From these inputs, the firing package sends the proper
outputs to control the direction and speed of each traction
motor. To prevent machine overloads, a maximum sump
speed adjustment limits the tram speed when the cutter
motors are running. Additional speed reduction is
obtained through the cutter-motor-current feedback input,
in which motor speed is reduced as the load on the cutter
motors increases. The amount of speed reduction can be
varied by selector switches in each of these two circuits.

Fuses

It has been mentioned that certain fuses are being
monitored in the control circuit in spite of the fact that a
blown fuse does not strictly represent a failed component.
The reason for including these fuses is to add additional
diagnostic capabilities to the traction circuit of the miner
and to reduce diagnostic time for traction-circuit failures.
‘The two fuses selected are in the traction circuit: The first
one is a 5-A fuse that supplies the entire traction circuit,
and the second one is a 2-A fuse that protects the firing
package. Other fuses that are in series with components
diagnosed in the control circuit are implicitly included in
the diagnostics as well.

Directional Interlocks

There are eight directional interlocks on the prototype
machine that serve to control the directional signal that
the firing package receives and sends. Four are located on
the input side of the firing package and four are located
on the output side. These interlocks help to prevent the
firing package from sending (or receiving) a forward and
reverse signal simultaneously to the same traction motor.
Figure 10 illustrates how this is accomplished, and a brief
explanation is provided here. A left forward input to the
firing package energizes the eFOR coil; this coil operates
two sets of normally closed eFOR contacts. One set is in
series with the left reverse input to the firing package and
the other is in series with the eREV output of the firing
package. This prevents the firing package from receiving
or sending a left reverse signal while there is a left forward
signal, An analogous situation also exists for a reverse
signal; however, the left and right traction circuits are
designed to operate independently of one other.

Demultiplexer

The demultiplexer is not being diagnosed; however,
many of its outputs are monitored to establish the

15

Left Left Left
Left reverse reverse, forwards of
forward —}— forward
inpu outpui
Left —w | Left Left
reverse forward reverse
input, Left Left
forward reverse
output
Right Right
reverse fovea
Input FIRING output
power PACKAGE | Right Right
forward FeverseRight
Right reverse
Right reverse output
forward —N__]
input
Right
reverse —N—
input Right
forward

Figure 10.—Directional interlocks in traction circuit.

operational function that is intended for the machine, The
applicable outputs from the demultiplexer are listed below:

Start pump Run pump
Start cutter Safety switch
Right forward Right reverse
Left forward Left reverse

Right forward or right reverse
(for high-speed operation)
Fast (tram)

Start conveyor

Run conveyor reverse

Left forward or left reverse
{for high-speed operation)
Run cutters

Run conveyor forward

In addition, control power to the demultiplexer is
monitored.

Remote Tram interlocks

The remote tram interlocks act (collectively) as a safety
switch in the traction circuit for remote operation of the
continuous miner. As such, their function is similar to that
of the foot switch during manual operation. The remote
tram interlocks open and close the circuit between the
demultiplexer and the firing package. When a traction
switch (cither left or right) on the remote control station
is activated, the remote tram interlocks close the circuit
between the demultiplexer and the firing package so that


are the controller outputs. Terminals 2 and 3 establish the
intended function to be performed by the circuit, while
terminals 4 to 7 can be used to determine the condition of
the controller within the circuit.

Monitoring terminals 2 and 3, in this example, appears
to be redundant for diagnosing the directional controller,
but both of these points can be used to establish the con-
tinuity of the circuit between the switch and controller.
In an actual machine, a similar path may be composed of
50 ft of control wiring and several interlocks. In that case,
verifying continuity could be a great time-saving action.
Despite the simplicity of this example, it illustrates how the
electrical location of sensors is determined.

The foregoing discussion has focused on a procedure
for identifying the electrical points that must be monitored
in the control circuit. The physical location of these points
can be quite different than their location on an electrical
print. Accordingly, the next step is to locate the points on
the machine and to verify the feasibility of connecting a
sensor at that physical point. All of the control circuitry,
other than interconnecting cables, is located inside of con-
trol cases on the testbed machine. These are the master

Left-hand controller,

3
=

Master station’

Right-hand controller

Figure 3.—Plan view of testbed machine.

Figure 4.—Master control station.

control station and the right-hand and left-hand control
cases; their location on the machine is shown in figure 3.

The master control station, which is located in the op-
erator’s platform, is shown in figure 4. Space constraints
prevent full opening of its access panel without first
removing the hydraulic control levers and other devices
within the cab area. The left-hand control case is shown
in figure 5. It should be noted that removing the access
panel to this case requires the removal of 22 bolts. Fig-
ure 6 shows the right-hand control case with the access
panel removed and one of its swingout panels exposed.

The locations of the sensing points are illustrated in
figures 7 to 9. These locations have also been tabulated,
and they are shown in tables 2 and 3. As will be discussed
later, because it may be desirable to implement this system
in two phases, the sensor locations have been separated by
the implementation phase. Phase 1 sensor locations are
given in table 2, and phase 2 sensors are listed in table 3;
a summary of these points by physical location on the ma-
chine is given in table 4.

Table 2.Phase 1 sensors and locations

(Maximum expected peak-to-peak voltage, 340 V)

Sensor

Location

‘Supply side of 2-A firing-package fuse.
Load side of 2-A firing-package fuse.
Left forward input to firing package.
Left reverse output to eREV coil.

Left forward output to eFOR coil.

Right reverse output to fREV coil.

Right forward output to {FOR coil.

Left 2d speed input to firing package.
Left 3d speed input to firing package.
Right 2d speed input to firing package.
Right 3d speed input to firing package.
Right forward input to firing package.
Left reverse input to firing package.
Cutter switch terminal 2,

Left reverse output on firing package.
Left forward output on firing package.
Right forward output on firing package.
Right reverse input on firing package.
Right reverse output on firing package.
Left-hand traction switch terminal 7.
Left-hand traction switch terminal 8.
Right-hand traction switch terminal 7.
Right-hand traction switch terminal 8.
Supply side of 5-A traction circuit fuse.
Left-hand traction switch terminal 5.
Right-hand traction switch terminal 5.
Left-hand traction switch terminal 3.
Right-hand traction switch terminal 3.
Line 2 connection on SCB (common return)



19

Table 7.Diagnostic information for phase 2 fallures—Continued

Problem

Failure

Mode Sensor condition

High Low

Diagnostic logic

Cutters start but will not run
in remote.

Conveyor will not start (for-
ward) in remote.

Unknown, possible jump from

‘Open connection demux term

4 to cutter term 8 (line 86) or
failed cutter switch.

Open connection demux term 3

to conveyor term 7, or failed
conveyor switch.

HSR failed closed line 40A-40 ...

1st to 3d speed.

HSR failed closed line 140A-140

Cutters will not start .. ‘Open connection cutter term 7

to pump term 3.

Open connection pump term 4
to cutter term 12.

55 71, 72, 73, 70, 18, 34, 35
66, 36

P71 n P72. n P73.n P70 0
P66 n P36 -n ~P1B 4
~P34 n ~P35 — M55.
P71 9 P72 n P67 9 PEB A
P19 9 P20 .n ~P1B A
~P27 — M56.

56 71, 72, 67, 68,
19, 20

18, 27

NA P2n P18n P64n (P44 U
P45) nm (P53-U (P53 0
P55) n ~P71 9 ~P720
~P58 —+ M60.

P2 n P18 n P63 n (P46 U
P47) nm (P54 U (P54 9
P56) 9 ~P71 9 ~P72.0
~P58 —+ M61,

P18 mn P19 n P21. P34n
P20 n P35 n P49 n
~P32 n ~P33.n ~P36 0
~P37 — M65.

P18 n P19 0 P21 n P34.
P20 n P35 n P49 n P32
1 P33. 0 ~P36 n ~P37
— M66.

61 NA

65 18, 19, 21, 34, 32, 33, 36,
20, 35, 49 37

66 18, 19, 21, 34,
20, 35, 49, 32,
33

demux
HSR High-speed remote.
NA Not available.

RT Remote tram.

term Terminal.

‘Logic symbols are as follows: n denotes logical AND, U denotes logical OR, ~ denotes logical NOT, and —» denotes THEN.

This interlocking is achieved by using auxiliary contacts
on power contactors. For example, suppose that the
bridge output in figure 11, which is labeled "Forward
output," will energize the coil of a contactor that will then
allow current to flow to the tram motors. Also assume
that this contactor has one or more auxiliary contacts.
Two of these are normally closed and are labeled as
"forward" in the figure. Thus, when the coil of the con-
tactor is energized, the power contacts will close and
current will flow to the motors, and at the same time, the
normally closed auxiliary contacts will open. As can be
seen in the figure, when these contacts open, it is impos-
ible for a "reverse" output signal to occur, thereby
achieving a control-circuit objective. Given this explana-
tion of the circuit, an example of the diagnostic logic can
now be shown.

The circled numbers correspond to sensor points, Con-
sider a failure of the interlock, located between sensor
points 5 and 7, that causes the "reverse" contact to remain
open. For this failure mode, control voltage will exist at
points 1, 3, and 5, and will not exist at points 7, 2, 4, 6,

and 8. As such a logic equation can be written in terms of
the sensed voltages, as follows:

PIN P3N PSN ~P2N ~P4N ~P6N ~P7N ~P8— M1,

where Pi. = control voltage present at sensor point
i,
9 = and,
U = or,
~ = not,

— = then, and

Mj = failure mode j.

There are, of course, several other failure modes that
could be diagnosed, but this should serve to illustrate how
the equations operate on the sensed data.


UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT
A ampere s second
ft foot Vv volt

ft/min foot per minute



16

the directional signals from the demultiplexer can be
received by the firing package.

High-Speed Remote Interlocks

During manual operation of the continuous miner, tram
speed is determined by the position of the traction levers.
Each point on the traction lever corresponds to a specified
tram speed. The first point is first speed (16 ft/min), the
second point is second speed (35 ft/min), and the third
point is third speed (68 ft/min). During remote operation
of the machine, only two speeds are available, slow (first)
and fast (third). In addition, these speeds are not deter-
mined by the position of the traction switch. Instead, they
are determined by a separate speed selector switch on the
remote control station. When the operator sclects high
speed on the selector switch, the high-speed remote inter-
locks close, and any forward or reverse signal becomes a
third speed input to the firing package.

Open Connections

Depending on the location of the monitoring points, it
is impossible to distinguish between a failed component
and a broken connection. Therefore, some failures are
best diagnosed as failed component OR an open connec-
tion (to the component). By doing this, the remote chance
of a broken connection is not overlooked.

In other cases, the sensor locations for the determina-
tion of failed components may allow for additional diag-
nostics not originally planned. In most cases, these will
consist of open connections. If some of these broken
connections can be included with little additional effort,
they should be. Although they are relatively rare, open
connections are known to occur predominantly after
components are replaced, indicating that certain
connections either may not have been made during the
repair process or were poorly made.

Failure Modes

As described in the general procedure, failure modes
must be considered when diagnosing component failures.
In many cases, the inclusion of multiple modes of failures
increases the complexity of the diagnostic system con-
siderably. If each mode of failure is to be detectable, then
the sensing, data acquisition, and algorithmic requirements
may become prohibitive. In some cases, this problem can
be alleviated by choosing to detect the common modes of
failure, or to monitor parameters that are indicative of
many modes of failure. This type of problem is most
common on sealed electronic devices that actually contain
many components, such as a firing package.

Failures of the firing package could not be simulated;
therefore, firing-package failures are diagnosed from the
presence or absence of directional outputs. If the inputs
to the firing package are correct but the proper outputs
are absent, the firing package is considered to be failed.
This procedure should not misdiagnose any failures; how-
ever, other failures may exist in the firing package that will
go undetected.

Diagnostic Logic

Logic equations using ON-OFF control-voltage signals
were developed for the diagnostic algorithm. The logic
symbols used are as follows: M denotes logical AND, U
denotes logical OR, ~ denotes logical NOT, — denotes
THEN, and M denotes failure mode. One equation exists
for each mode of failure diagnosed. At the present time,
19 modes of failure in the traction circuit are diagnosed
using 28 of the sensing points (plus one for the reference)
for the phase 1 diagnostics; they are listed in table 6. This
table includes the problem that the operator would experi-
ence, the failure mode, the sensing-point condition (high
or low), and the logic to diagnose the failure. Phase 2
diagnostic information is listed in table 7 for the additional
21 modes of failure that have been developed. An exam-
ple is given below to illustrate the logic.

The form and function of the logic equations that are
used to diagnose failures can be illustrated with the sim-
plified traction-control circuit shown in figure 11. The
main purpose of this circuit, which is similar to one on the
testbed machine, is to output a signal that will cause the
mining machine to tram either forward or reverse. One
function of the control circuit is to ensure that the tram
controller docs not simultaneously attempt to move the
machine both forward and reverse. Such an undesirable
event is prevented by interlocking both the input and the
output of the tram controller.

Forward
Forward
output

Reverse

jeverse
Reverse
output

TRACTION
SWITCH

TRAM
CONTROLLER

Figure 11.—Simplified diagram of traction-control circuit.
Circled numbers correspond to sensor points.


Existing expert-system shells can evaluate Boolean
expressions quite casily, and accordingly, such a shell could
be selected for this implementation, if there were some
other mitigating factor. For example, the shells generally
support extensive graphics and permit effortless interfacing
with other software. Had any of these capabilities been
required, an expert-system shell would have been a viable
option, However, in the absence of any such factors, the
shell requires significantly more computer resources, with
the attendant cost. Thus, after considerable study, an
algorithmic structure was selected as the more appropriate
approach.

Although an HWL implementation would have been
possible, a microprocessor-based system was specified for
the following reasons:

1. The time required to process the logic equations is
very small; thus the speed of the diagnostic system, i.e, the
throughput rate, would be satisfactory without the use of
HWL.

2. The development time for a hard-wired system
would have been unacceptably long for the constraints of
this project.

3. The cost savings that can be realized with an HWL
implementation would be offset by the development cost.
However, if several copies of the diagnostic system were to
be fabricated, it would be practical to utilize HWL.

4, The test program objectives of the project would be
served best if it were possible to alter the logic used in
the diagnostic system. Additional information required for
specification of the microprocessor-based system is given
in the appendix.

INFORMATION MANAGEMENT ISSUES

The research summarized to this point has focused on
the types of failures that are to be detected and the means
of sensing and diagnosing them. After a diagnosis is
made, the information must be communicated to someone
in a useful form. The "who" and "what" aspects of this are
crucial information issues, and the resolution of these will
directly impact the utility of the developed system. Cer-
tainly there are many different people to whom this infor-
mation might be conveyed, and similarly, a variety of for-
mats for communicating the information can be imagined.
The reality of cost and time constraints, however, require
better defined goals.

A good starting point is to define why the system exists;
in this case it is to diagnose control-circuit failures.
Normally, when the machine malfunctions or "breaks
down," an electrician or mechanic is summoned to investi-
gate the problem. Time is required to remove control

21

case covers and to perform troubleshooting activities.
After the problem has been diagnosed, parts are obtained,
and the required corrective actions are performed. It is
apparent that significant time savings could be achieved,
resulting in less downtime, if the failure could be automat-
ically diagnosed.

Knowledge of the failure mode would allow the elec-
trician to remove only the covers necessary to access the
failed component and would eliminate the time spent at-
tempting to diagnose the problem. Further, if a part is
needed, the procurement could occur at the same time
that the failed part is being accessed and removed. Based
on the foregoing, it is clear that the diagnostic system
must, at the very least, display the failed component, e.g.,
"eFOR interlock." Other information could be displayed
such as the name of the control case enclosing the failed
component. A schematic of the control circuit relevant to
the failed component could even be displayed.

Although a variety of additional information could be
displayed or otherwise made available, there would be no
useful purpose served by doing so. Additionally, there
would be a large cost in both system complexity and cost.
Any electrician who would be authorized to make the re-
pairs would know the location of the failed component and
would rarely have a need to examine a schematic prior to
executing the maintenance action. Thus, the objective of
the onboard diagnostic system will be achieved if it simply
displays the name of the failed component or an equiv-
alent diagnostic code. The location of such a display is the
next consideration.

The information could be displayed on a small light-
emitting diode (LED) or liquid crystal display (LCD)
screen in the operator’s cab, and it could be telemetered
to remote locations, While the information may serve
some useful purpose in a remote location, such as for
creation of a data base, it will be of most importance to
people on the working section. Therefore, a display in the
operator’s cab of the machine is essential. Thus, when a
failure occurs, the section mechanic will immediately know
what has to be repaired; if the mechanic is unavailable, the
section foreman can phone for assistance, and can also
communicate the name of the failed component. The pro-
totype uses an LCD display, mounted near the operator’s
cab, to display the code number of the failure. A more
complete display, giving the actual name of the failure,
might be desirable in a production-style system.

IMPLEMENTATION BY PHASES
An important aspect of the implementation is the

determination of the component failures to be diagnosed.
Although the surveys of component failures serve as


18

Table 7.—Diagnostic information for phase 2 failures

Problem Failure Mode Sensor conditi Diagnostic logic?

No left forward remote tram AT interlock open, fuse in line 40 58, 44, 5, 45, P2.n P71. n P72 n PSB
10C-10B, open connection 15, 39,40, P61 P64 ~P44.n ~P5
10-C-10A. 0 ~P45 n ~P15 n ~P39

1m ~P40 — M40,

No left reverse remote tram RT interlock open, fuse in line a 2,71, 72, 58, 44, 5, 45, P2 n P71 n P72 n P5B
15C-15B, open connection 15C- 62, 64 15,39, 40 P62. P64 n ~P44 n ~PS,
15A. 9 ~P45 n ~P15 9 ~P39

9 ~P40 + M4.

No right forward remote RT interlock open, fuse in line 42 2, 71, 72, 58, 46, 14, 47, P2.n P71. 0 P72 n PSB

tram. 110C-110B, open connection 59, 63 42,41,43 PB _n P63 n ~PAB
110C-110A. ~P14 mn ~P47 n ~P42 0

~P41 n ~P43 + M42.

No right reverse remote RT interlock open, fuse line 43 2, 71, 72, 58, 46, 14, 47, P2 m P71 n P72 n PSB

tram. 115C-115B, open connection 60, 63 42,4143 P60 _n P63 n ~P46 n
115C-115A. ~P14 n ~P47 n ~P42 9

~P41 n ~P43 — M43.

