Anti-stall control for electrical hydrostatic transmission control system

A control system for a hydrostatic transmission is disclosed of the type including an engine driven fluid pump and a fluid motor. The control system includes a main control operable in response to an electrical command signal to vary the displacement of the pump, and a command signal generator for generating the command signal. The anti-stall control includes means for comparing electrical signals representative of engine speed and a reference speed, and generating an electrical anti-stall signal representative of the maximum percentage of commanded pump displacement which is permissible without causing the engine to drop below the reference speed. The anti-stall control includes means for electrically multiplying the anti-stall signal and the command signal, downstream of the shaping and rate limiting circuits. The invention provides a simple, inexpensive anti-stall control which can be adjusted to have the maximum possible gain (responsiveness) without inducing circuit instability.

BACKGROUND OF THE DISCLOSURE 
The present invention relates to hydrostatic transmission control systems, 
and more particularly, to an anti-stall control for use in a control 
system which is responsive to an electrical command signal. 
In hydrostatic transmission control systems of the type in which the 
displacement of the fluid pump is controlled by means of variations in an 
electrical command signal, it is generally well known to generate 
electrical signals representative of actual engine speed and of minimum 
desired engine speed (or some reference speed), the two speed signals then 
being compared to generate an anti-stall signal. In such control systems, 
the command signal to control pump displacement normally originates at a 
command signal generator, similar to a potentiometer, in which the manual 
input controls the wiper position. 
In typical prior art anti-stall controls the comparison of the two speed 
signals is performed by an amplifier, the output of which is a DC signal. 
In one prior art approach, the DC anti-stall signal is used to change the 
excitation of the command signal generator potentiometer. Typically, 
control systems which utilize electrical command signals include signal 
shaping circuits and rate limiting circuits downstream of the signal 
generator. As a result, using the anti-stall signal to change the 
excitation of the potentiometer, i.e., upstream of the shaping and rate 
limiting circuits, causes the response of the system to be relatively 
slow. 
Another approach to using the DC anti-stall signal is illustrated in U.S. 
Pat. No. 3,914,938, assigned to the assignee of the present invention. In 
the anti-stall system of the cited patent, the DC command signal and the 
DC anti-stall signal are inputs to a summing amplifier, with the DC output 
of the amplifier representing the modified command signal. Although the 
performance of this prior art control has been generally satisfactory, the 
adding and subtracting of the command and anti-stall signals require the 
presence of certain protective circuitry to prevent unintended commands, 
for example, inadvertently commanding reverse. Such protective circuitry 
adds substantially to the complexity and expense of the anti-stall 
control. 
Accordingly, it is an object of the present invention to provide an 
improved anti-stall control for use with a hydrostatic transmission 
control system operable in response to an electrical command signal. 
It is a further object of the invention to provide an anti-stall control in 
which the anti-stall signal acts on the electrical command signal 
downstream of the shaping and rate limiting circuitry, while avoiding the 
necessity of the type of protective circuitry required when the electrical 
command signal and the anti-stall signal are the inputs to a summing 
amplifier. 
It is another object of the present invention to provide an anti-stall 
control which generates an electrical anti-stall signal which is 
representative of a percentage change in the instantaneously commanded 
displacement of the variable unit (pump or motor) necessary to prevent the 
engine speed from dropping below a reference speed, such as a 
predetermined minimum speed. 
One problem common to many known anti-stall systems is instability. 
Instability is typically caused by the fact that the time required to make 
necessary adjustments in the swashplate position is much greater than the 
time necessary to generate a modified command signal, such that changes in 
the engine speed and changes in the swashplate position may be out of 
phase, causing the engine speed to oscillate. In order to eliminate 
oscillation from the prior art anti-stall controls, it has generally been 
necessary to substitute various circuit components, such as capacitors and 
resistors, after the system is installed on the vehicle. 
Accordingly, it is an object of the present invention to provide an 
anti-stall control in which instability can be eliminated by means of a 
simple adjustment, after installation of the system on the vehicle. 
