BACKGROUND OF THE INVENTION This invention relates in general to vehicular braking systems, and in particular is concerned with a low-friction sleeve and armature subassembly for a control valve in vehicular braking systems.

Hydraulic braking systems for vehicles are well known. A typical hydraulic braking system includes a master cylinder, fluid conduit arranged into a desired circuit, and wheel brake cylinders. The master cylinder generates hydraulic forces in the brake circuit by pressurizing brake fluid when the driver steps on the brake pedal. The pressurized fluid travels through the fluid conduit in the circuit to actuate brake cylinders at the wheels and slow the vehicle.

Anti-lock braking systems for vehicles are also well known hydraulic systems. A hydraulic control unit or housing, containing control valves and other components such as a pump, is located between the master cylinder and the wheel brake assemblies. Through an electronic controller, the control valves and other components selectively control pressure to the wheel brake assemblies to provide a desired braking response of the vehicle.

Many control valves are formed as electronically controlled solenoid valves. A typical solenoid valve includes an armature which reacts to magnetic

flux generated by a coil subassembly of the solenoid valve. An armature can be formed as a cylindrical element slidably mounted in a tube or sleeve. Many anti- lock braking systems include both normally open solenoid valves (isolation valves) and normally closed solenoid valves (dump/hold valves).

Solenoid control valves use lateral magnetic circuits and operate at high frequencies. White lateral magnetic circuits provide the axial output force versus displacement characteristics need for proportional hydraulic control, they usually suffer from significant detrimental-lateral forces that can cause undesirable levels of hysteresis and slow performance. A reduction in such hysteresis can improve armature performance, and thus provide improved performance of a braking system. Various coatings of-low friction materials have been applied to the outer surface of armatures to reduce hysteresis. However such coatings can be expensive and may be limited in thickness. Furthermore, it is desirable to improve the construction of control valves to reduce costs and the time required for assembly.

SUMMARY OF THE INVENTION This invention relates to vehicular braking systems and solenoid control valves for such systems. The invention includes a sleeve and armature subassembly for control valves and a method for forming the subassembly. A sleeve is fitted on the armature to reduce hysteresis between the armature and a flux tube, also known as a sleeve, which slidably receives the armature.

Furthermore, the sleeve can be formed from a relatively thick film (e. g., 0.20 mm) with eccentricity controlled (e. g., +/-Or. 0025 on the armature and +/-0.0050 mm on the flux tube) to provide a very desirable armature film thickness/eccentricity ratio.

In a preferred embodiment, a control'valve for a vehicular braking system includes a valve subassembly including an armature slidably received in a flux

tube. A solenoid subassembly is mounted on the valve subassembly for inducing a magnetic flux to slide the armature. A sleeve is fitted onto the armature to reduce friction between the armature and the flux tube. The sleeve is formed from a low-friction material A method is disclosed for forming the armature and sleeve subassembly. The present control valve has less hysteresis than previous control valves and can improve performance of a vehicular braking system.

The flux tube according to this invention can be formed as a three piece subassembly including a non-ferromagnetic section disposed between two ferromagnetic sections. Furthermore, the present valve subassembly includes a valve body having a cap. A fluid passageway is provided the valve body and the cap. The cap acts as a valve seat for the-armature andvfor a valve element to control fluid flow through the control valve.

Various objects and advantages of this invention witl become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional view of a control valve according to the present invention mounted on a hydraulic control unit of a vehicular anti-lock braking system.

FIG. Z is a partly sectional view illustrating an expanded tubing being slid onto an armature for forming a low-friction sleeve and armature subassembly for use in the control valve of FIG. 1.

FIG. 3 is a partly sectional view illustrating chamfered ends of the tubing after it has been slid onto the armature and heated as desired.

FIG. 4 is an enlarged, partial view of a sleeve and armature subassembly according to the present invention after grinding has reduced the thickness of a cylindrical portion of the sleeve as desired.

