Optimizing Performance and Reliability of Automated Machinery Control Systems and Instrumentation

ABSTRACT Control and measurement systems can comprise complex arrangement s of electro-mechanical component s which operate as sensor-to-readout chains. ’lo achieve maximum automation capability of the host engineering systems, these instrumentation suites present challenges regarding alignment, calibration, and associated logistics. Accuracy of all components is also critical for use by knowledge-based condition assessment systems to form judgments regarding plant operation and maintenance. This paper presents a successful effort which supports these requirements.

System calibration is the process that ensures that a measurement chain, consisting of inter-related components, is accurately monitoring an engineering parameter within a specified tolerance. This entails calibration of a measurement or control system as a unified entity, from endpoint to endpoint. having first performed any prerequisite alignments. This approach enhances system availability by reducing operational problems through preventive methods that ensure correct setting of control circuitry and alarms, as well as interchangeability of components and spares, and by providing troubleshooting techniques to solve certain emergent difficulties.

System-level calibration concepts are described, in the form of an existing Navy program. along with a summary of benefits to Fleet operators and instrumentation support engineers. Lessons learned from this application, as they pertain to design and specification considerations, are also addressed. Examples of the relationship of calibration to today’s and future technology are also given.

Optimizing Performance and Reliability of Automated Machinery Control Systems and - Instrumentation

Introduction

he role and effect of calibration management and i t s impact on today’s complex machinery mon-

itoring and control systems and the engineering plants they support are often misunderstood. It is well recognized conceptually that the life ex- pectancy of a shipboard technology is directly proportional to the logistic support planned in its development and implemented in its deployment. Though not considered an element of traditional logistics coverage, calibration (and related documentation and data management) is an integral part of the holistic support required to maintain optimum performance of shipboard instrumentation systems.

Large quantities of mechanical/ electricaVelectronic instrumentation are installed throughout the fleet; as populations grew, calibration management difficulties arose due to the sheer numbers and complexity of some of the measurement systems into which they were being com- bined. Concurrently, with the advent of automated machinery control systems, unanticipated operational problems emerged, the root cause of which was the resultant intricacy of the various types of instrumentation elements acting together as integrated systems. Management atten- tion was focused on these new and unique technical needs. To help remedy the control system performance

deticiencies, an engineered resolution was devised. This structured method was necessary for the elab- orate multi-component propulsion control systems. It was also found to be the optimal solution to the calibration logistics problems (such as documentation of configuration, location, calibration recall, scheduling, and re- porting of individual components) arising from the many measurement and control instruments and systems now in existence. The Navy program is titled the Shipboard Instrumenta- tion and Systems Calibration (SIS- CAL) Program.

The SISCAL process entails, for each ship class, a documentation re- view and engineering analysis of the application, function, and location of all instrumentskystems to determine the need for periodic calibration. For each of these, instrument system- level or component procedures, as applicable, a re developed to place easy-to-use information, in techca l manual form, in the hands of specialized waterfront teams for accom- plishment of system alignment and calibration.

Terminology The following defines terms which will be used throughout the text:

Alignment: The proper adjustment of the components of a control or measurement system for coordinated performance.

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Optimizing Performance and Reliability oi Automated Machinery Conlml Systems and Instrumentation

Automated Machinery Control System: Hybrid integrated/distrib- uted analog-digital systems which monitor and control Hull, Mechanical, and Electrical (HM&E) propulsion and auxiliary machinery, electrical plant, certain steering systems, and selected components of the Damage Control (DC) system.

Calibration: The comparison of a measurement system or device of unverified accuracy with a measurement system of known and greater accuracy (smaller uncertainty). to detect and correct any deviation from specifications of the unverified measurement system or device. This entails obtaining quantitative estimates of the difference between the value in- dicated by an instrument and the actual value. Within this paper and the program described here, the defini- tion of calibration includes adjustments to the unit under test for correction of deviations.

Calibration Procedure: The specific steps and operations to be followed by qualified personnel in the performance of an instrument or system alignment and/or calibration.

Installed Instrumentation: Any device, permanently installed aboard ship, which is used to measure or monitor a parameter of a ship system. These may be mechanical, electrical/ electronic, or a combination thereof. Examples of types of such instrumentation are given in Table 1.

For the remainder of this paper, the term installed, when absent, is im- plied. Table 1 lists two types of installed instrumentation:

Instruments, such as pressure gages and thermometers, which are self-contained and direct-reading; and

W Components of measurement and control systems.

These a r e not mutually exclusive groups in their application, since both types are often used to monitor the same physical parameter.

52 November 1995

Measurement and/or Control System: Two or more components of installed instrumentation which act together as a “chain” to measure or process functional or static parametric engineering data. Automated machinery control systems invariably comprise multiple-component chains; such chains can also be used in pas- sive roles as measurement systems only

Background

THE UNITED STATES NAVY’S APPROACH TO CALIBRATION Engineering processes depend heav- ily on the science of measurement, and calibration is the foundation for

the integrity of any measurement. Within the U.S. Navy, the need for calibration is recognized and sup- ported. All measurement devices must be calibrated when first placed in service or after repairs are performed. Factors such as function or redundancy may result in a No Cali- bration Kequired (NCK) designation, which still necessitates calibration of the instrument when new or repaired, or when its condition or indication is suspect. Items requiring periodic calibration are managed in several ways. Only those portions of the Navy’s calibration program, which pertain to shipboard instrumentation will be mentioned here.

