LDSS Air Shutter Actuator

Contents

• Introduction

• Important notes

• Old shutter system

• Upgrade overview

• Front panel layout

• Component layout

• Testing

• Controller components 

• Battery box

• LDSS shutter air path

• Existing shutter mechanical modifications

• Maintenance, troubleshooting, and repair

Introduction

Important notes

1. The electronic air valves have a maximum pressure rating of 25 psi. Normal operating air pressure is 16-20 psi. The valves will leak at pressures greater than 25 psi, but they will not be damaged.

2. The shutter input signal is on a pair of insulated wires. These must remain ungrounded all the way to the solid state relay inputs.

3. The Bimba air cylinder is intended to be fed with lubricated (oiled) air but we are supplying clean, dry air. We do not anticipate a problem but if the the cylinder becomes sluggish or sticky, a tiny amount of light machine oil injected into each of the air input holes might help.

Old shutter system

The original shutter had a custom-built linkage that connected the shutter motion port to the on-board bearing stud. This linkage allowed manual shutter actuation and provided pickup points for shutter position sensors. It's the brass item that extends over the tabletop to the right in this picture:

Figure: Original LDSS shutter configuration with custom linkage (photo by Gabriel Martin)

Upgrade overview

The benefits of the new system:

1. The mechanical linkage is replaced. The existing system has been seen to bind and cause mechanical failures.

2. The air cylinder dumps almost no waste heat into the instrument.

3. The solenoid controller circuit, which provides a high-current activation pulse followed by a lower holding current, is eliminated. Much time has been spent understanding and tuning this circuit. Because the new electric valves do not need this two-state system, we won't have to worry about it.

4. The long shutter cable run to the motion control box, a potential noise pickup situation, is eliminated. instead, the shutter cable now runs from the CCD saddlebag to the instrument shutter control panel less than a meter away.

For operations and maintenance convenience, the following features will also be provided:

1. An on-board air pressure regulator reduces the >100 psi source pressure to the 16-20 psi needed to operate the shutter.

2. A panel-mounted air pressure gauge to set and monitor the shutter air pressure is provided.

3. A panel-mounted digital voltmeter to set and monitor the voltage supplied to the two 3-way poppet valves is provided. A switch selects which of the valves to measure.

4. A battery box capable of actuating the shutter without the CCD electronics is provided. This feature is not built into the new instrument shutter control to avoid accidentally disabling the shutter during observing.

Here is a diagram of the new shutter control (pdf version):

Front panel layout

The controller components are mounted on a cast aluminum panel that replaces the LDSS shutter access port cover. The panel is attached to LDSS with four 10-32 SHCS that thread into new tapped holes on the steel body of the instrument. A light barrier on the back side of the panel nests into the existing access hole. (AutoCAD DWG file). 

Testing

In this movie (movie-1) you'll see the shutter opening and closing on commands from a square wave generator. A lab DC power supply puts 23 volts at the electric air valves. Lab air at about 100 PSI was input to the the regulator, which was set for about 20 PSI. A red retroreflector and red light beam are used to sense the open/close state of the shutter.

The opening time is slightly different from the closing time because we're using a single-ended cylinder that has the effect of supplying more force in one direction (close) than the other because the connecting rod is only on one side of the piston, reducing the piston surface area. A double-ended cylinder has a shaft that sticks out the back end that might interfere with the LDSS structure so we chose the single-ended cylinder. Frame-by-frame viewing of the 30 fps video shows that a full open or close operation finishes in three video frames, or 1/10 sec.

For reliability testing, we used the signal generator to simulate 2.5 second exposures initiated every five seconds (0.2 Hz frequency square wave). We wired this signal in parallel with the shutter input to a USB port converter that sensed and recorded the shutter signal state as ON or OFF. This tells us whether or not the shutter is supposed to be open or closed. To sense the true shutter state (open or closed), we used a retroreflector interrupt beam (Banner Q45VR3LVQ borrowed from Alan Bagish) shining through the shutter opening to a retroreflector. This signal was connected to another input on the USB port converter and we recorded the shutter as OPEN or CLOSED depending on whether or not the sensor saw its reflected beam.

