Fault Detection for Diesel Engine Actuator - A Benchmark for FDI

Copyright (!;) IFAC Fault Detection. Supervision and Safety for Technical Processes. Espoo. Finland. 1994

FAULT DETECTION FOR DIESEL ENGINE ACTUATOR - A BENCHMARK FOR FDI by Mogens Blanke l • SI/Iren Bl/lgh l , Rikke Bille Jl/lrgensen l , and Ronald J. Patton 2 J

2

Aalborg University. Dept. of Control Engineering, Fredrik Bajers Vej 7, DK 9220 Aalborg 0, Denmark University of York, Dept. of Electronics, YOJ 5DD York, United Kingdom Abstract. Feedback control systems are vulnerable to faults in control loop sensors and actuators , because feedback actions may cause abrupt responses and process damage when faults occur. Such faults can be detected by methods for fault detection and isolation (FDI) but research results have not been widely accepted in industry . One reason has been scarcity of realistic examples. This paper introduces an electro-mechanical position servo, used in speed control of large diesel engines. as a benchmark example . The purpose is to provide a simple, industrial system as a platform for comparison of FDI methods and gathering of design experience. The system is quite simple in its basic structure, but FDI design is intricate. The paper describes the benchmark system and the requirements to fault detection. Further. two mathematical models are provide: a simple for use in design and a nonlinear description for simulation and verification. The models and laboratory data offered to interested groups has made it possible to make benchmark testing of new methods with realistic comparison of properties. Key Words: Fault detection, fault handling, automatic control, benchmark, industrial automation, actuators.

1. INTRODUCTION The governor is a device that controls the shaft speed on a diesel engine. It regulates the amount of fuel loaded into each cylinder by controlling the position of a common control rod. The rod can be moved by an actuator, which is part of the governor. The current and velocity of the actuator motor are controlled by a power drive made in analog and transistor switch mode technology for reasons of bandwidth and cost. The position of the rod is controlled by a microprocessor based digital controller. The system is thus a combination of continuous and discrete time components.

Research results within fault detection and isolation (FDI) methods have not yet gained wide acceptance in industry . In other areas of control theory where theory has materialised into extensive practical use, development included the use of numerous industrial processes as examples for control performance assessment and as a stage for refinement of the methods towards industrial use. One reason that this has not yet happened with the FDI methods has been scarcity of realistic examples for testing of new ideas against industrial systems . This paper introduces an electro-mechanical position servo, used in speed control of large diesel engines, as a benchmark example. The purpose is to introduce a real application with a genuine fault scenario as a common platform for researchers working with fault detection and isolation.

The test facility offers the prospect of repeated fault scenarios. Potential faults include malfunction of velocity or position measurements, and breakdown of the motor power drive. The faults can be both incipient or abrupt, depending on the type and the timing introduced for each fault event. The equipment also supports studies in robustness issues including model uncertainty, measurement noise, and unknown input.

The benchmark is based on an electro-mechanical test facility , which has been built by the authors at Aalborg University . The equipment simulates a part of a speed governor for large diesel engines (Blanke and Busk Nielsen (1990» .

471

the 12t is monitored. If 12t exceeds the limit, drive current will be limited to protect the motor.

The purpose of this paper is to describe the benchmark in sufficient detail to enable other researchers to continue from a well established industrial example and thereby create a platform for exchange of ideas and further development. It is the hope that enhanced methods and new approaches within FDI will be developed for this category of electro-mechanical systems. The incentive for the research community is that an estimate of one million of similar loops are in daily use in manufacturing and process industries worldwide. These loops fail occasionally and some faults have serious consequences for production and safety . Enhanced FDI methods and industrial awareness of the results could prevent many of these faults from developing into failures .

An electro-magnetic brake inside the motor is disengaged on power-up and will be engaged in case of loss of electrical power, e.g. , due to an unrecoverable error with power supply. The epicyclic gear has very little backlash (3 to 6 arc minutes) . The gear is designed to withstand repeated motion within a small angular range as is the case when a diesel engine runs a ship on an ocean passage.

