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XPS-Q8 Controller
Motion Tutorial
Note
When changing the PCO pulse settling time you must limit the maximum velocity
of the stage accordingly otherwise you will loose the PCO position and generate the
wrong number of pulses at wrong positions. As per the above table, if you set the
pulse settling time to 4 µs, the maximum PCO encoder frequency needs to be
limited to less than 0.25 / 4x 10
= 62.5 kHz.
-6
So, if EncoderScalePitch = 0.004 mm and HardInterpolatorFactor = 200 then the
ScanVelocity must ≤ 62.5 x 10
* 0.004 / 200 = 1.25 mm/s, otherwise the PCO will
3
not work properly.
13.3.2
Even Distance Spaced Pulses Position Compare
In the even distance spaced pulse configuration, one first output pulse is generated when
the positioner enters the defined position window. This is independent of the positioner
entering the window from the minimum position or from the maximum position. From
this first pulse position, a new pulse is generated at every position step until the stage
exits the window.
NOTE
To make sure that the trigger pulses are always at the same positions independent
of the positioner entering the window from the minimum or from the maximum
window position, the difference between the minimum and the maximum window
position should be an integer multiple of the position step.
The duration of the trigger pulse is 200 nsec by default and can be modified using the
function PositionerPositionComparePulseParametersSet (PositionerName,
PCOPulseWidth, EncoderSettlingTime). Possible values for PCOPulseWidth are: 0.2
(default), 1, 2.5 and 10 (µs). Please note, that only the falling edge of the trigger pulse is
precise and only this edge should be used for synchronization irrespactable from the
PCOPulseWidth setting. Note also, that the duration of the pulse detected by your
electronics may be longer depending on the time constant of your RC circuit.
Successive trigger pulses should have a minimum time lag equivalent to the
PCOPulseWidth time multiplied two.
The second parameter, EncoderSettlingTime applies a filter to the encoder signals for
the trigger pulse generation. Possible values are: 0.075 (default), 1, 4, 12 (µs). The
setting of this EncoderSettlingTime should be done in relation to the application, in
particular speed and encoder resolution, and the encoder/position noise. For most
applications, the default value works fine. At very low speed, with high encoder
resolution, and significant encoder/position noise, however, it may be possible that
additional trigger pulses are generated where no trigger pulse should be generated from
the application. In these cases, a higher value setting for the EncoderSettlingTime could
avoid these unwanted extra pulses. The value for the EncoderSettlingTime, however,
should not exceed the value for the Encoder resolution divided by the speed. Please note
also, that the EncoderSettlingTime adds a nominal delay between the encoder transition
and the trigger pulse.
Example
With XM stages and the hardware interpolator set to 200 (see function
PositionerHardInterpolatorFactorSet ()) the resolution of the trigger pulses is 20 nm (4
µm encoder scale pitch / 200). At continuous speed motion with 20 µm/s speed, the
nominal time between successive encoder counts is 1 ms (20 nm / 20 µm/s). In a not
optimum environment of the XM stages, it is possible, that the actual position detected
by the trigger circuitry is not continuously increasing, but flickering around one encoder
count (20 nm) from time to time. When using the default setting for the
EncoderSettlingTime (0.075 µs) under these conditions, it is very likely that more than
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XPSDocumentation V1.4.x (EDH0301En1060 — 10/17)
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