U.S. patent number 3,896,346 [Application Number 05/377,956] was granted by the patent office on 1975-07-22 for high speed electromagnet control circuit.
This patent grant is currently assigned to Electronic Camshaft Corporation. Invention is credited to Louis A. Ule.
United States Patent |
3,896,346 |
Ule |
July 22, 1975 |
High speed electromagnet control circuit
Abstract
A solid state electrical switching circuit of high efficiency is
employed to increase the speed with which an electromagnet operated
device, such as a relay or a hydraulic valve, can be actuated
without increasing the power required to maintain the device in the
actuated state. The circuit can also be made to provide the
momentary increased electrical current required to obtain a given
mechanical force when the magnetic gap between the solenoid core
and the movable pole piece is open. Both actuation and de-actuation
speed of the electromagnet are increased and rapid deactuation is
achieved either by a high reverse voltage applied to the solenoid
and the return of its energy to the power source or by a diode and
capacitor network which transfers the magnetic energy to a second
solenoid which thereupon becomes energized.
Inventors: |
Ule; Louis A. (Rolling Hills,
CA) |
Assignee: |
Electronic Camshaft Corporation
(Rolling Hills, CA)
|
Family
ID: |
27432004 |
Appl.
No.: |
05/377,956 |
Filed: |
July 10, 1973 |
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
308268 |
Nov 21, 1972 |
|
|
|
|
Current U.S.
Class: |
361/154 |
Current CPC
Class: |
H01H
47/325 (20130101); F02D 41/20 (20130101); F02D
2041/2017 (20130101); F02D 2041/2058 (20130101); F02D
2041/201 (20130101) |
Current International
Class: |
H01H
47/32 (20060101); H01H 47/22 (20060101); F02D
41/20 (20060101); H01h 047/32 () |
Field of
Search: |
;317/DIG.4,148.5R,154,157,DIG.6 ;307/131 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Miller; J. D.
Assistant Examiner: Moose, Jr.; Harry E.
Attorney, Agent or Firm: O'Neil; William T.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATION
This is a continuation-in-part of Application Ser. No. 308,268
filed Nov. 21, 1972, now abandoned.
Claims
I claim:
1. In combination with an electromagnetically operated device, a
circuit for producing rapid current growth in the coil of said
device, and for maintaining a predetermined level of current in
said coil, an electrical circuit comprising:
a source of relatively high voltage capable of effecting rapid
current growth in said coil;
semiconductor switching means responsive to external control for
applying said high voltage source to energize said coil, said
semiconductor switching means operating in a switching mode as a
substantially non-dissipative element;
first means for sensing the current in said coil to open said
semiconductor switching means when a predetermined coil current
value is reached, and for closing said switch intermittently
thereafter, to maintain said predetermined coil current value;
and second means subject to external control for applying said high
voltage source to said coil in opposite polarity, thereby
accelerating the rate of current decline in said coil.
2. Apparatus according to claim 1 in which a shunt diode path is
provided to permit continued current flow through said coil, said
diode being poled so as not to conduct in response to forward
energization from said source, but to conduct in response to the
reverse coil voltage extant between applications of said source
intermittently to said coil by said first means.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the art of increasing the speed,
force and efficiency of devices which employ an electromagnet to
produce a mechanical force or motion in response to electrical
signals.
2. Description of Prior Art
An electromagnet type actuator normally employs an electrical
winding or solenoid of copper wire over a core of ferrormagnetic
material. The core consists of two parts, one over which the
solenoid is wound, and the other movable, completing the magnetic
path when actuated. The mechanical force between the fixed and
movable cores is equal to ##EQU1## WHERE B is the magnetic
induction in gauss and A is the cross-sectional area of the gap or
pole face in square centimeters. For the ideal case, ignoring
fringing magnetic fields, the magnetic induction B is equal to
##EQU2## WHERE N is the number of turns on the solenoid, I is the
current in amperes, `w` is the width of the air gap in centimeters,
1.sub.i is the length of the magnetic path in the core (both
parts), A is the cross-sectional area of the core (assumed constant
for this explanation), in square centimeters, and .mu. is the
magnetic permeability of the core. The above expression for the
magnetic induction B demonstrates that the permeability .mu. should
be as large as possible if large values of B are desired. In a
typical electromagnet actuator the value of `w` or width of the air
gap is relatively large compared to the term 1.sub.i /.mu., prior
to actuation, and for this case ##EQU3## Thus, for a given current
I in the winding, the magnetic induction B is inversely
proportional to the gap width `w`, and therefore the mechanical
force produced itself varies inversely as the square of the gap
width. It is therefore very difficult to obtain a constant
mechanical force over any appreciable distance of travel of the
pole piece and the force is the smallest when the gap is fully
open, contrary to what is usually desired.
