Input circuit for an electronic vehicle speed control unit

An improved input circuit for an electronic vehicle speed control unit is disclosed. An ignition current sensor coil is adapted to have a voltage differential signal induced therein each time an ignition pulse is generated by the vehicle engine. Electrical noise signals generated by the vehicle engine appear as common mode signals across the ignition current sensor coil. The ignition current sensor coil is connected at each end to a capacitor, both of which are connected to ground potential. The ends of the ignition current sensor coil are further connected to the inputs of a differential amplifier. The capacitors appear in a series loop with the ignition current sensor coil for the differential voltage signals and, consequently, provide tuning for inductance. However, the capacitors appear in parallel with one another to ground potential with respect to the common mode noise signals, thereby providing high frequency noise rejection for the input circuit. The differential amplifier further rejects the common mode noise signal applied to the inputs thereof.

BACKGROUND OF THE INVENTION 
The present invention relates in general to vehicle speed control systems 
and in particular to an improved input circuit for an electronic vehicle 
speed control unit. 
Vehicle speed control units are widely known in the art and are adapted to 
maintain a vehicle at a constant predetermined speed despite varying 
engine loads, such as are imposed by the course of the road or the wind. 
Because of their reliability, accuracy, and inexpensiveness, 
fluid-actuated vehicle speed control units have become the dominant factor 
in the marketplace. Such speed control units typically utilize vacuum from 
the vehicle engine manifold as the actuating force. The vacuum is 
selectively supplied to a bellows connected to the engine throttle 
linkage, thereby increasing or decreasing the throttle position to advance 
or retard the vehicle speed. The control unit for such a system typically 
compares a signal representing the actual vehicle speed with a signal 
representing the desired vehicle speed and adjusts the vacuum level 
supplied to the bellows accordingly. 
Frequently, electronic control circuits are utilized to control the supply 
of vacuum to the bellows. However, such electronic control circuits are 
subject to unreliable operation or failure because of the large amount of 
electrical noise present in the vehicle engine. Such noise is typically 
generated by the vehicle ignition system and can adversely affect the 
signal representing the actual speed of the vehicle. Accordingly, it would 
be desirable to provide an input circuit for the electronic vehicle speed 
control unit which is resistant to such spurious noise signals. 
SUMMARY OF THE INVENTION 
The present invention relates to an improved input circuit for an 
electronic vehicle speed control unit. An ignition current sensor coil is 
adapted to have a voltage differential signal induced therein each time an 
ignition pulse is generated by the vehicle engine. Electrical noise 
signals generated by the vehicle engine appear as common mode signals 
across the ignition current sensor coil. The ignition current sensor coil 
is connected at each end to a capacitor, both of which are connected to 
ground potential. The ends of the ignition current sensor coil are further 
connected to the inputs of a differential amplifier. The capacitors appear 
in a series loop with the ignition current sensor coil for the 
differential voltage signals and, consequently, provide tuning for its 
inductance. However, the capacitors appear in parallel with one another to 
ground potential with respect to the common mode noise signals, thereby 
providing high frequency noise rejection for the input circuit. The 
differential amplifier further rejects the common mode noise signals 
applied to the inputs thereof. 
It is an object of the present invention to provide an improved input 
circuit for an electronic vehicle speed control unit. 
It is another object of the present invention to provide such an input 
circuit with a good common mode noise signal rejection capability. 
It is a further object of the present invention to provide such an input 
circuit including a differential amplifier. 
Other objects and advantages of the present invention will become apparent 
to those skilled in the art from the following detailed description of the 
preferred embodiment, when read in light of the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1, there is illustrated an improved input circuit for 
an electronic vehicle speed control unit (not shown). The input circuit is 
adapted to generate a signal representing the actual speed of the vehicle 
to the electronic vehicle speed control unit. An ignition current sensor 
coil 10 is provided which is adapted to be wound about an iron or ferrite 
ring or other toroidal core (not shown) in a manner described and 
illustrated in U. S. Pat. No. 4,495,913, issued Jan. 29, 1985, the 
disclosure of which is hereby incorporated by reference. When the ring is 
disposed about an ignition cable in the vehicle engine, a voltage 
differential is induced in the ignition current sensor coil 10 each time 
an ignition pulse is generated in the ignition cable by the vehicle 
ignition system. Thus, the ignition current sensor coil 10 provides a 
pulsating electrical voltage signal which is proportional in frequency to 
the actual speed of the vehicle engine. 