Waterfront and afloat calibration re- sources are currently resident at both

Individual Components Gages Meters (Analog and Digital)

Pressure Am meters Vacuum Voltmeters Temperature Ammeters

Wattmeters Switches Frequency Meters

Pressure Vacuum Thermocouples Temperature Flow Thermometers Level

Tachometers Transducers

Pressure Resistance Temperature Detectors (RTDs) Vacuum Vibration Display Devices Flow Digital Indicators Level Indicator Lamps

Conductivitv Cells

Components of MeasurementlControl Systems Converters Control Logic Permissive Circuitry

AID DIA Alarm Threshold Circuitry DUF FIDC Signal Conditioners Synchro/D

Power Supplies

Feedback Potentiometers

Throttle/Pitch Controls

Buffers

Multiplexers

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Optimizing Performance and Reliability of Automated Machinery Control Systems and Instrumentation

the organizational and intermediate maintenance echelons. Selected classes of ships have been designated as Field Calibration Activities (FCAs), and are manned with trained personnel and equipped to perform routine temperature, pressure, and torque calibrations of moderate complexity. Intermediate-level laboratories have been established at most major sites and aboard material support ships. Equipment to be calibrated at these laboratories must be brought to the facility, since the technicians do not travel to other locations. A s a result of the SISCAL program, a unique Shore-based Inter- mediate Maintenance Activity (SIMA) system-level calibration support capability was created, separate from the pre-existing in-house component- oriented calibration laboratory facili- ties, in the form of dedicated teams, which visit program ships. Particulars are given below.

A CALIBRATION MANAGEMENT PROBLEM EMERGES Prior to the introduction of gas turbine powered ships, instrumentation applications were relatively simple, and thus calibration support requirements were commensurately funda- mental. However, management of installed instrumentation was not without problems. For example, due to lack of awareness and clear guid- ance, temperature and pressure switches were frequently omitted from calibration programs. Basic configuration control was difficult for various reasons. Unfiltered data could be entered into the calibration recall database by such sources as ship’s force, builders, and off-ship calibration activities. In the absence of standard- ization, differences in nomenclature abounded (e. g. Gage, Gauge, Press, Pressure; Meter, AC, DC, Ammeter; many permutations of these singly and in random combination are possible). With uneven quality and com- pleteness of identification data, locat-

ing an instrument in an engineering space from the entries on a calibration recall form could be difficult, with the possibility of compounding configuration errors. As instrument populations grew, so did the calibration bur- den. Existing policy required the periodic calibration of all instrumentation, all at various intervals. This created increasing financial pressure to selectively define essential calibration actions.

The concept of criticality was in- troduced in 1979 in the form of Criti- cal Instrument Lists, which a t - tempted to generically categorize and limit instrument calibration requirements. Although the use of these list- ings reduced the calibration workload somewhat, the definitions and criteria were subject to interpretation, and thus led to confusion and misuse. The determination of what was critical was left to the discretion of individual ships. It was not unusual for ships of the same class to have large variances in the number of instruments included in their recall inventories.

THE IMPACT OF CONTROL SYSTEMS With rudimentary, strictly-measurement devices and systems, there is an element of forgiveness. By experience and intuition, an operator may be able to compensate for some system errors. It is possible to roughly calculate by feel or by relating other measurements or parameters that an out-of-calibration indication of 200 gallons is really closer to 250. When control is involved, as with an exact speed requirement for a landing or un- derway replenishment operation, precision becomes considerably more important.

The technology of discrete components for measurement began evolving into electro-mechanical and electronic measurement systems. Al- though automatic controls had been developed and used for boilers on steam driven ships, the introduction of gas turbine systems and the ex-

NAVAL E N G I N E E R S J O U R N A L

panded use of automated machinery control systems yielded increased de- mands on the technical support estab- lishment. Many of today’s engineering plants include automated measurement and control systems. Within each of these shipboard systems there are measurement chains consisting of such components as sensors or transducers attached to the equipment whose parameters are being monitored and signal conditioners which change the outputs of the sensors to voltages or currents suitable for further processing and display (these include any associated power supplies and printed wire boards). These signals may be fed to buffers, control electronics, and permissive circuits. In addition, the same parameter may be monitored at various control consoles, local operating panels, stations, switchboards and by associated alarms at different locations aboard the ship where the parameter is displayed. A block diagram of a typical, straightforward installation (a gas turbine lube oil supply pressure control system) is shown in Figure 1; some measurement chains can be considerably more complex.

Today’s measurement and control systems depend on the accuracy of the instrumentation that is integral to them and the data processed by them. An operator expects to be able to select and view a variety of system parameters by turning a thumb wheel on a Demand Display Indicator (DDI, a digital readout device), or by calling up a page of data on a plasma display

Precision measurements are now relied upon for automated decisions regarding starting of pumps and shutdown of entire systems, to cite a few high-visibility uses. Feedback signals such as pitch or valve position are critical to enabling ship systems to meet full operational requirements and to provide bumpless transfer of control from one control station to an- other. As a case in point, controls for engine throttle, propeller pitch, fuel valve position, overspeed and torque limits, and overspeed trips are integrated to perform as an ensemble.

November 1995 53

Oplimizing Performance and Reliabilily ol Aulomaled Machinery Conlml Syslems and Instrumenlalion

Panel Motor T*iTI I

.

p - 1 Low P m r u n

I Low PNSDUN

Shutdown

EPROM modification. An example of this is the CG 47 Class gas turbine generator Local Control Operating Panel, which has no potentiometers available for adjustments of the temperature measuring circuits. The signal conditioners receive their inputs from conventional Resistance Tem- perature Detectors (RTDs). Signal conditioner outputs are measured by an analog to digital converter, processed and relayed to the electric plant control system by digital to analog converters. Therefore, any errors in the signal conditioner will be carried throughout the rest of the measurement chain. It is possible to reprogram the gainwords stored in the EPROMs to compensate for any errors in the signal conditioners.