A further difficulty associated with electromagnet actuators is
that a substantial amount of magnetic energy must be delivered to
the open gap before the desired force is obtained, this
predelivered energy being equal to
F .times. (w) .times. 10.sup.-.sup.7 joules,
where F is the desired force in dynes. The instantaneous power
required to achieve this force in a desired time of T seconds is
##EQU4## For example, to provide 15 pounds of force in a gap 0.1
inches wide in one millesecond requires about 175 watts of power.
Yet, once the gap is closed (after actuation) the electrical power
required to maintain a constant force of 15 pounds depends only on
ohmic losses and may be a small fraction of a watt.
Many ingenious methods are available to ameliorate the above
difficulties. The magnetic circuit may be provided with shaped pole
faces to make the mechanical force more uniform, mechanical
linkages may be employed to increase the force with the gap open,
or the voltage applied to the electromagnet winding may be made
high initially and reduced after actuation. A thermally variable
resistance such as an incandescent lamp may be connected in series
with the winding to produce a high inrush of current when the lamp
is cold and its filament resistance is low and to limit the current
when the lamp comes up to temperature and its resistance is high.
If the electromagnet actuated device is a relay, a pair of relay
contacts may be used to reduce the solenoid current after
actuation.
SUMMARY OF THE INVENTION
The present invention employs switching transistors and diodes to
apply a relatively high voltage to the winding of an electromagnet
to increase the speed with which the current in the winding will
rise to a desired value. Once the desired value of current has bee
attained a further increase is prevented by negative current
feedback, the current being maintained at near the desired value at
very high efficiency by intermittent application of the same high
voltage to the winding. To de-energize the electromagnet, the same
high voltage is applied to the winding in the reverse direction to
produce a cutoff speed equal to the turn-on speed, recovering, in
the process, the energy stored in the electromagnet. The addition
of a single capacitor to the circuit allows the initial current to
rise momentarily to a much higher value than the desired
steady-state holding current, this to provide a high mechanical
force with the gap of the electromagnet open. When a pair of
electromagnets are to alternately energized, energy is transferred
from one to the other by a diode and capacitor network at
arbitrarily high speed. Except for switching losses, which are low,
and ohmic losses in the circuits, a theoretical efficiency of 100%
is attainable, the power consumption being only that required for
mechanical actuation.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a complete schematic diagram of the high speed
electromagnet control circuit.
FIG. 2 is a graph comparing the currents produced in the winding of
an electromagnet for both high and low values of applied voltage
with the current produced by the circuit of FIG. 1.
FIG. 3 is a graph of the surge current produceable by the circuit
of FIG. 1.
FIG. 4 is a simplified equivalent circuit for the circuit of FIG.
1.
FIG. 5 is a schematic diagram employing two circuits such as shown
in FIG. 1 to alternately energize two electromagnets.
FIG. 6 is a simplified equivalent circuit for the circuit of FIG. 5
for the interval of energy transfer.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, the reference numeral 1 indicates an
electromagnet having a moveable pole piece 3 which is used to
actuate a mechanical device such as a relay or hydraulic valve. The
voltage of the DC power source 6 is much higher than that required
to provide the current necessary for the electromagnet when this
current is limited only by the winding resistance. Transistor 10
acts as a switch responsive to an input control voltage to energize
and de-energize the electromagnet 1. When transistor 10 is in the
non-conducting state, the electromagnet 1 and the battery are
connected together in series with diodes 2 and 4, both of which act
to prevent the flow of current to the winding of electromagnet 1.
When transistor 10 is caused to conduct by application of a control
voltage to the junction of resistors 7 and 8, the electromagnet
becomes conntected to the supply 6 in series with transistor 5. If
transistor 5 is also caused to conduct, the full battery voltage,
less the collector-emitter voltages of transistors 5 and 10, is
impressed across the electromagnet winding. The current in this
winding rises initially at the rate of E/L amperes per second,
where E is the supply voltage of voltage source 6, and L is the
inductance of the electromagnet. The winding current would
eventually attain the value E/R amperes, where is the winding
resistance, a value which, if allowed to continue, would possibly
cause overheating of the winding or, if the magnetic circuit is
closed by movement of pole-piece 3, would be much higher than that
required to provide the desired mechanical force. The relationship
between the desired holding current and the excessive current
produced by the high voltage energizing of the winding of
electromagnet 1 is shown as curve 21 in FIG. 2. Curve 20 of FIG. 2
shows the current waveform produced when the winding is energized
from a source of low voltage. It is seen that the low voltage,
though it eventually produces the desired winding current, requires
a much longer time to do so, whereas the high voltage, though it
causes the desired current to be obtained more quickly, would, if
continually applied, result in an excessive winding current. The
circuit of FIG. 1 acts to provide the rapid current rise of curve
21 to the desired value and to prevent the current from exceeding
this desired value thereafter, producing a current waveform such as
curve 22 of FIG. 2.
