Logging Data at a Specific Rate with VirtualBench-Logger and NI 435x Primary Software: Other NI Software>>VirtualBenchPrimary Software Version: 2.51 Primary Software Fixed Version: N/A Secondary Software:
Problem: I want to log my data readings to disk every minute with VirtualBench-Logger and my NI 4350. I can specify to log every X intervals, but for the reading rate my choices are 'Fast' and 'Slow'. How does this translate into seconds or time? Solution: The NI 435x High-Precision Loggers are different in the way they have their reading rate set. It is dependent on the Filter Setting selected. The table below (from the NI 4350/4351 User Manual) lists the different reading rates based on the filter setting. Also from the manual: "In single-channel measurements, the reading rate is the same as the notch filter frequency 10, 50, or 60 readings/s. In multiple-channel measurements, the reading rates adjust to allow the analog and digital filters to settle to the specified accuracy. To determine the reading rate per channel when scanning multiple channels, divide the multiple-channel measurement reading rate by the number of channels in the scan." Note: If you are using a DAQ device other than the NI 435x, the reading rate is going to be dependent on the specifications of your DAQ device. If you need more control over your logging, VI Logger allows you set scan rate. So how do you set this in VirtualBench-Logger? In the Timing Configuration the 'Reading Rate' selects whether 10 Hz (slow) or 50/60 Hz (fast) is being used. If fast is being used the '50/60 Hz Power Line Freq' radio button in the main Settings dialog determines which filter is being used. If slow is being used this radio button is ignored. The 'Start and 'End Channel's in the main Settings dialog determine if a single channel or multiple channels are being read. To translate the VirtualBench settings into seconds use the following formula:
i.e. If you are reading 8 channels using the Slow setting and logging every interval you will have 2.86 seconds between your log entries ( ( 8 / 2.8 ) * 1 ). A more helpful formula below, allows you to calculate the number of Time Intervals you need if you know the number of seconds you want between log entries and you know the number of channels you need to log and have selected a filter setting.
So if you want to log 6 channels every 60 seconds and want to filter only 60 Hz Power-Line Frequency (Fast & 60 Hz), then you need to set your Log to Disk every X Time Intervals to 97 ( 60 * ( 9.7 / 6 ). The Log to Disk every X Time Intervals must be even, so round up if you want to insure that your number of seconds will take place. The above assumes a resistance of less than 50 K Ohms and multiple channels. For single channels use the Single-Channel Measurement reading Rate and the Number of Channels equals one. If you have a TBX-68T, TC-2190, CB-27T or PSH32-TC6 configured as your accessory for the 435X in the Measurement and Automation Explorer you will scan 2 additional channels when logging thermocouples and scan 1 additional channel all other times. These additional channels need to be included in the above calculations for correct results. Since the time stamp in the log file is not generated by the hardware it will vary from the actual time and the log time stamps be slightly longer if VirtualBench is interrupted or other Tasks take up CPU time. Related Links: NI 4350/4351 User Manual KnowledgeBase 11EH3L5Q: How Fast Can I Go with the VirtualBench-Logger? Attachments:
Report Date: 07/06/1999 Last Updated: 01/16/2003 Document ID: 1N5FJ4TS |
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