Mining dump truck PID electro-hydraulic proportional valve control algorithm based on pressure feedforward

Abstract

The invention discloses a PID (Proportion Integration Differentiation) electro-hydraulic proportional valve control algorithm of a mining dump truck based on pressure feed-forward, which realizes a closed-loop control technology of pressure feed-forward and PID composite application on the basis of taking a PLC (Programmable Logic Controller) as a control platform, an electro-hydraulic proportional valve as a control device and a brake and a lifting oil cylinder as executing mechanisms. The problem that the hydraulic pressure response of a traditional PID electro-hydraulic proportional valve control braking and lifting terminal executing mechanism is slow is solved. Application results show that the system pressure output meets the basic requirements of drive-by-wire control, the purpose of quick response of the control system is achieved, and the system has great popularization and application value in the field of drive-by-wire transformation control of mining dump trucks.

Description

Technical Field

[0001] The invention relates to a PID electro-hydraulic proportional valve control algorithm for a mining dump truck based on pressure feedforward, and belongs to the technical field of engineering machinery. Background Art

[0002] Mining dump trucks are one of the important key equipment in open-pit mining and large-scale earthwork construction, and are mainly used for the transportation of various loose materials. After the hydraulic system design of the vehicle is completed, the frequency of the proportional valve, brake, lifting cylinder, damping ratio, effective action area of ​​the brake and lifting cylinder, and the proportional coefficient of the proportional valve cannot be changed after the design is completed, and the dynamic performance of the entire control system is basically determined. This brings up a problem. The control parameters of the system are limited, and the response index of the system may not be achieved by the traditional PID control link alone. In the wire control of mining dump trucks, the response time of the brake electro-hydraulic proportional valve and the lifting electro-hydraulic proportional valve itself meets the requirements of conventional application scenarios, but because the establishment of actual pressure is affected by a variety of nonlinear factors, the pressure response of the actuator lags after the proportional valve is in place. Summary of the invention

[0003] The traditional PID electro-hydraulic proportional valve control method can achieve rapid action response of the solenoid valve, but it cannot ensure that the hydraulic pressure of all hydraulic system terminal actuators is quickly established and actuated. In order to solve the problem of slow response of terminal hydraulic pressure, based on PLC as the control platform, electro-hydraulic proportional valve as the control device, brake and lifting cylinder as the actuator, the present invention does not involve any external pipeline modification, and does not change the established system parameters. Under the condition, the terminal actuator pressure can be quickly established and responded through a new control algorithm.

[0004] The technical solution is as follows:

[0005] A PID electro-hydraulic proportional valve control algorithm for a mining dump truck based on pressure feedforward is proposed. First, the PID transfer function of the electro-hydraulic proportional valve is established and equivalent to the corresponding mathematical model; secondly, the mathematical model of the actuator and the corresponding transfer function are established, and the parameter value of the control variable is determined in combination with the design value of the hydraulic system of the actuator of the whole vehicle; the open-loop transfer function of the whole system is calculated again, and the restricted link of the system is confirmed; finally, the feedforward transfer function is established and introduced according to the restricted link of the system, a controllable variable is added at the control level, and the feedforward instruction and the input instruction are transitionally connected and controlled to improve the response speed of the hydraulic system of the actuator.

[0006] Furthermore, the selection of the feedforward transfer function takes into account both the action limit capability of the proportional valve solenoid and the response speed of the entire hydraulic brake system.

[0007] Furthermore, the mathematical model of the electro-hydraulic proportional valve is:

[0008] ,

[0009] Where: is the natural frequency of the electro-hydraulic proportional valve, ; is the damping ratio of the electro-hydraulic proportional valve, generally ranging from 0.5 to 0.7; is the gain of the electro-hydraulic proportional valve, .

[0010] Furthermore, the transfer function of the actuator is

[0011] ,

[0012] Where: is the effective working area of ​​the brake hydraulic cylinder, ; is the damping ratio of the load mass system, which is generally 0.1 to 0.2; is the natural frequency of the load mass system, .

