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The present invention relates to an arrangement in connection with
centrifugal separators for sensing, during operation, a displacement of an
interface formed between two media within the centrifuge rotor.
In the 1930's a method was proposed for sensing electrically, in a
centrifugal separator, when so much sludge has been separated therein from
a sludge containing liquid that the centrifuge rotor must be emptied. This
prior method resides in conducting an electric current through a resistor
placed in a suitable place within the separating chamber of the centrifuge
rotor, so that heat developed in the resistor at the beginning (i.e.,
before a substantial amount of sludge has been separated within the
centrifuge rotor) is removed by the liquid flowing through the centrifuge
rotor but thereafter is accumulated therein when a certain amount of
sludge has been separated. The separated sludge thus prevents an effective
cooling of the resistor, thereby causing a temperature rise in the
resistor. This led to a change of the resistance for the electric current,
which could be registered and used as an indication that the centifuge
rotor must be emptied of sludge.
Other methods have subsequently been suggested for electrical sensing of a
displacement of an interface formed between two media in a centrifugal
separator. For instance, it has been attempted to sense by means of a
sensing member, situated in a suitable place within the centrifuge rotor,
the dielectric constant or some other property of the medium being in
contact with the sensing member. In this way, it would be possible to
sense when an interface between two media, for instance oil and water,
reaches a certain level within the centrifuge rotor.
However, neither the above-mentioned nor other proposed methods of sensing
electrically a displacement of an interface between two media in the
centrifugal rotor have led to any practically usable arrangement for this
purpose. Instead, another method also proposed in the 1930's, has been
adopted, for instance for sensing sludge. This prior method resides in
creating a liquid flow in the radially outer part of the separating
chamber of the centrifuge rotor, it being sensed when sludge separated in
the separating chamber prevents this flow of liquid. However, the last
mentioned method, in spite of the fact that it has been practically used,
has a number of disadvantages. For example, it necessitates a substantial
complication of the centrifugal rotor construction. For sensing
displacements of an interface between oil and water, for instance,
comparisons are still made between pressures which are sensed in different
outlets from the separating chamber of the centrifuge rotor.
The primary reason why previously proposed methods of sensing such
interface displacement electrically have not been practically realizable
is believed to be the difficulty of solving the problem of transferring
information of an indication from a sensing member, rotating together with
the centrifuge rotor, to a stationary apparatus which may be intended
automatically to initiate or perform an operation in response to the
indication.
An object of the present invention is to provide a sensing arrangement by
which this information transfer problem is solved in a practically
realizable way.
This object is fulfilled according to the invention by an arrangement
characterized by a first unit forming an electric resonance circuit
rotating together with the centrifuge rotor and having at least one
variable oscillation property, a sensing member rotating together with the
centrifuge rotor being arranged to sense a displacement of the said
interface and to change, upon a sensed displacement, the variable
oscillation property of the resonance circuit, and a second unit separate
from the centrifuge rotor and comprising equipment for inductively
transferring a signal to the resonance circuit and means for sensing a
change of the load by the resonance circuit on this equipment as a
consequence of a change of the variable oscillation property of the
resonance circuit.
The invention is based on the concept that the sensing first unit, which
rotates together with the centrifuge rotor, shall be entirely passive in
that it shall not emit a signal by itself to the second unit separate from
the rotor due to a sensing. Instead, there shall be sensed in the second
unit changes of the load thereon by the first unit as a consequence of
changes of an oscillation property (oscillation characteristic) of the
resonance circuit in the first unit, which latter changes have been caused
by the sensing member. It will thus be possible to transfer to the second
unit, separate from the centrifuge rotor, information about very small
variations of a magnitude sensible on the centrifuge rotor, which
variations have taken place owing to a displacement of an interface
between two media within the centrifuge rotor. Thus, the invention makes
it possible for the first time, in connection with sludge sensing, to use
a small temperature change at the centrifuge rotor, which will arise
automatically, little by little, when sludge is separated therein. This
temperature change is too small to be certainly sensed by means of sensing
methods proposed heretofore.
Within the scope of the invention, the sensing member rotating together
with the centrifuge rotor may be of different kinds. It may be a separate
element arranged to influence, in dependence upon a sensed value of some
physical magnitude, one or more of the components which form the said
resonance circuit, such that the oscillation properties of the circuit are
changed. If possible, however, one of said components preferably is
utilized as the sensing member of the arrangement.
