Precision speed switch control

A speed switch control comprising a subfractional permanent magnet AC generator generating an AC signal varying in amplitude with saft speed variations; the AC signal is rectified and compared with a regulated DC reference to generate first and second threshold signals indicative of shaft speeds over and under a critical value. A switch actuator circuit develops "on" and "off" switch actuator signals, from the threshold signals to actuate a two-terminal solid state switch between on and off conditions; the "off" signal is continuous but the "on" signal is a high-duty-cycle semi-continuous signal including brief recurring "off" intervals. A power storage/supply circuit, connected in parallel with the switch terminals, which recharges during switch "off" intervals, affords the power supply for the threshold and switch actuator circuits.

BACKGROUND OF THE INVENTION 
There are numerous applications in which it is necessary or desirable to 
control some safety interlock or other device in accordance with the speed 
of a rotating shaft. Vehicular applications are common; for example, a 
control to latch or close the doors of a passenger vehicle whenever the 
vehicle is moving above a predetermined threshold speed, or an 
electrically actuated interlock mechanism to preclude shifting the 
transmission into reverse whenever the vehicle is moving forward at even a 
limited threshold speed, or vice versa. Similar speed switch control 
needs, based upon the rotational speed of a shaft or like rotary member, 
are also commonly encountered in machine tools and other industrial 
equipment. 
The requirements imposed upon speed switch controls, particularly those 
employed in vehicular applications, are frequently quite severe. Thus, the 
control may be subjected to high levels of vibration and to substantial 
shock forces. Electrical transients of substantial magnitude may be 
encountered. Because the control does not perform a primary operational 
function, it is frequently subject to severe cost limitations. In 
addition, it is highly desirable that the electrical connections to the 
control be as simple as possible, preferably constituting a simple 
two-terminal connection, to minimize cost and to facilitate replacement 
when necessary. 
A variety of different speed switch controls have been devised for use in 
applications of this kind; many start with an input signal derived from a 
small AC generator driven by the shaft or like rotary member being 
monitored. The circuits of these devices have often been undesirably 
complex and costly, particularly when operation is based upon the 
frequency of the AC input signal, requiring a frequency/voltage conversion 
stage as a part of the control circuitry. These controls are difficult to 
construct in a form rugged enough for vehicular applications, in large 
part due to the circuit complexities introduced by frequency/voltage 
conversion. Moreover, many of these speed switch controls require three or 
more terminal connections. These problems are particularly acute in speed 
controls applied to vehicles. 
Inexpensive precision two-terminal speed switch controls are described in 
the aforementioned co-pending United States applications of M.A. Lace, 
Ser. Nos. 732,332 (now U.S. Pat. No. 4,086,647) and 745,453 (now 
abandoned). But those controls are not satisfactory for critical speeds 
below ten revolutions per minute, where the output amplitude of the AC 
generator is quite small. This is particularly true in passenger vehicle 
safety control applications, where a shaft speed of two rpm or even less 
may constitute the critical speed. 
Moreover, a number of other important operating characteristics have been 
difficult and sometimes impossible to realize with previously known speed 
switch controls. Thus, it is highly desirable to provide a single basic 
speed switch control circuit that can be readily converted from operation 
as a normally open switch to operation as a normally closed switch, and 
vice versa, to meet the varying requirements of different safety devices 
and other loads. It is equally desirable to have a single basic speed 
switch control circuit that is capable of operation over a broad range of 
critical rotational speeds, from nearly zero rpm to hundreds of 
revolutions per minute, to minimize custom design of circuits to fit 
individual applications. Another critical requirement, in many 
applications, is the limination of "hunting" when the rotational speed of 
the input shaft is subject to substantial variation over a brief period of 
time. In addition, a practical and effective precision speed switch 
control should require only a low power drain but should be capable of 
handling relatively high currents, so that it can be readily adapted to a 
variety of different specific applications. 
SUMMARY OF THE INVENTION 
It is a principal object of the present invention, therefore, to provide a 
new and improved solid state electronic speed switch control, utilizing an 
input signal from a subfractional permanent magnet AC generator driven by 
a rotating shaft or like rotating member, that effectively overcomes or 
minimizes the problems and difficulties of previously known controls as 
described above. 
