Discussion of MPS PIC module trip settings and ASTS readout tolerances:
(HVS 8/95)
ADDENDUM: Much of the "legwork" about setting camparitor thrsesholds
(HVS has been accomplished by SCP software. As additional
3/02) complication, the recovery thresholds depend not only upon
repetition rate (below "FULLRATE" is always assumed to be
120 Hz; which for the present is not generally true).
Further additional complication surfaces when considering
the time filtering done in the module (programmable to be
either 1 S, 500mS, 100mS or 50mS).
For an at-complete-nauseum discussion of how repetition
rate and PIC time filtering effect the determination of
the recovery thresholds, see the URL:
picrecovery.html
Otherwise; what follows is very useful for becoming
familiarized with the way the PICS system works.
END OF 3/02 ADDENDUM
Each PIC module has five IC channels. Each of these channels has
four programmable comparitor levels (trip thresholds). These four comparitor
trip levels have been assigned the following meaning:
1) FULLRATE This level corresponds to to maximum allowable
radiation level. If this level is reached,
the beam rate is to be reduced so as to lower
the radiation level.
2) LIMIT_HI This level corresponds to 1/12 of the FULLRATE
level (pedistal subtracted). The purpose of
this threshold is to indicate that if beam rate
is increased by a factor of 12 (10Hz->120 Hz),
then the radiation level is not expected to
exceed the FULLRATE level. MPS algorithms
impliment this comparison level to determine
whether to allow transition 10Hz -> 120 Hz.
3) LIMIT_LO This level corresponds to 1/10 of the FULLRATE
level (pedistal subtracted). The purpose of
this threshold is to indicate that if beam rate
is increased by a factor of 10 (1 Hz-> 10 Hz),
then the radiation level is not expected to
exceed the FULLRATE level. MPS algorithms
impliment this comparison level to determine
whether to allow transition 1 Hz -> 10 Hz.
4) TLV_TEST This comparison is not used during normal
operation. It is used during an MPS system
test called "TRIP LOGIC VERIFICATION" in
which MPS software excercises the trip
logic/functionality of it's modules.
See HELP under MPS TERMS panel for a further
explaination of Trip Logic Verification.
In order to set appropriate trip levels, the following considerations
apply:
1) Ion chambers are intended to measure ionizing radiation level.
For a specific radiation level, the ionized gas inside
the chamber generates a signal current. This current is
measured by the PIC module. The sensitivity of the Ion Chambers
used by MPS is generally taken to be
1 rad/hr ~ 0.25 nA (for 1 liter of argon
Bazarko/Turk 6/91)
2) The PIC module digitizes this current using a 16 bit ADC.
Two scales are available, but to date only one is used.
This digitization is 20uA full scale...corresponding to
65535 counts so that:
1 count ~ 0.30 nA ~ 1.2 rad/hr (readout ADC)
Therefore, the ASTS readout is scaled by a factor 1.2 such
that the SCP display is normalized to rad/hr. There is an
offset term in the ASTS readout scaling such that the house-
keeping current is subtracted as well. SCAL(1)=offset and
SCAL(2)=1.2. In general the displayed ASTSS data value is
determined by the relation:
DATA = SCAL(1) + SCAL(2) * COUNTS
Where DATA is the value seen on the SCP, COUNTS
is the RAW ADC readout, and SCAL values are the
scale factor terms mentioned above.
3) The PIC compares the measured current to each of the four
programmable trip settings discussed above. These trip
thresholds are set by a TWELVE BIT DAC. It is important to
recognize the distinction between the twelve bit trip
threshold DAC settings and the sixteen bit readout ADC
sensitivity when setting trip thresholds. The trip threshold
DAC has also a 20 uA full scale...corresponding to 4096 counts
so that
1 count ~ 5nA ~ 20 rad/hr (Trip Thresh DAC)
4) The PIC threshold levels are entered into the SCP. They are
in units of Trip Threshold DAC counts.
Historically, ion chamber trip settings have been cataloged in the
form of IC current (uA). For the BSY ion chambers, a typical value of 3.5uA
has been used historically. Let's walk through how to set the various trip
levels based upon this.
FULLRATE: Since 4096 counts correspond to 20uA for the trip
comparison DAC, we can just scale the 3.5uA trip
level by 4096 counts/20uA to determine a trip setting
of:
3.5 uA ~ 717 counts (FULLRATE)
or in general:
FULLRATE = TRIPLEVEL * ( 4096/20 )
Where FULLRATE is in units of trip
threshold DAC counts, and TRIPLEVEL is
in units of uA.
LIMIT_HI Since this threshold is merely 1/12 of the FULLRATE
setting, we could set a level of 717/12 = 60 counts.
However, the housekeeping current is typically
significant (~40 counts of trip level DAC) compared
to this. Experience has shown that it is best to
OFFSET the LIMIT_HI and LIMIT_LO DAC trip settings
in order to compensate the housekeeping current.
The following formulation can be used in general:
LIMIT_HI = FULLRATE/12 + OFFSET
OFFSET is best determined by watching history buffer
of housekeeping current. OFFSET should be expressed
in terms of the trip threshold DAC counts. ASTS
readout is in rad/hr. This readout should get "unscaled"
to readout ADC counts, and then scaled by the ratio
of bit resolution between the readout ADC and the trip
threshold DAC.
So in general, if the housekeeping current is
HOUSECURRENT from the History Buffer, then
(HOUSECURRENT - SCAL(1)) (4096)
OFFSET = ------------------------ * ------
SCAL(2) (65536)
Where SCAL(1) is the offset term of the ASTS SCAL
primary, and SCAL(2) is the slope term.
LIMIT_LO Based on the above arguements, the LIMIT_LO threshold
can simply be expressed as:
LIMIT_HI = FULLRATE/12 + OFFSET
Where OFFSET is determined by housekeeping current in
the same manner as expressed above.
TLV_TEST This value is always set to the fullscale value of
4095 counts.
It is also useful to use the History Buffer measurement of housekeeping
current to adjust the SCAL(1) value such that the ASTS readout is zero with
just the housekeeping current present. This can be done by merely replacing
the old SCAL(1) value with that value minus the HOUSECURRENT determined from
History Buffer:
new SCAL(1) = old SCAL(1) - HOUSECURRENT
Where HOUSECURRENT is in units of scaled SCP
ADC readout.
The ASTS limits should also be considered. The upper ASTS limit should
be 90% of the FULLRATE trip value...expressed in units of ASTS readout. This
value can be determined by expressing the FULLRATE trip value in terms of raw
ADC value, scaling this by the ASTS SCAL factors, and taking 90% of that value:
|- -|
| 65536 |
LIMS(2) = 0.9* | SCAL(1) + SCAL(2)*(FULLRATE * ----- ) |
| 4096 |
|- -|
where FULLRATE is the 12 bit DAC value entered
into the SCP for the FULLRATE trip threshold,
the SCAL values are the ASTS readout scale
factors, and LIMS(2) is the ASTS readout upper
limit.
The lower ASTS limit, LIMS(1), is typically taken to be about
-100 rad/hr. This corresponds to about 80 ADC counts variation away from
nominal housekeeping current. Housekeeping current stability is typically
5-10 times better than this.