Note: Descriptions are shown in the official language in which they were submitted.
~7;~5~3
13130:CRH
DIRECT CURRENT CONTROL IN
INDUCTIVE LOADS
Background of the Invention
1. Field of the Invention
This invention relates to methods and apparatus
for controlling the current in ,an inductive device from
a source of direct current electric power with high energy
efficiency, even though the supply voltage from the source
may be subject to conæiderable variations.
2.~ The Prior Art
The presently existing techniques for controlling
the DC current in an inductive device may be placed in
three major classes as follows:
a) Varying the Effective ~oltage at the Load
This technique makes use of the relation-
ship that the current, through a fixed resistance, is
directly proportional to the voltage applied. The voltage
applied to the load is monitored and the power source is
directly or indirectly adjusted to maintain the required
voltage. ~irect control of the power source (e.g. adjust-
ing the current in the field winding of a generator) is
most often impractical or impossible. Indirect control
is accomplished by placing some form of electronic voltage
1~ 735(~3
13130:CRII -2-
1 regulator between the power source and the load. A series
type regulator allows for rapid and precise control of
the applied voltage but often dissipates 60~ - 90~ of the
supplied power. A switching type regulator normally dis-
sipates less than 20% of the supplied power but suffersfrom slow response to step changes in output voltage re-
quirements. The power inductors and the filter capacitors
are often exotic and bulky for high current applications,
although this is offset by the reduced needs of heat sinks.
A limitation of using voltage control to set
the current in the load is if the load resistance changes
(within one unit or from unit to unit), the current will
chanc,~e in inverse proportion since the feedback circuits
that are usually employed have no means of detecting this
change of resistance.
b) Passively Limiting the Current Through the
Load
This technique makes use of the relationship
that current is inversely proportional to rosistance with
a constant applied voltage. Although the least energy
efficient of the three techniques, this approach is most
commonly used due to its simplicity. A resistor is in-
stalled between the load and the power source. The value
of the resistor is chosen such that the combined series
resistance of the control resistor and the load limit the
current to the desired value. If different values of
currents are needed in one application, different resistor
values are installed in the circuit via mechanical or
electronic switches. This approach suffers several serious
setbacks. Load resistance changes will change the load
current, although not to the same extent as with voltage
control. The resistor will dissipate as much or more power
than a series regulator does (it is in fact a very simple
series regulator). The most important limitation occurs
when the supply voltage varies~
3S~g3
13L30:CRI-1 -3-
1 Since the resistance value of the load and
control resistor combination remains constant, the supplied
load current will vary proportionally with the supply
voltage. This lack of regulation can be intolerable in
most situations. This approach is also lacking in energy
and volume efficiencies. The resistor value is designed
to produce the desired current at the minimum supplied
voltage. As the voltage increases, the supplied current
increases proportionately. The power dissipation, however,
increases according to the square of the current change
(e.g., a doubling of the input voltage doubles the current,
but the power dissipation increase by four times). The
resistor, then must be of a power rating to withstand the
stresses at the maximum voltage.
c) Active~ Limiting the Current Through the
Load
In this technique" an active device (e.g., a
transistor) is used to limit the current ~upplied to the
load. A current sensing element (e.g., a resistor) is
placed in series with the load and the voltage across this
element is monitored. The control device is then set via
electrondcs to adjust its effective resistance to limit the
current to the desired amount. This system has merits in
that the supplied current remains constant whether the
load resistance changes or the input voltage varies. The
power dissipation is similar to that of a series pass
regulator. If the current range required is large, the
sensing element may present a problem. A value that
develops sufficient feedback voltage at low currents may
be too large to allow the high end of the current range to
be used at minimum supply voltage (too much resistance in
the line).
~'7;~ `3
Summary of the Invention
In accordance with the present invention there is
provided a current regulator for controlling the flow of current
from a source of direct current power comprising:
(a.) an inductor for supplying elecromagnetic energy to
a mechanical load,
(b) switching means for supplying power from the source
of direct current power to the inductor,
(c) means for actuating said switching means to apply
electric current from the source to the inductor for
predetermined fixed periods of time during all charging cycles,
so that the charging cycles are of fixed time duration
irrespective of changes in the load or of changes in the voltage
of the source of direct current power,
(d) means for discharging electric current from the
inductor during periods of time constituting discharging cycles
intermediate the charging cycles, and
(e) sensing means responsive to the electric current
which is discharged from the inductor during each discharging
cycle for actuating said switching means to apply electric
current from the source to the inductor to initiate a charge
cycle and to terminate each discharge cycle when the electric
current which is discharged from the inductor during discharge
cycles decays to a predetermined value.