FTG.'5is a schematic view of one embodiment of a vehicular braking system in which the control valve of FIG 1 can be incorporated.

FIG. 6 is a sectional view of a second embodiment of the present control valve illustrated as both a normally open isolation valve and a normally closed Toldump valve in a schematically illustrated vehicular braking system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A fluid control valve according to the present invention is indicated generally at 10 in FIG. 1. The valve 10, which can be a proportional valve, is particularly suited to regulate brake fluidpressure in vehicular braking systems such as an anti-lock braking system (ABS), a traction control braking system (TCS), or an electronic brake management system (EBM). The valve 10 includes a solenoid subassembly 12 connected or secured to a valve subassembly 14.

The solenoid subassembly T2 includes a cup-shaped housing or casing 16.

Preferably, the casing t6 is a drawn member having a generally large area and a planar end surface IS. A coil 20, preferably a bobbin-less coil, is placed inside the casing 16 At its outer diameter, the coil 20 can be wrapped with a suitable tape (not illustrated) for protection and retention of a coil lead tower washer (not illustrated) A flux ring 22 is pressed into the casing 16 to secure the coil 20. A pair of winding terminals 24a and 24b extend outwardly through respective openings in therend-surfåce tg ofthe casing 16 and are adapted to be connected

to an electronic control module (not illustrated) for inducing a magnetic field in a well known manner.

The solenoid subassembly 12 is pressed onto a flux tube or sleeve 25 of the valve subassembly 14 ; The flux tube 25 can be formed as a three-piece subassembly having a first, cup-shaped ferromagnetic section 26, a second, cylindrical non-ferromagnetic section 28 ; and a third ; cylindrical ferromagnetic section 30. The sections Z6, 28, and 30 can be friction welded together and then machined to a desired finish. Alternatively, the sections 26,28, and 3Q can be oven brazed or laser weIded together. Also, the sections 26,28, and 30 may be made as a metal injection molding. A radiatflange 3I is formed about a lower portion of the third section 30 which extends downwardly beyond the flux ring 2Z when the solenoid subassembly t2 is pressed onto the flux tube 25.

A cylindrical armature 3Z is slidably received within the flux tube 25.

The armature 3Scan be formed as an iron cylinder. Preferably, the armature 32 is formed with at least one chamfered end 32a. Preferably, a non-magnetic sleeve 33 is snugly fitted about at least the axiat length of the armature 32. The. sleeve 3Tis formed-from a low-friction material and preferably has a substantially uniform waX thickness. A preferred material for the sleeve 33 is polytetrafluoroethylene, commonly known by the trademark Teflon. Other preferred materials for the sleeve 3Tinctudefluorocarbons. A chamfered ends 33a of the sleeve 33-may be rounded over an upper end of the armature 32. In other embodiments, the sleeve 33 can be cut even with the ends of the armature 32. Preferably, the armature 32 and sleeve 33 are formed as a subassembly prior to positioning within the flux tube 25. The sleeve 33 reduces friction between the armature 32 and flux tube 25 and reduces hysteresis of the armature 32.

Furthermore, the sleeve 33 established the magnetic gap between the armature 32 and the flux tube 25. This magnetic gap is substantially uniform due to the substantiany uniform thickness of the sleeve 33. Also, the sleeve 33 is relatively

thick when compared to coatings and a desired sleeve thickness/eccentricity ratio.

When the coil 20 is energized, a magnetic field is generated causing magnetic flux to pass from the third section 30-through the sleeve 33 to the armature 3 :, and then through the sleeve 33 to the first section 26. The magnetic flux does not travel through the second section 28 The flux tube 25, flux ring 22 and casing f6 cooperate together to provide a complete path for the magnetic flux.

