Delta Tau GEO BRICK LV User Manual

Turbo PMAC has two programmable acceleration limits for each motor, one for jogging, homing, and 
RAPID-mode moves (Ixx19), and one for LINEAR and CIRCLE-mode programmed moves (Ixx17).  
Both parameters are in units of counts per (millisecond-squared).  PVT and SPLINE-mode moves do not 
observe either of these limits.   

If the commanded acceleration requested of a motor by the change in velocity and the acceleration time 
parameters exceeds the limit for the motor, the acceleration time is extended so that the acceleration limit 
is not exceeded.  In a multi-axis programmed move, all axes in the coordinate system are slowed 
proportionally so that no change in path occurs.  Accelerations are compared to these limits assuming no 
feedrate override (% value of 100); if feedrate override (a.k.a. time-base control) is used, the acceleration 
limits scale with the square of the override percentage.   

Without the special lookahead buffer enabled, the Ixx17 acceleration limit works only on LINEAR-mode 
moves with segmentation disabled (Isx13=0).  In this mode of operation, the acceleration time can be 
extended only equal to the move time of the incoming move.  If this is not enough of an extension to 
observe the acceleration limit, the limit will be violated. 

The Ixx17 limit works on LINEAR and CIRCLE-mode moves executed with segmentation enabled 
(Isx13>0) and the special lookahead buffer active.  Ensuring that all these moves observe this acceleration 
limit is the most important feature of the special lookahead buffer. 

Turbo PMAC has a programmable output limit (on the command Turbo PMAC sends to the amplifier or 
the internal commutation algorithm) for each axis (Ixx69) that acts as a torque/force limit for current-
mode, sine-wave, or direct-PWM amplifiers, or a speed limit for velocity-mode amplifiers.  If this limit is 
engaged to change what the servo loop commands, Turbo PMAC’s 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 (the Ixx67 “position error” limit), which has the effect 
of slowing down too sudden a move in a controlled fashion. 

Turbo 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.  It can either 
integrate the square of current over time – commonly known as I

2

T (eye-squared-tee) protection, or 

integrate the absolute value of current over time – usually called |I|T (eye-tee) protection.  I

2

T protection 

is used when the most thermally sensitive components are resistive in nature (e.g. motor windings or FET 
transistors), because their power dissipation is proportional to the square of current. |I|T protection is used 
when the most thermally sensitive components have a constant voltage drop (e.g. bipolar transistors), 
because their power dissipation is proportional to the magnitude of current.  Because the use of the square 
of current is more common, this protection is generically referred to as “I

2

T.” 

Some amplifiers have their own internal integrated-current protection, but many others do not.  Turbo 
PMAC’s integrated-current protection can be used in either case.  It can be used with any amplifier for 
which Turbo PMAC computes current commands, whether or not Turbo PMAC also performs the 
commutation and/or digital current loop functions.  If Turbo PMAC is closing the current loop for the 
motor, this function uses the measured current values; otherwise it uses the commanded current values.  
This protection is not suitable for use in systems in which Turbo PMAC outputs a velocity command, 
either as an analog voltage or a pulse frequency.