Also in accordance with the invention there is provided
a current regulator for controlling the flow of current from a
source of direct current power comprising:
- 4a
5~3
(a) an inductive device for supplying electromagnetic
energy to a mechanical device,
(b) means for applying electric current from the source
to the inductive device periodically for fixed period of time
during all charging cycles irrespect ve of changes in the
electric power consumed by the inductive device or of changes in
the voltage of the source of direct current power, in which the
current changes from an initial value to a larger value
determined by the period of time during which the charging cycle
takes place,
(c) means including a freewheeling diode for
discharging electric current from the inductive device in
discharge cycles during periods of time intermediate the
charging cycles, and
(d) control means responsive to the electric current
which is discharged from the inductive device during each
discharge cycle, for initiating charging cycles and discharging
cycles alternately, with each charging cycle being initiated
when the current which is discharged from the inductive device
decays to a value that is substantially equal to the initial
value of the electric current at the beginning of a charge cycle.
Further in accordance with the invention there is
provided a current regulator for controlling the flow of current
from a source of direct current power comprising:
(a) an inductive load in which the electric power is to
be dissipated by transfer of the electromagnetic energy in the
load to a mechanical device,
- 4b
3S~3
(b) switching means for applying electric current from
the source to the inductive load periodically for predetermined
fixed periods of time during all charging cycles regardless of
the duty cycle caused by changes in the operating conditions, in
which the current changes from an initial value to a larger
value determined by the period of time during which the charging
cycle takes place,
(c) means for discharging electric current from the
inductive load through a sensing resistor.
(d) means responsive to the current that is discharged
from the inductive load through the sensing resistor for
actuating said switching means to apply electric current from
the source to the inductive load to initiate a charging cycle
and terminate a discharing cycle when the electric current which
is discharged from the inductive load decays to a value that is
substantially equal to said initial value of the electric
current at the beginning of a charge cycle.
Further in accordance with the invention there is
provided a current regulator for controlling the flow of current
from a source of direct current power comprising:
(a) an inductive load
(b) control means for applying electric current from
the source to the inductive load,
(c) said control means providing uniform charging
cycles of fixed periods of time for all operating conditions for
applying energy to the inductive load and discharging cycles
during which electric current is discharged from the inductive
~[} ,
- 4c
- 1~'73~3
load and no energy is provided to the inductive load from
sources other than the discharge current, with the charging
cycles being of less time duration than the duration of the
discharging cycles, and
(d) sensing means responsive to said discharge current
for actuating said control means to apply electric current rrom
the source to the inductive load to initiate a charging cycle
and terminate a discharging cycle when the electric current
which is discharged from the inductive load decays to a
predetermined value.
Further in accordance with the invention there is
provided a current regulator for controlling the flow of current
from a source of direct current power to an inductor comprising:
(a) an inductor for supplying electromagnetic energy to
a mechanical load,
(b) switching means for supplying power from the source
of direct current power to the inductor during charging cycles,
~ c) means for discharging electric current from the
inductor during discharging cycles when power is not supplied
through the switching means to the inductor,
(d) means for providing a reference signal
representative of the desired amount of current to be applied to
the inductor,
(e) means for providing a control signal that varies in
accordance with the decay of the current which is discharged
from the inductor during discharge cycles, and
(f) control means responsive to said reference and
r
- 4d - .
S~I! 3
control signals for actuating the switching means for fixed
period of time under all operating conditions to initiate
charging cycles when said control signal decays to a
predetermined value with respect to the reference signal.
Further in accordance with the invention there is
provided a current regulator for controlling the flow of current
from a source of direct current power to an electric motor
having inductive windings for receiving direct current power
comprising:
(a) switching means for supplying power from the source
of direct current power to the inductive windings of the
electric motor,
(b) means for actuating said switching means to apply
electric current from the source to said inductive windings for
predetermined fixed period of time during charging cycles, under
all operating conditions,
(c) means for discharging electric current from said
inductive windings during period of time constituting
discharging cycles intermediate the charging cycles,
(d) means for sensing the decay of the electric current
from said inductive windings during discharging cycles to
provide a first control signal,
(e) means for sensing the speed of rotation of the
electric motor to provide a second control signal, and
(f) control means responsive to said first and second
control signals for initiating charge cycles and terminating
discharge cycles when said control signals have a predetermined
relationship.
