GB2267685A

GB2267685A - Operator interface for controlling a drive engine and a steplessly adjustable gearbox.

Info

Publication number: GB2267685A
Application number: GB9311805A
Authority: GB
Inventors: Heinz Weiss
Original assignee: Deere and Co
Current assignee: Deere and Co
Priority date: 1992-06-11
Filing date: 1993-06-08
Publication date: 1993-12-15
Anticipated expiration: 2013-06-08
Also published as: GB9311805D0; DE4219050C2; DE4219050A1; GB2267685B

Abstract

The basic load or speed of rotation of the vehicle engine is set by a manual throttle lever 10 acting through a hydraulic coupling 14, 16 on an adjusting lever 18. The speed of a PTO shaft 52 is thus preset. A control device 64 for a steplessly variable gearbox can similarly be set by a speed adjusting lever 56 acting through a hydraulic coupling 58, 60 on an adjusting lever 62. The desired vehicle speed or gearbox output speed is thus pre-set. The brake pedal 40 and the accelerator pedal 42 can modulate the preset values by means of electromagnetic valves 26 and 70 which control separate hydraulic circuits for the couplings 14, 16 and 58, 60. Mode switches 48 and 80 associated with the levers 10 and 56 allow the couplings 14, 16 and 58, 60 respectively to be blocked to maintain the preset values or released for modulation by the pedals 40 and 42. <IMAGE>

Description

2267685 An operator interface for controlling a drive engine and a

steplessly adjustable gearbox This invention relates to an operator interface for consolidated control of the primary drive engine of a vehicle, by which at-- least one selectively engageable PTO shaft can be driven, and a gearbox which is connected downstream of the engine and is at least partially steplessly adjustable. The drive engine, the gearbox and the PTO drive each preferably has its own control or shift means. The operator interface is considered to be the integration of the control elements including the combining logic between the control elements and the controlled modules.

Agricultural, municipal and commercial vehicles frequently comprise, in addition to their control elements needed to drive the vehicle, a plurality of further adjusting elements, e.g. for front and is rear vehicle-mounted implements. The number of the control and adjusting elements reduces their clarity and needs greater concentration from the operator. In order not to tire the operator and to assist his concentration, especially during long spells of operation, it is desirable to keep the number of operating elements as small as possible.

The safety of the vehicle on the street and the attention of the operator in relation to the vehicle depend.in the'main on the actuation as directly as possible of the steering wheel, the brake pedal and the accelerator pedal by the operator and the direct response of the vehicle.

While the aforesaid functions must not be taken away from the operator for reasons of safety, there are other control functions and elements which are suitable for automation, such as the shifting of automatic gears in private and commercial vehicles for example.

In agricultural machines and tractors there are further power loads for the propulsion unit, such as the PTO drive for the implements mounted on the front and rear of the vehicle. The demands on the operator increase as a result, insofar as each power load has to be set optimally in relation to all the other power loads. This is not a simple problem for the operator and is not a trivial problem for automation.

U.S. Patent Specification No. 4 953 427 describes a control system for a vehicle such as a tractor which has a hand accelerator lever used for presetting the engine speed to drive a working implement at a constant rate, a stepless change speed device through which the vehicle is driven by the engine, the change speed device being controlled by a foot pedal, and a selectively engageable interlock which allows the rotational rate of the engine to be modified by.the foot pedal. The disengagement of the interlock is effected when an auxiliary change speed device downstream of the stepless change speed device is set for a low vehicle speed.

The object underlying the invention is seen in providing a consolidated operator interface of the kind initially referred to, through which the recited problems can be overcome, which permits simple operation and which is satisfactory for the requirements of both street travel and also the agricultural or commercial use, without affecting the operation safety and the driving comfort.

This object is met on the basis of the pre-characterizing part oil claim 1 by the characterizing features thereof. Advantageous embodiments and developments of the invention appear from the j 1 f dependent claims.

A set-point value for the basic load or basic speed of rotation of the primary drive engine, which is an internal combustion engine as a rule, is preselected by a first pre-selector device, e.g. a pivoted throttle lever with a pivot angle transducer. To control the engine speed an engine control device can be used which determines the fuel supply to the engine in dependence on the default value of the first pre-selector device. The engine control can also include a control circuit which effects a set-point versus actual comparison for control of the speed of rotation of the internal combustion engine. The pre selected signal of the first pre-selector device can be altered by operation of a first or a second control device. This alteration is effected for example by an accelerator pedal, whose operation increases the engine speed above the preset set-point value, or by a brake pedal, through which the engine speed can be reduced below the preset set-point value. In addition, a shift of the engine speed from the preset set-point value can be evoked by parameters derived from the operation of the vehicle. Limit values for load restriction with too hot an engine or for speed restriction can serve as derived parameters for example.

