Rockwell Automation 1761 MicroLogix 1000 Programmable Controllers - Manual (Page 379)

Application Example Programs
E–29
Once you have entered these 2 values the following information is provided:
•
N7:1 – Counts per last Rate Measurement Period. This value is updated each
end of Rate Measurement Period with the number of counts that have elapsed.
Use this value if your application requires high-speed calculations such as
velocity.
•
N7:4 – Frequency. This value is updated once per second with the number of
pulses that occurred in the last second. This value (frequency) is calculated:
Frequency (Hz)
# pulses
1 second
=
•
N7:5 – RPM. This value is calculated once per second using the frequency
value N7:4 together with the counts per revolution value N7:2. For example, if
N7:4 contained the value 2000 (indicates 2000 Hz) and you had specified a
1000 count encoder in N7:2, the RPM calculation for N7:5 would be 120. This
equates to 2 encoder revolutions per second. Refer to the following calculation:
RPM
# pulses
1 second
= x
1 revolution
# pulses
60 seconds
1 minute
x
120 RPM
2000 pulses
1 second
= x
1 revolution
1000 pulses
60 seconds
1 minute
x
To maintain validity, you must ensure that you cannot accumulate more pulses per
rate period than counts per revolution. For example, if you have selected a 1000
pulse encoder, you cannot have more than 999 counts occur in any 1 rate
measurement period. If you determine that you exceed this rule, simply lower your
Rate Measurement Period T4:0.PRE.
Reference