TELESCOPE OPERATIONS

The 43m Telescope Upgrade

The NRAO 43m telescope is the first Big Science instrument designed and built for the National Science Foundation. It is the first radio telescope designed for the NSF. The founding of the NRAO and design of the 43m began in the late 1950s. Multitudes of astronomical discoveries were made by this instrument for 4 decades. In the year 2000, the Green Bank (Radio) Telescope was christened, and thus the 43m was shutdown and mothballed.

The 43m telescope is a unique design in many ways. Due to its various complexities, and the era in which it was built, it was always operated by local telescope operators. The servo computer was a 1970's vintage Honeywell 316 computer, the software was in assembly language, and the control system’s input used relay logic from various operator and indicator switches at various places around the telescope. There were also meters, indicator lights and audio feedback. Those inputs, combined with frequent walk-throughs, gave the Operator the ability to monitor the health of the instrument. A seasoned Operator was required 24/7, given the fact that most of the monitor and control was manual. With the advent of the more advanced GBT, the costs to operate the 43m led to shutting it down and moth-balling it. There was always hope, within the NRAO staff, that the 43m would later be revived and re-instrumented so it could be efficiently controlled and remotely operated.

In 2004, Lincoln Labs inquired with the NRAO about the possibilities of using the 43m for research. To make a long story short, the new servo system was begun in early spring of 2005, with the first usable system operational in August. A new control system design was begun in October 2005, with the final integration and testing of the servo and control systems in March 2006. First operation of the 43m telescope, with the two new systems, was in mid-March 2005, with 24/6 operational use beginning the first of May 2006.

The NRAO 43m Servo System

The new 43m servo system is based upon a design for the NRAO 45ft telescope in 2004. Referring to Block Diagrams #1 and #2, the system is basically a personal computer (PC) running Real-Time Application Interface RTAI Linux and an off-the-shelf analog /digital I/O card. Servo software (containing position and rate loops for each axis) is written in C++. Output of the servo system is summed with the feedback velocity of the hydraulics system and output to the power drive printed circuit cards. The summing amplifier and power drive cards are part of the original system. The hydraulically-derived velocity feedback is from LVDT transducers which indicate the position of the hydraulic control arm. Position encoders are the original inductosyn encoders. Interface to the inductosyns are a fiber-optic-based design used by the NRAO on the VLA, designed by Bob Broillo. Design of the 43m encoders is different from the VLA encoders. This led us to modify the driver software to a more understandable and maintainable design.

The 43m hydraulic system can be operated in full-stroke (low rate) or quarter-stroke (high rate) modes. In the past, all observing was done in low rate, while slewing from one source to another was done in high rate. Tracking sources in high rate was seldom, if ever, done. The research the 43m will be doing for Lincoln Labs requires it to track objects as fast as 25 degrees/minute in each axis. The rate and position loops were designed and tuned to be flexible for tracking both low and high speed sources.

Progressing towards the engineering side of things, the position and rate loop parameters can be easily modified by the design engineer at will. Position loops run 50 times/sec, and rate loops run 500 times/sec. All parameters and computed values in the PID loops, with selected variables outside the PID loops, for each axis and each loop at the end of each loop, are stored in a circular buffer. This buffer is 40 seconds long and allows the design engineer to save it, at will, and edit / graph it with spreadsheet tools. The 40-second buffer is time-tagged and stored anytime the servo system detects a fault. These time-tagged files have been invaluable in tuning and debugging the system. The PID loops also allow velocity, acceleration and friction feed forward to be programmed into the loops.

A Linux xterm command line interface was initially built into the servo system to allow the design engineer to easily change parameters, log files, and execute interface commands. This main interface is built into the main servo software task. The key interface commands were also coded into Remote Procedure Calls (RPC) to allow remote users to control the servo system over an Ethernet. Only the main command line interface allows the local engineer to change parameters and perform various Servo tests. The RPC interface allows a remote user to dynamically command and monitor the servo system. Assuming the telescope is in the ready mode, the remote user can release and set the polar and declination gear reducer brakes, monitor critical servo statuses, and issue antenna position commands. Position commands can be issued up to 50 commands per second. The position loop responds to the new position commands within 20ms.