The engine advantageously drives at least one PTO shaft directly, i.e. without intervention of the steplessly adjustable gearbox. A mechanical gear for switching between several speeds of rotation of the PTO shaft and/or a PTO shaft clutch can be arranged between the engine and the PTO shaft. The speed of rotation of the PTO shaft together with the engine speed can thus be set or controlled independently of.th.e vehicle drive. The function of setting the engine speed Mevolves" directly upon that of the speed of rotation of t PTO shaft. - A first mode switch is preferably provided, through which switching over between a preset mode and a constant mode can be carried out. In the preset mode the engine speed is basically run at a preset speed. However, in this kind of operation the engine speed can be modulated by operation of the accelerator pedal, the brake pedal or by vehicle parameters, as will be described. In the constant mode however the engine speed does not react to such influences and remains fixed (programmed) at the set-point value selected by the first pre-selector device By means of a second pre-selector device, e.g. a pivoted speed -10 adjusting lever with a pivot angle transducer, a set-point value for the vehicle or gearbox output speed is pre-selected. A gear control device can be provided to control the gearbox output speed, through which device the transmission ratio of the steplessly adjustable gearbox is so adjusted in dependence on the default value of the second pre-selector device that the desired vehicle or gearbox output speed results. The latter can also be controlled through a set-point versus actual comparison. In a similar manner as already described in relation to the adjustment of the engine speed, the preset signal of the second pre-selector device can also be altered by operation of a first or a second control device. This alteration is effected for example by an accelerator pedal, through whose operation the vehicle speed can be increased above the preset set-point value, or by a brake pedal, through whose operation the vehicle speed can be reduced below the preset set-point value. Here again parameters derived from the vehicle operation can lead to a shift of the vehicle speed from the preset setpoint value.

There is preferably provided a second mode switch which allows switching between a preset mode and a constant mode. In the preset mode the vehicle is basically driven at the preset speed. In this mode of operation however, the vehicle speed can be modulated as described by operation of the accelerator pedal or the brake pedal or by vehicle parameters. In the constant mode however, the vehicle speed does not react to such influences and remains fixed (programmed) at the preset set-point value selected by the second pre-selector device. However, for reasons of safety, if the foot brake is stepped on beyond a predetermined pressure value, the drive wheels are automatically isolated from the drive train and there is braking of the vehicle.

The mode switches are preferably electrical microswitches arranged in the region of the respective hand grips of the manual throttle and the speed selector lever, through which switching between preset mode and constant mode is possible. The microswitches can be microswitches operable by one finger of the hand holding the hand grip, for example.

To facilitate operation, the operating states of the primary drive engine, of the switchable PTO shaft andlor of the steplessly adjustable gearbox are sensed as electrical actual values and the J preset values of the first and/or second pre-selector devices are sensed as electrical set-point values. The sensed setpoint and actual values can be displayed on a monitor in an analog set-point versus actual comparison diagram. The operator can thus determine at any time whelEher the values set by the pre-selector devices are actually being attained. The operating states of the mode switches can also be shown on the monitor.

The speed of the propulsion engine and of the PT0 shaft are preferably shown alternatively on the monitor, a suitable switch-over being effected. It is then advantageous to switch over the associated labelling and scaling of the set-point versus actual display on the monitor simultaneously with the switching of the display.

A rapid but also sensitive adjustment of the vehicle or gearbox output speed can preferably be effected by a spring centred speed adjusting lever movable in two directions and which comprises in each of the two directions a first pressure point, at which a slow adjustment of the preset value takes place, and a second pressure point, at which a rapid adjustment of the preset value takes place.

The second pre-selector device is thus formed like a kind of joystick.

This allows precise and sensitive adjustment within narrow speed ranges, as is necessary in creeper operation, and rapid shifting of the preset value over large speed ranges, as in desirable in transport operation.

