-------------------------Monitor------------------------ The Newsletter for PC-Based Data Acquisition and Control Issue 155 www.windmill.co.uk June 2011 --------------------ISSN 1472-0221---------------------- Welcome to Monitor, the newsletter for data acquisition and control. This month we are pleased to offer you our 751 data acquisition and control unit for half its normal price. For more details read on below or see https://www.windmillsoft.com/daqshop/daq-usb.html. Don't forget that you can download free copy of our ComDebug data logging software. This lets you log data from instruments connected to the PC's COM port using RS232, RS285 or Modbus. Finally check out our new Twitter account at http://twitter.com/DataAcquisition where we'll be posting data acquisition news and updates. We hope you find the newsletter useful, but should you wish to remove yourself from our mailing please go to Monitor Newsletter CONTENTS ======== * Windmill News: Over 50% off Multi-Function Unit * Differential or Single-Ended Inputs? * Excel Corner: Reading Big Files into Excel into Excel * DAQ News Round-up ________________________________________________________ ________________________________________________________ Windmill News: Multi-Function Unit Reduced in Price ________________________________________________________ This month we are very pleased to offer you a discount on the price of our 751 data acquisition and control unit, reducing it from £595 to £295 (336 Euro or 480 USD). The 751 unit can capture data from 16 sensors and probes of various types. It can also switch digital outputs and count pulses. The package includes the Windmill 7 logging, charting and control software and free technical support for life. You can connect up to eight 751s to one PC, giving 128 analogue inputs, 256 digital inputs & outputs and 8 counters. The 751 has been used to measure temperature, CO2 concentration, humidity, photosynthetically active radiation, pH, strain, pressure and other signals, although extra hardware is needed for some of these measurements. Designed for maximum versatility, you have a choice of four analogue input ranges, selected through Windmill software individually for each channel. Alternatively, choose automatic ranging and let the software match the input signal as closely as possible. You can also use the Windmill software to select the resolution of the A-D converter, from 12- to 18-bits. Choose high throughput, high resolution or something in-between For accurate measurements the 751 uses differential inputs, which reduce noise and ground errors. See the next article for a discussion of the differences between differential and single-ended inputs. At regular intervals Windmill uses a stable on-board reference voltage to recalibrate the 751. One of our most popular hardware packages, the 751 is an excellent multi-purpose unit. For more information on the 751 data acquisition and control unit, see https://www.windmillsoft.com/daqshop/daq-usb.html, e-mail [email protected] or telephone +44 (0)161 833 2190. ________________________________________________________ ________________________________________________________ Differential or Single-Ended Inputs? ________________________________________________________ When monitoring analogue signals such as pressure or vibration, you can choose single-ended or differential connections. What is the difference between the two and which should you use? * Single-Ended Inputs =================== Single-ended inputs give you twice as many analogue channels for your money - so why not always use them? The reason is that they might give inaccurate measurements. With single-ended inputs you connect one wire from each signal source to the data acquisition unit. The measurement is the difference between the signal and the ground or earth at the unit. This method relies on 1. the signal source being grounded (earthed) and 2. the signal source's ground and the unit's ground having the same value. Differences in Ground Levels We think of the ground as a constant 0V, but in reality the ground, or earth, is at a different level in different places. The closer together the places, the more likely the ground level will be the same. Make a connection between two grounds and the difference in levels can drive large currents, known as earth or ground loops. This may lead to errors when using single-ended inputs. Noise Errors Single-ended inputs are sensitive to noise errors. Noise (unwanted signal contamination) is added because signal wires act as aerials, picking up environmental electrical activity. With single-ended inputs you have no way of distinguishing between the signal and the noise. The ground and noise problems can be solved by differential inputs. When to Choose Single-Ended Inputs? Where you have short signal wires, close-together signal sources and signals larger than around 100 mV, then single-ended inputs are a cheaper