U.S. Department of Transportation

FEDERAL MOTOR CARRIER SAFETY ADMINISTRATION

FMCSA Contact: Deborah M. Freund, MC-PSV, (202) 366-5541

On-Board Sensors for Determining

Brake System Performance

Project Funding

Under Section 5117 of the Transportation Equity Act for the 21st Century of 1998

(TEA-21), Congress required the U.S. Department of Transportation to "conduct

research on the deployment of a system of advanced sensors and signal processors in

trucks and tractor-trailers to determine axle and wheel alignment, monitor collision

alarm, check tire pressure and tire balance conditions, measure and detect load

distribution in the vehicle, and adjust automatic braking systems."

As a result of a comprehensive technology scan, as well as numerous interviews with

key industry stakeholders such as truck manufacturers, fleet operators, suppliers, and

regulators, a variety of research areas were identified, including the design,

functionality, and performance of brake systems and brake sensors for commercial

motor vehicle applications. 

Background

The significant number of trucks operating on the highway with brake defects is a

situation that has plagued the industry and law enforcement for years, despite

attempts by many different groups to address the problem. Commercial vehicle

inspection data shows that about 19% of all inspected vehicles – nearly one in five –

were found to have one or more brake defects.* During Roadcheck 2002, a safety

strike force activity combining the resources of FMCSA, the States, and Canadian

Provinces, enforcement officials conducted a 72-hour intensive inspection of 49,032

vehicles in Canada, Mexico, and the United States from June 4-6, 2002. Enforcement

officials placed 22.1% of the vehicles inspected out of service due to various defects

and violations. Brake-related issues accounted for 53.3% of these vehicles being

placed out of service.

It is understood that commercial vehicle braking system design and operation are

directly linked to stopping distance, handling and, therefore, overall safety. Properly

maintaining and performing brakes are clearly the driver's best ally in preventing and

mitigating crash situations. Although vehicle defects on large trucks can rarely be

pinpointed as the causative factor(s) in crashes, when defects do occur, faulty brakes

tend to be at fault.

Optimally adjusted braking systems could help prevent or mitigate crashes even when

the braking system itself was not the initial cause of the crash. Eliminating or

mitigating key mechanical problems, including brake-related issues, would likely yield

a significant reduction in the number and seriousness of injuries sustained in

commercial vehicle-related crashes. 

Study Objective

The overall objective of this research study was to document the performance and

operational characteristics of leading-edge technological approaches to monitoring

commercial vehicle braking systems. The study focused on the ability of the various

sensors to detect abnormalities, defects, and/or misadjustments of the brake system. 

The U.S. Department of

Transportation has established a

goal to reduce the highway

fatality rate to no more than 1.0

fatalities per 100 million vehicle

miles traveled by the end of

2008. Each of the Department's

highway safety agencies have

individual reduction targets that

will contribute to this 2008

target. Federal Motor Carrier

Safety Administration's (FMCSA)

aim is to reduce the large truck

fatality rate by 41 percent from

1996 to 2008, to 1.65 fatalities in

truck crashes per 100 million

miles of truck travel.

The Office of Bus and Truck

Standards and Operations, which

produced this Tech Brief,

develops and promotes national

motor carrier safety program

goals, priorities, and initiatives.

It provides technical expertise

and advice in the development

and deployment of motor carrier

safety programs, including the

development of regulations. The

Office determines national motor

carrier safety operational

program requirements,

standards, and procedures for

vehicle and roadside operations,

driver and carrier operations,

and bus safety.

Office of Bus and Truck Standards and

Operations

400 Seventh St. SW

MC-PSV, Room 8301

Washington, DC 20590

Tech Brief

*The Motor Carrier Management Information System (MCMIS) is operated and maintained by FMCSA. The MCMIS

File contains the results of all driver-vehicle safety inspections of interstate commercial motor vehicles performed by

States participating in the Motor Carrier Safety Assistance Program (MCSAP). In FY 2003, 2.9 million inspections of

interstate vehicles were conducted.

U.S. Department of Transportation

Federal Motor Carrier Safety Administration
The brake sensing technologies examined in this study included: 

• Anchor strain measurement to determine brake force at each wheel,

• Air chamber stroke measurement to assess brake adjustment at each wheel,

• Wheel slip measurement (using wheel speed sensor data) to determine brake force at each wheel,

• Deceleration measurement to determine total vehicle braking force (limited results due to system

software issues), and

• Temperature measurement to determine brake "work" or energy balance.

There are safety benefits associated with having a sensor or "sensor package" on-board the commercial

vehicle that would objectively and accurately measure the stopping potential of the vehicle, and

dynamically and continuously measure the actual braking force at each wheel. Such a system would

potentially have three primary applications or benefits:

1. Warning the driver and/or maintenance personnel if braking ability degraded to an unsafe level – and

help with diagnosis of the specific problem.