It is a related object of the present invention to provide an anti-stall 
control in which the gain of the circuit (i.e., the degree of response to 
changes in engine speed), can be controlled by the same adjustment which 
is used to eliminate instability. 
SUMMARY OF THE INVENTION 
The above and other objects of the present invention are accomplished by 
the provision of an improved control system for a hydrostatic transmission 
of the type including an engine driven fluid pump and a fluid motor, 
either the pump or the motor being of the variable displacement type. The 
control system includes a main control operable in response to an 
electrical input command signal to vary the displacement of whichever of 
the units is variable. The control system further includes a command 
signal generator for generating an operative command signal, means 
providing a first electrical input signal representative of actual engine 
speed, and means providing a second electrical input signal representative 
of a reference engine speed (such as a predetermined minimum engine speed, 
or an unloaded engine speed). The improvement comprises means for 
comparing the first and second electrical input signals and generating an 
electrical anti-stall signal having a gain between 0.0 and 1.0, the gain 
being representative of a percentage change in the instantaneously 
commanded swashplate displacement necessary to prevent the engine speed 
from dropping below the reference speed. The improvement further comprises 
means for electrically multiplying the anti-stall signal and the operative 
command signal to generate a modified input command signal.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings, which are not intended to limit the 
invention, FIG. 1 illustrates a hydrostatic transmission and its 
associated control system. The hydrostatic transmission includes a 
variable displacement swashplate fluid pump 10, preferably of the axial 
piston type, hydraulically connected to a fluid motor 12 by means of 
conduits 14 and 16. Typically, the motor 12 is a fixed displacement, axial 
piston unit, although the motor 12 may be variable displacement, with the 
anti-stall control of the invention applied to the displacement controls 
of the motor 12. 
Input power to the hydrostatic transmission is supplied by an engine 18, by 
means of an input shaft 20, which drives the rotating group of the pump 10 
(as is well known in the art) and also drives a charge pump 22. One 
function of the charge pump 22 is to supply make-up fluid to the low 
pressure side of the system by means well known in the art and therefore, 
not shown in FIG. 1. The charge pump 22 receives its inlet fluid from a 
fluid reservoir 24. 
The pump 10 includes a swashplate 26 which is movable over-center in a 
known manner by a pair of stroking cylinders 28 and 30. The motor 12 
includes an output shaft 32. Various other standard controls, such as 
relief and shuttle valves, which are well known, and form no part of the 
present invention, have been omitted from FIG. 1 and the description. 
In the control system illustrated in FIG. 1, the displacement of the pump 
10, and thus the speed ratio between the input shaft 20 and output shaft 
32, is varied by a main control, generally designated 34. The control 34 
is capable of changing the position of the swashplate 26 by communicating 
control fluid, received from the charge pump 22 by means of a conduit 36, 
to one of the strokers 28 or 30, the control 34 communicating the other of 
the strokers to the reservoir 24. The control pressure fluid is 
communicated from the control 34 to the strokers 28 and 30 by means of 
conduits 38 and 40, respectively. The selective porting of fluid by the 
control 34 to the strokers 28 and 30 is in response to variations in an 
electrical input command signal 42. 
The control 34 may be of the type illustrated in U.S. Pat. No. 3,924,410, 
assigned to the assignee of the present invention, and which is 
incorporated herein by reference. However, the reference to U.S. Pat. No. 
3,924,410 is by way of example only and is not intended to limit the 
present invention. Within the scope of the present invention, it is 
essential only that the control 34 must be operable to vary the 
displacement of the variable displacement unit, in response to variations 
in the electrical input command signal 42. 
The input command signal 42 is transmitted to the control 34 from an 
anti-stall control, generally designated 44. The main input to the 
anti-stall control 44 is from a command signal generator, generally 
designated 46, by means of a wiper 48. Operator control of the hydrostatic 
transmission is accomplished by manual movement of the wiper 48 to effect 
variations in the magnitude of the command signal transmitted by the wiper 
48, as is well known in the art. 