The control of the electromagnet `holding` current to the desired
value is achieved by feedback control employing a resistor 11 of
relatively low resistance to produce a feedback voltage
proportional to the electromagnet winding current. This feedback
voltage is compared with a reference voltage appearing across
resistor 14 which represents the desired value of steady state
current in the winding of electromagnet 1. Resistor 14 is part of
the voltage divider network comprised of resistors 12 and 14
providing a desired fraction of the voltage appearing across Zener
diode 9 which is energized through resistor 8 by the same input
control voltage that is used to bias transistor 10 into conduction.
The current feedback voltage from resistor 11 is applied directly
to the inverting input of the integrated circuit operational
amplifier 17 while the voltage across resistor 14, representing the
desired winding current, is applied to the non-inverting input of
the same amplifier 17. Resistor 16 from the output of amplifier 17
to the non-inverting input, provides positive feedback of a
controlled amount to cause amplifier 17 to operate as a Schmidt
trigger. Typically, the feedback is adjusted so that the holding
current through the winding of electromagnet 1 is maintained within
about 10% of the desired value as shown in curve 22 of FIG. 2.
Amplifier 17 drives transistor 24 through its base resitor 18. When
transistor 24 is caused to conduct, the switching power transistor
5 is biased into conduction by current flow through its base
resistor 19.
Transistor 5 will be alternately conducting and non-conducting as
the output of the Schmidt trigger alternates polarity. While
transistor 5 is in the non-conducting state, transistor 10
meanwhile held in conduction continually by application of a
control voltage to the junction of resistors 7 and 8, the current
through the winding of the electromagnet 1, since it cannot stop
abruptly, is switched to diode 4. In this passive state, the only
voltage acting to reduce the current in the winding is that due to
the winding I .times. R voltage drop, the voltage across resistor
11 and the voltages across transistor 10 and diode 4 which are
relatively low. Typically the voltage acting to decrease the
current in the winding is a small fraction of that applied to the
winding to produce the current so that the current in the winding
of electromagnet 1 will decay at a much lower rate than it
recovers, exhibiting a waveform such as curve 20 of FIG. 2. Thus
the voltage supply 6 is applied to the solenoid only intermittently
to maintain the desired value of holding current, typically only 5%
of the time. In this manner relatively little power is required of
voltage source 6 to maintain the desired current in the winding;
only that required to supply the power losses in transistors 5 and
10, diode 4, resistor 11, and the ohmic losses in the winding of
the electromagnet 1.
For the electromagnet to remain energized in the manner described,
the control voltage appearing between ground and the junction of
resistors 7 and 8 must be maintained at a value sufficient both to
bias transistor 10 into conduction and to operate Zener diode 9 at
its breakdown voltage. When this control voltage is removed,
transistor 10 will cease to conduct and the voltage across the
Zener diode 9 will fall to zero. Either one of these two
conditions, a zero reference voltage to amplifier 17, or transistor
10 rendering non-conducting, would suffice to reduce the current in
the winding of the electromagnet 1 to zero. When both conditions
are present, as is the case when the control voltage is removed,
both transistors 5 and 10 are rendered non-conducting, the former
by virtue of a zero reference voltage supplied to amplifier 17, the
latter as described above. The current in the winding of
electromagnet 1, which cannot cease suddenly, is provided a path
now through diodes 2 and 4 in series with the supply 6 which in
this instance appears across the winding of the electromagnet with
a polarity reversed from that by which the winding was energized.
This relatively high voltage, appearing across the winding of the
electromagnet in a direction to reverse the current through it
causes the current to decay with the same high speed with which the
winding was initially energized, but the current is prevented from
reversing by both diodes 2 and 4. Thus the current in the winding
is reduced to zero with great speed and the energy corresponding to
this current is returned to the voltage source. Curve 23 in FIG. 2
showns the current waveform at cutoff.