The load on the second unit separate from the centrifuge rotor may be
changed in different ways depending upon which oscillation property of the
resonance circuit is variable by means of the sensing member. Preferably,
the resistance is maintained substantially unchanged in the resonance
circuit, while the capacitance is allowed to vary in response to changed
of some physical magnitude that is sensed by the sensing member. If the
arrangement is to be used for sensing temperature changes in the
centrifuge rotor or its content, there may be included in the resonance
circuit a capacitor constituting the said sensing member, the capacitance
of which varies with the temperature in a predetermined way. In this way,
the natural frequency of the resonance circuit may be pre-set so that a
striking change of the load on the second unit separate from the
centrifuge rotor is obtained when a predetermined temperature is sensed on
the centrifuge rotor by the sensing member (i.e., the capacitor), namely,
at the moment when the natural frequency of the resonance circuit has
adopted a value that corresponds to the frequency of the signal that is
emitted by the unit separate from the centrifuge rotor. Not until the
resonance circuit in the first unit resonates with the energy emitting
circuit in the second unit does it absorb a substantial amount of energy
from the latter.
If desired, the energy emitting second unit may comprise means for varying
the frequency of the signal that is emitted. For instance, the frequency
may be caused to sweep between two values. In this way, it is possible
substantially continuously to follow changes of the value of the physical
magnitude that is sensed in the centrifuge rotor. Then it is sensed at the
energy emitting second unit which frequency is prevailing at each moment
when a striking change of the load on this unit is registered.
Another possibility of performing the sensing is to allow the resistance in
the resonance circuit on the centrifuge rotor to vary in response to the
value of the physical magnitude sensed on the centrifuge rotor, whereas
the natural frequency of the circuit is maintained constant. In this case,
changes of the value of the physical magnitude may be directly sensed at
the energy emitting second unit as corresponding changes of the load
thereon. The sensed changes of this load then correspond to changes of the
damping effect by the variable resistance on the oscillations in the
resonance circuit, as a consequence of sensed changes of the value of the
physical magnitude. The frequency of the signal emitted from the second
unit in this case is maintained substantially constant, equal to the
natural frequency of the resonance circuit.
Within the scope of the invention, changes of the load on the unit separate
from the centrifuge rotor may be sensed in different ways. One possibility
is to sense a change of the amplitude of the oscillations created in this
unit. Another possibility is to sense a change of the energy consumption
by the means creating the oscillations in the unit.
The invention is described below with reference to the accompanying
drawings, in which:
FIG. 1 is a vertical sectional view of a centrifugal separator provided
with an arrangement according to the invention,
FIGS. 2 and 3 are schematic views showing the main principle of two
alternative embodiments of the arrangement according to the invention, and
FIG. 4 is a circuit diagram of a part of the arrangement according to FIG.
2.
In FIG. 1, a centrifuge rotor comprises a lower part 1 and an upper part 2.
The parts 1 and 2 are held together by means of a lock ring 3. The
centrifuge rotor, which is covered by a protective cover 4, is supported
by a driving shaft 5. Through a stationary inlet conduit 6, a mixture of
media which are to be separated within the rotor is conducted to the
center of the rotor. By a conical distributor 7, the mixture is directed
from the center of the rotor into the lower part of its separating chamber
8. During the separating operation, a separated light component of the
mixture flows radially inwards from this chamber and between the conical
discs in a conventional disc set 9 and thence through an opening 10 into a
chamber 11. From this chamber 11 the separated light component is removed
from the rotor by a paring means 12 through a conduit 13. Separated heavy
component of the mixture remains in the separating chamber 8 and forms a
gradually growing layer in the radially outer part thereof. To the left in
FIG. 1 there is shown a thin layer 14, illustrating that a separating
operation has just been started, whereas to the right in FIG. 1 there is
shown a thick layer 15, illustrating that the separating operation has
proceeded for a substantial period of time.
The lower rotor part 1 has a number of openings 16 around its periphery.
These are outlet openings for the heavy mixture component separated within
the rotor. Before a certain quantity of heavy component has been separated
in the separating chamber 8, these outlet openings 16 are closed by a
valve member 17 axially movable within the centrifuge rotor. The valve
member 17, which forms the bottom of the separating chamber 8, is arranged
to be held in close abutment around its periphery against the underneath
side of the upper rotor part 2 by means of a liquid pressure. This
pressure is created by a continuous supply of liquid to an interspace 18
beneath the valve member 17 between the latter and the lower rotor part 1.