[0013] Furthermore, the open-loop transfer function of the system is

[0014] ,

[0015] Where: is the open-loop gain of the closed-loop control system; is the pressure feedback coefficient.

[0016] Furthermore, the feedforward transfer function is

[0017] ,

[0018] Where: is a setting value related to the hydraulic system parameters, is the Laplace transform of the input quantity, is a function related to the output pressure, is the system output, is the sensor transfer function on the feedback channel.

[0019] Furthermore, in the feedforward transfer function and The selection takes into account both the action limit capacity of the proportional valve solenoid and the response speed of the entire hydraulic brake system.

[0020] Further, the control process: Combine the input command to determine whether the pressure transmission meets the function setting , if so, the feedforward instruction and the input instruction are controlled to transition;

[0021] If not, convert the command into a set value , further determine whether the system pressure feedback exceeds the limit. If so, perform transition connection control between the feedforward command and the input command; if not, convert the command into the set value again , make a judgment;

[0022] After the transition control is completed, the feedforward instruction of the feedforward transfer function Tuning with PID transfer function.

[0023] The beneficial effect of the present invention is that the PID electro-hydraulic proportional valve control algorithm for mining dump trucks based on pressure feedforward can achieve rapid response of the system hydraulic pressure and establish a set pressure value at the actuator terminal without involving any modification of external pipelines or changing the established system parameters, while retaining the rapid response of the solenoid valve itself. The implementation of this algorithm can solve the problems of difficult design and modification of some hydraulic systems, saving manpower and material resources, and also saving a lot of design and modification costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a structural diagram of a proportional valve of the present invention;

[0025] Figure 2 It is a block diagram of a PID control system of the present invention;

[0026] Figure 3 It is the principle block diagram of the PID control module of the present invention;

[0027] Figure 4 It is a block diagram of the proportional pressure valve PID control transfer function of the present invention;

[0028] Figure 5 It is the pressure feedforward PID proportional valve control principle of the present invention;

[0029] Figure 6 It is a pressure feedforward PID proportional valve control transfer function block diagram of the present invention;

[0030] Figure 7 It is a flow chart of the pressure feedforward PID proportional valve control process of the present invention. DETAILED DESCRIPTION

[0031] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0032] Based on PLC as the control platform, electro-hydraulic proportional valve as the control device, brakes and lifting cylinders as actuators, the principle of the PID electro-hydraulic proportional valve control algorithm for mining dump trucks based on pressure feedforward is as follows: First, a mathematical model of proportional valve PID control is established to find the reasons for the limited control of traditional PID electro-hydraulic proportional valves. In order to solve the problem of limited control, pressure feedforward control is introduced and the controlled parameters are increased to improve the problems of pressure pre-charging and slow pressure establishment in the hydraulic system.

[0033] Firstly, the PID transfer function of the electro-hydraulic proportional valve is established and equivalent to the corresponding mathematical model; secondly, the mathematical model of the actuator and the corresponding transfer function are established, and the parameter values ​​of the control variables are determined in combination with the design values ​​of the actuator system of the whole vehicle; then, the open-loop transfer function of the whole system is calculated and the restricted links of the system are confirmed; finally, according to the restricted links of the system, a feedforward transfer function is established and introduced, which adds a controllable variable from the control level to improve the response speed of the actuator hydraulic system.

[0034] In this embodiment, the actuator is described by taking an electro-hydraulic proportional valve controlling a hydraulic brake circuit as an example.

[0035] Establishing the mathematical model of electro-hydraulic proportional valve

[0036] Proportional valve working principle

[0037] The structure of the electro-hydraulic brake proportional valve for mining dump trucks is as follows: Figure 1 As shown in the figure, the electromagnetic force generated by the proportional valve solenoid 1 after being energized acts on the pressure valve core 3 through the push rod and the pressure spring 2, and compresses the spring 5. When the pressure in the right hydraulic valve is greater than the spring force, the safety valve core 4 is pushed open to overflow. The opening pressure of the pressure valve core can be continuously adjusted by inputting different currents into the solenoid to achieve proportional control of the pressure.