There are three different monitors to monitor the state of the servo system. The main operator interface is a GUI (Graphical User Interface) and will be discussed later in the section on the control system. This interface is used by the Operator to monitor and control the servo and control systems with one seamless tool. The other two interfaces were originally written to help debug and integrate the servo system.

The first monitor we developed is known as the Debug Monitor, which runs as a separate Linux process on the servo computer. This monitor is a simple xterm monitor interface for engineers and support personnel who want to look at data either on or off site. The Debug Monitor interfaces with the servo system through shared memory. For each Debug Monitor screen there is an associated Linux process. The second monitor was written primarily so the Operator could visualize where the telescope is relative to its limits. It was written in Python more for the fun of it than as a required tool. The monitor output is a graphical outline of the 43m limits, with the position and rates plotted as circles, dots and vectors. This allows the Operator to see where the telescope is, where it is going, and its relative speed.

The NRAO 43m Control System

Referring to Block Diagram #1, the control system is comprised of the Main PLC, with 5 remote I/O slave units. The Main PLC is located behind and to the right of the 43m Operators console. The Console I/O unit is located on top of the old 43m Operators console. The rest of the slave I/O units are located in the basement, pump room, north passage and declination house.

The Main PLC controls and monitors the 120 / 240 VAC telescope subsystems via TB105A-J. Most of the sensors and switches that control the 43m are controlled and monitored here. There are a few 24 VDC sensor / switch control and monitor points which also need to be dynamically controlled and monitored by the PLC. These are taken care of by the Console I/O unit. About 95% of the I/O from the Console I/O unit go directly to the Operators console. The remaining I/O go to the remaining few 24-volt indicator switches and the buzzer / Sensaphone units. Strictly speaking, the PLC only needs these I/O points (Main PLC and Console I/O units) to monitor and control the telescope locally. In order to safely operate the telescope remotely, and to monitor trends in the system, we added the remaining remote slave units.

The main PLC design was taken directly from the original relay logic. This logic guarantees if any critical subsystem fails that the telescope will be safely shutdown. One can take the original relay logic schematics and compare them to the PLC ladder drawings and immediately realize the similarity. On top of this design, we added the logic that enables the local console functions to be controlled from GUI screens. (Notice that all the Operator Console switches labeled S-XXX have a corresponding virtual switch VS-XXX. In line with these switches is the Control Transfer switch, which determines if you are in local or remote mode. This allows the Local mode switch to enable the S-XXX switches, and disable the VS-XXX switches when you are in local mode, and vise-versa.)

In the old design, the Operator had to press console switches to allow the Honeywell 316 computer to move the telescope. As an example, the Operator had to release the declination stow brakes, release the polar stow brakes, set the anti-backlash for both axes to on, enable the hydraulics, then release the gear-reducer brakes before the Honeywell 316 could servo the telescope. The same capability is still provided locally, through the original switches, and remotely, through the Console GUI. In addition, another layer has been built in the PLC to allow this sequence of key strokes to be performed by pressing a couple buttons. This capability is brought out to the Operator through the Citect PC GUI screens. (The Citect interface will be elaborated on later in this document.) This software is what we call stage or state programming. It is important to notice that the ladder logic, stage logic, and virtual switches, which replace and simplify the old relay logic, are totally contained in the Main and Console I/O units. At this point, none of the remaining units are used to control the telescope. After we have had time to analyze hydraulic and system trends from the other remote I/O, units we will undoubtedly add some of those monitored parameters into the shutdown circuitry. At this point in time, the remaining remote I/O units are only used for system monitoring and data trending.

The 43m Operator GUI Screens

Referring to Block Diagram #1, we see the Operator Interface / Graphical User Interfaces computer. This computer is a Windows PC and runs the Citect GUI software. It provides GUI screens for the Operator to monitor and control the antenna. The Citect software communicates with the 43m Control System (Main PLC) and the 43m Servo System through PLC memory via the Ethernet. The memory map is located in the file, ServoPlcInterface.xls, and is described later in this document.

