------------------------Monitor------------------------- The Newsletter for PC-Based Data Acquisition and Control Issue 30 www.windmill.co.uk January 2001 --------------------ISSN 1472-0221---------------------- Welcome to the New Year issue of Monitor. This month we would be very grateful if you could fill in our short survey. Our main article discusses the GPS and tells you how to interface your receiver to a PC. First of all, though, news of how Windmill is being used to monitor trains over the Internet. We only send this newsletter to people who have subscribed - should you wish to cancel your free subscription please visit http://www.windmill.co.uk/newsletter.html CONTENTS ======== * Windmill News: Monitoring Sliding Train Wheels over the Web * New Year Survey * The GPS - What is the GPS? - History of the GPS - How the GPS Works - Connecting GPS Receivers to PCs ________________________________________________________ ============================================ Windmill News: Monitoring Sliding Train Wheels over the Web ============================================ In slippery conditions, such as where snow, ice or leaves are on the line, trains are in danger of skidding along the tracks. This has serious safety implications. It also results in damaged wheels: the area in contact with the track being ground flat. These deformed wheels, predictably, are expensive to replace. A British train operating company wanted to quickly deal with skid conditions, and needed to know when and where wheels were slipping. They chose Windmill Remote software to do this, as it let them view live data in ordinary web browsers. The Windmill system monitors the wheel-slip-detection mechanism on the train (which is similar to anti-lock brakes in cars). Windmill also reads a GPS receiver: using satellites to pinpoint the location of the slippage. As well as this information - the state of the traction control, braking control wires, wheel speeds, bogie dump valves and time are all recorded and transmitted to an Internet server. Windmill saves data on the server in a series of files; each file holding, say, data from one week or one month. This prevents the data file from becoming too large, and minimises downloading time. When Windmill receives a request from a browser, it presents the data in a series of web pages. Railway engineers can choose to see live information, or historical data and a calendar of wheel slip events. Normal users can only view information. More senior staff, though, can download logged data files, change logging conditions and set new telephone numbers to be dialled in case of alarms. They can import the downloaded data into Excel, or other Windows program, for analysis and report generation. The Windmill system lets the train operating company know immediately where problems are occurring and quickly take remedial action. For more details of Windmill Remote software and monitoring wheel slippage in trains, contact Graham Collins at [email protected] ________________________________________________________ =============== New Year Survey =============== As an aid to improving Monitor, and our technical support service, we would be very grateful if you could fill in this brief survey. All information is confidential and won't be passed to any third parties. Miss out any questions you don't want to answer.(You'll be returned to this page after pressing Send Survey.) ________________________________________________________ ============================================= The GPS: History, Function and PC Interfacing ============================================= What is the GPS? ================ The Global Positioning System gives you accurate position and velocity information anywhere in the world. * History of the GPS ================== In the early 60s, the US Department of Defense decided it needed a global, all-weather, accurate, positioning and navigation system. The Navy and Air Force began investigating the possibility of using radio signals transmitted from satellites. These studies eventually evolved into the GPS, which is no longer solely for the US military but is used in a myriad of applications around the world. The first GPS satellite was launched in 1978. The first 10 were development satellites, called Block 1. From 1989 to 1994, 24 new satellites - Block II - were launched, completing the satellite coverage we have today. * How the GPS Works ================= Before we venture into 3 dimensions, let us start with a 2-dimensional example of the principles involved. Suppose you don't know where you are, but you know you are 267 km from Dublin, 605 km from Paris and 493 km from Amsterdam. You draw a circle on a map of Europe with its centre in Dublin and a radius of 267 km. You now know you are somewhere on the circumference of this circle. You draw another circle with its centre in Paris and radius of 605 km. As you know you are on the circumference of each circle, you must be on one of the two places where they intersect. You now draw a third circle, around Amsterdam. Assuming the distances were accurate, this circle will pass through one of the two possible