Unknown RT interlock closed line 10B-10A 442, 18, 19, 44, 58,7172  P2.n PIB AM P19. Pad

5, 51, 61 PS n P51 n P61 ~P5Bn
~P71 1m ~P72 + M44,

Do... RT interlock closed line 15B-15A 452, 18, 19, 45, 58,71,72 — P2.n PIB NM P19. n PAS
15, 51, 62 P15 1 P51 nm P62 n ~P58

19 ~P71 9 ~P72 + M45,
Do, RT interlock closed line 110B- 46 2, 18, 19, 46, 58, 71, 72 P2 n PIB n P19 n P46
110A. 14,51, 59 P14 -n P51 9 P59 n ~P5B

9 ~P71 9 ~P72 + M46,
RT interlock closed line 115B- 47 2, 18, 19, 47, 58, 71,72 P2 n PIB n P19 n P47 9
115A. 42, 51, 60 P42 n PS1 n P60 n ~P5B

0 ~P71 9 ~P72 + MA,

No left forward fast remote HR interlock open, fuse in line 48 2, 71, 72, 58, 53, 55, 10,  P2.n P71. n P72n P58

tram, 40B-40A, open connection line 61, 64, 65. 11, 15 P61 1 P64 n PES n PAO
80. 40 ~P53.n ~P55 n ~PI0 n

~P11n ~P15 + M48.

No left reverse fast remote HSR interlock open, fuse in line 49 2, 71, 72, 58, 53, 55, 10, P2 mn P71 m P72 1m P58

tram. 40B-40A, open connection line 62, 64, 65, 11,5 P62 n P64 n PES 0 PIS
80. 39 ~P53 0 ~P55.n ~P10 n

~P119 ~P5 — Mag.

No right forward fast remote HSR interlock open, fuse in line 50 2, 71, 72, 58, 54, 56, 12, P2.n P71 n P72 n PSB

tram, 140B-140A, open connection 41, 59, 63, 13, 42 P59 n P63 n PES n P41
line 80. 65 ~P54.n ~P5E n ~PI2 n

~P13.n ~P42 —+ M50.

No right reverse fast remote HR interlock open, fuse in line 51 2, 71, 72, 58, 54, 56, 12, P2 mn P71 P72 n PBB

tram, 140B-140A, open connection 60, 63,65,43 13, 14 P60 n P63 n P65 n P43
line 80. ~P54 0 ~P5E n ~P12 0

~P13.n ~P14— M51,

Pump will not start in re- Open connection line 7 (demux 5274, 72, 73. 18, 49 P71 9 P72 n P73.n ~P18

mote, term 10 to control term 8 to in ~P49 — M52.
pump term 10).

Pump will not start in re- Open connection pump term 10 53.—S71, 72, 73,49 18, 20 P71 1 P72 1 P73 n PAS

mote. to cutter term 1, or failed cutter ~P18 n ~P20 — MS3.
or traction switches.

Cutters will not start in re- Open connection demux term5 54 71, 72, 73,70, 18, 36 P71 n P72 n P73.n P70.0

mote. to cutter term 10 (line 87) or 66, 34 P66 n P34 n ~PIB n

See notes at end of table.

failed cutter switch.

~P36 + M54.


2
t

itoa "5

tH 0

i

)

sal HS 130

2
f

roa 8s

H 10
i

i

SWITCH

an)
L
t- 08
a
108

Thu
SELECTOR

D.

T
ws 201! 190! Sag
@cli>@

Figure 8—Sensor locations (circled numbers) In left-hand controller.



If any one of these conditions is not met, the machine will
not tram. And, of the four components involved, the first
two are known to fail quite frequently, while the third and
fourth are fairly reliable (note that a blown fuse is treated
as a component failure for diagnostic purposes). Further-
more, traction failures are quite common and difficult to
diagnose. Thus, it would be advantageous to monitor all
four of the components, even though two of them are
reliable.

The previous example illustrates two important points
relevant to the general case. First, it is sometimes nec-
essary to monitor components that rarely fail when they
are part of a sequence of component actions necessary for
the correct operation of some other component. Second,
if a fairly reliable component, such as a fuse, can be easily
monitored, and if diagnosing the failure is time consuming,
then serious consideration should be given to monitoring
it. In the case of the fuse, it is easy to diagnose the fail-
ure, but considerable time can be wasted in removing the
covers of explosion-proof enclosures to access fuses, and
therein lies the motivation for monitoring certain of them.

A few cases may be encountered in which expensive or
otherwise constrained sensors are required. Sometimes an
appropriate sensor is simply unavailable. Both of these
situations require another evaluation of the importance of
including a particular failure type in the system, and in
the end, it will probably be determined that it is not worth
the effort to include that one. Occasionally, a component
modification, e.g., replacing a four-pole switch with one of
five poles, would alleviate a sensing problem or facilitate
sensing a specific component, and accordingly, it may be
worthwhile to pursue such a course of action. However,
a modification by anyone other than the manufacturer is
strongly discouraged.

The final stage of the general procedure is to address
miscellaneous implementation issues. These include the
physical location of sensors and the routing of wires on the
machine. Obviously, if it is impossible to locate a sensor
in a certain control case, it will be necessary to alter
the diagnostic logic or to make some other arrangement.
Another issue that must be addressed is the possibility that
the circuit on a given machine is slightly different than the
one illustrated on the prints used to develop the system.
Such discrepancies may alter the logic, the interpretation
of sensor values, or may be inconsequential, but they
should be checked.

The description of this general procedure has provided
an overview of an approach toward the development of an
onboard diagnostic system. The remainder of this report
illustrates application of the system to the development of
a prototype system for a continuous mining machine.

CONTROL-CIRCUIT FAILURES
Prototype

The prototype diagnostic system was developed for a
Bureau-owned Joy 14CM09-10DX continuous miner. This
machine, shown in figure 1, is typical of commonly used
equipment in US. coal mines, and the control circuit
represents one of the more advanced circuits in mining
equipment.

The electrical circuit of the Joy 14CM continuous miner
is composed of two distinct parts: (1) the control circuit
and (2) the power circuit. The control circuit operates at
120 V ac and is isolated from the power circuit; conse-
quently, all control-circuit measurements must be referred
to the common return wire. The cutter, conveyor, pump,
and dust collector all operate on the machine’s three-phase
voltage. The traction circuit is composed of an ac-dc drive
that includes one independent three-phase full-wave recti-
fier for each of two traction motors.

Features included in the machine’s electrical circuit are
current regulation and cutter-motor feedback. Current
regulation is used during motor startup and plug reversals
to provide smooth operation and to minimize drive train
shock loads. The cutter-motor feedback system varies the
sump speed depending on the cutter-motor load; as the
load increases, the speed decreases.

The (solid state) components of the drive circuit on the
machine are identical to those on the 12CM continuous
miner and the 10SC22 and 21SC shuttle cars. The traction
circuits are updated frequently, with the result that dif-
ferences are based more on date of manufacture than on
machine type or customer preference. Minesite modifica-
tions to these circuits cannot be overlooked either, and
must be considered.

The operation of the 14CM continuous miner is divided
into five circuits:

1. Control circuit.

2. Pump circuit.

3. Conveyor circuit.

4, Cutter circuit (with or without the dust collector).
5. Traction circuit.

The operation of each of these circuits follows a distinct
hierarchy to avoid unsafe conditions. This is achieved by
requiring that the control switch be activated first. This
determines if the machine will be controlled manually or
remotely. Once the control switch is on, the pump must
be energized before any of the remaining circuits. This
gives an audible signal to nearby personnel that the ma-
chine has been activated. Consequently, the conveyor,
cutter, and tram circuits must be off before the pump will
start.


24

APPENDIX.—SYSTEM SPECIFICATIONS

This appendix provides a summary of the diagnostic
system specifications developed for the Joy 14CM09-10DX
continuous miner. These were used in the development of
the hardware for the prototype system. Information on
the synthesis and implementation aspects of the prototype
was presented in the main body of this report.

SELECTED COMPONENTS FOR DIAGNOSTIC.
ACTION

Table A-1 lists the control-circuit components and
modes of failure selected for the prototype system.

Table A-1.—Control-circuit components
to be diagnosed In prototype

Failure
Component mode
Firing package ... Many.
5-A traction-cirouit fuse Open.
2-A firing-package fuse Do.
Left reverse (@REV) interlock (2) Do.
Left forward (eFOR) interlock (2) . Do.
Right reverse (fREV) interlock (2) . Do.
Right forward (fFOR) interlock (2) Do.
Connections... Do.

Remote tram (RT) interlock
High-speed remote (HSR) interlock

‘Failure diagnosed by lack of proper directional outputs.

Open or closed.
Do.

SENSORS AND SENSOR LOCATION

The "sensors" are voltage dividers for detecting the
presence or absence of the control-voltage signals, Be-
cause the control circuit is ungrounded, the common re-
turn line is used as the reference. Table 4 in the main text
lists the number of sensing points contained in each of the
controller boxes. A description of each sensing point and
the maximum expected peak-to-peak ac voltage signals are
listed in tables 2 and 3 in the main text.

Each sensor should be connected under the existing
screw head with a ring terminal at the appropriate sensing
point. The wire should be fused with an in-line fuse
holder that is located at a convenient point within the
explosion-proof enclosure.

The most convenient location for the electrical diag-
nostic box is on top of the master station, if possible. For
the prototype system, the computing requirements and the
available sizes of explosion-proof boxes precluded using
this location. Consequently, a small display box will

be mounted on the master station; this unit will display a
two-digit number that corresponds to the failure modes
listed in tables 6 and 7 in the main text. The diagnostic
computer will be mounted above the right-hand controller
in an explosion-proof box.

INPUT REQUIREMENTS

The minimum input requirements for the microcom-
puter are listed here:

Number of sensors ....
Maximum expected voltage
Reference point ..
De offsets ..

55 + 1 for reference,
See tables 2 and 3, main text.
Monitoring point No. 57.
None at present time.

