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MRO Delay Line

VME Software Functional Description

The Cambridge Delay Line Team

rev 0.5 21 January 2008

Cavendish Laboratory JJ Thomson Avenue Cambridge CB3 0HE UK


Objective
To describe the overall architecture of the metrology system software running on the VME CPU.

Scope
This document describes the architecture and functional behaviour of the prototype control software running on the VME system. The role of this subsystem is to close the delay line metrology loop by sending a correction signal to the cat'seye over the lowlatency link, the correction being obtained by comparing the measured cat'seye position from the metrology system with a demanded trajectory received from the workstation. This document should be read in conjunction with:


"System Overview", which provides a highlevel description of the metrology loop and explains how the cat'seye is controlled onboard the trolley "Metrology System and VME Hardware Design Description", which describes the hardware used to measure the cat'seye position "Workstation Software Functional Description", which describes how the trajectory demand is calculated and transmitted to the VME system






1 Introduction
The VME system supports the following (axial) system modes of the delay line (these are introduced in the "System Overview" document). We now outline the behaviour of the VME system in these modes.

1.1

Stop/Idle Mode

In this mode the trolley is stopped by the workstation sending a DirectSlew command with a demand velocity of zero to the onboard trolley micro. This activates the cat'seye local loop which locks the cat'seye in the middle of its travel. The VME system is not required to do anything in this mode.

1.2

Datum Mode

In this mode the VME system transmits a sequence of slew velocities to the trolley micro in order to precisely position the cat'seye at the datum switch. The metrology count is then reset.

1.3

Follow Mode

In follow mode, the VME subsystem receives a 10 Hzsampled trajectory (position and velocity) demand from the workstation, interpolates it to 5 kHz sampling, and compares this with the actual cat'seye position obtained from the laser metrology system. Within the follow system mode, the VME/trolley micro combination may be in either track or slew mode as determined by the VME CPU:

1.3.1

Track Mode

If the position error is within fixed limits, the VME system switches the trolley to track mode (which deactivates the cat'seye local loop) and closes the metrology loop by outputting a control voltage to the cat's eye via the low latency link. The current velocity demand is transmitted to the trolley micro so that the carriage follows the cat'seye motion.

1.3.2

Slew Mode

If the position error exceeds track mode limits, the trolley is switched to slew mode (which activates the cat'seye local loop, locking the cat'seye in its centre position). The VME system calculates a velocity based on the magnitude of the position error, which is sent to the trolley in order to reposition it so that eventually track mode will be activated. If the position error is large (i.e. when moving between target objects) then a 'coarse' slew mode is enabled. Given a acceleration/deacceleration limit and allowing for the new track velocity, the algorithm computes the velocity profile necessary to bring the trolley to the correct place without overshoot.

2 Metrology Hardware
The metrology hardware is described fully in the document "Metrology System and VME Hardware Design Description". We include an outline description here to help clarify the interaction between the metrology hardware and software. The metrology system is based on a commercial laser interferometer ­ the Zygo displacement measuring system (ZMI 2000). and the reader is referred to the Zygo web site (www.zygo.com) for more technical details and the principle of operation. The hardware consists of: Laser head (7702),




Linear interferometer (7002), Retroreflector(7003), Various mirrors, Beam expanders, Fibre optic pickup, Measurement board (2002), Datum switch (Reflective beam sensor, STM RLF50RN)

A schematic layout is shown below in Figure 1. Optical fibres connect the receiver to the measurement board and connect the laser to the reference input on the measurement board.

The datum switch is a reflective beam sensor fitted on the outside of the delay line pipe looking through a small window. This detects the presence of the trolley and is connected via a separate amplifier to the interface board in the VME chassis. The VME chassis contains: Processor (Concurrent Technologies CPU x86 with Ethernet port, keyboard, mouse, VGA port, etc), Timing board (Symmetricom TTM635VME), Measurement board (Zygo 2002 as listed above), Industry pack carrier board with DAC and PIO, Interface board.

Figure 1: Schematic layout of the metrology system hardware


3 Development and Execution Environment
The operating system is QNX 6.1. This was chosen because it is a real time operating system with a fast, deterministic interrupt response time and also because we have previous experience with QNX at COAST. The interrupt performance is essential in optical interferometers as the cat's eye must be positioned and track to submicron accuracy. At maximum tracking speed of 15mm/sec a timing error of 10µsec translates to an error of 0.15 microns in cat's eye position. QNX is UNIXlike and follows various industry standards such POSIX and TCP/IP. The system is used to both develop and run the software, which is coded in C.

4 Program Architecture
The software is divided between several threads or processes which run concurrently. One thread deals with transmission of messages to the workstation and trolley micro; another deals with interrupts and there are two threads to deal with command and command data messages received from the workstation. The advantage of using threads is that if there is no interrupt/message then the other threads are free to run but immediately an interrupt occurs or message arrives that thread will run.