A recess ? 4 formed in an outer endofthe armature 32 receives a spring 36, illustrated as a coil spring. An opposite end of the spring 36 is received by the cup-shaped first section 26 of the flux tube 25. A hollow cylindrical spacer ? 8 is used to center the spring 36 within the first section 26. In this construction, spring 36provides a force to bias the armature 32 away form the outer end 18 of the casing 16. When the magnetic field is induced, the armature 32 is urged toward the outer end 18 of the casing t6 and compresses spring 36. Preferably the inner surfaces of the sections 26,28, and 30 of the flux tube 25 are burnished and the outer surface of the armature 32 is ground so that these surfaces are highly concentric to minimize lateral (radial) forces, coulomb friction and hysteresis. The sleeve 33 provides concentricity and resists friction to reduce hysteresis.

A drop-in assembly is desired for the spacer 3g, spring 36, and armature 32. These elements can be easily slid into the interior volume of the flux tube 25 without any special assembly techniques.

The inner end ofthe third section 30 of the flux tube 25 is placed against a valve body 4T. Preferably, the valve body 41 is formed from a non-magnetic material so that the valve body 4T is not part ofthermagnetic circuit formed when the coil 20 is energized. The valve body 41'may be formed from plastic, aluminum, or steel, as well as other desired materials.

In the embodiment illustrated, the valve body 41 is a generally cylindrical member having an axial passageway 42. An upper end-of the valve body 41 is formed as an outwardly projecting cap 44 : Preferably, an outer surface of the cap 4-terminates in an apex, illustrated in this embodiment as a conical portion.

A passageway 46is formed in the cap 44~ concentnc witbpassagsaL42. An outer surface ofthe cap 44 acts as a valve seat for a lower end surface of the armature 3Z If desired, a valve element such as a disc or a ball (neither of which is illustrated) can be disposed between the end surface of the armature 32 and the cap 44 to assist in bLocking fFuiNfhow. At least onefluid passageway 47 is formed in the valve body 4T radially outwardly from passageway 42. A-radial fLange 4Tis formed about an outer surface of the valve body 41. A reduced diameter portion 50 is formed in a lower portion of the valve body 41.

The valve body 41 is received in a stepped opening oLbore 1û4 formed in a hydraulic control-unit (HCU) r02-of a vehicular braking system. The bore 104 includes a first step or shoulder i06 which receives a tower surface of the flange 48 ; An inner reduced diameter portion ¢8 of the bore 104 receives the reduced diameter portion 50 of the valve body 4t. A seat 52 received in an annular groove 54 formed in an outer surface of the lower portion 50-ofthe valve body 41 provides a fluid seat between the valve body 41 and the HCU 102. A seal 56 is trapped between the ftange 3T of the ftux tube 25 andThe radial flange 48 of the valve body 4r to provide a fluid seal.

The control valve TO is significantly smaller than other known proportional solenoid valves. For example, valve TO can be constructed with a 24 mm diameter and a length of 52mm. Valve 1t is easier to assembly, more resistant to contamination, and uses less current than other known proportional control valves. For example, in one embodiment, valve i0 uses only 0. 35 amp during periods of pressure change

In a preferred'method of assembly, various components are stacked together and'then retained on the HCU"t02 by a single orbital swaging operation.

The spring 36 ; spacer 38, and-armature 32 are inserted into the flux tube 25. The valve body 4I with seal 52Tis pressed into place (preferably with a low force) into the bore r04 SeaT56 is trapped between the flux tube 25 and the valve body 41 ; An annuLar lip 9Ris formed from material-surrounding the bore 104.

The up 9 ? can be formed by any desired metal-forming technique, including orbitat swaging. The lip 99'fbimcd by a single process retains the valve 10 on the HCU 102 and holds the components of the valve 10 together, all in one operation.

The present valve la controls fluid flow with a poppet seat provided by cap 44 and disc 40. Such a poppet seat is highly resistant to contamination and is highly resistant to leakage. The solenoid valve sensitivity (i. e., valve flow opening/ {pressure demand-pressure output of valve M) is controlled by the spring rate of spring 36'and not by the magnetic circuit spring rate since the preferred magnetic spring rate is zero (flat). If valve sensitivity needs to be changedfbr larger brake systems, then merely a spring 36 with a different spring rate need be changed.