A particular object of the invention is to provide a new and improved speed 
switch control for a rotary shaft, employing simple solid state circuits 
that require only two terminals for both load and power supply 
connections, that affords precision operation over a broad speed range 
extending down to speeds as low as one or two rpm. 
Another specific object of the invention is to provide a new and improved 
speed switch control that is readily convertible from operation as a 
normally open switch to operation as a normally closed switch, and vice 
versa, by a simple interchange of two circuit connections. 
Another object of the invention is to provide a precision speed switch 
control, having a broad range of operating speeds, that can be readily 
adjusted for different dropout delays, the dropout delay constituting the 
time interval between deceleration of an input shaft below a critical 
speed and the actual reversion of the switch to an original operating 
condition. 
Accordingly, the invention relates to a two-terminal precision speed switch 
control actuated by changes in the rotational speed of a shaft and 
adaptable to operation over a broad speed range down to less than tem rpm. 
The control comprises a sub-fractional AC generator, connectible to a 
rotary shaft, for generating an AC signal having an amplitude which varies 
with changes in shaft speed; a threshold circuit is connected to the 
generator and develops first and second threshold signals, one indicative 
of an AC signal input exceeding a given threshold amplitude corresponding 
to a critical shaft speed and the other indicative of an AC signal input 
below the threshold amplitude. A switch actuator circuit is coupled to the 
threshold circuit, for developing ON and OFF switch actuation signals 
corresponding to the first and second threshold signals, the OFF signal 
being a continuous DC signal of high duty cycle including brief recurring 
OFF intervals. A solid-state switching circuit, having two switch 
terminals connectible in series with an external power supply in an 
operating circuit for a controlled load, has its actuation input connected 
to the switch actuator circuit, actuatable to an "on" condition in which 
the impedance across the switch terminals is very low, in response to the 
ON switch actuation signal, and actuatable to an "off" condition in which 
the impedance across the switch terminals is very high, in response to the 
OFF switch actuation signal. A power storage/supply circuit, is connected 
in parallel with the switch terminals, affording a power supply for the 
threshold circuit and the switch actuator circuit, and including a storage 
device which is re-charged during intervals in which the switching circuit 
is in its "off" condition.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 provides a block diagram of a precision speed switch control 10 
which incorporates some of the principles of the present invention. 
Control 10 comprises a multi-pole sub-fractional permanent magnet AC 
generator 11 having an input shaft 19. In a given application, by way of 
example, shaft 19 may be driven from the speedometer cable, the drive 
shaft, or some operating element in the transmission of a vehicle. On the 
other hand, shaft 19 may also be driven from the operating shaft of a 
machine tool or other industrial machine that must be monitored for a 
safety function or other control purpose. The preferred construction for 
generator 11 is the sixty pole permanent magnet AC generator disclosed and 
claimed in the co-pending U.S. patent application of M.A. Lace, Ser. No. 
729,272 filed Oct. 4, 1976, now U.S. Pat. No. 4,074,157. 
The output of the AC generator 11 is electrically connected to a threshold 
circuit 12 that rectifies the AC input and that develops an output 
comprising two different threshold signals. A first threshold signal is 
generated by circuit 12 in response to an AC signal input below a given 
threshold amplitude corresponding to a critical speed for shaft 19. The 
second threshold signal output from circuit 12 indicates an AC signal 
input corresponding to a shaft speed exceeding the critical speed. The 
output from threshold circuit 12 is supplied to a switch actuator circuit 
21. 
Actuator circuit 21 develops OFF and ON switch actuation signals 
corresponding to the first and second threshold signals from circuit 12. 
These switch actuation signals are in turn applied to a solid state 
switching circuit 13. Circuit 13 has two switch terminals 14 and 15, shown 
as connected to a pair of normally open switch contacts 16. Contacts 16 
are included in FIG. 1 merely for purposes of illustration; the solid 
state switch 13 actually does not include any mechanically actuatable 
contacts, but uses a solid state circuit to perform an equivalent 
switching function. In fact, a mechanical switch would not be usable for 
contact 16, due to inertia in operation, as will be apparent from the 
operational description set forth hereinafter. 
Switch terminals 14 and 15 are electrically connected in series with an 
external power supply 17 and a load 18. In a vehicular application, power 
supply 17 may comprise the vehicle battery. In an industrial application, 
a power supply circuit energized from a local utility or other source may 
be employed. An internal power storage/supply circuit 22 is connected in 
parallel with the switch terminals 14 and 15 and affords a power supply 
for both the threshold circuit 12 and the switch actuator circuit 21. 
