Instruction Manual

Rockwell Automation Publication 1769-UM006E-EN-P - July 2013 35
Module Operation Chapter 2
Rate Method Selectio n
By knowing when to use each method, an optimal rate determination can be
made.
Use the following information to choose the appropriate calculation method. In
general, consider the effect of having the count off by ±1 in each method at
frequencies of interest to see if the resulting inaccuracy is acceptable.
Per Pulse Method Example
If the frequency of interest has 100 counts (of the 1 μs clock) between pulses, an
error of 1 count results in a 1-in-100, or 1%, error. If there are 1000 counts
between pulses, then the error is 1-in-1000, or 0.1%. Error for a variety of pulse
values is shown below.
Cyclic Method
Because the update time is programmable, there is more flexibility in choosing the
correct fit when using the Cyclic Method.
Error estimates are shown below for a variety of update times.
TIP
Fractional rates are not reported by the module, but can be calculated
from Ctr[n].PulseInterval in your control program.
Table 7 - Per Pulse Errors
Actual 1 µs
Internal
Pulses
(1)
(1) 1.9999 can be rounded to 2 and so on.
Reported
Pulses
Real Frequency Reported
Frequency
% Error
2 1 500 kHz 1 MHz 100%
9 10 111 kHz 100 kHz 11.1%
101 100 9.901 kHz 10.000 kHz 1.00%
1001 1000 999 Hz 1000 Hz 0.10%
9999 10,000 100.01 Hz 100.00 Hz 0.010%
99,999 100,000 10.00010 Hz 10.00000 Hz 0.001%
Table 8 - Maximum Cyclic Rate Errors
CyclicRateUpdate
Time x Scalar
Frequency
100 Hz 1 kHz 10 kHz 100 kHz 1 MHz
1 N/A N/A 20.02% 2.011% 0.210%
10 N/A 20.11% 2.020% 0.210% 0.030%
100 20.01% 2.110% 0.220% 0.031% 0.012%
1000 3.010% 0.310% 0.040% 0.013% 0.010%
10,000 1.210% 0.130% 0.022% 0.011% 0.010%