The optional addition of capacitor 13 across resistor 12, shown by
the dotted lines in FIG. 1, will cause the initial surge current in
the winding of electromagnet 1 to be much larger than the desired
or permissible steady state value. This large initial current is
advantageous in providing a high mechanical force in the
electromagnet when a maximum gap separates the pole pieces 3 from
the core. Momentarily, the full voltage of zener diode 9 becomes
the reference voltage corresponding to a higher initial reference
current which will decay exponentially to the steady state
reference current as shown in curve 26 of FIG. 3. The magnitude of
this higher initial reference current and the associated time
constant can be adjusted to desired values by the proper choice of
capacitance value and suitable modifications of the voltage divider
network comprised of resistors 12 and 14 as is well known to those
skilled in the art. Once the surge current reaches the reference
value it would tend to continue except for a relatively slow
exponential decay as shown in curve 25 of FIG. 3, were it not for
the fact that at some value of current the pole piece 3 of
electromagnet 1 would move to close the gap. The motion of
pole-piece 3 under mechanical force of the magnetic field withdraws
energy from the field and results in a corresponding reduction in
the current in the winding of electromagnet 1, as shown in curve 27
of FIG. 3. After the the current in the winding is reduced to the
desired steady state value by whatever means, the voltage across
resistor 14 remains constant and the current waveform is as shown
by curve 22 of FIG. 2.
Should the pole-piece 3 fail to be actuated during the high initial
surge current through the winding of the electromagnet 1, the high
current in the winding, shown as curve 25 in FIG. 3 will tend to
persist but with no further drain upon power supply 6 until the
value decays to the reference steady state value. Thus a high
mechanical force remains to actuate pole-piece 3 for an appreciable
length of time, a desirable circumstance. If the pole-piece fails
to move, the energy it would have taken is dissipated as heat in
the winding of the electromagnet.
When two solenoids are to be operated alternately, as is often the
case, for example to achieve a bidirectional mechanical motion or
to control a three-way solenoid valve, the energy of one
electromagnet being de-energized can be used to produce the high
voltage to energize the second electromagnet at high speed without
the aid of a separate high voltage source. To clarify the mechanism
by which the energy transfer from one electromagnet to another is
made to take place, a simplified equivalent circuit of the circuit
of FIG. 1 is shown in FIG. 4, where, for greater ease of
comprehension, the switching transistors 5 and 10 of FIG. 1 are
represented respectively by on-off switches 5 and 10. When switches
5 and 10 in FIG. 4 are open, as shown, no current flows in the
solenoid 1. To energize solenoid 1 continuously, switch 10 is
closed continuously and switch 5 is closed intermittently as
required to bring and maintain the current in solenoid at a desired
value as described previously. When solenoid 1 is to be
de-energized, both switches 5 and 10 are opened and the current in
the solenoid 1 is then required to flow through diodes 2 and 4,
rendering them conducting and thereby applying the full voltage of
voltage source 6 across the solenoid 1 in a polarity opposing the
flow of current.
In FIG. 5 two circuits, such as shown in FIG. 4, are employed to
control two solenoids, circuit 35 controlling solenoid 1 with
simplified circuit elements 2, 4, 5 and 10 as described above, and
a second similar circuit 36 controlling a second solenoid 31 with
simplified circuit elements 32, 34, 33 and 30 corresponding in
function respectively to circuit elements 2, 4, 5 and 10 of circuit
35. The two solenoid control circuits 35 and 36 are coupled to a
voltage source 6 through diode 28 and are shunted by capacitor 29.
Not shown in FIG. 5 are the input signals and control circuitry
which control the operation of switches 5, 10, 30 and 33, these
being as shown in FIG. 1. Though with the circuit of FIG. 5,
solenoids 1 and 31 may be operated independently, it is required
for energy transfer from one to the other that they be operated
alternately, and in this respect when an "on" signal is applied to
circuit 35, an "off" signal is assumed applied simultaneously to
circuit 36, a mode of control most easily obtained by driving
circuits 35 and 36 with the complementary outputs of a flipflop
circuit. In this "either-or" mode of operation, where either one of
two electromagnets is to be energized but not both, one of the two
circuits 35 or 36 will be in the "on" state, say circuit 35, and in
this state switch 10 will be closed, switch 5 will operate
intermittently under current feedback control to maintain the
current in solenoid 1 at the desired value, and switches 30 and 33
of circuit 36 will remain open, solenoid 31 thereby remaining
un-energized. When solenoid 1 is to be de-energized and solenoid 31
simultaneously energized, switches 5 and 10 are opened under the
action of the effecting control signal, switch 30 is closed and
switch 33 is actuated under current feedback control to bring and
maintain the current in solenoid 31 to the desired value.