In a vehicular braking system, a pair ofvalvesFOcan-beusedto control braking at a wheel. One valve 10 can be configured as a normally closed valve as illustrated in FICF I, and one valve can be configured as a normally open valve as discussectberow. For example, in an electronically controlled hydraulic brake system, one valve ro can be used to control pressure apply while a second valve 1-0 can be used to control pressure release. Only one valve 10 is energized at a time and only during peak periods of pressure change. On long downhill grades with long periods of unchanged brake. pressure holding, the valve 10 will draw no current, thereby greatly reducing solenoulvalve he

During energization, the magnetic forces of valve 10 tend to be close to a linear function of current, rather than a function of currentsquared, as is the case with many magnetic circuits. This aspect of valve 10, which results from the configuration and sizing of the armature 32 anctftux tube 25 and associated primary air gap, produces a valve whose output sensitivity-to input control currents is substantially uniform (at bothhighandlowbrake fluid pressures), thus making pressure control'proportional to current.

A preferred method for forming the subassembly comprlswthiLsleeve 33 and the armature 32s illustrated in FIOS. 2-4 as successive steps. The method involves the use ofatooi200having a downwardly projecting annular step 202 and a passageway 204 preferably aligned with an axis of the step 202. The tool 200 can be positioned as desired with respect to an armature 32. Shrink tubing 206 formed from a flexible, low-friction material is used with the tool 200.

The armature 32 is provided. A predetermined length of tubing 206 is provided mounted about the step 202 :Pressurizedjairtravels through the passageway 204 to expand the predeterminedXengF of tubing 206 When unexpanded, the inner diameter of the tubing 206 is preferably less than the outer diameter of the armature 327 Also, pressurized air increases column strength of the tubing 206 to prevent buckling or collapsing as the armature 32 is slid into the tubing 206.

The tool 2Xwith the tubing 2M'is moved-downwardly toward the armature 32-so that the tubing 206 slides over the-chamfered end 32a of the armature 32 as illustrated in FIG. 2. The chamfered end 32a leads the tubing 206 onto the armature 32 and provides an initial pressure seal with the tubing 206.

The pressurized air expands the length andctiameter oftE tubiñg 2û6 for easier installation onto the armature 32 and resultsin residùal bop stress to retain the sleeve 33 on the armature 32. This procedure avoids unsymmetrical

circumferential shrinkage The pressurized air also acts-as an air bearing between the armature 32 and the tubing 206 to reduce installation-friction.

The tubing 206 is heated to form the subassembly of the armature 32 and sleeve 3S. As illustrated in FIG ? ;, heat allows additional conforming of the tubing Z56to the armature 3 and-causes the ends 206a to round-over around the ends of the armature, resulting in the chamfered ends 33a of the sleeve 33. Ends 236å relieves and bending stress at the end corners which could otherwise cause the cylindrical portion of the sleeve 33 to separate from the armature 32.

The armature 3'Z and~tubing 2o6 can undergo a grinding process to accurately set the subassembly diameter. For example, the grinding step can be accomplished by a centerIess grinding operation using a-grinding wheel and a follow wheel. The centertess grinding operation accurately reduces the thickness ofthe cyliiidricaTportion of the tubing 206as compared to the thickness of the ends ZO6å. A cutting process removes portion oftthe tubing 206 from an end of the armature 32 which engages the valve body 41, as illustrated in FIG. 4.-Once the proper diameter has been set and the end-portion removed, the armature 32 and sleeve 33 subassembly can be used with contror valve 10 as illustrated in FIG. 1.

Tubing 2tut6 formed from polytetrafluoroethylene has been successfully used ; Centerhess grinding has resultedtin accurate armature core diameter (+/- O. OoZ5 mm), accurate armature-sleeve subassembly diameter, (+/- 0. 0025 mm), and an accurate thickness, low-friction sleeve 33 that is well attached to the armature 32. By using O. ZU mm low-friction tubing 206 in this method, and +/- 0. 025-mm eccentricity on the armature 32~and~+A-0. 060 eccentricity on the flux tube 25, desirable results have been obtained. A control valve 10 with the present armature 32 and sleeve 33 produces a low hysteresis versus proportional output by having a high ratio of armature non-magnetic gap. thickness versus armature positional eccentricity.

