Combined Brake and Steering Actuator for Automatic Vehicle Control
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CALIFORNIA PATH PROGRAM INSTITUTE OF TRANSPORTATION STUDIES UNIVERSITY OF CALIFORNIA, BERKELEY Combined Brake and Steering Actuator for Automatic Vehicle Control R. Prohaska, P. Devlin California PATH Working Paper UCB-ITS-PWP-98-15 This work was performed as part of the California PATH Program of the University of California, in cooperation with the State of California Business, Transportation, and Housing Agency, Department of Trans- portation; and the United States Department Transportation, Federal Highway Administration. The contents of this report reflect the views of the authors who are responsible for the facts and the accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of the State of California. This report does not constitute a standard, specification, or regulation. Report for MOU 331 August 1998 ISSN 1055-1417 CALIFORNIA PARTNERS FOR ADVANCED TRANSIT AND HIGHWAYS Combined Brake and Steering Actuator for Automatic Vehicle Research y R. Prohaska and P. Devlin University of California PATH, 1357 S. 46th St. Richmond CA 94804 Decemb er 8, 1997 Abstract A simple, relatively inexp ensive combined steering and brake actuator system has b een develop ed for use in automatic vehicle control research. It allows a standard passenger car to switch from normal manual op eration to automatic op eration and back seamlessly using largely standard parts. The only critical comp onents required are twovalve sp o ols which can b e made on a small precision lathe. All other parts are either o the shelf or can b e fabricated using standard shop to ols. The system provides approximately 3 Hz small signal steering bandwidth combined with 90 ms large signal brake pressure risetime. 1 INTRODUCTION Exp erimental research on automated highway systems is presently somewhat constrained by the limited availabilityofvehicles equipp ed to b e controlled by automatic means. We present here a simple metho d for actuating brakes and steering on a standard passenger car. The apparatus describ ed should not b e employed on public roads. It is for exp erimental purp oses only. Throttle actuators in the form of cruise control have b een available for some years now, and high p erformance throttle actuators have b een develop ed. Mec hanisms to allo w automatic application of brakes and steering are com- paratively scarce, since appreciable p ower has to b e delivered and relatively short resp onse time .1 sec or less is required for precise control of vehicle sp eed and intervehicle spacing. The most direct approach to delivering and controlling the substantial forces required for steering and brake actuation is the use of hydraulic p ower assist. Mo dern p ower steering systems already contain most of the required elements This work was supp orted by the California DepartmentofTransp ortation y Permanent Address: 1384 Je erson Avenue, Akron Ohio 44313 1 and it happ ens that hydraulic brake assist is commercially available and easily mo di ed for automatic control. The use of existing ABS is app ealing, but most ABS systems are carefully optimized for cost under the assumption that activity will b e infrequent and brief. The demands of routine braking, and the consequences of controller mis- b ehavior, are almost certain to exceed the design capacity of an ABS system. Obviously high p erformance servomechanisms develop ed for aircraft and in- dustrial applications are available but their cost readily exceeds that of a mo d- ern automobile. Usual high-p erformance acutators are not easily integrated with the steering and brake systems of a pro duction vehicle since they usually rely on constant pressure hydraulic control. In constant pressure systems the control valves idle in the closed p osition and an accumulator stores hydraulic energy which can b e applied to move masses quickly. In the inactive state there is appreciable p ower dissipation from valve leakage and some sort of pressure regulation system is required. In contrast, standard hydraulic p ower assist schemes for automotive steer- ing and brakes use constant ow designs; they o er acceptable p erformance at minimum cost. In a constant ow system the control valves idle in the op en p osition with uid circulating freely through the pump and valves. The parts count is minimal and by careful design the idle state p ower dissipation can b e made thermally insigni cant. However there is no accumulator, limiting the p eak p ower available and therefore making the system resp onse slow. Wechose to implement a constant ow actuator system. This minimizes the mechanical disruption to the vehicle and also allows a smo oth transition