Revised 12/10/99

M. Radovan

Lick Observatory

Theory of Operation:

ESI uses air driven stages to move the entrance hatch and the science shutter. The stage control logic is slightly different for the two stages since the shutter stage has two independent bimba cylinders that are controlled by "hard wired" logic. Each cylinder is air driven to the "in" or "out" position . Air is continuously applied at a position to maintain the Bimba setting.

Air is switched to the cylinders by means of a Skinner valve. This valve has an internal shuttle that is spring actuated to one position and air driven to the opposite position. During a change of state the A and B lines simultaneously change from supply to exhaust settings as the shuttle in the Skinner valve moves. The exhaust from the Bimba cylinder exits the valve through port EA or EB. This exhaust in piped to the exterior of ESI. The Air supply to the Skinner valves must be set at the minimum value marked on the valve to insure that the air driven change of state can overcome the spring driven return motion. The valves used in ESI have a minimum recommended operating pressure of 30 psi although we have found that 35 to 40 psi should be used to insure 100 percent reliability.

Stage logic for the science CCD shutter is managed by a control box mounted inside electronics vault #2. This box has LED’s which report the limit conditions on the cylinders as seen by the magnetic read switches. Power to the LED’s can be shut of by a toggle switch mounted on the box for spectrograph operation. When a command at the user interface is give to change state on an air stage the electronics determine what to do based on the state of the limits. All cylinders must be reporting an "in" or "out" limit condition before a command is sent to the Skinner valve. If a cylinder is in an ambiguous state the shutter can not be moved from the user interface.

During power up to the electronics box for the shutter, the electronics box automatically drives both Bimbas "out". After both blades have reached the "out" limit the right blade is returned to the "in" position and the system is ready to receive commands from the UI. In the case of a cylinder failure on one side, it is possible to run the shutter in a right or left blade only mode by changing a toggle switch setting on the shutter control box.

The entrance hatch Bimba cylinder is controlled by the Galil electronics without an intermediate logic control box. If the stage ever fails to reach a limit a subsequent command to change state will not be executed until the previous command is completed as indicated by the limits or until the Galil times out while waiting for the last command to complete.

Limit switch position for the CCD shutter is set by driving the cylinder to the limit of travel and then sliding the switch along the cylinder until the LEDs on the control box indicate that the electronics are reading the switch. The state of the limits can be determined by looking at the OPTO-22 input relay LEDs, which are on Galil controller #1, located in electronics vault #3. There is a range of travel over which the LED will indicate that the limit is being read. The switch should be set in the middle of that range. Set the switch position by marking the cylinder with a pen at the ends of the range and position the switch so that it splits the difference.

The shutter and hatch stages each have some type of valving mechanism on the air lines to help soften the impact when the cylinder reaches the end of the motion range. The shutter uses Bimba Cushion air cylinders which have a built in adjustment on each end of the cylinder. This adjustment is changed with a small flat screwdriver. The hatch has throttle valves, which are plumbed to the air cylinder. These valves have a thumbscrew for changing their setting.

TROUBLE SHOOTING CURRENT ESI PROBLEMS:

Shutter: It is my understanding that the same blade failed both times when a command to close was given. In this condition the blade is assumed to be out but the limit does not know it. Attempts to cycle power of the electronic box failed because the blade was already out. When air was applied manually to move the blade out nothing happen. The stage was only freed after the blade was blown in then back out. The limit on this blade for the out position must be poorly set. Check to see if the limit is set in the middle of the range over which it reads. Compare this range to a different limit switch to make sure something funny isn’t happening with the limit switch. ( You have spares if the switch is acting up).

Hatch: The air leak on the stage needs to be fixed. If the regulator is leaking it should be changed. All air lines should be visually inspected to make sure there are no kinks. Air pressure at the cylinder can be checked with an inline gauge between the supply line and the cylinder for both the A and the B lines. The condition of the skinner valve can be checked by operating the hatch with the hand paddle while slowly decreasing the pressure to the skinner valve until it fails. If this occurs at a pressure over 40 psi then something could be wrong with the valve.