Load 18 is most frequently an alarm, a safety interlock circuit, or some 
other safety device. For example, in a bus or other passenger vehicle, 
load 18 may comprise an electrically-actuated interlock to close or at 
least to prevent opening of a door whenever the vehicle is moving above 
some critical speed. In some instances, that critical speed may be 
represented by a very low speed for shaft 19, such as one or two rpm. In 
another vehicular application, load 18 may comprise an interlock to 
prevent shifting of the transmission of the vehicle into reverse gear when 
the vehicle is moving in a forward direction, or vice versa. Again, the 
critical speed for shaft 19 at which the safety device of load 18 must be 
operated may be quite low. Of course, load 18 may also comprise a simple 
visual or audible alarm in either industrial or vehicular applications. 
In the operation of the speed switch control 10 of FIG. 1, generator 11 
develops an initial AC signal having an amplitude generally representative 
of the rotational speed of its shaft 19. The relationship between the 
amplitude of the output signal from generator 11 and the speed of shaft 19 
is seldom truly linear. More commonly, the peak-to-peak output voltage of 
generator 11 usually conforms to a characteristic similar to curve 51 in 
FIG. 5, with the voltage rising rapidly for low and moderate speeds and 
increasing much more gradually at higher speeds. However, over an initial 
speed range, in this instance from about zero to nearly 400 rpm, the 
speed-voltage curve 51 is a close approximation to a linear relationship, 
so that the output amplitude of the AC signal from generator 11 is 
essentially representative of the rotational speed of shaft 19. 
The AC signal from generator 11 (FIG. 1) is rectified in threshold circuit 
12 and is utilized in that circuit to develop first and second threshold 
signals, one indicative of shaft speeds exceeding a critical level and the 
other indicative of shaft speeds below that critical level. These two 
threshold signals are supplied to switch actuator circuit 21, which 
develops ON and OFF switch actuation signals corresponding to the first 
and second threshold signals, respectively. These ON and OFF output 
signals are illustrated in FIG. 4. As shown therein, the OFF signal is a 
continuous DC signal, whereas the ON signal is a semi-continuous DC signal 
of high duty cycle including a number of recurring OFF intervals. The ON 
and OFF signals from actuator circuit 21 are applied to the solid state 
switching circuit 13. 
When the OFF actuator signal from circuit 21 is being supplied to switching 
circuit 13, the switching circuit is actuated to an "off" condition in 
which the impedance across the switch terminals 14 and 15 is very high, 
comparable to an open condition for a set of mechanical switching contacts 
such as the contacts 16. For this high impedance "off" condition of 
circuit 13, the power storage/supply circuit 22 is continuously charged. 
Circuit 22 supplies suitable operating voltages to both threshold circuit 
12 and switch actuator circuit 21. Preferably, the storage/supply circuit 
22 incorporates a voltage regulator to maintain a constant supply level to 
threshold circuit 12 regardless of variations in the output voltage from 
the external power supply 17. 
Whenever solid state switching circuit 13 is actuated to its "on" 
condition, however, the impedance across its switch terminals 14 and 15 is 
very low, corresponding essentially to the operating condition for a 
mechanical switch with the switch contacts 16 closed. Because the 
storage/supply circuit 22 is connected in parallel with switch terminals 
14 and 15, it receives little or no power input from the external power 
supply 17 under these conditions. Consequently, circuit 22 would shortly 
become ineffective if the switch "on" condnition were maintained 
continuously for switching circuit 13 for any substantial period of time. 
This is the reason for the recurring OFF intervals in the switch actuating 
signal applied to circuit 13 from actuator circuit 21 for the "on" 
condition of the switching circuit. During these brief OFF intervals (FIG. 
4) a capacitor or other storage device in circuit 22 is recharged, and 
this enables the circuit to provide a continuous power supply for 
threshold circuit 12 and actuator circuit 21. 
