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Pressure_control_scheme_for_air_brakes_in_commercial_vehicles

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Pressure_control_scheme_for_air_brakes_in_commercial_vehicles

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IEE Proc. Intell. Transp. Syst. Vol. 153, No. 1, March 2006 21

    Pressure control scheme for air brakes in commercial vehicles

C.L. Bowlin, S.C. Subramanian, S. Darbha and K.R. Rajagopal

    Abstract: Air brake systems are widely used in commercial vehicles such as trucks, tractor-

    trailers and buses. In these brake systems, compressed air is used as the energy transmitting

    medium to actuate the foundation brakes mounted on the axles. Here, we present a control

    scheme for regulating the pressure of air in a brake chamber of these brake systems. This

    scheme is based on a non-linear model for predicting the pressure transients in the brake

    chamber that correlates the brake chamber pressure to the treadle valve (brake application

    valve) plunger displacement and the pressure of the air supplied to the brake system. The

    control scheme regulates the pressure in the brake chamber by modulating the displacement

    of the treadle valve plunger. We have implemented this control scheme on the brake testing

    facility at Texas A&M University and present results for a variety of test runs in which the

    controller tracks realistic desired pressure trajectories.

    E-mail: dswaroop@tamu.edu 1 Introduction

A properly operating brake system is critical for the safe

    operation of vehicles on the road. We shall focus here on

    air brake systems which are widely used in commercial

    vehicles like trucks, tractor-trailers, buses etc. In fact,

    most tractor-trailer vehicles with a gross vehicle weight

    rating (GVWR) over 19 000 lb, most single trucks with a

    GVWR of over 31 000 lb, most transit and inter-city

    buses and about half of all school buses are equipped

    with air brake systems [1]. These brake systems use

    compressed air as the energy transmitting medium to

    actuate the foundation brakes mounted on the axles.

    A cruise control system is a standard feature in most

    of the automobiles on the road today. It maintains

    the vehicle speed to the value set by the driver by

    automatically regulating the engine throttle [2, 3].

    A sensor that measures the vehicle speed is used to

    provide the feedback signal to this system. A cruise

    control system is usually activated by the driver when

    the vehicle is traveling at medium/high speeds in

    smoothly ?owing traf?c. A conventional cruise control

    system utilizes a proportional-integral-derivative con-

    troller to minimize the error between the desired and the

    measured vehicle speed [4].

    In recent years, studies have been carried out to

    develop ‘adaptive cruise control’ (ACC) systems or

    ‘autonomous intelligent cruise control’ systems. The

    objective of these systems is to maintain a constant

    distance between two consecutive vehicles by controlling

    the engine throttle and the brake system. These systems

    obtain the spacing between vehicles from sensors such as

    a radar mounted on the vehicle [5]. They are being

    developed so that they can be engaged at low speeds in

ª IEE 2006

    IEE Proceedings online no. 20055007

    doi:10.1049/ip-its: 20055007

    Paper received 8 December 2005

    The authors are with the Department of Mechanical Engineering, Texas A&M University, College Station, TX 77843, USA

22 IEE Proc. Intell. Transp. Syst. Vol. 153, No. 1, March 2006

moderate to heavy traf?c conditions to

     control the motion of the vehicle in a automatically

    safe manner. Studies have been recently carried out to evaluate and implement these systems on vehicles [68].

    While most of the research on ACC has focused on passenger cars, the bene?ts of implementing this

    system on heavy trucks have far reaching consequences [9]. A typical ACC system for heavy trucks controls the engine throttle, the transmission and the brake system and will be interfaced with existing systems like the antilock braking system (ABS), traction control system etc. A typical truck ABS monitors the speed of the wheels and modulates the brake system pressure in the event of an impending wheel lock-up [10]. The ABS consists of an electronic control unit that receives signals from the wheel speed sensors and processes this information to regulate the brake system pressure through

    modulator valves. It should be noted that ABS does not control the treadle valve to regulate the pressure in the brake system. It reduces the brake system pressure that is ‘commanded’ by the driver when it

    senses an impending wheel lock-up. It cannot provide a higher pressure than that corresponding to the pedal input from the driver.

