Run-off+Roof+Observatory+Design

=__Overview__=

The Run-off roof observatory has been through a number of design changes over the last year (2009 to 2010). The original design was a small run-off shed, but this had drive issues and was not going to be large enough, or robust enough for remote and automated operation. We needed to have a design that was as reliable, or better than the dome.

Advantages of a run-off roof over a small run-off shed/dome are:-
 * Access to the telescope and control panels etc when the weather is bad (small run-off shed was too small!)
 * Better wind protection for the telescope
 * Faster slew rates for the Meade mount over a dome (it is not limited by the dome slew speed)
 * Cheaper and cost effective
 * Simpler and probably more reliable
 * Relatively easy/quick to build (but there was a LOT equipment and software complexity required to make it remotely operable and semi-automated)

Disadvantages are:-
 * Does not provide protection from glare as a dome does
 * Does not provide protection from dew as a dome does
 * Limited North horizon with the roof running off in that direction with limited space for the roof to run off (not that important)

We have finally decided on a configuration, and the observatory was under construction in my garage (as of April 2010). It can be seen that the telescope is mounted in AltAz mode, but the final installation is now equitorial mode. The sides and roof atre covered in 5mm white Foamex, and not wood as show below. It is be located next to the dome. As of late October 2010 the Observatory and Telescope are in operation (at last!).

See the page Run-off Roof Observatory documentation for drawings and other documets on the observatory and telescope.



=__Drive Mechanism__=

The drive mechanism is quite simple and uses a gearhead motor with interfacing direct gears to a 'roller' drive that has two fixed rubber wheels at either end. This means that 'both wheels' drive the roof off on the aluminium rails. The wheels are connected via a long drive 'tube' that is made from 100mm diameter thick wall ducting.

The image below shows the drive 'tube' with the wheels fixed at each end. The motor is not yet connected. The wheels run on 3 x 3 inch 'T' section aluminum bought from Aalco in Liverpool.



The image below shows the drive mechanism being installed at an early stage. The winch has been stripped of its casing and you can see the two gears. One of which is connected driectly to the drive 'tube'.The other to the winch motor that is supplied by 12V from the main control panel. There is a manual release clutch on the winch. This drive motor has sinced been replaed with a gearhead motor controlled via a variable speed drive controller. Details of the reasons why this was replaced are outlined in the 'control syste' section below.



The weight of the roof cladding has increased the load on the wheels and hence the friction to prevent slippage. To improve load on the drive wheels we have added 2 x concrete flags to the drive end of the roof. We have done tests with the aluminium rails wet and dry and there is very little chance of slippage.

The image below shows the new motor in place (some painting is required yet!)



The south side of the observatory has a mechanically operated door. This door opens when the roof opens and closes when the roof closes. Two roller wheels connected to the roof engage with aluminium angle slides that are connected to the south facing upper door. The door folds down to approximately 20 degrees above the horizon and is limited mechanically. The advantage with this system is reduced complexity, control, motors and limit switches. The door has operated reliably so far.

Control of the roof opening and closing, including roof and weather status, is done using the Observatory Auxiliary Control application.

Image below shows the mechanism that closes the door and the closed limit switch. One of the roof safety brackets can also be seen. Four of these are installed to prevent the roof from lifting during high winds if the roof happened to stay open during failure of the wind sensor.



The image below shows the other side of the mechanism with a piece of 'bungie' to help the door stay in contact with the guide wheel when the door is lowered.



=__Control System__=

The original roof motor (modified 12V winch motor) and associated control system installed in April/May 2010 worked fine until I began to integrate the rest of the observatory software systems.

The original design ([|Wide_field_camera_power_control_R3.00.pdf]) employed a modified 12V DC winch motor powered from an inverter using change over relays to control direction. Bypass capacitors and ferrite cores were used as part of the method to reduce RF noise.

This method of control is flawed and causes too much RF to be transmitted. USB ports and devices would drop out randomly during operation of the roof motor, in particular when the motor suddenly changed direction.

This system was stripped out and replaced with an old variable speed drive (VSD) and gearhead motor that I had forgotten about. See Run-off Roof Observatory documentation for the latest drawings. The VSD has additional controls over the motor that the DC system did not have. But it also had some disadvantages:-


 * VSD Advantages:-**
 * Control over start up and run speeds
 * Acceleration and decelleration speed control
 * Injection braking control
 * Current limiting and overload protection
 * Quite in operation compared to DC winch motor and associated gear noise


 * VSD Disadvantages:-**
 * More complex control with increased risk of failure
 * More panels, cabling equipment
 * Microprocessor/control failure could leave the roof open with no backup to close
 * Increased cost

The control system was bench tested and installed early June 2010. Documentation has been updated to reflect the installation One part of the system I pondered over a LOT was the design of the roof limit switches. Industrial limit switches are located at either end of travel to control the open and close positions. At the moment (dome included) there is only ONE limit switch to prevent over travel. We intend to install TWO limit switches at either end of travel to provide redundancy in the event of mechanical or electrical failure.


 * The primary hazards are:-**
 * The closed limit switch fails to operate and the motor keeps turning the drive wheels in a static position on the aluminium rails. The wheels do 'slip' on the aluminium rails at the end of travel.
 * The opened limit switch fails to operate and the motor keeps turning the drive wheels in a static position on the aluminium rails. The wheels do 'slip' on the aluminium rails at the end of travel.


 * Consequences are:-**
 * Excessive wear of the drive wheels and/or tripping the supply on overload of the motor/VSD unit.