FIG. 2 affords a schematic diagram of a speed switch control 30 
corresponding to the control 10 illustrated in FIG. 1. In control 30, a 
subfractional permanent magnet AC generator 11 driven from an external 
shaft 19 has one output terminal connected to system ground with the other 
output terminal connected through a rectifier diode D1 to the 
non-inverting input terminal 36 of a first operational amplifier A1 in a 
threshold circuit 12A. A voltage-regulating Zener diode Z1 is connected 
from terminal 36 to system ground, in parallel with a resistor R1 and a 
capacitor C1. 
The inverting input 32 of amplifier A1 is connected to a voltage divider 
comprising two resistors R2 and R3. Resistor R3 is returned to system 
ground. Resistor R2 is connected to an output line 33 from a power 
storage/supply circuit 22A. Power supply connections are also provided for 
amplifier A1, from line 33 and to system ground. 
The power storage/supply circuit 22A of speed switch control 30, FIG. 2, 
comprises a resistor R4 connected in series from the output line 33 to an 
input line 34 that is connected through a blocking diode D4 to switch 
terminal 14 of a solid state switching circuit 13A. A Zener diode Z2 is 
connected from output line 33 to system ground. A storage capacitor C2 is 
connected from input line 34 to system ground. 
The switch actuator circuit 21A in the embodiment of FIG. 2 comprises an 
operational amplifier A2 having a power supply connection to output line 
33 of circuit 22A and another power supply connection to system ground. 
The noninverting input 35 of amplifier A2 is connected to the center 
terminal 35 of a voltage divider comprising two resistors R7 and R9. 
Resistor R7 is connected to the output of amplifier A1 in threshold 
circuit 12A. Resistor R9 is returned to system ground. A feedback resistor 
R8 is connected from the outut of amplifier A2 back to input 35. 
The inverting input 38 of amplifier A2 is connected to a capacitor C3 that 
is returned to system ground. There is also a feedback circuit from the 
output of amplifier A2 to input 38. This is a parallel circuit comprising, 
in one branch, a resistor R6, and in the other branch, the series 
combination of a resistor R5 and a diode D5. 
The solid state switching device 13A of speed switch control 30, FIG. 2, is 
a dual transistor Darlington amplifier. The input to switch 13A comprises 
a series resistor R10 connected from the output of amplifier A2 in 
actuator circuit 21A to the base of the first transistor in the Darlington 
amplifier. The collector and emitter of the second transistor provide the 
switch terminals 14 and 15. A transient protection circuit, comprising a 
Zener diode Z3 and a parallel capacitor C4, are connected from switch 
terminal 14 to the input of the Darlington amplifier. 
The external circuit connected to the speed switch control 30 in FIG. 2 
comprises a power supply 17 having its negative terminal connected to 
switch terminal 15, which is also connected to system ground. The positive 
terminal of power supply 17 is connected to load 18, generally indicated 
as a load resistor RL, which is in turn connected to the other switch 
terminal 14. 
In considering the operation of speed switch control 30, it may first be 
assumed that the shaft 19 driving AC generator 11 is not rotating so that 
there is no AC output signal from the generator. Under these 
circumstances, there is no effective signal at the input 36 of amplifier 
A1. The only effective input to amplifier A1 is a positive DC signal at 
the inverting input 32 of the amplifier. As a consequence, the output from 
amplifier A1 is held steady at about ground potential. 
Switch actuator circuit 21A is a conventional Schmitt trigger pulse 
generator, which requires a positive voltage at terminal 35 to produce an 
effective output signal. Consequently, the output from amplifier A2 is at 
about system ground, affording the continuous OFF signal shown in FIG. 4. 
For this output from amplifier A2, the Darlington amplifier 13A is cut 
off, with both transistors non-conductive, so that there is a very high 
impedance across switch terminals 14 and 15; the switch 13A is effectively 
open. With the switch open, capacitor C2 is charged from the external 
power supply 17. Zener diode Z2 maintains a steady operating voltage on 
line 33, regardless of fluctuations in the power supply 17 or the charge 
on capacitor C2. 
If the shaft 19 driving generator 11 now begins to rotate, a 
positive-polarity DC signal, increasing in amplitude with increasing 
speed, is developed in the input circuit connecting the AC generator to 
the non-inverting input 36 of amplifier A1. As long as the voltage at 
terminal 36 remains below the voltage at terminal 32, there is no 
essential change in operating conditions because the output from the 
differential amplifier A1 remains at about ground potential. It should be 
noted that the reference voltage at terminal 32 remains constant due to 
the effect of the voltage regulator, circuit 22A. 
