WO2001010692A1

WO2001010692A1 - Solenoid quick exhaust circuit for improved anti-lock performance in an electronic braking system

Info

Publication number: WO2001010692A1
Application number: PCT/US2000/021448
Authority: WO
Inventors: William P. Amato; William A. Hughes; Edward G. Ramsey; Roman O. Marchak; Stephen P. Moran; Cheryl L. Greenly
Original assignee: Alliedsignal Inc.
Priority date: 1999-08-06
Filing date: 2000-08-04
Publication date: 2001-02-15
Also published as: CA2379266A1; MXPA02001284A

Abstract

A system and method are provided for controlling an anti-lock braking system (ABS) in a motor vehicle where the ABS is performed by an electronic braking system (EBS) which includes a pulse width modulating (PWM) drive circuit connected to a solenoid (30) in a modulator valve (20). The modulator valve controls air from a pressurized air source (12) to a brake cylinder. The PWM drive circuit (100) allows the solenoid to accurately control the pressure to the brake cylinder during the braking operation to control the smoothness of the current and force profiles. During steady-state control operation of the system, a diode (146) in the drive circuit is connected in parallel with the solenoid. When quick dissipation occurs, the diode is eliminated from the circuit by opening the drive circuit with a high-side and low-side switch system (44, 120). This allows for optimal performance of the drive circuit when the EBS is in either the steady-state or quick dissipation mode.

Description

SOLENOID QUICK EXHAUST CIRCUIT FOR IMPROVED ANTI-LOCK PERFORMANCE IN AN ELECTRONIC BRAKING SYSTEM

Background of the Invention The present invention relates to the art of vehicle control systems, and more particularly to a method and apparatus for controlling the operation of vehicle brakes on a vehicle equipped with an electronic braking system (EBS) having an electronic control unit (ECU) with a drive circuit (for example, a pulse width modulated (PWM) drive circuit) controlling a solenoid in a modulating valve of an anti-lock braking system (ABS) . More particularly, the invention relates to a drive circuit that operates effectively in both a steady state control mode and a quick current dissipation mode .

Modern vehicles commonly include anti-lock brake systems (ABS) . In these systems, a brake modulator valve is connected between a source of pressurized air and a brake actuator. The modulator is typically a three-way valve that under normal service conditions receives pressurized air upon activation of the brakes (e.g., upon operator depression of a brake pedal) and conveys the air to the brake actuators. If an anti-lock event -- i.e., impending wheel lock -- is sensed by the associated ECU, the ECU sends electronic control signals to a solenoid valve assembly associated with the relevant modulator. The ECU controls the solenoid valve assembly so that the modulator valve controls the flow of pressurized air to the brake actuators in a manner that simulates brake pedal "pumping" at a repetition rate not obtainable by a human operator. The result is improved braking efficiency without loss of vehicle control associated with wheel lock or skid.

Frequently, the modulator is a proportional pressure control valve. To use this valve for both service brake applications and ABS control it must be able to provide a smooth and consistent pressure and be able to quickly change from one pressure to another. PWM is used to control the current, and therefore the force, produced by the solenoid. In most PWM drive circuits a diode is placed in parallel with the solenoid to produce smooth current and force profiles and minimize power dissipation. Though the diode is beneficial during steady-state operation, it limits the rate at which the current and force can dissipate. Conventional PWM drive circuits cannot perform optimally during the anti-lock brake control because the diode does not allow for rapid dissipation of the current .

The present invention is directed to a new and improved apparatus and method which overcomes the above- referenced problems and others and provides for a PWM drive circuit that is able to operate in both the quick dissipation mode and steady-state control mode.

Summary of the Invention In accordance with the present invention, there is provided a method and apparatus for controlling an anti-lock and service braking system in a motor vehicle using an EBS where a brake pedal sends an activation signal to an ECU and at least one sensor sends situational data to the ECU. The ECU outputs a control signal to a solenoid m a modulator valve to control air to the brake cylinder. The drive circuit includes a diode connected m parallel with the solenoid when operating in a steady-state control mode. When the EBS system is operating m a quick dissipation mode the diode is removed from the circuit .

According to another aspect of the invention, the current path to the diode is opened and the diode is effectively removed from the circuit when the ECU determines that a quick dissipation mode is required.

A primary advantage of the present invention is found in the ability to effectively operate the braking system in both a steady state control mode and a quick dissipation mode without limiting the current and force dissipation rate.

