U.S. patent application number 09/781762 was filed with the patent office on 2001-07-26 for method of providing overvoltage protection.
Invention is credited to Green, Charles A., Rice, Kevin E., Roseman, Ronald W., Snider, Dean A..
Application Number | 20010009363 09/781762 |
Document ID | / |
Family ID | 24986521 |
Filed Date | 2001-07-26 |
United States Patent
Application |
20010009363 |
Kind Code |
A1 |
Rice, Kevin E. ; et
al. |
July 26, 2001 |
Method of providing overvoltage protection
Abstract
A controller for a starter.generator used with an aircraft
engine, includes means for monitoring field return current in a
generator field winding; means for monitoring generator voltage;
and microprocessor control means for adjusting generator field
current as a function of the field current and voltage and field
return current. The field return current and output voltage
monitoring functions allow for open field, field integrity, over
voltage, field weakening and torque limiting functions to be
realized in an integrated programmable system configuration.
Inventors: |
Rice, Kevin E.; (Stow,
OH) ; Roseman, Ronald W.; (Walton Hills, OH) ;
Green, Charles A.; (Cleveland Heights, OH) ; Snider,
Dean A.; (Twinsburg, OH) |
Correspondence
Address: |
RANKIN, HILL, PORTER & CLARK, LLP
700 HUNTINGTON BUILDING
925 EUCLID AVENUE
CLEVELAND
OH
44115-1405
US
|
Family ID: |
24986521 |
Appl. No.: |
09/781762 |
Filed: |
February 12, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09781762 |
Feb 12, 2001 |
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09414039 |
Aug 18, 1997 |
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6188203 |
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09414039 |
Aug 18, 1997 |
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08742855 |
Nov 1, 1996 |
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6107775 |
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08742855 |
Nov 1, 1996 |
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08131196 |
Oct 1, 1993 |
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5583420 |
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Current U.S.
Class: |
322/28 ;
322/36 |
Current CPC
Class: |
F02N 11/04 20130101;
H02P 2101/30 20150115; H02H 7/06 20130101; H02P 9/30 20130101 |
Class at
Publication: |
322/28 ;
322/36 |
International
Class: |
H02H 007/06; H02P
009/00; H02P 011/00 |
Claims
What is claimed is:
1. Apparatus for detecting a generator overvoltage condition,
comprising: sensing means for detecting generator output voltage; a
voltage regulator circuit utilizing the generator's output voltage
for input and producing an output comprising a status signal
corresponding to the generator's operating mode, microprocessor
control means for detecting an overvoltage condition from said
sensing means, and means for monitoring the voltage regulator's
status signal, said microprocessor control means causing the
generator to de-energize if an overvoltage condition exists and the
status signal of the voltage regulator remains on.
2. Apparatus for detecting a generator overvoltage condition,
comprising: a sensing means for detecting generator output voltage;
a voltage regulator circuit utilizing the generator's output
voltage for input and producing an output comprising a status
signal corresponding to the generator's operating mode,
microprocessor control means for detecting an overvoltage condition
from said sensing means, and means for monitoring the voltage
regulator's status signal, wherein said monitoring means comprises
a free wheeling diode in shunt with the generator field winding,
and means for detecting current flow through said diode, said
microprocessor control means causing the generator to de-energize
if an overvoltage condition exists and the status signal of said
sensing means remains on.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a division of application Ser. No.
09/414,039, filed Aug. 18, 1997, which is a continuation of
application Ser. No. 08/742,885, filed Nov. 1, 1996, now U.S. Pat.
No. 6,107,775, which is a division of application Ser. No.
08/131,196, filed Oct. 1, 1993, now U.S. Pat. No. 5,583,420.
BACKGROUND OF THE INVENTION
[0002] The invention relates to controllers for generators, such as
are used with aircraft engine starters and generators.
[0003] It is well known to use generators as starters for motors
and engines. For example, in the aerospace industry, DC generators
are commonly used as engine starters on small aircraft engines such
as gas turbine engines. After the engine is started, the generator
typically is used as an electrical power source for the
aircraft.
[0004] Aircraft manufacturers and governmental regulations have,
over the years, gradually increased the demand for more monitoring
and control functions to be implemented for these generators,
particularly with respect to providing fast and accurate
indications of fault conditions to avoid catastrophic generator
failures and engine damage. However, space and weight
considerations can limit the availability and use of multiple
discrete logic circuits. Discrete control circuits also inherently
lack central processing control, thus limiting the number of
interdependent control functions that can be implemented.
[0005] Furthermore, various control and monitoring functions
heretofore used with generator discrete controllers can be
inadequate or unsuitable for protecting components that can be
overstressed or for facilitating acceptable generator operation.
For example, linear current limiters for devices subject to current
overloads can exhibit large power dissipation, and known switch
mode current limiters can deliver excessive currents and therefore
not adequately protect interconnect wiring. As another example, it
may be desirable in some applications to provide an automatic field
flash for the generator without the need for manual pilot
control.
