Delta Tau GEO BRICK LV User Manual

 

 

Use for Cascaded Loops 

Offset-mode following can be very useful for the implementation of cascaded servo loops.  The output of 
the outer loop is used as the master position for the inner position loop (and not sent directly to an 
actuator).  In this mode of operation, typically, the inner loop is a standard position loop; the outer loop 
closes a loop on force or tool height, or some other process variable.  The use of offset mode for this 
purpose permits normal trajectory commands to the inner loop, with the possibility for simultaneous 
corrections from the outer loop.  Refer to the Cascading Servo Loops and Setting up the Servo Loop 
sections for details. 

Changing Following Modes 

The following mode is used in determining how the calculations relating motor and axis position are 
performed, so changing the mode changes how these calculations would be done for subsequent moves, 
creating a mismatch between motor and axis position, which could cause a jump in the motor motion on 
the next move if not corrected.  This is true regardless of whether or not following is enabled.  To correct 
for this mismatch, a PMATCH position-matching command must be executed before the next motion 
program move command is processed.   
Turbo PMAC automatically executes a PMATCH command any time a R (run) or S (step) command is 
issued; however, if the mode is changed in the middle of a motion program, this on-line command must 
be issued explicitly.  A DWELL command should precede this command (to stop program lookahead); 
another DWELL command should trail this (to give the command time to execute from the on-line queue).   
The program sequence would look something like: 

DWELL0 

; Stop lookahead 

I106=3 

; Change following mode 

CMD"&1PMATCH" 

; Re-match positions 

DWELL10 

; Delay for PMATCH execution 

A more sophisticated method of coordination to external axes is that of time-base control, in which the 
input signal frequency controls the rate of execution of moves and programs.  Time-base control operates 
on an entire coordinate system together.  Specify which encoder register is receiving the input frequency, 
and the relationship between the input frequency and the program rate of execution.  This not only varies 
the speed of moves in proportion to the input frequency (all the way down to zero frequency), but also 
keeps total position synchronization.  This permits operations such as multi-pass screw threading. 

Turbo PMAC’s motion language expresses the position trajectories as functions of time.  Whether the 
moves are specified directly by time, or by speed, ultimately the trajectory is defined as a position-vs-time 
function.   
This is fine for a great number of applications.  However, in many applications, we wish to slave the 
Turbo PMAC axes to an external axis not under Turbo PMAC control (or occasionally, an independent 
axis under Turbo PMAC control in a different coordinate system).  In these applications, we want to 
define the Turbo PMAC trajectories as functions of master position, not of time. 

Real-Time Input Frequency 

Turbo PMAC’s method for doing this leaves the language expressing position as a function of time, but 
makes time proportional to the distance covered by the master.  This is done by defining a real-time input 
frequency (RTIF) from the master’s position sensor, in units of counts per millisecond.  For example, 
define an RTIF of 32 cts/msec.  Then, in time-base mode, when the program refers to a millisecond, what 
it is really referring to is 32 counts of the master encoder, whatever physical distance that is.  If a move is 
programmed in the slave program to take two seconds, it will really take 64,000 counts of the master 
encoder to complete.