On July 10, 2007, we put the shutter in a vertical orientation (more like what it sees at the telescope) and ran it for over two hours (movie-2). After some positioning adjustments to guarantee that we were able to sense unanticipated shutter openings as small as 3 mm in diameter, the system ran for 2 hours, 17 minutes, and 25 seconds, or 8245 seconds, or the equivalent of 1649 exposures of 2.5 seconds each. There were no failures. We did not test for "fully open" performance but occasional visual monitoring never showed opening malfunctions.

These tests do not cover a wide range of conditions (orientation, temperature, humidity, local barometric pressure, etc.) so we cannot predict long-term performance. However, the design fundamentals seem OK and we expect reliable performance.

Controller components

Following is a list of purchased components used in the LDSS shutter.

Omron Solid state relay

  

We are using an Omron G3NA-210B "hockey puck" style solid state relay (G3NA data sheet | local copy).This relay has an opto-isolated input to interrupt the electrical path from the instrument to the CCD. Its current capacity (10 amps) is much larger than we need (30 ma) but the package requires only two machine screws for mounting and has screw terminal blocks for easy maintenance. None of the connections contact case ground. We could have built our own circuit but in the end it would have had higher engineering and fabrication costs and enhanced spare parts requirements.

We made one custom modification to this device: we covered the LED indicator.

For more information, Omron has an applications sheet (local copy) .

Price: Approximately USD$25.00 each

Clippard electronic air valve

This is a Clippard EV0-3-24-H mouse-style Minimatic electronic valve (description). The 3-way action allows exhaust venting through the same hose that pressurizes a cylinder. In this model, the input pressurized air is enters at the "in" port and exhaust air exits at the top of the valve. All ports are threaded 10-32 holes that accept fittings with rubber O-rings. Our version has these  characteristics:

Please note that the maximum air pressure is 25 psi. The valves will leak (and the shutter may fail) if that pressure is exceeded.

Price: Approximately USD$23.00 each

Hirose HR10 receptacle

We use a 4-pin Hirose HR10 series receptacle to bring 24 VDC from the instrument onto the controller panel. The Digi-Key series number is HR1559.

Price: Approximately $10 each (the mating plug connector costs about $16 each).

Bimba Flat-1 air cylinder

We are using a Bimba FO-020.375-HDQSY, Flat-1 double-acting, single-ended air cylinder similar to the one in this photo (on-line information). An important design constraint for the shutter actuator was that the cylinder length in the "shutter-closed" position needed to fit within the existing solenoid actuator space on the Prontor shutter. When mounted in LDSS, there is little room behind the existing bracket holes so a short cylinder length was essential. (Local copy of Flat-1 catalog pages)

The single-ended cylinder makes opening and closing times a little different. A double-ended cylinder might not have fit in the space alloted, though..

The configuration we chose is described in this table. See  http://bimba.technicon.com/CC_host/pages/custom/templates/bimba/product_page.cfm?cc_nvl=((CC,bimba,FO,FO)) for more configuration information.

Air pressure regulator

This is a 0-30 psi panel-mounted air regulator, part number 41795K33 from McMaster-Carr. This sets the air pressure going to the Bimba cylinder. Expected operating pressure is 16-20 psi.

From the McMaster-Carr catalog:
"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."

The package markings on the part we received show it is made or distributed by Marsh/Bellofram, Route 2, Box 305, Newell, WV  26050-0607, part number 960-237-000, "T91 MINI REG., 0-30 PSI."

Price: Approximately USD$35.00 each

Air reservoir (not used)

We were worried that hose restrictions would prevent fast shutter action so we anticipated using this air reservoir from Clippard to maintain pressure during actuation. This turned out not to be necessary.

