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See http://www.ociw.edu/~au/LDSS/LDSS_Shutter_TechDoc/LDSS_Shutter_TechDoc.html for a current description of the new LDSS shutter.
The LDSS shutter fails occasionally. The cause isn't known, although many components have been accused. To fix this problem, we will replace the shutter actuators.
The existing shutter is a PRONTOR magnetic E/100 with a built-in spring-return solenoid and solenoid controller (model number 141177 6660 000). There are two models with 100-mm aperture; ours is the one that is 194 mm on a side (the other is square, 210 mm on a side). This shutter is a 6-blade leaf design powered by a solenoid with a spring return. The shortest exposure time is 60 ms. To open the solenoid, a full 24 VDC must be applied for 50 ms. For exposures longer than 60 ms, a holding voltage of 12 VDC is used to avoid overheating the solenoid. The minimum cycle time is 1 seconds between successive exposures. A small RC controlled electronic circuit came with the shutter to provide the proper pulse and holding currents.
The original shutter had a custom-built linkage that connected the shutter motion port to the on-board bearing stud. This linkage was designed to allow manual actuation of the shutter as well as provide pickup points for shutter position sensors.
Investigating an LDSS shutter failure near the time of the Gladders LDSS optics upgrade, we first found that the custom-built manual linkage used to hold the shutter open for testing could sometimes cause the shutter to hang. We disconnected that linkage.
Later, we found a failed part on the controller board. LCO electronics staff installed a repair but this required an external power supply and some haywired components in the LDSS controls box.
We decided to replace entirely the electronic controls. We had two options and pursued both:
We normally would have chosen the commercial solution only. However, the commercial controllers we wanted use a pulse-width-modulated amplifier to supply the lower holding voltage. Since the frequencies of such amplifiers are sometimes near those of CCD readout electronics, there was a worry of imprinting a noise signal onto the CCD readout. So Alan Bagish designed and built an analog circuit equivalent to the one from PRONTOR that we could use in case the commercial unit (an Optimal Engineering high Performance Solenoid Driver, Pick and Hold Module part number PH-ET-01) proved noisy.
Both controllers were installed and tested in the LDSS control box. An early failure of the commercial board led us to use the Bagish-designed board but at the next shutter failure, a switch back to the commercial board was done, with longer-term success. During this time, the spare shutter (solenoid and leaf mechanism) was installed in LDSS.
A serious failure leading to lost telescope time happened in early 2007. This failure may have been instigated by nod & shuffle observing modes that require frequent shutter cycling. An investigation led to a modification to the mechanical linkage that seems to have reduced the failure rate but there have been reports since that the shutter continues to fail at the 1:250 rate.
At this point, we decided to replace the shutter control system. Failures were infrequent enough that we could not unambiguously find a single item to blame, and there was evidence that there was more than one potential failure point.
We will keep the PRONTOR 6-blade leaf mechanism because it is already built into the instrument and seems reliable. Prontor literature claims one million cycles during testing and expected lifetimes considerably longer. Note that the LDSS shutter linkage was modified by the LDSS builders, so its lifetime isn't the same.
We will replace all on-board linkages and actuators, including the PRONTOR-built spring and solenoid, and eliminate the electronic solenoid controller. A new control system that accepts the control signal from the CCD saddlebag and drives the PRONTOR 6-blade leaf mechanism will be designed and tested.
The CCD differential shutter signal is +5 VDC to -5 VDC.
This will drive two solid state relays, one for each control valve of the double-acting air cylinder. The prototype relay is an Omron G3NA-D210B-DC5-24 (see the data sheet at ![[WWW]](/wiki/img/moin-www.gif "[WWW]"){kind=small}
Omron G3NA-D210B-DC5-24 and this G3NA image. This relay has an opto-isolated input and a built-in kickback diode to handle inductive loads. Operating conditions: -30 C to 80 C, 45% to 85% RH. It's overkill in the sense that it can handle a lot more current than needed, but no circuit board or connectors are needed. At our operating current and ambient environment no heat sink is needed.
Each solid state relay will control one 3-way poppet valve. The prototype device is a Clippard EV-3-24 (http://www.clippard.com/store/byo_electronic/byo_mouse_valves.asp). These draw 28 ma at 24 VDC to dissipate 0.67 watts. Open/close response time is 5-10 ms. Operating temperature range is -20 F to 180 F.
FO-020.375-HDQSY Flat-1 Bimba
The shutter control box will have an LCD voltmeter display to monitor the voltage on the electric air valves. This can be used to check for proper operation and proper voltage settings. Datel DMS-20LCD-0/1-DCM.
Panel-Mount Miniature Air Regulator
Super-compact regulator has inlet and outlet ports on the bottom, making it ideal for panel-mount applications (panel mount nut included). Regulator is relieving style, which reduces downstream pressure through a vent port when your system is blocked. Body is corrosion-resistant anodized aluminum. Accuracy is ±0.25 psi. Max. temperature is 160° F. Max. pressure is 250 psi. Inlet and outlet connections are NPT female. Gauge port has 10-32 UNF female threads (gauge not included).
Please specify regulating pressure range (psi): 0-5, 0-15, 0-30, 0-60, or 0-100.
Pipe Max. scfm O'all Size, Size @ 100 psi Ht. x Wd. Each
1/16" 2.5 3.33" x 0.88"
41795K3 $35.12
PROTOTYPE SHUTTER ACTUATOR FEATURES
A commercial astronomy-specific shutter is available but there is a long lead-time and it isn't known if it will fit into the space available in LDSS (but we will measure this space the next time we have LDSS open). So our prototype will be designed and tested at SBS.
acl 2007-04-16 <au AT ociw DOT edu>