F I G U R E 1. Typical measurement and control system block diagram

THE CATALYST FOR IMPROVEMENT

Alignment of these as a coordinated unit ensures that the ship can operate with its designed automation capability; thus, with the operating controls set to full power, all components will respond at their appropriate full power set points or indications. In the absence of proper system alignment, more operator interaction is required, which can cause problems at maneu- vering speeds and impact the ability to reach full power.

With present technology, knowledge-based systems for machinery di- agnostics use in-place measurement systems and added instrumentation to obtain data for optimization of plant operation, determination of maintenance requirements, and other pur- poses.

An additional term will now be de- fined. System Calibration: The process that ensures that a measurement chain, consisting of interrelated components, is accurately monitoring a parameter within a specified tolerance. This entails calibration of a measurement or control system as a unified entity, from endpoint to endpoint, having first performed any prerequisite alignments.

54 November 1995

Calibration of measurement chains consisting of multiple readout devices (e.g. pressure gages, digital indicators, panel meters) may result in a divergence of readings due to differences and additive effects of tolerances. For example, it would be difficult to achieve identical readings of the same parameter being simulta- neously measured by both a pressure gage and a chain consisting of a pressure transducer, signal conditioner, and panel meter. System calibration is performed while the control system is aligned, to minimize such divergence.

The use of software for signal conditioning requires adaptation by the system calibration process. It is er- roneously assumed by some that an instrument cannot be calibrated if there are no physical adjustments such as potentiometers, pointers, le- vers, etc. As mentioned earlier, calibration includes the determination of variation from a standard. Adjust- ments, when present, are provided as a means of minimizing any deviations. With the increasing use of micropro- cessor-based instrument and control systems, adjustments a r e made through the system’s software by

N A V A L E N G I N E E R S J O U R N A L

With the arrival of control and measurement systems, defining installed instrumentation was problematic and documenting of configuration became much more difficult. Figure 2 depicts the interrelationship of units and signal flow within a guided missile cruiser class propulsion and auxiliary machinery control equipment array, along with a similar, separate arrangement for the electric plant controls for power generation and distri- bution. Taking into account the vast constellation of sensors and other instruments that comprise these control systems, it is apparent that the task of defining and locating a measurement chain and its individual components becomes a bewildering and time-consuming experience, especially for one not intimately familiir with an individual ship class layout.

The deployment of the marine gas turbine with its control technology brought unanticipated problems which manifested themselves when the engines went into fleet service. Opera- tional difficulties which arose included system shutdowns, unwarranted alarms, and inability to achieve full power. Experience gained from naval

Optimizing Performance and Reliability ol Automated Machinery Control Systems and Instrumentation

I SHIP CONTROL EOUIPMNT 1SCEl I

PROPULSION Lso AUXKIIRI MACHINERY I)(rORIUTION SVSlEM E0UIP)ILNT

IPWISEI

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ENGINE ROOM No. 2 ENGINE ROOM no. I 5-114-Of

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F I G U R E 2. Arrangement configuration of CG 47 Class Engineering Control and Surveillance System

aviation applications proved not to be exportable due to differences in configuration and the overall complexity of the interrelated systems. Calibra- tion of these systems was at first fal- laciously thought not to be necessary At fleet request, troubleshooting and investigation of the situation was begun; this revealed that component calibrations of only selected elements alone were inadequate. Component and system tolerances, alignment, adjustments, and circuitry internal to the system chains were not being addressed. Resolution of initial problems led to discovery of other items in the documentation which required modification. System tolerances were undefined, and there were conflicts with other documentation and processes, namely equipment technical manuals, operating procedures, and preventive maintenance actions.

Development of a s t ructured method to remedy these problems was begun. It became apparent that formal procedures to check and adjust alarm states and control logic levels were required, and that discrepan- cies in existing alignment procedures would need resolution. Signal conditioning circuits were analyzed to determine accuracy requirements. Op- erational setpoints were venfied with the fleet and cognizant In-Service En- gineering Agents. Originally developed to support gas turbine ship classes, the program's broad appli- cability to steam and diesel equipment and many other ship systems resulted in the fleet's request to expand be- yond the gas turbine classes.

The continued evolution of this iterative process led to the development of what has become the SISCAL Program, developed collaboratively

NAVAL ENGINEERS JOURNAL

by the naval engineering community. The program presently includes most active surface ship classes, and en- compasses all installed instrumentation on HM&E equipment and systems that interface with combat systems and electronics.

SISCAL Program Products The refinement of the approach to system calibration resulted in two key documents, which will be described in their present form.

CALIBRATION REQUIREMENTS LIST (CRL) The SISCAL process begins with the compilation and classification of a standardized list of all instmmenta- tion installed on the ship class. This

November 1995 55


yields the first of the two main program products, the Calibration Re- quirements List (CRL), which provides the ability to easily and uniquely identify and locate all installed instruments, and indicates whether or not calibration is necessary The estab- lishment of the CRL concept was based in part on fiscal necessity The unbounded calibration workload had long since become an affordability is- sue. The CRL solution presented an engineering basis for defining the minimum scheduled calibration requirements and for standardizing requirements within each ship class.

