Engine speed control system and method

Abstract

An engine speed control system of a vehicle, uses a phase-lag compensator to derive a compensated error signal between an engine speed and a target engine speed. An engine speed controller controls the engine speed based on the compensated error signal. A gain scheduler is used to generate a gain factor and time constant for the phase lag compensator based on the engine speed and a vehicle speed. In this way, the vehicle speed can be taken into account in the engine speed control, for example to provide a different compensation transfer function for times when the engine is driving different loads with different inertia.

Description

TITLEENGINE SPEED CONTROL SYSTEM AND METHODFIELD

[0001] Embodiments of the present disclosure relate to the control of engine speed, for example for agricultural vehicles such as tractors.BACKGROUND

[0002] It is well known to use a feedback control system to regulate engine speed. The aim is to maintain a desired engine speed under varying conditions and loads. The actual speed of the engine is monitored, and inputs are adjusted such as the fuel, throttle, or air supply to ensure the engine operates at a stable and optimal speed, regardless of external disturbances.

[0003] In this way, the engine speed can be regulated to compensate for changes in load or environmental conditions (e.g., uphill driving, acceleration, or additional mechanical loads). For example, when an engine experiences changes in load, the engine tends to slow down due to increased resistance. The feedback system detects this speed reduction and increases the power output (e.g., adjusting throttle or fuel injection) to bring the speed back to the desired level.

[0004] By precisely controlling the engine speed, the system can optimize fuel delivery, ensuring the engine operates efficiently under various conditions. This contributes to improved fuel efficiency and reduced emissions since the engine is kept running within its optimal performance range. The feedback control also improves engine performance and response, for example ensuring that the engine responds accurately to changes in throttle input.

[0005] The feedback control sets response characteristics of the system by processing an error between the measured engine speed and a desired engine speed.

[0006] This disclosure is based on the recognition that some vehicles are intended to be driven at very different vehicle speeds for different operations. For example, a tractor is driven at low speed when working on a field and using an implement carried by, or towed by,the tractor and at high speed when driving on a road. When the tractor is driven at high speed, especially during road transport, the system dynamics are drastically different compared to operation at low speeds or with the drivetrain disengaged.

[0007] The inertia of a vehicle is effected on by the engine through the drivetrain, which changes the design requirements for the engine speed control. The presence of a torsional vibration damper between the engine and the transmission further complicates the system dynamics. These variations can result in the engine speed control giving rise to instability issues, which then requires more complex transmission control and / or leads to less accurate vehicle speed control which is clearly noticeable to the driver.

[0008] There is therefore a need for an improved engine speed control approach.BRIEF SUMMARY

[0009] The concept of this disclosure is defined by the claims. According to examples in accordance with this disclosure, there is provided an engine speed control system for a vehicle, comprising:

[0010] a phase-lag compensator for receiving an error signal between an engine speed and a target engine speed and for outputting a compensated error signal, wherein the phase-lag compensator applies a gain factor and a time constant to derive the compensated error signal; and

[0011] an engine speed controller for providing an engine speed control signal for controlling the engine speed based on the compensated error signal,

[0012] wherein the system further comprises a gain scheduler for generating the gain factor and time constant based on the engine speed and a vehicle speed.

[0013] In this system, the error signal provided to the engine speed controller is adapted depending not only on the engine speed, but also on the vehicle speed. In this way, different vehicle configurations at different speeds (e.g. driving on the road or using implements in a field) can be taken into account.

[0014] The engine control is effectively adjusted in real-time, for example based the actual transmission ratio between the engine speed and the vehicle speed.

[0015] The different inertial loads driven by the engine can be taken into account by considering the engine and vehicle speeds, so that the engine speed control transfer function is suitable for the particular operating conditions, when applied to the compensated error signal. This enables improved speed control (less variation over time), smoother engine control and improved fuel economy.

[0016] The engine speed controller for example applies a total gain when processing the compensated error signal, wherein the gain scheduler generates the total gain based on the engine speed and the vehicle speed. Thus, the dynamic and steady state parameters of the engine speed control loop are adapted in dependence on both the engine speed and the vehicle speed.

[0017] The gain scheduler is for example configured to apply a total gain such that the open loop gain is substantially constant at different vehicle speeds.

[0018] The engine speed controller for example controls engine control parameters for adjusting an engine torque. The engine control parameters are for example fuel and / or air intake control.

[0019] The engine speed controller for example comprises a PID controller. It has a gain that may be set by the gain scheduler and it operate on an input signal in the form of the compensated error signal.