This air reservoir, a Clippard MAT-.50, holds reserve air to make sure we get consistent action from the shutter even at fast cycle times. It's on the low pressure side of the regulator.

Price: Approximately USD$6.00 each

Air pressure gauge

           

This is McMaster-Carr part number 3847K732. Ours has a black bezel and a dial that reads from 0-30 psi, not 0-100 psi as in this photo. The item we received is built by Ashcroft Instruments of Stratford, CT., and is designated "15W 1005PH 01B XUC 30#." It has a 1-1/2 inch face, ABS plastic case, polycarbonate window, brass sockets with 1/8 NPT male fitting, and a range of 0-30 psi.

From the McMaster-Carr catalog:
"Multipurpose Gauges—±2% Mid-Scale Accuracy (Grade B):

• Use with compressed air, hydraulic oil, nitrogen, and ethyl alcohol

• Bourdon Pressure Tube Material: Bronze

• Connection: Brass NPT male

• Lens Material: Polycarbonate

• Temperature Ranges:  Ambient: -40° to +150° F; Process: -40° to +150° F"

Price: Approximately USD$13.00 each

Voltmeter

This is a small panel meter, DMS-20LCD-1-DCM-C from C&D Technologies (Newark part number 87K3630) used to indicate voltage on the Clippard electronic air valves. A switch on the control panel selects the valve ("open" or "closed") being measured. It is used to do the intitial adjustment of voltage to the Clippard electronic valves and to verify that power to the valves is available during maintenance and testing.

Here's the product information and data sheet (local copy) for the DMS-20LCD-1-DCM-C.

Price: Approximately USD$30.00 each

Panel screws (not used)

Our original plan used these captured panel screws  press-fitted into the aluminum panel. We decided to use cast aluminum instead of rolled 6061 for the panel, however, to reduce warping. We worried that the cast aluminum would break if we used the pressed-in screws so switched to ordinary 10-32 socket head cap screws to hold the panel on the instrument.

McMaster-Carr part number 92060A325. This is press-fit into the front panel and connects to LDSS on new 10-32 tapped holes in the instrument body. The panel needs 5/16 inch diameter holes to accept this captive fastener. Side detail here (but ours will be press-fit, not snap-in like this drawing). A=7/16", B=7/32", C=33/64"

Price: Approximately USD$2.60 each.

Battery box

Built by Jorge Estrada. This box  has four (4) AA batteries and a toggle switch. It can be used in place of the CCD saddlebag shutter signal so testing of the shutter can be done without operating the CCD system.

LDSS shutter air path

This is a description of the air path and plumbing that powers the LDSS shutter. In most cases, specific part numbers from the McMaster-Carr catalog are provided although the actual parts used in the instrument might be equivalents, rather than these parts specifically.

Existing shutter mechanical modifications

The Prontor shutter is modified in the following way:

1. Mount the Bimba cylinder on the custom L-bracket with long 6-32 machine screws, lock washers, and nuts. This thread should use Lock-tite.

2. Install an 8-32 setscrew on the Bimba cylinder shaft.

3. Put a #8 washer over the setscrew.

4. Attach the custom brass nose-piece tightly to the setscrew on the Bimba cylinder. Use a small ignition wrench (provided) to hold the Bimba cylinder shaft.
   

5. Attach two McMaster-Carr part #5779K246 tube-to-pipe fittings to the Bimba cylinder air holes.

6. Remove the electric solenoid and custom linkages from the original Prontor shutter. Save the circlip on the post and the stainless steel pin on the shutter actuator.

7. Install the new custom linkage from the post to the shutter pin.

8. Attach the Bimba cylinder L-bracket to the shutter using the same M4 screw holes that the solenoid used. Important: The inboard M4 screw must extend no more than 2 mm beyond the bottom of the L-bracket. Use an M4x6 with an M4 lockwasher under the screw. If this screw is too long, it will extend into the gap between the two shutter panels and interfere with the shutter blades.

