Vehicle road speed control system

A vehicle road speed control system includes a vehicle road speed detection means, parameter storage means, comparator processing means and control signal generation means. The system is arranged to compare measured vehicle road speed with predetermined road speeds obtained from stored parameters. When an upper threshold road speed is exceeded a control signal is generated for an associated transmission control system which causes the operative ratio of an associated vehicle transmission to be reduced such that the vehicle road speed is reduced to at least the upper threshold road speed. The road speed may also be controlled by altering the characteristics of an associated fuel injection such that the road speed is reduced to at least the upper threshold road speed. This may be done in place of controlling the operative ratio of the transmission or in addition.

BACKGROUND OF THE INVENTION 
The invention relates to a vehicle road speed control system, and more 
particularly to a system for limiting the vehicle road speed to a maximum. 
In many countries it is necessary to limit the road speed of some forms of 
vehicles in order to comply with legislation. In the prior art this has 
been accomplished by mechanical means within the transmission system of 
the vehicle which, for example, prevent the selection of certain gear 
ratios. Alternatively engine speed governors have also been used. 
If it is required to remove or change the road speed limitation the 
transmission system must be physically opened up and parts changed, if 
certain gear ratios have previously been blanked off, or the governor 
settings must be eliminated or reset. This can be a long process and thus 
incur extensive costs, particularly where transmission changes are 
involved. In addition, for commercial vehicles of any sort, there is an 
undesirably long period of time for which the vehicle is out of use. 
It is an object of the invention to provide an alternative form of vehicle 
road speed control system which ameliorates the above problems. 
It is a further object of the invention to provide an alternative method of 
controlling vehicle road speeds. 
SUMMARY OF THE INVENTION 
The invention therefore provides a vehicle road speed control system 
including: 
(a) vehicle road speed detection means; 
(b) parameter storage means; 
(c) comparator processing means; and 
(d) control signal generation means, 
the system being arranged to compare measured vehicle road speed with 
predetermined road speeds obtained from stored parameters and, when an 
upper threshold road speed is exceeded, to generate a control signal for 
an associated control system to alter an operative parameter or parameters 
of the vehicle such that, the vehicle road speed is reduced to at least 
the upper threshold road speed. 
The invention provides the advantages that it is relatively cheap to 
implement and, should the upper threshold road speed require changing 
after the system has been implemented this may be effected both cheaply 
and quickly. 
In a preferred embodiment the vehicle road speed control system is 
operative in only a selection of transmission ratios. 
The control signal preferably operates on a transmission control system to 
reduce the operative ratio of an associated vehicle transmission system. 
In alternative embodiments the control signal may operate on a fuel 
injection control system to alter the characteristics of fuel injection to 
an associated vehicle engine or on both a transmission control system and 
a fuel injection control system to both reduce the operative ratio of an 
associated transmission and alter the characteristics of fuel injection to 
an associated engine. 
If the measured road speed is above a target road speed but below the upper 
vehicle threshold road speed a warning may be given to the vehicle driver. 
In addition to the warning the selector means for the associated vehicle 
transmission may be rendered inoperable. A warning may also be given to 
the vehicle driver if the measured road speed is above a lower threshold 
road speed but below the target road speed. The warnings may be given in 
the form of a flashing light or lights. 
The upper threshold road speed may be dependent on the operative ratio of 
the associated vehicle transmission, or may alter during the operation of 
the road speed control system. 
The invention also provides a method of limiting the road speed of a 
vehicle including the steps of: 
(a) measuring the road speed of the vehicle; 
(b) comparing the measured road speed of the vehicle with a target road 
speed; and 
(c) if, the measured road speed exceeds an upper threshold road speed, 
generating a control signal which operates on an associated control system 
to alter an operative parameter or parameters of the vehicle such that the 
vehicle road speed is reduced to at least the upper threshold road speed. 
In a preferred method the control signal operates on a transmission control 
system to alter the operative ratio of an associated vehicle transmission, 
alternatively it may operate on a fuel injection control system to alter 
the characteristics of the fuel injection to an associated vehicle engine 
or on both a transmission control system and a fuel injection control 
system. 