- . . . .
- 4e
~7~3S~3
Further in accordance with the invention there is
provided a method of controlling the flow of current from a
source of direct current electric power to an inductive device
for supplying electromagentic energy to a mechanical device
compr:ising applying electric current from the source to the
inductive device periodically for fixed periods of time for all
operating conditions during charging cycles in which the current
changes from an initial value to a larger value determined by
the period of time during which the charging cycle takes place,
discharging electric current from the inductive device in
discharge cycles during periods of time intermediate the
charging cycles, and terminating each discharge cycle and decays
during the discharge cycle to a predetermined value.
Further in accordance with the invention there is
provided a method of supplying energy from an inductive device
to a mechanical device comprising causing the inductive device
to apply electromagnetic energy to the mechanical device during
charging cycles when the inductive device is energised from a
source of direct current power, causing the inductive device to
apply electromagnetic energy to the mechanical device during
discharge cycles when the inductive device is discharged by
applying current through a discharge path, and initiating
charging cycles and discharging cycles alternately, with each
charging cycle having a fixed period of time for all operating
conditions which is intitated when the current which is
discharged from the inductive device decays to a predetermined
value.
- 4f
~.7~S~?3
Efficient direct current control in an inductive load
is achieved in the present invention by applying the electric
power directly to the inductive load element through a switching
means which closes the circuit to apply a charging current to
the inductive load for fixed on periods of time. A discharge
path is provided for the inductive load when the switching means
opens the charging circuit. In the preferred embodiment the
discharge path comprises a freewheel diode and a resistor
connected across the inductive load. The current in the
discharge path is monitored, and when the
~73S~P3
- 4g
clischarge current has decayed to a selected value, the
switching means is again closed for the fixed on charge time.
Since the electric power is applied directly to the
load element with very little loss in the control circuitry,
substantially all of the electric power is dissipated in the
inductive load itself. The electromagnetic energy which is
stored in the inductive load may be employed to actuate a
mechanical device, such as a solenoid or motor.
1~ 73SC~3
13130:CR~ 5-
1 _ ief Descri~tion of the Drawings
FIG. 1 illustrates the principles upon which the
present invention are based,
FIG. 2 is a diagram which illustrates the broad
concept of the invention,
FIG. 3 shows the idealized waveforms for the
apparatus of Fig. 2,
FIG. 4 shows the circuit of a type ~airchild
78S40 integrated circuit,
FIG. 5 shows how the circuit of Fig. 4 may be em-
ployed to provide the switching action,
FIG. 6 shows how the circuit of Fig. 4 may be em-
ployed to provide the switching action for applying
higher power to an inductive load,
FIG. 7 shows how the circuit of Fig. 6 may be em-
ployed to provide the switching action for a solenoid,
and
FIG. 8 illustrates how the invention may be employed
to control a DC motor.
:1~'73S~)3
13130:CRI~ -6-
1 _escription of the Preferred Embodiments
Eig. l(A) shows an inductor L placed in a series
circuit with a swi~ch SWl and power source VIN. The free-
wheel diode Dl or other unidirectional conductor for
electric current is used to provide an alternate current
path for the inductor when SWl is opened. Assuming no
initial currents, the following holds true when SWl is
closed:
VL L di (1)
rt
iL = L ~ VL dt (2)
Due to the series circuit, when SWl is closed VL = VIN and
is constant. Therefore, Equation (2) becomes:
. .
iL = LL t (3)
Complications set in when one realizes that real-life
inducto~s exhibit a finite resistance RL as illustrated
in Fig. l(B). As current starts to flow, this resistance
drops part of the voltage VRL, leaving a lower value for
the inductance. Eventually VRL will equal VIN and the
current will remain at a constant value. Solving equation
(2) for this condition yields the standard charge curve:
iL = V~L~ (1 - e L ) (4)
If the time period that iL is examined is short in relation
to the time constant L/RL equation (4) can be approximated:
iL = Io + VIN Io RL t t2 ~ tl
1~73S~3
13130:CRII -7-
1 where Io~ is the current at the start of the period t2 - t
and t is a relative time from tl to t2. The current now
appears to be a series of linear ramps with slopes dependent
upon the current present in the inductor.