To develop the CRL, a baseline in- ventory of all installed instrumentation is created. A Reliability-Centered Maintenance (RCM) analysis of factors such as system safety, function, redundancy, and location, is then performed on each item to assess its requirement for periodic calibration. This process is detailed further below. The CRL, published as a class- specific Technical Manual, provides all the necessary identification data (e.g. type of instrument, ship system, range, model number, and function), assigns a unique reference number, and lists the applicable hulls, the shipboard location, and the engineering analysis result (calibration required: yes or no) for each instrument. For instruments which are determined to require periodic calibration, a cus- tomized system calibration procedure (described below) for the instrument and/or its host system is created. This system calibration procedure number, and echelon of the laboratory to perform it, are also listed. These fields enhance the CRL’s utility by linking each instrument to its requisite calibration procedure, and by re- moving any doubt as to responsibility for calibration action.

The CRL data, in tabular form, is sorted and presented by system, by location, and by CRL reference number. The system sort allows ship’s force and the various inspection teams to identify instrumentation within any engineering system. The location sort furnishes ship’s force and

56 November 1995

calibration personnel a list of instrumentation within a given space, which is especially useful for workload planning. The reference number provides a ready identifier for instruments in correspondence and telephone discus- sions. Instruments within the same measurement chain are numbered in sequence (e.g. 1234, 1234A, 1234B, etc.), hence the sort by this number shows all components of each measurement system.

The CRL is the first successful method to identify and standardize instrument system nomenclature and calibration requirements for the fleet. Other advantages include the bound- ing of the calibration workload to only those instruments which are essential to plant operation and personnel safety, and the ability to locate a component from the data record and vice versa. Without the CRL, the task of locating a calibratable component on the ship using configuration data alone (e.g. Pressure Gage, XYZ Mfg. Co., O-lO,OOO PSI) would be a major chal- lenge. Conversely, matching an instrument to its calibration documentation is made considerably easier by the CRL.

By way of illustration of the mag- nitude of installed instrumentation and the advantage provided by the engineering analysis, the guided missile cruiser class CRL lists 4,215 total instruments, including 226 parametric chains; of these, 2,566 require periodic calibration, and l, 649 are designated as NCR.

SYSTEM CALIBRATION PROCEDURE (SCP) The second main document produced by the SISCAL Program is the Sys- tem Calibration Procedure (SCP), which is also published as a class-specific Technical Manual. The SCP provides the methodology for performing a component or a system calibration for each instrument that requires calibration, a s identified in the CRL. SCPs are used by dedicated Navy teams who are trained and equipped to perform shipboard system calibrations.

NAVAL ENGINEERS J 0 U R N AL

The term SCP will hereinafter be used interchangeably to signify either an individual system calibration procedure (for one parametric chain) or the entire Technical Manual compen- dium of system calibration procedures for a ship class, dependent on the context.

As the sophistication of measurement and control system technology has increased, so has the complexity of the related engineering documentation. The SCP provides the technician with all the information required to adjust and calibrate the parametric loop undergoing test. This relieves the technician of the task of compiling and reviewing manufacturer’s data, technical manuals, wiring diagrams, and schematics from several different sources before beginning the calibration. Using a single document, the technician can progress through a logical sequence of operations to accomplish the calibration and adjustment of the measurement chain and any associated controls or alarms and record the resultant data.

The SCP begins with a system de- scription, which provides a brief over- view of the measurement chain and its function. This is followed by a list of the Navy-approved test equipment required to perform the calibration. A “preparation“ section lists equipment to be tagged out (if necessary) and the prerequisite procedures that must be completed prior to commencing the SCE These prerequisites are performed first to reestablish baseline conditions for power supplies, voltage regulators, and A/D converters prior to commencing calibration of measurement chains. This minimizes adjustments once system calibration begins and simplifies realignment of measurement or control chains if out of tolerance conditions are present. The system operation test provides the illustrated procedure for proper connection of test equipment to measurement points, and a three-point check (with tolerances) of the measurement chain from sensor to readout to establish the accuracy of the chain. Branching statements are fur-

Optimizing Performance and Reliability ol Automated Machinery Control Systems and Instrumentation

nished to lead personnel through paths in the alignment sequence to correct out of tolerance conditions. These are component-by-component tests in order of precedence, e.g. sensor, signal conditioner, buffers, and readouts. After successful verifi- cation of the accuracy of the measurement chain functions, logic and alarm tests check for proper setpoints of alarm and control logic functions and provide for adjustment if necessary. Each procedure includes a check sheet which, for each step, lists function tested, nominal value, calibration tolerances, and provides spaces for measured values and out of tolerance entries.

The system calibration philosophy ensures that only the minimum and most effective effort is expended. The process is designed to allow initial assessment of the entire system to iden- hfy necessary actions to be taken. An end-to-end check of the system is performed to verify satisfactory operating condition and correct indications. Only if this test is failed are individual components then calibrated. The fun- damental principle is that an individual circuit card is calibrated to its specified output voltage, then the resultant display reading is confirmed. If a reading is out of tolerance, the SCP provides the means to backtrack through the system if necessary (perhaps to a transducer or signal conditioner), to determine and cor rec t whatever source is adversely affecting the reading. This highhghts a key attribute of the SCP: the provision of the proper sequence of checks and calibrations of these open-ended systems to pre- clude erroneous adjustments (for example, the adjustment of an alarm prior to the calibration of a requisite A/D converter).