[0006] The need exists, therefore, for a generator controller that
can implement numerous and complex control and monitoring functions
to facilitate proper generator operation without a significant
increase in cost, space requirements or weight with an increased
reliability of generator operation and control.
SUMMARY OF THE INVENTION
[0007] In view of the aforementioned needs, the invention
contemplates, in one embodiment, a controller for a
starter.generator used with an engine, comprising means for
monitoring field free wheeling current in a generator field
winding; means for monitoring generator voltage and current; and
microprocessor control means for controlling generator field
current as a function of said monitored generator current,
generator voltage and field current.
[0008] The invention further contemplates the methods embodied in
the operation of such apparatus, as well as a method for detecting
field integrity in a generator comprising the steps of:
[0009] a. monitoring field free wheeling current in the generator
using a rectifier in shunt with the generator field winding;
and
[0010] b. de-energizing the generator when field free wheeling
current is absent during a time period when field free wheeling
current should be present.
[0011] In another embodiment, the invention contemplates a
switching driver overload protection circuit comprising means for
controlling current through the device, and a relaxation oscillator
for decreasing said current in response to increasing load.
[0012] These and other aspects and advantages of the present
invention will be readily understood and appreciated by those
skilled in the art from the following detailed description of the
preferred embodiments with the best mode contemplated for
practicing the invention in view of the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 (parts A and B) is a system level functional block
diagram of a generator control circuit according to the present
invention;
[0014] FIG. 2 is a more detailed schematic diagram of a field
integrity monitor circuit suitable for use with the control circuit
of FIG. 1;
[0015] FIG. 3 is a detailed schematic diagram of a field integrity
signal conditioning circuit used with the field integrity monitor
circuit of FIG. 2;
[0016] FIG. 4 is a flow diagram of the microprocessor routine for
field integrity check;
[0017] FIG. 5 is a schematic diagram of a torque limiting circuit
according to the present invention;
[0018] FIG. 6 is a schematic diagram of a field weakening circuit
according to the present invention;
[0019] FIG. 7 is a schematic in functional block diagram form of a
fail-safe voltage regulator biasing circuit in accordance with the
present invention;
[0020] FIGS. 8 and 9 are flow diagrams of an overvoltage protection
routing implemented in the microcontroller software for the system
of FIG. 1;
[0021] FIG. 10 is a schematic diagram of a differential ground
fault detection circuit with built in test capability in accordance
with the present invention;
[0022] FIG. 11 is a functional block diagram of a driver overload
protection circuit in accordance with the present invention;
[0023] FIG. 12 is a detailed schematic diagram of an embodiment of
the circuit in FIG. 11;
[0024] FIG. 13 is a conventional field flash circuit;
[0025] FIG. 14 is a field flash circuit according to the present
invention; and
[0026] FIG. 15 is a representative graph for certain generator
signals during torque limiting according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0027] With reference to FIG. 1, a microprocessor based controller
apparatus for a generator is generally designated with the numeral
10. Although the invention is described herein with particular
reference to a DC shunt generator such as can be used conveniently
as a starter.generator for an aircraft engine, this is for purposes
of explanation and should not be construed in a limiting sense.
Those skilled in the art will readily appreciate that the teachings
of the present invention can be applied in AC generators, for
example. Thus, rather than being application specific, the
invention is more broadly construed as being directed to monitoring
and control apparatus and methods for generators. As used herein,
the term "starter.generator" simply refers to the fact that a
generator such as described herein can be used as a starter for an
engine, and after engine start can be used as a power plant for the
aircraft. However, reference to the use of a controller according
to the invention with such a starter.generator is not intended to
be limited only to generators used as starters.
[0028] In accordance with an important aspect of the invention, the
generator controller 10 is designed for the use of a
microprocessor-based controller or programmable microcontroller 12.
The use of a microcontroller significantly increases the number of
monitoring and control functions that can be integrated into the
controller 10. This increased flexibility permits monitoring and
control functions in accordance with the invention that heretofore
were not used with known generator controllers, and particularly
for starter.generators used with aircraft engines. The monitoring
and control functions as taught herein thus facilitate the use of a
microcontroller for aircraft starter.generators, for example, where
otherwise the benefits of such use of a microcontroller would not
be apparent. As will be understood from the detailed description
herein, however, some of the teachings of the present invention
also have general application to generator controllers that are not
microprocessor based, although their use with a
microprocessor-based controller enhances their advantages.
[0029] With continued reference then to FIG. 1, the controller
apparatus 10 includes the microcontroller 12 that receives a
plurality of input signals from various monitoring circuits and
issues control signals to these circuits and other control devices
to regulate and control operation of the generator. The
microcontroller can be any suitable microprocessor-based device
such as part number 87C51 available from Intel. This device can be
programmed in a conventional manner according to the manufacturer's
specifications to implement the control functions as described
herein and.or set forth in the accompanying flow diagrams, as is
well known to those skilled in the art. Peripheral components such
as ROM and RAM memories, clock generators and so on which are
inherently used in combination with a microprocessor form no
particular part of the invention and thus have been omitted from
the drawings for clarity and understanding.