The ignition current sensor coil 10 is typically connected by a pair of 
conductors 11 to the input circuit of the electronic vehicle speed control 
unit. The conductors 11 typically extend in a twisted pair configuration 
about the vehicle engine from the ignition current sensor coil 10 to the 
input circuit. The input circuit includes a resistor 12, a resistor 14, a 
capacitor 16, a capacitor 18, a resistor 20, and a resistor 22, all of 
which are connected in series between a source of positive voltage +Vs and 
ground potential. One conductor 11 connected to the ignition current 
sensor coil 10 is connected to the junction between the resistor 14 and 
the capacitor 16, while the other conductor 11 connected to the ignition 
current sensor coil 10 is connected to the junction between the capacitor 
18 and the resistor 20. The junction between the capacitor 16 and the 
capacitor 18 is connected to ground potential and also to the anode of a 
diode 24. The cathode of the diode 24 is connected to the junction between 
the resistors 12 and 14 and also to the non-inverting input of a 
differential amplifier 26. The inverting input of the differential 
amplifier 26 is connected to the junction between the resistors 20 and 22 
and also to the anode of a Schottky diode 28. The cathode of the Schottky 
diode 28 is connected to the +Vs voltage source. 
The output of the differential amplifier 26 is connected through a 
capacitor 30 to ground potential. The output of the differential amplifier 
26 is also connected to the input of a conventional monostable 
multivibrator circuit 32. The output of the monostable multivibrator 
circuit 32 is connected through a resistor 34 to a capacitor 36 and a 
capacitor 38. The capacitor 36 is connected to ground potential, while the 
capacitor 38 is connected through a single pole, single throw switch 40 to 
ground potential. The resistor 34 is further connected to the junction 
between the anode of a diode 42, a resistor 44, and the cathode of a diode 
46. The cathode of the diode 42, the resistor 44, and the anode of the 
diode 46 are all connected to the junction between a capacitor 48 and a 
capacitor 50. The capacitor 48 is connected to ground potential, while the 
capacitor 50 is connected to the actual vehicle speed signal input of the 
electronic vehicle speed control unit (not shown). 
The above-described circuit elements 34 through 50 form a filter network 
which is described in detail in the patent previously referred to. The 
switch 40 is provided to selectively include or exclude the capacitor 38 
in the filter network. When the switch 40 is closed, the effective 
capacitance is increased, thereby providing a somewhat slower response in 
the filter network to signals of changing magnitude. Such slower response 
is desirable in certain instances, such as in vehicles having an automatic 
transmission, as opposed to a manual transmission. 
The resistors 12, 14, 20, and 22 are provided to generate bias signals to 
the inputs of the differential amplifier 26 at predetermined voltage 
levels defining a voltage range. It has been found desirable to select the 
resistors 12 and 22 of equal value, and further to select the resistors 14 
and 20 of equal value. Also, the values of the resistors 12 and 22 can be 
selected to be quite large with respect to the values of the resistors 14 
and 20. By selecting the values of the resistors 12, 14, 20, and 22 in 
this manner, the inverting and non-inverting inputs of the differential 
amplifier 26 will be biased at voltage levels which are less than and 
greater than, respectively, one-half of the voltage level of the +Vs 
voltage supply by a predetermined relatively small magnitude. 
