This invention relates to an electronically contro¬ lled hydraulic drive system for a vehicle such as a tractor.

Hitherto the major problem of hydraulically driven vehicles, such as tractors, with hydraulically driven wheel motors has been to control the hydraulic transmission in such a manner as to maintain a steady vehicle speed over varying terrain and to avoid such loads on the ^' prime mover such as a diesel engine, tending to cause the engine to stall. When the driven vehicle wheels encounter increased road or ground resistance, the engine load increases and there is a tendency for the engine speed to drop. In extreme cases, this results in stalling.

It is an object ot this invention to provide an electronically controlled hydraulic drive system which automatically compensates for variations in engine loads.

According to the present invention there is provided a hydraulic drive system for a vehicle such as a tractor, comprising an internal combustion engine coupled to drive a hydraulic transmission pump with a variable displacement controlled by a swash plate, motor means arranged to displace the swash plate , throttle control means for controlling the desired speed of the engine, means for encoding a first signal representative of the setting of the throttle control means, engine speed sensor means generating a second signal representative of the engine speed, computing means receiving the first and second signals electronically deriving from these signals a third control signal which is applied to control the motor means to displace the swash plate to a position in which the load placed by the hydraulic transmission pump on the engine enables the engine to maintain

engine speed directly related to the setting of the throttle control means.

In an embodiment, the computing means is arranged to receive a fourth signal indicative of the swash plate null position and a fifth signal indicating rotary dis¬ placement of the motor means and hence the swash plate, from which fourth and fifth signals the actual position of the swash plate is derived, from which position the required displacement of the swash plate to a desired position is computed according to the instantaneous value of said third control signal, and the displacement is translated into a sixth control signal which- is • applied directly to the motor means.

In this embodiment electrical switch means are incorporated in order to determine the polarity of a D.C. sixth control signal according to the intended direction of travel of a hydraulic wheel motor driven via the hydraulic transmission pump.

In an embodiment the computing means only generates ^a third control signal when the first and second signals indicate a differential between the desired engine speed and the actual engine speed of at least 200 r.p.m. It is preferred to program the computing means such that the relationship between actual engine speed from idling speed up to maximum engine speed results in a sixth control signal having a predetermined relationship between the angle © of swash plate displacement relative to the swash plate null position and the level of actual engine speed above idling speed. In one case said pre-? determined relationship is linear, although polynomial relationships are possible.

In one embodiment, the hydraulic transmission purπp is arranged to drive a pair of hydraulic wheel motors. The engine may also be arranged to drive an auxiliary hydraulic pump for "power-take-off" devices when the vehicle is required to drive auxiliary equipment. In

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this case, cpntrol signal to the motor means for the swash plate may compensate for the additional load on the engine attributable to the auxiliary pump.

In a modification a separate hydraulic transmission pump may be provided for each of a pair of hydraulic wheel motors and each motor may be controlled separately by the computing means. If the computing means is arranged to receive an encoded signal from the vehicle steering means, then the swash plate of one transmission 10 pump may be advanced relative to the other for the purpose of assisting the steering.

Embodiments of the invention will now be described by way of example only, with reference to the accompanying drawings, in which: 15 Fig. 1 shows schematically an electronically controlled hydraulic drive system for a tractor;

Fig. 2 shows schematically the electronic control system of Fig. 1;

Fig. 3 shows a graph illustrating a relationship 20. between engine speed and pump displacement; and,

Fig. shows an electronic control circuit utilising a commercially available microprocessor unit.

Figs. 1 and 2 show a hydraulic drive system for a tractor. A diesel engine 1 drives a hydraulic transmission

2 ^J 5 pump 2 with a variable displacement controlled by a swash plate 3- A D.C. motor displaces the swash plate 3- Throttle control means (not shown) control the desired speed of the engine 1. Encoding means generate a first signal A representative of the setting of the throttle

30 control means. Engine speed sensor- mea.ns 6 generate a second signal B representative of the engine speed. Computing means 10 receive the first and second signals A and B and electronically derive from these signals A and B a third control signal C (not shown) which is

35 applied to control the motor . to displace the swash plate 3 "to a position in which the load placed by

hydraulic transmission pump 2 on the engine 1 enables the engine 1 to maintain an engine speed directly related to the setting of the throttle control means. The computing means 10 received a fourth signal D indicative of the swash plate null position and a fifth signal E indicating rotary displacement of the motor 4 and hence the swash plate 3 - The computing means derives from the fourth and fifth signals D and E the actual position of the swash plate 3« From this position the required displacement of the swash plate 3 to a desired position is computed according to the instantaneous value of said third control signal C, and the displacement is translated into a sixth control signal F which ^" is applied directly to the motor 4. Electrical switch means 8 are incorporated in order to determine the polarity of a D.C. sixth control signal F according to the intended direction of travel of a hydraulic wheel motor 9 driven via the hydraulic trans¬ mission valve 11 coupled to transmission pump 2.