PROCESSING REQUIREMENTS

Input voltages are assigned a logic high (true) or low
(false). In a perfect system, high would be defined as
120 V and low as 0 V root mean square (rms). Given
voltage regulation problems on the machine, noise, etc., it
is necessary to define high as any voltage over 100 V and
low as any voltage under 80 V. Even this is problematic
because of unintentional backfeeding of voltage in the con-
trol circuit. Accordingly, it may be necessary to change
the definition of high and low on different channels after
some operating experience with the prototype has been
obtained; the software should allow such changes.

The method used to determine the magnitude of the
sine wave also requires attention. A pure sine will be
sensed at most points, allowing for a simple determination
of the magnitude, Sensor points 6, 7, 8, 9, 40, 41, and 43,
however, will yield harmonic-rich, ite., distorted, sine
waves, complicating the task of determining magnitude.

The state of the control circuit is defined by the pres-
ence or absence of voltages at specific points. Therefore,
it is essential that these channels be sampled rapidly
enough to minimize the possibility of sampling during a
state change. There are several alternatives available for
achieving this; each has advantages and disadvantages.
The sampling scheme currently used is to collect three
identical sets of samples consecutively before processing
for a failure. The total sampling time is 0.5 s for the
phase 1 logic.

At this time, 40 logic equations, corresponding to
40 modes of failure (not equivalent to component failures),
have been defined. When these equations are evaluated,
only one can be TRUE; the equation that is evaluated as



INTRODUCTION

The U.S. Bureau of Mines, through its Advanced Min-
ing Systems research program, seeks to improve the safety
of the underground workplace. The use of sophisticated
control technology would allow miners to be located safely
away from the face area. To accomplish this remote
operation, mining machines will become increasingly
complex. Improved diagnostic maintenance will be needed
to keep these productive, complex machines in operation.
In support of that goal, the Bureau is examining improved
electrical system diagnostics.

Mining machines are generally operated under severe
conditions that often stress the machines’ electrical,
hydraulic, and mechanical components to the limit of their
designed capabilities. At the same time, preventive main-
tenance practices frequently are less than ideal. Moreover,
these machines contain numerous components fashioned
into increasingly complex subsystems, and accordingly,
breakdowns of these machines on a regular basis are like-
ly. Normally a breakdown also idles a working section,
and because of this, there is significant pressure to diag-
nose the cause of the problem and to effect a repair as
quickly as is possible.

Fault diagnosis, in an ideal sense and regardless of
the domain, consists of a logical and disciplined approach
based on established procedures, knowledge of the in-
volved equipment, and personal experience. Fundamental
steps involved in this process usually consist of several or
all of the following steps:

Symptom analysis.
Equipment inspection.
Fault-stage location.
Circuit checks.
Replacement or repair.
Performance tests.

Symptom analysis involves the collection and evaluation
of all pertinent information about the fault. Once pre-
liminary conclusions are drawn from the symptom analysis,
close inspection of the suspected equipment is performed
for any obvious signs of failure. Fault-stage location
involves the process of systematically checking inputs and
outputs within the system until the faulty stage is found.
The circuit-checks stage is similar to fault-stage location,
except that the specific component or components are
located. Once located, repairs are initiated, and per-
formance tests are carried out on the repaired circuit?

Despite the time-tested advantages of following these
rigorous diagnostic steps, maintenance personnel often

3George Godwin Ltd. (London). Systematic Fault Analysis, 1982,
pp. 13-49,

attempt to abbreviate the diagnostic process. These short
cuts lead to varying degrees of success in the diagnosis of
the problem. Sometimes, the experience of the repair per-
son will allow a correct diagnosis at the symptom analysis
stage, while in other cases, the failure to follow a more
rigorous procedure results in wasted time as the repair
person, blinded or fooled by previous experiences, pursues
one erroneous hypothesis after another. As the frustration
level mounts, there is a tendency to work less safely, and
in extreme cases, safety devices may be defeated in an ef-
fort to get the equipment back on-line.

Clearly, diagnostic procedures will have to be improved
if faster and more accurate diagnoses are to be made with-
out compromising safety. These improvements may be as
direct as increased training for maintenance personnel at
the plant, or the increased use of technology such as an-
nunciators to improve the maintainability of the equip-
ment. Many mines, for example, have added annunciator
bulbs to the control circuit of their belt starters; the logical
reason for a belt stoppage is then easily determined by
observing which bulbs are lit, rather than by making and
interpreting a series of electrical measurements. Of
course, this process has been automated in modern mine-
monitoring systems, but regardless of the mode of imple-
mentation, diagnostic time is reduced through the use of
these annunciators.

The diagnosis of many problems cannot be accom-
plished by simply adding annunciators to the circuit, for
different reasons. In some cases the result would be an
incomprehensible bundle of wires and bulbs, while in oth-
ers more sophisticated procedures are required. The wide-
spread use of knowledge-based expert systems (KBES) in
the 1980's was, in large part, fueled by the desire for im-
proved diagnostic systems. The diagnosis of an ailment—
in the human body, in an automobile engine, or in a con-
trol circuit—is often a very artful process driven by both
the knowledge and intuition of the diagnostician. Some
people can make accurate diagnoses quickly, while others
require more time. Accordingly, there has been much in-
terest in capturing the knowledge of human experts and
making it more widely available as an expert system.

One such system was developed to aid in the diagnosis
of hydraulic system failures in a Joy 16CM continuous
miner.5 Another, known as SCAR (Shuttle Car), is a
KBES for the diagnosis of failures in the electrical circuit
of a Joy 21SC shuttle car. SCAR’s inference engine is

‘Reference to specific products does not imply endorsement by the
US. Bureau of Mines.

SMitchell, J. Diagnostic Expert System Techniques for Improving
Hydraulic Maintenance of a Continuous Mining Machine. Min. Eng,
v. 43, No. 4, April 1991, pp. 419-423.


A GENERAL DESIGN AND IMPLEMENTATION PROCEDURE
FOR SENSOR-BASED ELECTRICAL DIAGNOSTIC
SYSTEMS FOR MINING MACHINERY

By J. L. Kohler! and J. Sottile*

ABSTRACT

Component failures in the electrical control circuits of mining machines account for a large
percentage of the total downtime of the machine. Once a failure has occurred it is always a tedious and
usually a time-consuming task to locate the failed component. Moreover, the pressure to quickly locate
the cause of a delay can lead to compromises in safety. Thus an onboard diagnostic system, essential
for an automated machine, would be a very useful addition to existing machines.

This report details U.S. Bureau of Mines development of a generic procedure for synthesizing
diagnostic systems for electrical-control-circuit failures in mining machinery. A continuous mining
machine is used as the testbed to illustrate the application of the developed methodology.

Substantive differences among mining machine control circuits made it impossible to achieve a generic
diagnostic system, but a generic approach for the synthesis of the diagnostic system was possible. As
the research progressed, it became apparent that an algorithmic approach was better than an expert-
system-based implementation. A prototype system was constructed and used to evaluate the diagnostic
system. Prototype implementation issues are also examined.

‘Mining engineer, Pittsburgh Research Center, U.S. Bureau of Mines, Pittsburgh, PA; associate professor of mining engineering, Pennsylvania
State University, State College, PA.
?Mining engineer, Pittsburgh Research Center (now with University of Kentucky, Lexington, KY).


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Figure 9.—Sensor locations (circled numbers) in master station.


SYSTEM SYNTHESIS

A general procedure for the synthesis of a machine
diagnostic system is presented. This procedure is then
illustrated through its application to the development of a
diagnostic system for the control circuit of a Joy 14CM
continuous mining machine. Before introducing the gen-
eral procedure, however, relevant background on the for-
mulation of the problem domain will be presented.

Initially, the problem domain, i.c., diagnosing mining-
machine control-circuit failures, was formulated as an
expert-system application, The success of the previously
described systems for the diagnosis of electrical-circuit
failures on mining machines established their ability to
provide expert-level diagnoses. Their main weakness, as
noted earlier, was the need for manual measurements to
be made within the control circuit. However, this defi-
ciency could be overcome by developing a sensor-based
system, and one of the challenges of this work seemed to
be interfacing a knowledge-based diagnostic system with
onboard sensors. As this work progressed, it became
apparent that the result of such an approach would be
quite unwieldy and probably unworkable.

Using the number of sensors required to support the
KBES would have been impractical, as their presence
would have reduced both the reliability and the maintain-
ability of the machine. Accordingly, it was necessary to
establish criteria for reducing the number of sensors.
After some deliberation, two criteria were established:
First, a limited number of failure modes would be diag-
nosed rather than all possible modes; and second, only the
minimal set of sensed values needed to uniquely define a
failure mode would be used. Practically, this had the
effect of requiring that the system developers function like
an expert system, while the final system, reflecting the
developers’ expertise, would be algorithmic in nature.
Additional considerations related to the method of imple-
mentation, ¢.g., algorithmic versus nonalgorithmic or
KBES, are presented later in this report. However, as will
be seen in the next section, defining the two criteria con-
stitutes a major part of the general procedure.

GENERAL PROCEDURE

First, the purpose of the diagnostic system must be
clearly articulated. Should the system include both the
power circuit and the control circuit, or only one of these?
Will the diagnostic system have the capability to detect all
possible failures, or only those that are known to occur?
Practical constraints suggest that the diagnostic system
should be limited to failures that have a higher probability
of occurring and are difficult to diagnose, as opposed to

those that are theoretically possible or easy to diagnose.
The most critical constraint is the increasing number of
sensors required to diagnose each additional failure.
Other constraints include sampling requirements and reli-
ability of the diagnostic system with increasing size and
complexity.