between automatic and human-controlled mo des of op eration. Figure 1 gives an overall view of the engine compartment and makes clear that no substantial rearrangement is required. For the PATH steering and brake actuator system wehave used a standard hydraulic p ower assist unit Bendix Hydro-Bo ost for the brake, the sto ckpower steering b ox for the steering and the existing p ower steering pump. The normal hydraulic passages b etween the integral control valves and actuator cylinders are blo cked, and the lines brought out to external manifold blo cks containing direc- tional valves to switchbetween normal and automatic mo de and prop ortional valves which do the controlling in automatic mo de. The prop ortional valves are op erated by analog electronic circuits delivering a few watts of electrical p ower. Closed lo op control is achieved by adding a pressure sensor on the brake system and a p osition sensor on the steering system. The use of analog electronics on the inner lo op may seem primitive at rst, but it greatly simpli es initial setup and testing. The analog feedback circuit can b e easily replaced by a digital one if more sophisticated control is desired. The constant ow control valves use standard constant-pressure sp o ol valve b o dies but custom sp o ols. Valve b o dies are essentially imp ossible to craft by non-sp ecialists but an acceptable sp o ol can b e made by a skilled machinist with little more than a go o d lathe and some patience. 2 Figure 1: View of the left side of the engine compartment showing minimal disruption of the sto ck con guration. The only changes are removal of the cold air intake duct and removal of the original vacuum op erated cruise control. 2 Technical Description. The complete hydraulic diagram is shown in Figure 2. All valves are in p ositions corresp onding to human control, with no command applied. There is a free path from the pump, through the brake b o oster into the steering gear, whence the uid is returned to the tank. The lter and co oler are omitted for clarity. The descriptions b egin with normal human control of the vehicle and then elab orate on changes required to p ermit automatic control. 2.1 Brake Servo and Actuator Brake actuation is achieved through use of a Bendix Corp oration Hydro-Bo ost power brake servo, mo di ed for external control. The Bendix Hydro-Bo ost is a constant owhydraulic brake servo designed for placement upstream of standard power steering. Since it was meant to b e substituted in place of vacuum b o osters in existing vehicle designs it is relatively easy to adapt using b olt plates, spacers and p edal ro ds to a variety of cars. Some cars which are equipp ed with Hydro- Bo ost from the factory include late '70s Cadillac Eldorados, mid '80s Lincoln Mark 7 LSCs and mid to late '80s GM diesels. The units cost b etween $300 and 3 Bendix Hydro-Boost unit Apply Chamber Booster Piston Master cylinder pressure sense transducer piston to brakes Pedal Rod power brake control valve port added to power internal apply steering controls not chamber pump shown steering selector valves brake selector valves Pressure relief power steering valve rack piston power Brake steering actuator control control valve valve steering actuator control valve steering selector valve Figure 2: Complete hydraulic diagram for the combined actuators. See the sp o ol construction discussion for a more intuitive representation of the control valves. $500 as replacement parts. In manual op eration, the Hydro-Bo ost receives ow from the p ower steering pump. With no p edal e ort, uid travels freely through the Hydro-Bo ost control valve and enters the steering system. The apply chamber is vented to the tank, and an isolation valve allows the steering system to develop normal op erating pressure without inadvertently applying brakes. When the driver applies the brakes the p ower brake control valve senses displacement of the brake p edal closing the vent, op ening the isolation passage and restricting uid ow from the pump to develop pressure in the apply chamb er. The b o oster piston applies force to the brake master cylinder, while at the same time a sense piston on the p edal input ro d opp oses the driver's fo ot. When the hydraulic force develop ed on the p edal overcomes the driver's e ort, the p ower brake control valve returns to neutral p osition, halting the pressure rise and giving the driver tactile feedback. When the driver releases the p edal, a relatively soft spring returns the valve to the vent p osition, closing the isolation valve and releasing pressure in the apply chamb er. The hydraulic force gain is simply the ratio of apply piston 4 Figure 3: Figure 3. The brake manifold blo ck, lo oking toward the rear of the car.