    It is important to note that the ABS modulates the brake system pressure only under conditions when a wheel lock-up is impending. The ABS is disengaged during ‘normal’ braking operations. In

    fact, it has been pointed out in [11] that ABS is ‘passive during the vast majority of braking

    operations’. During such braking operations, the

    pressure of air in the brake system is the level that is commanded by the driver through the motion of the brake pedal (and consequently the motion of the treadle valve plunger). Hence, in order to imple-

    ment ACC systems on commercial vehicles, it is necessary to develop control schemes that will automatically reg- ulate the brake system pressure during all braking operations.

    The air brake system currently used in

    commercial vehicles is made up of two subsystems

    the pneumatic

    subsystem and the mechanical subsystem. The pneu- matic subsystem includes the compressor, storage

    IEE Proc. Intell. Transp. Syst. Vol. 153, No. 1, March 2006 23

reservoirs, treadle valve, brake hoses, relay valves, quick it may be necessary to modify (and retro?t) some of

    release valve (QRV), brake chambers etc. The mechan- these components such as the treadle and relay valves to ical subsystem starts from the brake chambers and accommodate for the operation of the brake system by includes push rods, slack adjusters, S-cams, brake pads both the driver and the ACC system. If such modi?ca-

    and brake drums. One of the most important differences tions are made in the future, models of the correspond- between a hydraulic brake system (used in passenger ing components have to be taken into account in cars) and an air brake system is in the mode of implementing the control scheme. Recently, electronic operation. In a hydraulic brake system, the force applied actuators have been retro?tted to modulate the relay

    by the driver on the brake pedal is transmitted through valve in the existing air brake system for the purpose of the brake ?uid to the wheel cylinders mounted on the controlling a tractor semi-trailer combination along the axles. The driver obtains a sensory feedback in the form longitudinal direction [13]. The authors have used a of a pressure on his/her foot. If there is a leak in the linear model relating the vehicle speed to the throttle hydraulic brake system, this pressure will decrease and input/brake pressure to obtain controllers for speed the driver can detect it through the relatively easy tracking and vehicle following. The same arrangement motion of the brake pedal. In an air brake system, the of the brake system has been used for lateral control of application of the brake pedal by the driver meters out the tractor semi-trailer combination in [14].

    compressed air from a supply reservoir to the brake In Section 4, we shall observe from experimental data chambers. The force applied by the driver on the brake that much of the range of the brake chamber pressure is

    pedal is utilized in opening certain ports in the treadle attained in a very small range of the motion of the valve and is not used to pressurize air in the brake treadle valve plunger (~0.002 m). This fact coupled with system. This leads to a lack of variation in the sensory the lack of variation in the sensory feedback to the feedback to the driver in the case of leaks, worn brake driver, makes it very dif?cult for a driver to regulate the pads and other defects in the brake system. pressure in the air brake system to a desired value in a In the following pages, we present a control scheme reliable manner. The scheme that we present can be used based on a model for the pressure transients in the brake to automatically control the brake chamber pressure chamber of the air brake system. This control scheme required in a commercial vehicle under all braking will be used for modulating the pressure in the brake conditions.

    chamber and thus can be used to control the amount of 2 A brief description of the air brake system braking required during both normal and emergency and the experimental setup brake applications. A non-linear model of the pneu-

    matic subsystem of the air brake system that relates the A layout of the air brake system found in a typical pressure in the brake chamber to the treadle valve plu- tractor is presented in Fig. 1. An engine-driven air nger displacement and the supply pressure to the treadle compressor is used to compress air and the compressed valve has been developed and presented in [12]. Our air is collected in storage reservoirs. The pressure of the control scheme based on the non-linear model devel- compressed air in the reservoirs is regulated by a oped in [12], regulates the pressure in the brake chamber governor. Compressed air is supplied from these by controlling the motion of the treadle valve plunger. reservoirs to the treadle and relay valves. The driver We implement this control scheme on our experimental applies the brake by pressing the brake pedal on the setup and illustrate the results for a variety of test runs. treadle valve. This action meters out the compressed air We have developed our control scheme to conform to from the supply port of the treadle valve to its delivery the hardware components present in the air brake port. Then, the compressed air travels from the delivery system currently used in commercial vehicles. For the port of the treadle valve through air hoses to the brake purpose of implementing ACC on commercial vehicles, chambers mounted on the axles.