[0030] Among the monitoring circuits shown in FIG. 1 is a field
current integrity circuit 14 that provides means for detecting the
presence or absence of a field winding current. The field integrity
circuit 14 produces an output 16 which in this case indicates the
presence or absence of a field return current. The field return
output signal 16 is provided as an input to the microcontroller for
use as will be more fully described hereinafter.
[0031] Another circuit that interfaces with the microcontroller 12
is a voltage regulator circuit 18. The voltage regulator circuit 18
is conventional in design and, in accordance with the invention,
can operate independent of the microcontroller 12 in the event that
the microcontroller fails. However, in further accordance with the
invention, the microcontroller is programmed to implement a
start-up and control function as described herein via an interface
to the regulator circuit 18.
[0032] In further accordance with the invention, the
microcontroller receives as an input a digitized representation of
the regulated generator output voltage, hereinafter referred to as
the point of regulation or POR 20. The POR may be connected to the
armature through a feeder cable. As shown at 22 in FIG. 1, the POR
analog value is input to a conventional analog to digital converter
24 that can receive a plurality of analog inputs 22 in a time
multiplexed manner. The digitized POR value is input to the
microcontroller 12. As will be described herein, the
microcontroller 12 uses the POR information in combination with the
field integrity signal 16 to implement an overvoltage protection
function that responds more quickly to overvoltage conditions than
heretofore known designs.
[0033] Still a further monitoring function of the controller 10 in
accordance with the invention is a ground fault detection circuit
26. The ground fault circuit 26 can be used as a stand alone
circuit in control circuits that do not use a microcontroller.
However, a particular advantage of the ground fault circuit 26 is
that it provides an output format that can conveniently be used to
implement a built in test that the microcontroller can use to
verify proper operation of the circuit.
[0034] The invention further contemplates the methods embodied in
the use of such circuits in combination with the microcontroller,
as will be apparent from the descriptions herein.
[0035] The generator control unit apparatus 10 further includes a
number of switching transistors 28, that are used for controlling
operation of the generator. The numeral 28' is used to designate
the field transistor that is used to control the field current
supplied by the armature in a shunt type generator. These switching
devices (the devices 28 and 28' can be the same device or different
devices) are susceptible to overload conditions that can damage and
degrade the devices over time or cause sudden device failure. In
accordance with the invention, these driver devices are protected
with an overload or short circuit protection circuit 30. Operation
of these circuits are not dependent on the use of a
microcontroller; but, if desired, the microcontroller can be used
to monitor the device output status and de-energize the device as
appropriate.
[0036] The microcontroller 12 also receives a plurality of digital
signals 32 from discrete logic circuits. The digital signals are
typically input to conditioning circuits 33 of conventional design
to transform the signals into a format compatible for input to the
microcontroller. These digital input signals provide information to
the microcontroller such as relay status that the microcontroller
uses to determine if conditions exist that require de-energizing
the generator or other control action as appropriate. These digital
monitor and control functions form no particular part of the
present invention because they tend to be somewhat design specific
and are largely a matter of design choice, other than to the extent
that the use of the microcontroller, facilitated by the monitor and
control functions of the present invention, also conveniently
permits these digital signals to be used by the same control
program without increased circuit complexity.
[0037] The microcontroller 12 controls overall operation of the
generator through a number of relays. For purposes of this
embodiment of the invention, the controller 10 includes a latching
field relay designated KFR in the drawings. The field relay
includes a first set of contacts 34 (KFR-1) that can be opened by
the microcontroller 12 by actuation of a relay trip signal 36.
These contacts when open interrupt the shunt circuit between the
generator armature winding 38 and the field winding 40, thus
protecting the generator and regulator 18 by shutting down the
field current. The field relay includes another set of contacts 42
that are closed when the KFR is tripped. A non-latching relay
designated KST with normally closed contacts is used for field
flash and is energized when armature voltage is present.
[0038] In general terms, the basic functions of the controller 10
include fault detection for field current, ground fault and
generator overvoltage conditions, as well as controlling field
current during and after engine start. These functions are
explained in greater detail in the following descriptions of the
detailed exemplary embodiments of the monitoring and control
circuits. As described hereinbefore, these circuits and methods
facilitate the use of a microcontroller with a starter.generator
such as used for aircraft engines.
[0039] With reference next to FIGS. 2 and 3, we show an embodiment
of a circuit 14 for monitoring field integrity. It is noted at this
time that the generator G is not shown in detail in the drawings
because the particular generator design forms no critical part of
the present invention. In the described embodiment, the generator
is a DC shunt generator such as generator 23080-013 available from
Lucas Aerospace Power Equipment Corporation. However, the field
integrity circuit 14 can also be used conveniently with series and
compound generators, for example.
[0040] Generally, a generator will have the field windings disposed
on a stator assembly and the armature windings disposed on the
armature rotor. In a generator, when used as a starter, a power
source such as a battery (B in FIG. 1) typically is connected to
the armature; and also connected by means of a switching device
such as a field transistor to the field winding. As a shunt type
generator, the field is supplied by the armature through the
switching device.
