Sloan Digital Sky Survey

Observing Operations | Reviews | Survey Management

Interlocks For the 2.5 Meter SDSS Telescope

Fermi National Accelerator Laboratory


Revision    History                      Date 



  1.0     Initial FNAL                 October 29, 1996

  1.1     Modified Motor Amplifier     October 30, 1996

  1.2     FNAL Comments                November 3, 1996

  1.3     Final FNAL Comments          November 4, 1996

Abstract

This document describes the proposed interlock system for the SDSS telescope. Included are the minimum set of initial low level equipment protection interlocks along with the expected high level interlocks.

Purpose

The purpose of the interlock system is to lower the risk of personnel injury and to protect the telescope, instruments, and related equipment near the telescope. The function of the interlock system is to monitor telescope behavior and inhibit unsafe actions. The interlock system will not provide control to any system.

Overview

The 2.5 Meter Sloan Digital Sky Survey telescope is being assembled at Apache Point Observatory, New Mexico at an elevation of 9200 feet. There are several issues related to the interlock system due to the location of the telescope.

First, the personnel designing and installing the interlock system will not permanently reside at the observatory. This brings up the issue of reparability. The interlock system needs to be repairable by the observatory staff. To facilitate this, complete and accurate documentation will be provided to the observatory. The minimum documentation set includes a complete written description, wiring diagrams, component parts list with manufacturer and model number, maintenance and testing procedures, and component bypass procedures.

Second, the system needs to be modular. All components in the system need to have spares available at Apache Point. This will help reduce the mean time to repair in the event of component failure. Observatory personnel will have the ability to easily replace modules at night.

Third, some elements within the system need to be easily bypassable. In the event of a switch failure for example, the switch should be easy to remove from the circuit to allow safe storage of the telescope until repairs are complete. The bypassing of an interlock must not be taken lightly. Large amounts of equipment must be operational just to stow the telescope. Only trained observatory personnel should be allowed to override an interlock.

Fourth, high level interlocks will be provided by a programmable logic controller (PLC). The ability for observatory staff to modify PLC code is essential. There will inevitably be times when a motor tachometer trip setting for example, needs to be adjusted slightly for normal operation.

Fifth, although not imperative, the ability for system design personnel to monitor the interlock system remotely would be highly desirable. This would allow interlock personnel at Fermilab to monitor and assist observatory personnel remotely. At a minimum, observatory personnel will be able to monitor interlock system status in the observatory control room.

Sixth, the observatory is highly susceptible to lightning. It is expected the telescope enclosure or control room may be struck by lightning occasionally. Protection of the interlock system from lightning induced voltage and current spikes is essential. All cabling between buildings will be fiber optic.

Seventh, are the environmental factors. The telescope is operated outside throughout the year. Equipment utilized must withstand temperature ranges of 0 to 100 degrees F. Switches mounted on the outside of the telescope and cabling need to be sealed. Although the possibility of water getting to the switches is remote, equipment may frequently be challenged by rapidly changing temperatures, high humidity, air-borne dust, and moth infestation.

LOW LEVEL INTERLOCKS

These interlocks are implemented in hardware as critical motion controls. Low level interlocks should be in place prior to motorized control of the telescope.

Emergency Shutdown

There will be emergency shutdown push button switches located in the SDSS control room, in the telescope equipment room, on the telescope, on the telescope platform, and in the telescope enclosure. These switches will have the ability to be illuminated for easy visibility where desired. The switches will be alternate action type switches. Pushing the emergency stop switch will disable the system; the tripped switch will have to be pulled out manually to re-enable the system. The activation of one of these switches will stop all telescope and windscreen motions. This is deemed as a panic disablement. A controller reset should be required following any emergency shutdown before normal operation is resumed. The switches will be in series with the motor control amplifier output signal path removing all power from the motors.

The switch in the SDSS control room area will need to communicate to the telescope via fiber optic cable. In the event of a power outage, the interlock system will initiate an emergency stop command due to loss of communications with the control room. If this is a local power outage, the switch may be manually overridden at the telescope to allow for stowage.