The second step, based on the foregoing, is to deter-
mine the failure types or modes that should be included in
the system’s diagnostic capabilities. The only way to ac-
complish this is to obtain failure data from mines and
machine manufacturers. These data, ic., the component
failures, form a starting point for the desired capability of
the diagnostic system. Selection of failure types for
inclusion should be based on both frequency of occurrence
and difficulty of diagnosis, For example, headlight failures
frequently occur, but they can be diagnosed quickly and
easily. As such, their inclusion in the diagnostic system
would be frivolous.

Next, it is necessary to gain an understanding of the
normal and abnormal functions of the targeted compo-
nents in the circuit, as well as their interaction with other
components in the circuit. Such an understanding is nec-
essary before sensor types and locations can be deter-
mined; the development of the diagnostic logic also re-
quires an accurate and detailed knowledge of component
functions.

After component operation and interaction are under-
stood, a method for detecting the failed component should
be developed. Essentially, this consists of two steps: de-
termining specific measurements, e.g, temperature, resist-
ance, voltage, and so on, that are symptomatic of a failure,
and then defining a set of logical relations that must exist
so that the specific failure can be uniquely defined. It is
at this stage of the process that it becomes necessary to
consider the benefit and feasibility of including each failure
type in the diagnostic system.

In some instances, the state of many components will
have to be monitored to define a specific component fail-
ure. This will increase the complexity of the diagnostic
system and, to the extent that this will increase the number
of sensors and interconnecting wires on the machine, may
be of questionable value. Nonetheless, further study may
reveal the value of including that failure, as illustrated in
the following example. Assume that for a machine to
tram, the following conditions must be met:

The traction switch must operate;
A safety switch must operate;
A fuse must complete the circuit; and

.
.
.
¢ An interlock must operate.


In this example, there first appears to be a great deal of
redundancy; however, the additional monitoring points can
be used to diagnose other components in this circuit. For
example, the tram controller could fail in many different
modes such that monitoring points 3 through 6 are used to
establish that the controller is performing its intended
function before any further diagnostics are performed.
Monitoring points 1 and 2 establish the prerequisite con-
ditions necessary to determine the intended function of the
circuit. In addition, they can be used to detect certain
types of failures in the traction switch (shorts).

In many cases, redundancy can be used for developing
additional confidence if the monitoring points are neces-
sary for detecting other types of failures as well. In ta-
ble 6, the redundant points are enclosed in parentheses in

the "sensor condition” column; however, these points were
not incorporated into the diagnostic logic of the prototype.

During the development of the diagnostic system, fail-
ures were simulated where possible, and the appropriate
measurements were taken to compare with the expected
values. Although the values compared well in most cases,
there were a few in which an expected value of 0 V was
closer to 50 or 60 V. Sometimes this is due to indicator
lamps that have been placed in the circuit to aid trouble-
shooting; other times the cause is more elusive. Conse-
quently, it is important to have an a priori knowledge of
this, so that the "absence" of a voltage can be correctly
defined. In the prototype system a value less than 80 V
was considered to be the same as 0 V.

SYSTEM IMPLEMENTATION

A diagnostic system was synthesized and a logical basis
for diagnosing failures, given certain sensory input, was
established. The evolution of this basis from mathematical
procedures to a functional system is described.

SYSTEM STRUCTURE

A variety of implementation methods are commonly
used to realize diagnostic systems, including hard-wired
logic (HWL) and computer-based systems. The latter are
further divided into algorithmic and nonalgorithmic classes,
depending on the type of software used by the system.
Each of these classes of implementation has its advantages
and criteria for appropriate application, although there is
often considerable overlap in these. HWL is favored if the
logical relationships between the system input and output
can be economically modeled with circuit elements, such
as logic gates. If very high throughput rates are required
in the diagnostic system, HWL systems are favored. Pro-
grammable controllers and other computer-based systems
can mimic an HWL system, but with a loss of speed, and
usually at a greater cost. These disadvantages are offset,
however, by the relative ease with which a working system
can be realized. Moreover, changes to the logic are
relatively simple to make with a computer-based system.
Thus, if only a few systems are to be constructed, the
computer-based system is almost always favored, as was
the case here.

There are many application areas or domains in which
it is not possible to establish mathematical relationships,
using Boolean logic or otherwise, between the system
inputs and the system outputs. In other cases, the
required inputs to the decision-making process may not be

easily quantified or may be heuristic in nature. Sometimes
one or more of the inputs may be unknown. In still other
cases the certainty of the data clements may vary and
these variations may alter the certainty of decisions made
by the system. The presence of one or more of the afore-
mentioned conditions tends to suggest that a nonalgorith-
mic approach, typically a KBES, may be more appropriate
for the application.

An algorithmic implementation was selected for this
prototype for the following reasons:

1. The input information required to diagnose a
control-circuit failure is obtainable from sensors; these
sensors are inexpensive and reliable, and they can be
located on the machine without compromising the
machine’s reliability or maintainability. It is unnecessary
to speculate about input values.

2. Various combinations of input values definitively
indicate the presence or absence of a specific failure with
a 100% certainty. There are no circumstances under
which the inputs would indicate the possibility that a com-
ponent has failed with less than 100% certainty.

3. Simple mathematical relations between the sensor
inputs and the various failures can be established.

Thus, the decision to select an algorithmic approach was
independent of the number of failures to be diagnosed or
the number of inputs required to make the diagnosis, The
facts that direct logical relationships between inputs and
failures could be defined and that the inputs could be
measured with certainty were central to the decision to
select an algorithmic solution to the diagnostic problem.


REPORT OF INVESTIGATIONS/1

A General Design and Implementation
Procedure for Sensor-Based Electrical

Diagnostic Systems for Mining
Machinery

By J. L. Kohler and J. Sottile

UNITED STATES DEPARTMENT OF THE INTERIOR

| BUREAU OF MINES |


If remote control operation is desired, all of the pre-
viously mentioned control operations are valid from the
remote control station. The requirements for remote
operation are that the control switch on the machine be
placed in the REMOTE position, and that each of the oth-
er control switches be placed in the OFF position. In this
mode of operation, signals from the remote control station
are transmitted to the machine, where the demultiplexer,
which is wired into the control circuit at the appropriate
points, controls machine action, One notable difference of
remote operation is that the machine has only first and
third speeds (low and high speeds). However, as with
manual operation, cutter-motor feedback limits speed
while the cutters are running. (Note that remote control
stations for other continuous miners may have three
speeds available.)

This brief review of the fundamental operation of the
14CM continuous miner applies equally to a 12CM miner
and other variations of the 14CM miner. If more detailed
information is required, the manufacturer’s manual,
entitled "Joy 14CM Dual 6 SCR (now System II Dual 6
SCR) Electrical Circuits and Components," should be
consulted.

Component Failure Rates

Failure rates for control-circuit components were
collected from mining companies and mine-machinery
manufacturers to determine the types of failures that are
occurring in everyday use. A preliminary list was obtained
through telephone conversations and personal interviews
at various mine sites. Then companies were invited to
submit written documentation of failure types and
frequency of control-circuit failures in 14CM mining
machines, Finally, this information was used to establish
priorities for including various types of failure within the
diagnostic capability of the prototype system. Obviously,
the more frequent a failure, the more important it is to
have the capability to diagnose it.

The information provided by each company varied.
Some companies provided failure type and frequency infor-
mation for each machine in each of their mines, while
others provided average data for all machines in each of
their mines. In total, data were obtained for approxi-
mately 80 machines operating in mines in Pennsylvania,
West Virginia, and Ohio.

These data are believed to be representative of failures
occurring in general on 14CM machines, as well as all con-
tinuous mining machines, although the sample size may
not be large enough to compute certain reliability meas-
ures such as mean time between failure and so forth.
Nonetheless, the purpose was not to establish reliability

measures, but rather to identify the most common failures,
which the survey did quite clearly. A summary of the
compiled information is given in table 1.

Table 1.—Control-circult failures for continuous miner

Failures per Circuits primarily
Component machine per —_affected by
year component
failure
Firing packagi T6 Traction.
Foot switch .. 4612 Do.
5-A traction-circuit fuse 2 Do.
2-A firing-package fuse 2 Do.
Cutter-motor (A,B) interlocks . 1 Cutter.
Left reverse (REV) interlock . 1 Traction.
Left forward (@FOR) interlock ... 1 Do,
Right reverse (fREV) interlock .. 1 Do.
Right forward (fFOR) interlock 1 Do.
Pump (0) interlock .. 1 Pump.
Instantaneous overload 1 Cutter,
Conveyor forward (cF) interlock 1 Conveyor,
Control switches .... 26 All.
Connections between switches 2 All
Demuttiplexer! <1 All.
Control transform <1 All,
Emergency stop switch 6 Not applicable,

‘Remote control.

The first column lists the specific entity that fails. In
some cases this is a simple component like a fuse or a
switch, while in others it is a complex subsystem such as
the firing package. However, these entities represent the
level at which a replacement would be made. Thus, using
the examples here, a fuse failure would result in the re-
placement of a fuse, and a firing-package failure would
result in the replacement of the entire module, since it
is not possible to replace individual circuit boards or com-
ponents within the module. The second column shows
the average number of failures that occurred per machine
per year in the data collected for this project. In some
cases two numbers are shown because there was a wide
spread in the data provided, and in these cases the num-
bers represent the average of the minimum and maximum
values for each mine. Finally, the third column identifies
the particular circuit that is affected by a failure of that
component.

This table shows that components in the traction circuit
account for the majority of failures on continuous miners,
and because these component failures are among the more
difficult ones to diagnose, their inclusion in a diagnostic
system should be given high priority. Accordingly, these
components were emphasized in the prototype diagnostic
system developed as a part of this project.


e:

Figure 1.—Continuous miner used as testbed.

