Note: Descriptions are shown in the official language in which they were submitted.
12~75Z~
SOLENOID RESPONSE DETECTOR
SUMMARY OF T~IE INVENTION
The present invention relates ~enerally to an
arr~ngemer.t for detecting actuation of a solenoid and more
particularly to such an arrangement for sensing solenoid
armature movement in a solenoid actuated valve.
Such solenoids are typically of the type having
an actuating coil, a movable armature rsciprocable along
an axis between first and second positions corresponding
to valve-closed and valve-open positions respectively. A
spring or other means normally biases the armature toward
the first position, with the actuating coil inducing a
force on the armature tending to move the armature from
the first position toward the second position in response
to current flow in the actuating coil.
~n illustrative preferred environment of the
present invention is in the control of electrohydraulic
actuators of the doser type, for example, as disclosed in
U.S. Patent No. 4,256,017 to Eastman. Briefly, when a
measured quantity or "dose" of hydraulic fluid is injected
into or exhausted from the control chamber of a
differential area piston actuator, the piston motion or
output is a step movement commensurate with the size of
the input dose. The dose may be controlled by controlling
the time duration of an enabling pulse to a solenoid
actuated valve. The smallest discrete movements of the
piston and, therefor, also the minimal or "quantum" dose
will occur for the shortest effective actuation interval
of the solenoid.
For precision positioning of a doser actuator, it
is highly desirable to accurately deliver pulses which are
of only slightly greater duration than the minimum
threshold pulse of for given solenoid valve. Such a
minimal duration pulse will be of sufficient duration to
ensure that the valve moves from the normally closed
position fully to the open position under all expected
operating conditions while a pulse of lesser duration may
not be sufficient to ensure full opening of the valve.
S2~i
Doser control circuits are well known. For example,
U.S. Patent 4,366,743 disclosed a circuit which supplies a
pulse of slightly shorter duration than the anticipated
threshold pulse and the incrementally increases the pulse
width each time the controller senses that the solenoid
threshold has not been exceeded. Since several increments are
usually required, this approach is inherently slow and has
significant time lag problems. The use of proportional
incrementation calculations in systems similar to the patented
device have increased the costs of such systems and only
partially alleviated the time lag problem.
Among the several features of the present invention
may be noted the provision of a simple and inexpensive
solenoid actuation detector; the provision of such a detector
which is easily retrofitted to existing solenoid control
loops; the provision of a circuit for providing a pulse to a
solenoid controlled, in part, by detection of the actuation of
that solenoid; the elimination of mechanical switches,
scheduling circuits, or pulse incrementation logic circuitry
typical of prior solenoid movement sensors; the provision of a
solenoid actuation detector having very rapid response
characteristics and reduced sensitivity to loading effects;
and the provision of a simplistic yet effective solenoid
response detector suitable for doser threshold detection
applications. These as well as other advanta~eous features of
the present invention will be in part apparent and in part
pointed out hereinafter.
Specifically, the invention relates to a pulse width
control circuit for energizing a solenoid having a moveable
rn/
~75;2fi
armature reciprocable along an axis between first and second
positions, biasing means for urging the armature toward the
first position and an actuating coil responsive to current
flow for inducing a force on the armature to overcome the
biasing means to move the armature from the first position
toward the second position, the pulse width control circuit
comprising: means for initiating current flow in the actuating
coil; means for sensing current flow to the actuating coil; a
resistance-capacitance circuit for differentiating sensed
current flow to the actuating coil; comparator means for
identifying the time at which the differentiated current is
zero; a variable direct current biasing circuit coupled to the
comparator means for shifting the differentiated current
thereby changing the time at which the differentiated current
is identified as being zero to provide an estimate of the
departure time of the armature from the first position; a
counter; a source of timing pulses coupled to the counter;
means for loading an initial count indicative of a
predetermined time into the counter; means initiating timed
counter operation at the estimated departure time to modify
the initial count as a function of time; and counter
responsive circuit means operable upon the count in the
counter reaching a predetermined final count and thereafter
terminating current flow to the actuating coil a predetermined
time after initial armature movement away from the first
position.