[0041] Thus, as shown in FIG. 2, the controller 10 includes the
field transistor 28' that is gated on and off by a pulse width
modulated (PWM) gate control or drive signal 50. The duty cycle of
the PWM gate drive 50 will be a function of the amount of current
required by the field in order for the controller to regulate the
generator output voltage (POR), as well as to regulate the armature
current and generator torque. The control of the PWM gate drive in
connection with other functions of the controller 10 will be
further explained herein.
[0042] For purposes of the field integrity circuit 14 operation,
the microcontroller is programmed to strobe the field transistor
28' off momentarily during a time period when the gate drive signal
normally has the transistor turned on. For example, during
start-up, the gate control signal 50 duty cycle will be 100%
(following a torque limiting period which will be explained
hereinafter). During such time that the gate drive 50 is on, the
microcontroller 12 cyclically strobes the signal off for a short
period of time, for example, about 1 millisecond. If the field
winding is not open (as in normal operation) a field return current
will flow through a free wheeling diode 52 (due to back EMF from
the inherent field inductance). If the field is open, no return
current will be detected and the microcontroller can cause the
start contactor 88 (see FIG. 5) to open, thus terminating the
engine start and disconnecting the generator armature from the
battery, thereby protecting the starter.generator from potential
damage due to an open field winding or connection.
[0043] The field integrity monitor function is accomplished in this
embodiment with the use of the free wheeling diode 52, such as part
no. 03-0064-30 available from Lucas Aerospace Power Equipment
Corporation. The free wheeling diode is shunted across the field
winding 40. This configuration is also represented in FIG. 1
wherein the diode 52 is connected to the field and field return
lines 56,58 of the generator field. The armature voltage that is
supplied to the field through the field transistor is designated by
the numeral 60 in FIG. 2.
[0044] With reference to FIG. 3, the field integrity circuit 14
further includes a differential comparator circuit 62 that detects
field return current flow through the free wheeling diode 52 by
sensing a positive voltage drop across the diode. The comparator
circuit 62 includes a differential amplifier 64 that is biased with
two resistors 66a,b connected to a reference voltage 68 (the
reference voltage can be obtained in a conventional manner for
example from the generator POR).
[0045] The output 16 (labelled OPEN SHUNT in FIG. 1) of the
amplifier output is normally logic high due to the presence of a
blocking diode 70 and the fact that during operation when the field
transistor 28' is turned on, the free wheeling diode is reversed
biased by the field voltage. When the transistor 28' is strobed
off, if the field integrity is good, current flows through the
diode 52 causing the amplifier output to go to a logic low state,
and the microcontroller detects the comparator output 16 during the
strobe period to verify the field integrity. If the field shunt is
open, however, current will not flow through the diode 52, and the
comparator output 16 will not change state during the strobe
period. This condition is also detected by the microcontroller 12
during the strobe period as an indication of the open shunt, and
the start contactor 88 can then be opened.
[0046] In addition to checking for an open shunt condition during
start-up and steady-state operation of the generator, this field
integrity monitor function also will detect generator failures
caused by an open field return, absence of field input power (such
as from the armature, for example) and a failed (open or shorted)
field transistor 28'. For example, if the field transistor is
shorted or latched on, current will not flow through the free
wheeling diode during the strobe period.
[0047] FIG. 4 is a flow diagram for a suitable microcontroller 12
routine for the field integrity monitor function as implemented
through the exemplary circuit of FIGS. 2 and 3. From the main
processing routine at step 400, the controller first checks at step
402 whether the generator is in a start-up mode and a torque
limiting feature described hereinafter has been completed. These
conditions correspond to the gate control signal 50 having a 100%
duty cycle. After a suitable delay period at step 404 from the
preceding strobe period, the microcontroller 12 strobes the field
transistor off at step 406. By choosing a suitable delay period at
step 404, this momentary interruption in field drive does not
significantly affect the generator starter performance. This field
integrity check is performed cyclically throughout the start cycle
during which the field transistor is fully on (PWM is 100%).
[0048] A particular advantage of using the free wheeling diode for
detecting field return current as opposed to a current sense
resistor, is that the latter provides a poor signal to noise ratio
because the current sense resistance must be small to minimize
losses.
[0049] Those skilled in the art will appreciate that the field
integrity monitor circuit and function can be used independent of
other starter.generator control functions described herein such as
torque limiting and field weakening.
[0050] If field return current is detected, as checked at step 408,
then normal operation is continued. If field return current is not
detected, then at step 410 the start-up sequence is terminated. The
field integrity check is particularly useful during generator
start-up to prevent energizing the armature for a long period of
time in an open shunt condition, for example. However, the field
integrity monitor function can also be used to detect improper
generator or control unit 10 (GCU) operation, improper wiring
connections and so on.