As is common to most anti-stall systems, the engine speed is continually 
compared to some sort of reference speed, for the purpose of sensing an 
imminent engine stall condition, indicated by a sudden reduction in engine 
speed, relative to the reference speed. Therefore, one of the inputs to 
the anti-stall control 44 is actual instantaneous engine speed. A toothed 
member 50 is mounted for rotation with the input shaft 20, and disposed 
adjacent the toothed member 50 is a magnetic pick-up 52. As is well known 
in the art, the magnetic pick-up 52 generates lines of magnetic flux which 
are cut by the teeth of the member 50 as it rotates, such that the pick-up 
52 generates an AC signal whose frequency is directly proportional to the 
speed of rotation of the member 50. The AC signal is transmitted from the 
pick-up 52 to the anti-stall control 44 by means of a lead 54. 
The other input to the anti-stall control 44 is the reference speed which, 
in the subject embodiment, is illustrated as being the desired speed of 
the engine 18, as selected by a throttle setting, generally designated 56. 
There is provided a throttle setting signal generator, generally 
designated 58, including a movable wiper 60, the position of which 
corresponds to the position of the throttle setting 56, as indicated by 
the dashed line in FIG. 1 connecting the setting 56 and wiper 60. 
Referring now to the circuit schematic in FIG. 2, the anti-stall control 44 
of the present invention is shown in greater detail. As described in 
connection with FIG. 1, the AC signal generated by the pick-up 52 is 
transmitted to the anti-stall control 44 by means of the lead 54. The AC 
sine-wave signal transmitted by the lead 54 is conducted to a lead 62 
through a resistor 64. The lead 62 is one of the inputs to a 
frequency-to-voltage converter circuit, generally designated 66, of the 
type which is well known and commercially available. Also connected as an 
input to the converter circuit 66 is a grounded lead 68, and connected in 
parallel between the leads 62 and 68 is a pair of diodes 70 and 72, the 
characteristics of which are selected to limit the amplitude of the signal 
transmitted to the converter circuit 66. The function of the converter 
circuit 66 is to convert the variable frequency AC signal transmitted by 
lead 62 into a DC signal having a voltage proportional to the frequency of 
the AC signal. 
The DC output of the converter circuit 66 is transmitted by a lead 74 to a 
gain adjustment potentiometer, generally designated 76. The other input to 
the potentiometer 76 is by means of a lead 78, connected to the output of 
an oscillator circuit, generally designated 80, which provides a 
triangular-wave (or saw-tooth) signal, of a known frequency. The output of 
the gain adjustment potentiometer 76 is transmitted by means of an 
adjustable wiper 82 to the plus input of a comparator circuit 84. The 
minus input of the comparator circuit 84 is connected to the wiper 60 of 
the throttle setting signal generator 58, by means of a lead 86. As will 
be described in greater detail subsequently, the function of the gain 
adjustment potentiometer 76 is to provide a signal on the wiper 82 which 
is somewhere between the DC signal on the lead 74 and the triangular-wave 
signal on the lead 78 (a weighted summation). For ease of description of 
the rest of FIG. 2, as well as the voltage graphs of FIGS. 3 and 4, all 
subsequent references to the various signals will be by means of the 
reference numerals used to identify the leads or wipers on which those 
signals appear. 
As an example of the operation of the potentiometer 76, if the wiper 82 is 
set at the mid-point of the potentiometer 76, the signal 82 will be the 
mathematical average of the signals 74 and 78. It should be noted that the 
voltage graphs shown on FIG. 2 are not intended to indicate actual or 
relative voltages, but are intended merely to illustrate generally the 
form of each of the signals. It should be understood that the relationship 
of the magnitudes of signals 74 and 86 is not the same as the relationship 
of the engine speeds represented thereby. 
