User's Manual

PMAC User Manual
Making Your Application Safe 117
MAKING THE APPLICATION SAFE
Responsibility for the Safety of a Control System
Delta Tau Data Systems has provided many safety features on the PMAC controller, and invested many
resources to make PMAC a safe product. However, the ultimate responsibility for the safety of a control
system using PMAC must lie with the system designer, utilizing the safety features on PMAC and in
other parts of the system.
Hardware Overtravel Limit Switches
PMAC has positive and negative hardware overtravel limit switch inputs associated with each encoder
input. These inputs are optically isolated, with fail-safe circuit design. The inputs must actively be held
low, with the card sourcing current (i.e. a normally closed switch) for PMAC to consider itself not into
the limit. The source of the current is either the external +15V input that powers the analog output stage,
the +12V bus power line if that is jumpered over to power the analog output stage (defeating the analog
optical isolation), or a +24V input brought in on the JMACH2 OPTO+V pin. Jumpers E89 and E90
control which of these sources is used. They must be set properly for the card to be able to move.
Each motor must be directed to look at one of these pairs of inputs — this is done by setting I-Variable
Ix25 (for motor #x) to the address of the register that holds the appropriate inputs. On hitting a limit,
PMAC decelerates the offending motor at a user-programmed rate
(see I-variable Ix15). If the motor is in
a coordinate system that is running a motion program at the time, all motors in the coordinate system are
decelerated to a stop at their own Ix15 rate. The effect is equivalent to issuing an A (abort) command.
The limit input pins are direction sensitive: the -LIM pin only stops positive direction moves (those
coming at it from the negative side), and the +LIM pin only stops negative direction moves (those coming
at it from the positive side). This makes it possible to command a move out of the limit that you have run
into. However, this also makes it essential to have your limit switches wired into the proper inputs, or
they will be useless.
The hardware limit function can be disabled for a motor by setting bit 17 of Ix25 to 1. This can be useful
for homing into a limit switch (which will not work if the limits are enabled, the default condition), or for
a system where hardware overtravel limit switches are not practical, such as rotary tables. If the address
of the flags in Ix25 were $C004, the value of Ix25 with limits disabled would be $2C004.
Note:
PMAC brings the commanded trajectory for the motor to a stop at the Ix15 rate as
soon as it detects a limit condition. If there is significant following error at the
time, the actual position can try to catch up to the commanded position for a long
period of time. With a large enough following error, it is possible that the
commanded position would be well past the limit and into the hard stop. It is
important to set a reasonable fatal following error limit and to allow sufficient
room past the limit switch to absorb errors up to that following error limit.
The polarity of the limit switches is the opposite of what many would consider
intuitive. That is the -LIMn input should be tied to a switch at the positive end of
travel, and the +LIMn input should be tied to a switch at the negative end of travel.
Make sure the polarity of the limit switches are tested carefully.