[0038] Proportional Valve Physical Model

[0039] Ignoring the friction of the valve core, the valve core is mainly subjected to three forces in steady state, namely the electromagnetic force of the electromagnet , hydraulic pressure and steady-state fluid dynamics .

[0040] The force balance equation of the valve core in steady state is:

[0041] , (1)

[0042] Where: is the hydraulic pressure at the proportional valve inlet, ; is the electromagnetic force of the proportional valve solenoid, ; is the steady-state fluid force, .

[0043] Hydraulic pressure

[0044] , (3)

[0045] Where: is the proportional valve inlet pressure, ; is the hydraulic pressure action area of ​​the proportional valve port, .

[0046] Electromagnetic force

[0047] , (2)

[0048] Where: is the electromagnetic force of the proportional valve solenoid, ; is the magnetic induction intensity of the proportional valve, ; is the effective length of the conductor perpendicular to the magnetic field, ; is the initial current, .

[0049] Since the pressure in the outlet area is much lower than that in the high pressure area, and the flow rate at the valve port is very small during the opening or closing of the valve, the hydraulic force can be ignored. Substituting equations (2) and (3) into equation (1), the relationship between the input current and the output voltage of the proportional valve can be obtained as follows:

[0050] (4)

[0051] In the formula and are all obtainable constants, so from formula (4), we can know that the output pressure of the proportional valve is Input current of proportional valve In linear proportion.

[0052] Establishing the PID mathematical model of electro-hydraulic proportional valve

[0053] The above article analyzes the input-output characteristics of the proportional valve under steady-state conditions. In practical applications, whether the proportional valve can respond to the input current command in a timely manner and complete the corresponding pressure output in the shortest time, its dynamic response is more worthy of attention.

[0054] In engineering applications, since PLC can output a maximum of 3A PWM for direct control of the electro-hydraulic proportional valve, the proportional amplification link is omitted in the system design. The PID control block diagram is as follows: Figure 2 shown.

[0055] PID Transfer Function

[0056] In the automatic control system, PID control is the most commonly used control method. The principle of its control module is as follows: Figure 3 shown.

[0057] The control law is to process the deviation signal between the set value and the feedback value in proportion, integration and differentiation.

[0058] , (5)

[0059] Where: for Input of time; is the proportionality coefficient; is the integration time; is the differential time.

[0060] From formula (5), the transfer function of PID is:

[0061] (6)

[0062] In engineering applications, it is generally impossible to obtain an accurate mathematical model in advance. Commonly used tuning methods include normalization method, attenuation curve method and trial and error method. This paper adopts the trial and error method in practical applications, that is, , ,Sure range, and then fine-tune , and finally fine-tune according to the situation The traditional PID control is the linear weighted sum of proportion, integration and differentiation. The PID transfer function accounts for the largest proportion, so the PID transfer function can be regarded as a proportional link.

[0063] Mathematical Model of Electro-hydraulic Proportional Valve

[0064] The transfer function of the electro-hydraulic proportional valve is extremely complex if analyzed in depth. The natural frequency of different power components determines the different forms of its transfer function. In this system, the dynamic response of the electro-hydraulic proportional valve (hundreds of Hz) is higher than the dynamic response of the power components (tens of Hz), so the electro-hydraulic proportional valve can be regarded as a typical second-order system. Then the current input-flow output second-order oscillation mathematical model of the electro-hydraulic proportional valve is:

[0065] , (7)

[0066] Where: is the natural frequency of the electro-hydraulic proportional valve, ; is the damping ratio of the electro-hydraulic proportional valve, generally ranging from 0.5 to 0.7; is the gain of the electro-hydraulic proportional valve, .

