WO2002087940A1

WO2002087940A1 - Hydraulic vehicle retarder

Info

Publication number: WO2002087940A1
Application number: PCT/SE2002/000811
Authority: WO
Inventors: Marcus STEÉN
Original assignee: Volvo Lastvagnar Ab; Samuelsson, Bengt
Priority date: 2001-04-27
Filing date: 2002-04-25
Publication date: 2002-11-07
Also published as: SE518966C2; SE0101496D0; SE0101496L

Abstract

Hydraulic retarder installation with a retarder which is controlled by an electronic control unit (8), which regulates, via an electrically operated proportional valve (9), the air pressure in a hydraulic fluid reservoir (6), said pressure determining the braking torque. The control unit controls the valve by sending out a control current with a superimposed alternating current, which gives the valve spool of the valve a reciprocating movement with short strokes, when the momentary pressure lies outside the tolerance range for predetermined pressure. When the momentary pressure is within the tolerance range, the superimposed alternating current is adjusted down to a predetermined value, which can be zero.

Description

Hydraulic Vehicle Retarder

The present invention relates to a hydraulic retarder installation for motor vehicles, comprising a retarder with a stator solidly mounted in a housing, a rotor rotatably mounted in the housing, a reservoir for hydraulic fluid, and an electrically operated proportional valve in a compressed mr connection between a source of compressed air and the reservoir, an electronic control unit, which controls the proportional valve by sending out a control current with a superimposed operating current to the valve as a function of signals from a brake control, and an air pressure sensor, which sends a signal to the control unit dependent on the air pressure on the outlet side of the proportional valve.

Retarder installations known up to now of this type have control units which, in response to a signal to activate the retarder to effect a certain braking torque, initially apply a current over the solenoid of the proportional valve, which is estimated to provide an air pressure corresponding to the requested braking torque. In order to obtain a rapid response, i.e. a rapid increasing pressure, the control unit initially directs a current of higher magnitude than what is required to maintain the desired braking torque. The air pressure feeds back the current value of the pressure obtain- ed, and the control unit then corrects the electrical control current to the proportional valve, so that the valve adjusts the current value of the pressure towards a predetermined pressure, which corresponds to the requested braking torque. In order to obtain a rapid and precise resetting of the valve, it is important that the valve spool not jam. By superimposing on the control current an operating current in a known man- ner, preferably a sinusoidal alternating current with a frequency of about 20 Hz, the valve spool will always be making short reciprocating movements, which ensure the lowest possible friction between the valve spool and its bore.

By virtue of the fact that the valve spool continually switches above and below a mean pressure and the valve thus alternatingly lets in more air than is required and then lets out air so that the air consumption and thus the vehicle fuel consumption will be greater than if one could keep the valve spool completely stationary at a set pressure. This is, however, not possible in practice, i.a. because there will always be a certain amount of leakage in the system, making it necessary for maintaining nearly constant braking torque to add air from time to time.

The purpose of the present invention is to achieve a hydraulic retarder installation of the type described by way of introduction, which uses less air than corresponding known installations without sacrificing the requirement of rapid and precise move- ment of the valve.

This is achieved according to the invention by virtue of the fact that the control unit is arranged, upon actuation of the brake control for activating the retarder, to reduce the superimposed operating current to a predetermined value when the signal from the air pressure sensor indicates a pressure lying between lower and upper tolerance limits for a predetermined pressure corresponding to the braking torque requested by the position of the brake control.

In a preferred embodiment the control unit can be arranged to shut off the superim- posed operating current completely when the air pressure sensor indicates a pressure within the pressure range between the tolerance limits, and to turn on the operating current and increase the strength gradually with increasing distance to the pressure range between the tolerance limits.

The invention will be described in more detail below with reference to examples shown in the accompanying drawings, where Fig. 1 shows a schematic perspective view of a known hydraulic retarder, Fig. 2 shows a schematic diagram of one embodiment of a retarder installation according to the invention, Fig. 3 is a diagram of the response of the proportional valve, and Fig. 4 is an enlargement of a portion of the diagram in Fig. 3. The retarder 1 shown in Fig. 1 is a hydraulic retarder which is known per se and comprises a housing 2 with a stator 3 arranged solidly in the housing and a rotatably mounted rotor 4, which is connected, in a manner not shown in more detail here, to the output shaft of the vehicle gearbox and thus also to the vehicle propeller shaft 5. The retarder 1 has a tank 6 for hydraulic fluid and a cooler 7 for cooling the fluid.