2. Providing information to enforcement personnel for use during roadside inspections. 

3. Integrating brake performance-sensing technologies with an electronically controlled brake system

(ECBS) in a "closed-loop" fashion. The brake force information might be used to balance the braking

action at each wheel to improve service life, and/or provide an additional input for controlling braking

action at each wheel during emergency situations.

Overview of Project Approach

The various sensor systems were installed on a conventional tractor-trailer combination vehicle and tested

under controlled braking maneuvers on a test track. (All work was conducted at the Transportation

Research Center (TRC) in Columbus, OH by Radlinski and Associates, Inc.) The output of the various brake

sensor systems could then be compared on the same vehicle under identical testing conditions. This

approach facilitated objective, accurate comparison of the sensors, and eliminated problems associated

with test procedure repeatability when comparing different systems.

In addition, numerous industry stakeholders were contacted and interviewed during the study, including

suppliers of the various technologies examined. The companies and individuals were extremely helpful in

compiling the information contained in this report.

Summary of Results

The following are key observations and results from the testing of the aforementioned sensor technologies.

Anchor Pin Strain Gauges

Pre-production instrumented anchor pins (interchangeable with conventional S-cam brake anchor pins)

fitted with strain gauges are capable of measuring the shear stresses applied to the anchor pins of the

drum brake assemblies used on heavy-duty S-cam trucks and buses. Each anchor pin is fitted with two

strain gauges orientated 90 degrees apart, in the "X" and "Y" direction. The test vehicle was equipped

with four instrumented anchor pins, two on each of the

intermediate axle brake assemblies (one on the

upper/secondary and one on the lower/primary brake

shoes).

• Track testing shows a highly predictable relationship

between force data generated by instrumented (straingauged) anchor pins and the vehicle's deceleration rate.

Instrumented anchor pin force is therefore an accurate

measure of a vehicle's braking performance.

• Instrumented anchor pins can accurately detect brake

deficiencies in specific (individual) wheel assemblies,

including out-of-adjustment, disconnected, and/or oilsoaked shoe linings. They can also measure the effect of an out-of-adjustment brake on the other

(properly adjusted) brakes on a vehicle. This capability lends itself for application to advanced brake

balancing control schemes that may be possible with ECBS.

Secondary Anchor Pin

Primary Anchor Pin
• Instrumented anchor pins can accurately detect even low brake forces. By resolving the resultant force

into the "X" (friction force) and "Y" (normal force) directions, the instrument anchor pins can

differentiate between an out-of-adjustment brake and a brake with oil-soaked shoe linings. With an oilsoaked lining, less force is generated in the "X" direction when compared to an oil-free lining. This

capability could likely be leveraged to improve diagnostic efficiency and overall brake maintenance

planning.

• Instrumented anchor pins performed reliably throughout the testing.

Stroke Sensors

The test truck was equipped with two commercially available stroke sensor packages and two laboratorygrade linear potentiometers mounted on the intermediate drive axle to measure stroke. Key observations

and conclusions on the commercial brake stroke sensor packages, and on the utility of monitoring stroke in

general, are as follows:

• Commercial brake chamber stroke sensor packages can detect brake deficiencies. Their accuracy varies

depending on the load, deceleration rate, and type of brake deficiency. Both commercial systems tested

had the most difficulty detecting brake deficiencies with the trailer unloaded and at low deceleration

rates; however, both manufacturers state

that these systems are intended to

detect overstroke conditions during hard

braking applications.

• In-cab displays featuring indicator lights

for all ten brakes provide the driver with

valuable real-time data on the overall

condition of the vehicle's braking

system.

• Unlike the instrumented anchor pins, brake stroke monitoring cannot differentiate between out-ofadjustment brakes and oil-soaked shoe linings. For example, with oil-soaked shoe linings, the linear

potentiometers recorded an overstroke condition.

• The resolution and accuracy of stroke sensors make them well-suited for use in detecting brake

maintenance needs and potential brake safety issues, but they are probably not appropriate for use in

brake balancing systems.

Wheel-Speed Sensors

Wheel-speed sensors are a standard component of anti-lock braking systems (ABS) used on heavy-duty

trucks and buses. ABS wheel-speed sensors can be used to measure an individual wheel slip by comparing

the calculated speed of each wheel to the calculated average for all wheels or to some other "actual"

speed reference, such as a transmission signal or a contactless fifth wheel that measures ground speed. 

• In general, ABS wheel-speed sensors are highly accurate and track closely with "actual" vehicle speed as

measured by an instrumented fifth wheel.