The comparator circuit 84 compares the signal 82 (plus input) and the 
signal 86 (minus input), and generates a signal on its output lead 88 
which goes to positive saturation (V+) while the signal 82 is greater than 
the signal 86, and goes to negative saturation (ground) while the signal 
82 is less than the signal 86. Because the triangular-wave signal 78 has a 
known, constant frequency, the resulting square-wave signal 88 has the 
same constant frequency, and has a duty cycle (ratio of time at positive 
saturation to total cycle time) representative of the percentage of time 
that the signal 82 is greater than the signal 86. The duty cycle of the 
square-wave signal 88 is also representative of the change needed in the 
input command signal 42 (and hence, the displacement of the swashplate 26) 
in order to reduce the loading of the engine and prevent the engine speed 
from dropping below a reference speed, such as the engine speed 
represented by the throttle setting 56. 
Referring again to the gain adjustment potentiometer 76, the reasons for 
the signal 82 being a combination of the signals 74 and 78 should now be 
apparant. The amplitude of the triangular-wave 78 is constant, such that 
changes in magnitude of the DC signal 74, reflecting changes in 
instantaneous engine speed, result in a change in magnitude of the signal 
82. On the other hand, the triangular-wave 78 dictates the frequency of 
the signal 82 and the square-wave signal 88, while the alternately 
increasing and decreasing slopes of the signal 78 cause the signal 82 to 
be alternately above and below the reference signal 86, in turn causing 
the comparator circuit 84 to attain alternately positive and negative 
saturation. The effect of variations in engine speed (signal 74), 
reference speed (signal 86), or the gain adjustment of potentiometer 76 
will be discussed in detail in connection with FIGS. 3 and 4. 
As was described in connection with FIG. 1, the primary input to the 
anti-stall control 44 is from the command signal generator 46, by means of 
the wiper 48. The command signal is transmitted from the wiper 48 to a 
shaping circuit 90, which is well known and forms no part of the 
invention. As is indicated by the graph of voltage vs position of the 
wiper 48, the function of the shaping circuit 90 is to reduce the gain of 
the command signal around the neutral position of the wiper 48, and 
increase the gain of the signal further away from neutral. The output of 
the command signal 90 is trasmitted by a lead 92 to a rate limiting 
circuit 94 which also is well known and forms no part of the invention. 
The function of the rate limiting circuit 94 is to limit the rate of 
change of the electrical command signal as the position of the wiper 48 is 
changed. The output from the rate limiting circuit 94 will be referred to 
hereinafter as an "operative" command signal. For reasons which will 
become apparent subsequently, the phrase "operative command signal" will 
generally be understood to mean an electrical command signal which may be 
satisfactorily utilized by the main control 34, and in the subject 
embodiment, by way of example only, the command signal is "operative" only 
after the desired shaping and rate limiting has been performed. 
The operative command signal is transmitted over a lead 96 to the "HI" 
terminal of an analog switch 98, the "LO" terminal of the switch 98 being 
connected to a reference voltage V.sub.R. The analog switch 98 includes 
switching means represented schematically by a movable switching element 
100, which is connected to an output lead 102. The analog switch 98 may be 
viewed as having two alternating "states" or conditions. When the signal 
88 is at positive saturation, the lead 102 is connected by means of the 
switching element 100 to the HI terminal, receiving the operative command 
signal 96. When the signal 88 is at negative saturation, the lead 102 is 
connected through the switching element 100 to the LO terminal, receiving 
the reference voltage V.sub.R, representative of a zero or neutral command 
signal. Accordingly, the output of the analog switch 98 is a signal 
somewhere between the reference voltage V.sub.R and the operative command 
signal 96, depending upon the duty cycle of the signal 88. For example, if 
the signal 88 has a duty cycle of 80%, V.sub.R is 3.0 volts and the 
command signal 96 is 5.0 volts, the output of the switch 98 will be 4.6 
volts, i.e., 3.0 v+0.8 [5.0 v-3.0 v]. 
Therefore, in the subject embodiment, the duty cycle of the signal 88 
represents the maximum percentage of the instantaneous command signal 96 
which is permissible, without causing the engine speed to drop below a 
predetermined minimum engine speed (the reference speed indicated by the 
throttle setting 56). The analog switch 98 effectively performs an 
electrical multiplication of the operative command signal 96 and the duty 
cycle of the signal 88 to generate a reduced command signal which will 
result in a decreased displacement of the swashplate 26, and a reduced 
load on the engine, and will prevent the engine speed from dropping below 
the reference speed. 