Azimuth Limits

There will be three levels within the interlock system to limit the range of motion for the azimuth drives. The first will be a total rotational inhibit unless the altitude of the telescope is above 15 degrees and the elevation hard stop is engaged. This is to prevent the telescope from being driven into the surrounding guard rail.

The second will be a soft inhibit to the drive amplifiers. The inhibit will occur at + 280 degrees from center. This will allow the telescope to be driven back toward its normal operating range.

The third will be a hard drive inhibit removing power from the motors. This inhibit will occur at + 290 degrees from center. The telescope will have to be manually moved off the inhibit either by overriding the interlock locally at the telescope or by winching the telescope off the inhibit.

Altitude Limits

There will be three levels within the interlock system to limit the range of motion for the altitude drives. The first will be a soft inhibit to the drive amplifiers. The inhibits will occur at +25 and +95 degrees. This will allow the telescope to be driven back toward its normal operating range.

The second will be a hard drive inhibit removing power from the motors. This inhibit will occur at + 20 and +100 degrees. The telescope will have to be manually moved off the inhibit either by overriding the interlock locally at the telescope or by winching the telescope off the inhibit.

The third will be hard stops attached to the windscreen. The hard stop will prevent the windscreen from driving off the capstan drives. Deceleration dash pots will be used to soften the hard stop impact.

During stowing and unstowing, the MCP will have to provide "intent" of operations to the interlock system. Once the azimuth stow position has been reached, the interlock system will bypass the +25 and +20 degree inhibits to complete the stow or unstow operation.

At the time of this writing, the actual allowable range of altitude motion was not known. The inhibit points will have to be further examined for their validity.

Windscreen Limits

There will be collision detection switches mounted on the telescope to detect telescope to windscreen collisions. These switches will inhibit both telescope and windscreen motions in the axis of the collision. The windscreen altitude brake will be asserted whenever the windscreen altitude drives are disabled for any reason.

Instrument Rotator

There will be two levels within the interlock system to limit the range of motion for the instrument rotator drive. The first will be a soft inhibit to the drive amplifiers. The inhibit will occur at + 280 degrees from center. This will allow the instrument rotator to be driven back toward its normal operating range.

The second will be a hard drive inhibit removing power from the motors. This inhibit will occur at + 290 degrees from center. The instrument rotator will have to be manually moved off the inhibit either by overriding the interlock locally at the telescope or by winching the instrument rotator off the inhibit.

Building Controls

The current building controls need to be modified to detect the position of the Southwest overhead door. This door has the possibility of running into the telescope if not opened completely. The addition of a detection switch to sense the door position is required.

An additional modification needs to be done to stop door closing operations. There is a point in the operation of closing the door where the process can not be stopped or reversed. Modifications to the door controls will allow the door closing to be terminated at any point in the process and reversed by pressing the open button.

HIGH LEVEL INTERLOCKS

There is a point at which the implementation of relay logic is no longer practical. Elements in this category require intelligence to detect and monitor specific states or the difference between states. These high level interlocks will be provided by a programmable logic controller.

Sensor Interface

Wherever possible, independent sense monitoring switches will be used by the MCP and the interlock system to prevent common mode faults. However, in most cases the use of independent sensors is prohibited by space constraints and the sheer number of sensors required. All sensor outputs will be routed to a sensor interface chassis. This chassis will split sensor outputs to the MCP and the interlock system. Visual indicators for each sensor will be provided to assist in fault diagnosis.

Motor Amplifier

There are five motor drive amplifier control chassis for the telescope. Two each for the altitude and azimuth axis and one for the instrument rotator.

Each motor drive amplifier control chassis will provide an input signal to the interlock system indicating all internal motor drive amplifier interlocks are functioning correctly. This will be a closed relay contact closure when the amplifier control chassis is ready for motion commands.

Three different permit signals are required by each motor drive amplifier control chassis from the interlock system to allow for motion. One signal allows the drive amplifier to connect the motors to the control amplifier. One signal is used for forward motion and one is used for reverse motions. These signals are +24vdc supplied when motion is allowed.

The control chassis will also provide analog current and voltage information to both the MCP and interlock system. These signals will be used to compute motor winding temperature. The interlock system will shut down the axis drive motors if the winding temperature in the motor exceeds 130 degrees F.