When the rotational speed of generator 11 increases to a point at which the 
voltage at terminal 36 exceeds that at terminal 32, the output from 
amplifier A1 goes positive. This provides a positive input to the 
non-inverting input 35 of amplifier A2. As a consequence, the monostable 
trigger circuit 21A produces a positive output signal corresponding to the 
ON signal shown in FIG. 4, with recurring brief negative-going OFF 
intervals. Whenever the actuation signal from circuit 21A is positive, the 
switching circuit 13A is driven fully conductive, affording a very low 
impedance across the switch terminals 14 and 15 and energizing safety 
device 18 from power supply 17. 
During the extended intervals that the switching circuit 13A is conductive, 
the charge on capacitor C2 maintains the requisite output voltage on line 
33 to maintain amplifiers A1 and A2 in operation. Moreover, the reference 
voltage at terminal 32 is also maintained constant. During the "on" 
condition for control 30, capacitor C2 is recharged during those brief 
intervals in which the switching device 13A is cut off, by the OFF 
interval signals indicated in FIG. 4. Thus, control 30 can maintain 
operation in an "on" condition for an indefinite period. The duty cycle 
for switch actuator circuit 21A, and hence for switching circuit 13A, can 
be maintained at a very high level, usually 95% or more. Typically, the 
pulse frequency for the OFF intervals, FIG. 4, may be of the order of 300 
to 400 Hz, depending upon the circuit parameters and the voltage of power 
supply 17, with the period T1 ranging from about 2.5 to 3.5 milliseconds 
and the interval duration T2 being of the order of 75 microseconds. 
If generator shaft 19 now slows down, below the critical speed for control 
30, so that the voltage at terminal 36 drops below the voltage at terminal 
32, the output of amplifier A1 again drops to about ground potential. As a 
consequence, the voltage at terminal 35 drops below the level necessary to 
sustain operation of the pulse generator circuit 21A. The output of 
amplifier A2 drops to near system ground and switching device 13A is again 
cut off. However, this dropout action does not take place immediately when 
the generator slows to just below the critical speed. Instead, there is 
some dropout delay, determined by resistor R1 and capacitor C1. 
During operation of speed switch control 30, FIG. 2, Zener diode Z1 limits 
the input voltage applied to amplifier A1 from generator 11, through diode 
D1, and prevents damage to the amplifier. This is necessary in many 
applications, since the output voltage from generator 11 may reach 
relatively high values as shown in FIG. 5. 
The critical speed for generator 11 that is utilized to determine whether 
control 30 maintains its switch 13A "on" or "off" is determined by the 
ratio of resistors R2 and R3. By changing one of these resistors, a 
substantially different critical speed can be established. Of course, a 
variable resistors could be utilized if desired. On the other hand, to 
change the control from a normally open switch to a normally closed 
switch, it is only necessary to interchange the input connections to 
amplifier A1. Thus, if the connections for terminals 32 and 36 of 
amplifier A1 are reversed, control 30 will function as a normally closed 
switching device, with operation otherwise unchanged from that described 
above. By the same token, the dropout delay time during which the switch 
13A is maintained actuated after shaft 19 of generator 11 falls below the 
critical speed can be readily adjusted by changing the resistor R1 or the 
capacitor C1. 
In order to afford a more complete and concrete example of a speed switch 
control constructed in accordance with the present invention and utilizing 
the circuit 30 of FIG. 2, specific parameters are set forth below in Table 
I. With the values shown in Table I, speed switch control 30 functions as 
a normally open switch having a critical speed for the shaft of generator 
11 of one rpm, assuming a power supply 17 having a voltage of 6-40 volts. 
TABLE I 
______________________________________ 
R1, R2 1 megohm 
R3 10 kilohms 
R4 1 kilohm 
R5 47 kilohm 
R6 4.7 megohms 
R7, R8, R9 100 kilohms 
R10 2.2 kilohms 
C1 1 microfarad 
C2 22 microfarad 
C3 .001 microfarad 
C4 680 picofarad 
Z1, Z2 5.1 volts 
13A 2N6385 
A1, A2 LM358 
D1, D4 IN 5059 
D5 IN 4148 
______________________________________ 
To illustrate the range of critical speeds that can be attained merely by 
changing resistor R3, Table II correlates the critical speed with various 
values of resistor R3. 