This philosophy greatly helps to ensure the interchangeability of components if failures should occur during system operation between SISCAL visits, an important benefit to the ship. Prior to SISCAL, or in the absence of SCPs, persons working on these multi-element systems would typically adjust one of the separate

components at random to produce a desired reading or to bring the system into a deceptive state of equilibrium (viz. to pass an acceptance test, to suppress a troublesome alarm, or in an attempt to execute a misguided preventive maintenance requirement). This unsound method often results in faulty compensation for errors that exist in other components of the measurement chain without correct- ing those e r ro r s at their origin/ source. Transducers, circuit card as- semblies, and meters can all become targets for spurious adjustments, based on individual technicians’ pref- erences or hunches. If an inaccurately set component later fails, its properly adjusted replacement may degrade system performance. Through use of SISCAL system-level techniques, aU components are adjusted to their own specifications. Although this does not obviate the need for later calibration, this method facilitates substitution of a calibrated spare in the field, when necessary, with lowest risk of negative impact on the system.

The SISCAL approach also includes the practical concept of basing tolerance and accuracy requirements on design specifications of the system, rather than on manufacturer or component specifications, which can be more stringent. This is to avoid unnecessarily increasing the time and effort to perform the calibrations by making the acceptable band of as- found indication as wide as possible.

The SCP for the CG 47 Class con- tains 97 system calibration and 17 component calibration procedures. To depict its utility for locating components, Figure 3 shows a representative diagram from an SCP for a propulsion gas turbine module (GTM) lube oil scavenge temperature system, depicting RTD sensor and cable locations on the GTM lube and scavenge pump. Figure 4 depicts the test setup for connecting a pressure calibrator (a portable standard) to a transducer. Figures 5 and 6, from the SCP for the Landing Craft Air Cush- ion (LCAC) class, show the blow-in door board’s position in the Signal

F I G U R E 3. LM2500 Scavenge Temperature RTD Locations

NAVAL ENGINEERS JOURNAL November 1995 57

Optimizing Performance and Reliabilily ol Automaled Machinery Control Syslems and Instrumenlation

F I G U R E 4. Transducer calibration connection diagram

Conditioning Enclosure (SCE) and adjustment potentiometers for the circuit, along with the corresponding test point locations on the circuit card assembly

A System Calibration Example A representative case in point,of a relatively simple monitoring and control system calibration requirement is that of a guided missile frigate class GTM lube oil supply pressure system, which is vital to the engine’s ability to remain on-line. The lube oil supply pressure is monitored using a pressure transducer and a signal con-

ditioner card. The transducer is mounted underneath the GTM on the base penetration plate. System pressure is displayed on the Propulsion Control Console (PCC) and Local Op- erating Panel (LOP) panel meters and DDIs. At the PCC, the low pressure alarm is activated when the GTM is running and system pressure decreases to 15 PSIG. The low pressure automatic propulsion turbine shutdown is activated when system pressure decreases to 6 PSIG.

Table 2 summarizes the relevant SCP for the lube oil supply pressure system, one of 25 pertaining to the GTM, giving the prerequisite procedures to have been completed and the steps to be performed in accomplish-

ing the system-level calibration; each of these are detailed in the actual SCE

Prerequisite procedure PCSOOl (Propulsion Control System (PCS) Meters, step 1D of Table 2) is an example of a typical alignment action. The PCS meters are standard edge- wise panel meters; there are a total of 48 of them on the PCC and LOP for this ship class. As mechanical devices, these must first be physically zeroed with power secured. This is the only adjustment performed on these meters in the system calibration process. During the System Opera- tion Test (Table 2, step 2), a low pressure calibrator (a portable standard) is connected to the pressure transducer’s calibration port, and specified pressure values are set whde the applicable DDI and the LOP and PCC panel meters are monitored. Their indications are verified to be within listed tolerance limits. If the DDI indications are not w i t h limits, the applicable pressure transducer component-level calibration procedure (separate from this SCP) must be performed. If the panel meters are out of limits, Panel Meter Adjustment

Sample System Calibration Procedure Outline GTM014 GAS TURBINE LUBE OIL SUPPLY PRESSURE 1. Prerequisite Procedures

A. Free Standing Electronic Enclosure (FSEE) Voltage Regulators (FSEE001)

B. Local Operating Panel (LOP) Power Supplies (LOPOO1)

C. LOP Voltage Regulators (LOP002)

D. Propulsion Control System (PCS) Meters (PCSOO1)

E. Propulsion Control Console Power Supplies (PCSOOZ)

F. PCC Voltage Regulators (PCSOO3) G. PCS Analog to Digital Converters

(PCSOO4) 2. System Operation Test 3. Logic Test 4. Panel Meter Adjustment

(conditional; adjunct to Step 2) 5. Logic State Change Adjustment

(conditional; adjunct to Step 3)

58 November 1995 NAVAL ENGINEERS JOURNAL

Optimizing Perlormance and Reliability of Automated Machinery Control Systems and Instrumentation

A 4 CIRCUIT

CARD - Q

a J d E

Q -

Q

a J m E

F I G U R E 5. SCE front face (new style)

(step 4) is required. This entails opening the LOP console to access the card rack containing the meter driver circuits and adjusting the appropriate potentiometer(s) to obtain a panel meter indication within tolerance. The Logic Test (step 3) also accesses the LOP console card rack, as well as test points within the Free Standing Electronic Enclosure (FSEE), and involves insertion of a simulated "engine running" signal and the application of 20 PSIG to the pressure transducer. The logic output at the FSEE is monitored while the pressure is decreased to verify the required changes in logic states: first from a logic HIGH to a logic LOW at 15 PSIG, with the LOW ENGINE OIL PRESSURE indications verified to be illuminated at the PCC and LOP: then