Azimuth Limits

Slew rate differential limits will be used to detect capstan drive slippage. Tachometers or encoders will be used to monitor each drive motor and the drive disk rotation. The drive disk velocity will be scaled by a factor of 25 to equal motor capstan velocity. An allowable error band will need to be established.

Additionally a maximum velocity will be established at 3.5 degrees per second. A detected error will remove power from the motors.

Altitude Limits

Additionally a maximum velocity will be established at 3.5 degrees per second. A detected error will remove power from the motors.

Instrument Rotator Limits

Additionally a maximum velocity will be established at 3.5 degrees per second. A detected error will remove power from the motor.

Building Controls

There are two concerns requiring high level controls for the building. First, the telescope must be in the stowed position before moving the building over the telescope. Conversely, the building must be off of the telescope prior to moving the telescope. This requires communications to exist between the telescope and the building. The major problem is that the design of the building does not provide a way to extend copper wires from the telescope to the building.

The proposed approach would be to only monitor if the building is off the telescope. This will be accomplished with proximity sensors located on the safety railing outside the telescope enclosure. This would prevent telescope motions if the building is present. The use of multiple proximity sensors will allow for voted detection of the building, minimizing false trips due to environmental conditions.

The operator will need to insure the telescope has been stowed prior to moving the building over the telescope. Requiring an operator to stow the telescope prior to moving the building seems to be reasonable compared to the complexity of providing communications from the telescope to the building. There will be no system to inhibit running the building into the telescope.

Instrument Change

There are two main types of instruments for the telescope. The first is an imaging camera and the second is a spectrograph. Both of these instruments are moved to the telescope on instrument carts. The instruments are then lifted in place by an hydraulic lift and latched into place. There are numerous scenarios where the wrong type of cart could be used or an instrument unlatched at the wrong time and dropped. To assist in the prevention of damage to the instruments and telescope, the interlock system will monitor actions initiated by the MCP for correct operation and inhibit inappropriate actions.

Before an instrument cart with or without an instrument on it can be moved into place, the telescope must first be placed in the instrument change position. Once the altitude, azimuth, and instrument rotator are in the correct location, the telescope is locked in place and the drive motors are disconnected from the control amplifiers. The telescope remains in this position until the instrument change is complete.

Telescope Instrument Detection

The telescope has three sets of four switches located at the kinematic instrument mount points. These switches are used to detect the presence of an instrument and the instrument type through binary encoding. Three of the switches are normally open and the fourth is normally closed. The normally closed switch is used to detect a disconnected cable. The remaining three normally open switches are used to decode the instrument type. In the following table, a 0 indicates an open switch and a 1 indicates a closed switch. This translates to the actual logic levels seen by the controls and interlock system from Sensing Device Position and ID for Devices Mounted on the SDSS; Steve Bracker, SDSS Controls Mailing List Message 27.


Table 1.

Telescope Instrument Identification Switch Encoding



                             S1      S2      S3

ID  Mounted Device          1234    1234    1234



0   No Device               0001    0001    0001

                                        

1   Imager CCD Camera       1100    1000    1010

2   Corrector Lens Handler  1100    1000    1100

3   Collimation Camera      1000    1000    1110

                                        

4   Other Device            1100    1010    1000

5   Other Device            1000    1010    1010

6   Other Device            1000    1010    1100

7   Other Device            1100    1010    1110

8   Other Device            1100    1100    1000

9   Other Device            1000    1100    1010

10  Other Device            1000    1100    1100

11  Other Device            1100    1100    1110

12  Other Device            1000    1110    1000

13  Other Device            1100    1110    1010

14  Other Device            1100    1110    1100

15  Other Device            1000    1110    1110

                                        