An embodiment of the invention as applied to an agricultural tractor will 
now be described, by way of example only, with reference to the 
accompanying drawings in which:

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a tractor transmission is illustrated schematically. 
It incorporates an engine E which drives the front and rear wheels W1 and 
W2 via a main clutch CL, a planetary powershift gear unit P, a main 
gearbox G, a transfer box T and differentials D1 and D2. The main gearbox 
G will typically have four ratios and a forward/reverse direction selector 
train and may also include a high/low range facility indicated in dotted 
detail in FIG. 1. 
Referring now to FIG. 2, powershift unit P, which provides four ratios, A, 
B, C and D, is illustrated schematically. The powershift unit P will only 
be described very briefly here since it is the subject of a separate 
United Kingdom Patent Application, No. 9116856.7, published as GB 2247501, 
to which the reader is referred for a detailed description of the unit. 
The powershift unit P incorporates an input shaft 10 and a co-axial output 
shaft 11 and first and second planetary gear trains 12 and 13. It also 
includes first to fourth clutches C1, C2, C3 and C4 respectively. First 
and second clutches C1 and C2 are hydraulically-engaged by annular pistons 
14 and 15. The third and fourth clutches C3 and C4 are spring-engaged by 
belleville springs 16 and 17. The clutches operate in pairs C1 with C4 and 
C2 with C3, such that when the hydraulically-engaged clutch of a pair is 
engaged the associated spring-engaged clutch is automatically disengaged, 
and when the hydraulically engaged clutch is disengaged the associated 
spring-engaged clutch is automatically engaged. The engagement status of 
each clutch C1 to C4 for each ratio of the planetary gear unit P is given 
in tabular form in FIG. 3. It will be appreciated that the ratios A to D 
of the unit P operate as ranges of the main four ratio gearbox G. 
Referring now to FIG. 4, a vehicle road speed control system according to 
the invention is illustrated schematically in combination with a hydraulic 
control system for the powershift unit P and its normal manual selection 
means. The manual selection means 30 includes a lever 31 movable to each 
of four ratio selection positions A, B, C and D, as indicated by arrows 
32. The lever and its associated housing 33 are mounted on the steering 
column 34. When a manual selection of a new ratio is made a signal is 
passed down line 35 to a microprocessor 36. In response to the signal the 
microprocessor 36, generates appropriate control signals for the hydraulic 
control system 20 to engage and disengage the necessary clutches. Since 
the unit P is a powershift unit, actuation of the main clutch CL is not 
necessary when changing between ratios A to D. 
The hydraulic control system 20 will only be described very briefly here 
since it is the subject of a separate United Kingdom Patent Application, 
No. 9116851.8, published as GB 2247500, to which the reader is referred 
for a detailed description. The control system 20 incorporates a four-port 
two-position flow reversing valve 21 and a three-port two-position flow 
reversing valve 22 connected in series, a pump 23 and a reservoir 24. Both 
valves are biased to a first position by respective springs 21a and 22a 
and are movable by associated solenoids 21b and 22b to their respective 
second positions. Solenoids 21b and 22b are controlled, during manual 
operation, by the control signals generated by the microprocessor 36 via 
lines 37 and 38. 
The vehicle road speed control system incorporates a road speed detector 40 
of, for instance, well known radar type located on the underside of the 
tractor and electronic circuitry 41 located, for instance, in the tractor 
cab. The circuitry 41 incorporates parameter storage means 42, comparator 
processing means 43 and signal generation means 44. The above modules are 
connected via lines 45, 46 and 47. A four light indicator array 48, 
located, for instance, in the dashboard, is connected to the signal 
generation means 44 by lines 49 to 52. The signal generating means 44 is 
also connected to the microprocessor 36 by two lines 39a and 39b, the 
former taking signals from the signal generating means 44 to the 
microprocessor 36 and the latter from the microprocessor 36 to the signal 
generating means 44. 
