INPUTS AND OUTPUTS OF COMPRESSOR CONTROL SYSTEM

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INPUTS AND OUTPUTS OF COMPRESSOR CONTROL SYSTEM

Speed Inputs

The CCS can accept one, two or three speed inputs. Each speed input is monitored by two module’s. With six possible speed signals from which to control with, the CCS can withstand multiple speed input failures with no loss of control functionality. Only one of the possible six inputs is required for speed control. The control can accept a ‘slow speed’ detect probe on channel 4 which can be unique or different from the other speed signals.
All speed inputs are connected to the control, via termination modules (FTMs). An inputs termination module is
used to terminate customer control wiring and distribute each input signal to the two module’s. After the control receives their input values, the Application Software Redundancy Manager then compares each module’s voted
result to select a value to be used within the application logic.
A speed input signal is determined to be faulty and is taken out of the input voting logic when it is below its “Speed Failure Level” setting. This failure level setting is common to all inputs and can be adjusted via de CCT program’s Service mode.
An input deviation alarm is also used to annunciate if any of the three possible speed input channels is sensing a value that is different than the voted-good value used by the application. If an input channel’s sensed value deviates from the voted-good signal value, by a greater margin than the speed control’s “Max Deviation” setting, an input channel alarm will be issued.
This type of annunciation can be used to indicate when an input channel, or magnetic pickup unit is intermittently failing high or low. Max Deviation input settings are tunable via the CCT program’s Service mode, and are defaulted to 1% (deviation range = 0.01 to 20%) of the “Overspeed Limit” setting. If a deviation alarm condition occurs, the alarmed input is not removed from the control’s voting logic and still can be used to control with, in case all other channels fail.

Analog Inputs

The control can accept one or two transducer inputs for all critical parameters (ext/adm, aux, casc inputs). Only one input signal is accepted for non-critical functions (remote setpoint inputs). Each analgo input can withstand one failure with no loss of control functionality. If any of an analog input’s two “legs” are failed, the control uses the second healthy leg’s sensed input signal from which to control with.
All analog inputs are connected to the control, via field termination modules (FTMs). An input’s termination module is used to terminate customer control wiring and distribute each input signal to all two modules. After the control’s modules double exchange their input values, and vote out any erroneous values, the Application Software Redundancy Manager then compares each module’s voted result to select a value to be used within the application logic.
Optionally, each leg of an input channel can be tested and its calibration verified through the CCT program’s Service mode, by individually removing the other input leg.
An analog input signal is determined to be faulty when it is below its “Fail Low Value” setting, or above its “Fail High Value” setting. These failure level settings can be adjusted via the CCT program’s Service mode and are defaulted to values in engineering units which correspond to 2mA and 22mA respectively. If an input is determined to be failed, that input is removed from the control’s voting logic.

Input deviation alarms are used to annunciate if any of the input channels ore input legs are sensing a value that is different than the voted-good value used by the application. If an input channel’s sensed value deviates from the voted-good value, by a greater
margin that its “Max Deviation” setting, an input channel alarm will be issued. This type of annunciation can be used to indicate when an input channel or  system transducer is going out of calibration. Max Deviation settings are
tunable via the CCT program’s Service mode, and are defaulted to 1% (deviation range = 0.1 to 10%) of the configured input range. If a deviation alarm condition occurs the alarmed input is not removed from the control’s voting logic, and still can be used to control with, in case all other channels fail.

Analog Outputs

Each control readout can withstand one failure with no loss of output functionality. Any leg of an output channel can drive a readout’s full 4-20mA current signal. After each CPU generates an analog output signal, the signals are exchanged between modules, voted on, and sent to the Redundancy Manager for output. The Redundancy Manager divides the output signal based on the number of known good output channels and distributes each portion of the signal to the respective output channel.
Precision resistors are used in each channel’s readback circuitry to measure and verify the health of each output “leg”. If a fault condition is detected, the faulty output leg is isabled, and the Redundancy manger redistributes the output signal to the remaining legs. The Field Termination Module (FTM) combines each analog output signal from the two modules into one signal at the FTM’s terminal blocks.
An output is considered failed, and an alarm issued, if a channel’s combined output or any leg of the output measures a difference of more than 10% from the output demand. Optionally, each leg of a readout channel can be tested and its calibration verified trough the CCT program’s Service mode, by individually removing the other output leg.
With this output architecture, any single output driver failure results in the output signal only stepping to 50% of its original value. The time between when a failure is sensed and when the control corrects for it by redistributing current through the other driver can be as long as 50 milliseconds.
Upon the correction of an output failure and a “Control Reset” command, each failed output performs a continuity check on its external load before current is again redistributed evenly between all output drivers. This continuity check entails, the failed driver to output a small amount of current through its output load, and compare that value with what is read back. The time between when a continuity check is performed and when the control redistributes current through the drivers can be as long as 50 milliseconds.