Another advantage of the invention resides m the use of a PWM drive circuit that optimally performs during an ABS event, as well as when the braking system demands a rapid brake pressure decrease. Still other benefits and advantages of the present invention will become apparent to those of ordinary skill m the art upon reading and understanding the following detailed description of the preferred embodiments.

Brief Description of the Drawings

The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating a preferred embodiment and are not to be construed as limiting the invention, where:

FIGURE 1 illustrates a schematic representation of a braking system in accordance with the subject invention; FIGURE 2 is an elevational view, partly in cross-section of a typical solenoid operated, proportional modulator valve;

FIGURE 3 is a circuit representation of the pulse width modulating drive circuit;

FIGURE 4 illustrates the drive circuit's incorporation into a schematic representation of the ECU and the modulator valve ;

FIGURES 5a and 5b are graphical representations of the steady- state mode of operation;

FIGURES 6a and 6b are graphical representations of the operation with a conducting diode m the drive circuit; and

FIGURES 7a and 7b are graphical representations of the quick dissipation mode of operation with diode effectively removed from the drive circuit .

Detailed Description of the Invention

FIGURE 1 schematically illustrates an ABS/EBS braking system in which a brake cylinder 10 applies a braking force to a wheel (not shown) . The brake cylinder is m communication with a source of pressurized air 12 through modulator valve 20 having a solenoid actuator 30. As will be described m greater detail below, the valve 20 is electrically actuated when solenoid actuator 30 receives a control signal from a pulse width modulator circuit (PWM) , which is described m more detail below in reference to FIGURE 3, in an electronic control unit (ECU) 22. The onboard ECU 22 receives various electrical signal inputs from one or more sensors 24 and a signal from a foot pedal 26. The electronic signals are input to the ECU and, in response, a suitable control signal is provided through line 28 to the solenoid actuator 30 in modulator valve 20. The sensors, for example, monitor wheel skid conditions (anti-lock brake systems or ABS systems) and/or wheel slippage such as a traction control system. The pressure supplied to the brake cylinder may also be monitored and a suitable signal provided to the ECU 22 to prevent over pressure conditions. General details of these types of units are well known in the art (for example, U.S. Patent No. 5,404,303 which is commonly owned by the assignee of the present application and incorporated herein by reference) so that further description is deemed unnecessary to a full and complete understanding of the present invention.

Turning now to FIGURE 2, a typical proportional modulator 20 is illustrated and includes a solenoid actuator 30 having a piston 32 with an O-ring seal 34 at one end. The piston extends into valve cavity or supply chamber 36 in the modulator housing where the seal selectively engages valve seat 38 on one end of a spool valve 40. The spool valve is a hollow cylindrical arrangement having first and second seals 42, 44 defined at opposite ends thereof. A central, small diameter passage 50 extends through the spool valve and provides communication between a first or delivery port 52 and a small diameter exhaust port 54. In addition, an inlet or supply port 56 selectively communicates with the delivery port 52 for applying a pneumatic braking force to the brake cylinder associated with the wheel. A second or enlarged diameter exhaust port 70 is also provided and is in selective communication with the delivery port 52 via diaphragm 72. The diaphragm includes a bleed or pilot opening 74 that provides constant communication between the delivery port and the supply chamber 36.

When an operator depresses the foot pedal, an electrical signal is ultimately provided to the armature of the solenoid actuator. This urges the piston 32 rightwardly from its spaced association with the seat 34 (i.e., the valve is normally open) and into engagement with the left-hand end of the spool valve 40. Communication, therefore, between the exhaust port 54 and the delivery port 52 is closed. The spool valve then continues traveling rightwardly where seal member 44 is moved from its sealed position to an open position m which pressurized air from the supply port 56 communicates with the delivery port 52 and pressure increases in the brake cylinder. At the same time, the pressurized air also communicates through the small diameter passage 50 of the spool valve. This pressure counteracts the rightward movement of the piston caused by the energized armature and provides a proportioning action on the piston 32. In this manner, the proper pressure or proportioned pressure is provided to the delivery port 52.

Upon release of the foot pedal, pressure in the chamber 36 urges the piston leftwardly since the armature is no longer energized. This pressure, along with the biasing force provided by spring 80, urges the spool valve leftwardly to close off communication between the supply and delivery ports and opens the exhaust seat 38. In addition, differential pressure across the exhaust diaphragm lifts the diaphragm so that communication is established between delivery port 52 and the quick exhaust port 62. More particular details of this type of valve structure are shown and described in U.S. Patent No. 5,123,718, the details of which are incorporated herein by reference. Of course, it will be recognized that still other proportional modulator valve arrangements can be used without departing from the scope and intent of the subject invention.