[0020] This disclosure also provides a vehicle comprising:

[0021] an engine;

[0022] an engine speed sensor;

[0023] a vehicle speed sensor;

[0024] a drivetrain; and

[0025] an engine speed control system as defined above.

[0026] The vehicle for example comprises a tractor.

[0027] This disclosure also provides a method of controlling an engine speed of a vehicle, comprising:

[0028] receiving at a phase-lag compensator an error signal between an engine speed(<JL>) and a target engine speed;

[0029] generating using a gain scheduler a gain factor and a time constant based on the engine speed and a vehicle speed;

[0030] applying the gain factor and the time constant using the phase-lag compensator to derive a compensated error signal; and

[0031] providing an engine speed control signal for controlling the engine speed using an engine speed controller based on the compensated error signal.

[0032] The method may comprise applying a total gain at the engine speed controller when processing the compensated error signal, wherein the method comprises using the gain to generate the total gain based on the engine speed and a vehicle speed.

[0033] The method may comprise applying a total gain using the gain scheduler such that the open loop gain is substantially constant at different vehicle speeds.

[0034] The method may comprise using the engine speed controller to control engine control parameters for adjusting an engine torque. The engine speed controller for example implements PID control.BRIEF DESCRIPTION OF THE DRAWINGS

[0035] One or more embodiments of this disclosure will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0036] Fig. 1 shows an engine speed control system;

[0037] Fig. 2 shows an example Bode diagram;

[0038] Fig. 3 shows plots of the engine speed governor error versus time;

[0039] Fig. 4 shows plots of the instructed engine torque versus time;

[0040] Fig. 5 shows plots of the vehicle speed versus time;

[0041] Fig. 6 shows plots of the distance travelled by the vehicle versus time;

[0042] Fig. 7 shows further plots of the vehicle speed versus time; and

[0043] Fig. 8 shows plots of the fuel consumed versus time.DETAILED DESCRIPTION

[0044] The invention will be described with reference to the Figures.

[0045] It should be understood that the detailed description and specific examples, while indicating exemplary embodiments of the apparatus, systems and methods, are intended for purposes of illustration only and are not intended to limit the scope of the invention. These and other features, aspects, and advantages of the apparatus, systems and methods of the present invention will become better understood from the following description, appended claims, and accompanying drawings. It should be understood that the Figures are merely schematic and are not drawn to scale. It should also be understood that the same reference numerals are used throughout the Figures to indicate the same or similar parts.

[0046] This disclosure relates to an engine speed control system of a vehicle that uses a phase-lag compensator to derive a compensated error signal between an engine speed and a target engine speed. An engine speed controller controls the engine speed based on the compensated error signal. A gain scheduler is used to generating a gain factor and time constant for the phase lag compensator based on the engine speed and a vehicle speed (and hence depending on the transmission ratio between the engine speed and the vehicle speed). In this way, the vehicle speed can be taken into account in the engine speed control, for example to provide a different compensation transfer function for times when the engine is driving different loads with different inertia.

[0047] Fig. 1 shows an engine speed control system of a vehicle. The vehicle has an engine E which drives a transmission T through a drivetrain torsional vibration damper D.

[0048] A phase-lag compensator Clag receives an error signal e between an engine speed co and a target engine speed SP. The phase-lag compensator Clag outputs a compensated error signal elc. The phase-lag compensator Clag applies a gain factor a and a time constant r to derive the compensated error signal elc.

[0049] An engine speed controller G (i.e., a speed governor) provides an engine speed control signal trq for controlling the engine torque and hence speed.

[0050] The engine speed controller G thus does not simply use the difference between the engine speed co and the target engine speed SP as an error signal to be processed,but instead it uses a compensated error signal elc. The compensation to generate the compensated error signal elc results in different engine speed control depending on the function implemented by the phase lag compensator.

[0051] In particular, the generation of the compensated error signal elc takes account of both the engine speed co and the vehicle speed v. It therefore takes into account the transmission ration between the engine and the vehicle drive, which is for example set by a continuously variable transmission (CVT) of the vehicle.

[0052] For this purpose, a gain scheduler Fgs is provided for generating the gain factor a and time constant r, and also a total gain Ktot, based on the engine speed co and the vehicle speed v. The total gain Ktot is provided to the engine speed controller G and the engine speed controller G applies the total gain Ktot to the compensated error signal elc.

