1 Instrument Control System

 

1.1 Changes Since CoDR

 

1.1.1 Sensor Support Board

 

The NIRI Sensor Support Board (SSB) has been redesigned for use within NIFS. The redesign was necessary primarily because of the unavailability of Burr Brown INA141 instrumentation amplifiers in a DIP package – they can now only be purchased in a SMD package. The unavailability of up to date editable versions of NIRI schematics and PCB designs also influenced this decision.

 

The SSB now incorporates a full surface mount design as well as a number of design changes to improve integration into the control system. The general configuration is essentially the same as the NIRI SSB, with 13 Hall effect sensor amplifier channels available per board (a limit imposed by the 55 pin circular Hall effect sensor input connectors). Provision for thermistor support has been removed since these are not used on NIFS. There are now four instead of six home signal comparator channels per SSB due to the reduced requirements of NIFS. Cross-point switches have been added to allow the connection of any Hall effect sensor channel to any of the comparator channels, as well as any of the A/D converter channels. This allows the front panel cabling arrangements to be simplified by daisy chaining successive SSB boards and their connections to the XVME-566 A/D board as well as the Phytron Backplane.

 

The new SSB also has a reduced power requirement, and the on-board power supply arrangements have been optimized accordingly.

 

1.1.2 Phytron Backplane

 

The unavailability of editable versions of NIRI schematics and PCB designs has necessitated regeneration of the Phytron Backplane design documentation. Only minor changes to accommodate daisy chained cabling from the SSBs have been made.

 

1.1.3 NIFS Drive Mechanism Changes

 

Minor changes to the control system layout have been necessary to incorporate the different drive mechanisms used in NIFS. Changes are basically limited to the location of Hall effect sensors and their cabling. Control of the new friction controlled drives can be accommodated by the control system without major modification. Position calibration accuracy is expected to be adequate, but Hall effect sensors will be included on the first gear of each gear train to allow for higher precision calibration.

 

1.1.4 Detector Thermal Control

 

Tests on the spectrograph detector temperature controller have shown that temperature control of the detector thermal mass to the millikelvin level is feasible.

 

1.2 Overview

 

The NIFS instrument is a complex arrangement of optical elements, mask wheels, filter wheels, gratings, and sensors which need to be thermally controlled as well as configured with high precision in order to assure maximum scientific potential and versatility. It is the function of the Instrument Control System (ICS) to control these operations. Broadly speaking, the ICS may be thought of as two separate control systems operating under the same VME single-board processor. The detailed operation of the Components Controller (CC) and the Temperature Controller (TC) is described in this section.

 

NIFS will use the same motors and encoding philosophy as NIRI. This allows the NIFS ICS to be nearly a direct copy of the NIRI ICS. This section describes the operation of the NIFS ICS, and highlights areas where the NIFS ICS will differ from that used in NIRI.

 

1.3 Components Controller  Electronics

 

The Components Controller is responsible for moving and positioning all of the optical elements associated with the NIFS cryostat. There are two major assemblies within the cryostat; the OIWFS and the NIFS spectrograph. Externally there is also an Environmental Cover.

 

All movable elements within the NIFS cryostat can be categorized as:

  1. rotary positioned,
  2. linearly positioned, or
  3. open/closed (or in/out).

 

The NIFS CC will control and position eight active elements. Table 60 shows a list of NIRI mechanisms with their corresponding NIFS mechanisms and the mode of actuation.

 

Table 60: NIRI and NIFS Motor-Controlled Mechanisms and their Mode of Actuation.

