Rockwell Automation AADvance Controller Safety Manual User Manual
Page 61
Document: 553630
ICSTT-RM446K-EN-P Issue: 10
_C
4-5
Sensor delay: 250ms
Time for actuator (an ESD valve) to fully operate: 1750ms
In this example therefore, the setting of PST for the controller should be less
than or equal to 3000ms.
Choosing Controller PST Settings
The response time allocated to a logic solver such as the AADvance controller
needs to take account of delays within the operation of sensors and actuators.
In addition, the system's scan time should be considerably less than the process
safety time.
The value of the PST shall form part of the safety considerations for the
system. The value is defined by the process design authority; the system
integrator shall calculate and verify that the process safety time meets the
stated requirements.
In an AADvance system the PST value is assigned to the system and can be
assigned to individual modules. The system PST value is enforced by the
processor modules and has priority over the module PST values. When
the system PST is not met the processor modules will fail-safe.
The input PST is also enforced by the processor modules; when the PST is
not met, the processors will present fail-safe input values to the application
logic.
Output PST is enforced by the output modules and when the output PST
is not met, the output module will assume the fail-safe state.
Note: The fail-safe state for all AADvance modules is de-energized.
You must specify the process safety time for the whole controller. If desired,
you can specify additional process safety times for individual groups of I/O
modules. The setting for the whole controller is a top level setting, which you
make once for all the 9110 processor modules. Groups of I/O modules can
inherit this setting or, if desired, use individual process safety times instead.
Notes:
The minimum controller's PST must be at least twice the application scan
time.
If you choose to specify a process safety time for a group of I/O modules,
the I/O modules use this setting instead of the top level setting.
If you do not specify a process safety time for a group of I/O modules, the
I/O modules use the top level setting.
If you do not specify any process safety time, the controller will use a
default value of 2,500ms throughout.