MainConsole GUI Screen

The primary Operator interface is through the MainConsole GUI screen, Figure 1. This screen provides the Operator with crucial servo command and feedback data, such as: Present Positions, Command Positions, Position Errors, Command Rates, Present Rates and the Rate Mode for the polar and declination axes. Incorporated into the design for each GUI screen are certain color codes. Red always indicates the telescope cannot be moved because of the state indicated. Green indicates the telescope is free to move because of the indicated state. White is meant to be benign. Yellow is a warning that axes are still allowed to move but the system is in the warning state indicated. Examples include the Stow and Gear Brakes. Red indicates the axis is not able to move. Green indicates the axis is free to move. Present Positions are normally white. Yellow means the commanding computer is commanding them into a limit, but the servo system is inhibiting them from proceeding further into the limit. (This is common when the user is commanding the telescope to wait for a source at the horizon.)

Buttons at the bottom of the MainConsole screen, figure 1, allow the operator control of the telescope. Citect GUI software determines which buttons are valid to be pressed and either grays-out or omits the others. As an example, if the LOCAL CONTROL button is pressed, the only buttons remaining on the screen are LOCAL CONTROL and REMOTE CONTROL buttons. If the AUTO button is pressed, only the AUTO, HALT and LOG buttons are enabled. LOW/HIGH RATE buttons are enabled only in manual, and the gear reducer brakes are set. The STOW, SNOW EAST, SNOW WEST and SERVICE POSITION buttons are enabled only when System Ready is active and the gear reducer brakes are set. Remembering which buttons to press, and when, can be confusing at times. The intelligent GUI screens cut the confusion down and burden the Operator only with necessary and pertinent options.

Main Console Buttons

Remote Control

When this button is pressed, the local console is disabled and the CITECT GUI R virtual buttons are enabled. You must be in REMOTE CONTROL to operate in any type of computer control.

Local Control

When this button is pressed, the local console is enabled, and all computer control is disabled. Computer control is defined as any computer communicating in the system that would normally issue commands and cause the PLC to set/release brakes, or do any simple hardware command. Note that the PLC is always in control and is the computer that processes the LOCAL CONTROL console buttons. It also replaces the original relay logic that controlled the telescope before. LOCAL CONTROL does not prevent the PLC from functioning. It just keeps the PLC from issuing Virtual Switch commands that duplicate the local console switch commands.

System Idle

The SYSTEM IDLE button commands the PLC to take the system to the System Idle mode. System Idle is defined to be when the system pre-charge is on, the ceclination and polar drive motors are running, the stow brakes are set, and the hydraulics are set to bleed.

If the system has been down and pre-charge is up, pressing this button will start up the drive motors, set anti-back to off for both axes, and set the hydraulics to bleed.

If the system is in the Ready state, pressing this button will set the stow brakes for each axis, set anti-backlash to off for both axes and set the hydraulics to bleed.

System Ready

The SYSTEM READY button commands the PLC to take the system to the Ready mode. System Ready is defined to be the same as System Idle, except the stow brakes for both axes are released. Pressing the SYSTEM READY button from System Idle just basically releases the stow brakes. When the telescope is in the Active state, all brakes are released, and the anti-backlash and hydraulics is on. The SYSTEM READY indicator button will be lit but grayed-out. In this state, the servo computer is controlling the movement of the telescope. When the end user commands the system to disable, the gear brakes are set and the system returns to the System Ready mode.

Manual

The MANUAL button is basically the Operator’s Take Control button. This mode allows the Operator to move the antenna to Stow, Snow East, Snow West, or Service positions. It also allows the Operator to enter into the Position Designate mode. While the Operator has control, no other external system can get control. (An engineer can share control with the Operator through the Main xterm window, described previously, but this requires he be located at the servo computer in the local control room.)

The MANUAL button has priority over the AUTO button and will take control from the Observer. However, the MANUAL button is not enabled when the AUTO button is active and the gear brakes are released. The Operator must press the HALT button to take control from the Observer, if the Observer’s program is actively moving the telescope.

Auto

The AUTO button allows the Observer’s program to Get Control of the antenna and to begin observing. This button can only be pressed when the system is in the Ready mode. The Observer’s program can now communicate to the system through the RPC interface provided through the servo system.

While in the Auto mode, when the observing program requests the servo system to enable, the gear reducer brakes are released, anti-backlash is turned on and the hydraulic control is turned on. The servo system then servos the telescope to the positions commanded by the observing program. When the observing program requests the servo system to disable, the system reverts to the System Ready mode.