A vehicular bracing system indicated generally at 300 in FIG. 5 represents one example of a braking system which can incorporate the'control valve 10 with its armature 32 and sIeeve 33 presented above. In system 300, a brake pedal 310 is connected-to a master cylinder 32 to provide pressurized brake fluid to a wheel brake 314. In the embodiment illustrated in FIG. 5, the wheel brake 314 is illustratedas a disc assembly ; however, the wheerbrake 3-4 may be any type found on vehicles.

A hydraulic control unit (HCU) 302 is a housing for valves and other components as described below. For purposes of clarity ofiUustration, only one set of components are illustrated in FIG 5. Typically, however, the HCU 302 also houses corresponding components for other wheels of the vehicle.

The HCU 302-inctucres a normally open isolation-valve 316 disposed between the master cylinder 3T2 andthe wheefbrake 314, at least one low pressure accumulator 3T8, a normally closed hold/dump valve 320 disposed between the wheel'brake 3-1-4 and the low pressure accumulator 318, and a hydraulic pump 322-connected-low pressure accumulator 318 and an inlet to the isolation valve 316. The HCU 302 may alsodndude-an attenuator 3Z4 between an output ofthe pump 322-*anctthe inlet-to the isolation valve 316 to limit and smooth nuidftbw from the pump 322hack to the master cylinder 312.

Various fluid passageways are provided in the HCU 3022 tn_ connect the various components.

The isolation valve 316 and the hold/dump valve 320 are control valves preferably formed as electrically actuated-solenoid valves_ The isnlas n vaLve 316-is preferably formed as a two position, solenoid valve having a normally open configuration. The hold/dump valve 320 is preferably formed as a two position, solenoid valve having a normally closed configuration. The valves 316 and 320 are switched by an electronic control module (not. illustrated) to provide anti-lock braking in well known manner-

As described above, control valve 10 is a solenoid valve having a normally closed configuration. Control valve 10 can be substituted for hotd/dump valve 320-to perform in system 300. The low hysteresis provided by controrvatve IQ results in better perfwcrof system 300. The pressure output of control'vatve fU is a function of current to the valve lO times the. input pressure to the valve TO. By varying the current and knowing the input pressure, the output pressure can be controlled.

In other embodiments, a control valve 10 according to this invention incorporating the above-described armature 32 and sleeve 33 can be configured as a normally open solenoid valve. For example, a second-vehicular braking system is indicated generally at 400 and illustrated in JIG. 6. As described below, system 400includes a normally open isolation valve 422 incorporating an armature and sleeve as described above.

In system 40t, a brake pedS412 is connected-tua master cylinder 414.

A fluid conduit 416isconnectedbetweenthemastercylinder 414 and an inlet port 4W'formed in a hydraulic control unit (HCU) 420. Preferably.. the. HCU 420 is a housing formed from a suitable lightweight material which includes bores for receiving various components and internaL conduits connecting such components.

A first solenoid valve 422 funcuuns as an isolation and proportional reapply valve as describedbelbw. Isolation valve 422 includes a valve body 424 received in a first bore 426-of of me HCU 420 The valve body 424 has a radial flange 428, a first internal channel 430'and a second internal channel 432. After the valve body 424 has been inserted into the bore 426, an annual lip 434 is formedby swaging material of the HCU 420 adjacent the. bore 426 to retain the valve body 424 on the HCU 420. The second channel 432 is aligned with an outlet port 436 formed in the HCU 420.'The outlet port 436 is in fluid communication with a fluid conduit 437 : A seaL43 8 is received in a-groove

formed in an outer surface of the valve body 424 to provide a seal between the inlet port 418 and the outlet port 436.