According to a preferred embodiment of the invention there are provided a reversing switch for switching between forward and reverse travel and a controller which provides a timed course of the switchover such that the time interval for attaining the preset forward or reverse speed is dependent on the position of the first pre-selector device. If for example the first pre-selector device is in the vicinity of the idle setting, there is a slow approach to the preset vehicle speed value. With increasing engine load or engine speed, the time interval within which the preset vehicle speed is actually attained falls off. In the full throttle setting of the first pre-selector device there is a rapid approach to the preset vehicle speed value.

By means of the solution according to the invention the operator can be relieved of many operating steps, in that the control device automatically holds constant the engine speed and thus the PTO shaft speed and/or the vehicle speed.

The control device according to the invention facilitates in particular the following operating modes:

A. Constant mode both for the vehicle speed and the engine speed.

For this both the vehicle speed and the engine speed are fixedly preset by the operator. They are not affected by either the accelerator pedal or the brake pedal. Such a mode of operation is for selection when a fixed PTO shaft speed is desired with the vehicle stationary for example. It can also be desirable 35 when muck spreading.

B. Constant mode for vehicle speed and preset mode for engine speed. The preset vehicle speed is fixed while the preset engine speed and thus the PTO shaft speed can be affected by actuation of the accelerator or brake pedal. This mode of operation allows control with a view to the smallest fuel consumption, insofar as the engine is not driven under full load. It can be used for example in planting out seedlings.

1 . i C. Preset mode for the vehicle speed and constant mode for the engine speed. The preset engine speed and thus the PTO shaft speed is fixed while the preset vehicle speed can be affected by actuation of the accelerator or brake pedal. This mode of operation is advantageous with PTO work with heavy soil engagement and on hilly ground.

D. Preset mode for the vehicle speed and also for the engine speed.

The preset engine speed and the preset vehicle speed can be affected by actuation of the accelerator or brake pedal. This mode of operation can be advantageously used in transport driving, in which the vehicle speed can be modified by actuation of the accelerator pedal.

The first and the second pre-selector device can each be so connected to the control members of the gearbox or the engine through a mechanical transmission mechanism that the basic setting of the gearbox and of the engine is preset thereby. By means of an electrical and/or hydraulic auxiliary control the mechanical transmission;mechanism can be adjusted in dependence on accelerator pedal actuation, brake pedal actuation and/or derived parameters. The basic setting for the vehicle speed and for the engine speed can thus be modulated by the auxiliary control. The operability of the contol members is ensured by this design even with failure of the electrical or hydraulic systems.

The mechanical transmission mechanism is preferably so designed that it will also function without an auxiliary control and that it can be selectively supplemented by an auxiliary control, whereby the set value of the mechanical transmission mechanism can be overridden by the auxiliary control. This design makes it possible with the use of identical transmission mechanisms to provide in the alternative a simple, economical embodiment without a control device according to the invention as well as a luxury embodiment with the control device according to the invention. The identical design of the transmission mechanism results in cost advantages in manufacture.

Suitable gear and engine control strategies are advantageously integrated in the engine control and the gearbox control.

The invention and further advantages and advantageous developments will now be described in more detail and explained with reference to the drawings, which show embodiments of the invention and in which:

Figure 1 shows the hydraulic and electrical circuit diagram of an operator interface according to the invention, Figure 2 is the image on a monitor which displays the operating state of the vehicle, Figure 3 is a schematic view illustrating the adjusting range of the manual throttle, and Figure 4 is a schematic view illustrating the adjusting possibilities of the speed adjusting lever.

The operator interface shown in Figure 1 includes a manual throttle lever 10 which can pivot in the direction of the arrow, mounted at a pivot point 12. Away from the pivot point 12 the manual throttle lever 10 engages a cylinder 14 of a hydraulic actuator so that the cylinder 14 is displaced axially on pivoting the manual throttle lever 10. Within the cylinder 14 is a piston 16 which engages a pivotally mounted adjusting lever 18. The adjusting lever 18 is coupled to an engine control device 20, not shown in detail. By pivoting the adjusting lever 18 a set-point value for the engine speed can be set. The engine control device 20 detects the actual value of the engine speed, effects a set-point versus actual comparison of the engine speed and regulates the engine speed to the value determined by the adjusting lever 18 by control of the fuel supply.

The two ends of the hydraulic cylinder are connected by hydraulic lines 22, 24 to a hydraulic, double-ended magnetically actuated, spring centred 4/3-way valve 26. With the electromagnets 28, not excited, the valve 26 is set in its central position by spring force. In this position the two hydraulic lines 22, 24 are blocked, so that the Diston 16 cannot shift relative to the cylinder 14 and the manual throttle lever 10 is rigidly coupled to the adjusting lever 18, so that pivotal movement of the manual throttle lever 10 results directly in pivoting the adjusting lever 18.