The diodes 24 and 28 are provided to limit the voltage levels applied to 
the non-inverting and inverting inputs, respectively, of the differential 
amplifier 26. Thus, the minimum voltage which can be applied to the 
non-inverting input will be a negative voltage having a magnitude equal to 
the voltage required to forward bias the diode 24 (typically 0.6 volts) 
below ground potential. Similarly, the maximum voltage which can be 
applied to the non-inverting input will be a positive voltage having a 
magnitude equal to the voltage required to forward bias the Schottky diode 
26 (typically 0.2 volts) above the +Vs voltage source level. 
In operation, when an ignition pulse is generated in the ignition cable by 
the vehicle ignition system, a voltage differential is induced across the 
ignition current sensor coil 10. The capacitors 16 and 18 form a loop with 
the ignition current sensor coil 10 and provide tuning for its inductance. 
Thus, the capacitors 16 and 18 appear in series across the ignition 
current sensor coil 10 with respect to the induced voltage differential 
signal. Accordingly, a "ringing" sinusoidal voltage differential signal of 
exponentially diminishing magnitude will be generated across the ignition 
current sensor coil 10 each time an ignition pulse is generated. As 
described above, the ends of the ignition current sensor coil 10 are 
connected through the conductors 11 and the resistors 14 and 20 to the 
non-inverting and inverting inputs, respectively, of the differential 
amplifier 26. Since the inputs to the differential amplifier 26 are biased 
at predetermined voltage levels defining a voltage range, those portions 
of the "ringing" signal which have a greater voltage differential than the 
voltage range will cause the normally low output of the differential 
amplifier 26 to go high. The capacitor 30 filters the output signal of the 
differential amplifier 26 such that only a single pulse is applied to the 
input of the monostable multivibrator circuit 32 for each ignition pulse. 
As mentioned above, undesirable electrical noise is often generated by a 
vehicle engine, which noise can cause unreliable operation or failure of 
the speed control unit by distorting the actual vehicle speed signal. 
Despite their twisted pair configuration, the conductors 11 connecting the 
ignition current sensor coil 10 to the input circuit of the electronic 
vehicle speed control unit are vulnerable to such noise, since they are 
relatively lengthy and necessarily extend about the vehicle engine. One 
characteristic of such electrical noise, however, is that virtually 
identical noise signals will be generated simultaneously on each of the 
conductors 11. Thus, the noise signals are common mode signals with 
respect to the capacitors 16 and 18. The capacitors 16 and 18 appear in 
parallel with one another with respect to such common mode signals, since 
they are each connected to ground potential. The increased effective 
capacitance provides a short circuit for some of the high frequency 
components of the noise signals to ground potential. More importantly, 
however, since the common mode signals are superimposed on the voltage 
differential signals and applied to the inputs of the differential 
amplifier 26, the noise signals will have little effect, if any, on the 
output signal from the differential amplifier 26. This is because the 
differential amplifier 26, as is well known, is responsive only to input 
signals having different voltage levels. The diodes 24 and 28 provide a 
signal clamping means for the inputs of the differential amplifier 26, as 
described above, and prevent them from being driven beyond their common 
mode range. Thus, the input circuit of the present invention provides an 
effective means for rejecting of such common mode noise signals. 
Referring now to FIG. 2, there is illustrated a first alternate embodiment 
of the present invention. As illustrated therein, the input circuit 
includes an ignition current sensor coil 52 which is connected by a pair 
of conductors 54 to the input circuit as described above. The input 
circuit includes a plurality of resistors 56, 58, 60, 62, 64, and 66, all 
of which are connected in series between a source of positive voltage +Vs 
and ground potential. One conductor 54 connected to the ignition current 
sensor coil 52 is connected to the junction between the resistor 58 and 
the resistor 60, while the other conductor 54 connected to the ignition 
current sensor coil 52 is connected to the junction between the resistor 
62 and the resistor 64. A capacitor 68 is connected between the two 
conductors 54. The junction between the resistor 60 and the resistor 62 is 
connected through a capacitor 70 to ground potential. The junction between 
the resistor 56 and the resistor 58 is connected to the cathode of a diode 
72 and also to the non-inverting input of the above-discussed differential 
amplifier 26. The anode of the diode 72 is connected to ground potential. 