[0067] Establishing the Actuator Transfer Function

[0068] In engineering applications, actuators and controlled objects with mainly inertial loads and no or very small elastic loads are generally regarded as a combination of an integral and a second-order link. Taking the brake as an example, its hydraulic flow input-brake pressure output transfer function is

[0069] , (8)

[0070] Where: is the effective working area of ​​the brake hydraulic cylinder, ; is the damping ratio of the load mass system, which is generally 0.1 to 0.2; is the natural frequency of the load mass system, .

[0071] , (9)

[0072] Where: is the elastic modulus of hydraulic oil, Pa; is the sum of the mass of the load and the hydraulic actuator, ; is the total volume from the electro-hydraulic proportional valve to the brake hydraulic cylinder, .

[0073] Based on this, the parameter values ​​of the control variables are determined in combination with the design values ​​of the vehicle hydraulic system.

[0074] Build the open-loop transfer function of the system

[0075] Based on the above analysis, the block diagram of the pressure control system transfer function with only inertial load is obtained, as shown in Figure 4 shown.

[0076] Depend on Figure 4 The open-loop transfer function of the system is

[0077] , (10)

[0078] Where: is the open-loop gain of the closed-loop control system; is the pressure feedback coefficient.

[0079] From the analysis of formula (9), we can know that is the total volume from the electro-hydraulic proportional valve to the brake cylinder. However, in fact, the displacement of the brake cylinder is very small, and the pipe capacity from the proportional valve to the brake has become the absolute proportion of this parameter, resulting in The brake natural frequency is very small, and the system's quick response is poor. More importantly, due to the gap between the brake disc and the brake friction pad, and the elastic deformation of the brake line, the brake hydraulic pressure must first overcome the resistance of the long-distance pipeline, then fill and eliminate the brake gap and the deformation of the brake line, and finally establish the real brake pressure. Therefore, the total volume from the electro-hydraulic proportional valve to the brake hydraulic cylinder is It is the restricted part of the system.

[0080] Constructing the pressure feedforward transfer function

[0081] Without changing the original hydraulic system design, pressure feedforward control is introduced to improve the problems of system pressure pre-charging and slow pressure establishment. The control principle of pressure feedforward PID proportional valve is as follows: Figure 5 shown.

[0082] The feedforward controller has two functions in the entire braking system:

[0083] (1) Shorten the pre-charging time of the brake system pressure and minimize the time for eliminating the brake clearance and brake line deformation each time;

[0084] (2) After the system braking pressure begins to build up, the feedforward command is transitioned to the input command to ensure that there is no shock in the system pressure.

[0085] In order to achieve the above two functions, the feedforward controller is designed as a piecewise function:

[0086] (11)

[0087] Where: It is a setting value related to the hydraulic system parameters and is a key parameter for shortening the pre-charge time of the brake system pressure. is the Laplace transform of the input quantity, is a function related to the output pressure, is the system output, is the sensor transfer function on the feedback channel, and the appropriate It is possible to shorten the pre-charging time of the brake system pressure while ensuring smooth system pressure connection without transition shock.

[0088] Combining (11) with Figure 5 , we can get the transfer function block diagram of the system, as Figure 6 shown.

[0089] The control transfer function of this system and The selection should consider both the action limit capacity of the proportional valve solenoid and the response speed of the entire hydraulic brake system.

[0090] like Figure 7 As shown in the figure, the part in the dotted box is the pressure feedforward control process part, and the specific control process is:

[0091] Combine the input command to determine whether the pressure transmission meets the function setting ; If so, the feedforward instruction and the input instruction are transitionally connected and controlled.

[0092] If not, convert the command into a set value , further determine whether the system pressure feedback exceeds the limit. If so, perform transition connection control between the feedforward command and the input command; if not, convert the command into the set value again , make a judgment.

[0093] After the transition control is completed, the feedforward instruction of the feedforward transfer function Tuning with PID transfer function.

