PEP-II LLRF Gap Voltage Control Panel
Overview
This panel contains the various constants necessary to control the station
gap voltage. This is an interesting task since the actuator changes
depending on if the direct RF feedback loop is closed or open. With
the direct loop open, the actuator is the HVPS output voltage.
With the direct loop closed, the reference IQ voltages (either from the
gap module or from the RF Processing (RFP) module
if it is configured to operate without a gap module) controls the gap voltage.
The gap voltage panel can be accessed from the
Feedback panel or some
of the other buried panels.
TUNE or ON State with Direct Loop OFF
In TUNE, ON_CW, or ON_FM state with the direct loop
OFF, the gap voltage is controlled by the HVPS output voltage. The
following equation converts the gap voltage error into HVPS output voltage
change. There is a restriction on how much the HVPS setpoint can
change on each iteration:
Delta HVPS voltage = Err * Gain Factor * Conversion
- HVPS Loop Mode - Normally ON. Refer
to the Main Station panel for details.
- Min Delta Gap Volt (kV) - Deadband for the
loop calculation. Will not change HVPS setpoint
if the magnitude of the error is below this limit. Nominal value
= 2 kV.
- HVPS Voltage Control (kV) - This slider
type control/readback is normally just viewed as a monitor of the present
HVPS output voltage but is actually the control for the HVPS voltage setpoint.
With the "HVPS Loop" set to OFF or PROC (process), the user can alter the
HVPS setpoint by left clicking the mouse on the slider and dragging the
setpoint. Up/Down arrow keys also work after the slider had been
selected. A right mouse click allows changing slider control options.
- Gain Factor - Loop gain term which allows
changing gain without affecting the measured "Conversion" term. We
have always run with unity loop gain and "Conversion" value might not be
the actual gap voltage error kV to HVPS output kV value. Nominal
value = 1.00.
- Max Delta Voltage (kV) - Sets the maximum
change in HVPS voltage allowed on each iteration. Acts as a slew
rate limiter. Nominal value = 0.50 kV.
- Conversion (kV) to (kV) - Scale factor to
convert error in gap voltage kV to HVPS output kV.
Manually configured by changing the gap voltage setpoint and observing
the step response using the station voltage plot tool at the top of the
page. Nominal value = 0.0010 kV/kV.
- Initial Voltage (kV) - When the loop is
activated,
this HVPS output is preloaded to provide sufficient voltage for the klystron
to begin to produce measurable power. Nominal value = 45 kV.
ON_CW State - Direct Loop ON - GVF Module Missing
In ON_CW state with the direct loop ON, the gap
voltage is controlled by the baseband IQ reference voltages. The
station can be configured to operate without a Gap module by changing a
pair of jumpers in the RFP module.
The following constants support this condition.
- Min Delta Gap Volt (kV) - Deadband for the
loop calculation. Will not change HVPS setpoint
if the magnitude of the error is below this limit. Nominal value
= 2 kV.
- DAC Counts Control - Using the mouse, the
user can manually set the DAC output for the baseband reference voltages
IQ voltages. With the "Loop Control" set to OFF, the user can alter
the setpoint by left clicking the mouse on the slider and dragging the
setpoint. Up/Down arrow keys also work after the slider had been
selected. A right mouse click allows changing slider control options.
- Gain Factor - Loop gain term which allows
changing gain without affecting the measured "Conversion" term. We
have always run with unity loop gain and "Conversion" value might not be
the actual gap voltage error kV to DAC counts value. Nominal value
= 1.00.
- Calibrate Button - Used to measure the
present "Conversion (Counts/kV)" scale factor and update the Conversion
factor entry box. We never really used this feature but it does as
described. We usually manually enter in a "Conversion" factor which
results in a nice response to a step change in the gap voltage setpoint.
- Max Delta Counts - Sets the maximum change
in DAC counts allowed on each iteration. Acts as a slew rate limiter.
Nominal value = 20 counts.
- Conversion (kV) to (Counts) - Scale factor
to convert error in gap voltage kV to DAC counts.
Manually configured by changing the gap voltage
setpoint and observing the response, the "Conversion" factor is altered
to obtain a nice step response. Nominal value = 0.0400 Counts/kV.
ON_CW State - Direct Loop ON - GVF Module Available
In ON_CW state with the direct loop ON, the gap
voltage is controlled by the baseband IQ reference voltages. The
following parameters are used to setup the normal hardware configuration
which includes a Gap module to produce the baseband IQ reference voltages.
The following constants support this condition.
- Min Delta Gap Volt (kV) - Deadband for the
loop calculation. Will not change HVPS setpoint
if the magnitude of the error is below this limit. Nominal value
= 2 kV.
- DAC Counts Control- Using the mouse the
user can manually set the DAC output for the baseband reference voltages
IQ voltages. With the "Loop Control" set to OFF, the user can alter
the setpoint by left clicking the mouse on the slider and dragging the
setpoint. Up/Down arrow keys also work after the slider had been
selected. A right mouse click allows changing slider control options.
- Gain Factor - Loop gain term which allows
changing gain without affecting the measured "Conversion" term. We
use the "Calibrate" button to measure the "Conversion" scale factor and
then adjust this gain term to obtain a nice response to a change in gap
voltage setpoint. Nominal value = 0.50.
- Calibrate Button - Used to measure the
present "Conversion (Counts/kV)" scale factor and update the Conversion
factor entry box. We never really used this feature but it does as
described. We usually manually enter in a "Conversion" factor which
results in a nice response to a step change in the gap voltage setpoint.
- Max Delta Counts - Sets the maximum change
in DAC counts allowed on each iteration. Acts as a slew rate limiter.
Nominal value = 25 counts.
- Conversion (kV) to (Counts) - Scale factor
to convert error in gap voltage kV to DAC counts.
Manually configured by changing the gap voltage
setpoint and observing the response, the "Conversion" factor is altered
to obtain a nice step response. The gain of the direct loop also
contributes to the determination of the DAC counts calculation so that
the proper gain is achieved as the direct loop gain is ramped up during
a slow turnon sequence. Nominal value = 0.0400 Counts/kV.
- Gap Volt Feed Fwd Control - Opens or
closes the DSP based learning loop which allows the baseband
IQ references to follow the beam induced gap transients. Also located
on the "Feedback" panel. Normally ON.
- Faston GAP counts - Used to preload the
baseband IQ references during a "faston" sequence.
This value is determined by observing the DAC counts when the station is
operating at the desired gap voltage, direct loop closed with no beam.
We sometimes reduce this value by 100 counts depending on how successful
a few faston trials go. Nominal range = 700 to 1000 counts.