16  Fiber Cartridge #1      1110    1000    1000

17  Fiber Cartridge #2      1010    1000    1010

18  Fiber Cartridge #3      1010    1000    1100

19  Fiber Cartridge #4      1110    1000    1110

20  Fiber Cartridge #5      1010    1010    1000

21  Fiber Cartridge #6      1110    1010    1010

22  Fiber Cartridge #7      1110    1010    1100

23  Fiber Cartridge #8      1010    1010    1110

24  Fiber Cartridge #9      1010    1100    1000

25  Fiber Cartridge #10     1110    1100    1010

26  Fiber Cartridge #11     1110    1100    1100

27  Fiber Cartridge #12     1010    1100    1110

28  Fiber Cartridge #13     1110    1110    1000

29  Fiber Cartridge #14     1010    1110    1010

30  Fiber Cartridge #15     1010    1110    1100

31  Fiber Cartridge #16     1110    1110    1110

There are six switches to detect the presence of the spectroscopy lens, one normally open and one normally closed at each of the three mounting points.

There are six switches to detect the presence of the imager saddle, one normally open and one normally closed at each of the three mounting points.

There are three primary and three secondary camera pneumatic latches with sense switches to detect latch open and latch closed positions. The primary latches are also used to mount the fiber cartridge to the telescope. The secondary camera latches are also used to mount the spectroscopy lens.

There are two camera saddle latches again with sense switches for both the open and closed positions. The saddle latches hold the camera saddle to the camera. The saddle latches are not capable of holding the camera and saddle to the telescope.

The two spectrographs are permanently mounted to the bottom of the telescope and are not routinely removed. Each spectrograph provides slithead door open or closed position information and slit head latch open.

Instrument Lift

The instrument lift will normally be controlled by the MCP. An emergency override will be provided for personnel safety reasons. The MCP control signal will pass through the interlock system for concurrence of motion. Two types of monitoring will be provided by the lift, absolute position and load force. Both the lift position and the load cell will be monitored by the interlock system. This is to protect the camera, fiber cartridge, and spectroscopy lens from excessive mounting force by the lift. The interlock system will also insure a minimum mounting pressure before allowing the instrument latches to be engaged.

Four proximity sensors mounted on the instrument lift plate detect the presence of an instrument or parts of an instrument on the cart. These sensors will be binary encoded similar to the instrument mount position switches. Three sensors will detect the instrument type or parts of an instrument. The fourth sensor will be used to detect correct contact with the cart floor. In the following table, a 0 indicates an open switch and a 1 indicates a closed switch.


Table 2.

Instrument Lift Identification Switch Encoding



                                           Switch

          ID  Mounted Instrument            1234



          0   Lift Down                     0000

                        

          1   Cart Detected, No Instrument  0001

          2   TBD                           0011

          3   TBD                           0101

          4   TBD                           0111

          5   TBD                           1001

          6   TBD                           1011

          7   TBD                           1101

          8   TBD                           1111

Camera Operations Cart

The imaging camera cart is moved into place by an operator and locked in place. The presence of the cart is detected by a proximity sensor. This sensor conveys the cart type as the operations camera cart.

Camera Service Cart

The camera service cart is similar to the operations cart except the saddle is not placed on the service cart. Again a proximity sensor indicates the presence of the service cart and the cart type.

Camera

The camera has an internal sense switch to detect that the CCD array has been lifted off its kinematic mounts. This operation is only valid at the instrument change position.

Spectrograph Cartridge Cart

The spectrograph cartridge cart has a proximity sensor to detect each of the cartridge cart position transfer locations. The proximity sensor is activated when the cart is locked in place by the operator.

Sensor Table

The following table lists the sense switches used for instrument mount and dismount logic. The mnemonic corresponds with the PLC program symbol.