In vehicle braking systems such as that described above and U.S. Patent No. 5,404,303, drive circuits have been unable to operate optimally in both the quick dissipation and steady state modes. Specifically, the diode that is beneficial during the steady- state operation to smooth the current and force profiles adversely limits the current and force dissipation when quick dissipation is desired. The present invention overcomes this problem by effectively removing the diode from the drive circuit during quick dissipation mode of the EBS.

Turning now to FIGURES 3 and 4, a PWM drive circuit 100 in ECU 22, where the drive circuit 100 is used to activate the solenoid 30 for braking. The solenoid 30 may be of any known type in the art, and is preferably shown as being comprised of a series connected inductor 102 and resistor 104 for modeling purposes only. The solenoid 30 is connected to a pulse width modulating input controlled voltage source 106, which is controlled by the ECU software, through a low side switching circuit 110. The low side switching circuit 110 includes a low side switch 120, preferably a field effect transistor (FET) or MOSFET, which is connected to the solenoid through its drain and is connected m parallel with series connected zener diode 122 and diode 124 at its gate. The pulse width modulator input voltage 106 flows to the switching circuit through a resistor 130. Resistor 132 is connected to ground. The solenoid 30 is connected to a second voltage source 140 through a second switching system, which is series connected with the solenoid 30.

The second switching circuit includes a zener diode 142, which goes to ground, and a high side switch 144, which is also controlled by the ECU software. The high side switch 144 and diode 146 connect at node 148, and a third input to the node 148 includes a diode 150 m series with the voltage source 140. In accordance with the present invention, the method of effectively removing diode 146 from circuit 100 during the quick dissipation mode and allowing diode 146 to be m parallel connection with the solenoid 30 during the steady- state control mode will be discussed. In the steady-state control mode, the ECU 22 applies a pulse width modulated signal from source 106 to the gate of the low side switch 120 through the switching circuit 110. The ECU also energizes high side switch 144. In this manner, both switches are closed, completing the circuit to connect diode 146 in parallel with proportional solenoid 30.

Conversely, during the quick dissipation mode, when the automatic braking system needs to rapidly change the air pressure flowing from chamber 12 to brake cylinder 10 through modulator valve 20 by rapidly activating and deactivating solenoid 30, the diode 146 is optimally removed from the drive circuit 100. During the quick dissipation mode the ECU de- energizes the high side switch 144 and de-energizes the low side switch 110. This opens circuit 100, as is best shown in FIGURE 4, effectively removing the diode 146 from the circuit 100. Zener diode 122 is at approximately twenty volts, for example, which allows the lower side of the coil 102 to reach approximately twenty to twenty four volts. The energy stored in the coil is more quickly dissipated since there is a greater voltage across it. The lower side of the coil rises to the clamp voltage of zener diode 122. The top side of the coil is free to go to a negative voltage when switch 144 opens. Thus, the top side of the coil 102 in the steady state mode was at twelve volts. However, opening the switch 144 in the quick dissipation mode and the provision of zener diode 142 allows the top side of the coil to drop from, for example, twelve volts to a negative level. The higher voltage potential across the coil thereby dissipates the energy more quickly - - which is desired with the rapid pulsing/actuating of the brakes in an ABS mode. The reason the lower side of the coil can reach an elevated level is based in part on the provision of diode 150. The reverse bias of the diode 150 allows node 148 to go to a higher voltage. Opening high switch 144 starts the process, breaks the recirculating current path with diode 146, and permits the increased voltage potential across the coil to be attained which, in turn, more quickly dissipates the energy therefrom. This allows the current to rapidly dissipate through solenoid 30 as will be described in greater detail with reference to FIGURES 7a and 7b.

FIGURES 5a and 5b graphically represent a steady- state mode of voltage and current through the proportional solenoid 30. In FIGURE 5a, channel 1 represents the current flowing through the proportional solenoid 30 and channel 2 represents the low side voltage at the proportional solenoid 30. In FIGURE 5b channel 1 represents the current flowing through the solenoid 30 and channel 2 represents the pressure sensor output, which is approximately 2.3 V in the exemplary arrangement, although it will be understood that other values may be advantageously used without departing from the scope and intent of the subject invention. With additional reference to FIGURES 6a and 6b and

FIGURES 7a and 7b, the difference between the current flowing though the solenoid 30 with the recirculating diode 146 conducting (FIGURES 6a-b) and without the recirculating diode 146 conducting (FIGURES 7a-b) during the quick dissipation mode of the system is more clearly illustrated. Channel 1 in both FIGURES 6a and 6b represents the current through the solenoid during quick dissipation mode with the diode 146 conducting. Channel 2 in FIGURE 6a is the pressure sensor output, while channel 2 in FIGURE 6b is the low side voltage at the proportional solenoid 30.