Although in this embodiment a separate microprocessor 36 and circuitry 41 
are described it will be appreciated that embodiments may be constructed 
in which a single semiconductor chip or integrated circuit performs all 
the functions of these components. Indeed, this will normally be the case. 
This embodiment of the vehicle road speed control system operates only when 
the vehicle is in fourth gear in the main gearbox G and also in C or D 
ratio in the powershift unit P, (i.e. ratios 4C and 4D) since these are 
the only ratios in which road speeds above the desired maximum are 
attainable. 
The system operates as follows. The road speed detector 40 measures the 
road speed and passes a signal corresponding to the speed via line 45 to 
the comparator processor 43 which compares it with speeds calculated from 
various parameters stored in the parameter storage means 42. Depending on 
the result of the comparison a signal is sent to the signal generation 
means 44 which then generates the appropriate signals to control the 
valves 21 and 22, the indicator array 48, and the microprocessor 36. In 
normal conditions the indicator light A to D corresponding to the ratio 
currently in operation is illuminated, when the vehicle road speed control 
system is in operation the indicator array is used to provide warnings to 
the driver, as will be described later. When the vehicle is not in 4C or 
4D signals from the microprocessor 36 to the signal generation means 44 
via line 39b prevent the generation of control signals. The comparative 
calculations are repeated every 0.5 seconds. 
Referring now also to FIG. 5, the operating sequence of the vehicle road 
speed control system is illustrated schematically. The measured road speed 
is compared with three different values, a target speed S, an upper 
threshold speed S1.sub.1, or S1.sub.2 equal to the target speed plus a 
respective first percentage, and a lower threshold speed S2 equal to the 
target speed less a second percentage. In this embodiment the speeds are 
S=30 km/h, S1.sub.1 =32 km/h, S1.sub.2 =33 km/h and S2=28 km/h. There are 
a number of stored parameters stored in the parameter storage means 42, 
these are the target speed S, and the first and second percentages. The 
first percentage may vary, for instance it may be a function of the 
operative ratio or of whether the system has made a first ratio change or 
not. In this case there are two values of the first percentage, one in 
operation when no ratio change has yet been made from which S1.sub.1 is 
generated and the other when a first ratio change has been made from which 
S1.sub.2 is generated. 
If the measured road speed is above S2 but below S then a first warning is 
given to the driver. This warning takes the form of flashing light C or D, 
corresponding to the ratio in operation at the time, in the indicator 
array 48. If the ratio selected is 4C or 4D and the measured road speed is 
above S but below S1.sub.1 all four lights A to D in the indicator array 
48 flash, and the manual powershift ratio selector is rendered inoperable 
such that the vehicle driver can no longer change the powershift unit 
ratio. If the measured road speed continues to rise and exceeds S1.sub.1 
then the ratio is reduced by the speed control system from 4C to 4B or 4D 
to 4C as appropriate. 
If the operative ratio is now 4B, having been reduced from 4C by the speed 
control system, light B is lit continuously, and the manual powershift 
ratio selector is rendered operable again once the road speed drops below 
the target speed S, so that the vehicle operates normally. If the 
operative ratio is now 4C, having been reduced from 4D by the speed 
control system, the operation of the indicator lights depends on the road 
speed and is essentially as previously described. Thus if the road speed 
is below S2 light C is lit continuously, if the road speed is between S2 
and S light C flashes and if the road speed is above S but below S1.sub.2 
all four lights in the indicator array 48 flash, and the manual powershift 
ratio selector continues to be rendered inoperable. If the measured road 
speed continues to rise, and exceeds S1.sub.2 then the ratio is reduced to 
4B. The road speed may have continued to rise after the change to 4C 
either, because the road speed in 4C corresponding to the set engine speed 
has yet to be reached, or because the driver increased the engine speed 
further. When the change from 4C to 4B has been made light B is lit 
continuously, and the manual powershift ratio selector is rendered 
operable again once the road speed drops below the target road speed S, so 
that the vehicle operates normally again. 