• Wheel-speed sensors are sufficiently accurate to detect grossly out-of-adjustment and disconnected

brakes. Wheel-speed sensors do not provide sufficient accuracy to detect brakes that are 1/8” or less

beyond the readjustment limit.

• Wheel-speed sensors are sufficiently accurate to detect a problem due to oil-soaked brake linings.

However, unlike instrumented anchor pins, wheel-speed sensors cannot differentiate between out-ofadjustment brakes and oil-soaked linings.

• Wheel-speed data broadcast on the J1939 network was significantly

less accurate than data from actual ABS wheel-speed sensors. 

• Although the wheel-speed sensor data broadcast over the J1939

network was less accurate than data from actual sensors, it was

sufficient for detecting grossly out-of-adjustment, disconnected, and

poorly performing brakes.

Brake

Chamber

Sensor

Dashboard

Fault Light

Exterior

Display

Wheel Speed Sensor
U.S. Department of Transportation

Researchers

This study was performed by

Booz-Allen-Hamilton, Inc.,

McLean, VA, and Radlinski and

Associates, Inc., Columbus, OH,

by Robert M. Kreeb, Brian T.

Nicosia, Douglas Skorupski, and

Richard Radlinski. Contract No.

DTFH61-99-C-00025.

Availability

The study final report (FMCSAPSV-04-001), is available from

the U.S. Department of

Transportation and the National

Technical Information Service,

www.ntis.gov.

Key Words

Advanced sensor, brake, braking

system, sensor study, signal

processor

Notice

This Tech Brief is disseminated

under the sponsorship of the

Department of Transportation in

the interest of information

exchange. The Tech Brief

provides a synopsis of the

study's final publication. The

Tech Brief does not establish

policies or regulations, nor does

it imply USDOT endorsement of

the conclusions or

recommendations. The U.S.

Government assumes no liability

for its contents or their use.

Website

All FMCSA Tech and Analysis

Briefs may be accessed at

www.fmcsa.dot.gov.

Technical Writer

Michael Lang, C2 Technologies, Inc.

July 2005

Publication No. FMCSA-PSV-05-001

Brake Shoe Thermocouples

Standard Type J thermocouples were installed and tested as part of this program.

These tests had two objectives: (1) evaluate the thermocouples to determine whether

they could reliably be used to detect brake defects, and (2) use the thermocouples to

assist in evaluating the other sensor "packages". Thermocouples were mounted at

varying depths within the shoe lining to test their sensitivity for determining brake

deficiencies.

• Response time of thermocouples in general is not sufficient to detect brake

problems during singular, discrete braking events. 

• Because of the unpredictable variations in initial brake temperature, the

comparatively slow response time of thermocouples, and the general inaccuracies

inherent with thermocouples, their ability to detect and differentiate brake

deficiencies during discrete braking events was found to be very limited.

• During the simulated mountain testing, temperature patterns were detected and

used to identify various brake deficiencies.

Potential Sensor Applications

Several applications for the sensor technologies were identified during the study, and

described in this section.

Brake Balance Systems

The instrumented anchor pins were proven to accurately detect brake deficiencies and

provided sufficiently accurate data to measure the increase in work done by the

remaining brakes on a vehicle. This makes them ideal for use in brake balance

applications with advanced "brake-by-wire" technologies. In this application, brake

pressure could be tailored to individual brakes based on brake force output readings.

The benefits include increased brake life due to improved brake lining wear and the

ability to perform minor brake adjustments in real time.

Wireless Transfer of Brake Data

Companies in the transportation industry market products capable of wirelessly

transferring maintenance data from the vehicle to a central data processing computer

in a maintenance yard. These systems are currently configured to wirelessly transfer

engine and transmission fault codes, for example, from the vehicle's network. The

information generated from the commercial stroke sensor packages and instrumented

anchor pins could be broadcast to the vehicle's network and similarly transferred to

the maintenance yard. The data could assist in improving vehicle brake safety,

scheduling brake work, and tailoring brake rebuild schedules.

Improving Regenerative Braking in Hybrid Applications

Many hybrid propulsion manufacturers currently use an open-loop approach to

combining regenerative braking and friction braking. The initial application of the

brake treadle valve is regenerative. Exceeding a preset limit energizes the friction

brakes. This open-loop control methodology results in an arbitrary amount of

regenerative braking force being applied, and less-than-optimal energy being

captured during a braking event. Instrumented anchor pins can measure the

beginning of a friction braking application and its applied force. By factoring in this

measurement data, regenerative braking algorithms can be "closed-loop" in nature.

A closed-loop regenerative braking system, although still isolated from the service

brakes, can optimize the braking energy recovered as well as reduce emissions,

improve brake wear, and improve fuel economy.

U.S. Department of Transportation

Federal Motor Carrier Safety Administration