IT201800004932A1 - SPEED CONTROL METHOD OF AN INTERNAL COMBUSTION ENGINE - Google Patents

Description

DESCRIPTION

of the patent for industrial invention entitled:

"SPEED CONTROL METHOD OF AN INTERNAL COMBUSTION ENGINE"

Field of application of the invention

The present invention relates to the field of operating point control methods of internal combustion engines.

State of the art

It is known that the accelerator pedal allows the driver to cause a change in the rotation speed of the internal combustion engine.

This is evidently the consequence of a variation of a torque delivered by the internal combustion engine. The accelerator pedal can therefore relate to the engine point, i.e. engine speed / torque, of the internal combustion engine functioning, representing, with a relative inclination, a required torque value or a target engine speed.

Engine speed and torque are clearly interrelated quantities, nevertheless, a speed control implies very rapid reactions from the internal combustion engine.

Speed control on the internal combustion engine is particularly delicate due to the intrinsic response delay of the engine due to various factors, including the capacitive effect of the intake manifold, which exhibits slow dynamics.

During a gear change or a maneuver, or the sudden insertion of a load, such as a pump or a vehicle compressor, or a sudden change in the slope of the road, the maximum speed and accuracy of response of the speed control is required.

A speed control scheme is based on the fact of calculating a speed error with respect to a reference value and consequently calculating the necessary command variable (the torque) of the controlled system, so as to vary the response of the internal combustion engine, to reduce or cancel this error. This is obviously a feedback control.

The typical controller used by the known art to control the motor speed is of the PI type (proportional, integral). In other words, the error is corrected between a reference value and the measured or estimated value of the system output, through a proportional controller and an integral controller, which, on the basis of said error, generate respective control signals. The sum of the respective outputs, that is, of the respective control signals generated, is applied to the system input to make the system output converge to the reference value, which in this case is a reference for the motor speed.

It would be extremely advantageous to also use a Derivative contribution, realizing a complete PID, as the derivative contribution would allow a more prompt reaction, since the derivative term is an anticipatory control component. Unfortunately, the derivative term typically introduces high frequency dynamics into the system, making the controller unstable.

A complete PID controller requires that the outputs of P, I and D controllers add up and the result is applied to the input of the system to be controlled.

Due to the aforementioned instability, the derivative contribution is generally not used.

Summary of the invention

The object of the present invention is to provide a control method for an internal combustion engine which allows to implement a speed control that is fast but at the same time stable and fluid.

The basic idea of the present invention is to realize a feedback speed control by means of a PID controller, comprising a proportional, integral and derivative contribution only when a motor speed error is caused by a change in the speed reference value. , otherwise the controller is PI, that is, it includes only a proportional and integral contribution.

In other words, when the above operating condition occurs, a complete PID controller is used, in which the sum of the respective outputs, i.e. of the respective generated control signals, is applied to the input of the internal combustion engine to make the output converge of the motor to the speed reference value.

This implies that when the speed error is caused by an external disturbance, for example a rapid change in the slope of the road, or the activation of a device connected to an engine power take-off, etc ... the derivative contribution is not added together. to the proportional and integral contributions.

To discriminate the case in which the error depends on a change in the reference signal or a change in the load, it is sufficient to compare a sign of said motor rotation speed error with a sign of a derivative of the motor rotation speed. In particular, if the signs are discordant then the speed error is caused by a load variation. Hereinafter "speed" means the "motor rotation speed".

In fact, when it is found that the derivative of the speed is positive and the error is negative, it means that the engine is accelerating beyond the reference value. This generally occurs when a load is suddenly released.

Conversely, when the derivative of speed is negative and the speed error is positive, it means that the torque delivered by the motor is not sufficient to cope with the applied load, for example suddenly.

Therefore, according to the present invention, the derivative contribution is added to the outputs of the proportional and integral control when the error and derivative signs agree.

Conversely, when the signs are discordant, the derivative contribution is not added to the integral and proportional contributions.

In relation to the specific implementations of the invention, it can be decided to use the derivative contribution when the aforementioned signs are in agreement and positive or in agreement and negative.

Making use of the derivative contribution according to the present invention makes the motor extremely ready, but at the same time avoids introducing high frequency dynamics into the control system which can make the controller unstable.

By “controller” we mean the speed control operated on the internal combustion engine in its entirety.

According to a preferred variant of the invention, not only the signs must agree, but the absolute value of the speed error must exceed a predetermined first threshold and the absolute value of the derivative of the speed must exceed a second predetermined threshold.

