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SDM-SW8A Switch Closure Input Module
8.2 SPST Switch Closure
Single-pole single-throw switches are either open (high state) or connected to
ground (low state). Typical SPST switches include contact closure (reed
switch) anemometers, tipping bucket rain gauges, and open collectors
(semiconductor switches). Many control devices utilize open collectors and
provide terminals for monitoring the switch.
For all SW8A measurements, the 100 Hz maximum input frequency and 5 ms
pulse width specifications must be taken into consideration. To illustrate,
consider the Met-One 014A Cup Anemometer which is an SPST-type sensor.
The 014A calibration is:
MPH = 1.789 * f + 1
where: MPH = miles per hour
f = pulse frequency in Hz
The 100 Hz maximum input frequency to the SW8A equates to 180 MPH.
The duty cycle of the 014A as measured by the SW8A is 35%. A 5 ms pulse
width becomes a limitation at 70 Hz (0.35/0.005sec), or about 126 MPH. The
SW8A's maximum input frequency and 5 ms minimum pulse width
specifications are not a limitation given the 014A's maximum calibrated speed
of 100 MPH.
8.3 DC Voltage Pulse
Voltage pulse transitions from below 0.9 V to above 4.0V, not exceeding
±20V, with a minimum pulse width of 5ms, are counted accurately at any
frequency less than or equal to 100 Hz.
8.4 Duty Cycle
Duty cycle is not an exact measurement due to the SW8A input filtering and 2
ms sampling frequency. Signal magnitude also affects duty cycle
measurements. Optimum duty cycle measurements result if an integral number
of cycles are measured per datalogger scan interval, and the scan interval is at
least 1 second.
Input Filtering and Sample Frequency Error - In a "worst case" analysis,
input filtering will distort the time that the signal is high by ±2.5 ms. If the
input filtering is at worst case, the 2 ms sampling frequency can create an error
of ±2 duty cycle samples on any measurable cycle.
Equations given below estimate the "worst case" duty cycle measurement error
for a 50% duty cycle and the minimum/maximum measurable duty cycle for a
given frequency. The error limits are calculated, assuming a sample interval of
a single cycle. The error may be significantly reduced by allowing the SW8A
to measure duty cycle over several cycles. As shown by the equations, the
error decreases with decreasing frequency.
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