2.1. Performance Characteristics Main characteristics for the actuator are:

2. INDUSTRIAL ACTUATOR

Peak power to servo motor Time to full speed (0-3000 RPM) Full stroke of output (0-100%) Velocity loop time constant Max load torque

The actuator comprises a brushless synchronous DC motor that connects to a rod through an epicycIic gear train and an arm , see fig. I. To simulate external load, a similar arrangement is mounted in parallel. With this load motor a desired load torque can be programmed . Power to the motors is delivered by two high efficiency switching mode power drives that also have some safety arrangements. The power drive has internal current and velocity control (PI) of the actuator motor, whereas the load motor is coupled only with a current controller. The torque is proportional to the motor current. A current control loop within the power drive can be neglected because its bandwidth is several times faster than the remaining system . The output is mechanically limited, and to avoid damage two micro switches serve as end stop detectors. Activation of a switch inhibits current from the power drive in that direction.

2.5 kW 22 msec 270 msec 4.6 msec 650 Nm

2.2. Block Diagram of Actuator The basic system is very simple indeed: a velocity control within the power drive, actuator motor, and gear. A block diagram is shown in fig. 2. Values of parameters and ranges for variables are listed in appendix A. Inputs are speed reference n"f and load torque Q/. Measurable outputs are motor velocity n~ and gear output position s;. The load from fuel rack is only known within wide limits, and will vary significantly depending on the maintenance condition of the engine. The load torque Q/ is hence unknown input. Generic faults in electronics and other hardware for the position control loop include: position measurement (feedback element) velocity measurement (feedback element) velocity reference (reference) current (actuator)

-

These faults are shown as additive input in the block diagram, fig . 2.

2.3. Requirements to FDl Performance Figure I. Electro-mechanical actuator for diesel engine with mounting fixture.

The requirements to FDI design are the following :

1) Detection time. Feedback fault within 10 msec, preferably within one sample of the position controller. Reference fault within five samples. Drive fault within 5 samples.

The current generator in the power drive has adjustable peak and mean values. The average of

472

d(i2(t)) dt

2) False detection rate. Below lout of 10. The

--

load input is a potential cause of a false detection. The FDI should, therefore, be insensitive to the load input.

Kv Tv

_ (n

~

At) -

"J

n (t)) m

(2)

3) Robust design . Model uncertainty is specially important for the parameters total inertia IWI and total friction f lol ' In the simple linear model these include the load inertia and friction which are usually unknown. The inertia is varying due to backlash.

Motor input is the current i~. Equations for the motor and gear are

dn (t) 1 1 +, ~ I(-J;ol nm(t)+Kq Tl i';'(t)+ N Q/(t)) 101

3. FAULT SCENARIO

ds o(t) __ dt

The occurrence of a generic fault can have several reasons. In the benchmark, we consider two very realistic events. Both events are non-trivial because they are difficult to locate for maintenance personnel. Computerized fault detection could therefore save the costs of bringing experienced staff to a possibly remote corner of the world.

(3)

I _ n (t) N m

~

The continuous time state space description for the system, written in the standard form, is

(4)

J. A sensor fault in a feedback element as a fault in !l.n. The wiper of a feedback potentiometer loses contact with the resistance element. The fault is intermediate, and lasts for only 0.2 s.

~

yet)

Cx(t) + Du(t) + F, f.(t)

=

where x = [ i2 ' nm ' solT, u n"f the control demand (output from position controller), d Q/ is a vector of unknown disturbances, fa = !lim additive actuator fault, f. !!.so additive sensor fault, and y [nm , s;l the measurement.

The particular fault might cause an overspeed of the diesel engine with a shut down of the engine as the result. If this happens during manoeuvring of the ship consequences could easily be serious.

=

=

=

The system matrices are

2. An actuator fault in !li m. An end-stop switch suddenly malfunction. The reason is a broken wire or a defect in the switch element due to heavy mechanical vibration. As a result, the power drive can only deliver positive current.

