U.S. patent application number 14/930344 was filed with the patent office on 2016-05-05 for detection of plunger movement in dc solenoids through current sense technique.
The applicant listed for this patent is Texas Instruments Incorporated. Invention is credited to Manu Balakrishnan, Navaneeth Kumar Narayanasamy.
Application Number | 20160125993 14/930344 |
Document ID | / |
Family ID | 55853415 |
Filed Date | 2016-05-05 |
United States Patent
Application |
20160125993 |
Kind Code |
A1 |
Narayanasamy; Navaneeth Kumar ;
et al. |
May 5, 2016 |
DETECTION OF PLUNGER MOVEMENT IN DC SOLENOIDS THROUGH CURRENT SENSE
TECHNIQUE
Abstract
An apparatus and method of detecting movement of a plunger of
the solenoid includes detecting a peak (I.sub.PEAK) in a current
signal applied to a coil of the solenoid. A predetermined threshold
is added to the current signal applied to the coil of the solenoid
to generate a level shifted signal. The level shifted signal and
the peak signal are compared to detect movement of a plunger of the
solenoid.
Inventors: |
Narayanasamy; Navaneeth Kumar;
(TamilNadu, IN) ; Balakrishnan; Manu; (Kerala,
IN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Texas Instruments Incorporated |
Dallas |
TX |
US |
|
|
Family ID: |
55853415 |
Appl. No.: |
14/930344 |
Filed: |
November 2, 2015 |
Current U.S.
Class: |
361/160 ;
324/207.12 |
Current CPC
Class: |
G01R 31/72 20200101;
H01F 7/1844 20130101; G01R 31/2829 20130101; H01F 2007/185
20130101; G01D 5/2006 20130101 |
International
Class: |
H01F 7/18 20060101
H01F007/18; G01R 31/06 20060101 G01R031/06; G01D 5/20 20060101
G01D005/20 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 3, 2014 |
IN |
5512/CHE/2014 |
Claims
1. A method of detecting movement of a plunger of the solenoid
comprising: detecting a peak (I.sub.PEAK) in a current signal
applied to a coil of the solenoid; detecting a valley in the
current signal applied to the coil of the solenoid to generate a
valley signal (I.sub.VALLEY); adding a predetermined threshold to
the current signal applied to the coil of the solenoid to generate
a level shifted signal; comparing the level shifted signal and the
peak signal to detect movement of a plunger of the solenoid.
2. The method of claim 1 wherein the threshold is related to an
absolute value of the difference between I.sub.PEAK and
I.sub.VALLEY.
3. The method of claim 2 within the threshold is determined by
measuring characteristics of the solenoid.
4. The method of claim 1 wherein if the current signal does not
have a dip at least equal to the predetermined threshold, a fault
signal is generated.
5. The method of claim 1 wherein if the current signal has a dip at
least equal to the predetermined threshold, current to the solenoid
is reduced to its hold value.
6. The method of claim 5 wherein the threshold is added to a
voltage representing coil current which is compared with an output
of an active peak detector, if the level shifted voltage matches
the output of the active peak detector, complete solenoid movement
has been detected.
7. The method of claim 4 wherein a dip less than the threshold
indicates an unacceptably slower plunger movement.
8. In a system for operating a solenoid, a circuit to detect
complete movement of the plunger of the solenoid, comprising: a
device for measuring current through the solenoid and generating a
first current signal; an active peak detector circuit receiving the
first current signal for detecting a peak thereof and generating a
peak detection signal; a level shifter circuit coupled in parallel
with the active peak detector and receiving the first current
signal, the level shifter circuit adding a threshold voltage to the
first current signal to generate a level shifted signal; and a
comparator comparing the peak detection signal with the level
shifted signal, whereby if the level shifted signal matches the
peak detection signal, complete movement of the solenoid is
detected.
9. The system of claim 8 wherein the threshold is related to an
absolute value of the difference between a current peak and a
current valley.
10. The system of claim 8 wherein the threshold is determined by
measuring characteristics of the solenoid.
11. The system of claim 8 wherein if the first current signal does
not have a dip at least equal to the predetermined threshold, a
fault signal is generated.
12. The system of claim 8 wherein if the first current signal does
not have a dip at least equal to the predetermined threshold, the
solenoid current controller continues to drive the solenoid at its
nominal current.