It should be understood that the output of the analog switch 98 is not a 
smooth, DC signal, but rather, a stepped waveform. Therefore, the lead 102 
is connected to a filter circuit, generally designated 104, which changes 
the stepped waveform into a signal which is basically a DC signal with 
some "ripple." The command signal is then transmitted to a current 
amplifier circuit, generally designated 106, the output of which is the 
input command signal 42 which is then transmitted to the main control 34 
as described in connection with FIG. 1. 
Referring to FIGS. 3 and 4, the effect of variations in the engine speed 
(signal 74), the reference speed (signal 86), and the gain of 
potentiometer 76 will now be described in some detail. Before specific 
reference is made to the graphs of FIGS. 3 and 4, some general 
observations will be made. Unlike the voltage graphs in FIG. 2, the graphs 
in FIGS. 3 and 4 are intended to indicate relative voltages of the various 
signals but, as indicated in connection with the graphs in FIG. 2, the 
actual magnitudes of the signals are not important. However, for ease of 
reference in describing FIGS. 3(A), 3(B), 4(A), and 4(B), each line on the 
voltage coordinate will be considered as representing 1 v. For example, 
the triangular-wave signal 78, which is always the same, varies between 1 
v and 4 v. 
The primary difference between the graphs of FIG. 3, and those of FIG. 4 is 
the different gain setting of the gain adjustment potentiometer 76. In 
FIG. 3, the gain is 75%, i.e., the wiper 82 is positioned 75% of the way 
up from input signal 78 (or 25% of the way down from the input signal 74), 
such that the instantaneous amplitude of the signal 82 is represented by 
the following equation: 
##EQU1## 
For the graphs of FIG. 4, the gain of the potentiometer 76 is 50%, i.e., 
the wiper 82 is positioned at the midpoint between the input signals 78 
and 74, such that the instantaneous amplitude of the signal 82 is 
represented by the following equation: 
##EQU2## 
As should be understood by those skilled in the art, in hydrostatic 
transmission control systems of the type disclosed herein, there may not 
always be sufficient load on the engine to necessitate a reduction of the 
operative command signal 96. When this situation occurs, the duty cycle of 
the signal 88 is 100%, and the input command signal 42 is substantially 
identical to the command signal 96, such that the displacement of the 
swashplate 26 will correspond substantially to the displacement indicated 
by the position of the wiper 48. The 100% duty cycle condition is 
illustrated in graphs 3(A) and 4(A), in both of which the engine speed 
signal 74 is 3.67 v. With the potentiometer 76 at the higner gain setting 
of FIG. 3(A), the signal 82 varies between 3.0 v and 3.75 v. Accordingly, 
the duty cycle of the signal 88 will be 100% because the signal 82 is 
always at least equal to the reference signal 86. By comparison, with the 
potentiometer 76 at the lower gain setting of FIG. 4(A), the signal 82 
varies between 2.33 v and 3.83 v, thus illustrating that, as the gain of 
the potentiometer 76 is decreased, the signal 82 becomes more similar to 
the signal 78. In FIG. 4(A) it may be seen that the signal 88 will remain 
at 100% duty cycle only as long as the signal 82 is at least equal to the 
reference signal 86, or conversely, only as long as the reference signal 
86 does not exceed 2.33 v. Therefore, a comprison of FIGS. 3(A) and 4(A) 
indicates that with the higher gain setting, the predetermined minimum 
engine speed (reference signal 86) may be set higher, without dropping 
below 100% duty cycle for the signal 88. 
Referring now to FIG. 3(B), and comparing it to FIG. 3(A), it may be seen 
that the reference signal 86 is still 3.0 v, but the engine speed signal 
74 has dropped from 3.67 v to 3.15 v, indicating excessive engine loading. 