4.1

Main program

When the main program starts it sets up a common data area for use in the interrupt routine, initializes VME hardware, instantiates status and telemetry objects, and establishes a socket connection with the trolley in order to send command data messages. The various threads are then started: two lowerpriority threads to deal with command and data messages respectively, and a higherpriority thread to handle interrupts. Note that eventually there would also be another thread to handle communication with the fringe tracker. The main program thread then loops, sending messages when the appropriate intervals have elapsed, as follows:


Status messages are sent to the workstation at 0.1s intervals. Command data messages are sent to the trolley at 0.1s intervals. These are either Slew or Track messages depending upon the value of a flag set by the interrupt routine. Both message types contain a demand velocity for the carriage. In track mode this is equal to the demand velocity received from the workstation, whereas in slew mode the velocity is calculated by the VME system on the basis of the current position error. Telemetry messages are sent to the workstation at 1 second intervals.



The operation of the main program thread is preempted by the other threads described below.

4.2

Interrupt Routine

The interrupt routine comprises two parts: the interrupt service routine (ISR), which runs when a hardware interrupt occurs, and a second part which runs after the ISR returns. The ISR uses the complete resources of the processor and therefore runs deterministically so that a minimum time elapses between sampling the measurement board and outputting a drive voltage to the cat's eye. Floating point operations are not available in the ISR and so all arithmetic is done using integers, performing carries explicitly where necessary. The timing board (which will be synchronised to a GPS receiver) generates a signal at 5 kHz rate which samples the Zygo measurement board and is also connected to the "SYSFAIL" signal on the VME bus. This method of generating the CPU interrupt was chosen, rather then a VME bus interrupt, because the "SYSFAIL" signal is fed directly to the processor interrupt lines and so gives minimum response times.


The ISR immediately reads the measurement board, then compares the metrology value with the previous value to check if the measurement board has overflowed1. The routine then calculates a position error by subtracting the position demand for the time the interrupt occurred (inferred by counting interrupts) from the measured position. If fringe tracking is enabled, the tracking offset is added to the the position demand before calculating the error. Provided the delay line is in follow mode, the subsequent ISR behaviour depends on the magnitude of the position error:


If the error exceeds fixed limits, slew mode is flagged, causing the main thread to send Slew messages to the trolley micro Otherwise, track mode is flagged, causing the main thread to send Track messages to the trolley micro. The position error is converted into a rate demand by adding the product of the demand velocity and the interrupt period (0.2ms) and the result transferred to the DAC for transmission to the cat'seye



If a command to go to the datum has been received the measured position of the cat's eye is ignored and the trolley is switched to slew mode and driven to the datum position as described in the next section. Finally the demanded position and velocity for the next interrupt are calculated. This is done by interpolating the trajectory demand received from the workstation assuming a constant velocity over each 0.1s interval between supplied values (this is sufficiently accurate given the small accelerations required to follow a sidereal trajectory). If the interrupt is coincident with the one second tick, the interrupt count is reset and a "second" flag is set. The following operations are performed when the ISR returns:


Telemetry data are stored in a buffer for use by the main program thread Every tenth of a second, a new demand position and velocity are read from the received message buffer for action by the next ISR If the "second" flag is set:




The "telemetry buffer full" flag is set indicating that this block of data should be sent to the workstation and the storing of telemetry data is switched to a second buffer The appropriate set of demand positions and velocities to use in the subsequent second are selected based on the time stamps of the buffered messages



4.3

Datum Routine

The datum routine is entered on receipt of a command from the workstation. This is necessary at start up and if the metrology loses count for any reason. The VME system commands the following sequence of operations in order to seek the datum: 1. Slew at maximum speed towards datum 2. Stop the trolley when the datum switch closes 3. At this point the trolley will be past the datum switch 4. Drive slowly back until the datum opens 1 The Zygo measurement board is designed to count ±21.2 m and has 36bit storage so for 200 metres an extra 4 bits are required which are added in software. The board is programmed to carry on counting after an overflow has occurred.


5. Store the metrology position at the instant the datum switch opens. 6. Reclose the metrology/cats eye loop and drive to stored position 7. When stored position is reached reset metrology counter If the trolley is already past the datum when commanded to go to the datum then the trolley will drive slowly back as in step 4 above.

4.4

Message Routine

The format for sending and receiving messages has been described fully in the document "Control Software Architecture". Received command messages are checked for which trolley they apply to and the appropriate flag set. In the case of received command data messages, each message is stored in a circular buffer that can store up to ten messages. Messages are sent by the workstation at 1Hz and each contains ten positions and velocities, plus timestamp information specifying when the positions/velocities should be realised. When a "second" event is detected in the ISR, the code that runs when the ISR returns checks the time stamps of the messages in the buffer and selects the appropriate set of positions and velocities to use in the subsequent second. Hence data can be sent sometime in advance and performance will not be affected by network delays.