Msc Control And Instrumentation Computer Science Essay

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The new drive control is aimed at being able to drive stages and targets to within 1µm. In order to achieve this motors have to be in a closed loop with an encoder. However this is only useful if the control system can self calibrate to a known position if the power is lost. The control system should be able to be used on any existing laser facility and advanced enough for controlling the new 10 Petawatt (PW) laser.

Brief:

Looking at the table 1.0 the drive characteristics for commonly used motors, the ability to achieve +/- 1µm is not ideal, especially if the power is lost. One method would be to use expensive absolute encoders, but the high electromagnetic pulse (EMP) environment is not ideal for many of these devices.

Experience gained at CLF. (Central Laser Facility) over the years in high EMP environments, has favoured a number of devices for their reliability and performance. Control of these devices has had mixed reception. This has ranged from stepper encoder systems developed by "Thorlabs", to simply reading "Sony Magnescales" position devices while monitoring the moving motor. The later system has gone through many hardware and software iterations to keep pace with the demands on the facility.

Table 1.0 drive characteristics

Drives and Magnescales

Drive To Limits

Drive to Software Stop

Power Cycle

New Home Switch*

Moving to 1µm

Hit mechanical stop

Magnescales

N/A

N/A

lost position

+/- 1µm

Excellent

+/- 500µm

Thorlabs DC Drives No Encoders

Sticks on lead screw

+/- 1µm

lost position

+/- 1µm

Excellent

N/A

Thorlabs DC Drives With Encoders

Sticks on lead screw

+/- 1µm subject to PID

lost position

+/- 1µm

Poor subject to PID/load

lost position

Thorlabs Stepper Drives

+/- 4µm

+/- 1µm

lost position

+/- 1µm

Good/better with Encoder

lost position

PI DC Drives With Encoders

+/- 4µm

+/- 0.5µm subject to PID

lost position

+/- 1µm

Poor subject to PID/load

lost position

National Aperture DC Drive With Encoder

Sticks on lead screw

+/- 1µm subject to PID

lost position

+/- 1µm

Poor subject to PID/load

lost position

* to be fully tested

CLF has invested considerable resources in many of these motor drives, Magnescales and hardware Infrastructure. Ideally a new drive system should make use of this hardware and optimize each device for the best performance.

Existing software for the drive system has evolved on a range of different PC's (Personal computers) over the years as PC's architecture and operating systems also matured. This has lead to a range of solutions each trying to keep pace with users demand. Much of the software has not been at the facilities core competence, as simple functionality and immediate implementation have been key drivers. Little time has been left for detail on user interface and more importantly "closed loop" motor repeatable position control.

Implementation (Hardware):

The new drive system will use all the existing hardware, but will use optimization for performance shown in table 1.1. Key to achieving repositioning reliability is the use of a home device. This can be a "Hall effect" magnet switch or a high quality switch. This home switch will be retro fitted to all drives, and targets that need power loss position realignment. Note where limit switches have been factory fitted which lock the drive on a lead screw a extra limit switch is also recommended.

Each motor, stage or assembly will have a special small aluminium black anodized tube. This tube will serve a number of functions:-

A laser etched barcode on the outside. (Now tested and working) The barcode links important information in a database about electrical and mechanical capabilities of the devices. Later this barcode will also be used as an asset tracking number.

The tube will house the wiring joints needed to convert all drive systems to a common connector and cable length. Something that has been a major problem and expense with the existing system.

The whole assemble will withstand high vacuum conditions (Ideally 109 subject to what connected to it?)

Table 1.1 Required drive improvements

Drives and Magnescales

Drive To Limits

New requirements

Home Switch

To home *

Expected Closed Loop

Dc Drive With Magnescales

Must have 1 limit switch

New Magnescales to Parker encoder

New home switch

+/- 1µm

+/- 1µm

Thorlabs DC Drives With Encoders

Must have 1 limit switch

Software limit added

New home switch

+/- 1µm

+/- 1µm & PID

Thorlabs Stepper Drives

Must have 1 limit switch

Extra encoder added

New home switch

+/- 1µm

+/- 1µm

PI DC Drives With Encoders

Use internal limit switch

Software limit added

New home switch

+/- 1µm

+/- 1µm

National Aperture DC Drive With Encoder

Must have 1 limit switch

Software limit added

New home switch

+/- 1µm

+/- 1µm & PID

* to be fully tested

The Parker 6K8 drive controllers will now have their own 2Amp 24volt power supplies independent of the motor drive power. An additional per Parker 5volt 2amp power unit used for reference voltages, Hall Effect switches, switch limits and encoders is also required.

Subject to a max 2amp per DC motor budget, a single high current 24Volt switch mode power supply unit (PSU) will drive the DC motors. The stepper motors should have a 5Amp 24Volt DC per motor budget on separate high current PSU. Each single motor driver should have its respective 2-5amp slow blow fuse with an LED indicator.

Implementation (Hardware Cont/...):

The new drive system has the following hardware architecture (See figure 1.0):-

Figure 1.0 System Architecture

Centre to the system is the "Dell" Server/Master PC this is an Intel icore7 system with 8 GB DDR3 memory and powerful 1 GB "Nvidia GeForce" Graphics card. The PC runs Windows 7 professional 64bit operating system all selected for its Multithreading ability. The Server/Master has to poll all the parkers, remote devices and its own touch screen. The server runs as its own standalone drive controller or can add slave PC's or 802.11g/n wireless hand held devices. Visiting users can use their own Iphone (Not provided) by downloading a special application or using a site TBS-Recon rugged hand held (Running Windows Mobile).

