User guide

Chapter 3. Basic Operation Setup 113
Position Capture Accuracy
Servos: If you are capturing the position/value of a feedback source (encoder, ANI, LDT)
that is currently selected with the SFB command, the position capture accuracy
is ±50 µs ∗ current velocity.
If you are capturing the position of a device that is not selected with the SFB
command, the last sampled position is simply stored as the captured position.
Exceptions :
• The captured commanded position is always interpolated from the last
sampled position (of the feedback device selected with the SFB command)
and the position error, and the time elapsed since the last sample.
• Regardless of the SFB selection, one encoder position is latched in
hardware within ±1 encoder count (at max. encoder frequency) after its
dedicated trigger input is activated (see table below). Triggers A-D are
dedicated to encoders 1-4, respectively. This encoder capture
method offers the highest accuracy.
Encoder AT6250 AT6450 615n * 625n 6270 OEM6250
Encoder #1 TRG-A TRG-A TRG-A TRG-A TRG-A TRG-A
Encoder #2 TRG-B TRG-B TRG-B TRG-B n/a TRG-B
Encoder #3 TRG-C TRG-C n/a TRG-C n/a n/a
Encoder #4 n/a TRG-D n/a n/a n/a n/a
* 615n: TRG-A captures the internal resolver and TRG-B captures the external encoder.
Steppers: There is a time delay of up to 50 µs between activating the trigger interrupt input
and capturing the position; therefore, the accuracy of the captured position is
equal to 50 µs multiplied by the velocity of the axis at the time the input was
activated.
System status bits #25 through #28, reported with the TSSF, TSS and SS commands, are set
to 1 when the positions have been captured on trigger inputs A through D, respectively. As
soon as the captured information is transferred or assigned/compared (see command list above),
the respective system status bit is cleared, but the information is still available from the
register until it is overwritten by a new latch from the trigger input.
Captured values are offset by the PSET command. If scaling is enabled (SCALE1), the
captured values are scaled by the programmed SCLD value.
Trigger Functions The Trigger Functions command (TRGFN) allows you to assign additional functions to trigger
inputs that have been defined as trigger interrupt inputs (INFNCi-H):
In the TRGFN command syntax, each field of 8 enable bits is for one axis. The “c” in the first
data field is for specifying the trigger input (TRG-A through TRG-D). There are two possible
functions, corresponding to the first 2 enable bits in the syntax—the other 6 enable bits per
axis are reserved (“1” enables the function, “Ø” disables the function, “x” leaves the function
bit unchanged):
TRGFNc bbbbbbbb bbbbbbbb bbbbbbbb . . .
Trigger Letter (“A” for TRG-A , “B” for TRG-B , etc.)
FMCNEW Function (see description below)
reserved
Axis 1
GOWHEN Function (see description below)
Axis 2 Axis 3
GOWHEN Function: (TRGFNc1xxxxxx) Suspends execution of the next move until the
specified trigger input (c) goes active. If you need execution to be
triggered by other factors (e.g., master position, encoder position, etc.)
use the GOWHEN command. Refer to page 186 or to the GOWHEN
command description for additional details. Axis status bit #26 (reported
with TASF, TAS, or AS) is set to one (1) when there is a pending
GOWHEN condition initiated by a TRGFNc1xxxxxx command; this bit
is cleared when the trigger is activated or when a stop or kill command is
issued.