Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
6 Case 255
This invention relates to the determination
of a slipping condition of a wheel in a traction vehicle
such as a locomotive and the providing of a control ~;
signal to prevent or limit slipping.
Traction vehicles, such as locomotives,
commonly have a plurality of individually powered
axles. For each wheel there is a coefficient of friction
existing between that wheel and the surface on which ~
it is rolling. If the coeffieient of friction becomes ~ -
less than that required to keep the wheel in rolling
contact with the rail or other surface on which it is
rolling, there will be a loss of adhesion and a slipping
or skidding of the wheel. The present invention is
primarily concerned with a determination of slipping.
Slipping occurs when the accelerating torque applied
to a wheel becomes too large for the frictional forces
existing between the wheel and the supporting surface
and consequently the peripheral velocity of the wheel
becomes greater than the corresponding vehicle velocity. ~ ;
In other words, the wheel turns faster than required
to maintain rolling contact. Uncorrected slipping not ~;`
only causes loss of performance but may result in damage
to the wheels or rails or other related apparatus. It is
therefore desirable to detect the occurrence of slipping
as soon as possible and to correct or control the slipping.
It is, in fact, desirable to detect slipping within
less than a fraction of a wheel revolution, for example `
in 12 inches of a slip in a wheel of 40 inches in
diameter, and then to correct the condition.
There are two types of slipping. In the first
type the wheels of one of the individually powered axles
of a vehicle may slip. In the second type, all the ~ ;
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wheels slip more or less simultaneously. This second
type of slipping, referred to as synchronous slipping,
tends to occur more frequently in systems where the
source of driving power is alternating current (a.c.)
motors where the speed of the motors is a function of
the frequency of the electrical power supplied to the
motors from a common source. When the source of
driving power is direct current (d.c.) motors, it is
possible for all the wheels to slip at the same time
but due to small differences in individual motors, -
coefficient of friction, etc., it is unlikely they
will all start to ~lip at the same instant and maintain
a more or less equivalent slip. The present invention
takes advantage of this by detecting differences
between the actions of the wheels as will be discussed
hereinafter.
Many ways have been devised in the past for
detecting or determining wheel slip. As an example,
one prior apparatus detects wheel slip by obtaining
wheel speed from several wheels to determine an average
wheel speed. Then the wheel speed from an individually
monitored wheel (there may be several wheels that
are individually monitored) is compared with the average
speed to derive a difference signal. This difference
signal is then compared with a reference signal which
may be representative of either the speed of an
individual wheel or the average wheel speed. The result
of this comparison provides a control signal for controlling
motor drive acceleration or braking deceleration. Thus,
the reference signal and the resulting control signal
have a dependency upon, or a relationship with, wheel
speed. IIn other words, the control system response is
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adjusted to be more or less sensitive in accordance
with speed. Reference is made to Canadian Patent No.
993,984 - Adde, issued July 27, 1976 for a description
of this form of control system.
The present inven-tion does not re~uire an
average wheel speed. The present invention, in its -
basic form, compares signals representing speeds of all the
desired wheels and derives a first signal representing
the speed aifference between the highest and the lowest
wheel speeds, and it derives a second signal representing
the highest rate of acceleration of the wheels. The
first and second signals may be compensated or modified, ~-
and are then summed to provide a control signal to control ;
the driving torque. Thus, the response to wheel slip
is adjusted to be~mor~ or less sensitive in accordance
with acceleration. The present invention is able to
detect wheel slip within a fraction of a revolution `~
of a slipping wheel.
It is therefore a feature of the present
invention to provide an improved method and apparatus
for determining wheel slip using a comparison of wheel
speeds from a plurality of wheels and deriving a
control signal related to the difference between the -
highest and lowest.
In accordance with one form of the invention
there is provided apparatus for determining and controlling
wheel slip in a traction vehicle powered by d.c. motors,
comprising sensor means for sensing the speed of
individual wheels driven by different motors and providing ;
first signals representing individual wheel speeds, first
comparison means for receiving said first signals and
deriving a second signal representing the difference
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between the highest and lowest sensed wheel speeds,
conversion means for receiving said first signals and
deriving respective third signals representing rates
of acc~leration of said individual wheels, selector
means for receiving said third signals and providing
a fourth signal representing the highest rate, summing
means for receiving said second and fourth signals and
providing a fifth signal representing a fault signal,
and signal modifying means for receiving said fifth
signal and providing as an output control signal for
said motors a signal corresponding to said fifty signal
with a controlled rate of decay. `
The invention will be described in one form `
with reference to the accompanying drawings, in which:
Figure 1 is a simplified block diagram
of circuitry according to a basic form of the invention, ;~
and
Figure 2 is a simplified circuit diagram of one
embodiment of the invention.