locations and pin-point your position. The GPS follows these principles. However we're now using 3, not 2 dimensions and so are dealing with spheres. This also means we need 4 position references, not 3. The GPS satellites broadcast radio signals containing their position and time, which your GPS receiver picks up. The receiver knows exactly where in the sky the satellite is, it just doesn't know exactly where on earth it is, until it determines the distance from the satellite. It does this by calculating the time it took for the signal to reach it. It knows that the radio waves should travel at the speed of light, and it thinks it knows the elapsed time since the signal set off. It calculates its distances from 4 or more satellites and checks whether the spheres intersect at one point. If they do not it assumes its clock in inaccurate (satellites have atomic clocks, receivers quartz clocks) and adjusts the clock to find the exact location. So, why do GPS receivers sometimes tell you you've arrived at your destination when instead you know very well you are in a muddy field with some bad tempered animals? The two major sources of error are the US government, who can degrade the signal if they choose, and the speed of the radio signals. The signals may be delayed by particles in the atmosphere, mountain ranges, tall buildings, etc, as they bounce off all these obstructions. * Connecting GPS Receivers to PCs =============================== When connected to computers, GPS receivers become very versatile tools. You need some software to read data from the receiver and save and present it on screen. Our free Windmill software will do this for you. This is useful in all sorts of situations. We briefly mentioned one in the train monitoring story above. In a completely different application, a marine biology survey is using Windmill and the GPS as part of a project to map and monitor marine habitats. The portable system (an old laptop running the software inside a plastic box) can quickly be transferred to the inflatable boat they use for mapping small areas. A similar system could be used in SCUBA diving, coral reef conservation, salvage and marine archeology. For more details see http://www.windmill.co.uk/mapping.html * How do I Get Data from the GPS into My PC? ========================================== Windmill will typically collect latitude and longitude data from the GPS receiver. It does this as 2 "channels" of data (or as many channels as there are types of data). Here are brief details of the steps to take, for a fuller explanation see http://www.windmill.co.uk/gps.html 1. Plug the GPS receiver into your computer's COM port, using the data output cable (you need one with a serial connector). 2. Verify the communications between the GPS and the PC with Hyperterminal. Details of how to do this are in the labiml.hlp file, which you'll find in the windmill folder. These communications settings will generally work with NMEA instruments, but check your GPS Manual to make sure. Baud rate = 4800 Bits = 8 Parity = none Stop bits = 1 Flow control = Xon/Xoff or Hardware 3. Run Windmill ConfIML and enter your GPS settings. These should work. Number of channels = 2 (latitude, longitude) Reading protocol = Continuous flow Data persistence = 2 seconds The data will arrive at the PC as a string from which you can extract your desired information. 4. Use Windmill SetupIML to give your channels names, change the measurement units, and such like. 5. Use the Windmill Logger, Chart or DDE Panel programs to view and save the GPS data, or send it straight to Excel, a mapping program, or other Windows application. Once you have set up the system, you can dispense with steps 1 to 4 and simply use your chosen software to collect the data as in step 5. If you need more help getting the data from your GPS receiver, see our gps page and our Windmill FAQ. Any questions not answered there please fill in the technical support form at the bottom of the FAQ page. http://www.windmill.co.uk/techsupp.html * Further Reading =============== Understanding Gps : Principles and Applications ed Kaplan, E.D., 1995. Artech House Telecommunications Library The Global Positioning System - Assessing National Policies Marshall Brain's How Stuff Works Joe Mehaffey and Jack Yeazel's GPS Information Resources ________________________________________________________ * Copyright Windmill Software Ltd * Reprinting permitted with this notice included * For more articles see http://www.windmill.co.uk We are happy for you to copy and distribute this newsletter, and use extracts from it on your own web site or publication, providing the above notice is included and a link back to our website is in place. An archive of previous issues is at http://www.windmill.co.uk/newsletter.html and an index of articles at http://www.windmill.co.uk/newsletter.html Windmill Software Ltd, PO Box 58, North District Office, Manchester, M8 8QR, UK Telephone: +44 (0)161 833 2782 Facsimile: +44 (0)161 833 2190 E-mail: [email protected] http://www.windmill.co.uk/ https://www.windmillsoft.com/
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