The initial step in machine operation is to place the
control switch to the REMOTE or MANUAL position.
This supplies control power to the emergency stop and
pump switches. The pump switch may then be rotated to
the START position, energizing the pump motor and giv-
ing an audible signal that the machine has been activated.
The cutter and the traction circuits must be off in order to
energize the pump. If these conditions are met, the pump
will be energized and the pump switch can be placed in
the RUN position. Any of the three remaining circuits
(conveyor, cutter, or traction) may now be energized based
on the operator’s need,

The conveyor switch has three active positions: START,
RUN, and REVERSE (RUN implies forward direction).
In order to run the conveyor in the forward direction, it is
necessary to first place the conveyor switch to the START
position. In the START position, the conveyor forward
(cF) coil is energized, which closes the cF contacts com-
pleting the conveyor forward circuit for the RUN position.

To operate the conveyor in the reverse direction, the
conveyor switch must be held in the REVERSE position.

For safety purposes, the cutter motors can only be
started by simultaneously moving the pump and the cutter
switches to the START position. In this position, the in-
stantancous overload, a unitized overload device, reduces
cutter-motor starting current by providing sequenced start-
ing of the motors. Once the cutters are running, this unit
provides overload protection by monitoring the cutter-
motor current and deenergizing the motors if the instanta-
neous current level is exceeded for a specified time. This
instantaneous value, known as the instantaneous overload,
is determined by the manufacturer and is not adjustable.

The traction circuit is activated by operating the left
and right hand tram levers while simultaneously depressing
the foot switch, Current Joy miners have three speeds in
forward and reverse. Forward speed is limited to second
if the cutters are running. Additional speed reduction is
possible with the maximum sump speed adjustment and
cutter feedback adjustment.


goal driven. The user of the system supplies the necessary
inputs, such as voltage measurements or continuity checks,
based on system-generated prompts. Model-based tech-
niques, i.e., knowledge and behavior of the circuitry, were
used to establish the knowledge base.

SCAR is a comprehensive and powerful diagnostic tool,
and it demonstrates the efficacy of such systems in mining-
electrical applications. The system does have some
notable limitations, which were cited by the developers of
the software’

1, The system does not support the use of certainty
factors or unknown responses to questions. This is not a
limitation in the application, because the requested
information can be unambiguously established. However,
it is not always easy to get the requested information, and
moreover, that information may not add significantly to the
certainty of the diagnosis.

2. No generalized systematic procedure for creating
and organizing the knowledge base was made. Instead,
potential faults were considered in the sequential order
that the components occurred in the circuit, and it was
assumed that all faults were equally likely to occur.

3. The program requires the user to refer to a hard
copy of the schematic because of graphic limitations with
the specific software used in this development.

Another expert-system program for diagnosing control-
circuit-component failures has been developed for a Joy

16CM continuous miner.’ The development of this KBES
was based on the following criteria,

1. The expert system should lead the user through a
logical diagnostic procedure by emulating a qualified ex-
pert troubleshooter.

2. The expert system should indicate physical locations
of components and test points that the user is required to
access.

3. Pertinent sections of the control system should be
displayed as necessary.

4. Access to a data base of part numbers, quantity and
location of spares, and interchangeability of components
should be readily available.

This system is also goal driven. As with SCAR, it gen-
erates appropriate requests for user inputs, such as meas-
urements and observations. This system has been success-
fully utilized by novice mechanics to diagnose complex
electrical problems both quickly and accurately.

These systems have the demonstrated advantage of pro-
viding expert-level diagnoses, and their potential benefits
as training tools should not be overlooked. Nonetheless,
they have a serious limitation: They require the user to
make a series of electrical measurements during the con-
sultation. This in itself is time consuming, and it exposes
the electrician to certain electrical hazards. Thus, the next
logical step would be to develop a sensor-based diagnostic
system that could make diagnoses without any user input.

RESEARCH OBJECTIVE

The objective of this research was to develop a sensor-
based diagnostic system for generic application to mining
machinery, using a KBES implementation. The efficacy of
the system was to be evaluated by constructing a prototype
and installing it on an actual mining machine. A Joy
14CM continuous mining machine was to be used for the
prototype development.

It became apparent in the earliest stages of the project
that it might be impossible to develop one diagnostic
system that would work on many different mining ma-
chines, such as all continuous miners. The control circuits
of many different models and manufacturers were studied,
and it was found that significant differences can exist even

among machines with the same model number. These dif-
ferences are due to customer-specified options, technolog-
ical improvements, and even date of manufacture. Certain
portions of the circuit, particularly the traction-related
components, tend to vary more than others.
Concurrently, however, it became apparent that if the
development of a generic piece of hardware was not pos-
sible, a generic approach to the development of the hard-
ware might be, and if so, the approach could be demon-
strated through the development of a prototype for the Joy
14CM continuous miner. Such an approach was devel-
oped; it and its application are the subject of this report.

Novak, T., J. R. Meigs, and R. L. Sanford. Development of an
Expert System for Diagnosing Component-Level Failures in a Shuttle
Car. IBEE Trans, Ind. Appl., v. IA-24, No. 4, 1989, pp. 691-698.

7Berzonsky, B. E. A Knowledge-Based Electrical Diagnostic System
for Mining Machine Maintenance. IEEE Trans. Ind. Appl., v. IA-26,
No. 2, 1990, pp. 342-346.


Table 6.—Diagnostic information for phase 1 failures

17

Problem Failure Mode Sensor con Diagnostic logic”
High
No tram? Foot switch ..........cccceeeeee 1 1,2, (20),52 51 P1 nm P2 nm P52 n ~P51
= M1.
2. firing-package fuse . 2 1,20), (62) 2 Pin ~P2—M2.
5-A traction circuit fuse ...... 3 (20), 52 1,@), (61) P52n ~P1 M3,
Broken connection: Cutter 3 0 (1), @, 52 P20n ~P52—+ M32.
switch and 5-A fuse,
No left forward tram’ Firing package ... 6 (1, 44, 51), 2, 15, 40, (45) P2.n PS n ~PI15 n ~P4O
5 = M6.

No left reverse tram* «do. 7 (1, 45, 51), 2, 5,39, (44) P29 PIS n ~P5 n ~P39
15 = M7.

No right forward tram’ ...... «do. B (1, 46, 51), 2, 41,42, (47) P2n P14. ~PAt nm ~PA2
14 = MB.

No right reverse tram* do, 9 (1, 47, 51), 2, 14,43, (46) P29 P42 n ~P14. n ~P43
42 = M9,

No left forward tram .. eREV NC interlock Line 10A- 10 2, 44, (1, 51) 5, 6, 15, 39, P2.n P44. ~P5 Nn ~PE
10 failed open (or open con- (45) ~P15 0 ~P39 > M10.
nection).

No left reverse tram ... FOR NC interlock Line 150-15 11 2,45, (1,51) 5,7, 15,40, P2n Pan ~PSn ~P1S A
failed open (or open con- (44) ~P7 n ~PA0 + Mit.
nection).

No right forward tram ... fREV NC interlock Line 110A- 12 2, 46, (1,51) 8, 14, 42, P2n P46 ~PB ~PI4n
110 failed open (or open con- 43, (47) ~P42 n ~P43 — M12.
nection).

No right reverse tram . {FOR NC interlock Line 115A 13 2,47, (1,51) 9, 14, 41, P2n P47 n ~PON~P14n
115 failed open (or open con- 42, (46) =P41 mn ~P42 + M13.
nection).

No left 2d speed ............ Broken connection: Left tram 27 (1), 2, 53 10 P2.n P53 n ~P10 + M27.
switch to left 2d speed input.

No left 3d speed ... Broken connection: Left tram 30 (1), 2, 10, 53,11 P2 n P10 n P53 n PSS 9
switch to left 3d speed input. 55 ~Ptt > M30.

No right 2d speed ... Broken connection: Right tram 28 +~—(1), 2, 54 12 2m P54. ~P12 + M28.
switch to right 2d speed input.

No right 3d speed .. Broken connection: Right tram 29 — (1), 2, 12, 84, 13 P2 n P12 n P54.n PSB n
switch to right 3d speed input. 56 ~P13 + M29.

No tram> : Tripped EOL or FOL overload... 31

No left forward tram . @REV interlock Line 11 failed 36 — (1, 44, 51), 2, 6,7, 15,39 P2.n P5.n PAO n ~PB
open. 5, 40 ~P7 9 ~PIS n ~P39

— M36.

No left reverse tram .. eFOR interlock Line 16 failed 37 (1, 45, 51), 2, 5,6,7,40 — P2.n P15.n P39_n ~PS A

open. 5, ~P6_n ~P7 mn ~P40
— M37.

No right forward tram ... fREV interlock Line 111 failed 38 (1, 46, 51), 2, 8,9,42,43  P2.n P14 n Pat n ~P8 A

open. 14,41 ~P9 _n ~P42 n ~P43
— M38.
No right reverse tram {FOR interlock Line 116 failed 39 8,9, 14,41 P2 n P42 n P43 n ~PB

open.

(1, 47, 81), 2,
3

=P9 9 ~P14 n ~PA1
— M39.

EOL

NC Normally closed,

Left overload relay.
FOL Right overload relay.

Parentheses indicate redundant points.
2Logic symbols are as follows: n denotes logical AND, U denotes logical OR, ~ denotes logical NOT, and ~+ denotes THEN.
3Diagnostic system must recognize that the foot switch is engaged.
‘Must check effects of cutter-motor feedback, feedback adjustment, and maximum sump speed adjustment.