option than differential inputs. You could choose our 750 data acquisition unit for example: details at https://www.windmillsoft.com/daqshop/analogue-input.html. This is still at the low price of £150 (240 USD / 170 Euro). * Differential Inputs =================== With differential inputs, two signal wires run from each signal source to the data acquisition unit. One goes to a + input and one to a - input. Two high- impedance amplifiers monitor the voltage between the input and the unit's ground. The outputs of the two amplifiers are then subtracted by a third amplifier to give the difference between the + and - inputs, meaning that any voltage common to both wires is removed. This can solve both of the problems caused by single- ended connections. It means that differences in grounds are irrelevant (as long as they aren't too large for the amplifier to handle). It also reduces noise - if you twist the wires then any noise picked up will be the same for each wire. Floating Signals A common problem when using differential inputs is neglecting any connection to ground. For example, battery-powered instruments and thermocouples have no connection to a building's ground. You could connect a battery, for instance, between the data acquisition unit's + and - inputs. The two input amplifiers will try to monitor the voltages + to earth and - to ground. However, as there is no connection between the battery and ground, these voltages to ground could be any value and may be too large for the amplifier to handle. For these "floating" signal sources you should provide a reference. The data acquisition units like the 751 have a socket labelled 0V, REF or GND. Run a wire from, say, the - wire to this ground socket, either directly or via a resistor. If your signal source is itself grounded you don't need to make a connection to the interface's ground socket. Amplifier Ability and Operating Range The three amplifiers used for differential inputs are collectively known as an "instrumentation amplifier". Ideally, as previously described, any voltage common to both wires (common mode voltage) is cancelled. In practice the two input amplifiers are not perfectly matched so a fraction of the common mode voltage may appear. How closely the instrumentation amplifier approaches the ideal is expressed as the common mode rejection ratio (cmrr). This is the reciprocal of the fraction let through and is usually given in decibels. The higher the rejection ratio the better. Another specification to look for is the common mode range. This is the maximum contamination voltage with which the amplifier can cope. If the difference in ground levels between your data acquisition unit and signal source exceeds this value, your measurement will be inaccurate. Be aware that your hardware operating range may be given as higher than the common mode range, but the operating range just guarantees that your hardware won't be damaged, not that it will work properly. Less Signals with Differential Inputs? A disadvantage of differential inputs is that you need twice as many wires and differential units tend to be more costly. Our 751 provides 16 differential inputs and is on promotion for £295 (470 USD / 330 Euro). More details at https://www.windmillsoft.com/daqshop/daq-usb.html With some data acquisition interfaces you can choose whether to use the inputs in differential or single-ended mode, for example the Microlink 3050 module (http://www.microlink.co.uk/analog.html#3050). * Pseudo-Differential Inputs ========================== Some manufacturers offer pseudo-differential inputs. These may be used when the signal sources are close together and share a common ground. Pseudo-differential is similar to single-ended but the signal source's ground is isolated from the data acquisition interface's. A wire runs from this ground to the interface. By subtracting the interface ground from the signal ground, differences are removed from the measurement. However, this method is no use for reducing noise. * Summary: Making your Choice Between Single-Ended and Differential ================================================================= 1. Signal leads over a few metres in length? Choose differential to reduce noise. 2. Small signals under 100 mV? Choose differential to reduce ground and noise errors. 3. Signals with different grounds to each other, as happens when signals are remote from one another? Choose differential to remove ground errors. 4. Sensors with high resistance such as strain gauges? Choose differential to remove common mode voltage. High resistance gives greater pick-up and thus higher common mode voltage. 