[0053] By adjusting the engine speed control total gain based on the engine and vehicle speeds, the effect of increased inertia of the system is counteracted. The phase-lag compensator adjusts the controller dynamics to suppress the oscillations of engine speed and torque, which could for example arise due to the use of the drivetrain torsional vibration damper D. The relevant parameters of the phase-lag compensator are adjusted based on the engine speed and vehicle speed to achieve satisfactory behavior (total open-loop gain and stability) in different vehicle states.

[0054] The phase lag compensator Clag is a known component used to improve the stability and accuracy of an engine speed controller. It introduces a desired phase shift (specifically lagging the phase) at certain frequencies, helping to shape the system's dynamic response. By increasing the low-frequency gain relative to the high frequency gain, steady-state performance is improved, and system stability is improved. The phase lag compensator Clag modifies the control system transfer function, altering how the engine speed controller responds to changes in engine speed.

[0055] In the following equations, a((i),v) is the gain term a of the lag compensator as a function of engine speed cu and vehicle speed v and r(a>,v) is the time factor T of the compensator as a function of engine speed cu and vehicle speed v. s is the Laplace frequency.

[0056] The phase lag compensator Clag for example has a Laplace transfer functionClagfco, v) as a function of engine speed co and vehicle speed v given by:

[0057] The transfer function introduces a low-pass filter behavior, with higher gain at lower frequencies. The compensator introduces a phase lag over a certain frequency range (typically mid to high frequencies).

[0058] The relatively higher gain at low frequencies (which tends to 1 as s tends to zero) improves the steady-state error in engine speed control, particularly when the system is trying to reach or maintain a target speed. As the frequency increases, the compensator introduces a phase lag, meaning the response becomes slower in terms of reacting to changes. The gain tends to a as s tends to infinity. This provides increased stability to avoid overshooting or oscillations in the speed control system.

[0059] The phase lag compensator Clag thus modifies the error signal e by a factor between a and 1 depending on the engine speed and vehicle speed. The engine speed governor then introduces a gain Ktot. The engine speed governor G can be a PID-type governor or other suitable controller, which acts on the engine speed deviation from the setpoint by adjusting the engine torque appropriately.

[0060] The engine speed controller G is for example optimized for stable operation with only the bare engine. Such situation would be, for example, idling with the drivetrain disengaged. In such a situation, the total effective inertia of the system is the inertia of the engine, most of which is in the engine flywheel. The response, or the transfer function, of such a system is:&j(s) 1 1G(s) J' s

[0061] where )(s) is the angular engine speed in the Laplace domain, G(s) is the speed governor output, hence the torque acting on the physical system, and J is the inertia of the governed system.

[0062] This is the transfer function of the (idealized) physical system. It relates the engine speed (co) to engine speed governor output (engine torque). The inertia of the engine flywheel (and vehicle mass) is treated as an integrator, denoted by 1 / s.

[0063] The aim of the control is to retain the transfer function co(s) / G(s) as closely as possible to constant even when effective inertia J is changing.

[0064] To retain the response regardless of the inertia, the total gain factor Ktot needs to be added for the system. This total gain factor Ktot ensures that the response of the controller scales with the total inertia of the governed system. The aim of this feature is to keep the open-loop gain of the system constant regardless of the vehicle speed.

[0065] In the following equations, R is the radius of the driven wheel, Jfw is the inertia of the engine flywheel, and m is the mass of the vehicle.

[0066] The effective (angular) transmission ratio n as a function of the engine speed cu and vehicle speed v is: a> a>R n(a), v) = — — = - v / R v

[0067] The effective moment of inertia Jeff affecting the engine as a function of engine speed cu and vehicle speed v is:

[0068] This equation translates the mass of the vehicle to an equivalent moment of inertia (representing the vehicle mass as another flywheel coupled to the engine flywheel with1:1 ratio). It is well-known that the transmission ratio n affects the effective mass by factor of 1 / n2.

[0069] The torque produced by the engine effects acceleration of the engine flywheel, mounted directly at the end of the crankshaft, and acceleration of the vehicle, with a torque multiplied by the transmission ratio. This torque produces a force, which is the torque at the wheel hub divided by radius of the driven wheel. The angular velocity of the driveshaft is equal to the vehicle speed divided by wheel radius and to the engine speed divided by the transmission ratio. Thus v - coR / n.

[0070] The result is two equations of motion, one for the engine and another for the vehicle:Tvdv mR da>T.F = n • — = m— = - —R dt n dt mR2da> ” n2dt

[0071] Substituting this torque value Tvinto the first equation leads to an effective inertia term that resists the acceleration effected by the engine torque.

