Hydraulic distributor control via stepper motor

Abstract

The invention consists of controlling the direction and opening of a hydraulic distributor with a stepper motor acting on it via a reversible transmission. This economical and reliable solution allows for precise positioning of the distributor spool during normal operation, and a return to the neutral position under the action of the return spring(s) in the event of a power failure. Figure 8 (for the abstract)

Description

Title of the invention: Hydraulic distributor control by stepper motor technical field

[0001] The invention relates to the field of hydraulic machine technology. A typical hydraulic installation comprises at least three categories of components.

[0002] The first category aims to provide a flow of hydraulic fluid under pressure. This is referred to as a hydraulic power unit or hydraulic pumping station. At a minimum, it consists of a hydraulic pump driven by an electric or thermal motor, and a reservoir from which the hydraulic fluid is drawn and sent under pressure to the installation, and to which the fluid is returned after use.

[0003] The second category aims to open the passage of pressurized fluid to user devices (actuators) on demand. If not used, the fluid flow is simply returned to the reservoir. The present invention relates to this category, which includes manually or electrically controlled distributors. [Fig.1] illustrates these two types of distributors.

[0004] The third category aims to transform the flow of pressurized fluid into mechanical power directly usable by a machine. This category comprises actuators, typically hydraulic cylinders and hydraulic motors. The subject of this patent relates to the second category and more specifically to electrically controlled distributors. Previous technique

[0005] A hydraulic distributor typically operates by moving a spool that opens and closes fluid passages. A typical application is the control of the cylinders of a mechanical excavator, which allows for slow and precise approach movements as well as fast and powerful movements. [Fig.2] shows, by way of illustration, the diagram of a three-position distributor which allows a hydraulic cylinder or motor to be operated in both directions, by sending the pressurized fluid present at the inlet P either to A and the fluid then returns via B to the reservoir T, or to B and the fluid then returns via A to the reservoir T. In neutral, A and B are blocked, preventing any circulation of fluid in the actuator. [Fig. 3] shows a cross-section of a three-position distributor conforming to the previous diagram. The two return springs installed at the ends of the spool return it to neutral when no actuation force is applied. [Fig.4] schematically represents this distributor at rest, with ports A and B closed, preventing any circulation through the actuator. [Fig. 5] shows the configuration that corresponds to a small displacement of the spool in one direction. A reduced cross-section allows a limited flow of fluid to circulate from the supply P to the port A, and back from the port B to the reservoir T. [Fig.6] presents the wide open configuration in the same direction, the passage sections are maximal, allowing a maximum flow rate to pass through the actuator.

[0006] The control of this drawer can be manual, that is to say that the drawer is moved by a person acting on a lever, or it can be electric.

[0007] In the case of an electrical control, this is typically achieved today by two axial coils mounted at the ends of the spool, one for each direction. Each coil, carrying an electric current, applies an electromagnetic force to a movable soft iron element which tends to maximize the overall magnetic permeability of the magnetic flux loop by closing a soft iron circuit. The magnetic circuit can close either by the movement of a movable soft iron tab, like an electromagnet, or by the axial displacement of a sliding soft iron core. Technical problem

[0008] With regard to electrically controlled valves, good practice, as recalled by European machine regulations, requires that an unexpected absence of electrical energy or its establishment must not be able to lead to potentially dangerous movements.

[0009] A power outage must lead to a complete halt of all movement. For a hydraulic machine, this translates into the blocking of all fluid circulation through the actuators. Restoring power should lead to a restart with all movements stopped.

[0010] In practice, this translates at the distributor level into the return of the spool to the neutral position under the action of one or more return springs. If an electromagnet-type device does not mechanically prevent the drawer from returning to the neutral position in the event of a power outage, the question arises for a drawer driven by an electric motor, essentially because of the gear reduction which must be reversible (the return spring must be able to bring the drawer back to the neutral position without being prevented by the geared motor).

[0011] The difficulty associated with using an electromagnet-type device is ensuring the accuracy of the drawer's movement. Indeed, the drawer's position is a direct consequence of the current flowing through the coil and the resulting electromagnetic force.

[0012] The difficulty related to the use of a geared motor is to guarantee that with a reversible reduction, it is able to drive the drawer in both directions, by pushing the return springs or on the contrary by braking their action, and to maintain the position reached. As a reminder, a transmission is reversible if it does not prevent the motor from being driven by the load. Technical solution

[0013] The use of an electric motor is relevant insofar as it allows for the generation of continuous displacement (linear or angular) without significant change in the torque or effort characteristics along this displacement, which allows for the easy implementation of a reduction gear to reduce the power required.