Reference is made to Fig. 3« The computing means 10 only generates a third control signal C when the first and second signals A and B indicate a differential between the desired engine speed ND and the actual engine speed NA of at least 200 r,p.m. It is preferred to program the computing means 10 such that the relationship

N between actual engine speed A and desired engine speed

N D from idling speed up to maximum engine speed Nm results in a sixth control signal F having a predetermined relationship between angle ft< of swash plate displacement relative to the swash plate null position and the level of actual engine speed NA above idling speed.

In one embodiment, the hydraulic transmission pump is arranged to drive a pair of hydraulic wheel motors.

The engine may also be arranged to drive an auxiliary hydraulic pump for "power-take-o f" devices when the vehicle is required to drive auxiliary equipment. In this case, control signal to the motor means for the

• swash plate may compensate for ^' the additional load on the engine attributable to the auxiliary pump.

As indicated above, the variable stroke hydraulic pump 2 is driven by a diesel engine 1 and supplies oil to two radial hydraulic wheel motors 9 via a transmission valve 11.

Auxiliary facilities such as power-take-off motors and hydraulic lift (3-point linkage) are supplied with oil from a pump 12. The control unit based on computing means 10 permits automatic drive for the .transmission system.

N Control of the tractor travelling speed A is effected through the pedal or hand throttle control means.

With this system sensitive and accurate control is

N possible. The engine speed A remains constant for a given throttle position even if the road resistance changes or the tractor meets with an obstacle. This is achieved because the load on the engine 1 is monitored at all times. If the engine 1 becomes overloaded and its

N speed A starts to fall, then the oil flow rate from the transmission pump 2 is reduced in order to reduce the load on the engine 1.

When the auxiliary hydraulic facilities are used, such as the hydraulic lift or PT0, the transmission pump automatically destrokes to prevent the engine 1 being overloaded.

As seen from Fig. 2 the control of the system is based on the micro-computor or computing means 10. This is energised by the battery 13 _* The computing means 10 by means of signals A, B and F monitors engine speed, throttle setting and provides control of the swash plate angle ^~ * on the transmission pump 2.

The operation of the system is as follows: Signals A and B are monitored by the computing means 10. The resulting signal is proportionally translated into an instruction signal F so that the swash plate 3 is driven

N to an angle & so that the engine speed A which is con-. stantly being monitored, does not fall by more than a specified amount below the desired speed. Electrical switch 8 is used to select forward, neutral and reverse. Signal B is derived by means 6 situated on the periphery of a fly-wheel of the engine 1. Sensor means 6 is provided by a small hole in the periphery of the fly¬ wheel (not shown) which passes over a coil (not shown) mounted close the the fly-wheel. This generates a pulse 0 once for eVery engine revolution. Signal B then comprises a series of pulses sent via an amplifier (not shown) and an analogue to digital converter 20 to the computing means 10.

The throttl.e control means (not shown) is linked to 5 the engine 1 in a conventional manner with either a cable or linkage mechanism and the throttle control position is monitored by a rotary encoder 5 which provides signal A to the computing means 10.

The swash plate 3 is driven by the 12 V DC motor 4 0 through a worm gearbox arrangement (not shown). The rotary motion of the motor is monitored by an emcoder l4 which generates signal E which is a measure of the movement of the swash plate. Since this encoder l4 is not intended to indicate the absolute position of the swash plate 3 but onl 5 encode the relative movement of the swash plate 3, means a provided to indicate the null position of the swash plate. This null position is determined by two pressure transduce 11 which are situated in the main hydraulic feed lines 15, l6 either side of the transmission pump 2. The signal D is Q the resultant of the indication of one transducer output subtracted from that of the other transducer. If the oil flow through pump 2 is in one direction then the signal D is positive and for the other flow direction signal D is negative. In the null position there is no resultant signal 5 D. Means 18 is an analogue to digital converter for signal D.

Consequently, if the direction switch 8 is in the neutral position or the throttle pedal (not shown) is in the zero speed position, then the computing means 10 generates a signal F which via motor voltage supply switch 19 causes the swash plate 3 to be displaced to the null position by providing a signal F which energises motor 4 until signal D is zero.