TABLE II 
______________________________________ 
R3 (kilohms) 
10 47 100 220 470 1000 
Critical speed (rpm) 
1 2.5 4.5 9 11 25 
______________________________________ 
To afford a more complete illustration of the wide range of dropout time 
values that can be attained with limited changes in the parameters of 
resistor R1 and capacitor C1, Table III is provided. 
TABLE III 
______________________________________ 
R1 C1 Drop Out 
(megohms) (microfarads) Time (Sec.) 
______________________________________ 
0.015 1.0 instantaneous 
0.1 1.0 0.3 
0.22 1.0 1.0 
0.47 1.0 1.5 
1.0 1.0 2.6 
2.2 1.0 4.5 
4.7 1.0 7.0 
10.0 1.0 9.0 
.infin. 1.0 14.0 
3.3 3.3 22.0 
10.0 3.3 33.0 
.infin. 3.3 52.0 
10.0 10.0 90.0 
.infin. 10.0 137.0 
______________________________________ 
The data in Table III was determined with an external power supply 17 of 
twelve volts and a load current, through R.sub.L, of approximately two 
amperes. 
Speed switch control 30, constructed with the circuit values of Table I, 
provides a normally open switch with a critical shaft speed of one rpm, 
highly suitable for use in a safety interlock control for the doors of a 
passenger vehicle such as a bus. However, the same circuit, with only 
limited modifications, can be applied to an entirely different 
application. Thus, to change control 30 to function as a normally closed 
switch having a critical speed of 667 rpm, only the following changes need 
be made: 
Change resistor R2 to 680 kilohms. 
Change resistor R3 to 330 kilohms. 
Add a feedback resistor of 4.7 megohms from the output of amplifier A1 to 
terminal 32. 
Reverse the connections of input terminals 32 and 36 of amplifier A1. 
Change resistor R10 to 22 kilohms. 
Add an additional transistor, type MPS-A43, as an input stage to switching 
circuit 13A, maintaining the Darlington configuration. 
Speed switch control 30, FIG. 2, while performing well in many 
applications, is sensitive to changes in load conditions. That is, changes 
in the external circuit comprising power supply 17 and load 18 may affect 
the performance more than is desirable. This difficulty is effectively 
eliminated in the speed switch control circuit 40 illustrated in FIG. 3, 
which provides other additional advantages as well. Thus, the circuit 40 
of FIG. 3 provides more precise voltage regulation, allowing greater 
precision in control. In addition, it incorporates a single adjustable 
resistor that allows for changes in the critical speed, over a broad range 
of approximately one rpm to 500 rpm, without change of other circuit 
components. 
Speed switch control 40, FIG. 3, is arranged for normally-closed switching 
operation. The AC generator 11 has one terminal connected to a resistor 
R11 and the other terminal connected to system ground; a 
voltage-regulating Zener diode Z11 is connected from the other terminal of 
resistor R11 back to ground. A series capacitor C11 and a diode D12 
connect resistor R11 to the inverting input 42 of an operational amplifier 
A1 in the threshold circuit 12B. An adjustable resistor R12 is connected 
between the common terminal of capacitor C11 and diode D12 and system 
ground, in parallel with a diode D11. Diodes D11 and D12 constitute a 
conventional voltage doubler circuit. A capacitor C12 and a resistor R13 
are connected in parallel from terminal 42 to system ground. 
The non-inverting input 46 of amplifier A1 is connected to a voltage 
divider comprising two resistors R14 and R15 connected between the output 
line 43 of a power storage/supply circuit 22B and system ground. 
Appropriate power supply connections are also made to amplifier A1 from 
line 43 and from system ground. A feedback resistor R16 is connected from 
the output terminal 49 of amplifier A1 back to input terminal 46. The 
amplifier output terminal 49 is also connected to a resistor R17 in turn 
connected to line 43. 
The power storage/supply circuit 22B comprises a transistor Q11 having its 
emitter connected to output line 43 and having its input, line 44, 
connected through a blocking diode D4 to one switch terminal 14 of a solid 
state switch 13B, which again constitutes a Darlington amplifier. The base 
of transistor Q11 is connected to system ground through the series 
combination of a diode D13 and a Zener diode Z12. A resistor R18 is 
connected between the base and collector of transistor Q11. A storage 
capacitor C13 is connected from input 44 to system ground. 