In general and in one form of the invention, the pulse
width control circuit for a solenoid includes an arrangement
for initiating current flow in the solenoid actuating coil
rn/~
3a
along with circuitry for determining the arrival time at which
the armature arrives at the second or valve open position.
This arrival time may then be used to estimate the departure
time of initial armature movement away from the first
position. Current flow in the actuating coil is then
terminated a predetermined time after initial armature
movement away from the first position. The circuitry for
determining arrival time may include a small resistor for
sensing actuating coil current flow, a resistance-capacitance
lo circuit for differentiating the sensed actuating coiled
current flow, and a comparator for identifying the time at
which the differentiated current is zero. Utilizing the
arrival time to estimate the departure time may be
accomplished by a variable direct current biasing circuit
coupled to the comparator for shifting the time at which the
differentiated current is identified as being zero. The
circuitry for terminating actuating coil current flow includes
a source of timing pulses, a decrementable counter, control
circuitry for loading a number indicative of the predetermined
time into the counter and for initiating counter
decrementation at the estimated departure time, and a counter
responsive circuit which is operable upon the count in the
counter reaching zero for interrupting the current flow in the
actuating coil. There may be a plurality of counter
responsive circuits with the pulse width control circuit being
shared by a like plurality of solenoids. The counter
responsive circuit and the arrangement for initiating current
flow
rn/
1297~2~i
-- 4
may share at least one common circuit element such as a
silicon controlled rectifier or similar on/off switching
device.
~RIEF D~ CRIPTION OF THE DRAWINGS
Figure 1 is a schematic diagram of an electrical
circuit suitable for the practice of the present invention;
Figure 2 is a collection of voltage or current
waveforms on a common time scale at various points within
the circuit of Figure l;
Figure 3 is an enlarged view of a typical
solenoid current waveform; and
Figure 4 is a waveform illustrating the effect of
variation of the bias in the circuit of Figure 1.
Corresponding reference characters indicate
corresponding parts throughout the several views of the
drawing.
The exemplifications set out herein illustrate a
preferred embodiment of the invention in one form thereof
and such exemplifications are not to be construed as
limiting the scope of the disclosure or the scope of the
invention in any manner.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring first to Figure 3 wherein the current
flow in the coil of an illustrative solenoid i9 depicted,
when a step voltage is applied at time T0 the coil of the
solenoid, the current in the coil begins to in~rease along
the familiar exponentially increasing waveform 11 with
time constant L/R. If the solenoid armature fails to
move, the time constant is unchanged and the current
continues to increase to a steady state value along this
same exponential curve, i.e., along the curve port~on 13.
However, if the solenoid armature begins to move at time
Tl in response to the current in the coil, the inductance
of the coil is changed to a new not necessarily constant
value L' by this armature movement and the current
deviates to follow a new generally exponentially
increasing curve 15 with the new time constant L'/~. When
the armature comes to rest at time T2 in its new (valve
1~97~
open) position, the inductance reverts to a constant value
near the original value L, but there is a temporary back
EMF generated in the coil which opposes the applied
voltage and actually results in a temporary decrease in
coil current as along the curve portion 17. Thereafter,
at time T3, the current again increases exponentially with
the original L/R time constant along curve segment l9. If
the applied voltage is a pulse rather than a step of
voltage, a somewhat similar decay of the coil current
occurs upon the termination of the applied voltage.