When a certain quantity of heavy component has been separated in the
centrifuge rotor (i.e., when the said layer 14, 15 has reached a certain
thickness), the outlet openings 16 are uncovered for a short period of
time. This uncovering is obtained by brief interruption of the liquid flow
to the interspace 18. The liquid that has arrived in the interspace 18
then leaves through throttled openings 19 at the periphery of the rotor
part 1, whereby the pressure from the liquid situated in the separating
chamber 18 forces the valve member 17 downwards, so that a part of the
rotor content is thrown out through openings 16. When the flow of liquid
to the interspace 18 is reestablished, the valve member 17 is pressed back
to sealing engagement with the rotor part 2, so that the outlet openings
16 are closed.
The present invention relates to an arrangement for sensing when the said
layer of separated heavy component of the mixture has reached a
predetermined thickness in the separating chamber 8, so that the rotor
periphery outlets 16 are to be opened. In the following, one embodiment of
an arrangement of this kind is described, it being assumed that the
separated heavy component is constituted by sludge or solid particles. The
centrifuge rotor then will be influenced in the following manner as a
consequence of a gradual accumulation of sludge in the separating chamber
8.
When the centrifuge rotor rotates, heat is generated owing to friction
between the rotor and the surrounding air, which heat is conducted inwards
through the rotor wall towards the separating chamber 8. The heat is
further transferred from the rotor wall to the medium that is situated in
the separating chamber. Before any sludge has been separated in the
separating chamber, or as long as the sludge quantity is relatively small,
the greater part of the rotor wall inside is contacted by liquid that
subsequently leaves the separating chamber. The friction heat generated on
the outside of the rotor wall is thus led away with the liquid leaving the
rotor, whereby the temperature of the rotor wall is maintained
substantially constant. However, when the sludge layer in the separating
chamber 8 grows thicker, it forms an insulation between a part of the
rotor wall and the liquid flowing through the rotor, whereby the removal
of heat from this part of the rotor wall will be less effective. This
results in a temperature increase of this part of the rotor wall and the
sludge separated in the separating chamber. It is this temperature
increase that is to be sensed by means of the arrangement according to the
invention. (If the medium in the centrifuge rotor is sufficiently hot,
there is instead a heat flow from this medium via the rotor wall to the
atmosphere surrounding the centrifuge rotor. In this case the insulation
by sludge, separated in the centrifuge rotor, will cause a temperature
decrease of the radially outer parts of the centrifuge rotor. The
arrangement according to the invention then can be used for sensing this
temperature decrease.)
The arrangement according to the invention comprises a first unit A firmly
connected with the rotor part 2, and a second unit B fastened on the in
ide of the protecting cover 4 so that the unit A will pass exactly
opposite to unit B when the centrifuge rotor is in rotation. By means of
electric lines 20, the unit B is connected to an apparatus 21 which is
arranged to perform three functions, namely (1) to provide the unit B with
necessary energy, (2) to sense a varying load on the unit B, and (3) to
supply a signal to the control unit of the centrifugal separator for
initiating a sludge discharge operation when a certain quantity of sludge
has been separated in the separating chamber.
The units A and B in FIG. 1 are shown at A1 and B1, respectively, in the
FIG. 2 embodiment and at A1 and B2, respectively, in the FIG. 3
embodiment.
In FIG. 2, the unit A1 connected to the centrifuge rotor consists of
members which form together a passive oscillatory circuit. These members
are a cable 22, an inductor 23 and a capacitor 24. The capacitor 24
constitutes the sensing member in the rotor wall and is of a kind the
capacitance of which is strongly dependent on the temperature to which the
capacitor is subjected. The natural frequency of the oscillatory circuit
is thus dependent on the temperature to which the capacitor 24 is
subjected. The unit B1, which is fastened to the inside of the protective
cover 4, comprises an oxcillator 25 with frequency determining elements in
the form of an inductor 26 and a capacitor 27. By means of conventional
equipment 28, the capacitance of the capacitor 27 may be caused to vary,
automatically or manually, between two predetermined values. The
connection between the equipment 28 and the capacitor 27 is illustrated in
the drawing by a dotted line 29. By means of cables 20, the oscillator 25
is connected to the apparatus 21 (FIG. 1) which among other things
supplies energy to the oscillator.