In the interval of transition, during which solenoid 1 becomes
de-energized and solenoid 31 becomes energized, diode 28 prevents
the return of energy from solenoid 1 to the voltage source 6,
whereupon the current from solenoid 1 is discharged into capacitor
29, raising its voltage to a value which may be as high as desired
for a suitably small value of capacitance of capacitor 29, and
which voltage is impressed across solenoid 31, likewise energizing
it as quickly as desired within the voltage breakdown limits of the
circuit elements employed. Since, during the interval of
transition, diodes 28, 32 and 34 are non-conducting and may be
replaced by open circuits, and correspondingly transistors 5 and 10
may also be replaced by open circuits and the elements represented
by numerals 2, 4, 30 and 33 are all conducting and may be
represented by short circuits, the mechanism of energy transfer
from one solenoid to another is most readily made apparent by
recourse to the simplified equivalent circuit of FIG. 6 where e is
the instantaneous voltage across the capacitor 29, the latter
having a capacity of C farads, L is the inductance in henries of
the two solenoids 1 and 31, each assumed, for purposes of
illustration to have an inductance of L henries, and the voltage of
the voltage supply is assumed to be E volts. The two currents in
the solenoids 1 and 31 flow in the direction shown and are denoted
respectively by the variables i.sub.1 and i.sub.31, the initial
value (at the instant that the process of energy transfer begins)
of i.sub.1 being I amperes, the desired value of current in either
of the solenoids in the "on" state, and the initial value of
i.sub.31 being zero amperes. The dynamic behavior of the simplified
circuit of FIG. 6 is easily obtained by solving the relevant
differential equations for the initial conditions and circuit
element values given, the voltage e and the currents i.sub.1 and
i.sub.31, being as follows: ##EQU5## where ##EQU6## is the angular
frequency in radians per second and t is the time in seconds. Since
it is obvious from the above that i.sub.1 + i.sub.31 = I, the
current i.sub.31 in solenoid 31 will reach the value I when the
current i.sub.1 in solenoid 1 reaches the value zero. The time T,
in seconds, required for this complete transfer of energy from
solenoid 1 to solenoid 31 can be obtained by setting i.sub.1 equal
to zero in the above equations, resulting in the following
expression for the switchover time T: ##EQU7## For an extremely
large value of C, or in effect a very large capacitor 29, we may
use the approximation that for small angles, the trigonometric
tangent of an angle is equal to its value in radians, for which
case the switchover time T can be given explicitly as ##EQU8##
which, to give an example, would be 4.167 milleseconds for typical
values of supply voltage E of 12.0 volts, solenoid inductance of
0.1 henries and solenoid current of 0.5 amperes. This switchover
time, using a very large capacitor, is the same switchover time
that would be obtained with the circuit of FIG. 1 for one solenoid
having a voltage supply 6 of 12.0 volts. For smaller values of
capacitance C of capacitor 29, the switchover time T may be made as
small as desired, provided safe voltage limits of the circuit
elements are not exceeded. Thus, for a desired switchover time of
one millesecond, the first expression given above for the
switchover time T can be solved for the requisite value of C from
the known values of circuit parameters I, E, L and T, which, for
the values assumed in the present example, results in a value C for
capacitor 29 of 4.0 microfarads.
When a small value of capacitance C is used to obtain a short
switchover time T from one solenoid to another, it is important
that the peak voltage appearing across capacitor 29 not be
excessive. The peak value e.sub.peak of the voltage e may easily
shown to be given by the following expression: ##EQU9## For a very
large value of C, this voltage will be equal to the voltage E of
the voltage supply 6, and for values of E, L and C of the above
example, the value of e.sub.peak would be only 57 volts. To produce
a switchover time of one millesecond in this example with the
circuit of FIG. 1 only, would have required a voltage supply 6 of
50 volts, yet it was produced with the circuit of FIG. 4 with a
supply voltage E of only 12.0 volts and with a peak voltage not
much in excess of the above mentioned 50.0 volts. Thus the addition
of one capacitor 29 and one diode 28 to a pair of basic circuits 35
and 36 provides for energy transfer from one solenoid to another at
high speed using only a single low voltage supply. An advantage of
the lower supply voltage is the reduced probability of damaging the
switching transistors in short circuit conditions and the greater
ease with which the switching transistors may be protected;
further, since all transistors are employed in a switching mode
they dissipate relatively little power and low power transistors
can be used to control relatively high power solenoids.
* * * * *