[0051] With reference next to FIG. 5, we show a start-up torque
limiting circuit 77 that can be used conveniently in conjunction
with the voltage regulator circuit 18 of FIG. 1. The basic function
of the circuit 77 is to utilize a PWM control signal 72, for the
field transistor 28' gate drive, that has a gradually increasing
duty cycle from a selected minimum, for example about 25%, to about
100%. This PWM control signal can be produced by the
microcontroller 12, for example, and used as an over ride signal 72
(VR INHIBIT in FIG. 1) that is ANDed as at 74 with the regulator
output PWM drive signal 76.
[0052] At start-up, under normal conditions the duty cycle of the
voltage regulator PWM signal 76 will be 100% in order to provide
maximum field excitation current. However, this can cause large
starting torque which may, for example, stress the engine gearbox.
Consequently, the circuit of FIG. 5 is used to limit the starting
torque by gradually increasing the field current by means of
gradually increasing the duty cycle of the field transistor gate
control signal.
[0053] In the embodiment of FIG. 5, the microcontroller 12 is
programmed to produce a PWM drive signal 78 that has a linearly
increasing duty cycle from about 25% to about 100%. By ANDing this
signal with the regulator PWM drive signal 76, such as with AND
gate 74, the regulator drive signal is temporarily overridden until
the duty cycle of the torque limiting signal 72 reaches 100%.
Thereafter, the voltage regulator PWM signal can be used to control
the field transistor 28'.
[0054] Note that in FIG. 5 we show the torque limiting PWM signal
72 as directly feeding the field winding through a functional block
84 (without showing, for example, the field drive transistor that
in actual practice would be part of the block 84). We show this
configuration because, from an operational standpoint, that is the
basic function and effect of the torque limiting signal 72, i.e. to
control a gradual increase in the field current. Furthermore, the
torque limiting PWM signal 72 does not have to exhibit a linearly
increasing duty cycle, and in fact a significant benefit of the
microcontroller 12 is that it can be programmed to produce a time
variant duty cycle of any desired format depending on the
particular application.
[0055] In the circuit of FIG. 5, the armature or starter current is
sensed from the voltage developed across the interpole winding by a
voltage monitor 87 which converts the interpole winding voltage to
a digital input to the microcontroller. The torque limiting
function is thus de-energized by means of a software gate 94 until
closure of the starter contacts 88 (the starter contacts are
controlled by the microcontroller in response to actuation of the
manual start switch 90 shown in FIG. 1). When the starter contacts
88 close, a comparator (realized in software) 92 detects the
initial starter current and thus enables the gate 94 to pass the
torque limiting PWM field control signal 72 through to the field
transistor. The circuit 92 and the functions 78 and 94 can be
realized conveniently in the microcontroller 12 software, if so
desired.
[0056] With reference to FIG. 15, we show a graph of representative
response curves for starting torque, generator voltage and field
current. Note that the torque limiting function according to the
invention causes a gradually increasing field current in direct
proportion to the generator voltage so as to limit the starting
torque.
[0057] A concept similar to the starting torque limiting circuit 77
of FIG. 5 is used to implement a field weakening function. During
engine start-up, field weakening is used to lower the field current
to reduce starter back EMF to thereby increase armature current and
torque. Field weakening thus helps maintain starting torque at a
desired level.
[0058] With reference to FIG. 6 then, a field weakening circuit in
accordance with the present invention is generally designated with
the numeral 100. Certain aspects of the circuit 100 are similar to
the torque limiting circuit 77, and in fact those skilled in the
art will readily appreciate that the two circuits can conveniently
be implemented together by selective use of logic circuits such as
AND gates. Thus, although some of the circuit components for the
circuits of FIGS. 1, 5 and 6 can be the same, we assign different
reference designators in the Figures because the individual
circuits can be implemented as stand alone circuits, combined as
appropriate or omitted as appropriate based on each particular
application.
[0059] The field weakening circuit 100 monitors the armature
current by detecting the voltage drop across the generator
interpole winding 86. This voltage (V.sub.1 in FIGS. 5 and 6) is
then converted to a digital input for the microcontroller 12 using
a conventional analog to digital (A.D) converter device 102 (that
corresponds to the monitor circuit 87 in FIG. 5. It will be
appreciated that the A.D circuit 102 can conveniently be the same
as the A.D 24 described with respect to FIG. 1, with the signal
INTERPOLE digitally converted in a time multiplexed manner with the
other analog input signals 22 as previously described.
[0060] The microcontroller 12 uses the armature current input from
the interpole winding to develop a PWM control signal 104 (.gamma.)
that is input to an AND gate 106. The AND gate 106 can conveniently
be the control AND gate 74 shown in FIG. 1, with the PWM signal
.gamma. being part of the PWM voltage regulator control signal VR
INHIBIT.
