above.
measured aluminum plate distance before adjusting aluminum plate

13",26 19/16"

13",23 7/8
18mar05 12
I put all jacks at 13" and then measured the dist from bottom of aluminum plate to top of ridge on jacks (see luis's picture). These look like the old values back in 2001.
13", 26 3/8"
13",25 15/16"
17mar05 12 finalLim12:(23.28,0.86 ) preLim(22.69,1.57)
finalLim12:(23.21, 1.19) preLim (22.62, 1.9  ) old values
final limits computed from previous prelim,flim differences.
td 12 jack replaced (see above)

28feb05 4 finalLim4:(23.19,1.37)  preLim(22.82,2.01
Prelim measured, flim computed from old flim and distance change for prelim.

13. , 37 "
13may04 4 finalLim4:(23.24,1.28) preLim:(22.87,1.91)
jacklimLow: went to .057 inches then stopped. Had not hit limt yet. 
jackLimHi: went to 23.875 then stopped. currents started to go up.

13" , 36 15/16"
10may04 4 jack broke, had to replace. this required the encoder to be removed as well as the limits.


    13" , 35 15/16" 
13" , 36 13/16" 
13" , 36 7/8"
06aug03 4 finalLim4:(23.2,1.2) preLim4:(22.7,x.xx)
jackLim0:went to 0" enc , did not hit end of travel. 
jackLim24: stopped at 23.9998 inches. 
full speed into final limit stopped at 23.26 
full speed into prelimt stopped at 22.77

06aug03 4 rotated limit switches to face away from box. moved mounting bracket for limit switches so previous tape measurements are no longer valid.

04aug03 12 finalLim12:(23.21, 1.19) preLim12:(22.62,1.9)
jackLim0:hit endOf travel a little beyone 0"(enc) 
jackLim24:hit end of travel a little beyond 24"(enc). 
full speed into final limit (no tension) stopped at 1.177 enc 
full speed into prelim (no tension) stopped at 1.7 enc. 
full speed into prelim stopped at 22.72 (enc)


8 finalLim8:(23.2088, 1.19)  prelimit8:(22.6,1.9)
jackLim0:ran jack to  0" (enc) didn't hit end of travel
jackLim24:ran jack to 24"(enc) didn't end of travel. 
Full speed into prelimit (no tension) cpu stopped within about .2 inches.
2003??   replaced all limit switches.Previous limit switch positions no longer valid    


24sep01 8 finalLim8:(22.20,1.08)   (13.07, 25 3/4)

4 finalLim4:(22.82, 1.92)   (14.44, 25 1/4)

12 finalLim12:(22.29, 1.85   (14.70, 24 7/8)


08feb00 8 finalLim8:(22.20, 1.19)    


12 finalLim12:(22.29, 1.43)    


28aug98 8 finalLim8:(21.79 , 1.20) (17       , 37 9/16)  

4 finalLim4:(23.21 , 1.95) (16 5/8, 37 7/8)  

12 finalLim12:(22.14 , 1.52) (17 1/2, 38 1/8)  

    The software limits used to limit the requested positions are .2 inches inside the final limits. They are in the file /home/phil/h/tieProg.h
software request position limits.
start date td12 td8 td4
08feb00 (22.29,1.43) (22.87,1.93) (22.20,1.19)
xxx (21.79,1.2) (22.14,1.52) (23.21,1.95)

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