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:


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.



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.



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.



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

  1. The Simpson F45 modules display a self test sequence on power-up. Observation that both displays indicate < -250.0 +/- .2 > mv. when the TCP to TIB control cable is temporarily disconnected provides an additional check of their operation and initial scaling.
  2. Proper operation of the lower display unit and TCP current sending circuit combination is verified by turning the threshold control dial to " 0.0 " which should produce a display of < 0.0 + / - .2 > mv. No adjustment is possible. Full scale calibration is accomplished by setting the panel control dial to fully clockwise to " 10.0 " and adjusting the F45 display unit calibration so that < 1000.0 > mv. is seen.
  3. TIB performance is confirmed and calibration done by connecting a voltmeter to its output. A threshold dial setting of " 0 " should result in < 0 + / -0.8 mv. No adjustment is possible. System calibration at full scale is set by adjusting TIB trimpot "C" for 1000.0 mv output with the threshold dial fully clockwise at "10.0".
  4. Full scale calibration of the upper display or "Readback" indicator is achieved by simply adjusting the TIB trimpot labled "R" until the value measured by the voltmeter is seen.