[0094] Feedforward Instructions After the PID transfer function is tuned, it can be seen from the measured data that compared with the traditional PID control algorithm, the proportional valve action response time remains unchanged, and the system pressure pre-charge time and the system terminal execution structure pressure establishment time have been greatly shortened. If the response time of the hydraulic system is to be further reduced, the layout of the hydraulic pipeline, related valve group and actuator can only be adjusted from the design source.

[0095] The PLC pressure feedforward PID electro-hydraulic proportional valve control method proposed in the present invention can effectively explore the maximum potential of the hydraulic system's rapid response. Compared with the traditional PID control method, the pressure feedforward control can effectively shorten the time to eliminate the brake clearance and the time for the hydraulic pipeline to tighten and deform, and ensure the rapid and smooth transition between the feedforward command and the input command. It has great promotion and application value in the field of wire-controlled transformation and control of mining dump trucks.

Claims

1. A PID electro-hydraulic proportional valve control algorithm for a mining dump truck based on pressure feedforward, characterized in that: Firstly, the PID transfer function of the electro-hydraulic proportional valve is established and equivalent to the corresponding mathematical model; secondly, the mathematical model of the actuator and the corresponding transfer function are established, and the parameter value of the control variable is determined in combination with the design value of the hydraulic system of the actuator of the whole vehicle; again, the open-loop transfer function of the whole system is calculated to confirm the restricted link of the system; finally, according to the restricted link of the system, a feedforward transfer function is established and introduced, a controllable variable is added at the control level, the feedforward instruction and the input instruction are transitionally connected and controlled, and the response speed of the hydraulic system of the actuator is improved.

2. According to the pressure feedforward-based PID electro-hydraulic proportional valve control algorithm for a mining dump truck according to claim 1, it is characterized in that: The selection of the feedforward transfer function takes into account both the action limit capability of the proportional valve solenoid and the response speed of the entire hydraulic brake system.

3. The PID electro-hydraulic proportional valve control algorithm for a mining dump truck based on pressure feedforward according to claim 1 is characterized in that: The mathematical model of the electro-hydraulic proportional valve is: ， Where: is the natural frequency of the electro-hydraulic proportional valve, ; is the damping ratio of the electro-hydraulic proportional valve, generally ranging from 0.5 to 0.7; is the gain of the electro-hydraulic proportional valve, .

4. The PID electro-hydraulic proportional valve control algorithm for a mining dump truck based on pressure feedforward according to claim 1 is characterized in that: The transfer function of the actuator is ， Where: is the effective working area of ​​the brake hydraulic cylinder, ; is the damping ratio of the load mass system, which is generally 0.1 to 0.2; is the natural frequency of the load mass system, .

5. The PID electro-hydraulic proportional valve control algorithm for a mining dump truck based on pressure feedforward according to claim 1 is characterized in that: The open-loop transfer function of the system is ， Where: is the open-loop gain of the closed-loop control system; is the pressure feedback coefficient.

6. The PID electro-hydraulic proportional valve control algorithm for a mining dump truck based on pressure feedforward according to claim 1 is characterized in that: The feedforward transfer function is ， Where: is a setting value related to the hydraulic system parameters, is the Laplace transform of the input quantity, is a function related to the output pressure, is the system output, is the sensor transfer function on the feedback channel.

7. The PID electro-hydraulic proportional valve control algorithm for a mining dump truck based on pressure feedforward according to claim 1 is characterized in that: The feedforward transfer function and The selection takes into account both the action limit capacity of the proportional valve solenoid and the response speed of the entire hydraulic brake system.

8. The PID electro-hydraulic proportional valve control algorithm for a mining dump truck based on pressure feedforward according to claim 1 is characterized in that: Its control process: Combine the input command to determine whether the pressure transmission meets the function setting , if so, the feedforward instruction and the input instruction are controlled to transition; If not, convert the command into a set value , further determine whether the system pressure feedback exceeds the limit. If so, perform transition connection control between the feedforward command and the input command; if not, convert the command into the set value again , make a judgment; After the transition control is completed, the feedforward instruction of the feedforward transfer function Tuning with PID transfer function.

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