In the operation of the arrangement according to FIG. 2, the capacitance of
the capacitor 27 in unit B1 is caused to vary slowly in a stepless way, by
means of the equipment 28, from a certain maximum value to a certain
minimum value and to restart thereafter from the maximum value. In this
way the frequency of the oscillation in the inductor 26, caused by the
oscillator 25, is caused to sweep slowly between two predetermined values.
The unit A1 by its construction has a certain natural frequency, which is
changed, however, in dependence upon the temperature to which the
capacitor 24 is subject, i.e., in dependence upon how much sludge has been
separated in the separating chamber 8 (FIG. 1). The frequency field over
which the frequency sweeping takes place in the unit B1 is chosen so that
it comprises the frequency values which the natural frequency of the unit
A1 will adopt during the temperature (sludge) sensing operation in
question.
As long as the natural frequency of the circuit in the unit A1 differs from
the frequency of the oscillation in the inductor 26 at the moment, the
circuit cannot be caused to oscillate to any substantial extent by the
unit B1, when the two units A1 and B1 are situated opposite to each other.
The unit A1 is no substantial load on the unit B1 during this period.
However, at the very moment when the oscillation in the inductor 26 of the
unit B1 has a frequency that corresponds to the natural frequency of the
circuit in the unit A1, the latter circuit is caused to oscillate when it
passes the unit B1; in other words, it will be made to resonate with the
circuit in the unit B1. This means that it is suddenly sensed by the unit
B1 as a substantial load (i.e., it gets from the unit B1 the energy that
is necessary for its oscillation). At the moment when a load increase can
be registered in the unit B1, the frequency of the oscillation prevailing
in the unit B1 corresponds to a predetermined temperature at the capacitor
24 in the unit A1, i.e., in the rotor wall.
By letting the frequency of the oscillation in the unit B1 sweep in the
described manner repeatedly between two predetermined values, it is
possible to follow temperature changes in the rotor wall at the unit A in
FIG. 1 and, thus, when a predetermined temperature (frequency) is
achieved, or a predetermined temperature (frequency) change has occurred,
manually or automatically initiate a sludge discharge operation. Normally
it is of little interest, however, to follow continuously temperature
changes in the rotor wall. Usually it is sufficient to obtain information
about when a certain temperature has been achieved or a certain
temperature change has occurred. Therefore, the equipment 28 may be so
constructed that a certain constant value of the capacitance of the
capacitor 27 can be set, corresponding to the temperature at the capacitor
24 (i.e., in the rotor wall) at which a sludge discharge operation should
be performed. As a trigger signal for automatic initiation of the sludge
discharge operation, a signal can then be used coming from means arranged
to sense a load increase by the unit A1 on the unit B1. A load increase of
this kind will not occur until it is time to initiate a sludge discharge
operation.
A load increase in the unit B1 may be sensed in different ways. One
possibility is to sense a decrease of the amplitude of the oscillations in
the inductor 26 of the unit B1, caused by the load increase. Another
possibility is to sense a change of the energy consumption of the
oscillator 25. In the embodiment in FIG. 2, a load increase in the unit B1
is intended to be sensed by means of equipment included in the apparatus
21.
The drawing shows no means for indicating the frequency of the oscillation
in the unit B1. If desired, an indicator of this kind may be connected at
any suitable place, for instance, connected to the equipment 28. As a
rule, information about the magnitude of the frequency is desired only
when a change of the load on the unit B1 occurs. In the case where the
frequency in the unit B1 is caused to sweep between two values, this
information may be achieved, for instance, in the way that the said
equipment for sensing a change of the load on the unit B1 is arranged to
open a connection to a frequency (temperature) indicator only when a load
increase is registered. In the embodiment according to FIG. 1, no
frequency (temperature) indicator is required. The apparatus 21 is instead
arranged to emit a signal to the control unit of the centrifugal
separator, when a certain temperature change has been registered in the
centrifuge rotor, thereby initiating a sludge discharge operation.
The frequency (temperature) reference value which must be set in a
frequency indicator or the equipment 28, if the frequency of the
oscillations in the unit B1 should be constant, may be fixed empirically.