[0072] The effect of changing transmission ratio is that the inertia resisting the acceleration changes. The change can be continuous or happen in discrete steps.

[0073] The approach of this disclosure is for example suitable for use with a continuously variable transmission (CVT), which allows a continuum of ratios. However, the same approach may be applied to other transmissions such as a dual-clutch (DCT) or powershift (PS) transmission, where the change would happen in discrete steps.

[0074] One possible implementation of the total gain to be applied by the gain scheduling function, as a function of engine speed cu and vehicle speed v, and which preserves the open-loop gain, is:

[0075] This function can for example be implemented by calculating the effective inertia based on the vehicle speed, or it can use a two-dimensional look-up table with interpolation.

[0076] The equation - is modified by adding the Ktot term and replacing theG(s) J s total inertia with effective inertia:6 (s) _ Ktot(g), v) 1 _ 1 1 G(s) S Jfws

[0077] The transfer function is preserved for different engine and vehicle speeds.

[0078] In practice, the drivetrain is not a rigid system. Flexible elements complicate the dynamics and introduce potential resonances into the system. These resonant modes will be amplified by the total gain term, rendering the system potentially unstable.

[0079] Determining the resonances of the driveline is a complex topic and there are various methods for it. In the example of Fig. 1, there is a torsional vibration damper (TVD) between the engine flywheel and the transmission input shaft, which introduces resonances into the system. The TVD acts as a (nonlinear) damped oscillator coupled to two rotating elements, each with its own inertia: the engine flywheel on one side, and the mass of the vehicle on the other.

[0080] As explained above, the mass of the vehicle is seen as effective moment of inertia, the magnitude of which depends on the transmission ratio. Hence, the resonance frequency of such system shifts with changing transmission ratio. The lag compensator parameters thus also have to change based on the transmission ratio. The phase lag compensation attenuates the resonances and retains system stability.

[0081] The transfer function that includes the lag compensator and a model of the coupled inertias M(w,v) is:

[0082] It is not possible, in a general sense, to retain the transfer function shape across all frequencies. Hence optimization may be made based on different sets of constraints. For example, these constraints are the open-loop gain, and sufficient stability and robustness given certain variation of vehicle parameters.

[0083] Figure 2 shows an example Bode diagram for an arbitrary pair of values of co and v. The top plot shows a function of amplitude versus frequency. The x-axis shows the Laplace frequency of the engine speed error term and the y-axis shows the amplitude of the error term into the engine speed. The bottom plot shows a function of phase versus frequency. The x-axis again shows the Laplace frequency of the engine speed error term and the y-axis shows the phase shift of the error term into the engine speed.

[0084] Plots 10 are the response curves for a model of the transmission structure ofFigure 1 with TVD. Plots 12 are the response curves for a model of a rigid system. Both models have comparable total inertia.

[0085] Figure 2 is used to show the existence of a resonance (pole) in the system.

[0086] The resonant modes of the drivetrain set the constraints for the phase-lag compensator design. The damping ratio (;( J,V) and the resonance frequencyv) of the resonant modes can be determined, for example, with the use of a drivetrain model or by experimental characterization.

[0087] The resonance can be clearly seen at approximately 30 rad / s.

[0088] By way of example, a state-space model may be used to model for M(<JD,V) and the control loop can be optimized for discrete pairs of values (co,v). The resulting parameters (Ktot, a, r) may then be applied to look-up table and may be interpolated in real-time based on measured values of (co,v).

[0089] To effectively attenuate the resonances of the drivetrain, the gain factor should satisfy the inequality:

[0090] where ^( ,V) is the damping ratio of the resonant mode.

[0091] The time factor T(O),V) should be adjusted in such a way that the following inequality is satisfied, to ensure sufficient stability margin: a(o), v) • T(O>, v) « f(c , v)

[0092] f (co, v) is the resonant frequency of the resonant modes. This treatment assumes that the total moment of inertia of the vehicle drivetrain components, such as transmission internal components, driveshafts, and wheels, is negligible compared to the inertia of the engine flywheel and the effective inertia due to the mass of the vehicle. The model can be modified to include terms to account for the inertia of other drivetrain components.

[0093] Figs. 3 to 5 how results of a first modeling of the performance of the system and method of this disclosure.

[0094] Fig. 3 shows plots of the engine speed governor error elc versus time for a conventional engine speed control system (plot 20) and for the engine speed control system of this disclosure (plot 22).

[0095] Fig. 4 shows plots of the instructed engine torque versus time for a conventional engine speed control system (plot 30) and for the engine speed control system of this disclosure (plot 32).

