Rockwell Automation 1756-CFM ControlLogix Configurable Flowmeter Module - Manual (Page 36)

36 Rockwell Automation Publication 1756-UM010B-EN-P - December 2011
36 Configurable Flowmeter Module Features and Operational Modes
Operating in
High Resolution
Frequency Mode
In High Resolution Frequency mode, the CFM module calculates frequency
over a user-defined time period up to 2 seconds. Frequency sampling begins
on the leading edge of the first pulse and ends on the next pulse to occur after
the sampling period expires.
This figure shows a CFM module in a turbine shaft speed monitoring
application in high resolution frequency mode.
You must calculate a resolution for this mode. Use the following equation to
calculate resolution.
This table lists sample F
in
and T
sample
values and the corresponding resolution.
The values listed in Table 4 are listed for example purposes. Specific values will
change according to your application.
42731
CV
Pulse output
Fuel
Monitors fuel flow
and total gallons
Generator
Electricity
Shaft
encoder
Turbine
Shaft speed
1756-CFM
Logix controller:
 monitors shaft speed
 performs PID calculations
 adjusts valve by a 1756-OF6VI
analog output module.
1756-OF6VI
Table 4 - Sample Resolutions
F
in
T
sample
Resolution
60Hz 0.033 seconds 0.00045Hz
5000Hz 0.020 seconds 0.0625Hz
50,000Hz 0.050 seconds 0.25Hz
F
in
(250 x 10 -9 )
T
sample
Resolution =
Where:
 F
in
is the input frequency
 T
sample
is the sample time period