Also an automatic setting (control) may be provided by connecting the
frequency indicator or the equipment 28 to suitable means such as an
equipment which continuously senses either the temperature of the mixture
of components supplied to the centrifuge rotor for separation, or the
temperature in the part of the rotor wall that is not covered by sludge
separated in the separating chamber. An equipment of this kind may
comprise two units of the same kind as the previously described units A
and B, the unit of the equipment rotating with the rotor being mounted in
the rotor wall radially inside the unit A in FIG. 1 at a place to which
the interface between liquid and separated sludge never reaches. Two units
A1 of the same kind may be fastened to the rotor in a way such that the
inductors 23 of the respective units A1 are spaced circumferentially of
the rotor, but still situated at the same distance from the axis of
rotation of the rotor, so that they may cooperate during the rotation of
the rotor with one and the same inductor in one single stationary unit
corresponding to the unit B1. The temperature sensing member (e.g., the
capacitor 24) in one of the said units A1 is then placed closer to the
rotor axis than the corresponding sensing member in the other unit A1. The
common stationary unit must then be provided with means for selectively
receiving and comparing information obtained during rotation of the rotor
regarding the temperatures sensed by the two units A1.
A further possible arrangement for an automatically performed adjustment of
a reference value, in response to temperature changes of the liquid
mixture, supplied to the centrifuge rotor, is the following: In an
embodiment of the invention according to FIG. 1, a further capacitor may
be connected in parallel, or in series, with the capacitor 24 in the unit
A1. In FIG. 2 a further capacitor 24a of this kind, connected in parallel,
is illustrated by dotted lines. This further capacitor 24a may be situated
in the centrifuge rotor at a place where it is constantly influenced by
the temperature of the liquid mixture supplied to the rotor, i.e., it may
be situated at such a radial distance inside the capacitor 24 that it will
not be substantially influenced by a temperature change caused by a
certain quantity of sludge having been separated in the centrifuge rotor.
The capacitor 24a is of a kind having a temperature/capacitance
characteristic such that if both of the capacitors 24 and 24a are
subjected to exactly the same temperature change, then the resulting
capacitance of the resonance circuit of the unit A1 will remain unchanged.
In other words, if for instance the capacitor 24 has a characteristic such
that its capacitance will increase as a consequence of a certain
temperature change, then the capacitance 24a should have a characteristic
such that its capacitance will decrease to the same extend for the same
temperature change, or vice versa. This means that if the temperature of
the liquid mixture supplied to the centrifuge rotor varies, then the
natural frequency of the resonance circuit of the unit A1 will not be
influenced, since both of the capacitors are subjected to these
temperature variations. Not until a temperature change of a certain
magnitude is sensed only by the capacitor 24, i.e., when a certain
quantity of sludge has been separated in the centrifuge rotor and has
caused a temperature change of this kind only around the capacitor 24,
does the natural frequency of the resonance circuit of the unit A1 change,
which may be sensed in the unit B1.
The diagram of FIG. 3 illustrates another embodiment of the arrangement
according to the invention for sensing temperature changes in the rotor
wall or in sludge separated inside this wall. This embodiment comprises
two units A2 and B2. The unit A2 consists of members which together form a
passive oscillatory circuit. As in the embodiment according to FIG. 2,
there are also an inductor 30 and a capacitor 31. In FIG. 3, however,
there is also a particular resistor 32 connnected in parallel with the
capacitor and the inductor. The resistor 32 in this embodiment constitutes
the temperature sensing member and is therefore of a kind the resistance
of which varies in a predetermined way with the temperature to which it is
subjected. Differing from the capacitor 24 in FIG. 2, the capacitor 31 in
FIG. 3 is of a kind the capacitance of which is substantially constant
independent of the temperature to which it is subjected. However, owing to
the variable resistance of the resistor 32, the amplitude of the
oscillations which may be generated in the oscillatory circuit of the unit
A2 will be dependent upon (i.e., stand in a certain relation to) the
temperature to which the resistor 32 is subjected.
The unit B2 comprises an oscillator 33 with an oscillatory circuit
including an inductor 34 and a capacitor 35. For its energy supply, the
oscillator 33 is connected to the apparatus 21 (FIG. 1) which contains, as
in the embodiment according to FIG. 2, equipment for sensing changes of
the load on the unit B2. Load changes of this kind are sensed in this case
as changes of the amplitude of the oscillations which are generated in the
unit B2. The capacitor 35 is of a kind the capacitance of which is
maintained constant. The oscillatory circuit is thus caused to oscillate
by the oscillator 33 with a constant frequency.