The two electromagnets 28, 30 of the valve 26 are each connected to the tap of an associated potentiometer 32, 34. The potentiometers 32, 34 are connected between vehicle earth 36 and the positive terminal of a vehicle battery 38. The one potentiometer 32 is adjustable by actuation of the brake pedal 40 and the other potentiometer 34 by actuation of the accelerator pedal 42, as indicated by broken action lines. If the brake pedal 40 and the accelerator pedal 42 are not actuated, the respective taps of the potentiometers 32, 34 are at the potential of the vehicle earth. Thus the electromagnets 28, 30 are not energised and the valve 26 assumes its illustrated central position.

With increasing actuation of the brake pedal 40, the electromagnet 28 is excited and the valve 26 is displaced more and more to its illustrated left position, in which the hydraulic line 22 is connected to a pump 44 and the hydraulic line 24 is connected to a reservoir 46. on account of the reduction of the hydraulic pressure of the hydraulic pump 44 effected by the valve 26, the piston 16 is moved to the right relative to the cylinder 14 in correspondence with the actuation of the brake. In this way the adjusting lever 18 is so pivoted, with the manual throttle lever 10 staying still, that the engine control device 20 reduces the engine speed.

If however the accelerator pedal 42 is actuated, the electromagnet 30 responds and moves the valve 26 into its illustrated right position, so that the hydraulic line 22 is connected to the reservoir 46 and the hydraulic line 24 to the hydraulic pump 44. As a result of the reduction in the hydraulic pressure of the hydraulic pump 44 effected through the valve 26, the piston 16 is now moved to the left relative to the cylinder 14, which results in an increase in the engine speed with the manual throttle lever 10 in fixed position.

Accordingly an engine speed can be pre-selected with the manual throttle lever 10 but which can be altered by actuation of the brake pedal 40 or of the accelerator pedal 42. By means of a microswitch 48, which can be arranged in the region of the hand grip 50 of the manual throttle lever 10, the potentiometers 28, 30 can be disconnected from the vehicle battery 38, so that actuation of the brake pedal 40 and of the accelerator pedal 42 has no effect on the position of the valve 26 and the setting of the engine speed.

The microswitch thus serves to switch over between a preset mode, in which the engine speed set by the manual throttle lever 10 can be affected by actuation of the brake pedal 40 or the accelerator pedal 42, and a constant mode in which no such influence is allowed.

In Figure 1 there is also shown a speed adjusting lever 56 pivotal in the direction of the arrow and mounted on a pivot point 54 and on which the cylinder 58 of a hydraulic actuator engages.

Similarly to what has already been described in connection with the engine speed, a movable piston 60 is arranged inside the cylinder 58 and is connected to a adjusting lever 62, which in turn provides the input signal"(set-point signal) for a gearbox control device 64, so that, by pivoting the adjusting lever 62, the transmission ratio of the steplessly variable gearbox, not shown in detail, can be adjusted.

The gearbox can be a hydro-mechanical power splitting gear as is described in EP-A 0 302 188 or EP-A 0 444 472 for example.

Here also the two ends of the cylinder 58 are connected through hydraulic lines 66, 68 to a double ended magnetically actuated, spring centred 4/3-way valve 70, which subjects the two ends of the cylinder 58 selectively to the pressure of a hydraulic pump 44 or of a reservoir 46. The electrical power for the electromagnets 72, 74 of the valve 70 is set through the potentiometers 76 and 78, whose taps are adjustable by the accelerator pedal 42 or the brake pedal 40.

With increasing actuation of the accelerator pedal 42 the electromagnet 76 is energised and the valve 70 is moved into its left position, in which the ydraulic line 66 is connected to the hydraulic pump 44 and the hydraulic line 68 to the reservo..Lr 46. Through the hydraulic pressure derived from the hydraulic pump the piston 60 moves relative to the cylinder 58 to the right. The adjusting lever 62 is thus so pivoted with the speed adjusting lever 56 fixed that the gearbox control device 64 increases the vehicle or gearbox output speed in correspondence with the actuation of the accelerator pedal.