Fig. 1 A general layout of a tractor air brake system

    24 IEE Proc. Intell. Transp. Syst. Vol. 153, No. 1, March 2006

diaphragm generating a force resulting in the motion of The S-cam foundation brake, found in more than 85%

    the push rod. The motion of the push rod serves to of the air-braked vehicles in the United States [1], is

    rotate, through the slack adjuster, a splined shaft on illustrated in Fig. 2. Compressed air from the treadle

    valve enters the brake chamber and acts against the which a cam in the shape of an ‘S’ is mounted. The ends of two brake shoes rest on the pro?le of the S-cam and the rotation of the S-cam pushes the brake shoes outwards so that the brake pads make contact with the rotating drum. This action results in the deceleration of the rotating drum. When the brake pedal is released by the driver, air is exhausted from the brake chamber and the push rod strokes back into the brake chamber thereby rotating the S-cam in the opposite direction. The contact between the brake pads and the drum is

    now broken and the brake is thus released.

    A schematic of the experimental setup at Texas A&M University is provided in Fig. 3. Two ‘Type-20’ brake

    chambers (having an effective cross-sectional area of 220 in.) are mounted on a front axle of a tractor and two ‘Type-30’ brake chambers (having an effective cross- 2sectional area of 30 in.) are mounted on a ?xture designed to simulate the rear axle of a tractor. The air Fig. 2 The S-cam foundation brake supply to the system is provided by means of two

    IEE Proc. Intell. Transp. Syst. Vol. 153, No. 1, March 2006 25

Fig. 3 A schematic of the experimental facility

26 IEE Proc. Intell. Transp. Syst. Vol. 153, No. 1, March 2006

Fig. 4 A sectional view of the treadle valve

    compressors and storage reservoirs. The reservoirs are system is tuned to obtain the desired performance using chosen such that their volume is more than 12 times the IDC’s Servo Tuner software program [19].

    volume of the brake chambers that they provide air to, A pressure transducer is mounted at the entrance of as required by the Federal Motor Vehicle Safety each of the four brake chambers by means of a custom Standard 121 [15]. Pressure regulators are mounted at designed and fabricated pitot tube ?xture. A displace-

    the delivery ports of the reservoirs to control the supply ment transducer is mounted on each of the two front pressure to the treadle valve. A cross-sectional view of brake chamber push rods through appropriately fabri-the treadle valve used in the experiments is illustrated in cated ?xtures in order to measure the push rod stroke. Fig. 4. The treadle valve consists of two circuitsthe All the transducers are interfaced with a connector block primary circuit and the secondary circuit. The delivery through shielded cables. The connector block is

    port of the primary circuit is connected to the control connected to a PCI-MIO-16E-4 DAQ board [20]

    port of the relay valve (referred to as the service relay (mounted on a PCI slot inside a desktop computer)

    valve in Fig. 1) and the delivery ports of the relay valve that collects the data during brake application and are connected to the two rear brake chambers. The relay release. An application program is used to collect and valve has a separate port for obtaining compressed air store the data in the computer.

    supply from the reservoir. The delivery port of the secondary circuit is connected to the QRV and the 3 Control scheme delivery ports of the QRV are connected to the two front brake chambers. The control scheme developed in this section is based on

    the model of the pressure transients in the brake A closed loop position feedback control system is

    used to regulate the displacement of the treadle valve chamber presented in [12]. In this section, we shall plunger. An EC2 electric cylinder (mounted with a B23 provide a brief description of this model and then derive

    the control scheme for regulating the brake chamber brushless servo motor) manufactured by Industrial

    pressure. When the driver presses the brake pedal, the Devices Corporation/Danaher Motion is used for

    primary piston in the treadle valve (see Fig. 4) ?rst closes actuation [16]. The actuator shaft is interfaced with the

    servo motor through a belt drive and lead screw the primary exhaust port (by moving a distance equal assembly. The actuator is controlled by a B8501 Servo to x) and then opens up the primary inlet port pt