NIRI Mechanism

NIFS Mechanism

Actuation

OIWFS X-Axis Gimbal

OIWFS X-Axis Gimbal

Linear

OIWFS Y-Axis Gimbal

OIWFS Y-Axis Gimbal

Linear

OIWFS Filter Wheel

OIWFS Filter Wheel

Rotary

OIWFS Focus

N/A

 

Imager Focal Plane Mask wheel

Spectrograph Focal Plane Mask Wheel

Rotary

Entrance Beam Splitter

N/A

 

Imager Pupil Mask/Filter Wheel

Spectrograph Flip Mirror

In/out

Imager Pupil Viewer

Spectrograph Grating Drive

Rotary

Imager Filter Wheel #1

Spectrograph Order Blocking Filter Wheel

 

Imager Filter Wheel #2

N/A

 

Imager Focus Stage

N/A

 

Imager Beam Steerer #1

N/A

 

Imager Beam Steerer #2

N/A

 

Environmental Cover

Environmental Cover

Open/closed

 

 

1.3.1 Control Strategy

 

An open loop scheme using stepper motors and dead reckoning is employed using Hall effect sensors to provide "home" or "datum" positions. Once a zero-point is established, the number of steps taken by the stepper motor determines position.

 

Hall effect sensors consist of a small block of semiconductor material, such as indium, sandwiched between two electrodes. They produce a weak DC voltage, typically 10 millivolts per kilogauss across the electrodes, in the presence of a magnetic field. The polarity of the potential difference is a function of the "pole orientation" of the magnet field (north-seeking or south-seeking).

 

The position monitoring and calibration system will use stationary Hall effect sensors and small position-indicating magnets mounted on specific moving parts of all stepper motor driven optical component mechanisms. Additional (redundant) Hall effect sensors, which are software selectable, will be incorporated to improve reliability.

 

1.3.1.1 Rotary Positioned Elements

 

The output from the Hall effect sensor, as the magnet passes under it, rises to a peak as more magnetic flux passes through the area of the Hall effect sensor and then declines again as the magnet moves away (Figure 113).

 

Figure 113: Hall effect sensor output voltage versus magnet angle.

 

The "Home" position is found by finding the midpoint between two points of identical fluxes (output voltage from the Hall effect sensor). This should provide a great deal of repeatability and be tolerant to sensor sensitivity and DC drifts or offsets introduced by the Hall effect sensor amplifiers, as well as the non-linear wheel motion resulting from the use of Geneva mechanisms in the OIWFS.

 

Redundancy is provided by placing another Hall effect sensor in parallel with the first. Rather than mounting it on top of the primary, it is placed to the side on a slightly different radius. By then adding additional magnets of variable field strengths, a two track system quasi-positioning system can be incorporated with little additional effort. At each mechanism position, the output voltages of the Hall effect sensors are measured and compared with those stored in a calibration table. If these values are out of tolerance, an error is signalled.

 

The position indicating magnets are samarium cobalt rod shaped magnets, which produce a magnetic field intensity of approximately one kilogauss. Intensity will be controlled by adjusting the depth of the mounting hole.

 

Being a rotary system there is no need for limit switches or hard stops to prevent excess travel.

 

1.3.1.2 Linear Positioned Elements

 

The OIWFS X-Axis Positioner and OIWFS Y-Axis Positioner are linearly positioned elements. All linear elements move back and forth on lead screws over a finite travel distance. Position is determined by a combination of Hall effect sensors and magnets (Figure 114) defining coarse and fine positioning.

 

Figure 114: Encoding scheme for linear positioned elements.

 

Coarse position is determined by measuring the flux between a North-South pair of magnets. The opposite polarities of the coarse positioning, or "linear", magnets produce two adjacent peaks, one positive the other negative. Between the peaks there is a ramp, which is (almost) linear (Figure 115). The stage is designed so that it is always operated in the linear region, providing a useful although not extremely accurate encoding of the position stage. Note that any changes in the sensitivity of the sensor electronics or DC offset can affect this encoding scheme. This is adequate, because the encoding scheme needs only to position the stage datum to within one turn of the main gear. A zero-crossing comparator is provided to rapidly find the "Coarse Home" position, the point in the middle of the two magnets where the Hall effect output crosses from negative to positive or vice versa.

 

Figure 115: "Coarse Home" and "Datum" positions for linear positioned elements.

