User's Manual

PMAC User Manual
118 Making Your Application Safe
Software Overtravel Limits
PMAC also has positive and negative software limits for each motor to complement or replace the
hardware limits. The user-set values (Ix13 and Ix14 parameters for motor #x) for these limits cannot be
saved in EAROM on battery-backed boards; they are held in battery-backed RAM. The behavior on
hitting these limits is the same as for hardware limits. A value of zero in these parameters disables the
limit.
The software limits are disabled automatically during homing search moves, until the homing trigger is
found. As soon as the trigger is found, the software limits are reactivated, using the new home position as
the reference. If overtravel limits are used, they should be far enough from the home position to allow for
deceleration and turnaround after the trigger is found.
Following Error Limits
PMAC has three following error limits for each motor (following error is the difference between the
commanded position and the actual position at any time). The following error limit is an important
protection against serious system faults, such as loss of feedback that can cause dangerous conditions like
full speed runaway.
Fatal Following Error Limit
One of these limits (Ix11) is a fatal limit, which causes a shutdown of the system – zero output
commanded, amplifier disabled (i.e., the motor is killed), moves for the motor and programs for the
motor's coordinate system aborted.
Which amplifiers get disabled – only the offending motor, all in the coordinate system, or all in PMAC –
is determined by Ix25. This limit is intended for conditions where something has gone seriously wrong
(e.g., loss of feedback or power stage) and all operation should cease.
After the motor or motors have been killed due to the fatal following error limit, closed-loop control can
be re-established with the J/ command (single motor), the A command (coordinate system), or the
<CTRL-A> command (entire card).
Warning Following Error Limit
The second limit (Ix12) is a warning limit – when exceeded, PMAC sets status bits for the motor and the
motor's coordinate system, and can set output lines on the control panel connector, the machine
connectors, and through the programmable interrupt controller (for PMAC PC and PMAC STD). This
permits special action to be taken, either by PMAC itself through a PLC program, by the host, which can
find out through an interrupt or by polling the card, or by an operator notified with one of the external
signals.
These limits may be disabled by setting the parameter to zero, but this is strongly discouraged in any
application that has the potential to kill or injure people, or even to cause property damage. Disabling the
fatal limit removes an important protection against serious fault conditions that can cause runaway
situations, bringing the system to full power output faster than anybody could react.
Integrated Following Error Protection
In addition to the normal following error protection provided by the Ix11 variable for each motor, PMAC
can shut down the motor if the time-integrated value of the following error exceeds a preset value. This
integrated error feature can protect against those cases where the magnitude of the measured following
error never gets very large — for example, a loss of feedback followed by a very short commanded move.