Referring now to Figure 1, there are shown -
three lines or conductors 10, 11 and 12 with a fourth ;
conductor 14 partly indicated with a broken line to
show there may be any number, and these lines or conductors
come from sensors (not shown) which,provide signals
vl, v2, V3 - - vn each representing the velocity of an
individual locomotive wheel. The sensores (not shown) ; ~ -
may be any type that provide a voltage proportional to
wheel speed, i.e. to rpm. For example, the sensors
(not shown) may be a magnetic pick-up sensitive to teeth ~;
on a wheel that rotates with a locomotive wheel or
drive wheel where the frequency of sensed pulses is
proportional to wheel speed, and a frequency-to-voltage
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converter to provide a voltage signal varying with wheel
speed. Alternatively the sensors may be photo-tachmeters
which are a sealed unit having a light source and an
opposed photodetector with a transparent disc between.
The disc bears radial lines which interrupt the light
to provide pulses from the photodetector. The pulses
vary in frequency with speed. The pulses are applied
to a frequency-to-voltage converter to obtain a voltage
signal varying with wheel speed.
Conductors 10, 11, 12 and 14 are connected
to a voltage comparator and summer 15 which compares
the signals vl - - v to derive a signal representing
the differnce between the highest and lowest and then ~ :~
sums the difference over a short term. The signal,
appearing on conductor 16, will be zero or some other
reference level when there is no slip. In a preferred
form the velocity difference comparator and summer 15
includes a biasing or set point means which inhibits
an output signal until a predetermined minimum level
is reached to avoid causing an output signal with
very minor and insignificant fluctuations.
The conductors 10 - 12 (and 14 as required)
are connected to respective rate converters 20 - 22.
The rate converters 20 - 22 provide output signals on
conductors 23 - 25 respectively, which represent the
rate of acceleration, and these are applied to rate
selector and summer 26. Rate selector 26 selects the ~ `~
signal representing the highest acceleration, sums the
signal over a short term, and places this signal on a
conductor 27. A rate compensator 28 is connected to
rate selector and summer 26 to compensate or modify ;
the signal in any manner as required. For example, ;
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it has been found desirable to be able to reduce the
effect of acceleration on the final control signal
under some circumstances and this can be done in the
compensator 28. In other circumstances it may be
desirable to increase the effect of acceleration on the
final signal at the lower end of the acceleration range
and decrease it at the higher end. In a preferred
form the rate selector and summer 26 includes a biasing
or set point means which inhibits the output signal
until a predetermined minimum level is reached to avoid
insignificant fluctuations in acceleration affecting
the output.
Conductors 16 and 27 are connected as inputs ~-~
to a slip summer 33 which adds the signals and provides -
a summed signal on conductor 34. In a preferred form
the slip summer 33 may include an amplifier means
whose gain may be set by a control 35 to adapt the ~;
apparatus for different conditions. Conductor 34 is
connected to slope generator 36 which is used to
control decay or recovery. As is well known in the
control art, it may cause problems if a correcting ~-
control signal is removed immediately the correction
is applied. The fault may recur or an oscillation may
start. The slope generator 36 provides the necessary `
controlled delay in reducing the control signal after
the fault is corrected. When a wheel slip is detected ;
a fault signa:L appears on conductor 34 and a corresponding
control signal is applied via conductor 37 to the
driving motors (not shown) to reduce the torque. The
torque is reduced and the wheel slip stops but the
correcting control signal on conductor 37 is not imme-
diately removed. The slope generator 36 provides a
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sloped signal that reduces with time, that is the signal
~reduces at a controlled rate or slope. The slope generator
has two controls 40 and 41. Control 40 sets the decay
rate or rate of decrease (i.e. the slope), and control
41 sets the starting point for the decay or decrease.
That is, the starting point for the controlled decrease
may be set at the maximum control signal that can be
achieved, or it can be set at a lower value.
It is believed that the operation of the
Figure 1 circuit will be apparent from the preceding
description. Very briefly, ~f a locomotive is
accelerating and one wheel begins to slip and that wheel
accelerates very quickly, there will be a difference
in velocity between that wheel and the rest of the wheels.
This is detected by comparator 15 and the difference
summed. Also there will be a rapid increase in
acceleration for one wheel which will provide an
increasing signal, selected by selector 26 and summed. ~-
It will be recalled that the locomotive was itself
accelerating and there would consequently be a signal ;
representing this on conductor 27. However rate ~ ;
compensator`28 may be set or arranged to reduce this
comparatively low level acceleration below the limit
of acceleration set into rate selector and summer 26 so
that the locomotive acceleration, i.e. the slow accelera-
tion of all wheels of the locomotive, does not cause a
signal on conductor 27. The rapid acceleration of
the single wheel is high there will be a large signal on
conductor 27 and if it is a more moderate acceleration
the signal on conductor 27 will be more moderate to
adapt the con-trol response to the severity of the slip.