5. Need twice as many inputs, and have none of the above problems? Choose single-ended. For data acquisition units offering single-ended and differential inputs see https://www.windmillsoft.com/daqshop/data-acquisition.html ________________________________________________________ ________________________________________________________ Excel Corner: Reading Big Files into Excel ________________________________________________________ Some versions of Excel have a limit of 65536 rows in a data sheet. When you are collecting data over a long period of time, this may not be sufficient. With Windmill you can set Logger to automatically close one data file and open another one to prevent files getting too big. Alternatively you can write a macro in Excel to read data from the log file and open a new worksheet to hold the data when 65536 rows are reached. An example of a macro, which was written by John Morton of Poole Hospital and very kindly shared with us, is below. Thanks John. ------------------------------------------------------------- Importing more than 65,000 rows of data into EXCEL Reading data from a file located in "c:\windmill\data.wl" the macro loops until it reads EOF End of file message. Data is entered on sheet 0 until the pointer goes above limit 65,000 (worksheets actually have 65536 rows). The pointer is reset to 1, and a new worksheet is added to the workbook and this worksheet starts to receive data from file. Maximum number of worksheets is 255, so you could read in 255 * 65536 (16,711,680) lines of data from a file. ScreenUpdating is turned off to prevent screen flicker during import. Sub newImport() ' ' newImport Macro ' Macro created 24/11/2008 by Morton ' myfile = "c:\windmill\data.wl" Open myfile For Input As #1 Application.ScreenUpdating = False pointer = 1 sheetNo = 1 Worksheets(sheetNo).Select Do While Not EOF(1) Line Input #1, myLine Cells(pointer, 1) = myLine pointer = pointer + 1 If pointer > 500 Then Worksheets.Add after:=Sheets(sheetNo) pointer = 1 sheetNo = sheetNo + 1 If sheetNo > 255 Then MsgBox "Maximum number of worksheets have been added - STOP !" ThisWorkbook.Worksheets(sheetNo).Select End If Loop Application.ScreenUpdating = True Close #1 ' End Sub ------------------------------------------------------------- Any Excel questions, or tips, please get in touch Further Reading: Using Excel for Data Acquisition https://www.windmill.co.uk/excel/ ________________________________________________________ ________________________________________________________ DAQ News Roundup ________________________________________________________ Welcome to our roundup of the data acquisition and control news. If you would like to receive more timely DAQ news updates then grab our RSS newsfeed at https://www.windmillsoft.com/monitor.xml. Read https://www.windmill.co.uk/newsfeed.php for notes on how to display the live news on your own web site. Final Version of Industrial Control Systems Security Guide Published The National Institute of Standards and Technology has issued the final version of its Guide to Industrial Control Systems (ICS) Security, intended to help managers of critical infrastructures to secure their systems while addressing their unique performance, reliability, and safety requirements. Source: NIST https://www.nist.gov/el/isd/ics-062111.cfm 'Orca ears' inspire researchers to develop ultrasensitive undersea microphone Researchers have developed a microphone that can be used at any depth in the ocean, even under crushing pressure, and is sensitive to a wide range of sounds, from a whisper in a library to an explosion of TNT. They modeled their device after the extraordinarily acute hearing of orcas. Source: Stanford University News http://news.stanford.edu/.html New Sensor To Measure Structural Stresses Can Fix Itself Researchers have designed a sensor that can measure strain in structural materials and is capable of fixing itself when broken: an important advance for collecting data to help make informed decisions about structural safety in the wake of earthquakes, explosions or other unexpected events. Source: North Carolina State University http://news.ncsu.edu/releases/wmspeterssensor/ Industrial Automation Electronics Equipment Market Set to Surpass Pre-recession levels in 2011 Following initial reports of a strong first quarter for industrial automation electronics equipment (IAEE), IMS Research has upgraded its forecast to 12.9% market growth in 2011. The global IAEE market is now projected to be worth $97 billion in 2011, according to a new report, “The World Market for Industrial Automation Electronics Equipment”. Source: IMS Research http://imsresearch.com/ ________________________________________________________ ________________________________________________________ * Copyright Windmill Software Ltd * Reprinting permitted with this notice included * For more articles see https://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 https://www.windmill.co.uk/newsletter.html and an index of articles at https://www.windmill.co.uk/monitorindex.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] https://www.windmill.co.uk/ https://www.windmillsoft.com/
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