[0014] The choice of a stepper motor allows for precise control of the speed and positioning of the drawer in open loop (as opposed to closed-loop operation which implements a control system based on a displacement measurement). It also allows for maintaining its angular position by maintaining the power supply. [Fig. 7] is a diagram illustrating the operating principle of a bipolar stepper motor with 4 steps per revolution. Each of the two fixed coils A and B (the motor stator) can be energized by passing current in either direction, forming two electromagnets, each capable of generating a north pole on one side and a south pole on the other. The rotor's permanent magnet then aligns with the resulting magnetic field. By successively energizing the coils with appropriate polarities, the magnet can be rotated in either direction at the desired speed. Its angular position can also be fixed by maintaining one coil under voltage. The desired angular position of the motor is determined by counting the steps in each direction.

[0015] The object of this invention is therefore to control the drawer with a stepper motor by means of a reversible reduction kinematic.

[0016] The major characteristics of the stepper motor are recalled below.

[0017] Its reliability and repeatability are widely demonstrated on paper printers and 3D printers, which can operate for hours without loss of steps.

[0018] It is simple to implement thanks to its ability to operate in open loop. There is no need to implement either a displacement sensor or a position control device based on the difference between a setpoint and the measurement. With the stepper motor, each commanded step, in either direction, is assumed to be executed (this only works in the absence of mechanical blocking).

[0019] Its mode of operation is such that in the event of a mechanical blockage, the steps are simply skipped without mechanical or electrical damage to the motor or its control.

[0020] It typically has a resolution of 200 steps per revolution, which allows for good positioning accuracy. For example, a reduction of 5 of such a stepper motor makes it possible to achieve a resolution of 1000 steps per revolution, or almost 3 steps per degree of angle.

[0021] Its ability to maintain its angular position by providing an electromagnetic holding torque prevents drift when no movement is commanded. As a corollary, the absence of electrical power to the motor significantly reduces the holding torque, which makes it possible not to impede the action of the return springs (thanks to reversible kinematics and an appropriate gear ratio).

[0022] Its ability to enable the driving of a resisting load, and also the braking of a driving load.

[0023] Its operation is based on the alternating energization of coils; each coil is either energized or not energized (binary control). The positioning accuracy of the motor is not affected by any voltage variations beyond the minimum operating voltage.

[0024] Advantages with regard to a hydraulic distributor control

[0025] The stepper motor is capable of generating significant torque relative to its dimensions, which allows for limited gear reduction; it can drive or brake; it is reliable; it is precise; and its ability to operate in open loop simplifies control. Furthermore, it is capable of maintaining its angular position as long as it is powered and of providing weak resistance when it is no longer powered.

[0026] The motor itself does not keep track of its current angular position; keeping the step counter up to date (image of its angular position) is the responsibility of the control electronics, which commands the steps in one direction or the other.

[0027] All these characteristics make it particularly suitable for controlling a hydraulic distributor:

[0028] In the absence of power, the motor position is not maintained, and the distributor return spring can ensure the return and holding in the neutral position without being impeded by the motor. At the same time, since the control electronics are no longer powered, the step counter value is lost. This value will be reset to zero at the next start-up.

[0029] The restoration of the electrical supply therefore does not risk causing an untimely opening of the hydraulic distributor which could lead to potentially dangerous movements: the system is re-energized in a neutral and stable configuration (distributor and control electronics). Example of implementation of the invention

[0030] This example illustrates the conversion of a manual distributor into an electrically controlled distributor by adding a motor. [Fig.8] shows an installation built with a stepper motor that moves the hydraulic distributor spool via a mechanical transmission consisting of gears and levers linked by a connecting rod. In this configuration, the control handle of a manual distributor (6) has been replaced by the lever (5). The latter is the final link in the external mechanical transmission to the distributor, allowing the stepper motor (1) to operate in place of manual action. The gear reducer (2) mounted in line with the stepper motor, the lever (3) at the end of the reducer, the connecting rod (4), and the final lever (5) constitute the reversible transmission.

[0031] In normal operation, the geared motor assembly is designed so that the motor torque is sufficient to ensure the movement and holding of the drawer despite the force opposed by the return springs when the drawer is moved away from its neutral position, and despite the driving force applied by the return springs when the drawer is brought closer to its neutral position.

[0032] In the absence of power supply, the connecting rod and geared motor assembly is designed so that the resistance it opposes to the movement of the spool is less than the force applied by the return springs of the distributor.

Claims

6. Claims

1. Control device for a hydraulic distributor characterized in that the distributor spool is moved and held in position by a stepper motor.

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