 

Fine positioning is determined by measuring the rotary position of the lead screw in a method similar to that detailed in §7.3.1.1. A single magnet and Hall effect sensor pair is used to seek the peak response while travelling outwards from the "Coarse Home" position. Backlash in the gears is compensated for by travelling in only one direction. This lead screw peak position is used as the "Home" or "Datum" position for all further screw displacements (Figure 115).

 

Multiple positions for a single magnet are milled into the lead screw spur gear so the choice of "Home" position is made to provide the most versatile "Home" position in relationship to the "Coarse Home" which may move slightly due to DC drift. This is determined empirically.

 

For redundancy, all Hall effect sensors operate in pairs, mounted in parallel with a software switch choosing between primary or backup sensor. Should one Hall effect sensor fail, the computer will switch to the backup sensor. There will be a slight difference in peak position because it is not possible to ensure that the sensors are at the same angle with respect to the axis. This must be compensated for in software by recalibration.

 

The OIWFS Gimbal mechanism includes limit switches to protect it from damage in the case of over travel. The use of limit switches in cryogenic conditions may not be very reliable.

 

1.3.1.3 Open/Closed Elements

 

The Environmental Cover is an open/closed element. The cover is a two vane sliding cover, driven by a flexible belt with limit switches signalling the end of travel (Figure 116).

 

Figure 116: Environmental Cover assembly drawing.

 

In the "Closed" position, the cover should be light tight and protect the cryostat window from dirt, dust, and moisture likely to be encountered during storage or shipping. In the "Open" position during operation, the cover should be retracted and provide an unobstructed path for light to enter the cryostat.

 

An Ioniser Sprayer Head (item 14 in Figure 116) is separate to the mechanism for opening and closing the Environmental Cover but part of the same sub-assembly. This head is responsible for venting de-ionized, dry, non-turbulent air over the cryostat window in order to prevent moisture condensing during astronomical observations.

 

A Cleaning Spray Bar (item 8 in Figure 116), is also part of the Environmental Cover sub-assembly. It uses a high-pressure jet of dry air to clean the lower vane and cryostat window of large detritus. The control system provides power to the Ioniser Sprayer Head and controls the operation of the Cleaning Spray Bar.

 

1.4 Temperature Controller

 

The Temperature Controller is responsible for temperature regulation and temperature control during cool-down and during warm-up. The NIFS cryostat consists of three main thermal sub-assemblies; the OIWFS detector, the spectrograph detector; and the Cold Work Surface (CWS) plate.

 

The CWS plate and OIWFS detector sub-assemblies are regulated by two Omega CYC321 Temperature Controllers. The spectrograph detector temperature has to be regulated to the milliKelvin level (§6.5.8). NIFS uses a Lake Shore Model 340 Temperature Controller for the spectrograph detector. This is a dual PID controller. The primary control loop will control the detector thermal mass. The secondary control loop will control the detector housing. The temperature set-point, heater On/Off, curve data, and output data for each temperature controller will be controlled over a RS232 link via a XYCOM XVME-490 Serial I/O board located in the CC VME crate.

 

Additional temperature sensors will be mounted at various locations within the cryostat for use during Accelerated Warm-up and for thermal diagnosis. These sensors are monitored by an Omega CYD208 8-channel Digital Thermometer located in Drawer 2 of the CC thermal enclosure. The CYD208 relays temperature readings via RS-232 to the XYCOM XVME-490 Serial I/O board located in the CC VME crate.

 

The two Leybold Coolpower 130 closed-cycle helium cryocoolers are attached with cold straps to internal mechanisms. Varying the motor speed within each cryocooler head regulates the rate of cooling. During normal cool-down operation, the speed of the cryocooler will be set at 140 rpm, which is the factory recommended nominal operating speed. During emergency cool-down operation only (minimum cool-down time), the speed will be set to 200 rpm. Once a nominal cryostat operating temperature has been achieved, the speed of the cryocooler will be reduced to a value consistent with normal operation heat-load requirements. It is estimated that reducing the cryocooler speed to ~ 30% of maximum would be a nominal operating speed. The actual value will be determined during NIFS commissioning.