Waveform A in Figure 2 is a waveform of the
voltage aCross a small current monitoring resistor 21 in
the circuit of Figure l ~hich accurately de~icts the current
flow in one of the solenoid coils 35 or 37. Comparing
Figures 2 and 3, the time interval between T0 and T2
represents the time required for the solenoid to operate
after the voltage is applied, i.e., the opening delay. In
the event the armature bounces or rebounds from its open
or actuated position as is frequently the case, it will be
repeatedly driven back toward that open position by the
magnetic field generated by coil current and there will be
a series of repetitions of the sequency of events
occurring after T3 in Figure 3. A number of such rebounds
or bounces are seen just subsequent to T3 in waveform A of
Figure 2 before the coil current reaches its steady state
value. The coil voltage pulse terminates at T4 and the
armature returns to its initial position at T5. Coil
current decay follows the same sequency of events as
described in conjunction with Figure 3. Armature bounce
and the resulting oscillations may occur upon
deenergization, or depending on armature damping, the
restorative force and other design parameters, no
oscillation may appear. The solenoid closing delay for
the particular illustrative solenoid is greater than the
opening delay and no oscillations on closing are depicted
in Figure 2.
7S2~i
In the circuit of Figure 1, a small resistance
21, such as one ohm, is inserted in series with the supply
and return solenoids. The current in the operating
solenoid may then be monitored by sensing the voltage drop
(shown in Figure 1, waveform A) across this resistor 21.
To approximate the point at which solenoid
armature movement begins, the point at which the first
oscillation occurs is determined by capacitor 23 and
resistor 25 which differentiate the voltage across
resistor 21 (Figure 2, waveform A). This differentiated
voltage is depicted as waveform E in Figure 2 where each
zero crossing or time when dv/dt=0 corresponds to a peak
(maximum or minimum) of the solenoid coil current.
Waveform E is illustrated in somewhat exaggerated form in
Figure 4. The first such zero crossing occurs at the time
T2 when the armature reaches its full stroke, but may be
used employing the variable bias adjustment potentiometer
27 to estimate the time Tl at which the armature begins to
move. When potentiometer 27 is set so that the voltage on
line 41 is the same as on line 39, comparator 29 is
unbiased and zero crossing occurs at T2 in Figure ~. An
increase in the setting of potentiometer 27 raises the
zero voltage line in Figure 4 upwardly as, for example, to
line 45 thereby also indicating an earlier zero crossing
at Tl. The difference between Tl and T2 is preferably on
the order of 30 microseconds.
~ omparator 29, which may be a type LM 319 with
the pin number connections shown within the triangle,
functions to compare the voltages on lines 31 and 33, and
to provide an output signal in the form of a change in the
output voltage level (waveform C) on line 36 upon the
occurrence of each zero crossing, that is, when the two
input voltages on lines 31 and 33 are the same. The 12
volt solenoid power supply on line 43 is utilized to
operate the comparator 29 and a 5 volt low impedance
reference with respect to the 12 volt return on line 45 is
provided by a 5 volt return on line 45 is provided by a 5
volt regulator such as a LM78L05. A 4N33 photo coupler 47
1~75~
-- 7 --
provides electrical isolation between the high current
transitions of the solenoid drivers (pins 1 and 2) and the
logic circuitry connected to pins 4 and 5 thereof.
At the start of each sampling period, the pulse
width necessary for delivery to the proper solenoid is
calculated from a linear relationship (request minus
position at each sampling time) and that solenoid is
turned on by the solenoid driver circuit 53 enabling the
corresponding silicon controlled rectifier 49 or 51. The
enable flipflop 55, for example, a type 74LS109, is held
in a reset state by applying an inverted carry signal
(waveform B) to its reset input pin. The pulse width
counter 57 dwells in the carry state until new pulse width
data is loaded from the control computer 59 into the
counter at which time the reset signal is removed from
the flipflop 55. The next zero crossing level change on
line 36 will set the flipflop (signal C') to its high
state and an output (waveform D) will enable the counter
57 to count up from the preloaded count and when the
counter reaches the carry state, solenoid driver 53
disables the corresponding silicon controlled rectifier.
From the forgoing, it is now apparent that novel
solenoid actuating and actuation detecting arrangements
have been disclosed meeting the objects and advantageous
features set out hereinbefore as well as others, and that
numerous modifications as to the precise shapes,
configurations and details may be made by those having
ordinary skill in the art without departing from the
spirit of the invention or the scope thereof as set out by
the claims which follow.
' f