[0061] The PWM field weakening signal .gamma. is ANDed with the
output of the voltage regulator 18, which during start-up and
normal operating conditions will usually be a PWM signal near 100%
duty cycle. Thus, the PWM signal .gamma. can be used to control the
field excitation and thus armature current and torque according to
the programmed field weakening algorithm, which can be any suitable
PWM profile adapted for each particular application. As shown in
FIG. 6, a suitable algorithm includes calculating an error term
e=V.sub.1-SP, then multiplying the error signal e by a gain factor
Av to arrive at duty cycle gamma, where gamma is limited to between
0% and 100%. Note that V.sub.1 is the interpole voltage, SP is a
selected setpoint and Av is a gain factor. These parameters are
application specific and chosen based on generator rating and
aircraft engine characteristics, as will be understood by those
skilled in the art.
[0062] Note that the field weakening signal .gamma. (and thus the
microcontroller 12) has control of the field transistor 28'
whenever the POR value is below the voltage regulator threshold
reference 21 (which causes a logic high output from the regulator
18). However, when the POR value goes above the threshold, the
regulator 18 output becomes a PWM signal with a duty cycle based on
the regulator control function, and can override the PWM field
weakening signal because of the AND function.
[0063] In addition to controlling the field excitation during
start-up, as with the circuit implementations of FIGS. 5 and 6, the
microcontroller can also be used to augment the POR regulation of
the voltage regulator circuit 18. Such a control function may be
desirable in applications, for example, where small adjustments to
the POR voltage are useful such as for load equalization. This
added regulation function is accomplished in the described
embodiment using a regulator biasing circuit 110 illustrated in
FIG. 7 (note that for clarity the implementation of FIG. 7 is
partially shown in the system diagram of FIG. 1).
[0064] An important consideration when using the microcontroller 12
as part of the generator output voltage (POR) regulation control is
that a failure of the digital control logic such as the
microcontroller 12 should not be allowed to produce an unsafe
condition or override basic regulator functions. Therefore, the
circuit of FIG. 7 is particularly advantageous in that it provides
a fail-safe biasing approach in which if there is a failure in the
microcontroller circuit, the voltage regulator 18 can continue to
operate in a normal manner. In general, this is accomplished by
implementing a zero bias condition from the microcontroller in the
event that the device fails.
[0065] In FIG. 7, the microcontroller 12 produces two biasing
signals 112 and L14 which respectively are bias high and bias low
signals. These bias signals are PWM signals, and each signal is
passed through a low pass filter 116,118 (which effectively
implement the D.A function 110a in FIG. 1A) to produce
corresponding DC high and low bias signals 120,122. The bias low
signal 122 in this case is summed with the voltage regulator
reference signal 21 by a summation circuit 124, and the bias high
signal 124 is summed with the POR signal by a summation circuit
126. Note that the summation for the circuits 124 and.or 126 may
include an inversion of the bias signal depending on the particular
implementation of the biasing scheme. In the described embodiment,
the bias low signal is subtracted from the reference signal, and
the bias high signal is subtracted from the POR signal, because the
POR signal is applied to the inverting input of the voltage
regulator 18.
[0066] To implement the fail-safe feature, the bias signals are
preferably unidirectional signals as represented in FIG. 7, such as
can be achieved, for example, by the use of open collector switches
128. In the event of a microcontroller 12 failure, the typical fail
mode is such that the PWM signals 112,114 will be the same state.
In this case the bias from the adjust low signal is canceled by the
signal from the adjust high signal by the common mode rejection of
the circuit 18, thus removing any biasing influence of the failed
microcontroller 12 from the voltage regulator circuit 18.
Consequently, the voltage regulator 18 can continue to regulate the
generator output voltage even if the microcontroller 12 fails.
[0067] The generator output voltage (POR) regulation function is,
of course, an important function for the control unit 10. Discrete
control systems used heretofore typically include an overvoltage
condition sensing function so that if the generator output voltage
exceeds a predetermined threshold the generator is de-energized.
Various specifications including government specifications such as
MIL-STD-704 and RTCA-DO-160 specify overvoltage protective
functions in terms of a voltage.inverse time delay characteristics.
This approach limits the time interval during which an overvoltage
condition is allowed to exist inversely proportional to its
magnitude. In this control scenario, overvoltage conditions are
detected for magnitude and time duration, and a common
implementation is to time integrate the voltage transients. A
problem with conventional control schemes is that the discrete
circuits must be able to allow a maximum specified transient but
prevent a minimum or higher overvoltage condition. The inherent
weakness of this approach is that during a transient that in
actuality is an overvoltage condition (i.e. not an acceptable
transient), the overvoltage condition is allowed to increase in
magnitude until the control circuit time integration determines
that in fact it is an overvoltage and not a transient.
[0068] In order to overcome this problem, the microcontroller 12
implements a control routine preferably in software that more
quickly identifies or distinguishes an overvoltage condition from a
transient. This is realized by monitoring the regulator 18 status
and the overvoltage condition concurrently. FIGS. 8 and 9 show a
microcontroller routine for the overvoltage protection, with FIG. 8
showing the basic routine and FIG. 9 showing a specific
implementation thereof.