The isolation valve 422-also includes a coil assembly 440. The coil assembly 440 is inserted onto the valve body 424 and can be retained by a cover assembly or an electronic contror module assembly (neither of which is illustrated) or the like, which is pushed downwardly onto the coil assembly 440.

A cup-shaped housing or casing 442provides a flux return path. A coil 444, preferably an epoxy bound bobbin-less coil, is placed'around-a-flux tube-446. A fl'ux ring 44Sis pressed''into the casing 442 to secure the coil 418 and complete the flux return path. Preferably, the flux ring 4Dis machined to minimize air gaps between the casing 442 and the flux tube 446. A pair of winding terminals 450FA and'450B extend outwardly through respective openings in the casing 442 and are adapted to be connected to an electronic control module (not illustrated) for inducing a magnetic field through the flux tube 446 in a well known manner.

Preferably, the flux tube 446 is formed as a three-piece cylindrical member having a first, cup-shaped ferromagnetic section 452, a second,. cylindrical non-ferromagnetic section 454 ; and a third, cylindrical ferromagnetic section 456. When induced by the coil 444, the magnetic field passes through the first and third sections 452 and 456, and not thp second section 454.

A cylindrical armature 459 is slidably received within the flux tube 446.

A spring 4Ris seated at one end on a spring seat 462 formed on the lower end of the armature 458. The spring 460 is seated at its opposite end on a spring seat 464 formed on an upper end of the valve body 424. The solenoid valve 422 is normally open as the spring 460 urges the armature 45S away from the valve body 424 with a low spring force. When the coil 444 is energized, the armature 45S is urged downwardly against the spring 460 by magnetic forces until spring seat 462 contacts spring. seat 464 to close the isolation valve 422. The magnetic

force works against the pressure differential across a valve seat formed by the spring seats 462 and 464.

Preferably, the flux tube 446 is roll burnished and the outer diameter of the armature 45tis ground-to provide very precise positional concentricity of the armature 45;, thereby minimizing lateral-forces. Furdlermore, a low-friction sleeve 465 fitted on the armature 458- (like sleeve 3Ton armature 32) will provide low friction between the armature 458 and the flux tube 446 and minimize hysteresis. The magnetic configuration of the isolation valve 422 provides a lateral working air gap in which the magnetic permeance is a linear function of axial traveL. Thus, the axial magnetic force is proportional to current and independent of the traverofthe armature 458 in its operating-range. In this manner, proportional control of-the isolation valve 422 is stable and controlled.

A second solenoid valve 466 functions as a proporoonaLdumpvalve-as described below. Dump valve 466 is a normally closed valve having a valve body 468 and a coil assembly 470 valve body 46tis preferably retained in a bore 472 in the HCU 420 by an annular swaged lip 474. A first channel 476 is in fluid communication with an inlet port 478 formed in the HCU 420. A second channel 480 is in fluid communication with an outlet port 482 formed in the HC'U 4Z0. Outlet port 48Z is in fluid communication with an inlet line 484 of a hydraulic pump 486.

The coil assembly 470 includes a casing 489, a coiL490, a flux tube 492, and an armature 494 slidably received inside the flux tube 492. Preferably, the flux tube 492 is formed as a three-piece cylindrical member having a first, cup- shaped ferromagnetic section 496, a second, cylindrioal non-ferromagnetic section 498 ; and a third, cylindrical ferromagnetic section 500. When induced by the coil 490, the magnetic field passes through the first and third sections 496 and 500, and not the second section 498. A low-friction sleeve 501 is fitted on the armature 494 (Hke sleeve 33 on armature 32) to provide low friction between

the armature 494and the flux tube 492 and minimize hysteresis. A spring 502 is received in a cavity 504 formed in an upper portion of the armature 494 and urges the armature downwardly so that a lower surface 5D6 of the armature 494 contacts a valve seat 50S formed on an upper surface of the valve body 468.