 

Heater sub-systems will be used to control heat flow such that the OIWFS and science detectors are always maintained at a slightly positive temperature differential with respect to their surrounding environments in order to minimize sublimation of any residual matter onto the detector surfaces.

 

The first stages of each cryocooler head, which has the greater refrigeration capacity, are cold-strapped to the CWS plate. The second stage of one cryocooler head, which can achieve a lower temperature, is cold-strapped to the spectrograph detector. The OIWFS detector is cooled by a cold strap to the first stage of the other cryocooler head.

 

Controlled warming of the NIFS cryostat will be accomplished by turning off the coolers and raising the set point of the thermal regulation systems. Accelerated warm-up is possible by diverting power normally dedicated to the stepper motors to additional heater resistors located on the CWS plate. An additional 800 W will become available to provide the accelerated warm-up.

 

The CC IOC controls the temperatures of the CWS plate and the detector thermal masses during the accelerated warm-up operation so that positive detector temperature differentials are maintained to prevent detector contamination. During accelerated warm-up operation, up to one kilowatt of power will be applied to the NIFS cold surfaces under the control of the CC Heater System. The CC utilizes the computer-independent Auto-shutdown and Interlock Safety System to prevent overheating of critical cryostat parts should any of the accelerated warm-up systems fail.

 

Initiation of the accelerated warm-up operation requires active human action. To start an accelerated warm-up operation, the following must be accomplished:

1. A panel circuit-breaker has been set.

2. A panel timer has been set.

3. All cryostat thermostat switches are below 40 C (100 F).

4. A panel spring-loaded normally-off key-switch has been momentarily switched on.

5. UPS and telescope mains power are present.

 

The safety interlock system will:

1. Prevent accidental turn-on of the high power heater system.

2. Require periodic "human verification" that cryostat temperatures are within normal limits.

3. Automatically shut-down the high power heater system if an UPS power failure should occur which could affect the accelerated warm-up operation.

4. Automatically shut-down the high power heater system if cryostat temperatures exceed 40 C (100 F).

5. Provide a computer-independent temperature display of cryostat temperatures so that a human being can verify nominal warm-up operation.

 

In addition to the safety interlock system, the stepper control signals are monitored by the cryocooler Pump Backup Module. This is a custom designed board that monitors the integrity of the cryocooler stepper motor signals. In its basic mode of operation, the Pump Backup Module will take over the generation of the stepper motor signals should they fall out of specification. This protects the cryostat from unexpected warmup in the case of control system malfunctions.

 

During accelerated warm-up, the Auto-shutdown System has the ability to disconnect the mains voltage to the power supply powering the cryostat heaters. Thermal switches, located inside the NIFS cryostat, will trigger the shutdown. The Auto-shutdown System will also be equipped with a timer that must be periodically reset in order to continue the warm-up operation. This part of the Auto-shutdown System will terminate the warm-up operation should weather conditions prevent observatory access. Additionally, the Omega CYD-208 Digital Thermometer will allow crew personnel to visually check the cryostat temperatures.

 

1.5 Thermal Enclosures

 

The CC and DC thermal enclosure layout diagrams are presented in Figure 8 and Figure 9 of §3.10. Their power requirements are listed in Table 26 and Table 27 of §3.11.

 

Figure 117 and Figure 118 show an overview of the CC and its wiring to the cryostat. Figure 119 shows an overview of the DC and its wiring to the cryostat. Figure 120 shows the cabling between the NIFS instrument control system and the Cassegrain Connector Panel.

 

A detailed listing of thermal enclosure internal cabling, cryostat internal wiring, and thermal enclosure to cryostat cabling is contained in Appendix F (§15). Schematics of the internal cryostat wiring are contained in Vol. 2 of the CDR documentation.

 

Figure 117: Components Controller mechanism control wiring.

 

Figure 118: Components Controller thermal control wiring.

 

Figure 119: Detector Controller wiring.