SNot diagnosed because control power is present in circuit, but current is limited to values less than the amount needed to close

pump interlock.


22

guidelines for the likely components for inclusion, the
developers must ultimately make the determination. As
described before, the final selections are based on many
conditions, e.g., frequency of failure, input requirements,
circuit complexity, and circuit updates. In most cases,
however, both hardware and economic constraints will dic-
tate the number and type of failures that can be diagnosed
by the system, It is simply impractical to monitor and
diagnose every theoretically possible failure.

Similarly, it is often impractical to attempt developing
the ultimate system in the first trial. A more effective and
prudent approach is to establish multiple levels of develop-
ment and implementation in which increased capabilities
and sophistication are added. Such an approach is par-
ticularly desirable in the prototype stage since it allows for
changes to evolve in an orderly fashion, rather than
through major modifications and “fixes” to the "final
product." Here, the work was divided into phases, thereby
allowing for thorough testing and debugging of each stage
while developing the next one.

Prototype development over multiple phases also offers
opportunities to study certain aspects of interaction be-
tween the machine and the diagnostic system, and then to
use this information to improve the subsequent system. In
particular, there is some uncertainty over the char-
acteristics of state changes that occur in the control circuit
of the miner. If state changes occur (primarily in the
traction circuit) while sampling is occurring, there is a
chance of misdiagnosis or, at minimum, the collection of
useless information. This is an aspect that should be
investigated whenever the procedures outlined in this
report are applied to a different mining machine. Very
closely related to this issue is one of signal corruption;

implementation in phases will allow for possible refine-
ments in sampling procedures. The mineworthiness of
hardware components can also be evaluated in the first
phase of implementation, and modifications made in the
second phase. Consequently, the experience gained with
a phase 1 prototype will be invaluable in the development
of a phase 2 unit that will have to diagnose a larger set of
failures. Of course, it is possible to divide the implemen-
tation into additional phases, depending on the goals of
the system developers.

Many of the failures previously listed in table 1 were
selected for the phase 1 development, and are shown in
table 5. All of the failures in this group are related to the
traction circuit, and they were selected for two reasons:
First, taken as a group, they account for the largest
percentage of control-circuit failures on the continuous
miners; and second, they are the most difficult to diagnose.
Another desirable characteristic of this group is that they
are applicable for all machine operating modes, i.e., re-
mote and manual.

Generally, the phase 2 diagnostics detect failures of
additional components that are required for remote opera-
tion of the machine. Although the number of additional
failures that are detected is limited, the demultiplexer must
be monitored to define its operational state. Accordingly,
most of the phase 2 monitoring points are located on the
demultiplexer output. Failures of fuses, interlocks, and
connections between the demultiplexer and the rest of the
control circuit are also diagnosed, although the logic does
not distinguish them individually. The additional sensing
requirements that this would impose is not justified by a
corresponding time savings from such a specific diagnosis.
Certain switch connections are also diagnosed.

TEST PROGRAM

The development and implementation of a diagnostic
system requires testing at various stages of the work.
First, it is necessary to make measurements on the ma-
chine to verify the proposed logic. Sometimes there are
subtle but important differences between the schematic
and the actual clectrical implementation on the machine.
These nced to be detected and noted at this stage as well.
Once the accuracy of the diagnostic logic has been veri-
fied, the microprocessor can be programmed and the sen-
sors can be installed on the machine. Then it will be
necessary to verify the integrity of the programming by
testing it on the machine. Of course, testing in this sense
means to simulate cach failure on the machine and to
check for a correct diagnostic response from the micro-
Processor. An incorrect response indicates an error in the
programming or in the wiring of the sensors, assuming

that the logic equations were previously verified. The final
stage of testing would be to place the machine, with the
installed diagnostic system, into the mine. Based on the
outcome of this test it may be necessary to alter the diag-
nostic logic or some other aspect of the system, It is
important to note that the addition of the diagnostic sys-
tem to a mining machine will likely necessitate obtaining
certain Mine Safety and Health Administration (MSHA)
approvals, due to wiring or other hardware additions to
the machine,

The test program for the development of the prototype
system was more involved than suggested by the foregoing,
because of the experimental nature of the work. Initially,
a significant effort was made to study the electrical behav-
ior and performance of the control circuit, to determine
the feasibility of implementing a diagnostic system of the


CONTENTS

Abstract ...
Introduction .
Research objective .
System synthesis .
General procedure .
Control-circuit failures ....
Prototype ............005
Component failure rates ......
Identification of sensing points ...
Relationships between sensor data and failures ...
Component descriptions .
Failure modes ......
Diagnostic logic ..........
System implementation .
System structure ....
Information management i issues
Implementation by phases . .
Test program ... .
Conclusions .... .
Appendix—System specifications ..........

Continuous miner used as testbed ....... eee eeee
Simplified diagram of switch and directional controller .
Plan view of testbed machine ......

Master control station . .
Left-hand control box .
Right-hand control box .
Sensor locations in right-han control ler.
Sensor locations in left-hand controller ..
Sensor locations in master station
Directional interlocks in traction circuit ..........
Simplified diagram of traction-control circuit ....

Be
BS cor awaynyp

Control-circuit failures for continuous miner .
Phase 1 sensors and locations .
Phase 2 sensors and locations ...........+
Number of sensing points in controller boxes .
Control-circuit components to be diagnosed .....
Diagnostic information for phase 1 failures .
Diagnostic information for phase 2 failures . .
Control-circuit components to be diagnosed in prototype...

PNAMAYNE


10

Table 3.—Phase 2 sensors and locations

(Maximum expected peak-to-peak
voltage, 340 V)

Sensor

Location

Pump switch terminal 5.
Pump switch terminal 6,
Cutter switch terminal 4,
Conveyor switch terminal 2.
Pump switch terminal 3,
Pump switch terminal 4,
Cutter switch terminal 3,
Cutter switch terminal 7.
Cutter switch terminal 11.
Cutter switch terminal 12.
Pump switch terminal 10.
Control switch terminal 4,
Demultiplexer terminal 21.
Demultiplexer terminal 20.
Demultiplexer terminal 19.
Demultiplexer terminal 18.
Demultiplexer terminal 17.
Demultiplexer terminal 16.
Demuttiptexer terminal 15.
Demultiptexer terminal 14,
Demultiplexer terminal 4,
Demultiplexer terminal 3.
Demultiplexer terminal 2.
Demultiplexer terminal 5.
Demultiplexer terminal 27.
Demultiplexer terminal 9.
Demultiplexer terminal 10.

Table 4.—Number of sensing points in controller boxes

Controller Phase 1 _Phase2 ‘Total

Right hand 1 15 16
Left hand 18 C) 18
Master station 10 12 22
Total 29 27 56

NOTE.—Phases refer to development phases of the prototype.

RELATIONSHIPS BETWEEN SENSOR
DATA AND FAILURES

A group of components was selected from table 1 for
diagnosis. The selections were made based on considera-
tions mentioned in the general procedure; the main con-
siderations are listed below:

1. Most of the components are associated with the
traction circuit, where failures are difficult to diagnose;

2. Sensing requirements are reasonable;

3. Sensors arc reliable;

4, These components account for a large number of
control-circuit failures; and

5. No machine modifications are necessary.

Table 5 lists the components that were selected. A brief
description of the components is given in the next section.

Table 5—Control-circult components to be diagnosed

Component

Phase

Firing package ...
5-A traction-circuit fuse?
2-A firing-package fuse’
Left reverse (@REV) interlock (2)' .
Left forward (eFOR) interlock (2)!
Right reverse (fREV) interlock (2)
Right forward (fFOR) interlock (2)!
Remote tram (RT) interlock or fuse?
High-speed remote (HSA) interlock or fuse
Open connections

1
1
1
1
1
1
1
2
2
2

‘Diagnosis of these components also includes the pos-
sibility of an open connection.

Component Descriptions
Firing Package

The drive unit for each traction motor consists of a sil
con controlled rectifier (SCR) bridge in the power circuit
and the firing package in the control circuit, The firing
package is a solid-state, self-contained circuit that (1) con-
trols tram speed by sending gate signals to each SCR in
the bridge, and (2) controls tram direction through outputs
to directional contactors in the power circuit. Inputs to
the firing package are listed below:

© Control power.
e Direction:
Left (forward, reverse).
Right (forward, reverse).
© Speed:
Left (first, second, third).
Right (first, second, third).
© Cutter-motor-current feedback.


12

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Library of Congress Cataloging in Publication Data:

Kohler, Jeffery L.
A general design and implementation procedure for sensor-based electrical
diagnostic systems for mining machinery / by J.L. Kohler and J. Sottile.
Pp. cm. — (Report of investigations: [128.23]; 9443)
Includes bibliographical references,
1. Mining machinery—Electric equipment. 2. Electric fault location. I. Sottile,

J. (Joseph). IL. Title. IE. Series: Report of investigations (United States. Bureau
‘of Mines); [128.23]; 9443.

TN23.U43— [TN345] 622 s—de20.— [622] 92-23709 CIP



Existing expert-system shells can evaluate Boolean
expressions quite casily, and accordingly, such a shell could
be selected for this implementation, if there were some
other mitigating factor. For example, the shells generally
support extensive graphics and permit effortless interfacing
with other software. Had any of these capabilities been
required, an expert-system shell would have been a viable
option, However, in the absence of any such factors, the
shell requires significantly more computer resources, with
the attendant cost. Thus, after considerable study, an
algorithmic structure was selected as the more appropriate
approach.

Although an HWL implementation would have been
possible, a microprocessor-based system was specified for
the following reasons:

1. The time required to process the logic equations is
very small; thus the speed of the diagnostic system, i.e, the
throughput rate, would be satisfactory without the use of
HWL.