A direct linkage between the throttle control and the engine 1 then ensures that the engine speed will be directly proportional to throttle pedal position. The swash plate angle and hence the vehicle speed will depend not only on the pedal position but on the load on the engine 1. These relationships are explained with reference to Fig. 3-

N Let the desired engine speed be D, as given by the

N throttle pedal position, A be the actual engine speed, as

N indicated by the sensor 6. If 1 is the engine idling speed then the pump displacement is not ^' increased until the engine

N N \N speed has reached 2 under no load conditions where 2. / 1 by

500 rpm. The swash plate angle θ is then increased linearly

N with the engine speed until the maximum engine speed max

(3000 rpm) is reached when the swash plate 3 s at its maximum angle. Computing means 10 is programmed so that

N N

D = A within 200 rpm at all times in the region above 2.

N N Computing means 10 computes D - A at all times and if

ND - NA 200 rpm then either o signal will be sent to the swash plate control motor 4 or a signal be sent so that the

N N swash plate angle is reduced until D - A 200 rpm. Thus the couputing means 10 increases the swash plate angle -& to give maximum torque in the hydraulic motors 9 at a given

N engine speed A.

If the auxiliaries such as a power take off are used then the computing means 10 may be arranged to make the necessary adjustments to ensure that the engine 1 is not overloaded.

In order to more fully understand the nature and function of the computing means 10 reference is now made t Fig. 4.

Integers with like references are as described above. It will be noted that the drawing shows an analogue to digital converter 20 for signal B from the sensor 6.

The input/output signals A,B,D and F are applied to th computing means 10 which is a MOTOROLA MICRO PROCESSOR 680 "via a parallel 1/0 interface. Also connected to the 1/0 interface is the switch 8 for controlling the direction of current to the D.C. motor and hence the tractor direction.

The computing means performs the following computation functions:-

(1) It counts the number of pulses from the sensor 6 over set period of time (t) and stores the total number of pulses NA.

(2) It stores the throttle position by reading the signal A from digital encoder 5 on the throttle control means. The value given by the encoder is multiplied by a calibration factor to give the equivalent of engine speed D on the same basis as that in (l).

(3) After each cycle of time (t) the computer subtracts

A from ⁱ D and compares the result with 200 (rpm). If

N D - 200 then the D.C. Motor 4 driving the swash plat

3 in switched on via a signal F to a solenoid operated control switch 19 and the motor 4 is allowed to run until

^' ?N

N N jr

D - A^200 when the current to the motor is switched off.

N N If D - A <-_, 200 no action is taken.

(4) The direction of motion of the tractor is selected by switch 8 which controls the direction of current through the D.C. motor. This is performed via the micro computer I/O.

N

(5) If the desired speed D is zero then the computing means

10 monitors the pressure difference across the variable stroke pump 2. Signals from two pressure transducers 17 in the fluid flow lines 15 i l6 either side of the pump 2 are subtracted, and the result digitised via an ADC l8. This value (P) and sign of this pressure difference from the transducers is also determined over a cycle time (t) and stored. P is examined by the computer and compared with a small pressure p (= 5 psi).

If p - p> +5 then the D..C. motor 4 is switched on and driven until p - p<+5. If p - p> -5 then the motor 4 is driven until p - p<-5«

This action ensures that the tractor remains stationary when the throttle position is at zero.

(6) A start/run switch 21 is a safety device which ensures that with this switch in the Start position the engine can only be started with the swash plate 3 n the null position. Afterwards the switch 21 is turned to Run. This switch 21 ensures that the engine starter cannot be operated until switch 21 is in the Start position.

A micro switch (not shown) is arranged to be actuated in the null position of the swash plate and this micro switch is connected in the engine starter circuit so that the engine cannot be started until the swash plate is in the null position.

Once the engine is started, the swash plate is accurately positioned at the null point via the method described in (5) above. -

(7) If the engine fails when the tractor is at speed, the the swash plate may return to the null position by turnin the Start/Run switch to Start. This causes the motor 4 to run until the null sensing micro-switch is activated.

Braking may be effected using the hydrostatic transmission system in conjunction with a special brake pedal which is independent of the conventional braking system of the tractor. The special brake pedal controls encoder 22, (brake position transducer 22, Fig. 2), simil to the throttle position encoder 5 - _% such that when the pe is pressed a signal is sent to the micro computer 10 • indicating' the position of the pedal. When the pedal is moved and the micro computer 10 senses the movement, the swash plate 3 is progressively moved towards the null position so that with the pedal fully depressed, the swas plate 3 s in the null position. The braking system operates both in forward and reverse transmission. The braking instructions have priority over the throttle cont system.

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