From the discussion above, we see the observing program can enable or disable the system. When the system is enabled, the observing program can send dynamic position commands to the servo system. To enable the system means to go from the System Ready state to releasing the gear reducer brakes, turning on the anti-backlash, and turning on the hydraulics. If the observing program does not command a position, the servo system commands the system to the present position. To disable the system means to go from the enabled state to the System Ready state; thus, the gear brakes are set, the anti-backlash is turned off and the hydraulics are set to .

The setting of the stow brakes is always initiated by the Operator through the GUI interface, or by the PLC, when it detects a fault. The latter occurs if the system is in the System Ready mode and the PLC detects one of the axes has moved over 0.2 degrees. In this case, the PLC goes to the System Idle state. The PLC will also go to System Idle if the servo system watchdog is not updated properly

HALT

The HALT button is always enabled. When the HALT button is pressed, the system goes from the Active state to the Ready state. If the system is already in the Ready or Idle state, then physically nothing happens. However, the HALT button also causes the system to go to Manual mode. In other words, the HALT button means Take Control.

Stow / Snow East / Snow West / Service Position

These buttons are active only when you are in Manual mode, System Ready, and the axes are disabled. They are essentially preset positions the Operator can move the telescope to. In particular: The STOW button will move the antenna to the Survival position at the rate selected, high or low. The SNOW EAST and SNOW WEST buttons move the antenna to the designated Snow Dump position in Low Rate mode. The SERVICE POSITION button moves the antenna to the Snow East position in Low Rate. When both axes are in position, it moves to the Service position at 3 deg/min in both axes. When moving out of the Service position, the Servo system will only allow movement at 3 deg/min. Therefore, it is recommended you move to Snow East from Service. Move from Snow East to your final destination, after changing to High Rate, if you intended to do so.

Low Rate / High Rate

The LOW RATE/HIGH RATE buttons allow you to select if the telescope is in the Low Rate, 10 deg/min max, or High Rate (25 deg/min) modes. Low Rate was originally referred to as Full Stroke mode. High Rate was also referred to as Quarter Stroke mode.

LOG

The LOG button, via PLC memory, commands the Servo System to save the last 40 seconds of PID servo data to disk for both axes. This button is for the Operator to press anytime they see a servo anomaly they want the servo engineer to assess.

Crawl Active

This button allows the Operator to slow down the telescope to 3 deg/min. It is a maintenance tool to allow the Operator to slow down instead of stop the antenna.

Position Designate

The POSITION DESIGNATE button is not as obvious as the other buttons. It is enabled when you are in the Manual and System Ready modes. It is a maintenance popup that allows the operator to enable each axis individually and command their position and rates manually. To activate this mode, move the mouse button over the Command Position label in the monitor area of this window. You will notice this area of the screen becomes active as the mouse enters the invisible button area. When you press this active area, a Position Designate window pops up. If the window does not pop up, then too many Citect windows are open. Try closing one and clicking on this area again.

When the Position Designate window pops up, the commanded positions and rates are preset. The commanded position is preset with the present position and the commanded rate is preset to zero. Enter the desired position and rate, then press ENABLE ACCESS. The gear reducer brakes release, and the servo system servos around the present position. It does not yet move to the commanded position. The ENABLE AXIS button then changes to MOVE W/RATE. Pressing the MOVE W/RATE button causes the system to move to the designated position with the specified rate. When the servo system has moved the selected axis to the designated position, it remains enabled and continues to servo the telescope around that position. To set the brakes for that axis, press DISABLE AXIS. If perchance you exit the Position Designate window before you disable the axis / axes, the software automatically disables the axis / axes and sets the maximum rate for each axis to what it was before entering the Position Designate mode.

Remote Console

The Remote Console screen can be selected from the MainConsole screen by clicking on the second icon on the top left of the MainConsole screen.

The Remote Console is laid out similar to the local Operator console. Not much needs explaining on this page, since it operates identical to the local console, with a couple exceptions.

First, the PRECHARGE OFF and PRECHARGE ON buttons are always shaded out. You cannot turn PRECHARGE on or off remotely. Second, the RESET FAULTS button does not function exactly as it does on the local console. On the local console, the RESET FAULTS button has a dual purpose. First it allows the Operator to reset limit 3, limit 2 and 5th pad faults. Second, it allows the Operator to silence the buzzer and inhibit any present faults from being forwarded to the Sensaphone. The RESET FAULTS button on the Remote Console will only silence the buzzer and inhibit any present faults from being forwarded to the Sensaphone. You are not allowed to reset limit 3, limit 2, or 5th pad faults remotely.