If on the other hand the brake pedal is actuated, the electromagnet 74 responds, which results in pressurisation of the right side of the cylinder 58 and displacement of the piston 60 to the left. The gearbox control device 64 reduces the vehicle or gearbox output speed in this case in correspondence with the brake pedal actuation. If the brake pedal is stepped on beyond a certain amount, a brake switch, not shown, is actuated and provides a corresponding control signal to the gearbox control device 64, so that this isolates the gearbox from the final drive of the vehicle. The vehicle is thus no longer driven by the engine and can be brought to a standstill.

Accordingly a specific vehicle speed can be set with the speed adjusting lever 56 and this can be modified by actuation of the brake pedal 40 or the accelerator pedal 42. A microswitch 80 is also provided in this circuit, through which the potentiometers 76, 78 can be isolated from the battery 38, so that actuation of the accelerator pedal 42 or the brake pedal 40 has no effect on the gearbox adjustment. The described brake switch however still reta:ins its function, so that emergency braking is always available.

The microswitch 80, which can be arranged in vicinity of the hand grip 82 of the speed adjusting lever 56, serves to switch over between a preset mode, in which the vehicle speed set by the speed adjusting lever 56 can be affected by actuation of the accelerator pedal 42 or the brake pedal 40, and a constant mode, in which no such influence is allowed.

In Figure 2 can be seen the picture of a monitor 84 with a digital engine speed display 86 and a digital vehicle speed display 88. Below the two displays 86, 88 there is an associated twopart analog bar display 90, 92, whose left bar shows the magnitude of the set-point signal which is set by the manual throttle lever 10 or the speed adjusting lever 56 and is modulated by the brake pedal 40 and the accelerator pedal 42, and whose right bar shows the magnitude of the measured actual value signal. Beside each bar display there can be a scale, not shown, which allows the indicated values to be read off.

The engine speed display 86 and the scaling of the associated bar display 90 can be designed to be switched, so that, with a suitable switch, either the engine speed or the speed of rotation of the PTO shaft can be read and displayed on the monitor.

Beside each of the bar displays 90, 92 is an indicator 94, 96; these indicate the switching state of the microswitches 48, 80 and indicate whether the respective preset mode or constant mode is set.

Figure 3 illustrates the adjusting range of the manual throttle lever 10, which is adjustable between an idling position A and a setting B for maximum engine and PTO shaft speeds. A throttle vane, not shown, of the engine is set in dependence on the adjustment of the manual throttle lever 10. A scale 98 in the region of the manual throttle lever 10 facilitates the adjustment of the desired engine speed. The engine or PTO shaft speed is shown digitally by the engine speed display 86 or as an analogue by the bar display 90 on the monitor 84.

With the constant mode set, an automatically operating engine control device 20 maintains the pre-selected engine speed even with changing load. The set, constant engine speed cannot then be overridden by actuation of the accelerator pedal 42 or the brake pedal or by another algorithm.

If the preset mode is set by the microswitch 48. the engine speed set by the manual throttle lever 10 can be overridden by actuation of the accelerator pedal 42 or the brake pedal 40. This is indicated by the double arrow P in Figure 3. In the same manner as override by actuation of the accelerator pedal 42 or the brake pedal 40, the setting of the manual throttle lever can be overridden by any other vehicle parameter. Such parameters can for example be derived from the load condition of the engine or a requirement for the smallest possible fuel consumption.

Figure 4 is a view of a speed adjusting lever, which is formed like a joystick and carries one of the microswitches in the region of its hand grip. The speed adjusting lever is in its central, neutral position N and can pivot out of its central position N in two directions in a plane. Through this the pressure points D, D+, T and T+ can be approached. With an approach to the pressure points D and D+ the originally set set-point value for the vehicle speed is slowly reduced or increased. The setting of the se'L-.-point value can be effected sensitively and be observed on the vehicle speed display 88 of the monitor 84. with an approach to the pressure points T and T+ the set-point value of the vehicle speed is reduced or increased rapidly. This allows a rapid, coarse approach to the desired set point value. The vehicle speed can be adjusted between stationary and a maximum value with the speed adjusting lever.

Although the invention has merely been described with reference to the preceding embodiments, many alternatives, modifications and variants which fall within the scope of the present invention will occur to the man skilled in the art in the light of the preceding description. Thus the electro-hydraulic determination of the set point value according to Figure 1 can also be effected electronically with the manual throttle lever, the brake pedal and the accelerator pedal each provided with an electrical transducer with provides electric signals corresponding to the present operating swing to an electronic control unit. The speed adjusting lever can be formed like a joystick, as described with reference to Figure 4 and be connected to the control unit. The control unit further detects the switching state of the microswitches through which the operating mode can be set, as well as the actual value signals of the engine or PTO shaft speed and the gearbox output or vehicle speed. The control unit is so programmed that the functions described above are carried out.

