ANTILOCK BRAKING SYSTEMS
APPARATUS: THE SHOPS
It was late in the afternoon on a dark, overcast day. a light rain was falling. This usually routinely “slow” tour at the fire station was interrupted by an alarm of a structure fire not far from quarters. The pumper was responding with warning lights on and siren sounding when a school bus unexpectedly appeared and was crossing its path. The apparatus chauffeur applied the brakes in a panic stop situation, locking up all four wheels. The 30,OOO-pound pumper was hurling do ten the wet pavement directly at its target, with no usable steering and even less chance of avoiding disaster…
ANTILOCK BRAKING SYSTEMS
A relatively new feature in fire service apparatus is the electronically controlled antilock braking system (ABS). Its ability to increase vehicle safety generally has been recognized by the public. Most new automobiles either are equipped with an ABS as a standard feature or have it available as an option. In Europe, all new commercial trucks are required to have an ABS installed.
Do not confuse or compare the modern ABS with the original, unreliable “121” attempt at antilock brakes made in the 1970s. Vast strides have been made in computerization and electronics since then, making the present systems highly dependable.
Several air brake, axle, and electronic original equipment manufacturers produce antilock braking systems for commercial vehicles and fire apparatus. The main purpose of this system is to allow the driver of the vehicle to stop in the shortest possible distance while maintaining full control of the vehicle. This is possible because the wheels do not lock while the brakes are being applied. Locked wheels can cause the rear of the vehicle to spin around, and since the front tires are not rotating, steering capability is lost and the vehicle goes out of control. Slowing the wheels while maintaining a rolling friction surface with the road—as occurs with an ABS—helps the vehicle to retain stability and to stop while still under control.
COMPONENTS
The major components of the antilock braking system are as follows:
- Electronic control microprocessor. This miniature computer, mounted in the cab. compares and processes a signal it receives from each wheel and, in turn, controls the air pressure to the brake chamber from the modulation valve. Evaluation and control take place in milliseconds and change as conditions warrant.
- Exciter or pulse ring. Attached to the axle or wheel hub, it turns at the same speed as the wheel.
- Wheel speed sensor. A small induction coil mounted in close proximity to the pulse ring, it generates— in conjunction with the teeth on the spinning ring—an impulse to the electronic controller, where the microprocessor analyzes the pulses and determines the speed at which each wheel is turning.
Modulation valves. They control air pressure to the brake chambers on command from the electronic control. As quickly as five times per second, these valves have the ability to apply, release, or hold air pressure to the brake chamber, which controls the braking action at each wheel.
All these components are wired together. A shielded harness prevents interference. Several common electrical components such as relays, fuses, or circuit breakers accomplish the job.
THE OPERATION
Several ABS variations are available; some control each wheel individually, and others brake by axle.
The typical system on a two-axle vehicle consists of four channels (control sensors) arranged as two diagonal circuits in an “X” pattern. One circuit constitutes the right front and left rear wheels and the other, the left front wheel coupled with the right rear wheel. On tandem rear axles, the two wheels on the same side of the tandem can be connected together, using only one sensor on each side of the sets (four-channel system). A more costly and complicated six-channel system, with sensors located at each wheel, can be installed if desired.
As the air brakes are applied, the computer samples speed information from each wheel sensor and determines if more or less air pressure should be applied to the brakes at a particular wheel. If the vehicle is attempting to stop in a situation where one side is on a dry road surface and the other side is locking up in a skid, such as on ice or wet leaves, the computer rapidly releases and applies the brakes of the locked wheel to maintain its rotation and help bring the vehicle to a safe, even stop. The wheels on the dry surface continue the normal steady brake application.
The ABS is compatible with all current air brake systems, including disc, drum, and wedge types, and requires little more in the way of air volume to accomplish the apply and release technique.
The ABS is designed to interface with engine brakes and retarders installed on the vehicle. If a wheel spin or lockup is detected, the controller automatically disables the retarder or engine brake until the situation is under control.
If a component in the ABS fails, the affected circuit returns to normal braking, while the remainder of the system continues to operate as designed. If the entire system fails, the vehicle returns to standard braking without losing any of its normal stopping ability.
The control unit has a nonvolatile memory and electronically records the fault for the technician to troubleshoot. This is an especially important feature when an intermittent problem is present but almost never appears while the vehicle is in the shop.
TRACTION CONTROL
Another system that can be integrated into the ABS is automatic traction control (ATC). It uses the signals from the wheel sensors to determine whether spinning of the drive wheels is because of excess power being applied.
The ATC, depending on the model in use, controls the situation in one of two ways. One system automatically applies brake control air pressure to the spinning wheel, causing it to slow down until surface traction is restored. The other system signals the vehicle engine to reduce the throttle application until the spinning wheel regains traction. Both of these systems activate automatically without any change in operation on the part of the driver.
The ATC has several safety advantages, as it maintains traction on slippery road surfaces, while driving on hills and curves, when increasing speed, and when accelerating from a stopped condition. The system can reduce maintenance costs by decreasing the instances of broken axles, driveshafts, or differentials that occur when a spinning drive wheel makes contact with dry pavement.
COST
Adding an antilock braking system to a new apparatus costs approximately S3,500 to S4,()00. The cost of adding automatic traction control to an ABS-equipped vehicle is minimal. The cost is relatively small when considering the large sums of money routinely awarded in vehicle accident liability suits.
The high-performance driving required of fire apparatus and the varied skill levels of their drivers warrant an ABS. It helps improve control and stability, allows steering during brake applications, and in most cases reduces stopping distances, especially on wet, icy, or loose-gravel surfaces.
If the pumper in the scenario at the beginning of this article had been equipped with an antilock braking system, chances are the apparatus would have braked to a safe stop or could have been steered around the obstruction.