According to a preferred variant of the invention, which is combined with the previous ones, when only the proportional and integral contributions are used, the derivative of the motor rotation speed (which is normally multiplied by a predetermined parameter defines the aforementioned derivative contribution), is used to modify an integral controller operating parameter so as to compensate for sudden variations due to loads.

In particular, the integral controller comprises a saturator which saturates the command signal to the internal combustion engine, on the basis of a speed error and on the basis of the derivative of the speed of the internal combustion engine itself.

The derivative contribution responds much more quickly than the other proportional and integral contributions, but its contribution is used to intervene on the integral contribution by minimizing the over and under elongations of the response of the internal combustion engine, without compromising its stability.

The claims describe preferred variants of the invention, forming an integral part of this description.

Brief description of the figures

Further objects and advantages of the present invention will become clear from the following detailed description of an example of its embodiment (and its variants) and from the annexed drawings given purely for explanatory and non-limiting purposes, in which:

Figure 1 shows a control scheme based on an example of implementation of the present invention;

Figures 2a and 2b show two equivalent control schemes used in a mutually exclusive way according to another example of implementation of the present invention;

Figure 3 shows in detail a block of the control scheme of Figure 2b.

The same reference numbers and letters in the figures identify the same elements or components.

In the context of this description, the term "second" component does not imply the presence of a "first" component. These terms are in fact used only for clarity and should not be understood in a limiting way.

Detailed description of examples of realization

Figure 1 shows an example of a control example with a block diagram.

The control is performed recursively at discrete time, according to the operating frequency of the processing unit ECU that controls the internal combustion engine.

The internal combustion engine is represented in figure 1 with the “Engine” block.

A speed sensor, for example a phonic wheel applied to the crankshaft, allows you to obtain a measurement of the "Speed" of the internal combustion engine.

In addition, the accelerator pedal or any vehicle device suitable for imparting a reference speed, such as a robotic gearbox or a power take-off generates the reference signal Ref in terms of target engine speed.

The measured speed Speed is subtracted from the target speed Ref generating an Err (speed) error by the first summing node S1 to the left of Figure 1.

The same error value (of speed) is brought to the input of the blocks, P and I, while a speed signal is brought to the input of the D block realizing a PID control.

The outputs of controllers P, I and D converge in the adder node S2 to the right of Figure 1 to control the internal combustion engine Engine. Generally, the controllers receive speed signals in input and generate command signals relating to the percentage of torque that the motor must deliver, with respect to the relative nominal torque.

The derivative contribution D is given by the derivative of the motor rotation speed ΔSpeed multiplied by a fixed or calibratable parameter KD.

The Sign block receives the aforementioned error as input and also the derivative of the speed ΔSpeed from the derivative block D and compares the relative signs allowing or not, by means of the "Switch" switch, at the output of the derivative D to flow into the summing node S2.

Therefore, when the switch enables the derivative output, a complete PID control is obtained, otherwise a PI control is obtained.

Preferably, the Sign block enables the contribution of the derivative controller both when both signs are positive and when they are both negative.

According to a preferred variant of the invention, the Sign block not only compares the signs but also verifies that when both are positive the speed error exceeds a first predetermined positive threshold and the derivative exceeds a second predetermined positive threshold. When both conditions, on the agreement of the signs and on the overcoming of the respective thresholds, are satisfied, then the derivative contribution is used.

Therefore, the verification also of the exceeding (positive and / or negative) of the thresholds represents a more restrictive condition than the mere agreement of the signs. Conversely, when both signs are negative, the Sign block verifies that the speed error is less than a third predetermined negative threshold and the derivative contribution is less than a fourth predetermined negative threshold. When both conditions, on the agreement of the signs and on the overcoming of the respective thresholds, are satisfied, then the derivative contribution is used. When the modulus (absolute value) of the first and third thresholds are equal to each other and the second and fourth thresholds are equal to each other then the Sign block enables said derivative control when the aforementioned signs agree, and the absolute value of the velocity exceeds a predetermined first threshold and the absolute value of the derivative of velocity exceeds a second predetermined threshold.

Consequently, a first interval comprised between the first and third threshold and a second interval comprised between the second and fourth threshold is defined, in which the derivative controller is disconnected from the summing node S2. In a 3 x 3 error / derivative of turns matrix, a so-called "dead band" is therefore defined in which the derivative controller is disconnected from the summing node S2.

The following table shows an example of application of the activation mechanism of the derivative contribution as a function of signs and thresholds.

