NC safety functions
A monitoring function (watchdog) is provided in order to monitor the operation and regular updating of the cyclical interface between the PLC and the NC. In addition, there is task runtime overrun monitoring for each task and position lag monitoring and end position monitoring for the NC axes.
Notice | |
No safety functions in the sense of safety technology The NC safety functionalities are purely functional settings that are not safety functions in the sense of safety technology. |
There are further monitoring options on the hardware side.
Also find out about other safety measures for axis commissioning.
Watchdog for cyclic axis interface between PLC and NC
The watchdog (function monitoring) functionality between the cyclic axis interface of the PLC and NC should always be activated. This is the case if any value other than zero is entered for the watchdog. The value specifies the number of sequential task cycles following which the watchdog will trigger if no new information has been transferred between the NC and the PLC. If the watchdog is triggered, the corresponding axis interface (PlcToNc or NcToPlc) is cleared, i.e. zeroed.
Example: When the watchdog is active, it is ensured in the following cases that the NC axes are stopped by means of the removal of the controller and feed enable caused by the watchdog. When:
- the PLC is stopped
- an infinite loop has been programmed within the PLC
- an FPU exception occurs
Task time-out monitoring
For purposes of diagnosis and analysis the task time-out monitoring should be activated. This is true both for the SAF task (I/O task of the NC) and for the SVB task of the NC. As regards content this monitoring has no effect, but should there be an occasion where an unexpected task time-out occurs, the response is in the form of a message box and an additional entry in the event display.
Position lag monitoring, end position monitoring, target position control
Right from the start of any operations each axis should be driven with both "position lag monitoring" and "end position monitoring" active. Even if an axis that has not yet been optimized travels with sometimes very large lag errors, these elementary monitoring mechanisms should not be switched off, but should be parameterized accordingly (Position Lag Monitoring).
Furthermore, there is the option of automatically checking the target position (Target Position Monitoring).
Direction inversion, direction monitoring
There are functionalities, e.g. position compensation on a master axis, which can cause an inversion of the direction of movement. To avoid an unwanted direction of travel, there is a direction-dependent feed enable that stops the axis instantaneously if it travels in the wrong direction.
Maximum velocity
Furthermore, there is the possibility to define the maximum allowed velocity of an axis in the axis parameters or to limit the output of the drive in percent.
For example, it can happen that, by mistake, the direction of actuation of the axis control loop (positive feedback) is altered as a result of changeover of drive or encoder polarity, and the axis, with full logical consistency, drives towards the mechanical end position at maximum output value.
In the following situations the maximum velocity can be exceeded:
- by position compensation of the master or slave axes
- by setting or changing the coupling factor of a slave axis or (indirectly) of the flying saw
- by externally generated data in the FIFO or the table slave axes
Stop
All master axes can be stopped at any time.
Slave axes can be stopped by disconnecting them online and converting them into master axes, which can then be stopped. The FIFO axes and the NCI group can be stopped.
Hardware monitoring
It may be that a facility in an emergency situation (emergency stop, watchdog, etc.) must not be allowed under any circumstances, for mechanical or other reasons, to halt abruptly its axes in the next I/O cycle (e.g. to output 0 V suddenly). Such behavior can only be ensured via the drive hardware that is present. To this end most manufacturers offer simple digital circuitry options that ensure that an axis is brought to a halt in a defined way (braking ramp, standstill window for electrical deactivation of the control system and activation of the brakes, etc.).