K,.

r:

0

-1,0< - K,K. T\

Kl' A= C

I::;

Both faults are multiplicative by nature, but we have chosen a description as additive faults because most FDI methods are based on this type of model and only few theoretic methods support multiplicative faults.

0

-102 0

=

f,o<

0

N

K

[1:1

-171

1.12 ,10-2

~]

0

(5)

"

r: [I m] = 1~3

B C = K,KqT\ -1-

4 . A MODEL FOR LINEAR DESIGN

tul

0

The differential equation covering the velocity controller without current saturation is 0

E

I C

=

NI ,ol 0

c

473

=[·H

=[0o I o~78l· 0

GC

f" I

101

F

D

=

0

0 ;

F,

=[+1 =[o~78l

The actuator drives a load through a gear and a bearing. The gear has a small amount of backlash and the rod connecting it with the load is slightly elastic, although this is not intended. For this reason the total system is divided into the motor with gear and the load mass as two sub systems connected through a spring.

The discrete time representation is x(k+ 1):Ax(k) +Bu(k) + Ed(k) +F.f.(k) (7)

y(k):Cx(k) +Du(k) +F,f,(k)

Where the discrete time system matrices A , B, E, and F. are, for a sampling time of Ts = 10 ms:

Load dynamics can be described similarly to the motor : A load torque QI through a stiction function accelerates the load inertia II to the load velocity nl • The load is also subject to coulomb and viscous frictions when n l is nonzero . The load position is denoted by SI' When the spring between the actuator motor and the load is loaded (sort0), the effect is a torque acting on both the actuator motor and the load. A spring torque Q.• thus adds to the driving torque. The spring torque constant has been found to be different between loading and releasing the spring. LOAD.

A

{

0.507 0.635

-0.358 0] {0'380] -9.15'10-2 0 ; B 1.064

5.42'10-5 3.93 '10-5

1

7.07'10-5

(8)

-1.21 '10-2] { -0.493] 1.55 '10-2 ; F. 0.635

E

{

1.33·1O~

5.42'10-5

5. NON-LINEAR MODEL FOR DESIGN The equations covering the entire system are presented below. They can be easily implemented in a non-linear simulation program.

The structure of a non-linear model of the position loop is presented in fig . 3. Position control is digital. The control signal n"j is thus quantizised and measurements are assessed through AID conversion.

Power Drive Input Filter:

COMPUTER.

(9)

The actuator motor current im is controlled by a PI velocity controller. The input to the velocity controller is a velocity reference n"jJilt, filtered through a low pass input filter, and the measured velocity nmv ' The maximum motor current is limited by the power drive electronics due to current saturation. The peak current limits are ±30 A. Continuous current is limited to ±12 A. The integral term in the velocity controller is limited to ±60 A by voltage saturation in an operational amplifier. POWER DRIVE.

Velocity Controller:

K

di 2(t) : T: (n"'lj~, (t) -

norv

(I»

if ((i2(t):;:;-ilim)A(diit)<0» V

((i2(t)~ilim)A(diit»0»

d(iit» : 0 dt else

» : d'2t '()

d(ii t -dt

MOTOR & GEAR. The motor current generates a motor torque Qm. The motor efficiency depends on the motor shaft position. The effect is a ripple on the motor torque.

(l0)

Motor peak and continuous current limits: if (E2(t)<.E",..) i ..... (t) : i"... else i,...(t) : i mu

The shaft is subject to stiction . When QmsO is below the stiction value the velocity remains zero. The total acting torque QlOt.m accelerates the motor inertia Im to the velocity n m. A special integrator function is required because numerical problems arise in connection with stiction simulation when the velocity approaches zero.

i.,(t) : max( -i..... (t), min(i,...(t),i.,n(l)))

(11)

With non zero velocity, coulomb and viscous friction give damping torque opposite to the driving torque. The velocity integrates to a position through the gear with a gear ratio of N.

474

Motor Efficiency Variations and Gear Efficiency:

if (i';'(t)

~O)

else Qm(t) =

Ip(t) Ip(t)

=
Analog to Digital Conversion with quantisation : n (k)

(12)

."