R12 H73 R59c)-C P4 WIPER WIPER WIPKR

U13 ' PIN I

u9 'PIN 7

F I G U R E 6. Blow-in door A6 circuit card, foil side

NAVAL ENGINEERS JOURNAL November 1995 59

Optimizing Performance and Reliability ol Automated Machinery Conlml Systems and lnslrumentation

from a logic HIGH to LOW again at 6 PSIG to verify that the LUBE OIL AUTO SHUTDOWN is illuminated at the PCC. If the LPC indications are not within tolerance, Logic State Change Adjustment (step 5) is performed, in which the logic output setpoints are adjusted at the FSEE. After all necessary adjustments have been made, the System Operation Test is performed again to verify that the entire measurement chain is within tolerance. This sequence is re- peated for each GTM.

Other Cali brat ion Support Considerations Technical personnel with responsibility for a priori or in-service engineering requirements related to measurement and control systems should bear in mind the following issues which may impact decisions to be made in the design, acquisition, and logistic support phases.

SELF-CALIBRATION Manufacturers and vendors may state or imply that their products are self- calibrating, or are so inherently accurate or stable that calibration is not required. Whether intentional or not, this can create an impression of heightened automation capability or decreased support costs associated with calibration. Such claims should be critically evaluated to avoid being misled or deluded into a false sense of confidence in component or system accuracy System effectiveness de- pends on equipment reliability, which in turn relies on periodic testing with accurate measurement equipment that is calibrated to traceable stan- dards. In many cases, self-calibration is a misnomer: self-testing may be a legitimate functional capability, but should not be confused with calibration. Self-checks, when present, may merely test overall functionality in go/ no-go terms (such as by feeding a signal through the network to confirm that all components are responding),

60 November 1995

or may check selected individual components (e.g. output drivers, A/D converters, or software). Self-checks rarely test sensors or input ampli- fiers, thereby leaving uncertainty with major portions of the measurement chain. Even seemingly innocuous fea- tures, such as clock frequencies used to measure tachometer signals, should be assessed for functional requirements prior to procurement.

Drift is an unavoidable physical attribute of analog circuits; hence, using internal circuitry to check other internal circuitry can lead to undetectable errors. Valid internal checks therefore require a very stable internal source (whether current, voltage, frequency, or resistance) which is itself periodi- cally calibrated externally to provide confidence in such a process.

In systems with automated Sam- pling routines, micro-processors scan numerous signal channels, perform A/D conversions, and process data. Self-checks may be included as a reg- ular part of this repetitive operating cycle and can interrupt or slow down the data acquisition process, which is one reason why they are often limited to those described above. Designers and those specifying such systems should be aware of and sensitive to these issues; alternatives can then be considered. These include reducing the periodicity of automated self- checks, providing a separate calibration mode, and making the calibration process a totally off-line function.

SOFTWARE CALIBRATION The incorrect perception exists that calibration is not possible or required if there are no physical adjustments available (as with the CG 47 Local Control Operating Panel signal conditioner example mentioned previ- ously). By design, adjustments are made through software, with calibration constants or correction factors stored in memory. If, for reasons other than calibration (such as control system revisions), the software is modified and/or the memory circuits are changed, the calibration constants


will likely return to default or un- known values, causing measurement errors. This results in a logistics support requirement, preferably solved by establishing a central point of contact to maintain calibration data for each installation so that proper constants are downloaded when revisions are made. A second option would provide for the local downloading of any existing calibration data into replacement hardware or software at the time of installation, whether for replacement or upgrade. Alternatively, in lieu of making calibration software changes, replacement of circuit boards (signal conditioners and/or A/D converters) could be the standard methodology Costs, risks, and benefits are associated with each approach. For example, if software calibration were the selected approach, interchangeability of components could be lost due to the customization of the measurement chain, with each channel having its own unique calibration curve.

SYSTEM ACCURACY AND TOLERANCE REQUIREMENTS As noted above, control system design specifications are a source of accuracies and tolerances, and ideally these are set as liberally as possible, since they relate directly to acquisition and maintenance costs. Such costs are multiplied as populations grow. There can be occasions when design specifications are more stringent than in-service conditions require. The SISCAL process looks at the accuracy of the entire parametric chain and performs circuit analysis to ensure that the tolerances specified are what the entire chain can deliver. An unexpected finding of one such analysis occurred with the current- to-voltage signal conditioner circuit card assembly used on FFG 7, AO 177, and MCM 1 Class ships. The technical manual and acceptance test procedure documentation show a tolerance of t 2% of full scale. An analysis of the circuit schematic shown in Figure 7 (taking into account individ-


-12 vdc

-15 vdc

R l R 2 105u 1% 3.091 0.1% U 1 A

P r o m - X d c r R 3 a4 4 - 20 mAdC 7 . ~ 7 ~ im 30s 0 . ? %

L M 7 q I

5

- I S vec

I O K 0.1%

RS l o * 0.1%

F I G U R E 7. Current-to-voltage converter schematic

ual resistor tolerances, op-amp specifications, voltage regulator accuracy and temperature coefficients) reveals that an accuracy of 2 0.5% is achiev- able. To procure this card at the greater accuracy would raise the cost unnecessarily, since the excess capability is not needed.