When the ratio changes are made the engine speed is not altered by the 
control system, but it will rise briefly as it will be driven up by the 
rotation of the wheels at the high road speed, and will then fall again to 
the engine speed corresponding to the throttle setting. Therefore the road 
speed of the vehicle is reduced to below the target speed S. The maximum 
speed attainable at maximum engine speed in this embodiment is 33 km/h, 
(i.e. the target speed 30 km/h plus 10% error allowance). 
The second ratio change, from C to B after a first ratio change from D to 
C, is made at a higher road speed than the first ratio change in order to 
protect the transmission from damage and make the changes smoother. This 
is necessary because the road speed does not reduce immediately the ratio 
change is made and the circuitry 41 repeats the calculations every 0.5 
seconds, and performs them rapidly. Thus if the second change was made at 
the same road speed as the first it would occur as a result of the next 
calculation, and in effect follow immediately after the first resulting in 
severe jerking of the vehicle and possibly in damage to the transmission. 
An alternative way of preventing these problems is to arrange for a 
minimum delay of a predetermined length of time between first and second 
changes. 
A particular sequence of operation has been described, however, the 
invention is not limited to such a sequence and embodiments may be 
constructed to operate when different ratios are engaged, to different 
target and threshold speeds, and with, for instance, different threshold 
speeds operative in each ratio. In addition, the above embodiment has been 
described as taking various actions at particular road speeds, it will, 
however, be appreciated that some degree of overshoot will in general 
occur before an action takes effect. The degree of overshoot will depend 
on the detail of the construction of the system. 
A system such as this may, for instance, be used in a country which limits 
the speed of certain types of vehicles, such as tractors. A tractor may be 
capable of 40 km/h but in one country the speed limit may be 30 km/h, but 
with a 10% error allowed. Thus, either a mechanical speed limiting means 
must be incorporated in the transmission (by, for example, preventing 
selection of certain ratios) or the speed of the engine must be governed, 
with their accompanying problems, or a system according to the invention 
may be employed. 
The system according to the invention described above provides the 
advantage that the road speed of the vehicle is limited without losing the 
ability to use the full performance of the engine in lower gear ratios. 
Prior art speed governors limit the engine speed to the same value 
independent of the transmission ratio selected, and thus engine 
performance which is of considerable use in lower ratios is lost. The 
system described above also provides the advantage that it does not reduce 
the number of transmission ratios which may be selected, as is the case in 
some prior art systems. 
Systems according to the invention provide the further advantage that they 
are relatively cheap to implement and, should the speed limit change, the 
time taken to modify the system is minimal compared to the prior art since 
only the parameters stored in the storage means need be changed. In 
addition, where vehicle operators could relatively easily remove prior art 
mechanical speed limiters, thus making their vehicles illegal, 
modification of this system is only achievable with specialist equipment 
provided to authorised agents. 
Vehicle road speed control systems according to the invention are not 
limited to operating by means of reducing the operative transmission ratio 
of powershift transmissions. They can also be constructed for use with 
continuously variable transmissions, commonly known as CVT, in which they 
would also operate by means of changing the operative transmission ratio. 
They can also be constructed for use with electronically controlled fuel 
injected engines, and to operate by means of altering the characteristics 
of the fuel injection to the engine, thus limiting the engine speed and 
with it the road speed. 
A road speed control system according to the invention arranged to operate 
by means of controlling the characteristics of fuel injection may be 
constructed as now described with reference to FIG. 4. The hydraulic 
control system 20 is replaced by an electronic fuel injection control 
system and the clutches C1 and C2 by a fuel pump. The system operates 
substantially as previously described and operation common to the 
foregoing embodiment will not be described. A control signal, or signals 
as appropriate, are generated by the signal generation means 44 and pass 
via lines such as 37 to the fuel injection control system. The control 
system then alters, for instance, the flow of fuel, and/or the timing of 
its injection, to the vehicle engine, such that the desired slowing of the 
vehicle is achieved. Clearly a system such as this must be appropriately 
managed in order to prevent the vehicle engine stalling, and will not be 
appropriate in all kinds of vehicles. 
Embodiments may also be constructed which operate by means of changing the 
operative ratio of the vehicle transmission in combination with altering 
the characteristics of the fuel injection.