F Systems Mengenmessung En

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FAQ on F-Systems

How can quantities be detected failsafe using

SIMATIC F-Systems ?

FAQ



Failsafe recording of quantities and

Integration of analog signals

Quantity measurement with F-Systems Entry ID: 24359548

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Table of Contents

Table of Contents ......................................................................................................... 2 Question 2

How can quantities be detected failsafe using SIMATIC F-Systems ? ........ 2 Answer 2

The instructions and notes listed in this document provide a detailedanswer to this question. ............................................................................... 2

1 Task.................................................................................................................. 3 2 Solutions ......................................................................................................... 4 2.1 Quantity measurement by means of evaluating recorded pulses..................... 4 2.2 Quantity measurement by evaluating an analog signal .................................... 6 3 History ............................................................................................................. 8

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Question

How can quantities be detected failsafe using SIMATIC F-Systems ?

Answer

The instructions and notes listed in this document provide a detailedanswer to this question.






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1 Task

Worum geht es?

For critical processes, where the failsafe SIMATIC F-Systems is often used,failsafe detecting and summing of quantities is required. There are twodistinctive ways to do this:

Methods for quantity measurement

The quantities are measured by means of evaluation of

• recorded pulses or

• an analog signal representing the quantity.

Both methods are each briefly discussed below.

Method 1: Quantity measurement by means of evaluating recorded pulses

The quantity measurement delivers a certain number of pulses per quantityunit.

Recording of pulses is realized via a failsafe digital input and subsequentpulse count. The maximum recordable pulse frequency here depends onthe form of the signal (pulse / pause ratio). Depending on this ratio, theinput driver and counter must be integrated into a respective watchdoginterrupt cycle.

The maximum recordable frequency is relatively low, therefore there is onlya limited application for this method. The maximum frequency isapproximately 10 Hz at a pulse / pause ratio of 1:1 and a programprocessing cycle of 20ms.

Method 2: Quantity measurement by evaluating an analog signal

The signal representing the quantity is available as an analog signal.

Most modern quantity measuring devices additionally supply the currentflow rate as analog signal, otherwise a frequency / current converter isused. In this case, the signal is read via an analog input driver andintegrated in the system. However, the block libraries of F-Systems does

not provide an integrator. This function must therefore be emulated usingthe existing blocks.




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2 Solutions

What will you find here?

The previous chapter mentioned and briefly discussed two methods ofquantity measurement. For both methods

• quantity measurement by means of evaluating recorded pulses(chapt. 2.1)

• quantity measurement by evaluating an analog signal (chapt. 2.2)

a solution option on program level is illustrated.

2.1 Quantity measurement by means of evaluating recorded pulses

Program description

The pulses are recorded with a DI driver and counted with an F counterblock F_CTUD. The block can count up as well as down and has a resetinput. The group signal is available at output CV as a failsafe value. Theoutput is a failsafe integer value which is limited to the value 32.767.

Note A number overflow at the output of the counter must be prevented.Further infos on the F_CTUD block are available in the online help of theblock.

Counter with F_CTUP block




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Application of an adderThe restriction of the counter block can be by-passed by using an adder. Atthe input signal an edge evaluation is performed and with each rising edgea memory is increased by 1. This is realized with the F_ADD_R block whichworks with numbers of the REAL type, therefore the maximum count valuepossible is 8 388 607.

Counter with adder




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2.2 Quantity measurement by evaluating an analog signal

Program description

The following program description refers to the figure specified furtherbelow.

The input signal from the AI driver is added to the previously accumulatedvalue in each cycle and then related to the time.

In the logic diagram shown here, the integrator can be reset at the input INof the RESET_NOT block using a "1" signal. As long as this "1" signal ispending, the output of the integrator remains on"0" signal.

To enable the integrator, input IN3 of the AND block ENABLE can be used.The enable is given by generating a "1" signal at the input IN3.

The minimum quantity is set at the input MAX of the limit value blockMIN_FLOW. This enables suppressing smaller quantities (for examplefluctuations around the value 0). This block might be void if the integrator iscontrolled via an enable sign (for example if the enable only exists when avalve is open).

The smaller the time interval in which the input signal is recorded, the fasterthe count value increases at the adder and the larger the count value usedfor the division to create the relation with the integration time. A result might

be that after some time, the “small“ input signal at the adder is only addedwith rounding error or is not at all added to the group counter (not likely forcharge counters).

Generally, the maximum count value should only be 1 000 000 times largerthan the smallest significant digit.

Example:

• Measuring range 0 to 100

• smallest significant digit 0.1 (0.1% tolerance)

The resulting maximum count value is 100 000.

The output of the group counter is restricted to 8 388 607 (mantissa) due tothe number display in REAL.




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Integrator

Solution with additional memory

Depending on the size of the input signal and the cycle of the OB block, theremay be large differences between the input signal and the count value at theadding block ADD_VALUE, as well as an overflow at the output.To prevent this, the following solution contains an additional storage whichprevents the count value for the integration from becoming too large.

WARNING In this solution the block order must be followed.

The maximum value of the integrator (ADD_VALUE block) is set at the MAX

input of the VALUE_MAX block. When reaching the maximum value, the valuefrom the output of the DIV_TIME block is added to the ADD_OUT_MEMmemory. The output of the integrator is made up of the total of theADD_OUT_MEM buffer and the DIV_TIME output.




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Integrator with second memory

3 History

Version Data Change

V 1.0 15.01.2007 First edition

F Systems Mengenmessung En

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