Over-engineered specifications and excessively tight tolerances may be found in the field to be unnecessary, and calibration tolerances can compensate for this, with a resultant sav- ing in life-cycle cost. Conversely, field experience may point out a need for tighter accuracy requirements than were specified in the original design. This is of most concern in systems with automated decision making functions, such as shutdowns or limiting. An example of this is gas generator and power turbine speed circuits on gas turbines, where not shutting

down fast enough can quickly cause dangerous overspeed situations.

THE OPERATIONAL IMPACT OF I NSUFFlCl ENT ACCURACY Several existing ship classes have transducers that are not inherently accurate enough for optimal operation of the gas turbine. Ships with LM2500 engines use various devices to protect the turbine during operation. One of these devices is a torque computer which calculates engine load and thereby, horsepower. The output of the computer is used to provide torque limiting to prevent excessive loading, which could result in damage or decreased life of the engine.

The LM2500 torque computer relies on four inputs to calculate engine torque. For a DD 963 or CG 47 Class

ship, this is calculated by the following formulas:

Q = 1900 . (PT,, - PT,) - (NPT/TT,) . (4.3 . PT,, + 1.7 . PT,)

and

TT, = 459.6 + T, where

Q pounds

PT,,, = power turbine inlet pressure in PSIA

PT, = gas generator d e t pressure in PSIA

NPT = power turbine speed in RPM

TT, = gas generator inlet temperature in "Ranlune

T, = gas generator inlet temperature in "Fahrenheit

= calculated torque in foot-

NAVAL ENGINEERS J OU R M A 1 November 1995 61


Each of the four inputs is a measurement chain consisting of a sensor, a signal conditioner and an analog to digtal W D ) converter. The accuracy of each measurement chain is a con- tributing factor in the overall accuracy of the torque computer. The change in torque with reference to the change in the four inputs is as follows:

6Q/WT, I = 1900 - 4.3 . NPT/TT, 6Q/6PT, = - 1900 - 1.7 . NPTITT,

SQhTT, = (NPT/TT,’) . (4.3 . PT, I

+ 1.7. PT,)

6Q/6NPT = -(l/TT,) . (4.3 . PT, I

+ 1.7. PT,)

The accuracy of the pressure transducers used for PT, $ and PT, is * 1% of full scale from MIL-P-24212. The accepted accuracy of the type “E” printed wire board (PWB) signal conditioner in the Free Standing Elec- tronics Enclosure (FSEE) is 20.15 VDC, from the LM2500 manual S9234-AD-MMO-O50/LM2500. The AID converter used in the torque computer is an 8 bit converter, thus its accuracy can be no better than ? 1 bit, or 20.4%.

Assume the following operating conditions:

PT, , = 30 PSIA PT, = 14.5 PSIA NPT = 3500 RPM T1 = 77 “F; then

Q = 28,573 foot-pounds,

which at 3500 RPM is equivalent to 19,041 horsepower. By factoring in each measurement chain component’s tolerance, the effect on the overall torque computer accuracy can be determined. Table 3 provides a chart showing the effect each component’s accuracy has on the calculated torque. Taking the square root of the sum of the squares and using a confidence level of 20 yields an overall torque uncertainty of 6900 foot- pounds, which can have significant impact. Consequences include the ship’s inability to make full power,

Torque Computer Errors Due to Component Errors

Measurement Accuracy in Torque Error in Component Calculated

Chain Component Engineering Units Foot-Pounds

Transducer 0.75 PSlA 1407 PTS 4 Signal Conditioner 2.25 PSlA 4220

A/D Converter 0.30 PSlA 563

Transducer 0.16 PSlA 306 PT, Signal Conditioner 0.48 PSlA 917


Speed Pickup 0 RPM N PT Signal Conditioner 150 RPM

A/D Converter 20 RPM

0 38

5

RTD 2.00”F T2 Signal Conditioner 5.70”F

A/D Converter 0.76“F

3 8 1

premature torque limiting or, worse, not limiting torque soon enough to prevent possible engine damage, and/ or improper balance or load sharing between engines.

It can be seen from Table 3 that the largest errors in calculated torque are associated with PT, -I and PT,. It can also be seen from the torque formula that under ambient conditions preced- ing an engine start where P T , , and PT, should be approximately equal, if the tolerances allow the value of PT,, , to be less than the PT, reading, the calculated torque will be a negative value. Hence, even with both transducers a t specified accuracy and within tolerance, the torque computer can interpret power turbine inlet pressure as slightly lower than com- pressor inlet pressure. This results in Power Lever Angle actuator (PLA)

failure, wherein the engine will not respond to throttle commands and consequently cannot be lit off. It is therefore necessary to calibrate these two signals as accurately as possible. The A/D converter is a fixed device, hence no improvement to this part of the measurement chain is possible. Examination of the type “E” PWB circuit components’ tolerances shows that the signal conditioner tolerance of ? 0.15 VDC is a very conservative figure. If more accurate pressure transducers can be used, and/or if the pressure transducer and signal conditioner can be calibrated as a single component (although this sacrifices interchangeability) at an accuracy of ? 0.5% the accuracy of the torque calculations can be greatly improved. Table 4 shows the improvement in the torque error with an increase in PT,,,

Torque Computer Errors Due to Component Errors

Measurement Accuracy in Torque Error in Component Calculated

Chain Component Engineering Units Foot-Pounds

Transducer/ PTS 4 Signal Conditioner 0.375 PSlA 703


Transducer/ Signal Conditioner 0.08 PSlA

A/D Converter 0.06 PSlA

PT2 153

122

62 November 1995 NAVAL ENGINEERS JOURNAL

and PT, accuracy. The overall calculated torque uncertainty will improve from 2 6900 to -c 1400 foot-pounds.