[

•

(t), n ' D' n'[)n ]

[ .,

•

n

:; max -n ALWuu' mm n ADnoar' lOt

[n-n;;;..:(t;4r} AD ] (20)

Kq(t + A msin(21tu1.,So(t) + Ip(t»)}..,i,;,(t)

s~v(k)

Stiction: Q/m(t) = -

= Quantisation[n

=

Quantisation[s~v(t),

SAD' SALWuu]

n,,/t) = Quantisation[n,,/ (k) , n DA , n DAmar ]

~ Q,(t)

where int is the integer function. QmsO(t) = Qm(t) + Q/m(t)

if (n)t» = 0)

Integration of angular velocity considering stiction and Coulomb friction:

(13)

Q .. ,(t) = max(O, Qmsfj(t)-Q'Slic.•,) + min(O, Qmsfj(t)-Q-SliC,m) else

(21)

n / is derived as n ..

Qm,(t) = QmsO(t)

(14)

QIOI.m

Q/, is calculated as Q/m above

Q",,(t)=a.mn.,(t)2+b.mn .. (t)+Q«0./omb ...

if (nm(t)
Q",,(t)=a-..n ..(tf+b _.. n ..(t) +Q-r;oolomb,m (15)

d(n /(t» dt else d(nlt» -d-t-

n/ is calculated as n .. above

t N ..

=0 QIOJI) --[-I-

(22)

Position Integration: = _n (t)

Q/t)

/\ (Q-co./omb.l < Q/sO < Q
if (nm(t»O)

des (t» 0_ dt

+

*' sign(n/(t-D.t») /\ (n/t-D.t) *' 0)

if «sign(n/(t»

Coulomb and Viscous Friction:

_

= Q/,(t)

White noise added to measurements and reference input to analog velocity controller:

(16)

(23)

(17)

Here v(t) is a white noise sequence. independent of all three inputs.

Simulation of load torque: solt) = so(t) - s/(t)

All symbols are described in appendix B. See also B0gh et.a!. (1993a).

d( I(so/(t) I> ft.t) = -.....,....dt

if (j{t) else

~

0) K(t) = K,

(18)

K(t) = K_

Q,(t) = K(t)[max(O, so/ (t)-~SboctJl1Jh) +

min(O, So/(/)+ ~SboctJl1Jh)] Position fault and measurement scaling: (19)

475

Figure 2. Block diagram of servomotor with velocity control loop of the power drive. The speed reference is generated from a digital position controller. The figure also shows the various generic faults considered.

L __ _

Whit. nols. v{l}

+

r---

-----l

I I I

I I

I

I I I

:

I I I ISO

l.4olor and Gear

I Effiency I I I I I I I ____________ _ L

Actuator Coulomb f riclion and Viscosity

_______________________________________

..OTOR AND GEAR

I I I I I I I JI

+

Os

------------------------------------ - ---,

Load Coulomb friclion and Viscosity Ofl

I I I I I I I I

ISI

L ________________________________ _ __________

LOAD

Figure 3. Detailed model of the actuator including load dynamics and nonlinear effects. 476

~

7. SUMMARY

6. BENCHMARK DEFINITION

An industrial position control loop with electromechanical components was presented as a benchmark example. It has several characteristics which are generic for this type of feedback control systems. The paper provided a simple mathematical model of this system for use in the design of fault handling methods and a complete, nonlinear description for simulation and verification work. These models should make it possible to make benchmark testing of new methods in FDI theory.

6.1. Fault Sequences Data sequences are delivered to represent the different models . Three sets are based on the same input sequence, whereas the fourth has a different input pattern and larger stimulation so the current saturation is more in effect. Each set include all input and output signals sampled every lO'th millisecond. The sequences comprise a position fault, a current fault, and load disturbance. The time of fault events and load step disturbances are

Event

Start time

Position fault 0.7 s 1.2 s Load input Current fault 2.7 s

The benchmark was defined, and data sequences described that enable an accurate comparison of the results obtained with different methods contributed by different research groups.