The presence of a "1" indicates that the derivative contribution is summed in the summing node S2, while the "0" indicates that the derivative contribution is not summed and therefore it is disconnected from the summing node S2.

According to another preferred variant of the invention which is combined with any of the previous variants, when the Sign block disconnects the first output of the derivative control D from the second summing node S2, and connects a second output of the derivative control D to the integral controller, in particular to the saturator Sat_1 with reference to figure 3, which limits the contribution of the integral controller I. The first output of the derivative control supplies KD * ΔSpeed, while the second output supplies only ΔSpeed.

Therefore figures 2a and 2b correspond to the equivalent schemes that occur when, respectively, the Sign block enables or disables the connection of the derivative control output with the second summing node S2.

Therefore, the integral contribution is saturated according to a function F (Err, ΔSpeed) of the speed error Err and the derivative of the speed.

The presence of a "-1" indicates that the derivative contribution is not used for motor control, according to the present variant, and a second output of the derivative block, which generates the derivative of the motor rotation speed ΔSpeed, interacts with the saturator of the integral controller as described below.

Also to allow the interaction of the derivative block (ΔSpeed) with the saturator in the integral controller, it is possible to consider thresholds that the speed error and the derivative of the speed must exceed (both positive and negative), defining the aforementioned dead band symmetric with respect to the two diagonals, principal and secondary of the square matrix.

An integral controller I (discrete time), according to a preferred variant of the present invention, can be schematized, with reference to Figure 3, as a "Memory" which contains an Int-1 value generated by the same integral controller I in the previous step (therefore "Int-1" indicates that it is generated at "step-1"), to which the current value of the velocity error Err, suitably multiplied by the integral coefficient KI, is added iteratively by means of the summing node S3 the multiplier node M1. The result of the sum, carried out by the summing node S3, represents the output of the integral controller I, i.e. the aforementioned command signal, and at the same time the Int-0 input of the Memory memory (i.e. at the "step-0" i.e. current step) which stores it for the next integration step.

The saturator Sat_1, according to the present invention, is arranged between the summing node S3 and the output of the integral controller, so that the limitation is not operated only on the output of the integral controller, but also on the "Memory" block contained therein.

According to a preferred variant of the present invention, the saturator Sat_1 achieves a symmetrical saturation with respect to zero and the saturation module is given by the following formula:

| Saturation | = | K * RadQ {[exp (- | Err | / A)] * [exp (- | ΔSpeed | / B)]} |

Where is it:

- RadQ represents the square root operator

- Exp represents the exponential

- | Error | represents the Err error module described above

- | ΔSpeed | represents the module of the derivative of the motor speed ΔSpeed.

K, A and B represent constant values.

The derivative of the engine speed can be expressed using Newton's equation:

ΔSpeed = Cost * [(Torque delivered by the motor) - (Resistant torque due to external loads)]

Where Cost is generally the reciprocal of the moment of inertia J of the internal combustion engine.

The external loads include, for example, the slope of the road traveled by the vehicle driven by the internal combustion engine or the resistant torque offered by an electric generator, a compressor, a pump connected to a relative PTO (power take-off).

The CALC block based on the inputs:

- Err speed error;

- Derivative of the speed ΔSpeed.

Apply the above formula by calculating the saturation module | Saturation |.

The CALC block has two outputs, each addressed to the two inputs: high and low of the saturator Sat_1.

A positive or negative saturation is applied only when the Err and ΔSpeed signs are discordant, otherwise the signal is saturated at / - 100% of the actuator authority. A second saturation Sat_2 is preferably arranged between the multiplier node M1 and the summing node S3 in order to limit the command signal to 100% and -100% of the so-called "actuator authority". The reasons for the implementation of saturations to the authority of the actuator are known to the person skilled in the art and basically have the purpose of avoiding unnecessarily requesting a performance from the actuator (Engine) in excess of its nominal characteristics. In this case, the actuator is the internal combustion engine which cannot deliver more than its nominal torque in the relative nominal speed / torque map.

According to a preferred variant of the invention, the above formula is implemented in the CALC block by means of a Look up Table, with the advantage of greater flexibility because it allows to modify the coefficients of the table itself, introducing deviations from the output of the above formula, which better fit on the specific internal combustion engine. In addition, the look up table allows you to reduce the computational burden.

An example of a Look up Table with | ΔSpeed | as inputs is shown below and | Err |.