13. The system of claim 8 wherein if the first current signal has a
dip at least equal to the predetermined threshold, current to the
solenoid is reduced to its hold value.
14. The system of claim 8 wherein the threshold voltage is added to
a voltage representing coil current which is compared with an
output of an active peak detector, if the level shifted voltage
matches the output of the active peak detector, complete solenoid
movement has been detected.
15. A control circuit for operating a solenoid comprising: a
circuit for applying a voltage across a solenoid coil and measuring
current through the coil to generate a first signal; a detector
circuit detecting a peak in the current through the coil
represented by the first signal; a circuit for detecting a valley
in the first signal; an adder circuit for adding a predetermined
threshold to the first signal; a comparator comparing output of the
adder circuit to the peak signal to detect movement of the plunger
of the solenoid.
16. The control circuit of claim 15 wherein the threshold is
related to an absolute value of the difference between the peak and
valley currents.
17. The control circuit of claim 16 wherein the threshold is
determined by measuring characteristics of the solenoid.
18. The control circuit of claim 15 wherein the current signal does
not have a dip at least equal to the predetermined threshold, a
fault signal is generated.
19. The control circuit of claim 15 wherein if the first current
signal does not have a dip at least equal to the predetermined
threshold, the solenoid current controller continues to drive the
solenoid at its nominal current.
20. The control circuit of claim 15 wherein if the current signal
has a dip at least equal to the predetermined threshold, current to
the solenoid is reduced to its hold value.
21. The control circuit of claim 15 wherein the threshold is added
to a voltage representing coil current which is compared with a
output of active peak detector, if the level shifted voltage
matches the output of the active peak detector, complete solenoid
movement has been detected.
22. The control circuit of claim 18 wherein a dip less than the
threshold indicates an unacceptably slower plunger movement.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This patent application claims priority from Provisional
Indian Patent Application No. 5512/CHE/2014, filed Nov. 3, 2014,
which is incorporated herein by reference in its entirety for all
purposes.
FIELD
[0002] The invention generally relates to a solenoid controller
and, more specifically to a solenoid detector that can detect
complete plunger movement
BACKGROUND
[0003] Linear solenoids are electromechanical devices that convert
electrical energy into a linear mechanical motion, which is used to
control electrical, pneumatic or hydraulic systems. Solenoids are
used in valves, relays and contactors.
[0004] Electromechanical solenoids consist of an
electromagnetically inductive coil that is wound to encircle a
movable steel or iron slug, termed "the armature" or "plunger." The
coil is shaped such that the plunger can be moved in and out of its
center, altering the inductance of the coil. The plunger is used to
provide a mechanical force to activate the control mechanism, for
example opening and closing of a valve.
[0005] A solenoid coil needs a higher current during activation,
called the pull--in current, to pull the plunger into the solenoid.
However, once the plunger has moved completely, the solenoid coil
needs only approximately 30% of its nominal current, called "the
hold current," to keep the plunger in the same position. DC
solenoids having coils that operate continuously at their nominal
current, which is limited by the resistance of the coil, will have
an increase in temperature of the coil due to the higher power
dissipation. Once the complete plunger movement is detected, the
steady-state current can be reduced to the hold current to minimize
the power consumption in the solenoid. The detection of the plunger
movement is required in safety-critical applications to detect
proper operation of the valve, relays or contactors. Movement of
the plunger can be slow, due to factors such as friction, rusting
and other mechanical impediments to the movement of the
plunger.
[0006] FIG. 1 shows an example of a known solenoid drive circuit,
shown generally as 100. A DC input voltage at 102 is applied to one
terminal of a solenoid coil 104, the other terminal of the solenoid
coil is connected to a transistor 108, controlling current through
the solenoid, which is sensed by sense resistor R.sub.SENSE 110.
Transistor 108 is controlled by current controlled solenoid driver
112, which will drive the solenoid at its nominal current until the
plunger has moved completely, at which time the current can be
reduced to its hold value. Freewheeling diode 106 is used to
eliminate the sudden voltage spike seen across the transistor when
it is switched off by the current controlled solenoid driver.