9. Provide air, adjust the pressure to 16-20 psi, and check the operation of the shutter. Make sure that the blades open and close fully without binding. Here's the parts list for modifying an existing Prontor shutter (we will need to do this to the shutter currently in use):

Here are two custom brass components for the LDSS shutter. On the left is a linkage that defines the arc along which the shutter actuator pin travels. On the right is the part that connects the end of the actuator rod to the shutter actuator pin. Note that this part is not symmetrical and must be attached in the correct orientation.

The modified Prontor shutter with the brass linkages attached. The solenoid and spring have been replaced with the pneumatic cylinder:

The following is a photo of the cylinder mounting. Note that the inboard M4x6 screw must be no longer than 6 mm:

Here's an edge-on view of the L-bracket shutter cylinder mount. The M4 screw on the right must not extend into the interior space of the shutter mechanism or it will interfere with the blade motions.

Maintenance, troubleshooting, and repair

Testing the shutter controller

If a problem occurs, the following procedure might be useful to identify the reason:

1. Check that air pressure is available. The gauge should read between 16 and 20 psi. If not, check that air pressure is available to the instrument. If air pressure is available, unlock the regulator (locking nut is under the knob) and adjust to 20 psi. Note that if this adjustment is necessary you should suspect that something else might be wrong with the air system. Note also that the maximum pressure before the valves leak is 25 psi.

2. Check that the LDSS controller chassis under the platform is turned ON.

3. Disconnect the shutter cable from the CCD and connect the battery box (check that the batteries are good).

4. Use the switches on the battery box and the controller box to check that 24 VDC is being applied to the electronic valves and that the voltage is applied and removed appropriately.

5. If the voltage is not 24 VDC, the SOLA 24 VDC SDP-2-24-100T can be adjusted by opening the LDSS controller chassis under the platform. Note taht if this adjustment is necessary, you should suspect that something else is wrong with the system or that the SOLA power supply is faulty.

6. Remove the four 10-32 mounting screws (with red knobs) and lift the controller panel far enough away from the instrument to view the shutter mechanism. Operate the battery box to make sure the shutter motion is correct. The total stroke should be about 1 cm.

Removing the shutter controller from LDSS

If the controller is faulty, it can be replaced on the platform. Two complete controllers were built so it might be easier to replace the controller panel and repair the failed one later. Use the following procedure to remove a shutter controller from the instrument:

1. Turn off the air pressure to LDSS.

2. Turn off the LDSS controller under the platform.

3. Disconnect the LDSS shutter cable from the control panel.

4. Remove and store the four red-knobed 10-32 panel mounting screws.

5. Lift the panel from the instrument and disconnect the 24 VDC Hirose connector.

6. Remove the two air lines going to the Bimba air cylinder.

7. Remove the input hose on the pressure regulator.

8. Reverse these instructions to install a new controller.

Repairing the shutter controller in the lab

The electronics and plumbing on the LDSS shutter control is simple and enough spare parts are provided to replace any single failed component.

Repairing the shutter mechanisms

The mechanical mechanism requires some care in disassembly and assembly. It is possible to assemble it such that it does not operate well.

Check that the mechanical parts on the Prontor shutter mechanism are solidly connected. Vibrations may cause some parts to unscrew. Use Loctite on all threaded fasteners on-board the Prontor shutter mechanism.

The Bimba air cylinders are designed to be used with lubricated (oiled) air. The LDSS air supply is oil-free and dry, so it is possible that friction in the cylinders may slow the actuation times after a  while. If this occurs, we suggest replacing the cylinder with a new one. It is possible to add oil to an old cylinder, but replacing them might be a better option.

A general characteristic of air cylinders is that they tend to leak at low temperatures. While this cylinder was designed to operate down to -40 F, it might still leak at low temperatures resulting in slower actuator times.