Threshold Control System
The Threshold Control Panel (TCP) and Threshold Interface
Board (TIB) provide a means to accurately set detector tray LED threshold level
from the electronics platform. Key features of the system include redundant
measurement of the control signal at the tray, failsafe default to a fixed local
threshold, and a reverse or "Readback" indication of the actual
applied threshold voltage. In addition, the analog current signal transmission
(4-20ma loop) technique used was chosen because of its simple implementation and
proven reliability in a wide range of industrial and process control
environments. Operational / Installation Points: TCP: Operator interface at the TCP consists of precision 10 turn
dial potentiometer and two 4-1/2 digit displays. Adjustment of the desired
threshold voltage is through observation of the lower display that is scaled in
whole and fractional millivolts. Markings on the potentiometer dial serve as a
rough check of system operation by interpretation as percent full scale of TIB
output voltage that in this case is 1 volt. The upper display indicates the actual threshold voltage
measured or "Readback" from TIB output. Values on the two displays
will normally be within +/- .5 mv. During TCP, control line or possible TIB
receiver failure, the lower display will indicate the local tray default
threshold which has been set there. TIB: Conversion of the TCP control signal to a voltage level
suitable for use as the LED threshold occurs at the TIB. While the output
voltage from TIB is normally determined by the TCP, the facility to adjust the
"default" LED threshold is provided by a trimmer type potentiometer
on the board. The "default" function is enabled under either of
two conditions. First, when possible failure of current receiver I.C. is
suggested by a difference between its output and that of the redundant device.
Second, if the TCP control current falls significantly below the "0"
scale value of 4 ma. as most likely due to a broken or shorted connecting cable
or TCP power supply failure. The local tray "default" setting can
thus also be selected by intentionally disconnecting the control cable although
the Readback" signal indication is also disabled. Signals sent to and from the TIB are sent as analog current
signals and are galvanically isolated from the LED power system to eliminate TCP
to TIB ground current generated error. Power is derived from the + and 5
volt LED supplies present in the detector tray. Output LED threshold voltage is
referenced to TIB power common which should be physically connected in the
detector tray at a point which minimizes local ground current error at the
individual LED boards. Circuit / Device Description: The Remote Threshold Control sub-system is based on using a
precision, controlled current to represent the desired LED threshold voltage
value. The use of current as the signal and offsetting the zero value allows
for simple detection of a broken or shorted connecting circuit. Process control
industry standards for such systems commonly assign a current of 4 ma as the low
value and 20 ma as the full scale or 100% representation. TCP: Generation of the current signal at the TCP utilizes an
Analog Devices AD-694 integrated circuit whose input is voltage from the 10 turn
dial potentiometer on the panel. A galvanically isolated 12 volt power supply
for this circuit comes from the lower Simpson F-45 "Process Indicator"
display module. That unit measures the output signal and scales its indication
to allow precise adjustment of the implied output voltage at the TIB output. TIB: Measurement of the current signal at the TIB is accomplished
by a Burr-Brown RCV-420 device which removes the 4 ma zero offset and presents a
0 to 5 volt output signal. The difference between that and a second redundant
RCV-420 measuring the same current in series is examined by two CMP-04
comparator sections. Design of the comparator circuit is such that the
allowable error window varies from about 5 mv. at zero scale to 10 mv. at full
scale. Comparator output is used to control a DG-436 SPDT switch I.C. The TCP
set threshold from the active RCV-420 calibrated to 0-1 volt is selected if
error conditions are acceptable while the "default" threshold set by
the on board trimpot, "T"is enabled if limits are exceeded. A third
CMP-04 comparator is configured to cause selection of the default threshold when
control line current is significantly less than 4 ma. as would be caused by TCP
power or connecting cable failure. Troubleshooting: / Calibration