[0007] FIG. 2 shows the known excitation current waveform of a
solenoid, generally as 200. As soon as the solenoid is energized at
202, the current begins to increase as shown at 204. When the
current reaches I.sub.PEAK at 206, the plunger starts moving
because of the sufficient magnetic field created by the solenoid
coil. The movement of the plunger induces back EMF in the coil, and
hence, the solenoid current starts dropping. At 208, the plunger
has moved completely and the current dips to I.sub.VALLEY. After
the plunger strokes, the current continues on its normal upward
path, as shown at 210, to its maximum value, as shown at 212, which
is limited by the resistance of the coil. The prominent dip in the
excitation curve from I.sub.PEAK to I.sub.VALLEY is an indication
of plunger movement.
[0008] One known plunger position sensing method includes hall
sensors to detect the position of the plunger. The mechanical
mounting of these sensors are complex and their performance is
affected by ageing and external field. In addition, the hall sensor
will provide a signal at the end of the plunger movement, and
therefore, cannot detect slow movement of the plunger.
[0009] Other plunger movement detection logic uses fixed references
for detecting peak and valley current, or utilize algorithmic
solutions. These algorithms may fail during temperature variation
or during slow movement of plunger.
[0010] There is a need for a simple, low-cost and reliable
technique for detecting complete solenoid movement that can detect
plunger movement over wide variation of temperature and also detect
slow-moving plungers.
SUMMARY
[0011] A method of detecting movement of a plunger of the solenoid,
including detecting a peak (I.sub.PEAK) in a current signal applied
to a coil of the solenoid. A valley (I.sub.VALLEY) in a current
signal applied to a coil of the solenoid is detected. A predefined
threshold is defined as the absolute difference between the peak
and valley current signal. The predetermined threshold is added to
the current signal applied to the coil of the solenoid to generate
a level shifted signal. The level shifted signal and the peak
signal are compared to detect movement of a plunger of the
solenoid.
[0012] In a system for operating a solenoid, a circuit to detect
complete movement of the plunger of the solenoid, includes a device
for measuring current through the solenoid and generating a first
current signal. An active peak detector circuit receives the first
current signal for detecting a peak thereof and generating a peak
detection signal. A level shifter circuit is coupled in parallel
with the active peak detector and receives the first current
signal, the level shifter circuit adding a threshold voltage to the
first current signal to generate a level shifted signal. A
comparator compares the peak detection signal with the level
shifted signal, whereby if the level shifted signal matches the
peak detection signal, complete movement of the solenoid is
detected.
[0013] A control circuit for operating a solenoid includes a
circuit applying a voltage across a solenoid coil and measuring
current through the coil to generate a first signal. A detector
circuit detects a peak in the current through the coil represented
by the first signal. An adder circuit adds a predetermined
threshold to the first signal. A comparator compares the output of
the adder circuit to the output of the detector circuit to detect
movement of the plunger of the solenoid.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] Further aspects of the invention will appear from the
appending claims and from the following detailed description given
with reference to the appending drawings:
[0015] FIG. 1 shows an example of a known solenoid drive
circuit;
[0016] FIG. 2 shows the excitation current of a solenoid;
[0017] FIG. 3 shows the current drawn by the solenoid at different
temperatures;
[0018] FIG. 4 shows the difference between I.sub.PEAK and
I.sub.VALLEY (predefined threshold);
[0019] FIG. 5 is a block diagram of solenoid plunger position
detection logic;
[0020] FIG. 6 shows the plunger position detection circuit
consisting of an amplifier, active peak detector, op-amp adder
circuit and a comparator with latch;
[0021] FIG. 7 shows the peak detector circuit;
[0022] FIG. 8 shows the solenoid current level shifting
circuit;
[0023] FIG. 9 shows the comparator circuit generating the signal
I.sub.TRIP;
[0024] FIG. 10 shows plunger movement circuit waveforms;
[0025] FIG. 11 shows finding the threshold from the solenoid
excitation current characteristics; and
[0026] FIG. 12 compares the excitation characteristics of a normal
and a slow-moving plunger in a solenoid.
DETAILED DESCRIPTION
[0027] The method and apparatus disclosed herein is based on change
in current wave shape due to back EMF generated by plunger
movement. In an embodiment, the detection circuit comprises of
current sense amplifier, peak detector, level shifter and
comparator.
[0028] The current, drawn by the solenoid just before start of
plunger movement, is held by a peak detector. The circuit tracks
the dip in current due to the back EMF generated by the plunger
movement. If the dip in current is more than the predefined
threshold, it is interpreted as complete movement of plunger. The
threshold is set based on plunger characteristics.