In FIG. 3(B), the signal 82 has the same shape as in FIG. 3(A), because 
the gain setting is still the same, but the magnitude of the signal 82 is 
decreased by 75% of the decrease in the engine speed signal 74 (because 
the gain setting is 75%). As a result, the signal 82 is less than the 
reference signal 86 for part of each cycle, causing the signal 88 to have 
a duty cycle less than 100%. As may be seen from FIG. 3(C), the magnitudes 
of the signals for FIG. 3(B) have been selected to yield a signal 88 
having a duty cycle of 50%. 
Referring now also to FIGS. 4(B) and 4(C), it may be seen by comparing FIG. 
4(B) and 4(A) that the reference speed signal 86 has remained at 2.33 v. 
However, as was indicated in the comparison of FIGS. 3(A) and 3(B), the 
engine speed signal 74 has decreased, from 3.67 v to 2.15 v. In comparing 
FIGS. 4(A) and 4(B), it should again be noted that the signal 82 has the 
same shape, because the gain is still the same, but that its magnitude is 
reduced by an amount equal to 50% of the reduction in the speed signal 74 
(because the gain setting is 50%). Again, the magnitudes in FIG. 4(B) have 
been selected to yield a signal 88 having a duty cycle of 50%. As a 
result, it may be seen that with the potentiometer 76 at the 75% gain 
setting, a reduction in the engine speed signal 74 of 0.52 v causes the 
signal 88 to go from 100% duty cycle down to 50% duty cycle, whereas, with 
the potentiometer 76 at the 50% gain setting, the same reduction to a 50% 
duty cycle for the signal 88 requires a reduction in the engine speed 
signal 74 of 1.52 v. Thus, in general, the higher the gain setting of the 
potentiometer 76, the greater the change in the duty cycle of signal 88 
for a given change in the engine speed signal 74. 
The practical significance of the adjustability of the gain setting is that 
after the system is installed on a vehicle, it is possible to "optimize" 
the performance of the anti-stall control by a simple adjustment of the 
potentiometer 76. Starting with the gain setting near the minimum (wiper 
82 near lead 78), the wiper 82 may be slowly moved in a direction of 
increased gain setting, until the gain setting reaches a point where it 
begins to induce instability in the anti-stall control, as evidenced by an 
oscillating engine speed. From the point at which instability first 
occurs, the gain setting should then be reduced to a setting slightly 
below the setting at which the instability is eliminated. The result is an 
anti-stall control having the highest gain possible (quickest response to 
imminent engine stall), without causing instability. 
When the anti-stall control 44 of the present invention is used in a system 
such as that shown in FIG. 1 wherein the signal 86 is representative of 
the throttle setting 56, the above-described adjustment of the gain 
setting of the potentiometer 76 is the only major system adjustment 
required after installation of the control on the vehicle. However, in 
many applications of such anti-stall controls, the engine is intended to 
run at a constant speed at all times during the operation of the vehicle, 
in which case, it would be preferable for the position of the wiper 60 of 
the signal generator 58 to be an adjustable setting, rather than being 
connected to the throttle setting 56. In the type of arrangement just 
described, the position of the wiper 60 would be adjusted after 
installation of the control on the vehicle, and would be set to correspond 
to a predetermined minimum engine speed (previously referred to as the 
reference speed). For example, in a typical installation, the throttle of 
the vehicle engine might be set so that the engine would run continuously 
at 2200 rpm, with the wiper 60 being adjusted to correspond to an engine 
speed of 2000 rpm. Again, it should be remembered that the relationship 
between these engine speeds is not necessarily the same as the 
relationship between the magnitude of the signals 74 and 86. In general, 
the adjustment of the wiper 60 might be considered to represent a minimum 
"unloaded" engine speed, with a drop in actual engine speed below the 
unloaded engine speed indicating an excessive or undesired load on the 
engine. 
It should be apparent that various modifications of the system may be made, 
within the scope of the invention. For example, if the anti-stall control 
is part of a control system for a variable displacement fluid motor, the 
anti-stall signal 88 will represent an increase in motor displacement 
(rather than decrease in pump displacement) necessary to prevent the 
engine speed from dropping below the reference speed.