PEP-II LLRF Tuners Panel
Each RF cavity has a mechanical tuner to control the resonant frequency
as temperature and loading conditions change. The position of each
tuner is controlled by a stepping motor. A linear potentiometer mounted
on the tuner assembly provides an absolute read back of the present tuner
position. Each tuner motor is driven by a Allen Bradley tuner motor
controller (type 1746-HSTP1) driving a Superior Electric stepper motor
translator (model SS2000MD4) housed in the tuner driver chassis.
This combination of hardware handles the basics such as limit switches
and current limiting for the motors. An EPICS driver was written
for the motor controller, allowing a fully programmable tuner loop to be
The Tuners panel is accessed from the
Main Station panel or the
Feedback panel and provide controls for
the following loops.
Main Tuner Loop
This loop compares
the phase of the forward RF being delivered to the cavity with the phase
of the RF measured via the cavity probe (after the appropriate phase offsets
are taken into account) and moves the tuner to keep this phase error nulled.
Loading Angle Offset Loop
This loop adds
a phase offset to the normal tuner loop phase error signal based on the
voltage distribution observed on the cavities. Each cavity is assigned
a setpoint in percent, representing the portion of the total station gap
voltage it is expected to produce. Once the beam current reaches
the preset minimum ("Min Beam Curr (mA)" - nominally 300 mA), the loop will
slowly add a phase offset to the main tuner loop to achieve the desired
cavity contribution. This is an extremely useful feature, allowing
the gap voltage on any cavity to be reduced to help prevent faults.
This loop also compensates for longitudinal misalignment of the cavities
and can easily correct for 10 degree alignment errors.
Cavity Parking Loop
When the station is set to the PARK state, this loop will set the tuner
resonant frequency, estimated by a polynomial, to the
"Desired PARK Frequency (kHz)" located on the
Inp&Consts (input and constants) Tuner panel.
Controls and Monitors for All Tuners in the Station
- Loop Control - Turns the tuner loop ON/OFF.
Before the tuner loop can be
enabled, the tuners must be configured as instructed in the
Station Configuration Procedure.
- Loop Interval - Using a right mouse click,
the tuner loop sample rate can be changed. The loop is presently
configured for a 2 second sample rate. Altering the rate would most
likely require changing the tuner loop gains and is not recommended (or
deemed necessary) at this time. Nominal sample rate is 2 seconds.
- Klys Frwd Pwr Limit (kW) - Sets the minimum
klystron output power that the tuner loop will operate. Below this
power level, the tuners will not move. Nominal value = 1 kW.
- Min Beam Curr (mA) - Sets the minimum beam
current required before the loading angle offset loop will operate.
Nominal value = 300 mA.
- Loading Angle Offset Control - Turns the
loading angle offset loop option ON/OFF. Normally ON.
- Park state: Go Home - Forces
the station's tuners to move to their park home positions. The position
is defined by the "PARK Home" entry located on the
Inp&Consts Tuner panel.
- Park state: Set Home - Updates the park home
position for each cavity in the station. This is done after
a cavity or tuner assembly is replaced. Before activating, the station
must be placed in PARK state with the correct "Desired PARK Frequency (kHz)"
value entered for each cavity.
- TUNE/ON state: Go Home - Forces
the station's tuners to move to their TUNE/ON home position. The position
is defined by the "TUNE/ON Home" entry located on the
Inp&Consts Tuner panel.
- TUNE/ON state: Set Home - Updates the
TUNE/ON home position for each cavity in the station. This is done
automatically during tuner configuration which is a part of
station configuration but can be
done manually when there is no beam.
- Avg Freq Offs (kHz) - The average of the
individual cavity resonant frequencies estimated by the tuner polynomials.
It is used to determine the phase offset required to keep the
direct loop properly
phased as the cavities detune. This feature is enabled
by the "Freq Offs Tracking" ON/OFF button on the Direct Loop or Feedback panel.
The average frequency offset is also used to direct the tuners to their park
positions when the station is placed in the PARK state.
Controls and Monitors for Each Tuner
- Fixed Offs (Deg) - Allows the user
to place a constant phase offset into the tuner loop.
Intended to be used as a diagnostic but not currently used.
Normally 0.00 Deg.
- Setpoint (%) - Set point for the loading
angle offset loop. Determines the percent of the total station gap
voltage a single cavity will contribute (once the "Min Beam Curr" limit
has been reached). For example, in a 4-cavity station, each cavity
will normally be set to 25%. If one cavity is prone to RF breakdown,
its set point can be reduced to 22% and the remaining three raised to
26%. Important note: The sum of
all the station cavity set points must equal 100%.
- Position Control (mm) -
Each slider indicates the present value of the tuner according to the software
in the EPICS tuner driver. This value is not the potentiometer read
back. User can turn OFF the tuner loop, and use a left mouse click
or up/down keys to manually move the tuner. A right mouse click will display
more options. To the left of each slider is the position read back
from the linear potentiometer. It is not important that the two agree
but will be calibrated during tuner configuration.
- Temperatures - The present value
of the temperature for the fixed tuner, movable tuner, and movable tuner
bellows are displayed in degrees C.
- Stepper Motor - Opens the Step Motor
panel. On this panel, the stepper motor controller can be reinitialized
by left mouse button click in the "Stop & Init" button. This
also forces the controller software task control position to match the
present potentiometer read back value.
- Loop Status - Should normally read GOOD.
Important - If it ever reads "DRV_LIMT", it
is likely that the controller is instructing the tuner to move but nothing
is happening. This condition can be caused by the stepper motor
translator faulting. We have observed this condition on several
If the translator has faulted, a red LED fault light will appear on the
unit which is located in the Tuner Driver Chassis. The translator
can be reset (even while the station is operating) by removing the large
round MS type connector on the rear of the Tuner Driver Chassis (this removes
the 24 power to the unit) and then reconnecting. If this does not
solve the "DRV_LIMT" error condition, the tuner assembly might have a broken
shaft pin or some other mechanical problem. Before committing to
an access, the station's crate processor should be rebooted once to eliminate
the possibility of an obscure software glitch causing the problem.
- Ld Angle Error (Deg) - Current value of
the tuner loop phase error. Will turn yellow
if the value exceeds the "Ld Angle Err Limit (Deg)" entry on the
Inp&Consts Tuner panel. This should
normally remain below the "Error Deadband (Deg)" parameter on the
Inp&Consts Tuner panel.
Nominal value for the deadband = 0.25 degrees.
- Ld Angle Offs (Deg) - Current value of
the phase offset created by the loading angle offset loop. If all
cavities are set for equal "Setpoint (%)" (25% in a 4-cavity station) and there
is sufficient beam in the ring, this value can be used to determined if
the adjustable waveguide bellows are properly adjusted. The offset
is reset to zero when the station goes to the OFF state.
- Cav Strength (%) - Percent contribution
the cavity is making to the total station gap voltage.
- Freq Offset (kHz) - Estimated cavity resonant
frequency based on the tuner polynomials. Typically the polynomials
are accurate to ~10 kHz which has proven to be sufficient.
- Diag Plots - Brings up an EPICS plot which
displays the loop phase error, tuner position, and loading angle offset.
A right mouse click in the plot window will allow auto scaling the plot
to the current data.
- Inp&Consts - Opens up the
Inp&Consts Tuner panel where most of
the detailed tuner parameters including
the tuner polynomial coefficients are stored.