Omega Engineering OMB-NETSCAN, Руководство пользователя

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Configuration Aspects for Programmers Appendix C
C-4 NetScan User’s Manual
Normal Mode
NetScan has the ability to reject noise resulting from AC line pick-up when an Average weight setting of 16 or
more measurements per line cycle is used. This noise rejection factor is important, as it allows for higher
accuracy. When an average weight setting of 32 or more is used, NetScan is operating in a line cycle integration
manner and can provide readings for DC voltage, AC voltage, and thermocouple-based temperatures at a rate of
up to 44 channels per second. Noise filtering improves by selecting higher average weight settings; but reduces
the overall response time of the unit.
You can use the following formulas to compute the amount of time needed to acquire one scan.
For 60 Hz systems:
Scan time = #channels x (#samples/c 12) x 520.83 µs
For 50 Hz systems:
Scan time = #channels x (#samples/c 12) x 625.00 µs
To use the Normal scan mode, such that noise filtering (line cycle integration) is enabled, use the
M#
and
W#
commands, or use ChartView (see Chapter 4).
Example:
M#0 ‘Select normal mode
W#16 ‘Select 16 samples over which to average
Note: The number of samples in the above example could also be one of the following: 64, 128, or
256. However, using the weight of 256 will limit the number of active channels in an
acquisition to 122 due to hardware constraints.
If noise resulting from AC voltage line pick-up is not a problem, you can set the system to average 1, 2, 4, or 8
line cycles per channel for faster scanning throughput (high-speed, multiple channel application). For example,
if the unit is configured to take 1 sample per channel, it will scan 147 channels per second.
Note: In regard to AC voltage measurements, NetScan only supports AC voltages having an input signal
frequency which is an integer multiple of the AC line cycle. For this reason, sampling weights less than
32 can yield incorrect results.
To use the Normal scan mode for high-speed, multiple channel applications (when noise filtering is not needed)
use the
M#
and
W#
commands, or use ChartView (see Chapter 4).
Example:
M#0 ‘Select normal mode
W#1 ‘Select 1 sample over which to average
Note: The number of samples in the above example could also be one of the following: 2, 4, or 8.
Reading inaccuracies can result if common mode voltages on adjacent channels are widely dissimilar. This is
due to inadequate settling time at the instrumentation amplifier when the unit is scanning between channels.
Settling time is defined as the time measured to a point at which a transient voltage value is within a specified
percentage of the final voltage value. Settling time is usually measured in nanoseconds. To eliminate these
inaccuracies, the settling time can be increase with use of the
D#
command as described in Appendix A,
API Command Reference.