# Controlling the platform height

#### 25may16

platform motions:
tiedowns,distomats use
tietrk command
tiepos (temp) command
tieposoff (inches)   command.

# Platform motions:

• The platform is suspended by long metal cables.
• When it gets hot, the cables get longer.. this causes the platform to move lower.
• When it gets cold the cables get shorter.. this raises the platform up.
• We measure the air temperature.
• The cables get longer, shorter depending on the cable metal temperature.
• at night these two are equal.
• In this case 5 deg F is about  1 inch vertical platform motion.
• During the day, the cable temperature can be very different from the air temperature.

# Tiedowns, distomats used to control height:

We use the tiedowns to try and keep the platform at the correct height: 1256.22 feet.
The procedure is:

• The distomats measure the platform height every two  minutes (it starts on the even minute).
• We need 6 measurements to measure the platform positon. There are only 6 distomats, so if 1 distomat fails, we can't compute the platform height.
• I did write a routine to use only 3 distomats. It's problem was that it mapped platform rotations into height changs.
• We have pretensioned the tiedowns.
• with temps around 75F the platform is in focus and there is tension in the tiedowns.
• As it gets colder,  the main cables shrink and the platform moves up..
• To counter this we end up pulling on the tiedown cables to remain in focus.
• As it gets warmer, the main cables get longer so the platform goes below the focus height.
• To counter this, we release some tension in the tiedowns so the platform will move back up to the focus height.
• This works as long as there is tension in all 3 tiedown cables.

# The tietrk command

We normally run the tietrk command. This will:

• every two minutes, it looks at the distomat measurements.
• If the platform in more than .05 inches out of focus, it will move the tiedowns to correct for this.
• The motion takes about 45 seconds.

Problems with this method occur when:

• As the temperature increases, the platform lowers, and the tietrk routine will continue to move the tiedowns up (releasing tension) so the platform remains in focus.
• at some point we loose tension in 1 of the tiedown cables..
• After this, the tietrk command continues to try to release tension in the tiedown cables.
• If 1 cable has lost tension, two of the tiedowns will still be able to release tension (their platform corners will go up ) while the cable with no tension will not move (can't push up on a loose cable). This tilts the platform.
• This problem is worsened when the dome is at a high za and passes close to one of the tiedowns..
• The dome will tilt the platform toward the tiedown and cause the cable to loose tension earlier.

In this case letting the tietrk program continue to run will tilt the platform even more than normal.. this can change the pointing by 1 or 2 arcminutes...

# The tiepos  (temp)  command

This will move the tiedowns to a fixed position given the input temperature. eg:

• tiepos 83
• Will move the tiedowns to the position it would be at when the nighttime temperature is 83 degF
• This routine fails miserably during the day.. because the air temperature we measure can be 10 deg F different than the actual cable temperature.

# The tieposoff (inches) command

This command will move the tiedown the requested number of inches from where it is currently at. eg:
• Suppose the tiedowns are at: 10.040 11.437  11.729
• Suppose that one of the tiedowns have lost tension
• We can move the tiedowns down 1 inch using:
• tieposoff  1.
• the tiedowns will move to 11.040 12.437  12.729 inches.
• A positive number  N will move the tiedowns down N inches (increasing tension)
• A negative number -N will move the tieodowns up N inches (decreasing tension).
• You can move N inches at a time but.. it's probably best to move by 1 inch at a time till you get some tension back in the loose tiedown

When to use tieposoff:

• You loose tension in a tiedown.. it's either warm, or the dome  is at a high za close to a tiedown direction.
• tieposoff 1.
• then look to see if the tension increased in the loose tiedown
• tieposoff 1.
• keep going till you see a tension increase in the cable
• During the day, you probably can stop trying to get tension when the tiedown positions get to 12-13 inches.. (although continuing will not cause any structural problems).
• Beware:
• The tiedown tensions have measurement offsets
• 25may16:
• td4 tensions read  2.76, .5  when there is no tension..
• So when you get close to 0 (say 2 or 3 kips) you should start to think about using tieposoff.
• A good way to check this is:
• do a tieposoff 1.
• the tiedown will move down 1 inch.
• the tension on all the tiedowns should increase.
• If the tension is a tiedown does not change, then you know that it has no tension.

Once you get sufficient tension in the cables (say > 6 kips) you can switch back to tietrk...
But beware.. when the dome is at a high za and passes near a tiedown, you will again loose tension