Additionally, in FIGURES 7a and 7b channel 1 is a graphical representation of the current through the solenoid 30 during quick dissipation mode with the diode 146 out of the circuit 100 based on the present invention' s ability to open the circuit by de-energizing the high side switch 144. The signal on channel 2 in FIGURE 7a is the low side voltage at the proportional solenoid 30 and the signal on channel 2 in FIGURE 7b is the pressure sensor output.

It is evident from FIGURES 7a and 7b that by opening the switches 120, 144 and effectively removing the diode 146 from the circuit during quick dissipation mode, the current can dissipate remarkably faster than when the diode is left to conduct in the drive circuit, as in seen in FIGURES 6a and 6b. This allows the drive circuit 100 to optimally operate to produce a smooth current and force profile during the normal pressure changes in the braking system experienced with service braking application, as well as rapidly changing conditions associated with ABS braking. By using this pulse width modulator drive circuit 100 in the ECU to drive the solenoid, a smooth and consistent pressure is provided for both service brake and ABS brake control, and the valve is able to quickly change from one pressure to another. The invention has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the proceeding detailed description of the preferred embodiment. For example, more than one ECU can be used to control the operating parameters of the braking system. Likewise, the circuit described above was developed for modeling purposes and thus it will be appreciated tha a number of components can be varied, or even eliminated, in accordance with known circuit design principles, such as resistors 130, 132, zener diodes 122, 142, and diodes 124, 150. It is intended that the invention be construed as including all such alterations and modifications in so far as they come within the scope of the appended claims or he equivalence thereof.

Claims

What is claimed is:

1. A braking system for a motor vehicle comprising: a source of pressurized air; a brake actuator selectively receiving air from the source ; a pressure control valve operatively interposed between the air source and the brake actuator, the valve including a circuit having a coil powered by a voltage source that is operatively associated with a valve member and a device operatively associated with the coil for controlling smoothness and force application to the valve member; and a switch assembly for selectively controlling the rate at which the current is dissipated, in normal operation mode the device is connected with the coil and in quick dissipation mode the device is effectively removed from the circuit .

2. The braking system of claim 1 wherein the device is connected in parallel with the coil.

3. The braking system of claim 2 wherein the device is a diode .

4. The braking system of claim 1 wherein the circuit includes a first, low side switch and a second high side switch, and both the high side and low side switches are de- energized in a quick dissipation mode to effectively remove the device from the circuit .

5. The braking system of claim 1 wherein the circuit includes a pulse width modulator.

6. The braking system of claim 1 wherein the quick dissipation mode is associated with an anti-lock braking system.

7. A method for controlling an anti-lock braking system (ABS) in a motor vehicle, the method comprising the steps of: sending an activation signal from a brake pedal to an electronic control unit (ECU) ; sending situational data to the ECU from sensors ; processing the situational data in the ECU after the ECU has received the activation signal from the brake pedal and outputting a control signal from a drive circuit in the ECU to a solenoid in a modulator valve wherein the drive circuit includes a diode, ; and, receiving the control signal in the solenoid of the modulator valve to control pressurized air traveling through the modulator valve from a source of pressurized air to a brake cylinder, the solenoid regulating the pressurized air to produce a braking force in the brake cylinder; and essentially removing the diode from the drive circuit when the EBS is in quick dissipation mode.

8. The method according to claim 7, the method further including the steps of : using a pulse width modulating (PWM) drive circuit as the drive circuit, a high side switch, and a low side switch; and, wherein the diode is connected in parallel with the solenoid when the EBS is in steady-state control mode by applying a PWM signal to the low side switch from the ECU and energizing the high side switch.

9. The method of claim 8 comprising the further steps of closing the high side switch and the low side switch during the steady state mode to connect a diode in parallel with a solenoid, where the solenoid controls air flow from a pressurized air chamber to a brake cylinder during braking based on an activation signal from a brake pedal to the electronic control unit; and, opening the high side switch by de-energizing it ^~with the ECU during the quick dissipation mode to open a section of the drive circuit containing the diode, thereby effectively removing the diode from the drive circuit so that the solenoid will optimally dissipate the current during rapid activation and de-activation to smoothly control a force on the brake cylinder.