[0029] If the solenoid is faulty or if the plunger did not move
fully, or moved very slowly, then the circuit can generate a fault
signal I.sub.TRIP that is equal to zero.
[0030] In order to study the effect of temperature on the
excitation current, a typical solenoid is characterized at
different temperatures. FIG. 3 shows the current drawn by the
solenoid at 3 different temperatures, -30.degree. C., 0.degree. C.
and +45.degree. C. The curves shift up as the temperature decreases
because of the reduction in resistance of the solenoid coil.
However, it should be noticed that the difference between the peak
(I.sub.PEAK) and valley (I.sub.VALLEY) of the solenoid current, due
to back EMF, remains constant irrespective of the temperature. This
characteristic of the solenoid current is utilized by the present
disclosure in detecting the movement of the plunger of the
solenoid.
[0031] Referring now to FIG. 4, a solenoid current characteristic
is shown generally as 400. When a voltage is applied to the
solenoid, the current rises at 404 to a peak 406 and then drops
back to a valley at 408 due to the back EMF of the solenoid coil.
Once the plunger moves completely, the current will increase, as
shown at 410, until it reaches its maximum at 412, which is
determined by the coil resistance. The absolute difference between
I.sub.PEAK at 406 and I.sub.VALLEY at 408 is measured and is used
to define the threshold for the detection of plunger movement. The
threshold is set slightly above I.sub.VALLEY and is referenced to
I.sub.PEAK so as to reduce the effect of temperature on the
detection circuit.
[0032] FIG. 5 shows a block diagram of a plunger movement detection
circuit consisting of a gain stage, an active peak detector, an
op-amp adder circuit and a comparator. This detection logic can be
implemented utilizing simple op-amp circuits rather than using a
sensor or controller. The solenoid current is detected using the
sense resistor R.sub.SENSE shown in FIG. 1, which is applied to
gain stage 504 at 502. The voltage across the resistor is filtered
and amplified by the gain stage 504, which is configured as a
differential amplifier, amplifying the voltage across the sense
resistor to the suitable high-value in order to improve the noise
immunity of the following circuits. The output of the differential
amplifier gain stage 504, is shown by waveform 520, and is fed
simultaneously to the active peak detector 514 and buffer 508. The
active peak detector 514 is used to track the differential
amplifier 504 output corresponding to the peak current (I.sub.PEAK)
during the excitation of the solenoid. The output of the peak
detector is shown at 526.
[0033] Buffer circuit 508 and adder circuit 510 form a level
shifter circuit 512, which is used to introduce a positive level
shift equivalent to the predefined threshold 414 in FIG. 4. The
threshold value that is added by adder circuit 510 to the output of
buffer 508 is shown at 506. The output of the level shifter circuit
is shown at 522. The signals shown at 522 and 526 are input to the
comparator 516. The output of the adder circuit is input to the
inverting input of the comparator 516 and the output of the active
peak detector 514 is input to the non-inverting input of the
comparator 516. Adding the threshold to the output of the gain
stage 504 and comparing this to the output of the active peak
detector results in a trip signal on the output 518 of comparator
516, when the two voltages substantially match. The output of the
active peak detector 514 will store the value of the first peak
such as peak 406 shown FIG. 4. As the solenoid current 400 drops to
form the valley at 408, the value of the signal 520, plus the
threshold, will substantially match the peak value of the signal
526, at the instant 408, triggering the comparator to produce the
trip signal I.sub.TRIP 518. The trip signal 518 indicates that the
plunger has completely moved within the solenoid.
[0034] FIG. 6 shows a schematic diagram of block diagram 500,
generally as 600. In FIG. 6, amplifier 604 receives the voltage
from the current sense resistor R.sub.SENSE shown in FIG. 1 and as
resistor R34 in FIG. 6, at the I.sub.SENSE+ and -I.sub.SENSE-
terminals. Differential amplifier 604 amplifies this voltage in
order to increase noise immunity in the following circuits.
Resistor R8, capacitor C1, and resistor R1, form a low pass filter
to smooth the voltage developed across R.sub.SENSE. The output of
amplifier 604 is fed simultaneously to active peak detector 614 and
buffer circuit 608. The output of the buffer circuit 608 is fed to
adder circuit 610 which adds the threshold value to the output of
the amplifier 604. The output of the adder circuit is fed to the
inverting terminal of comparator 616. The output of the peak
detector 614 is fed to the non-inverting input of comparator 616.
