Gemini GV6K and Gemini GT6K Programmer's Guide

Master Velocity
Relative to Master
Position Prediction;
Variation in Master Velocity: Although increasing master position filtering (increasing the
FFILT command value) eliminates the error due to sampling accuracy, it increases the error
due to variations in master speed when the master position prediction mode is enabled
(FPPEN1).
Most applications maintain a constant master speed, or change very slowly, so this effect is
minimal. But if the master is changing rapidly, there may be a significant master speed
measurement error. Because predicted master positions are in part based on master speed
measurement, the can result in an error in master position prediction mode (FPPEN1). This
effect will always be smaller than that due to the master position prediction mode being
disabled (FPPENØ).
Tuning;
(Servo Axes Only)
A servo system’s tuning has a direct impact on how well the follower axis can track the master
input. Overshoot, lag, oscillation, etc., can be devastating to Following performance. The best
tool to use for tuning the Gem6K series controller is Motion Planner’s Gemini Servo Tuner
utility.
Repeatability of the
Trigger Inputs and
Sensors;
Some applications may use the trigger inputs for functions like registration moves, GOWHENs,
or new cycles. For these applications, the repeatability of the trigger inputs and sensors add to
the overall position error.
Refer to page 99 for accuracy specifications on the trigger input position capture function.
Refer to your sensor manufacturer’s documentation for the sensor repeatability. In the Gem6K
controller, the position capture from the trigger inputs have approximately 100 µs
repeatability, and the sensor repeatability (SR) should be determined, too. Velocity ∗ time =
distance, so the error due to repeatability is (SR + 0.0001 seconds) ∗ speed = error. If the
sensor repeatability is given in terms of distance, that value can be added directly.
Preset vs. Continuous Following Moves
When a follower performs a preset (MCØ) move in Following mode (FOLEN1), the
commanded position is either incremental or absolute in nature, but it does have a commanded
endpoint. The direction traveled by the follower will be determined by the commanded
endpoint position, and the direction the master is counting.
Let’s illustrate this with an example. Assume all necessary set-up commands have been
previously issued for our follower (axis #1) and master so that distances specified are in
revolutions:
FOLRN3 ; Set Following follower-to-master ratio numerator to 3
FOLRD4 ; Set Following follower-to-master ratio denominator to 4
; (ratio is 3 revs on the follower to 4 revs on the master)
FOLEN1 ; Enable Following on axis #1
FOLMD10 ; Set preset move to take place over 10 master revolutions
MC0
; Set follower to preset positioning mode
MA1
; Set follower to absolute positioning mode
PSET0 ; Set current absolute position reference to zero
D5
; Set move distance to absolute position 5 revolutions
GO1 ; Initiate move to absolute position 5
If the master is stationary when the GO1 command is executed, the follower will remain
stationary also. If the master begins to move and master pulses are positive in direction, the
follower will begin the preset move in the positive direction. If the master pulses stop arriving
before 10 master revolutions have been traveled, the follower will also stop moving, but that
GO1 command will not be completed. If the master then starts to count in the negative
direction, the follower will follow in the negative direction, but only as far as it's starting
position. If the master continues to count negative, the follower will remain stationary. The
Chapter 7. Multi-Tasking 191