    Drive/Controller [17, 18]. A linear potentiometer is built (when x> x). This action serves to meter the pp ptinto the electric cylinder and its output is provided to the compressed air from the reservoir to the brake chamber. We shall refer to this phase as the ‘apply phase’. When servo drive. The servo drive also receives a feedback

    signal from an encoder mounted on the motor shaft to the pressure in the primary circuit increases to a level

    such that it balances the force applied by the driver, the regulate the torque input to the motor. The desired

    primary piston closes the primary inlet port with the plunger motion trajectory is provided from the compu- exhaust port also remaining closed (when x? x). We ter to the servo drive through a data acquisition (DAQ) pp pt

    shall refer to this phase as the ‘hold phase’. When the board. The servo drive compares the desired displace- driver releases the brake pedal the primary piston return ment and the measurement from the linear potenti-

    spring forces the primary piston to its initial position. ometer at each instant in time and provides the suitable

    control input to the actuator. The position control This action opens the exhaust port (when x< x) and pp pt

    24 IEE Proc. Intell. Transp. Syst. Vol. 153, No. 1, March 2006

     Data Fit 2500

    2000

    1500

     Load(in N)

    1000

    500

    0 0 1 2 3 4 5 6 3 Deflection (in m) x 10

    Fig. 5 Calibration curve of the rubber graduating spring

    air is exhausted from the brake chamber to the of the stem spring, the primary piston return spring and

    atmosphere. We shall refer to this phase as the ‘exhaust , Fand the primary valve assembly return spring, Fkssikppi phase’. Our control scheme will take into account these Fdenote respectively the pre-loads on the same, Akpvi pp is the net area of the primary piston exposed to the phases of operation to regulate the brake chamber pressurized air at the primary delivery port, Ais the pressure. In this article, we shall consider the con?gura- pv net cross-sectional area of the primary valve assembly tion of the brake system where the delivery port of the gasket exposed to the pressurized air at the primary primary circuit is directly connected to a front brake is the net cross-sectional area of the delivery port, Apv1 chamber. primary valve assembly gasket exposed to the pressur- ized air at the primary supply port, Fis the force gs transmitted from the plunger to the primary piston by 3.1 A model of the pressure transients in the rubber graduating spring, Pis the pressure of air ps the brake chamber being supplied to the primary circuit, Pis the pressure pd A lumped parameter model of the treadle valve has been of air at the primary delivery port and Pis the atm developed and presented in [12]. In this section, a atmospheric pressure. summary of the governing equations of this model is The mechanical response (the load–de?ection curve) presented. The treadle valve opening has been modeled of the rubber graduating spring is non-linear. It has as a nozzle. The friction at the sliding surfaces of the been tested to obtain the calibration curve illustrated in treadle valve is assumed to be negligible since these Fig. 5. The de?ection of the rubber graduating spring is surfaces are well lubricated. The springs in the treadle denoted by x(t) which is equal to (x(t) x(t)). From pdpppvalve have been tested and found to be linear in the the calibration curve, we obtained the following region of their operation (except the rubber graduating relationship between Fand x: gs pdspring). The governing equation of the primary piston during the apply and hold phases of the brake 8 mxþ nif xÇ l application process is given by 1pd 1 pd 1>< mxþ nif l< xÇ l 2pd 2 1 pd 2..2xðtÞ F? 2 ð4Þ dppgs axþ axþ aif l< xÇ l 12pd 3 2 pd 3ðMþ MÞ þ KxðtÞ ? KxðtÞþ FðtÞ pp pv2ppsspgs>2pd dt mxþ nif x> l : 4pd 4 pd 3