The values reported in the matrix, for example 5 x 5, reported in the previous table are those considered optimal for an implementation of the present invention. Nevertheless, they can be suitably varied. These values are positive when the error Err is negative and ΔSpeed is positive, while each of the values shown in the table is multiplied by "-1" when Err is positive and ΔSpeed is negative.

In other words, the saturator Sat_1 is symmetrical with respect to zero and when the error is negative and ΔSpeed is positive this indicates that the high (positive) portion of the saturator (upper limitation) is being manipulated, vice versa when Err is positive and ΔSpeed is negative, this indicates that the low (negative) portion of the saturator (lower limitation) is being manipulated.

The present method can be advantageously implemented by means of a processing unit ECU (Engine control Unit) for the control of the internal combustion engine, which processes the information of engine speed and pressure on the accelerator pedal or of requests for revolutions or torque to be other devices and controls the internal combustion engine accordingly as described above.

The present invention can be advantageously carried out by means of a computer program which comprises coding means for carrying out one or more steps of the method, when this program is executed on a computer. Therefore it is intended that the scope of protection extends to said computer program and further to computer readable means comprising a recorded message, said computer readable means comprising program coding means for carrying out one or more steps of the method. , when said program is run on a computer.

Implementation variants of the described non-limiting example are possible, without however departing from the scope of protection of the present invention, including all equivalent embodiments for a person skilled in the art.

From the above description, the person skilled in the art is able to realize the object of the invention without introducing further construction details. The elements and features illustrated in the various preferred embodiments, including the drawings, can be combined with each other without however departing from the scope of protection of the present application. What is described in the chapter relating to the state of the art is needed only for a better understanding of the invention and does not represent a declaration of existence of what is described. Furthermore, if not specifically excluded in the detailed description, what is described in the state of the art chapter is to be considered as an integral part of the detailed description.

Claims (11)

CLAIMS 1. Method of controlling an internal combustion engine by means of a feedback control of the speed of the engine itself, based on an error (Err) of said engine speed, calculated between a reference value (Ref) and a measured value (Speed ) of the speed, in which said control comprises a sum (S2) of a proportional contribution (P) and an integral contribution (I), the method comprising a step of adding also a derivative contribution (D) only when it results at least that a first sign of said error and a second sign of a derivative of said engine speed are mutually agreed. Method according to claim 1, wherein said derivative contribution (D) is added when said speed error exceeds a predetermined first positive threshold and said engine speed derivative exceeds a second predetermined positive threshold and / or when said speed error it is lower than a third predetermined negative threshold and said derivative contribution is lower than a fourth predetermined negative threshold. Method according to claim 2, in which a module of said first threshold and a module of said third threshold coincide and in which a module of said second threshold and a module of said fourth threshold coincide. Method according to any one of the preceding claims 1 - 3, wherein the method comprises a step of saturating said integral contribution as a function (F (Err, ΔSpeed)) of: - a derivative (ΔSpeed) of said measured value of the speed of the motor and of - called speed error (Err), when said first sign of said error and said second sign of said derivative contribution are at least mutually inconsistent. 5. Method according to claim 4, wherein said integral contribution (I) consists of the sum (S3) of - an error value (Err) calculated at the current step, multiplied by an integral coefficient (KI) e - a value of the integral contribution (Int-1) generated in the immediately preceding step, and wherein said saturator (Sat_1) is applied to said sum. Method according to one of claims 4 or 5, wherein an absolute value of said saturation is given by the following formula: | Saturation | = | K * RadQ {[exp (- | Err | / A)] * [exp (- | ΔSpeed | / B)]} | Where is it: - RadQ represents the square root operator, - Exp represents the exponential operator, - | Err | represents the module of the speed error Err described above, - | ΔSpeed | represents the module of the derivative of the motor speed ΔSpeed, - K, A and B represent constant values. 7. Method according to any one of the preceding claims, in which said absolute value of said saturation is given by a Look up table, in which relative values are included, in absolute value, between 0 and 100% of a nominal torque of the combustion engine internal. A computer program comprising program coding means adapted to carry out all the steps of any one of claims 1 to 7, when said program is run on a computer. 9. Computer readable means comprising a recorded program, said computer readable means comprising program coding means adapted to perform all steps of any one of claims 1 to 7, when said program is run on a computer. 10. Internal combustion engine comprising - a device for measuring relative speed, - a processing unit (ECU) configured to control said internal combustion engine on the basis of a speed reference signal (Ref), said processing unit being configured to carry out the control method according to any one of the preceding claims 1 to 7. 11. Vehicle or fixed installation comprising the internal combustion engine according to claim 10.