Hydraulic Screens

The hydraulic screens for the basement, pump room, north passage and declination house remote I/O units can be selected from the MainConsole screen by clicking on the first icon on the top left of the MainConsole screen.

The hydraulic screens can be paged between by clicking on the up or down arrow icons on the left side of the hydraulic screens. Faults are indicated by red icons on the screen and warnings are indicated by yellow icons. Note that, at times, the software cannot determine which subsystem should be on or off, when parallel subsystems can be selected by the telescope mechanic. As an example, on the Basement Subsystems screen, the Pre-Charge pumps are in pairs. The telescope mechanic will typically turn one on and leave the other off. At least one must be on. In this case, if one is on, it will be green and the other will be light gray. If both are on, they will be green. If both are off, something is wrong and they will be red. It is up to the telescope mechanic to determine which pump is at fault.

One common warning from the basement and pump rooms is the temperature of the air being too high. This normally indicates an air-conditioning problem, or the fact that the telescope mechanic turned it off to perform maintenance. The Operator needs to determine when this is a fault and when it is just a nuisance.

You exit the hydraulic screens by clicking on the bottom icon at the left of the screen. The icon is an open door with an arrow, indicating you are exiting.

Alarms

The Alarms screen automatically comes up when the 43m Citect GUI system is started. This screen cannot be exited and cannot be sent to the back of any other screen. This screen shows the present state of the control and servo systems. The first 7 rows show fault indicators which, when red, indicate a disabling system fault. The last row is warning indicators which, when yellow, indicate something needs to be looked at in the near future.

When a fault occurs, one or more of the alarm indicators will appear flashing red. Each fault can be cleared individually by clicking on the flashing icon, or simultaneously cleared by clicking on the ACK button. If the fault is still present, the red flashing icon will become a solid red icon. If the fault clears, the icon will also clear. When clicking the flashing red icon, if the fault is no longer present, the icon will immediately clear. There is one exception to this rule. Servo computer faults stay latched until the servo computer is re-enabled. In other words, if the servo computer flags a fault, it will continue until it is re-enabled. As an example, if the servo computer determined it was over-speeding, it would fault-out and request the PLC to disable. The gear-reducer brakes would be set and the Operator would see a flashing red servo computer icon on the Alarms screen. Although the servo computer faulted-out, it can run. Clicking on the ACK button on the Alarms screen does not clear the fault, it only acknowledges it. The red icon will not go away until the servo computer is re-enabled.

If you click on a warning indicator, the system will pop up a screen of the indicated subsystem. If the warning indicator is yellow, the pop-up screen should also have a subsystem which is indicating a warning yellow. Operators should note that there are multiple screens for the pump room. Operators should page through the different pump-room screens to see the whole picture.

Diagnosing Servo and Control System Faults

When the control system PLC senses a fault, it both flags it and shuts down the appropriate subsystems. In many cases, this shutting-down means that the PLC shuts down the drive motors for both axes, sets the gear-reducer brakes and stow brakes. When the drive motors are shutdown, there is loss of hydraulic pressures. This causes a snowball effect of generating more fault conditions, and therefore more flashing red icons. What’s an Operator to do? The only way to determine what actually started the whole mess is to look at the data log. The data logs for the PLC control system are located on the Citect PC on C:\Data\AlarmLog.001 through AlarmLog.010, and AlarmLog.txt. AlarmLog.txt is always the most current file, with AlarmLog.001 the next current and AlarmLog.010 the least current. By inspecting these logs, one can check the time of the fault and see which fault occurred first. This will be the one that originally faulted the system. When you inspect an AlarmLog.xxx file, note column 5. The PLC constantly logs two types of data – PLC States and Fault States. You are probably not interested in the PLC states. This is used by the engineer to see exactly how the PLC is responding to certain conditions. If the 5th column does not say PLC, you are looking at a Fault State. The first Fault State at the time of your fault will tell you what faulted the control system.