In speed switch control 40, the switch actuator circuit 21B includes an 
operational amplifier provided with power supply connections to line 43 
and to system ground. The non-inverting input 45 of amplifier A2 is 
connected through a resistor R19 to a terminal 48 that is directly 
connected to the line 44. Resistor R19 is a part of a voltage divider 
which includes an additional resistor R21 that is returned to system 
ground. Terminal 45 is also connected through a diode D14 to terminal 49. 
In actuator circuit 21B, terminal 48 is further connected through a 
resistor R20 to the inverting input 47 of amplifier A2. Input terminal 47 
is also connected to a Zener diode Z13 that is returned to system ground. 
The switching deivce 13B of FIG. 3 remains unchanged, in its construction 
and external connections, from the device 13A shown in FIG. 2. This 
includes the external connections to the power supply 17 and the load 
safety device 18 through the switch terminals 14 and 15, as well as the 
input connection through resistor R10. 
In considering the operation of speed switch control 40, FIG. 3, it may be 
assumed that the control is incorporated in a safety circuit for a vehicle 
and that a switch SW in the load circuit is closed as an incident to 
actuation of the ignition switch for the vehicle, at a time when the shaft 
19 of generator 11 is stationary. When switch SW is closed, capacitor C13 
is charged from power supply 17, through diode D4, and transistor Q11 in 
circuit 22B begins to conduct. As the charge on capacitor C13 builds up, 
the voltage at terminal 48 in actuator circuit 21B rises. The voltage at 
terminal 45 increases proportionally, depending upon the ratio of the 
voltage divider R19,R21. Usually, the two resistors R19 and R21 are 
approximately equal so that the voltage at terminal 45 is approximately 
one-half the voltage at terminal 48. Initially, the parallel circuit from 
terminal 48 to ground, through resistor R20 and Zener diode Z13, does not 
affect the voltage at terminal 45 because the Zener diode is 
non-conductive. 
As the voltage at terminal 48 rises, the corresponding increase in the 
voltage at terminal 47 ultimately reaches the breakdown level for diode 
Z13. Once terminal 47 reaches that level, the Zener diode holds terminal 
47 at its breakdown voltage. Moreover, there is now a voltage drop across 
resistor R20. With Zener diode Z13 conducting, the voltage at terminal 48 
exceeds the voltage at terminal 47 by the drop across resistor R20. 
The voltage of power supply 17 is substantially higher than the breakdown 
voltage of diode Z13. Consequently, as capacitor C13 continues to charge, 
the voltage at terminal 48 continues to increase, as does the voltage at 
terminal 45, until the voltage at terminal 45 exceeds the voltage at 
terminal 47. At this point, amplifier A2, which has previously had an OFF 
output, at approximately ground potential, develops an ON output which, as 
applied to solid state switch 13B through resistor R10, drives the 
Darlington amplifier switch conductive. When this occurs, the charging 
circuit for capacitor C13 is effectively shunted to ground through the 
very low impedance afforded by the output transistor of device 13B. 
The charge previously stored in capacitor C13 maintains transistor Q11 
conductive and also maintains amplifier A2 in the operating condition to 
produce an ON actuation signal. However, the charge on capacitor C13 
slowly dissipates. As it does, the potential at terminal 45 is gradually 
reduced and ultimately falls back below the potential at terminal 47, 
which remains at the breakdown voltage for diode Z13. When the voltage at 
terminal 45 drops below that at terminal 47, amplifier A2 is cut off and 
its output drops to near ground potential, with the result that device 13B 
is switched "off". This produces a brief OFF interval (FIG. 4) during 
which capacitor C13 recharges to a point at which the voltage at terminal 
45 again exceeds that at terminal 47, when the control returns to its "on" 
condition. Thus, the ON signal output of switch actuator 21B again 
corresponds to that shown in FIG. 4, and this is the initial operating 
condition for the actuator circuit, making control 40 a normally closed 
switch control. 
To establish control 40 in its "off" condition it is necessary to maintain 
the voltage at terminal 45 in actuator circuit 21B below the voltage of 
terminal 47. For this condition, the output from actuator circuit 21B 
corresponds to the OFF signal of FIG. 4. This action is effected by 
threshold circuit 12B and the connection from that circuit to actuator 
circuit 21B afforded by diode D14. 