 

Figure 120: Interconnections between the thermal enclosures and the Cassegrain connector panel.

1.6 Manufacture

 

1.6.1 Printed Circuit Boards

 

The printed circuit boards necessary for the NIFS ICS are listed in Table 61.

 

Table 61: Printed Circuit Boards to be Manufactured for NIFS.

Circuit Board Assembly

Number Required

Sensor Support Board

3

IDC64 – KPTO-55 Convertor

3

Phytron Backplane

1

Cryocooler Pump Backup

1

Thermal Isolation Board

8

 

The Sensor Support Boards are based on the boards designed by IfA. A new design was generated primarily because of the unavailability of some components in the required packages as used by IfA. The new design incorporates mainly surface mount components and a modular interconnect design between amplifier, comparitor, and output stages to allow for simplified external wiring.

 

The IDC64 – KPTO-55 Convertor is used to adapt the 64 pin IDC connector from the SSB VME backplane connector to a military style circular connector on the thermal enclosure bulkhead.

 

The Phytron Backplane PCB serves a number of purposes in the ICS. It provides the VME mounting connectors for the Phytron 42-40 Mini stepper motor driver modules. The stepper motor control signals from the Oregon VME8-8 stepper controller modules and home signals derived from the Sensor Support Boards are grouped and routed through DB-37 connectors to the various parts of the cryostat. Separate circuitry supports the routing of the OIWFS Gimbal limit switch signals and the safety interlock system.

 

The Cryocooler pump backup PCB is a board designed by IfA to provide a safety backup for the cryocooler stepper motor controller should it fail in operation.

 

Schematics for the Sensor Support Boards and the Phytron Backplane can be found in Vol. 2 of the CDR documentation.

 

1.6.2 Cabling

 

The NIFS cabling requirements are detailed in Appendix F (§15).

 

1.6.3 Electronics Hardware

 

1.6.3.1 Components Controller Thermal Enclosure

 

The contents of the Components Controller thermal enclosure are listed in Table 62.

 

Table 62: Components Controller Thermal Enclosure Contents.

Crate

Description

Size

1

CC VME Electronics

8U

2

Stepper Motor Drive and Heater Control

9U

3

Cryocooler Drive Electronics

2U

4

Network Communications

4U

 

A diagram of the CC thermal enclosure front panel is shown in Figure 8 in §3.10. Most components will have their own front panel hardware and any open spaces will be covered by blank panels to ensure positive ventilation of the enclosure. The contents of each crate are described in the following sections.

 

1.6.3.1.1 CC VME Electronics Crate

 

The CC VME electronics crate is a 8U, 21 slot ELMA 12V-0920-RV21J21-P750-D VME frame. Eleven of the 21 slots will be occupied by the components shown in Table 63, with the remaining slots covered by blanking panels.

 

Table 63: CC VME Electronics Crate Contents.

Component

No.

Motorola MVME 2700 Single Board Computer

1

Datum Inc. BC635VME Time and Frequency Processor Board

1

Xycom XVME-240 TTL Digital I/O Card

1

Oregon Microsystems VME8-8 Stepper Motor Controllers

2

Sensor Support Board

3

Xycom XVME-566 Analog I/O Card

1

Motorola MVME 761-001 Transition Module

1

Xycom XVME-490-1 Serial Port Card

1

 

 

1.6.3.1.2 Stepper Motor Drive and Heater Control Crate

 

The contents of the 9U Stepper Motor Drive and Heater Control crate are listed in Table 64.

 

Table 64: Stepper Motor Drive and Heater Control Crate Contents.

Component

No.

Phytron ZSO 42-40 Mini Stepper Motor Driver Modules

8

Lambda FE-2000-48-RA 48Vdc 42A Power Supply

1

Phytron Backplane

1

Omega CYC321-01 Temperature Controller

2

Lambda ZUP60-3.5/U 60Vdc 3.5A remote Power Supply

1

 

Note that the Lambda FE-2000-48-RA power supply provides power to both the stepper motors and the thermal control circuit for accelerated warm-up. The Lambda ZUP60-3.5/U power supply is used for CWS plate thermal control.