If there is a Servo fault, the servo system prints out 40 seconds of internal data to /home/43m/servoLogs/AxisYear_Month_Day_Hour_Minute_Second, where Axis is either pol or dec. Typically, this log is only useful to the engineer. In general, if you have a servo computer fault, either the servo computer is dead or detected a fault. If it is dead, it will not respond to anything you do. You will have to reset the 43m-acu computer. If it is not dead, all you need to do is re-enable the axis and it will restart.

Position Limit Switches

Software Pre-Limits and Limits

The first set of limit switches are wired as shown in the file limits.dwg; however, they are no longer used. The servo system defines its own set of software pre-limits and limits. The AutoCad file, 43mLimits.dwg, graphically shows these limits with their polar and declination encoder values. When the servo system determines you have reached a software pre-limit, and the commanded position is further into the limit, it moves the telescope slightly into the pre-limit, and holds that position until the commanded position is out of the limit. The indicated position on the MainConsole GUI screen will turn yellow, indicating the telescope is in a position override in the offending axis. The Servo system should never allow the telescope further into the limits than a degree or so past the pre-limit, where it holds the telescope until the commanded position is out of the limit. This position limiting is implemented in the position-loop code.

If the servo system detects the telescope has passed the software pre-limit in the position-loop code, and has entered the software limit, it then faults-out, sending a code to the sequencer. When the sequencer detects the fault, it disables both axes and requests the PLC to set the brakes. At this point, the PLC will stop the antenna before it gets into the second hardware limit.

Hardware Limit #2

The second set of hardware limit switches are also wired differently than the original design. Refer to drawing limits.dwg for the specifics. These limit switches are individually and in pairs input into the PLC. If the right combination of polar and declination switches open, the PLC ladder logic will set the gear-reducer brakes and fault-out. In addition, other PLC code monitors the inductosyn encoders and shuts down the telescope, if either axis reading is further than 0.1 degrees past the second limit. When the gear reducer brakes are set, the servo system notices it and faults-out. This fault will cause the servo system to log the previous 40 seconds of PID data for the position and rate loops for both axes. Other debug data is also written to the log files for the engineer to diagnose. Notice that the servo system actions are not required to safely shutdown the telescope. The PLC is the system protecting the hardware at this point.

Hardware Limit #3

The third set of hardware limit switches are wired basically the same as they originally were, with a couple exceptions. Referring to the drawing limits.dwg, on the right side of limit # 3 switches, take note of the J30G terminal pin-outs. The PLC monitors each of these locations and determines when we are in a limit # 3 condition. Although the PLC takes action, no action is required. The 115 VAC powering these switches is routed to the Console E-Stop. If there is a disruption of this 115 VAC, due to a third limit condition, then the power is removed from the polar and declination drive motors, gear reducer brakes and stow brakes.

E-Stops & Lockouts

Referring to the drawings BlockDiagram_3.dwg and 43M E-Stop & Lockout, sheet 21/23 of the PLC drawings, the 115 VAC from the # 3 limit switches is routed to the local console E-Stop via TB105D-10. Note if the local E-Stop or the remote fiber optic E-Stop is activated, then the power to the polar and declination drives is removed, along with the power to the gear reducer and stow brakes.

The lockout switches remove power from the polar and declination gear reducer and stow brakes while allowing the drive motors to remain powered. Note that S9 and S10 are lockout switches that are installed in the polar and declination drive houses.

Control Drawings

To date, there are 23 control drawings labeled xxxxxx in the drawing tree. These control drawings are on a PLC module-by-module basis. Each module is given either an X, Y, GX or GY prefix. The X drawings refer to input signals to the Main PLC. The Y drawings refer to the output signals from the Main PLC. The GX drawings refer to the inputs to the remote I/O modules, while the GY drawings refer to outputs from the remote I/O modules. This nomenclature follows the name and address of the PLC module in DirectSoft convention. The X,Y,GX and GY addresses follow every module, I/O point and wire. The layout of the different modules, with their corresponding rack name, input addresses, output address, V-Memory addresses, and power input specifications, are in the file PLCModuleAddressing.xls. The different AC busses, along with fuse sizes and associated neutrals, are listed in a table on sheet X00. The first 20 drawings detail each modules wiring. Sheet 21 describes the E-Stops and Lockouts, while Sheet 22 describes the Servo Interface with the telescope hydraulics, in detail. Sheet 23 describes the wiring for limit 1, limit 2 and limit 3.

