User's Manual
PMAC User Manual
120 Making Your Application Safe
Ix17 is particularly useful to prevent unreasonable moves early in system development, when it is easy to
make large mistakes in scaling. In some systems, it can be used during the actual application to make
sure that accelerations always happen in the minimum time. In these applications, the TA and TS
acceleration times are set very small so that the Ix17 limit is always used.
Sometimes the Ix17 limit is too effective limiting when it is not wanted; other times it is not effective
enough, permitting the trajectory to violate the limit. For more details, reference Chapter 14, Writing
Programs for PMAC.
Command Output Limits
PMAC has a programmable output limit (on the command PMAC sends to the amplifier) for each axis
(Ix69), which acts as a torque limit for current-loop amplifiers, or an actual velocity limit for tachometer
amplifiers. If this limit is engaged to change what the servo loop commands, the PMAC anti-windup
protection activates to prevent oscillation when coming out of the limiting condition. In addition, there is
a limit on the size of the error that the feedback filter is permitted to see (Big Step limit: Ix67), which has
the effect of slowing down too sudden a move in a controlled fashion.
Integrated Current (I
2
T) Protection
PMAC can be set up to fault a motor if the time-integrated current levels exceed a certain threshold. This
can protect the amplifier and/or motor from damage due to overheating. This type of protection is
commonly known as I
2
T (eye-squared-tee) because it measures integrates the square of current over time
– power dissipation is proportional to the square of current.
Some amplifiers have their own internal I
2
T protection, but many others do not. The PMAC I
2
T
protection can be used in either case. It reads the PMAC commanded current registers to determine
current levels, so it can be used without bringing actual current measurement signals into PMAC. It can
be used with any amplifier for which PMAC computes current commands, whether or not PMAC also
performs the commutation and/or digital current loop functions. It is not suitable for use in systems
where PMAC outputs a velocity command, either analog velocity or a pulse frequency.
Note:
The I
2
T protection feature is not available on PMACs with the Option 6 Extended
Servo Algorithm firmware. Ix57 and Ix58 have other functions in that firmware
version.
Two I-variables control the functioning of the I
2
T protection for each motor. Ix57 is the continuous
current limit magnitude. It has the same units as the Ix69 instantaneous output limit, bits of a 16-bit DAC
(even if some other output device is used). Both have a range of 0 to 32,767, where 32,767 is the PMAC
maximum possible output magnitude. Generally, Ix57 will be 1/4 to 1/2 of the value of Ix69.
Ix58 is the integrated current limit parameter. If Ix58 is set to 0, this function is disabled. If Ix58 is
greater than 0, PMAC will compare the integrated current value to Ix58. When the integrated current
value exceeds this value, PMAC will fault this motor as if an amplifier fault had occurred. The offending
motor is killed; if it was in a coordinate system running a motion program, that motion program aborted;
other motors are killed according to the setting of bits 21 and 22 of Ix25.
The PMAC I
2
T function works according to the following equation:
t
2
32768
57Ix
2
32768
d
I
2
32768
q
I
SumSum
∆
−++=