As can be seen schematically in Fig. 2, the retarder 1 is controlled by an electronic control unit (control computer) 8, which can be the same unit as that controlling the various functions of the engine, the clutch and the transmission. The control unit 8 controls a regulator valve in the form of an electrically operated proportional valve 9, which is coupled between a compressed air source (accumulator tank) 10 and the hydraulic fluid tank 6 of the retarder 1. When compressed air is supplied to the tank 6, fluid is pressed in between the rotor 4 and the stator 3 of the retarder, thus generating braking torque, the magnitude of which is dependent on the volume of the fluid which is pressed by the compressed air into the chamber in which the rotor rotates. The magnitude of the fluid volume is dependent on the pressure of the compressed air supplied to the tank 6, and this pressure is determined by the setting of the proportional valve 9. This setting is in turn determined by the strength of the current which the control unit 8 sends to the solenoid of proportional valve 9.

When the retarder 1 is to be activated, the driver actuates a brake control 11, which can be a separate retarder control or the ordinary brake pedal of the vehicle. The control 11 sends a signal to the control unit 8 representing the braking torque requested by the driver. The control unit 8 then sends out a predetermined current de- pending on the requested braking torque. Manufacturing tolerances, resulting in a certain variation as regards the setting of the proportional valve 9 and thus the pressure which determines the braking torque, cause in this case variations in the initial braking torque between different retarder installations having identically programmed control units 8. By feedback to the control unit 8 by means of a pressure sensor 12, which senses the current value of the pressure, the control unit adjusts the strength of electric current to a value which provides the predetermined value of the pressure corresponding to the requested braking torque. In the diagram in Fig. 3, P_f designates a predeteriΗined value of the pressure which is to provide a certain requested braking torque, and I_f designates the strength of the electric current which is to provide the pressure P_f. The dashed lines P_! and P₂ indicate upper and lower tolerance limits to which the pressure P is allowed to deviate from the predetermined pressure P_f without an alternating current being superimposed on the direct current I determining the valve setting. The vertical lines r t₂, t₃, t , t₅ and t₆ indicate important points in time along the time axis of the diagram. As can be seen in the diagram, the pressure P is equal to the predetermined pressure P_f at ti. At t₂, the current pressure P exceeds the upper tolerance limit Pi, and a sinusoidal alternating current is superimposed on the control current I. When the pressure has dropped below the upper tolerance limit Pi at t₃, the super-imposed alternating current is reduced to zero. The pressure now drops and at t₄, the prevailing pressure is equal to the predetermined pressure P_f. When the pressure drops below the lower tolerance limit P₂ at t₅, alternating current is superimposed until the pressure is once again within the tolerance range P₂ - P_! at t₆, at which time the alternating current is reduced to zero. As is particularly evident in the diagram in Fig. 4, the amplitude, _ampi, of the superimposed alternating current is regulated so that it increases with increasing distance from the tolerance range P2 - Pi and drops as the distances closes reaching zero when the distance has dropped to zero.

Claims

Hydraulic retarder installation for motor vehicles, comprising a retarder (1) with a stator (3) solidly mounted in a housing (2), a rotor (4) rotatably mounted in the housing, a reservoir (6) for hydraulic fluid, and an electrically operated proportional valve (9) in a compressed air connection between a source of compressed air (10) and the reservoir, an electronic control unit (8), which controls the proportional valve by sending out an electric control current with a superimposed operating current to the valve as a function of signals from a brake control (11), and an air pressure sensor (12), which sends a signal to the control unit dependent on the air pressure on the outlet side of the proportional valve, characterized in that the control unit (8) is arranged, upon actuation of the brake control (11) for activating the retarder (1), to reduce the superimposed operating current to a predetermined value when the signal from the air pressure sensor indicates a pressure (P) lying between lower and upper tolerance limits (Pι-P₂) for a predetermined pressure (P_f) corresponding to the braking torque requested by the position of the brake control.

Hydraulic retarder installation according to claim 1, characterized in that the control unit (8) is arranged to reduce the superimposed operating current to zero, when the air pressure sensor (12) indicates a pressure between said lower and upper tolerance limits (Pι-P₂)-

Hydraulic retarder installation according to claim 1 or 2, characterized in that the control unit (8) is arranged to vary the strength of the superimposed operating current as a function of the distance to the pressure range between said tolerance limits (Pι-P₂). Hydraulic retarder installation according to one of claims 1-3, characterized in that the control unit (8) is disposed to superimpose a sinusoidal alternating current.