In the operation of the arrangement according to FIG. 3, the frequency at
which the oscillator 33 causes the oscillatory circuit of the unit B2 to
oscillate is set so that it corresponds to the natural frequency of the
oscillatory circuit in the unit A2. (This natural frequency, as previously
mentioned, is substantially independent of temperature changes occurring
around the unit A2 in the rotor wall.) The oscillatory circuit in the unit
A2 will then be caused to oscillate by and constitute a load on the unit
B2. The magnitude of the load depends on the energy loss that is absorbed
by the unit A2, and mainly by the resistor 32 therein. Upon a change of
the resistance of the resistor 32, caused by a change of the temperature
to which this resistor 32 is subjected, the load on the oscillator 33 in
the unit B2 is thus changed. This load change is sensed by checking the
amplitude of the oscillations in the oscillator 33, a certain oscillation
amplitude corresponding to a certain value of the resistance by the
resistor 32, i.e., a predetermined value of the temperature to which the
resistor 32 is subjected. The amplitude sensing equipment included in the
apparatus 21 may therefore be constructed in a way such that it can show
directly the temperature which prevails in the rotor wall at the unit A2.
As previously mentioned, FIGS. 2 and 3 show only the main principles of an
arrangement according to the invention. Within the scope of these main
principles, the arrangement may be changed in many different ways by a
person skilled in the art.
In the unit A1 or A2, the sensing member may be formed in many different
ways depending upon how the interface between two media in the centrifuge
rotor is to be sensed, and depending upon how the resonance circuit in the
unit A1 or A2 is to be influenced. The sensing member may be arranged so
that, in response to a sensed value of some physical magnitude in the
rotor or the media therein, it influences any (one or more) of the
components included in the unit A1 or A2. If a capacitor included in the
resonance circuit is intended to constitute the sensing member, and the
capacitance thereof it to be influenced, the medium within the centrifuge
rotor may be allowed to enter the interspace between the plates of the
capacitor. Also the inductance of an inductor may be influenced by letting
the medium in the centrifuge rotor flow freely between the windings of the
inductor. If temperature changes in the centrifuge rotor or its content
are to be sensed, it is not necessary to sense the automatically occurring
temperature changes, such as described above. Instead, heat may be
supplied in a suitable way by external means, for instance, to a part of
the rotor wall, the unit A (FIG. 1) being arranged to sense when this
supply of heat causes a temperature rise in the rotor wall owing to the
fact that a heavy component of the mixture, supplied to the rotor, has
been separated in the separating chamber and prevents an effective leading
away of the heat.
Also the unit B1 or B2 may be formed in many different ways by a person
skilled in the art of electronics. FIG. 4 shows an example of a circuit
diagram for a unit B1, there being in FIG. 4 a list of the different types
of the components included in the circuit diagram. D stands for a
resistor, E for a capacitor, F for a varactor, G for an inductor, H for a
transistor, and I for a Zenerdiode. In the circuit diagram, K stands for
oscillator, and L for amplifier.
In the circuit diagram of FIG. 4, which needs no detailed description here,
a variable resistor M corresponds to the equipment 28 in FIG. 2, and an
inductor N corresponds to the inductor 26 in FIG. 2. O and P designate the
connection points for a current source, and R designates one of the
connecting points (O constitutes the other one) for a control equipment of
a suitable kind.
The arrangement according to FIG. 4 operates in the following manner: By
means of energy from a direct current source connected at O and P, an
oscillation is generated having a certain frequency, which can be set by
means of the variable resistor M. The inductor 23 (FIG. 2) is arranged to
pass the inductor 26, i.e., the inductor N in FIG. 4, once for each turn
of the centrifuge rotor. As long as the natural frequency of the circuit
A1 differs from the frequency of the oscillation in the inductor N, the
oscillatory circuit including the inductor N is not influenced. However,
when the natural frequency of the circuit A1 corresponds to the frequency
in the inductor N, resonance is established between the circuits in the
units A1 and B1, when the inductor 23 passes the inductor N. In the
arrangement in FIG. 4, this causes a positive pulse at the connection
point R. This pulse may be used in any suitable way, as for causing a lamp
to give light or for automatically initiating a sludge discharge
operation.
Referring back to FIG. 1, the further sensing capacitor 24a previously
mentioned is shown with a connection 24b to the resonance circuit and is
located radially inside the sensing member (capacitor) of the unit A.
Also, FIG. 1 shows at AA the previously mentioned additional unit mounted
on the rotor and similar to the first unit A, these two units being spaced
from each other circumferentially of the rotor and each coacting with the
unit B. The sensing member 244 of the additional unit AA is shown at a
shorter distance from the rotor axis than the sensing member at A.
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