PULSE_GEN / PULSE_GEN_S - Pulse Generator

 

This pulse generator function block generates a pulse signal with a configurable pulse/pause ratio. The pulse/pause ratio is set using the function block inputs PTH and PTL.

The generated pulse signal can be used to control other safety-related and standard functions/function blocks.

This topic contains information on the following:

• Description of formal parameters

• Exception avoidance

• Timing diagram

• Application example

Formal parameters of PULSE_GEN/PULSE_GEN_S

Exception avoidance

Validating the entire safety function is the only way to detect incorrectly set time values at the PTL and PTH inputs.

Impermissible signals at the IN input:

If no further measures for exception avoidance are taken, signal levels at the IN input which change or toggle sporadically will cause the pulse generator to restart on each and every positive edge. A TRUE pulse at the Q output will be set to FALSE immediately on a negative edge.

Undesirable connection of the IN input:

If no further measures for exception avoidance are taken, the undesir­able connection of the IN input to a signal not designated for this purpose will cause the signal connected in error to control pulse generation. As a result, the pulse generator will be restarted on every positive edge. A TRUE pulse at the Q output will be set to FALSE immediately on a nega­tive edge.

Impermissible static TRUE signal at the IN input (operating current principle):

If no further measures for exception avoidance are taken, a static TRUE signal at the IN input will cause the pulse generator to run continuously and it will not be possible to stop it.

Impermissible static FALSE signal at the IN input (closed-circuit current principle):

If no further measures for exception avoidance are taken, a static FALSE signal at the IN input will cause the pulse generator to be idle continu­ously and it will not be possible to start it.

Connection with safety-related function block SF_TestableSafety­Sensor: Impermissible FALSE signal or no connection at the IN input.

If the pulse generator FB is connected to the safety-related function block SF_TestableSafetySensor (TSS) in such a way that a positive edge of its Q output signal at the StartTest input of the TSS function block will trigger the sensor test, the IN input of the pulse generator FB must always be connected to the TRUE constant as shown in the appli­cation example below. You must always validate the entire safety func­tion.

Causes of the named error sources and how to rectify them

The above mentioned signals may be caused by:

• Programming errors in the application program (user errors)

• Cross-circuit, short-circuit and cable break (user errors, wiring errors)

To prevent this, the following measures can be taken, depending on the safety function:

• Use of safety-related device signals

• Use of options for cross-circuit detection

• Verification of the safety-related logic in the code editor and subse­quent validation of the entire safety-related application code

Timing diagram

The following example shows the timing sequence for the pulse ratio 2:3 (for example, PTH = 2.0 s and PTL = 3.0 s).

Application example

In this example, the pulse generator starts a sensor test periodically. For this purpose the Q output of the pulse generator is connected to the StartTest input of the safety-related function block SF_TestableSafety­Sensor. A positive edge at this input starts the sensor test of the connected safety sensor.

Exception avoidance when using the pulse generator FB with the safety-related function block SF_TestableSafetySensor: With the correct connection shown in the example graphic below, a pulse at the Q output of PULSE_GEN does not guarantee that the sensor test will actually be carried out.

This is because in order for the sensor test to be carried out, the SF_TestableSafetySensor function block must permit it. In other words, the TestPossible TSS output must be set to TRUE when the pulse generated by PULSE_GEN is present at the StartTest TSS input. You must take this into account in the context of your risk analysis.

The frequency of the sensor test (test interval) is thus set using PTL input of the pulse generator. The pulse duration (PTH input) is of no relevance to this application.

The connected sensor is a single-channel connected light grid. The SF_TestableSafetySensor function block is connected/configured as follows:

• Both function blocks are permanently activated by the TRUE constant at the IN input (PULSE_GEN) and the Activate input (TSS).

• The sensor's status signal is connected to the input terminal I0 of the safety-related input device SDI 1. This terminal is assigned to the global input variable SI_1_0 which is fed to the function block input S_OSSD_In for evaluation.

• The function block output S_TestOut outputs the start signal for the sensor test to the sensor. The sensor's test input is connected to output O0 of the safety-related output device SDO 4 (assigned to the global output variable SQ_4_0).

• The function block output TestExecuted signals the successfully executed sensor test. It is connected to a standard global output variable which is in turn assigned to terminal NO0 of the standard output device DO 10. A signal lamp is connected to this terminal.

• Input S_StartReset = SAFEFALSE specifies a start-up inhibit after the controller has been started up or the function block has been activated. In addition, S_AutoRestart = SAFEFALSE specifies a restart inhibit for the function block after the TRUE signal returns at the S_OSSD_In input (in other words, the safety sensor's light beam is no longer interrupted). These two start-up inhibits are only removed when there is a positive signal edge at the Reset input. To this end, the S1 reset button is connected to terminal NI0 of the standard input device DI 7. This standard device terminal is assigned to the global standard input variable NSI_7_0.