 

1.6.3.1.3 Cryocooler Drive Electronics Crate

 

The contents of the 2U Cryocooler Drive Electronics crate are listed in Table 65.

 

Table 65: Cryocooler Drive Electronics Crate Contents.

Component

No.

Pacific Scientific 6410-001-N-H-K Microstepping Drive

2

Lambda FE0500-48 48Vdc 11A Power Supply

2

Omega CYD208 Digital Thermometer

1

 

 

1.6.3.1.4 Network Communications Crate.

 

The 4U Network Communications crate will house a Cisco Switch and an NX Terminal Server.

 

1.6.3.2 Detector Controller Thermal Enclosure

 

The contents of the Detector Controller thermal enclosure are listed in Table 66.

 

Table 66: Detector Controller Thermal Enclosure Contents.

Crate

Description

Size

1

DC VME Electronics

8U

2

Lake Shore 340 Temperature Controller

2U

3

SDSU Power Supplies

6U

 

A diagram of the DC thermal enclosure front panel is shown in Figure 9 in §3.10. Most components will have their own front panel hardware and any open spaces will be covered by blank panels to ensure positive ventilation of the enclosure. The contents of the DC VME Electronics crate are described in the following section. The Lake Shore 340 is mounted directly in the thermal enclosure and the SDSU-2 power supplies will be mounted on a shelf.

 

1.6.3.2.1 DC VME Electronics Crate

 

The DC VME electronics crate is a 8U, 21 slot ELMA 12V-0920-RV21J21-P750-D VME frame. Three of the 21 slots will be occupied by the components shown in Table 67, with the remaining slots covered by blanking panels.

 

Table 67: DC VME Electronics Crate Contents.

Component

No.

Synergy SVGM5 Single Board Computer

1

Datum Inc. BC635VME Time and Frequency Processor Board

1

SDSU-2 VME Interface Card

1

 

 

1.6.4 Integration

 

Progressive integration and testing of NIFS will largely be governed by the optical and mechanical design schedule. As mechanisms are manufactured and assembled and ready for testing, the appropriate sub-sections of the control system will need to be ready to test each assembly. It is anticipated that the integration and testing requirements of the control system will take the following schedule:

 

  1. Cryocooler testing: This will require either full stepper driver setup (i.e., Omega VME8-8 Controller, Pacific Scientific Drivers, Lambda power supplies, and associated wiring) or alternatively a simple pulse driver setup. The Omega CYD-208 with temperature sensors and wiring will also be necessary.
  2. Run OIWFS mechanisms on the bench: This will require Omega VME8-8 plus Phytron Backplane and Phytron ZSO 42-40 Drivers with associated cabling to drive stepper motors. Sensor Support Boards will also be needed to test Hall effect sensor operation.
  3. Grating Drive bench test: This will require the stepper drive system plus SSBs. Higher level computer control will be needed to calibrate Hall effect sensors.
  4. Cryostat temperature control: This will require heating resistors plus wiring and Omega controllers. Accelerated warm up will be tested as well.
  5. Complete system integration and test: This will require the entire instrument control system.

 

1.7 Instrument Control System Risks

 

1.7.1 Mechanism Encoding Method

 

Problems with the magnetic position encoding methods were uncovered during NIRI testing. Any NIRI mechanisms that are to be duplicated for use in NIFS must incorporate the NIRI design changes needed to make the encoding methods reliable. The mechanism encoding method represents a small risk until it is demonstrated to perform at a satisfactory level in NIRI.

 

1.7.2 Science Detector Temperature Control

 

Controlling the science detector temperature to 1 mK has been demonstrated effectively in the laboratory. The Lake Shore 340 temperature controller is capable of controlling the detector thermal block to this level when sensors and heaters are mounted carefully. The detector needs to be closely coupled to this block to prevent self-heating effects causing temperature variation at the detector.