End time 0.9 s 2.3 s 3.0 s

Several groups have expressed interest in participating in this benchmark and have contributed with results . Those interested have received a kit consisting of

Two data sequences are shown in fig. 4 and fig . 5. One is for small, the other is for large signal excitation . Both are generated with the nonlinear simulation model.

a description of a linear model for design a nonlinear model written in SIMULABo a set of simulated sequences described above

6.2. Benchmark Test This material has enabled a genuine comparison of ideas and a broad forum for development of fault detection and isolation theory .

The main task is to design an algorithm that

enables detection and isolation of the two faults which is also robust to load changes, noise, and model uncertainty.

8. REFERENCES

The generally available data are velocity reference n,." velocity measurement n mv , and position measurement with fault and noise s~v retrieved on discrete sampling instants every lO'th millisecond, These signals can be used for FDI. The additional variables provided in the data files are for intermediate analysis only.

Blanke, M. and P. Busk Nielsen (1990) : The Marine Engine Governor. Proceedings Second Interna-

tional Conference on Maritime Communications and Control. London 21-23 November 1990, Society of Marine Engineers, UK. pp. I 1-20.

To make the benchmark tests uniform, the following strategy for each applied FDI method is recommended:

B0gh, S., M. Blanke, and R . Bille J0rgensen (1993a) : Nonlinear Characterization and Simulation of Industrial Position Control Loop. Aalborg University, Dept. of Control Engineering Report no. R93-4017 .

Design FDI. Test with data set I to verify design. Test with data set 3 to examine influence from model uncertainty and noise. Test with data set 4 to examine with different and more challenging signals.

B0gh , S . , R. Patton, M. Blanke, and R . Bille J0rgensen (1993b). Industrial Actuator Benchmark Test. Aalborg University, Department of Control Engineering Report no. R93-4020.

The tests with data set 3 and 4 are important because the model generating these sequences includes hard non-linearities.

477

A. DA TA SEQUENCES Load Input

Data Sequence' 3

a_,

o.ta Sequence' 4

600~--~--~~~========~~--~ Current... fauh '&---

400

200

E

POSition

fault

Load Input

a- I

'e--<>

E

~

~.200

200

Current I....

-400

°0~----~0~.5~~--~--~--~1~.5~-----7----~~2~.5~----~

~O

0.5

TIme!s)

1.5

2

~

2.5

Time!s)

Actuator Motor V~octty Reference "Jef and Measurement "_m

Actuator Motor Velocity Reference "_fe' and Measurement "_m

400

~

.:.

-400 0

0.5

1.5

2

2.5

2

2.5

TIme!s) Lo.d Velocity n_'

Load Velocity n_'

4

0.5

¥

.:.

0

-0.5 ·1 0

0.5

1.5

2.5

TIme!s)

20

0

~ ·20

-40

0

0.5

1.5 T.... !s)

2

0.5

2.5

1.5 T"",,!s)

Measured Actuator Position s_o, wi\II F.uIt s·_o, .nd Load Position 5_1

Measured AclUator Position s_o, wi\II F.... s·_o , and Load Position s_'

02

O.~r-----~------~------~------~------~----~

0.1 '6"

£.

.1).1

.I).o2oL----~0"'":.5~----~-------,,....5=-=::::::::::::!~-.::::..:::2~.::-5--'----!

-0 2 0

T"",,!s)

0.5

1.5

2

TIme!s)

Figure 5. Data sequence for large signal excitation.

Figure 4. Data sequence for small signal excitation.

478

APPENDIX A. LINEAR MODEL

a., b... b.,

The values of parameters and range of variables in the linear design modes are all readily available from manufacturer data. Par. Value

Unit

Description

19.7 .]()) 2.53·1()) 0.54 0.9 N 89 a., 0.978 TJ 0.85 T, 8.8·1())

Nmlradls kgm] NmlA A/radls

Total friction referred to servo motor Total inenia referred to servo motor Torque constant of servo motor Gain of the speed controller Gear ratio Measurement scaling factor Gear efficiency Integral time of the speed controller

fw, 1"" K. K,

s

N

Unit A radls rad/s Nm Nm rad

nDAMIU

314

n.o..u s.o..u no. n. o

342 0.50 0.153 0.167 2.44-J(r 1 5'J(r 0.1

K,o K"u/

Variables Unit

D.t

s

APPENDIX B. NON-LINEAR MODEL Par.