Comparator 616 produces a signal I.sub.mp which identifies that the
plunger of the solenoid has moved completely.
[0035] The peak detector circuit shown in FIG. 6 is shown
separately in FIG. 7, generally as 700. The output of differential
amplifier U2A is fed to the active peak detector formed by the
op-amp U2B, diode D2, resistor R26 and capacitor C4. The peak
detector detects the maximum value of the signal over a period of
time. A simple diode and capacitor can form a peak detector;
however, adding an op-amp with feedback can eliminate the diode
drop and make a more precise peak detector.
[0036] The output of differential amplifier U2A at ISENSE-AMP is
fed to the non-inverting input of the op-amp U2B. The output of the
op-amp U2B is connected to capacitor C4 through diode D2 and
resistor R26. The node formed by resistor R26 and capacitor C4 is
connected to the inverting input of op-amp U2B. The high
differential gain of op-amp U2B causes the capacitor C4 to charge
to a voltage that equals the non-inverting input voltage of the
op-amp.
[0037] The presence of diode D2 and the ultra-low input bias
current of op-amp U2B assures that the capacitor C4 will not
discharge even if the non-inverting input of the op-amp U2B goes
below the voltage across capacitor C4. This means that the voltage
across capacitor C4 will always track the maximum value at the
non-inverting terminal of the op-amp U2B.
[0038] A small resistor R26 is provided to increase the stability
of op-amp U2B as it is charging the capacitor. VIN is the power
supply to the op-amp. During times when the power is off, the VIN
voltage suddenly reduces to zero. The diode D1 makes sure that
during a power off situation, the capacitor C4 will discharge
immediately, which helps tracking the solenoid current during the
next power on sequence.
[0039] The level shifter circuit 612 of FIG. 6, is shown separately
in FIG. 8, generally as 800. The op-amp U2C is the voltage buffer
608 and the op-amp U2D is the adder circuit 610. The two op-amps,
U2C and U2D, are used to generate the level shifted waveform of the
current sense signal. The adder circuit is used to provide a level
shifting equal to the threshold defined by the characterization
current curve for the solenoid. The threshold is the voltage
equivalent of the difference between the peak and valley currents
defined in FIG. 4. The op-amp U2C is used as a buffer. The buffer
circuit ensures that the differential amplifier will not be loaded
by the circuit components in the adder circuit. The function of the
buffer U2C is to replicate the signal provided at its input to its
output. The buffer offers a high input impedance and a low output
impedance. The input of the buffer is the differential amplifier
(U2A) output. This signal will then appear at the output of the
buffer U2C.
[0040] The adder circuit is formed by op-amp U2D, resistors R18,
R19, R12 and R11. The threshold can be set by adjusting the values
of the resistors.
[0041] By selecting R11=R19 and R12=R18:
Output of U 2 D = I SENSE_AMP + ( V IN * R 11 R 12 )
##EQU00001##
[0042] This equation allows the values of the resistors to be
designed to add the required threshold to the output of the buffer.
This means that the threshold is added to the output of the buffer
to generate the level shifted waveform.
[0043] The comparator 616 shown in FIG. 6 is shown separately in
FIG. 9, generally as 900. The resistor R29 is used to avoid false
latching of the comparator U3 during power-on. The components
resistor R17 and capacitor C11 (see FIG. 6) also avoid power-on
latching of the comparator U3. The resistor R17 will pull the
inverting input of U3 to a positive voltage during power on, which
depends upon the value of R17 and R14 (see FIG. 6), to make sure
that the inverting input of U3 is greater than its non-inverting
input to make sure that there will be no latching of U3 during
power-on of the comparator.
[0044] In operation, the peak detector output is connected to the
non-inverting input of the comparator through the diode D3 and
resistor R28. The level shifted signal is connected to the
inverting input of the comparator. VIN is a supply voltage for the
comparator. The output of the comparator U3 will be a logic high,
equal to VIN, when the non-inverting input voltage of U3 is higher
than the inverting input voltage. Similarly, the output of the
comparator U3 would be zero when the non-inverting input voltage of
U3 is less than the inverting input voltage. The diode D8 will be
reverse biased when the output of U3 is zero. The output of the
comparator U3 is the signal
[0045] At the start of the solenoid energization, the peak detector
output voltage will be less than the level shifted signal.