Servo / PLC / Citect Interface Command Definitions

The Excel spreadsheet, ServoPlcInterface.xls, documents the interface between these three computer systems. This appendix describes that interface sheet in detail.

Citect GUI Commands to the PLC

Please refer to the section titled Citect GUI to PLC Command in the document ServoPlcInterface.xls. This section documents the push buttons on the GUI MainConsole which are handled by the PLC. When the Operator clicks one of these buttons, the Citect software sets the corresponding bit in the command field in PLC memory. When the PLC detects the command has been set, it clears the bit and sets the corresponding fault bit. The Citect system monitors the fault bits and causes the corresponding button on the GUI to flash when they are set. The flashing button indicates the system is working on completing the command. When the command is complete, the active for that button is set by the PLC, and the corresponding fault bit is reset. The Citect GUI stops flashing the button when the active bit is set. If the function never gets completed, the button will remain flashing until the Operator fixes the problem.

One should note the functions the PLC manage and the functions the Servo system manages. This will give you a feel of which system is responsible for what function.

Citect GUI Commands to the Servo System

Please refer to the section titled Citect GUI to Servo System Commands in the document ServoPlcInterface.xls. This section documents the push buttons on the GUI MainConsole which are handled by the servo system. When the Operator clicks one of these buttons, the Citect software sets the corresponding bit in the command field in the PLC memory. The PLC never references these memory locations. In the servo system, a PLC reader / writer task executes every 0.1 seconds. The sequencer executes every 0.5 seconds. At the end of the sequencer task, the software calls different PLC methods which allow the sequencer to respond to different commands, both from the PLC and from the Citect GUI. The sequencer also sets certain bits to alert the PLC and Citect GUI of certain events. When the sequencer recognizes a particular command is active from the Citect GUI, it sets the corresponding fault bit. The Citect GUI then flashes the corresponding button so the Operator knows something is happening. When the servo system completes the command, it resets the fault bit and sets the corresponding active bit. In the case of the position designate positions and rates, the 4 byte V-memory address is set to the corresponding position / rate. The active and fault bits are also not used for the position designate command bits.

Servo System Commands to the PLC

Please refer to the section titled Servo System to PLC Commands in the document ServoPlcInterface.xls. When a remote user is commanding the servo system via RPC commands, the servo system breaks them into various sub-commands. These sub-commands are sent in the right time order sequence to other servo tasks, to the PLC control system, or the Citect GUI by the sequencer. Although the command SystemReady is documented here, it is not implemented. This command would allow the servo system, and possibly the remote user, to release and set the stow brakes. In the present design, only the Operator, through the GUI and the PLC upon fault detection, command the stow brakes to be set. It should be noted that anytime the servo system issues a disable command to the PLC, the PLC releases the command signal relays for 2 seconds, and then sets the gear reducer brakes. During this 2-second period, the command to the summing amplifier, right before the power amplifier, is ramped from the current command to 0 volts. This gives the system a much smoother stop.

Servo System Data Monitored by the PLC

The data in this section is from the servo system being passed to either the PLC, or to the Citect GUI via the PLC memory. The Watchdog State is an integer which is incremented every second by the servo system watchdog monitor. This integer is monitored by the PLC. If it is not incremented in a timely manner, the PLC will disable both axes and set the brakes. The Position and Rate Commands, and the Servo Max Rates, are sent to the Citect GUI for display on the MainConsole. The position encoder data is monitored by the PLC to verify we never intrude into Limit # 2 region. If we do, the PLC disables both axes and sets the brakes.

While the gear reducer brakes are set, if the PLC recognizes a position jump of over 0.2 degrees in either axis, or if the watchdog is not properly updated, then the PLC sets the Stow brakes.

PLC Data Monitored by the Servo System

In order for the servo system to function properly, it needs to know certain telescope subsystem states. The rate loops need to know if the low or high rate modes are selected. If the brakes are set when they should be released, or if they are released when they should be set, the servo system needs to respond accordingly. This data is provided to the servo system through this area of PLC memory. The PLC is the manager of the data, the servo system PLC task reads the data, and various servo system tasks use this data.

This document is archived as: //prospero/doc/drawings/archive/29140/D002

-- DavidRose - 28 Feb 2008