The pump 486 outlet is connected to conduit 416-and delivers fluid to inlet port 4I-S : A check valve 5I permits only one-way fluid flow from conduit 437to conduit 4T6. A pressure transducer 512reduces the effects of varied pressure from the master cylinder 4T4 during certain modes of the system 400 as described below.

During normal braking ofthe system 400 ; i. e., not anti-lock braking, both solenoiFvaYves 4'2Z and 4'66 are not energized and-the pump 486 is not operated.

An operator presses the brake pedaT4T2to release fluid pressure from the master cylinder 414 to conduit 4T6 Ihcreasedfluid pressure travels inlet port 4rg and channel 430, past spaced-apart spring seats 464 and 462 to channel 432 and outlet port 43-6 to conduit 43T Pressurized-fluid reach-es a wheel brake 514, illustrated as a caliper, to cause braking of a wheel rotor 516.

During anti-rock braking, i. e., when wheellbckup is imminent, the pump 4876 is operated and the solenoid valves 42Z and 466 are energized By energizing the isolation solenoid valve 422, the armature 45t slides downwardly to engage the valve body 424 and block fluid Sow from the master cylinder 414 to the wheel brake sur4. The dump solenoid'valve 466 is opened-as-the armature 494 is pulled upwardly away from the valve body 468 so that fluid pressure is reLieved'from the wheel brake 5T4 through inlet port 478, outlet port 482 and conduit 484.

As the vehicle wheels start spinning up and pass a target slip level, the dump valve 466 is de-energized and the voltage to the isolation valve 422 is reduced to a predetermined stored value plus a second value which is-a function of the master cylinder 414 pressure. This causes the isolation-valve 422 to

quickly reapply a pressure to the brakes that is a function of the stored voltage.

The stored voltages are retained in the electronic control-module (not illustrated).

Afterwards, current to the isolation valve 422 is decreased at a controlled rate which is also a function of master cylinder 4t4 pressure, thereby causing brake pressure to increase at a relatively slow rate after a proportional quick reapply of the isolation valve 422.

If lockup is not again imminent after a predetermined interval(e approximately a'seconds), the isoIation valve 4SZcurlent is decreased at a faster rate, thereby providing a faster brake pressure reapply for this portion of the anti-lock braking cycle.

When lockup is again imminent, the voltage of the isolation valve 422 is first stored. Next, the isolation valve 422 is futly energized and the dump valve 466is energized.'The stored'voItage indicates the pressureneeded to lockup a when under the conditions of the previous cycle and is used to control the next reapply cycle.

For the next reapply cycle, a proportion of the stored voltage is quickly applied to the isolation valve 4ZZ, foltowed by a slowly decreasing voltage.

Thus, a fast proportional reapply is foltowed-by a slow reapply as described above.

Similar logic can be used for proportional dump control. A quick increase in dump valve 466 current can be followedby a slower increase until the vehicle wheels start spinning up again. Ifthe wheels do not start spinning up after a desired interval (e. g., approximately O. Z0 seconds), the dump current rate can be increased The value of the current at which the wheels start to spin up-is stored. and usedonthe next dump cycle, thus providing proportional pressure dump.

The isolation valve 422provides a reapply pressure that is an inverted function of current and is based on master cylinder pressure. Thus the pressure transducer 5T2is beneficiaLin providing input to adjust control current to

diminish effects of varied master cylinder pressure on reapply pressures to the wheel brake 514.

In the system 40U, proportional reapppy lKs aDwayspravided IlRing decreasing isolation valve 422voltage and the proportional release option is always provided'using increasing dump valve 466'voltage. Both magnetic and coulomb hysteresis are diminished since each valve 422-ad466 is cortller1 from one directiom AdditionaIIy, since the inputs are valve voltages and essential feedback is wheel behavior, variations from valve to valve and variations due to temperature can be substandallydiminished- In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment : However, it must be understood that this invention may be practiced otherwise than as specifically expLained and illustrated without departing from its spirit or scope.