To keep a handle on new remote hardware as hand held devices age so quickly, an "Apple Ipad" application plus "Ipad" will be available for registered users. The key advantage over other hand held devices is long battery life, large capacitance touch screen and low cost due to Apple's large consumer market.

Implementation (Software):

The main software is written using Microsoft Visual Studio 2008 and will be also complied with Visual Studio 2010 as this version was only realised during this report. The source code using visual basic dot net is highly documented and uses a "Class" and "Form" structure. Each class is designed to be easily identified as to its control function within the hole of the program. Key to the software objectives is modularity so that new classes can be added or deleted as hardware changes.

The code will be realised in three issues, Alpha, Beta and Final. Alpha code is used to prove the hardware with limited overall functionality, this is to test each key element and prove stable performance. Beta code is all the code complete, but still under performance trial. This version will be used ideally in conjunction with new hardware in racks so that live laser experiments can use the system. Should the Beta system fail the old drive system can simple be "plugged" back while the fault is identified?

The final code will still need some site dependency modules added but this will be proven during the Beta run. This version like all versions will be backed up using "Subversion".

The user interface is designed to be intuitive with simple touch control. The user can move drives, targets and stages in a number of ways.

Simple touch left or right

Slider control in absolute values

One touch for +/- 1 µm or 1mm

Numeric touchpad for absolute or relative values

The PC's use two screens one lower screen as touch entry and above screen as visual data, the above screen allows the user to even see a photograph of the stage under control.

Key to the system's performance is the databases, here explicit information needed to control the drive system is held. The database system as shown in Figure 1.0 has its own PC user interface "Drive and Stage editor". Any PC on site can access the drive system and add new experiment numbers with related experiment drive stages. This program also allows administrators to add new users, facilities, drive, drive assembles and targets.

It is hoped that this system is used as a planning tool and an asset management tool. The aim of this software is to reach a cross department audience each needing different functions from the databases. At its most basic level it allows CLF staff to add steps and stages to the drive system.

Figure 2.0 shows a screen shot of the PC planning tool. This helper screen allows the user to search, edit delete or add different aspects to the drive system.

For example:-

A mechanical engineer can find out the maximum travel on a linear motor.

A technician can find out what pin out a connector should be for an encoder

Figure 2.0 Drive System Planning Tool

From the above helper the respective database can be found. Further forms act as front ends to the Microsoft 2007 Access database engine. "Microsoft Access" was selected as almost all the desktop PC's at CLF have "Office" and "Access" installed, so no dependence on a dedicated SQL server was needed.

The database Classes used in the planning tool is the same as in the server/master so the code is simply duplicated. The last drive system software used heavily bound data controls which made the software very difficult to debug without real data. The new drive system server simply extracts the required data via a SQL type request into a common array. It is then easy to read the array by name or debug the data without XML / Html formatting and control loss.

The Individual database names are as follows:-

User Database. This stores information on user access rights for drive system and planning tool issues the user with a 4 number pin.

Experiments Database. This users the RAL/CLF facility year planer experiment number, Start date, End date and facility name. It helps to select the correct facility and experiment setup.

Step system Database. This stores a step by step selection of controls stages, and targets and photographs. It allows correct visual, controls, Magnescales and forms to be displayed on the drive system.

Drive stages Database. This stores barcode numbers on every asset/drive used by CLF. The data contains a host of information: PID, Max travel, Encoder data, limits, connectors, PDF's, scaling and much more. By simple pointing the bar code reader during the experiment design stage at the device the part is booked out and booked in to the experiment. If the part becomes faulty simply ticking the fault box alerts the user about its future status.

Parker Database. This holds each facilities motor drive controller (Parker 6k8) IP addresses, Mac Addresses, Channel number, Working status, connector details and limits details.

Implementation (Software Cont/....):

The following shows how easy it is to design an experiment.

The user logs in (Only CLF staff that have correct access rights to design an experiment can do this.)

Select an experiment number from RAL/CLF or make up a new 5 digit one. Select the facility and the start and finish date.

Open the experiment wizard. When asked scan the assembly or drive motor or select from a list the device to control. Give each axis a name then Click done or add another stage.

For legacy Magnescales you will have to select the drive it's used on. But this will be should have been phased out if the "PIC Magnescales to Parker Encoder hardware" is complete.

Finally any photo's that you have that are not in the database can be added to aid user stage recognition especially when many stages are called the same name e.g. XYZ.

The following shows how to get details on all the connectors.

The user logs in.

Select experiment number.

Click on connectors. Done....

Summary:

As this is a "living" document more details will be amended to it as time permits. At the time of writing much of the software and hardware has been developed and the software is at the Alpha stage. A limited trial is underway at ASTRA Gemini using just one Parker (8 Channels) to control a F20 and F2 stages.

Laser beams, targets and camera's have to be moved precisely within 1/50th of a human hair for a vast range of experiments carried out every day at CLF.

Over the years of controlling powerful lasers a range of different drive systems evolved, out of the need to control these motor drives.

Some of this control needed to get more repeatable and more accurate as the demand for multiple drives within laser experiments increased by users of the facility.

As technology leaders ourselves, we have to be mindful that the technologies "we use" changes. So the control system itself had to be upgraded to take advantage of this technology.

The new system has multi-touch screens and remote touch pads that control drives in a new "Closed loop" system that gives this repeatable performance with a simple wave of a finger!

A complete re-writes of the software written in the latest Microsoft languages. Takes advantage of Intel ICore7 processors for multi-threaded simultaneous motor control.

Even the way the user interacts with the system has changed to put the scientist back into control.

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