Rockwell-automation 1769-HSC Compact High Speed Counter Module Manual de usuario descargar pdf (Pagina 35)

Rockwell Automation Publication 1769-UM006E-EN-P - July 2013 35

Module Operation Chapter 2

Rate Method

Selectio

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%

1 2 ... 30 31 32 33 34 35 36 37 38 39 40 ... 169 170

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