When SWl is opened, the inductance opposes any change
in current and will develop a sourcing potential (back EMF
or inductive kick) to maintain the current. Since the coil
voltage changes polarity, Dl conducts. Assuming a perfect
diode, the inductor voltage and current become:
RL
VL = Io RL e L t ~6)
RL
iL = I e L (7)
where Io is the current through the inductor at the time
the switch is opened.
As~with the charging current, if the change in dis-
charge current examined is small in relation to the current
value, the discharge current curve portion can be approxi-
mated:
iL = Il - L t ¦ 2 tl (8)
where Il is the current at the start of the period t2 - tl.
1~L7~5~3
13130:CRH -8-
1 Fig. l(C) shows how the invention can be practiced
using a switch SW2 in the place of the diode Dl. When
SWl is closed to provide the charging current SW2 is
open, and when SWl is open SW2 is closed to provide the
discharge path.
The charging cycles to to tl will be of less duration
than the discharge cycles tl to t2. In order to obtain
more constant regulation, the charging cycle should be
very short, e.g., 200 microseconds.
The present invention operates on the above-outlined
principles as illustrated in broad concept in Figs. 2 and 3.
The switch SWl is turned on and off by a switch
control circuit 20. A resistor Rs is employed in the dis-
charge current leg to provide a control voltage Vs to the
control circuit which compares Vs to a preset reference
voltage VR and actuates SWl when Vs is equal to VR
The switch SWl is turned on for fixed periods of time
to to tl. It is turned off for variable periods of time
tl to t2 which are determined by the time required for
the IR drops across Rs to equal VR.
In operation, a known current Io is established and
flows i4 the inductor L when SWl is closed at time tO. The
switch is opened after a brief fixed time tl. At this time
the inductor current has reached a value Il. The current
in the inductor-diode loop is monitored, and when the
current has decayed bacX to the value of Io at time t2, the
switch is closed again for the fixed charge time. This
cycle is then repeated.
In order to avoid unwanted resistance during the
charge time, decay current sensing is done in the diode
leg of the circuit. If the time period tl - to is kept
short, the effective current in the inductor is Io~
Fig. 3 shows idealized current waveforms for the
inductor and the switch of Fig. 2. If the charge time
tl - to is short enough to keep the current change Il - Io
1~l73S~)3
13130:CR~ -9-
1 srnall compared to Io~ the inductor current is essentially
a fixed DC value. The average supply current effectively
becomes:
IIN Io tl - to
o
or the inductor current times the duty cycle (ratio of
on time to cycle time). The duty cycle can be approximated
(ignoring sense resistor loss):
tl - to Io~ (10)
t2 _ to VIN
The system in Fig. 2 achieves the desired results in
control~ing DC currents in inductive loads. The current
in the coil is maintained by means of charge and discharge
cycles, the circuit losses are minimized because the
control element operates in a switching mode, and current
sensing does not interfere with the main current source
path. The current from the source VIN is applied directly
to the load inductor L during the charge cycle to provide
power efficiently, and the current produced by the inductor
during the discharge cycle is also emloyed for efficiency.
The energy that is stored in the inductance provides an
efficiency that is not produced by the prior art devices.
RS has small resistance so that little power is lost in
developing Vs. No capacitive element is used in developing
VS because immediate response is needed, and all of the
1~'73S(~3
13130:CR~T -10-
1 stored engergy should be in the inductance.
The switch control circuit 20 may be a standard
commercially available integrated circuit for switch regu-
lation, such as the Fairchild 78S40, the Texas Instruments
TI 497, or the 1524 or 3524 that are available from
multiple sources. Such circuits may be connected to drive
a transistor switch, sense the voltage drop across the
current sense resistor RS and control the off time until
the preset reference level VR is reached.
Fig. 4 shows the circuit of the Fairchild 78S40.
It comprises a fixed on period switching regulator, a
voltage reference source, and an uncommitted operational
amplifier. The on time is controlled by the capacitance
Ct .
Fig. 5 shows the regulator of Fig. 4 in a circuit for
controlling the current in an inductive load. This circuit
can be used only if the input voltage and load currents are
within the operational limits of the regulator. In Fig. 5
the operational amplifier is used to correct the polarity
of the current sense signal (from negative-going to
positive-going) and adjust the effective value prèsented
to the ~egulator comparator (at the setpoint current the
voltage output of the amplifier will equal VR). Ct is
chosen to give an on-time consistent with the load time
constant (L/R) and the response times of the switch,
amplifier, and comparator. For a varied current output,
VR would be varied.