mnemonic                     Description



Alt_Chg                 Altitude at instrument change position

Az_Chg                  Azimuth at instrument change position

Rot_Chg                 Instrument rotator at instrument change position

Wdn_Lock                Wind screen altitude lock

Az_Lock                 Azimuth lock

Alt_Mtr_Inh_1           Altitude Motor 1 Inhibit

Alt_Mtr_Inh_2           Altitude Motor 2 Inhibit

Az_Mtr_Inh_1            Azimuth Motor 1 Inhibit

Az_Mtr_Inh_2            Azimuth Motor 2 Inhibit

Rot_Mtr_Inh_1           Instrument Rotator Motor 1 Inhibit



Inst_M1_1               Instrument mount position 1 switch 1 NO

Inst_M1_2               Instrument mount position 1 switch 2 NO

Inst_M1_3               Instrument mount position 1 switch 3 NO

Inst_M1_4               Instrument mount position 1 switch 4 NC

Inst_M2_1               Instrument mount position 2 switch 1 NO

Inst_M2_2               Instrument mount position 2 switch 2 NO

Inst_M2_3               Instrument mount position 2 switch 3 NO

Inst_M2_4               Instrument mount position 2 switch 4 NC

Inst_M3_1               Instrument mount position 3 switch 1 NO

Inst_M3_2               Instrument mount position 3 switch 2 NO

Inst_M3_3               Instrument mount position 3 switch 3 NO

Inst_M3_4               Instrument mount position 3 switch 4 NC



Spec_M1_1O              Spectroscopy Lens Mount position 1 NO

Spec_M1_1C              Spectroscopy Lens Mount position 1 NC

Spec_M2_1O              Spectroscopy Lens Mount position 2 NO

Spec_M2_1C              Spectroscopy Lens Mount position 2 NC

Spec_M3_1O              Spectroscopy Lens Mount position 3 NO

Spec_M3_1C              Spectroscopy Lens Mount position 3 NC



Pri_Latch_Cont          Primary Latch Control Signal

Pri_Latch_1O            Primary Latch 1 Open

Pri_Latch_1C            Primary Latch 1 Closed

Pri_Latch_2O            Primary Latch 2 Open

Pri_Latch_2C            Primary Latch 2 Closed

Pri_Latch_3O            Primary Latch 3 Open

Pri_Latch_3C            Primary Latch 3 Closed



Sec_Latch_Cont          Secondary Latch Control

Sec_Latch_1O            Secondary Latch 1 Open

Sec_Latch_1C            Secondary Latch 1 Closed

Sec_Latch_2O            Secondary Latch 2 Open

Sec_Latch_2C            Secondary Latch 2 Closed

Sec_Latch_3O            Secondary Latch 3 Open

Sec_Latch_3C            Secondary Latch 3 Closed



Sad_M1_1O               Saddle Mount position 1 NO

Sad_M1_1C               Saddle Mount position 1 NC

Sad_M2_1O               Saddle Mount position 2 NO

Sad_M2_1C               Saddle Mount position 2 NC

Sad_M3_1O               Saddle Mount position 3 NO

Sad_M3_1C               Saddle Mount position 3 NC



Sad_Latch_Cont          Saddle Latch Control

Sad_Latch_1O            Saddle Latch 1 Open

Sad_Latch_1C            Saddle Latch 1 Closed

Sad_Latch_2O            Saddle Latch 2 Open

Sad_Latch_2C            Saddle Latch 2 Closed



Slt_Door_1O             Slithead Door 1 Open

Slt_Door_1C             Slithead Door 1 Closed

Slt_Door_2O             Slithead Door 2 Open

Slt_Door_2C             Slithead Door 2 Closed

Slt_Latch1_Cont         Slithead Latch 1 Control

Slt_Latch2_Cont         Slithead Latch 2 Control

Slt_Latch1_O            Slithead Latch 1 Open

Slt_Latch2_O            Slithead Latch 2 Open



Lift_Position           Analog Value for Lift Elevation Position

Lift_Load               Analog Value for Lift Load Weight



Imm_Ops_Cart            Imager Operations Cart in Place

Imm_Ser_Cart            Imager Service Cart in Place



Cart_Cart_Pos1          Spectrograph Cartridge Cart in Position 1

Cart_Cart_Pos2          Spectrograph Cartridge Cart in Position 2



CCD_Aray_Up             CCD Array lifted off kinematic mounts



Lift_Plate_1            Lift Plate Instrument Detect Switch 1

Lift_Plate_2            Lift Plate Instrument Detect Switch 2

Lift_Plate_3            Lift Plate Instrument Detect Switch 3

Lift_Plate_4            Lift Plate Instrument Detect Switch 4

Instrument Mount Dismount Logic

Imager operations mount / dismount

To be added

Imager service mount / dismount

To be added

Fiber Cartridge mount / dismount

To be added