Therefore, the output of comparator U3 will be zero. When the
solenoid plunger moves completely, the output of the peak detector
would be higher than the level shifted signal, which makes the
non-inverting input voltage of U3 higher than the inverting input
voltage. Therefore, the output of U3 goes high. That means, the
signal I.sub.TRIP goes to the voltage level VIN. This causes diode
D8 to be forward-biased and hence non-inverting input of U3 becomes
equal to the output voltage, which is VIN, minus the diode drop.
This ensures that once the output of the comparator goes high, the
non-inverting input is always higher than the inverting input and
therefore the output latches to high. In other words, on complete
movement of the solenoid plunger, the signal I.sub.TRIP goes from
zero to high and latches there. The latching of I.sub.TRIP at the
high-value ensures that any other monitoring circuit provided to
monitor I.sub.TRIP, will have enough time to process the signal.
The state of I.sub.TRIP at VIN implies that the plunger has moved
completely. The state of I.sub.TRIP at zero implies that the
plunger has not moved or the plunger is faulty.
[0046] FIG. 10 shows a detail of the working principle of the
circuit. The output of the differential amplifier U2A, which is the
amplified solenoid current, is fed to the peak detector. The peak
detector output tracks the solenoid current until point 1, where
the solenoid plunger starts moving. After this point, the solenoid
current decreases because of the back EMF generated by the solenoid
and the solenoid current dips to point 2. The output of the peak
detector will remain at a value equal to the peak value at point 1
and is fed to the non-inverting input of the comparator U3. The
inverting input of the comparator U3 is fed with the level shifted
solenoid current signal. This waveform is derived by level shifting
the solenoid current waveform by a voltage equal to the difference
between the peak current at point 1 and the valley current at point
2, the threshold. At point 2, the non-inverting input voltage of
the comparator (the peak detector output) becomes higher than the
inverting input voltage (the output of the op-amp adder circuit)
and the comparator output goes high and latches there.
[0047] Referring back to FIG. 3 which shows the current drawn by
the solenoid at different temperatures, remember that the curves
shift up as temperature decreases because of the reduction in the
resistance of the solenoid coil. As stated in the description of
FIG. 3, the difference between the peak and the valley of the
solenoid current dip due to the back EMF remains constant
irrespective of the temperature. The absolute difference between
I.sub.PEAK and I.sub.VALLEY is measured and used to define the
threshold for detection of plunger movement for a particular
solenoid. The non-inverting input of the comparator is the output
from the peak detector and the inverting input is the level shifted
solenoid current waveform. The active peak detector output
connected to the comparator ensures that the threshold is always
referred to the peak current which makes the logic more immune to
temperature variation. In FIG. 10, at point 2, in order to make
sure that the level shifted waveform falls below the peak detector
output, for the comparator trigger and latch, the threshold is
referenced slightly above I.sub.VALLEY. This ensures that the
comparator inverting input will not be higher than the noninverting
input voltage for sufficient time, such as more than 1 .mu.s, for
the comparator to act. When the solenoid current drops below the
predetermined threshold, the comparator output will go high
indicating that the plunger has moved completely.
[0048] FIG. 11 shows how the threshold is determined for a
solenoid. The threshold is found by using the excitation current
characteristics of a "healthy" solenoid. A typical solenoid is
tested, experimentally, to find out the current characteristics.
The rated DC voltage is applied across the solenoid, the excitation
current waveform is captured and the difference between the peak
and valley currents are measured as shown in FIG. 11.
[0049] Prolonged operation of solenoid can cause a plunger movement
to become slow due to factors such as friction, rusting and other
factors. For example, if the plunger is expected to move 10 mm
within 10 ms, and if it does not move at the same speed as
expected, the back EMF, generated in the solenoid coil, will be
less. Hence, the magnitude of the current dip will not be equal to
that of a "healthy" solenoid. As shown in FIG. 12, generally as
1200, when the plunger movement is slow, that is the time T2 is
greater than T1, the magnitude of the current dip I.sub.DIP2 will
be less than the current dip I.sub.DIP1, as shown in FIG. 12. This
will cause the waveform to have a delayed and lower dip as shown in
FIG. 12 at time T2, which causes the comparator output to remain at
zero. This can be utilized to detect a faulty solenoid.
[0050] Although the invention has been described in detail, it
should be understood that various changes, substitutions and
alterations, may be made thereto without departing from the spirit
or scope of the invention as defined by the appended claims.
* * * * *