[0069] Referring to FIG. 8, an overvoltage detection method
according to the invention typically starts from a normal operating
mode at step 130. If an overvoltage condition is not detected at
step 132, normal operation continues. If an overvoltage condition
is detected, then at step 134 the controller 12 checks the on.off
status of the voltage regulator. If at step 136 the regulator is on
during the overvoltage condition then the generator is shut down at
step 138. If the regulator is off during the overvoltage condition,
then the overvoltage condition is integrated over time at step 140
in a conventional manner. If the overvoltage.inverse time test at
step 142 passes, then normal operation resumes because the
overvoltage condition was a transient. If the test fails at step
142, then de-energizing occurs.
[0070] FIG. 9 shows a specific and preferred implementation of the
basic control routine of FIG. 8. In this case, from normal
operation at step 130, if an overvoltage condition is detected at
step 132, a new voltage measurement is made at step 144 for
purposes of checking whether the overvoltage is increasing, as at
step 146. If the overvoltage condition is not increasing, normal
operation is resumed. If the overvoltage condition is increasing,
then at step 148 the microcontroller checks the status of the free
wheeling diode 52 (keeping in mind that this current status is an
input to the microcontroller 12 via the exemplary circuit of FIG.
3). A properly functioning regulator turns off the field transistor
meaning that free wheeling diode current should be present. If
current is detected in the free wheeling diode at step 150, then
normal operation is resumed. If current is not detected, then the
generator is de-energized at step 138.
[0071] The free wheeling diode current detection previously
described is particularly useful in the overvoltage control routine
of FIGS. 8 and 9 because the diode return current is an immediate
indication that the voltage regulator for the generator output
voltage POR has turned off in response to regulator 18 control. If
the regulator is still on, then the likelihood of an overvoltage
condition rather than a transient is substantially higher. However,
those skilled in the art will appreciate that the overvoltage
protection function can be realized with other techniques for
detecting the status of the regulator. For example, the on.off
status of the field transistor could be detected by the
microcontroller 12.
[0072] With reference next to FIG. 10, a differential ground fault
detection circuit 26 is illustrated. According to an important
aspect of the invention, this circuit includes built in test
capability that facilitates the use of the microcontroller 12 for
ground fault detection.
[0073] The ground fault detection circuit 26 monitors the
instantaneous difference between the generator G return current and
the generator feeder current. These currents are monitored using a
pair of current transformers 160,162 which produce a pulse at the
occurrence of a fault. The current transformers can each be device
no. 50866-000 or GC66 available from Lucas Aerospace Power
Equipment Corporation, for example.
[0074] The pulsed outputs of the current transformers are sensed
using a pair of differential amplifier circuits 164,166 which are
connected to complement each other. For example, the return current
transformer 162 is connected to the inverting input of amplifier
166 and the non-inverting input of amplifier 164. In a
complementary manner, the feeder current transformer 160 is
connected to the inverting input of the amplifier 164 and the
non-inverting input of the amplifier 166. In this manner, the two
currents are differentially sensed so that as long as there is no
imbalance in the currents, a ground fault is not indicated. The
outputs of the amplifiers 164,166 are connected together through
respective diodes 168,170 such that if either amplifier output goes
low (i.e. negative in the embodiment of FIG. 10), the circuit 26
output signal at node 172 is pulled low. If the outputs of both
amplifiers are high, then the output at node 172 of the circuit 26
is a logic high, in this case about 5 volts. A capacitor 174 is
used at the output to provide a sample and hold capability for the
current transformer pulse. This output is monitored by the
microcontroller such as with an A.D converter (see FIG. 1) because
the magnitude of the pulse detected at the sample and hold node is
proportional to the magnitude of the fault.
[0075] Additionally, each of the amplifier outputs 165,167 are
connected to a BIT circuit 175 realized with a voltage summing
network of resistors 175a,b,c. The output node of the BIT circuit
is indicated by the numeral 173.
[0076] Thus, under normal operating conditions the output of the
circuit 26 will be between +5 volts and zero (for the embodiment of
FIG. 10). In this embodiment, the output node 173 of the BIT test
circuit is designed to be at a selected value under normal
operating conditions of the circuit 26. This value can be selected,
for example, as a midpoint of the maximum excursions of the voltage
at the BIT node for all circuit failure modes.
[0077] If one or the other amplifier circuits 164,166 fails then
the output voltage at the BIT node 173 will be biased out of its
normal operating limits which condition can be detected by the
microcontroller 12. Thus, the circuit 26 contains an built in test
capability for the microcontroller to test the circuit 26 based on
the output voltage level at the output node 173 without the use of
a test stimulus or signal. For example, if the amplifier 164 fails
shorted to ground, the voltage at the node 173 will be about
one-half its normal value. As another example, if amplifier 164
fails high, then the voltage at the BIT node 173 is twice its
normal value.
[0078] The output of the circuit 26 is the inverse of the absolute
value of the current transformer signals. Differential ground fault
sensing in accordance with the invention provides inherent immunity
to common mode noise problems typically found in many environments
such as aircraft installations. The differential ground fault
sensing thus makes the circuit 26 immune from ground level shifts
and common mode noise coupling that could otherwise produce false
readings in conventional circuits that rely on absolute ground
level comparisons.
