------------------------Monitor------------------------- The Newsletter for PC-Based Data Acquisition and Control Issue 36 www.windmill.co.uk July 2001 --------------------ISSN 1472-0221---------------------- Hello and thank you for subscribing to Monitor. Should you wish to cancel your free subscription please visit http://www.windmill.co.uk/newsletter.html Our main article this month covers analogue output signals, whilst the Excel Corner gives tips on analysing only the most recently collected data. CONTENTS ========== * Windmill News: Windmill results in 90% efficiency improvement * Control and waves: Generating analogue signals * Excel corner ________________________________________________________ Windmill News: Windmill Results in 90% Efficiency Improvement ________________________________________________________ The free Windmill software has resulted in a 90% efficiency improvement for a company using a Quadra Check 2000 optical measuring system. You can now download Excel spreadsheet and DDE examples for the Quadra-Chek, plus the Windmill setup files, from our Setup File Library. http://www.windmill.co.uk/serial_driver.html Other files available from the library include those for the Ashtech GPS receiver, Ecom gas analyser, Furuno sonar, Mettler Toledo pH transmitter, Molyteck data logger, OMRON plc, Parallax BASIC Stamp and Sartorius balance. If you haven't yet downloaded the free Windmill 4.3 software with the LabIML serial driver, subscribe to Monitor for full instructions. ________________________________________________________ Control and Waves: Generating Analogue Signals ________________________________________________________ Analogue output signals tend to fall into one of two broad categories: control and waveform generation. When they are used as control signals, they are generally slowing changing. The computer software might be monitoring one or more input signals and using these to calculate a control signal. Alternatively, the computer may generate an output voltage as a stimulus prior to measuring the response. The second category of analogue output is that of waveform synthesis - where the output signal changes several million times a second. In both these categories, a digital-to-analogue converter is used to generate the signal. For waveform synthesis the analogue output will be a voltage signal. For control the output may be a voltage or a current. Analogue Output Hardware: Understanding the Specifications ============================================ Analogue output hardware comes in many guises, such as boards that plug into your PC or units which connect to the computer over (for example) a USB, Ethernet, GPIB or RS232 link. The hardware suppliers quote various specifications for these devices, including: Range, Resolution, Slew Rate, Settling Time and Linearity. It's not always clear, however, what they mean by these terms. * OUTPUT RANGE The maximum and minimum voltage or current that can be generated. The range may be bipolar, covering a range of -1 V to + 1 V for example; or unipolar, perhaps covering a range of 0 to 10 V. Many systems offer a choice of ranges, which you can select through either software or hardware. It's best to choose the smallest range spanning the output you need, as this optimises the resolution. * RESOLUTION The resolution is the number of steps into which the output range is divided. The resolution is normally expressed as bits (n) and the number of steps is 2n-1. This equates to 2n values. A converter with 12-bit resolution, for example, divides its output range into 212 or 4096 values. In this case a 0-10 V range will be resolved to 0.25 mV, and a 0-100 mV range to 0.025 mV. The output is thus not truly analogue but an approximation. How close the approximation depends on the resolution. * SLEW RATE Slew rate is the maximum rate of change of the output signal, measured by the rise in voltage divided by time. Slew rate is expressed as volts per microsecond. * SETTLING TIME When the D-A converter changes from its minimum output level to its maximum, the output signal swings through its "full scale". The settling time lets you know how long it will take the output to settle to its final voltage. It is normally given as the time to settle to a percentage of the full-scale voltage or current range, following a full-scale change in the voltage or current. * LINEARITY Ideally a D-A converter with n-bit resolution will convert the input range into equal steps. In practice the steps are not exactly equal, which leads to non- linearity in a plot of D-A output against output signal. This is expressed as a percentage of the output's full- scale range. * BUFFER AND MEMORY Waveform synthesis devices need a memory to hold the points used to generate the waveform. Memory may be expressed as the maximum number of points: 2 M (two million) for example. Alternatively the memory may be shown in bytes. For example, if you had a 12-bit D-A converter each point might occupy two 8-bit bytes and in our example the memory would then be 4 Mbytes. Typically you use software to synthesise the waveform, storing the points in the memory. The memory contents are then scanned into a D-A converter at the required speed - often regulated by hardware for precision. Lastly a buffer amplifier drives the actual output. * For more information on analogue outputs please contact Windmill Software at firstname.lastname@example.org ________________________________________________________ Excel Corner Filtering and Summing Columns: Showing only Recent Data ________________________________________________________ Suppose you've used Windmill to route data from your measurement instrument into Excel (see http://www.windmill.co.uk/excel/ for details how). You took readings every hour for three months and have 2208 rows of data. However, at the moment you are only interested in the readings from the previous week. To hide the superfluous data use Excel's AutoFilter. Select a cell in the date column. From Excel's Data menu select Filter, then AutoFilter. The column title becomes a drop-down list. Select the down arrow and choose Custom. You can now elect to only show information gathered on dates greater (or later) than a week ago. You can use the AutoFilter not just for dates but for any criterion you like. To filter data from two columns, such as temperatures over 70oC in the last week, use the Advanced Filter option. So far so good, but what if you were analysing your data - perhaps summing your data columns. When you filter the data the summed total remains for all the data, not just that now displayed. You can correct this and display a value which changes according to the rows you have chosen to display. To do this, instead of the SUM function use the SUBTOTAL function. SUBTOTAL will ignore any hidden rows that result from a list being filtered. Use SUBTOTAL(9, ref), where ref is the range of data cells in the column. You can also use SUBTOTAL for Average, Maximum, Minimum, Standard Deviation and other Functions. In these cases, instead of 9 use these numbers 1 AVERAGE 2 COUNT 3 COUNTA 4 MAX 5 MIN 6 PRODUCT 7 STDEV 8 STDEVP 9 SUM 10 VAR 11 VARP I hope you've found this tip useful. If you would like to share any tips for using Excel in data acquisition applications, please send them to email@example.com ________________________________________________________ * 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: firstname.lastname@example.org http://www.windmill.co.uk/ http://www.windmillsoft.com/
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