The signals on conductors 16 and 27 are summed
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by summer 33 to provide a control signal on conductor
37 to reduce the driving torque of the motors. The
gain control 35 is set to ensure the control signal
will be adequate for all situat:ions. As explained
previously, the slope generator 36 controls the rate of
return of the control signal to normal.
Assuming now that the locomotive moves on
to a section of track with a lesser coefficient of
friction and all the wheels begin to slip. Because
the different wheels are driven by different d.c. motors
the torque will not be precisely the same and the condi-
tions will not be identical. Consequently the wheels
will not all slip at identical speeds. There~ore '
there will be a difference in wheel velocity between
the wheels and the velocity comparator 15 will ~' ,
detect the difference and provide a signal representative
of the difference between the highest and the lowest
wheel speeds as previously described. There will, ~,,'
of course, be acceleration as the wheels begin to slip
and the rate selector and summer 26 will provide a '7
signal representative of the highest rate of wheel
acceleration. The two signals will be summed and a control ,,
signal applie~ via conductor 37 to reduce the driving ~,''
torque from the d.c. motors to stop the slip. Thus -;',
the slipping condition will be corrected whether one
wheel is slipping~,or more than one wheel is slipping.
If desired, as an added precaution in case all wheels ~;~
should slip in abso1~ute synchronism (an unlikely ' ,~
occurrence in a d.c. drive), the wheel speed may be ~
monitored and when wheel speed exceeds that which '
corresponds to maximum locomotive velocity a warning '~ ',
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or control device could be activated. `,
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Case 2558
As will be apparent from Figure 1, signals, such
as the fault or velocity signals, are available and
these signals could be used for auxiliary purposes. For
example, the fault signal or conductor 34 might be used
to initiate operation of a sanding device. The signal
for the slowest wheel speed, available from velocity
comparator/summer 15, might be used to drive a locomotive
speed indicator. The speed indicator might be driven
by a signal directly from the front axle as is done on
some locomotives at present, and this signal would be
available on one of the conductors 10-14.
It is believed that the invention will be
clearly understood from the description thus far.
However, one specific circuit is included in Figure ;~
2 as an example of one form or embodiment of the ;
invention.
Referring to Figure 2, the conductors 10,
11 and 12 are shown connected to pairs of oppositely poled
diodes 42-47 as shown. The diodes are connected to
provide on conductors 50 and 51 the highest and the
lowest signals as inputs to an operational amplifier
circuit 52 connected as a difference summer. A biasing
voltage is applied via resistor 53 to inhibit signals
below a certain level. The output is on conductor
16a and represents the sum of the difference between
the highest and lowest wheel speeds.
The conductors 10, 11 and 12 are connected --
by capacitors 54, 55 and 56 to rate converters 20a, ;~
20b and 20c respectively. The rate converters 20a, 20b
and 20c provide output signals representative of the
rate of acceleration of the respective wheels and these
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are applied via diodes 57, 58 and 59 and conductor 60.
~:~l23~; Case 2558
The signal or conductor 60 will thus represent the
largest accleration of the wheels whose speed is sensed.
This signal is applied as one irnput to operational
amplifier 61 which sums the signal and provides an
output on conductor 27a. A selectable bias can be set
into the summer via conductor 62 as shown.
The signal representing the summed velocity
difference on conductor 16a appears across potentiometer
64 and a selected portion of the signal is applied
via resistance 74 to an input of operational amplifier
65. Similarly, the signal representing the summed
greatest rate of accele~;ration on conductor 27a appears
B across potentiometer ~ and a selected portion' of
the signal is applied via resistance 67 to the same
input of amplifier 65. Amplifier 65 is connected as `
an amplifier/summer whose gain is controlled by gain
control 35a. This portion of the circuitry is indicated
as being generally equivalent to slip summer 33 of ;Figure 1.
The signal form amplifier 65 ~the fault signal)
is applied over conductor 34a to circuitry including
operational amplifier 68. This circuitry, generally
equivalent to slope generator 36 of Figure 1, provides `
the desired decay rate. Th~ feedback control 40a-controls
the rate of decay and the voltage selected by voltage ,~
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control 41a controls the starting level. The
conductor 37 carries the control signal as in Figure 1.
The Figure 2 circuit is included only as an
example of one embodiment and various alternatives are `-~
30 available as is apparent from the preceding description.
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