[0079] As described hereinbefore, the generator control unit 10 can
include one or more power switching driver devices, such as the
field transistor 28'. With reference to FIGS. 11 and 12, we show an
embodiment of a driver overload protection circuit 30 such as can
be used with the control unit 10 of FIG. 1. As illustrated in FIG.
11, the protection circuit 30 includes the use of a relaxation
oscillator 180 which pulses the control input of the driver
transistor with a duty cycle that decreases as output loading
increases above a fault threshold determined by a reference value
(194). See FIG. 16.
[0080] For purposes of explaining the protection circuit 30, we
will assume that the driver device being protected is the field
transistor 28'. However, those skilled in the art will readily
appreciate that the protection circuit 30 can be used in any
application for protecting a driver device from an overload
condition.
[0081] As previously described herein, the field transistor 28' is
controlled by means of a PWM gate control signal, 182 which may,
for example, include the starting torque limiting and field
weakening control features of FIGS. 5 and 6, as well as the basic
voltage regulator 18 control feature. The gate control signal 182
is ANDed with a PWM signal 186 from the relaxation oscillator 180,
as with AND gate 184. A current sensing circuit 188 of conventional
design is used to sense output load current in the device 28'. This
overload sensing signal is input to a comparator 190 that includes
hysteresis and threshold sensing. A delay circuit 192 delays
receipt of the overload signal from the current sensor. If the
overload signal, after the delay, exceeds the reference level 194,
the comparator trips, turning the device 28' off regardless of the
drive signal 182 state. As soon as the overload condition drops
below the reference level including the hysteresis value, the
comparator resets back again and the gate control signal again
takes over control of the device 28'. The OFF pulse time to the
device 28' from the comparator 190 will typically be fairly long
compared with the duty cycle of the control signal 182 because of
the delay function. The hysteresis and delay times can be selected
so that the device 28' operates at a preselected current limited
operating point. Under such conditions, the comparator 190 will
pulse at a fairly constant period, defined primarily by the delay
period and the hysteresis set points, as the device 28' operates
near the current limit point. The comparator with hysteresis thus
operates as a relaxation oscillator. The delay function can be used
to insure that the relaxation oscillator does not respond to every
minor overload condition, and generally sets the time constant of
the circuit 30.
[0082] FIG. 12 shows a detailed schematic of one embodiment of the
circuit of FIG. 11. In this embodiment, the AND gate 184 is
realized by the use of an optocoupler having a phototransistor 196
that is activated by a photodiode 198 responsive to the gate
control signal 182. The comparator 190 with hysteresis can be
realized with a conventional circuit design as illustrated in FIG.
12. Note that the output of the comparator 190 at node 200 provides
the collector drive signal for the phototransistor 196, thus
providing an AND function between the gate control signal 182 and
the output of the relaxation oscillator. The delay function 192 can
be realized conveniently by the use of an RC time constant
established by a resistor 202 and capacitor 204.
[0083] An amplifier 206 configured as a conventional comparator
thus produces the PWM gate drive signal to the field transistor
28'.
[0084] With reference next to FIG. 13, we show a conventional field
flash circuit. As those skilled in the art understand, field flash
refers to the operation in which a generator can be started using
the residual voltage of the generator without the use of an
independent voltage supply to the armature. This residual voltage
typically is a minimum of 0.5 volts DC for a standard aircraft
starter.generator. In the conventional design, a field flash
resistor 210 is connected between the field winding 40 and a
contact 212 of a generator control switch GCS through a starter
relay contacts 216 (KST-1). The KST relay is closed when the
generator is de-energized and opened when the generator is
energized by the armature voltage. Energizing the KST relay opens
the field flash path. In conventional field flash operation, the
GCS switch must be manually actuated by the pilot even when the
field relay KFR is in the reset position as represented in FIG. 13
and only minimal residual voltage is available. In other words, the
field flash path must be through the GCS switch.
[0085] In accordance with the present invention, as illustrated in
FIG. 14, the field flash resistor is connected between the field
relay contacts 214 and the field winding 40 so that the KFR
contacts are used to select the source of field flash current. When
the KFR is in the trip position (FIG. 14 shows the reset position
for the KFR relay, the de-energized position for the KST relay and
the off position for the GCS), the field flash path is selected
through the GCS switch as in the conventional arrangement. When the
KFR is in the reset position, the field flash path is selected to
be from the armature. In this manner, field flash can automatically
occur without the pilot having to manually toggle the GCS relay
switch when the KFR is reset which is the normal mode of
operation.
[0086] The invention thus provides a microprocessor based generator
control unit 10 that can control and monitor various generator
start-up functions and regulation functions with greater
responsiveness and control than previously realized.
[0087] While the invention has been shown and described with
respect to specific embodiments thereof, this is for the purpose of
illustration rather than limitation, and other variations and
modifications of the specific embodiments herein shown and
described will be apparent to those skilled in the art within the
intended spirit and scope of the invention as set forth in the
appended claims.
* * * * *