2. The development time for a hard-wired system
would have been unacceptably long for the constraints of
this project.

3. The cost savings that can be realized with an HWL
implementation would be offset by the development cost.
However, if several copies of the diagnostic system were to
be fabricated, it would be practical to utilize HWL.

4, The test program objectives of the project would be
served best if it were possible to alter the logic used in
the diagnostic system. Additional information required for
specification of the microprocessor-based system is given
in the appendix.

INFORMATION MANAGEMENT ISSUES

The research summarized to this point has focused on
the types of failures that are to be detected and the means
of sensing and diagnosing them. After a diagnosis is
made, the information must be communicated to someone
in a useful form. The "who" and "what" aspects of this are
crucial information issues, and the resolution of these will
directly impact the utility of the developed system. Cer-
tainly there are many different people to whom this infor-
mation might be conveyed, and similarly, a variety of for-
mats for communicating the information can be imagined.
The reality of cost and time constraints, however, require
better defined goals.

A good starting point is to define why the system exists;
in this case it is to diagnose control-circuit failures.
Normally, when the machine malfunctions or "breaks
down," an electrician or mechanic is summoned to investi-
gate the problem. Time is required to remove control

21

case covers and to perform troubleshooting activities.
After the problem has been diagnosed, parts are obtained,
and the required corrective actions are performed. It is
apparent that significant time savings could be achieved,
resulting in less downtime, if the failure could be automat-
ically diagnosed.

Knowledge of the failure mode would allow the elec-
trician to remove only the covers necessary to access the
failed component and would eliminate the time spent at-
tempting to diagnose the problem. Further, if a part is
needed, the procurement could occur at the same time
that the failed part is being accessed and removed. Based
on the foregoing, it is clear that the diagnostic system
must, at the very least, display the failed component, e.g.,
"eFOR interlock." Other information could be displayed
such as the name of the control case enclosing the failed
component. A schematic of the control circuit relevant to
the failed component could even be displayed.

Although a variety of additional information could be
displayed or otherwise made available, there would be no
useful purpose served by doing so. Additionally, there
would be a large cost in both system complexity and cost.
Any electrician who would be authorized to make the re-
pairs would know the location of the failed component and
would rarely have a need to examine a schematic prior to
executing the maintenance action. Thus, the objective of
the onboard diagnostic system will be achieved if it simply
displays the name of the failed component or an equiv-
alent diagnostic code. The location of such a display is the
next consideration.

The information could be displayed on a small light-
emitting diode (LED) or liquid crystal display (LCD)
screen in the operator’s cab, and it could be telemetered
to remote locations, While the information may serve
some useful purpose in a remote location, such as for
creation of a data base, it will be of most importance to
people on the working section. Therefore, a display in the
operator’s cab of the machine is essential. Thus, when a
failure occurs, the section mechanic will immediately know
what has to be repaired; if the mechanic is unavailable, the
section foreman can phone for assistance, and can also
communicate the name of the failed component. The pro-
totype uses an LCD display, mounted near the operator’s
cab, to display the code number of the failure. A more
complete display, giving the actual name of the failure,
might be desirable in a production-style system.

IMPLEMENTATION BY PHASES
An important aspect of the implementation is the

determination of the component failures to be diagnosed.
Although the surveys of component failures serve as


Report of Investigations 9443

A General Design and Implementation
Procedure for Sensor-Based Electrical
Diagnostic Systems for Mining
Machinery

By J. L. Kohler and J. Sottile

UNITED STATES DEPARTMENT OF THE INTERIOR
Bruce Babbitt

BUREAU OF MINES


type described in this report. In addition to utilizing the
testbed machine for this work, training panels were also
obtained and utilized.

A prototype system was then constructed, and the sen-
sors were installed on the machine. Next, the prototype
system was tested and modifications were made as re-
quired. Finally, the system was documented, and an
application was submitted to MSHA for an experimental
permit to use the system in the mine. The experimental
permit was not received prior to the testbed machine’s

23

departure to a West Virginia coal mine, where it is being
used in the Bureau’s computer-assisted mining project.
Consequently, in-mine testing of this system has been
delayed pending MSHA approval to mount the diagnostic
unit on the machine. However, it will be installed on the
testbed machine later this year when the machine is
returned to the Bureau for additional modifications. Once
the diagnostic system has been tested in the mine, it will
be possible to determine its usefulness to mine personnel,
in addition to its technical performance.

CONCLUSIONS

A general procedure for the development of onboard
diagnostic systems for mining machines has been devel-
oped, although it was impossible to develop one diagnostic
system for generic application to mining machines. The
application of this procedure was illustrated through the
actual development of a diagnostic system for a Joy
14CM09 continuous miner, the miner used in the Bureau’s
computer-assisted mining program.

Implementation issues were examined, and it was deter-
mined that an algorithmic approach would be more advan-
tageous than an expert system one. Information issues
related to the output of the diagnostic system and the use
of this output by mine workers were also addressed.

The developed prototype system is capable of diagnos-
ing approximately 40 different commonly occurring fail-
ures, and yet the system requires less than 60 measured
voltage points throughout the control circuit. This trans-
lates into relatively few wiring additions to the machine’s
control circuit, and furthermore, these additions do
nothing that would significantly change the machine’s
maintainability or reliability.

The logic unit is embodied in the prototype by a
microprocessor. This was an expedient choice for the
prototype, but for commercial units, the logic functions
could be easily implemented with HWL. Such an imple-
mentation would have minimal cost and would be rela-
tively trouble free.

The developed diagnostic system will be an important
module on future computer-assisted mining machines, and
it will be applicable for use on manually or remotely
operated continuous miners. The system could be added
to miners that are currently in service, and certainly it can
be made an integral part of new continuous miners. The
use of the system will result in an immediate improvement
in the maintainability of the machine, and will result in a
significant reduction in machine downtime. Moreover,
personnel safety will be enhanced.

The generic procedure presented in this report can be
used by mine personnel or machine manufacturers to
achieve these advantages.


IDENTIFICATION OF SENSING POINTS

In general, determining sensor type and location and
developing the diagnostic algorithm is an iterative proce-
dure based on many possible tradeoffs in complexity and
performance. Processing limitations and space constraints
must also be considered, as well as possible applications to
other pieces of equipment. With this in mind, additional
guidelines for determining sensor type and location are
listed below.

1. The sensors should be reliable and inexpensive, and
they must not reduce machine availability.

2. The sensors must fit within existing control boxes.

3. The sensors should not hinder repairs or replace-
ments,

4. The sensors must not create safety hazards.

5. Sensor wiring must not interfere with, or be mis-
taken for, existing control wire.

6, The sensors must not alter the behavior of the
control circuit.

The selection of specific sensor types is based on the
parameters that must be measured. If, for example, the
value of current flow at a point in the circuit can be used
to evaluate the integrity of a component, then a current
sensor must be placed at that location. Or, if the tem-
perature at a particular point is useful in detecting a
specific failure, then a temperature sensor must be
selected and located at that point. In the case of control-
circuit diagnostics, the state of the circuit can be defined
if voltages are known at key points throughout the circuit.
More complicated measurements could be made, including
hydraulic pressures, linear position, conveyor speed, and so
on, but these would not significantly improve the ability to
diagnose a problem. Moreover, it is not necessary to
distinguish between various magnitudes of voltage. Based
on the characteristics of electromechanical control circuits
of the type used in mining machinery, it is sufficient to
differentiate only between a voltage being "present" or
"absent."

Given that the required measurand is the presence or
absence of a control voltage, the sensor is simply a wire
connected to the point where the voltage is to be sensed.
This wire is then routed to a logic box, where the voltage
is reduced to a level that is compatible with the electronic
logic unit that will evaluate the signals. In each case, the
measured states, essentially “on" or "off," are evaluated in
a series of logic equations, which will be discussed later in
this report.

Once the types of failures that are to be detected have
been determined, it is a reasonably straightforward process
to identify the requisite sensing points. The process can
be summarized as follows. First, the location of the criti-
cal component is determined, and then the voltage levels
at the component, which define its state, are determined.
For example, suppose the state of a component is defined
by the voltage level on either side: If one side is high and
the other low, the component is fine. Thus, a sensing
point would be placed on either side of the component. In
most cases, however, other components connected in series
with the subject component may need to be monitored as
well. If the component is in series with an interlock, and
the interlock is open, then the voltage levels across only
the component will not be sufficient to determine if it has
failed, Sensing points must be added across the interlock,
and the corresponding voltages at these points must be
considered when assessing the integrity of the component.
Any parallel paths that interact with the component must
also be considered, and appropriate sensing points must be
added. An appropriate point is one where the voltage
level must be known before a component's integrity can be
ascertained. While not a difficult process, this aspect
of the general procedure can be very tedious and time
consuming.

As a further illustration of the process, consider the
circuit shown in figure 2, Potential sensor locations are
illustrated for a switch supplying an electronic device that
controls the directional outputs of a traction circuit. The
switch is designed to supply (exclusively) a forward, re-
verse, or no signal to the directional controller. In this
example, switch terminals 2 and 3 establish the expected
input to the controller, terminals 4 and 5 measure the
signal that the controller receives, and terminals 6 and 7

Control
power ©
r-=1 Forward Forward
input output
;
1
{Of Peverse @| Reverse
ane tput
SWITCH @ oun
DIRECTIONAL
CONTROLLER

Figure 2,-Simplified diagram of switch and directional con-
troller. Circled numbers correspond to sensor points.


11

Figure 6.—Right-hand control box.