The junction between the resistor 64 and the resistor 66 is connected to 
the anode of a Schottky diode 74 and also to the inverting input of the 
differential amplifier 26. The cathode of the Schottky diode 74 is 
connected to the +Vs voltage source. The output of the differential 
amplifier 26 is connected to the circuit elements 34 through 50 of the 
filter network as described above. 
In operation, when an ignition pulse is generated in the ignition cable by 
the vehicle ignition system, a voltage differential is induced across the 
ignition current sensor coil 52. The capacitor 68 and the resistors 60 and 
62 form a tank circuit with the ignition current sensor coil 52. The 
capacitor 68 provides tuning for the inductance of the ignition current 
sensor coil 52, thus producing a "ringing" sinusoidal voltage differential 
signal as described above. The resistors 60 and 62 appear in series for 
such voltage differential signal and tend to damp the "ringing" thereof 
considerably. As described above, those portions of the "ringing" signal 
which have a greater voltage differential than the voltage range 
determined by the resistive voltage divider network consisting of the 
resistors 56 through 66 will cause the normally low output of the 
differential amplifier 26 to go high. 
The resistors 60 and 62 appear in parallel with respect to common mode 
noise signals generated on the conductors 54. Thus, their effective 
resistance is diminished somewhat. For example, if the resistors 60 and 62 
are of equal value, their effective resistance with respect to the common 
mode noise signals will be one half of that value. Since the resistors 60 
and 62 are connected through a capacitor 70 to ground potential, the high 
frequency components of the common mode signals will be shorted to ground 
potential. Additionally, the remaining portions of the common mode noise 
signals will have little effect on the output signal from the differential 
amplifier 26 for the reasons described above. 
Referring now to FIG. 3, there is illustrated a second alternate embodiment 
of a portion of the improved input circuit illustrated in FIG. 1. As shown 
therein, a center-tapped ignition current sensor coil 76 is connected at 
its ends by a pair of conductors 78 to the input circuit of the electronic 
vehicle speed control unit. The center tap of the ignition current sensor 
coil 76 is connected by a third conductor 80 to the input circuit. The 
input circuit includes a resistor 82, a resistor 84, a capacitor 86, a 
resistor 88, and a resistor 90, all of which are connected in series 
between a source of positive voltage +Vs and ground potential. One 
conductor 78 connected to one end of the ignition current sensor coil 76 
is connected to the junction between the resistor 84 and the capacitor 86, 
while the other conductor 78 connected to the other end of the ignition 
current sensor coil 76 is connected to the junction between the capacitor 
86 and the resistor 88. The third conductor 80 is connected through a 
capacitor 92 to ground potential. The junction between the resistor 82 and 
the resistor 84 is connected to the cathode of a diode 94 and also to the 
non-inverting input of the above-discussed differential amplifier 26. The 
anode of the diode 94 is connected to ground potential. The junction 
between the resistor 88 and the resistor 90 is connected to the anode of a 
Schottky diode 96 and also to the inverting input of the differential 
amplifier 26. The cathode of the Schottky diode 96 is connected to the +Vs 
voltage source. The output of the differential amplifier 26 is connected 
to the above-described elements 34 through 50 of the filter network. 
In operation, when an ignition pulse is generated in the ignition cable by 
the vehicle engine ignition system, a voltage differential is induced 
across the ignition current sensor coil 76. The capacitor 86 forms a loop 
with the ignition current sensor coil 76 and provides tuning for its 
inductance. The capacitor 92 provides a short circuit to ground potential 
for the high frequency components of the common mode noise signals. More 
importantly, however, the common mode noise signals are applied to the 
inputs of the differential amplifier 26 as described above, having little 
or no effect on the output signal therefrom. 
In accordance with the provisions of the patent statutes, the principle and 
mode of operation of the present invention have been explained and 
illustrated in its preferred embodiment. However, it must be appreciated 
that the present invention can be practiced otherwise than as specifically 
explained and illustrated without departing from its spirit or scope.