Value

Unit

s;

Description

s;, s

Kp

0.01 200

s"

Tp

1000

s

R

75'lcf 4.7·]()9 0.9 8.8·]())

Ohm Farad

T,

C

Q.fllc,lfl

Q.sltC.1 Q.llir,lfl

Q,"ic.l

1.. I,

7.72 Nmr -8.0·1()) Nms

Discrete contr/. sampling time Position PI controller gain Position P1 integral time, continuous value Input filter resistance 1nput filter capacitance Velocity P1 controller gain Velocity PI integral time 1ntegral term saturation limit Peak current saturation Cont. cu"ent saturation Power drive max. dissipation Motor constant average value Motor constant sine variation amplitude Motor sine variation frequency Motor sine variation phase (+) Motor sine variation phase (.) Gear torque transfer efficiency Motor positive stiction torque Load positive stiction torque Motor negative stiction torque Load negative stiction torque Total motor system moment of inenia reftrred to motor shaft Total load system inertia Motor coulombic friction, (+) Load coulombic friction, (+) Motor coulombic friction, (-) Load coulombic friction, (-) Motor viscosity square term (+) Load viscosity square factor (+) Motor viscosity linear factor (+)

-58.0 Nrns -1.15·1()5 Nms]

Load viscosity linear factor (+) Motor viscosity square factor (-)

A A A A]s

0.54 0.20

NmA"

42.3 0.76 3.65 0.85 0.824 70.2 -0.824 -70.2 9.6 'J(r 8.7

-14.5 Q.-... 0.265 Q.-.1 22.6 1.25·1()5 a.m

a.m

s

60 30 12.2 1600

Q._.m -0.170

Q.-.,

As"

Nm Nm Nm Nm kgrns']

kgrns'] Nm Nm Nm Nm Nmr

e,

n..,

i_ i]

idiff E, E]

III Q.. , Q. Q,Nm Nm Q...,() Que Nm

Q...

Q... Q"

Nm

Q"",Q, Q"".m

,Nm Nm

Q"" .,

Nm

K

Nm

f

s"

v

479

Nrns

42.6·ld Nm 32.4·ld Nm 1())

K~

Description Motor current from power drive Shaft speed of servo motor Shaft speed reference Load torque referred to servo motor Torque developed by servo motor Shaft angular position after gear

-58.0 89 1.045

K. K.

s.o Var. Range -30 to 30 im ·314 to 314 nm -314 to 314 n", Q... -6 to 6 Qm -16 to 16 -0.4 to 0.4

-7.72 Nmr -8.0·1()) Nms

s" A s" s" s" s"

Load viscosity square factor (-) Motor viscosity linear factor (-) Load viscosity linear factor (-) Gear ratio Measurement scaling factor (a, is different from simple model) Spring constant for increase Spring constant for decrease Total backlash motor- load, referred to gear output DA converter saturation limit AD converter saturation limit AD converter saturation limit DA converter quantization error AD quantization error AD converter quantization error White noise gain White noise gain White noise gain

Description Simulation adaptive step size Position reference Gear output position Gear output position fault Gear output position with fault Gear output position with fault and noise Position control error Load position Difference motor and load position Velocity reference Velocity reference including noise Low pass filtered velocity reference Motor shaft velocity Motor shaft velocity including noise Load inertia velocity Velocity control error Requested current from controller Obtained current after saturation Current fault Obtained current after saturation and fault Power drive current saturation limit Velocity controller integral variable Current saturation: current variable Current saturation: energy variable 1 Current saturation: energy variable 2 Motor constant sine: variation phase Motor/load/spring torque Spring torque referred to motor shaft Spring & motorlload torque before stiction Spring and motor,load torque after stiction Motorlload Coulomb and Viscous torque Spring, mOlOr and friction torque Spring, load and friction torque Spring constant Spring constant selection parameter