Conclusion The large population of control and measurement instrumentation in the fleet is essential to the successful operation and maintenance of the many engineering systems served by this equipment. A substantial number of these systems depend upon integrated suites of instrumentation for monitoring and active control. Their technical complexity presented a chal- lenge to both operators and logistics personnel, requiring a retrofitted solution. The need for precision was met by a program tailored to emerg- ing gas turbine control systems requirements, evolving under central- ized leadership into an effective solution to this and other related, pre- existing calibration management problems. Advantages are summarized below. The SISCAL approach: H provides optimum system perfor-

mance and automation capability by ensuring design accuracies of measurement chains and by furnishing coordinated adjustment of control system components;

H enhances system availability by reducing operational problems through preventive methods that ensure correct setting of control circuitry and alarms as well as interchangeability of components and spares, and by providing troubleshooting techniques to solve certain emergent difficulties;

H controls and reduces the workload and costs of calibration by binding requirements to an essential sub-

Optimizing Performance and Reliability of Automate !d Machinery Control Systems and Instrumentation

set of all installed instrumentation and facilitating its location by calibration personnel, and by making tolerances fit t echca l needs;

H contributes to the configuration documentation and calibration management processes through standardized nomenclature, con- trolled inventories, structured recall and workload planning aids, and effective feedback methods; assists the fleet in its thrust for self-sufficiency by providing intermediate level procedures, equipment, and training to accomplish shipboard system calibration, with assistance to ship’s force in system operation, troubleshooting, component calibration, and scheduling; furnishes detailed, easy-to-use calibration documentation to calibration technicians, based on the minimum, most effective, sequenced effort necessary to perform and re- port system calibrations; benefits the larger Navy technical community by ensuring that all related engineering documentation is consistent, accurate, and com- plete. Modern process instrumentation

and signal conditioning bring challenges to the calibration process (and hence to system reliability) through the elimination of traditional physical adjustments and by improved component accuracies. These advances ne- cessitate a new awareness of calibration methodology by all development and support engineers. The system- level calibration concepts given here can be successfully adapted and ap- plied to all vital shipboard engineering systems of the future. 4-


~

Bruce Marshall is &he Digital Technol- ogies Branch Head at the Carderock Division, Naval Suflace Warfare Center (CDNS WCI , Philadelphia. PA. formerly the Naval Ship Svstems Engineenng Station (NAVSSES). He received his B.S.E.E. from Drexel University in 1972, beginning his career at NAVSSES as a shipboard vibration analysis prqect engineel: He then sewed as the Station’s Material Condition Assessment Program Manager developing and coordinating various machinery health monitoring and diagnostic effmts. His current responsibil- ities include management of shipboard instrumentation calibration technical documentation. engineering data acquisition and communication systems, and vibration measurement and analysis support fw various NAVSEA and Fleet programs, and the operation ofan in- house data base which provides real-time machinery vibration and p M m n c e data to participants in the Assessment of Equipment Condition (AEC) Program. He is a member of the American Society of Naval Engmeers (ASNE). Shawn Egnak is an Electronics Engi- neer at the Carderock Division. Naval Surface Watjhre Center (CDNSWC), Philadelphia, PA. He received his B.S. degree in Physics from Drexel University in 1972 beginning his career as a laboratory supervisw at Land Instruments Inc.. where he specialized in the calibration and troubleshooting of infrared thermanie- ter systems and temperature controls. He began at CDNSWC in 198.2 and is currently a technical specialist for the Digital Technologies Branch. His respon- sibilities include development of System Calibration Procedures for various ship classes, technical support of five in-house calibration laboratm’es and redesign and maintenance of a dish buted control system used in the Volumetric Flaw Cali- bration Laboratory. He is a member of the Society of Photo-Optical Instrumentation Engineers (SPIE) and the Instrumenta- tion Society ofAmerica ( I S ) .

,November 1995 63

Call for Papers

Tenth Annual NAVSEA/NAVSUP/SPAWAR Logistics Symposium The Changing Realities of Logistics

in cooperation with the

American Society of Naval Engineers Hosted by ASNE Mechanicsburg Section Mechanicsburg, Pennsylvania

7-9 May 1996 Harrisburg Hilton, Harrisburg, Pennsylvania

Product Areas Interactive Courseware LSA Life Cycle Cost Modeling Engineering Drawings Technical Documentation Configuration Management

Training Reliabili ty/Maintainabili ty

Supply support

Possible Topics Rightsizing Electronic Classroom Impact of Expert Systems From Government to COTS Privatization Contracts CALS Logistics Acquisition

Fleet and Industry Papers are Particularly Desired

Submit 1-2 page abstracts by 5 January 1996 to Mr. William L. Dugan (NAVSEALOGCEN, 7 17-790-740 1) c/o ASNE, 1452 Duke Street Alexandria, Virginia 223 14-3458

Fax 703-836-749 1 ; E-mail [email protected]

Call for Exhibitors An exhibit hall will be open to those attending the symposium. If you or your organization are interested in participating, contact Sally Cook, ASNE Meetings Manager, at 703-836- 6727. 8 x 10 booth $800 with deposit received by 1 April 1996; $900 after 1 April 1996.

64 November 1995 N A V A L E N G I N E E R S J O U R N A L

Optimizing Performance and Reliability of Automated Machinery Control Systems and Instrumentation

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