When control 40 is first placed in operation, with shaft 19 stationary, 
there is a constant input signal to terminal 46 from the regulated source 
afforded by circuit 22B, line 43, and voltage divider R14,R15. There is no 
effective input at the other terminal 42 of amplifier A1. In effect, the 
output terminal 49 of amplifier A1 remains at the power supply potential 
of line 43, due to the presence of the connecting resistor R17, and any 
current flow from terminal 45 to terminal 49 through diode D14 is 
precluded. 
When shaft 19 now begins to rotate, as when movement of the vehicle is 
started, a positive input signal is applied to terminal 42 of amplifier A1 
through the rectifier circuit comprising voltage doubler D11,D12, the 
applied voltage increasing with increasing speed. When the voltage at 
terminal 42 reaches the same level as the voltage at terminal 46, the 
output terminal 49 of amplifier A1 goes to approximately ground potential. 
This allows a current flow from terminal 45 through diode D14 to terminal 
49, so that terminal 45 is also driven to near ground potential. This 
maintains amplifier A2 in an operating condition in which its output is 
essentially at ground, the OFF signal of FIG. 4, actuating switch 13B to 
its "off" condition. 
Initially, the voltage at terminal 46 in threshold circuit 12B is fixed, 
since the voltage on line 43 is well regulated by the power storage/supply 
circuit 22B. However, as the voltage at the amplifier output terminal 49 
rises, the feedback resistor R16 causes some increase in the potential at 
terminal 46. This provides a limited hysteresis effect in the operation of 
threshold circuit 12B. 
As in the previously described embodiment, threshold circuit 12B affords a 
dropout delay time, in this instance determined by the parameters selected 
for capacitors C12 and resistor R13. The use of the voltage coupler 
circuit D11,D12 increases the sensitivity of the threshold circuit. The 
adjustable resistor D12 affords an effective control for the critical 
speed of control 40. This one adjustable circuit element makes it possible 
to adjust the control for operation at critical speeds from 500 rpm down 
to two rpm or even lower with no change of other circuit components. 
Furthermore, as noted above, the control 40 of FIG. 3 is not particularly 
load-sensitive. A decrease in the load resistance RL, or other load change 
causing an increased load current, simply decreases the duration T2 of of 
the brief OFF intervals in the ON signal output from actuator circuit 21B 
(see FIG. 4). If the circuit is properly constructed, with adequate 
charging time for capacitor C13 for relatively high load currents, it will 
function over a broad range of load current changes with no difficulty. 
There may also be some variation in the overall period T1 for the 
recurring OFF interval pulses in the ON signal output from actuator 
circuit 21B, but it is a simple matter to keep the frequency of these OFF 
pulses high enough so that they do not affect the operation of the load 
safety device 18. It will be recognized that control 40 can be changed 
from a normally closed switching device, as illustrated, to a normally 
open device, again merely by reversing the circuit connections to the 
input terminals 42 and 46 of amplifier A1. 
Typical circuit parameters for control 40, FIG. 3, for operation with a 
power supply of six to 40 volts DC and a maximum load current of five 
amperes, and with a critical speed range of approximately 2-500 rpm, are 
set forth in Table IV. It will be understood that the specific circuit 
parameters incorporated in the tables are presented solely by way of 
illustration and in no sense as a limitation on the invention. 
TABLE IV 
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R10 270 .OMEGA. ohms 
R11 10K .OMEGA. ohms 
R12 (adjustable) 1 megohm 
R13 220 Kilohms 
R14 15 Kilohms 
R15 18 Kilohms 
R16 22 Kilohms 
R17 10 Kilohms 
R18 1.2 Kilohms 
R19 12 Kilohms 
R20 1 Kilohm 
R21 10 Kilohms 
C11 .1 microfarad 
C12 1 microfarad 
C13 6.8 microfarad 
Z11 5.1 volts 
Z12 6.2 volts 
Z13 3.6 volts 
D4 Diode 1 ampere 
D11 Diode 1 ampere 
D12 Diode 1 ampere 
D13 Diode 1 ampere 
D14 Diode 1 ampere 
13A 2N6385 
A1, A2 LM358 
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