Fig. 6 shows a similar current regulator for use
with higher voltage and load current requirements. A
separate switch circuit (Ql~ Q2, Q3) was used because the
input voltage and load currents of this system exceeded
the operational limits of the 7~3S40. Rx was added to allow
for electronic system turn-off. When the ON/OFF line is
open, +V forces the output of the sense amplifier Al to
always exceed VR, thus keeping the regulator from turning
1~735~3
13130:C~II -11-
1 on. When the ON/OFF line is shorted to ground, the circuit
operates in the normal mode. Ct was chosen at .01 ~ f to
allow a 200 ~sec on time, which i5 considerably smaller
than the typical time constant of the chosen load (100
msec min.) but still slow enough to allow adequate operation
of Al and the switch system.
Several DC solenoids were used as loads. The circuit
controlled the currents at the desired value regardless
of the type of solenoid used or the nominal voltage rating
of the solenoid used. Ct was varied in order to determine
the change of efficiency resulting. Better circuit effi-
ciency was noticed as the on time was decreased until the
operational speeds of the amplifier and switch were reached.
The principle requirements of a solenoid driver are
lS a high-current drive to move the plunger and, once the
plunger has completed its travel, reduce the current to a
much lower value that is suffi~ient to hold the plunger in
place. Current practice is to apply the entire source
supply across the solenoid for the time needed to move the
solenoid. ~7hen this time is passed (either a fixed time
period or plunger motion sensed by some device), à load
reducti~n device (typically a resistor) is switched into
the circuit (e.g., Patent No. 3,766,432).
Fig. 7 shows a solenoid driver using the current
driver of Figure 6. When the "OPEN" signal is applied to
Q4, Q6 is turned on (allowing the regulator to operate).
Also the one-shot multivibrator OSl is fired for a fixed
period of time turning Q5 off during this time period.
This applies the entire +V to VR of the regulator (via
RA) and the RF/ RI feedback ratio in the driver sets the
value of the high pull-in current. The one-shot multi-
vibrator is reset by an external plunger motion sensing system
30 having a switch which is closed when the solenoid is
energized and moves its armature 31 upwardly. When the
one-shot time is reset Q5 turns on.~ This action reduces
1~'735~3
13130:CR~l -12-
1 VR by the resistance ratio of RB/(RA + RB). ~7ith a lower
reference voltage to meet, the regulator reduces the output
current by the same ratio so that Vs is substantially
equal to VR, thus deriving the required holding current.
~hen the "OPEN" signal is removed, Q6 is turned off
preventing the regulator from supplying any current to the
solenoid.
The circuit of Fig. 7 is particularly suitable for
use in well logging during the drilling operation for the
well, where the power for operating the solenoid is obtained
from a turbine generator that is actuated by the flow of mud
used in the drilling operation. In such applications
the voltage VIN may vary from 48 to 96 volts so that good
regulation is essential in order to obtain substantially
constant current for actuating the solenoid. It is
essential that overheating caused by the electric circuitry
be avoided, and the efficiency ,of the apparatus of this
invention satisfies that requirement.
Fig. 8 illustrates the application of the current
controller to operate a DC motor.
A RPM sensor 40 provides a voltage that is propor-
tional t~ the speed of rotation of the DC motor 42. The
sensor 40 may be a tachometer or an encoding disk. A
RPM set 44 serves to provide a voltage representative of
the desired speed of rotation, and that may be adjustable
if desired.
The outputs of RP~ sensor 40 and RPM set 44 are
applied to a comparing amplifier 46 and the resultant
control signal VR is applied to constant current driver
of the general type discussed above with reference to
Figs. 6 or 7 which supplies a regulated current to the
motor 42.
In this circuit, not only are the efficiencies of the
current regulator attained, but only the power needed to
maintain the requested speed is applied to the motor, there-
1~'7;~S~3
13130:CRH -13-
1 by providing a very efficient means of controlling a DC
motor that is not achieved by the prior art.
The DC motor control arrangement is especially suitable
for use in electric vehicles that are battery powered because
the efficiency of the circuit serves to conserve the power
of the batteries. With such an arrangement, the vehicle can
travel farther between charges than vehicles using other
existing circuit arrangements.
The preferred embodiments employ a freewheeling
diode or other unidirectional conductor means to pro-
vide the discharge path for the inductive load. However,a switching means electronically controlled by the control
circuits may be employed to provide the discharge path, as
illustrated in Fig. l(C).
