EL-1183Miscellaneous Drawings
CCD Shutter Controller,
EL-1183
Schematic: schematics/MULTIPURPOSE/EL-1183.sch.pdf
Page last updated: May 14, 2002
USER GUIDE
CONNECTORS:
A 10-pin ribbon connector accepts an open/close signal from a CCD controller (or other source) and returns shutter status in the form of separate open and close signals. A-7 pin MS connector sends control pulses to electrically toggle Skinner air solenoids which energize the Bimba air pistons which move the shutter blades in and out. Each air piston sends back limit signals to the controller's 5-pin MS connector that indicate which end of travel the air piston is at.
CONTROLS:
A row of 4 LED lamps shows the current state of the air piston limits. The toggle switch turns them off for dark operation. The push button switch is to help "jog" loose a stuck shutter blade by pulsing it repeatedly in the open direction. The toggle switch to the left of the jog push button is normally in the "center off" position. If for some reason (e.g.: one blade is stuck open) the astronomer wants to operate with only one shutter blade moving, the "One Blade" switch should be moved to the left for left-blade-only or moved to the right for right-blade-only.
TECHNICAL
The controller box provides 24 volt pulses of alternate polarity to a pair of Honeywell Skinner air valves. (P/N 73419AN2NN00M1J011C2, 24VDC) These valves latch into position and thus don't require a continuous signal to keep them activated. The Skinner valves have 5 air fittings on them as follows:
P: Pressure in (30-150 PSI)
A: Pressure to outboard air cylinder fitting (closes shutter blade)
B: Pressure to inboard air cylinder fitting (opens shutter blade)
EA: Exhaust air from A fitting
EB: Exhaust air from B fitting
The black wires from the solenoid coils must go to pins A & C and the red wires go to pins B & D on the 7-pin MS connector. Reversing the red & black wires or the A & B air lines will cause the shutter to fail, although it may seem to work for a few strokes. A positive voltage on the black solenoid wire should apply air to the B port of the skinner valve and should open the shutter blade.
The shutter blades themselves are pushed or pulled into position by Bimba air cylinders. Inside the air cylinders at each end are two magnets, one at each end of the internal piston, which can be used to actuate external reed switches to indicate the position of the piston. In this application the switches serve to indicate whether the shutter blade is fully in the light path (closed) or fully out of the light path (open).
The speed of the shutter blades is set by adjusting the flow control valves which are located at each end of the cylinder. They control the rate of air release from one end of a cylinder as pressure at the other end moves the piston. If the air is released without restriction, the piston can move fast enough to damage the shutter blade itself.
Another possible effect of "fast piston" is to jar the external switches loose so that they don't indicate that the piston is at one end or the other, leaving the controller unable to tell that the move has completed. If the solenoid pulse length was just set by the limit switches, a stuck blade would cause the power to remain applied to the solenoid coils overheating them. The coils have a 20% duty cycle with a maximum on time of 5 min. The solenoid driver chips would probably burn out too. On the other hand, if the switches are properly attached, they should not be able to be jarred loose. To avoid the overheating problem from this and other possible causes, a one-shot chip sets the maximum on-time to about 200mS.
Each solenoid coil is driven by H-Bridge transistor array, the SGS L6202. Given separate open & close input signals, the pair of outputs is either 24V & Ground or Ground & 24V. This provides current through the solenoid coils first in one direction and then in the other. The negative true inputs to the L6202 chips come from a programmable logic device, an Altera EP610. This chip is where most of the action takes place.
The EP610 accepts inputs from the 2 pairs of limit switches, the open/close command line, the reset circuit, the jog push button and the "One Blade" switch.
In addition to the open & close signals sent to the L6202 chips, the EP610 sends back to the CCD controller two negative-true signals, one indicating that the shutter is open and the other indicating that the shutter is closed. The lines from the 4 limit switches also are sent back, to allow software monitoring. In addition to a common signal ground and power ground, 5 volts is supplied to power a test box, (El-1233) which can be used to simulate the controller and/or the shutter.
The signal from the reset line, an R-C delay circuit, is used to establish the initial state of the shutter. First both blades are told to open, then the right blade is told to close.
The open/close input signal is normally low, keeping the shutter closed. When the signal goes high, the shutter will open and stay open for as long as the signal stays high.
A latch, internal to the EP610, remembers which blade was the last to open, which allows the controller to close the other blade, producing an alternating left-open/right-open pattern. Power is applied to the solenoid coil until the appropriate piston limit switch indicates movement is complete or for a maximum time set by the one-shot. This ensures that the pulse of current won't be too short to trip the solenoid.
The jog button provides solenoid current to the blade that's not open. By repeatedly pressing the button, the stuck blade may be encouraged to open. Once the stuck blade is fully open, the observer can continue to operate by putting the "One Blade" switch in the Left position if the right blade was stuck and vice-versa.