Digital inputs

The system is provided with 24 discrete inputs.
Each discrete input can withstand one failure with no loss of control functionality depending on the configuration. If one discrete input fails, the control used the second healthy leg’s sensed input signal from which to control with.
All discrete inputs are connected to the control via two Field Termination Modules (FTMs). Through marshalling each discrete input is connected to two FTMs, which distribute the signals to two modules. After the control’s modules double exchange their input values and vote out any erroneous inputs, the Application Software Redundancy Manager then compares each module’s voted result to select a value to use within the application logic.
A discrete input signal is determined to be faulty when it is determined to be different than the other input signal. If an input is determined faulty, the Redundancy Manager switches over to a default value, and an input channel alarm is issued. Once the input fault is corrected and both discrete input signals are equal, the alarm condition can be reset by issuing a “Control Reset” command.

Digital outputs

The system is provided with 12 discrete outputs. Each discrete output is connected in parallel with a second
discrete output on the second Field Termination Module (FTM).
Each discrete output can withstand one failure with no loss of control functionality. If one discrete output fails, the system layout will ensure the output is still utilized through the second FTM.
All discrete outputs are connected to the control via two FTMs. Through marshalling two discrete outputs are connected in parallel to form one discrete output to the field.
A discrete output is determined to be faulty when it is determined to be different when the Application Software determines a difference in command signal and read-back. On the output fail is corrected, the alarm condition which is issued to indicate this faulty state, can be reset by issuing a “Control Reset” command.

Actuator outputs

Each actuator output can withstand one failure with no loss of output functionality. Any leg of an output can driver an output’s full current signal (4-20mA or 20-160mA). After each module generates an actuator output signal, the signals are exchanged between modules, voted on, an sent to the Redundancy Manager for output. The redundancy manager divides the output signal based on the number of known good output channels and distributes each portion of the signal to the respective output channel.
Precision resistors are used in each channel’s readback circuitry to measure and verify the health of each output “leg”. If a fault condition is detected, the faulty output leg is disables, and the Redundancy Manger redistributes the output signal to the remaining leg. This good channel (leg) will drive the entire output.
An output is considered failed, and an alarm issued, if a channel’s combined output or any leg of the output measures a difference of more than 10% from the output demand. Optionally, each leg of a readout channel
can be tested and calibration verified through the CCT program’s Service mode, by individually removing the other output leg.
Actuator outputs, or HP and LP valve outputs, are treated the same way as the other analog outputs, with the
exception of an added precision resistor in the actuator output’s return path.
This resistor is used to measure and detect ground loops and coil shortages that are possible when interfacing to an actuator. If a single coil actuator is being driven, the dual coil terminal blocks are jumpered (wired) to the single
coil terminal blocks and the redundancy manager shares the current equally between the two modules. In the event of a fault, the Redundancy Manager will redistribute the load.
With this output architecture, any single output driver failure results in the output signal stepping to 50% of its original value. The time between when a failure is sensed and when the control corrects for it by redistributing current through the other driver can be as long as 50 milliseconds.

Upon the correction of an output failure, and a “Control Reset” command, each failed output performs a continuity check through the actuator before the current is again redistributed evenly between all output drivers. This continuity check entails, the failed driver to output a small amount of current through its output load, and compare that value with
what is read back. The time between when a continuity check is performed and when the control redistributes current through the drivers can be as long as 50 milliseconds.

Please read also:  Anti-Surge Control Theory and Quench Control Theory of Compressor

 

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