Claims

1. An operator interface for controlling the primary drive engine of a vehicle, by...which at least one selectively engageable PTO shaft can be driven, and a gearbox which is connected downstream of the engine and is at least partially steplessly adjustable, characterized by a first pre-selector device (10) for controlling a basic load or a basic speed of rotation of the primary drive engine, which drives the at least one PTO shaft (52) independently of the vehicle propulsion, a second pre- selector device (56) through which a desired vehicle or gearbox output speed can be selected independently of the drive speed of rotation, and at least one control device (40, 42) operable by an operator, through which the preset basic load or basic speed of rotation of the primary engine and/or the preset vehicle or gearbox output speed can be varied or modulat-ed in operation.

2. An operator interface according to claim 1, characterized in that at least one of the preset values can be varied or modulated by parameters derived from the operation of the vehicle.

3. An operator interface according to claim 1 or 2, characterized by a first mode switch (48), through which variation or modulation of the preset basic load or basic speed of rotation of the primary engine can be selectively prevented, andlor a second mode switch (80), through which variation or modulation of the preset vehicle speed or gearbox output speed can be selectively prevented.

4. An operator interface according to any of claims 1 to 3, characterized in that the first pre-selector device comprises a pivoted throttle lever (10) and/or the second pre-selector device comprises a pivoted speed adjusting lever (56), whose pivot angles are each sensed by an analog transducer.

5. An operator interface according to any of claims 1 to 4, characterized in that the at least one control device comprises an accelerator pedal (42) or a brake pedal (40).

6. An operator interface according to claim 4 or 5, characterized in that the mode switch is an electric microswitch (48, 80) arranged in the region of the grip (50, 82) of the manual throttle lever (10) or the speed adjusting lever (56).

7. An operator interface according to any of claims 1 to 6, characterized in that the operating states of the primary drive engine, of the switchable PTO shaft (52) and/or of the steplessly adjustable gearbox are sensed as actual values and the preset values of the first and/or second pre-selector devices (10, 56) as set-point values and in that the sensed set-point and actual values are contrasted on a monitor (84) in an analog set-point versus actual comparison.

8. An operator interface according to claim 7, characterized in that a switch-over device is provided, through which the scaling and labelling of the set-point versus actual display can be switched between the primary drive engine speed of rotation and the speed of rotation of the directly coupled PTO shaft (52).

9. An operator interface according to any of claims 1 to 8, characterized in that the second pre-selector device contains a spring centred speed adjusting lever (56) movable in two directions and which comprises in each of the two directions a first pressure point (D, D+), at which a slow adjustment of the preset value takes place, and a second pressure point (T-, T+), at which a rapid adjustment of the preset value takes place.

10. An operator interface according to any of claims 1 to 9, characterized in that there are provided a reversing switch, for switching between forward and reverse travel, and a controller and in that the controller provides a timed course of the switchover such that the time interval for attaining the preset forward or reverse speed is dependent on the position of the first pre-selector device.

11. A control system for controlling the primary drive engine of a vehicle, which drives a PTO shaft, and a gearbox which is downstream of the engine and at least partially steplessly adjustable, the system including a first preselector device for setting a basic load or basic speed of rotation of the engine to determine the speed of rotation of the PTO shaft independent of the required vehicle speed, a second preselector device to set the steplessly variable gearbox for a desired vehicle speed or gearbox output speed independent of the speed of the engine, and a control device operable by the operator through which the preset vehicle speed or gearbox output speed can be varied or modulated during operation.

12. A control system as claimed in claim 11 including a control device operable by the operator through which the preset base load or base speed of rotation of the engine can be varied or modulated during operation.

13. A control system as claimed in claim 11 or 12 in which the or each preselector comprises a settable member and a setting member coupled by a piston and cylinder unit which provides a rigid coupling for presentation but is operable by supply and removal of fluid pressure to vary the position of the settable member relative to the setting member.

14. A control system as claimed in claim 13 in which the supply and removal of fluid pressure to each preselector is controlled by a respective valve whose position is electromagnetically determined in accordance with movements of a brake pedal and an accelerator pedal of the vehicle.