     þF PðtÞðA AÞ PAþ PA 1 pdpp pvpspv1 atmpp ð1Þ where the calibration constants are obtained from Fig. 5. F? Kxþ F F Fð2Þ1 pvpt kssi kppi kpvi In the above equation, lis the value of the de?ection 1 of the rubber graduating spring until which its res- and ponse is described by the ?rst sub-equation. The second K? Kþ Kþ Kð3Þ 2 ss pp pv sub-equation describes the response of the rubber

    graduating spring when its de?ection lies between l 1where Mand Mdenote respectively the mass of the pp pv , the third sub-equation describes its response and l2when its de?ection lies between land l, and the fourth primary piston and the primary valve assembly gasket, 2 3xsub-equation describes its response when its de?ection isand xdenote respectively the displacement of the p pp treadle valve plunger and the primary piston from their greater than l. 3respective initial positions, xis the distance traveled by The mass of the primary piston was found out to be pt 0.16 kg and the magnitude of the spring and pressure the primary piston before it closes the primary exhaust, ~ 2 forces were found to be in the order of 10 N. Thus, the K, Kand Kdenote respectively the spring constants sspp pv

    IEE Proc. Intell. Transp. Syst. Vol. 153, No. 1, March 2006 25

acceleration required for the inertial forces to be Next, we will consider the ?ow of air in the brake

    comparable with the spring force and the pressure system. The treadle valve opening is modeled as a 23 2nozzle. The ?ow through the nozzle is assumed to be force terms has to be in the order of 1010m/s,

    one-dimensional and isentropic. We also assume that which is not the case. Hence the inertial forces are the ?uid properties are uniform at all sections in the neglected and (1) reduces to

    nozzle. Under the above assumptions, the part of the Kx? Kxþ Fþ F PðA AÞ 2pp ssp gs 1 pdpp pvpneumatic subsystem under consideration can be ð Þ 5PAþ PAvisualized as illustrated in Fig. 6. It should be noted pspv1 atmpp that when the primary delivery port is connected directlyDuring the exhaust phase, the equation of motion of to a front brake chamber, the term Pin the above pd the primary piston can be written as equations is taken to be the same as the pressure in the

    brake chamber (denoted by P). For this con?guration, b ..2xppdin the above the supply pressure term, denoted by Pps M? Fþ Fþ Kðx xÞ gs kssi ssp pppp 2dt ð6Þ equations, is denoted by Pin the equations that follow. o

    A cross-sectional view of the brake chamber is shown ðP PÞA Kx Fpd atmpp pppp kppi in Fig. 7. From recent experiments, it has been found

    It should be noted that at the start of the exhaust phase out that the evolution of the push rod stroke with the xis equal to xand decreases as the exhaust phase brake chamber pressure can be divided into three phases pp pt progresses. Neglecting the inertia of the primary piston (refer to Fig. 8). The push rod starts to move only after a

    the above equation can be simpli?ed to ‘threshold pressure’ (P) is reached in the brake th

    chamber. This is the ?rst phase and Prepresents the th Kx? Kxþ Fþ F PAþ PAð7Þ3pp ssp gs 2 pdpp atmpp amount of pressure needed to overcome the pre-load on

    the brake chamber diaphragm. In the second phase, the where push rod moves and rotates the S-cam such that the F? F Fð8Þ2 kssi kppi clearance between the brake pad and the brake drum K? Kþ Kð9Þ 3 ss pp decreases. The brake pad contacts the brake drum at a certain pressure in the brake chamber. We shall refer to this pressure as ‘contact pressure’ and denote it by P. ct In the third phase, further stroke of the push rod with

    increasing brake chamber pressure is caused due to the

    deformation of the mechanical components of the brake

    system. Thus, the total stroke of the push rod is made up

    one that is required to overcome of two components

    the clearance between the brake pad and the brake drum

    and another that is due to the deformation of the

    mechanical components after the brake pad makes

    contact with the brake drum. Thus, the total stroke of the

    push rod depends both on the brake pad to brake drum

    clearance and the steady state pressure in the brake

    chamber. We have included these effects in our model

    and approximated the corresponding regions with linear

    models (see Fig. 9) to obtain a calibration curve relating

    the push rod stroke to the brake chamber pressure.

    In Fig. 8, the arrows represent the steady state condition in each of the three sets of data presented. We note

    that the steady state push rod stroke in each case Fi g . 6 Th e s im pli f ie d p n e u m at ic su bs ys tem u nd er depends on the corresponding steady state brake consideration

Fig. 7 A sectional view of the brake chamber

    26 IEE Proc. Intell. Transp. Syst. Vol. 153, No. 1, March 2006

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