[0096] Fig. 5 shows plots of the vehicle speed versus time for a conventional engine speed control system (plot 40) and for the engine speed control system of this disclosure (plot 42).

[0097] It can be seen that the vehicle speed control is improved and unnecessary low frequency oscillations in engine speed and torque are reduced by using the control system and method of this disclosure.

[0098] Figs. 6 to 8 show results of a second modeling of the performance of the system and method of this disclosure.

[0099] Fig. 6 shows plots of the distance travelled by the vehicle versus time for a conventional engine speed control system (plot 50) and for the engine speed control system of this disclosure (plot 52).

[0100] Fig. 7 shows plots of the vehicle speed versus time for a conventional engine speed control system (plot 60) and for the engine speed control system of this disclosure (plot 62).

[0101] Fig. 8 shows plots of the fuel consumed versus time for a conventional engine speed control system (plot 70) and for the engine speed control system of this disclosure (plot 72).

[0102] It can again be seen that the vehicle speed control is improved and that fuel consumption is also reduced. In the example, fuel consumption dropped from 309 g / km to 291 g / km.

[0103] Thus, through the use of total gain adjustment and phase-lag compensation, a significant improvement in vehicle speed control accuracy is achieved, which improves the driving experience and drivetrain / vehicle performance. Additionally, this allows for a simpler and more robust transmission control algorithm. With improved stability and control, there is potential for improving the total efficiency of the vehicle, reducing fuel consumption and CO2 emissions.

[0104] The analysis above assumes a constant vehicle mass. However, the system may also take into account different vehicle mass, for example by providing a discrete set of parameters for e.g. high / low mass (e.g. trailer attached / not attached), or estimated PTO inertia.

[0105] Variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality.

[0106] The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

[0107] Any reference signs in the claims should not be construed as limiting the scope.

[0108] All references cited herein are incorporated herein in their entireties. If there is a conflict between definitions herein and in an incorporated reference, the definition herein shall control.

Claims

CLAIMSWhat is claimed is:

1. An engine speed control system for a vehicle, comprising: a phase-lag compensator (Clag) for receiving an error signal between an engine speed (<JL>) and a target engine speed (SP) and for outputting a compensated error signal (elc), wherein the phase-lag compensator applies a gain factor (a) and a time constant (r) to derive the compensated error signal (elc); and an engine speed controller (G) for providing an engine speed control signal (trq) for controlling the engine speed based on the compensated error signal (elc), wherein the system further comprises a gain scheduler for generating the gain factor (a) and time constant (r) based on the engine speed (co) and a vehicle speed (v).

2. The system of claim 1, wherein the engine speed controller (G) applies a total gain (Ktot) when processing the compensated error signal, wherein the gain scheduler also generates the total gain (Ktot) based on the engine speed (co) and a vehicle speed (v).

3. The system of claim 2, wherein the gain scheduler is configured to apply a total gain such that the open loop gain is substantially constant at different vehicle speeds.

4. The system of any one of claims 1 to 3, wherein the engine speed controller (G) is for controlling engine control parameters for adjusting an engine torque.

5. The system of any one of claims 1 to 4, wherein the engine speed controller (G) comprises a PID controller.

6. A vehicle comprising: an engine; an engine speed sensor; a vehicle speed sensor; a drivetrain; and an engine speed control system as claimed in any one of claims 1 to 5.

7. The vehicle of claim 6, comprising a tractor.

8. A method of controlling an engine speed of a vehicle, comprising: receiving at a phase-lag compensator (Clag) an error signal between an engine speed (co) and a target engine speed (SP); generating using a gain scheduler a gain factor (a) and a time constant (r) based on the engine speed (co) and a vehicle speed (v). applying the gain factor (a) and the time constant (r) using the phase-lag compensator to derive a compensated error signal (elc); and providing an engine speed control signal (trq) for controlling the engine speed using an engine speed controller (G) based on the compensated error signal (elc).

9. The method of claim 8, comprising applying a total gain (Ktot) at the engine speed controller when processing the compensated error signal, wherein the method comprises using the gain to generate the total gain (Ktot) based on the engine speed (co) and a vehicle speed (v).

10. The method of claim 9, comprising applying a total gain using the gain scheduler such that the open loop gain is substantially constant at different vehicle speeds.

11. The method of any one of claims 8 to 10, comprising using the engine speed controller(G) to controlling engine control parameters for adjusting an engine torque.

12. The method of any one of claims 8 to 11, comprising using the engine speed controller(G) to implement PID control.

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