 * There were two options de-selected. These were:-**
 * 1) Limit switches for position control and ultimate limit switches for safety by means of cutting the supply to the controller.
 * 2) Limit switches for position control and control of the supply to the controller from an ‘enable’ contactor. This ‘enable’ contactor could be on a timer to ‘limit’ the supply to the controller.


 * Solution implemented:-**
 * The new design has an 'enable' contactor turning the supply off to the VSD during quiescent periods.
 * Mechanical stops have been installed to prevent the roof from ever 'falling off' the end of travel.


 * Primary failure modes and consequences identified are:-**
 * Loss off mains supply enabling the UPS to the variable speed drive. The likelihood of this happening was considered low. The limit switches would be detected by the VSD and stop the roof motor.
 * Loss off mains supply enabling the UPS to the variable speed drive AND failure of the ‘enable’ contactor. The likelihood of this happening was considered VERY low. In this case the motor would remain running in a static position until the supply was isolated manually.As has been stated above, the drive wheels do slip on the aluminium rails at either end of travel.
 * Variable speed drive failure leading to the limit switches not being acted upon. The likelihood of controller failure is low. In this case the motor would stop after the ‘enable’ contactor was de-energised.
 * Variable speed drive failure leading to the limit switches not being acted upon AND failure of the ‘enable’ contactor. The likelihood of controller failure and failure of the ‘enable’ contactor to remove the supply is considered VERY low. In this case the motor would remain running until the supply was isolated manually.

There are other modes such as gears falling off, wires falling off and motor failure etc, but these cannot be accounted for in this design. Only good installation and regular inspections and maintenance can help reduce these sorts of failure mode. Gears are positively retained and should not fall off.


 * Hazards and consequences associated with the roof not closing are:-**
 * Exposure of the optics to sunlight causing damage to the CCD camera and possible fire.
 * Damage to the telescope, equipment and observatory from water.
 * Protective devices should isolate the supply before an electrical fire propagates.

Option 2 was chosen for simplicity, practicality of installing additional limit switches (no room and roof closed does not give travel to allow for ultimate switches) and likelihood of failure of components. The installation required extensive modifications to all panels and wiring. In addition, the motor required a new interface and gear coupler. Approximately 70% of the design had to be modified due to the change in a 12V DC motor system to a 230V VSD motor system.


 * Emergency stop:-**

An emergency stop is provided to stop the roof and telescope if required. The category of the emergency stop is ‘Category 1’. This category was chosen for simplicity. The system originally had a category 2 emergency stop system, but this was removed as the roof would not close in the event of power failure. The level of ‘risk’ was determined and deemed safe at category 1 for the purpose of this installation. The observatory is generally unmanned unless there is a fault or maintenance required. A useful document to understand the basics of the emergency stop and their categories can be seen here:-

[|http://www.hs-compliance.com/uploaded/documents/THE%20EMERGENCY%20STOP.pdf]

See the Run-off Roof Observatory documentation for drawings and schematics of the control system.


 * IMPORTANT SAFETY NOTES:-**
 * The door cannot be stopped part way open or closed unless the emergency stop is operated. This is located next to the control computer.
 * The gear train mechanism for the drive is VERY dangerous and may cause namputation of fingers if not isolated for maintenance BEFORE the protective covers are removed.
 * Isolate the mains at the main distribution panel and switch off the UPS nsupply at the main control panel.


 * The roof can be opened via:-**
 * A command is sent from the computer via the observatory auxiliary control screen to open.
 * ACP, either manually or via an ACP plan after the mount is powered up
 * Cartes du Ciel from a GOTO command after the mount is powered up
 * Manually from the open/close switch on the main control panel so long as the enable contactor is energeised.


 * The roof can be closed via:-**
 * BAD weather status from the observatory auxiliary system
 * BAD weather direct from the weather station hard wired
 * A command is sent from the computer via the observatory auxiliary control screen to close.
 * Parking the telescope in ACP (either manually or via script fail initiating the park script)
 * Parking the telescope in Cartes du Ciel
 * Power failure to the observatory initiating UPS intervention
 * Manually from the open/close switch on the main control panel so long as the enable contactor is energeised.
 * Axis limit activated


 * The roof will not open if:-**
 * Power to the observatory is lost and the door is closed
 * The main isolator on the power control panel is off and the roof is closed
 * BAD weather status from the observatory auxiliary system is reported
 * BAD weather direct from the weather station hard wired interlocks the control system
 * An Axis limted has been activated

=__Weather safety__=

The obsevatory is interlocked for weather safety to the weather station, see: Observatory control room and weather station =__Observatory control computer__=

The observatory control computer is an old Dell laptop that has all applications running including:-
 * ACP
 * MaximDL
 * Cartes Du Ciel
 * Observatory Auxiliary Control
 * Filezilla
 * VNC
 * Web cam



A protective cover (foam sheet yetto be added for heat retention) has been added to prevent dew build up in the winter. This folds down over the laptop when closed. The laptop is configured so that 'nothing happens' when lid is closed except the screen is not illuminated.

=__Materials__=

The main frame is made from 38 x 65mm planed wood 2.4M long and cut to suit.

The sides and roof are covered in 5mm PVC white foam.

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Product code: 05PALITESSWH (1220 x 2440 sized sheets for reduced cost)

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The aluminium rails are 3 x 3 inch from Aalco in liverpool.

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Most other materials are from B&Q and local hardware supplier TE Huges and Sons.

Total cost to date (not including the telescope, mount or CCD) is approximately £800

Stainless fixings and screws have been used were ever possible outside.

Plastic DPCmaterial has been used for the skirts. This is cheap and durable.