This article highlights the fact that engineers who design and test anti-lock brake systems (ABS) have been trying to come up with ways to minimize the noise and vibration that drivers hear and feel when they stomp on the brake pedals. The ABS engineers want drivers to do during a panic stop is to let their feet off the brakes. According to the engineers, braking should be the concern, because the less time the driver worries about stopping the car, the more time there is to concentrate on steering it. The mechanical components in both systems are functionally identical, consisting of a brake pedal, a master cylinder and booster, hydraulic lines and fluid, wheel calipers, brake pads, and rotors. In fact, unless the system is actuated by hard braking, ABS acts just like an ordinary disc brake system. Engine noise would only mask the ABS noise reaching the binaural head, which sits inside the passenger compartment where a driver would normally be.
THE LAST THING that anti-lock brake system engineers want drivers to do during a panic stop is to let their feet off the brakes. Braking should be our concern, say the engineers, because the less time the driver worries about stopping the car, the more time there is to concentrate on steering it.
Unfortunately, drivers can be surprised by noise and vibration when they step hard on the brake pedals and the anti-lock brakes take over. Confronted by strange sounds and pulses during a moment of panic, many drivers react by letting up on the pedal-the worst response possible when trying to stop a car.
Over the last few years, engineers who design and test anti-lock brake systems, or ABS, have been trying to come up with ways to minimize the noise and vibration that drivers hear and feel when they stomp on the brake pedals. Chad Johnson, an engineer working in systems design and analysis at Lucas Varity Aut.omotive in Livonia, Mich., a supplier of light and medium-duty truck brake systems for auto manufacturers worldwide, has been spending his days there working on the NVH program, an abbreviation for noise, vibration, and harshness. By removing these three elements from brake systems, or at least minimizing them, Johnson and his co-workers hope to lessen the chance of startling drivers during panic stops, and thus keep their feet planted firmly on the pedals.
NOISE IN THE SYSTEM
What makes ABS so much noisier than everyday disc brake systems? To answer that, a short explanation of ABS operation is in order. The mechanical components in both systems are functionally identical, consisting of a brake pedal, a master cylinder and booster, hydraulic lin es and fluid, wheel calip ers, brake pads, and rotors. In fact, unless the system is actuated by hard braking, ABS acts just like an ordinary disc brake system. That means a piston in the master cylinder forces hydraulic fluid out to a wheel caliper. There, the fluid pushes brake pads against a spinning rotor, slowing the rotor and wheel. When ABS is actuated, several more components jump into action: a couple of valves, a hydraulic pump, electronic sensors and circuitry, and a bit of programmed logic.
Johnson is quick to point out that for ABS "the stability of the vehicle is the primary goal." The objective is to slow the rotation of the tires as quickly as possible, with minimal skidding, so that the driver never loses the ability to steer. To achieve this end, ABS monitors wheel speeds and makes an estimate about the road surface over which the tires roll. "For most surfaces," Johnson said, "ABScontrolled vehicles will outperform a vehicle without such a system. ABS is designed to maintain stability and steering, and minimize stopping distance--in that order."
In the worst case, on ice, the wheel could go from 50 mph, say, to zero in an instant, the time in which two extra ABS valves must begin operating. The first valve shuts off the master cylinder from the brake pedal, isolating the brakes from any further foot pressure by the driver. The second valve begins dumping hydraulic pressure, relieving pressure on the wheel rotor and allowing it to start spinning again. As long as the driver keeps the pedal pressed hard, the ABS system functions automatically.
The ABS system at this point begins to home in on the maximum pressure it can apply to the brake rotor without making the wheel skid. The two ABS valves close and open in rapid succession. Johnson likens the vibration and noise stemming from the operation of these valves to water hammer, the all-too-familiar knocking in the pipes that occurs at home when the faucet is suddenly turned off. The noise that is so irritating at home is also every bit as annoying when it invades a car's passenger COITlpartment.
Another component p eculiar to ABS introduces a noise that's no less insidious. After the second ABS valve dumps hydraulic pressure, that fluid must be pumped back into the master cylinder. Otherwise, the brake pedal would gradually reach the floor. To do this, engineers specifY a hydraulic pump that kicks on whenever ABS is called into action. Unlike the hammer- like vibration of the valve pulses, the noise from the pump is more of a low growl.
INTO THE TEST LAB
To develop anti-lock brakes, Lucas Varity employs a variety of test fixtures, ranging from a "full vehicle simulator with vehicular dynamics" and computer-screen projections of virtual race tracks and virtual ice pads, to an engine- less truck cab that is fitted with brakes and what Johnson respectfully calls a "binaural head," which could be described in common parlance as a dummy human torso with microphones for ears.
Johnson uses mostly the engine-less truck for his investigations of ABS noise and vibration. Engine noise would only mask the ABS noise reaching the binaural head, which sits inside the passenger compartment where a driver would normally be.
Johnson and a colleague split responsibilities in most of the tests. Johnson monitors such things as wheel pressure and master cylinder pressure, the control of the hydraulic pump, as well as various voltages and currents. His associate takes care of the binaural head, making sure that its microphone ears are clear.
A test typically lasts for 30 to 45 seconds. In that time, the brakes are applied, initiating the ABS controller, which begins pulsing the valves that isolate the master cylinder and relieve brake pressure. The hydraulic pump starts, pumping fluid back to the master cylinder. The engineers record the change in fluid pressure as the ABS hunts to find the most force it can apply without locking up the wheels.
At one time, Johnson and his colleagues relied on strip chart recorders and two- and three-channel oscilloscopes to gather data. Then they began using an eight-channel oscilloscope/recorder that simplified data acquisition while expanding test content. The new scope, manufactured by Yokogawa Corp. ofNewnan, Ga., enables Johnson to store many waveforms for comparison an10ng different tests.
Using this new oscilloscope, Lucas Varity engineers now conduct tests on ABS that they could not even consider previously. Master cylinder and wheel pressures, voltages, currents, and controller signals, for example, can be recorded simultaneosly and synchronized. Johnson uses the instrument's internal hard drive to record waveforms for future analysis. Using the instrument's built-in interfaces, he can download the waveforms as well and thus keep a historical archive.
Johnson said they have discovered several new things about ABS noise, most of which Lucas Varity prefers to keep secret and out of the hands of its competitors. Johnson was forthcoming about one discovery, however. He said that eventually the systems they were testing began to seem less like anti-lock brakes and more like giant speakers aimed straight into the passenger compartn1ents.
VALVES AND SPEAKERS
The bulkhead that divides the engine compartment from the passenger cabin-what used to be called a firewall-is a vertical, somewhat flexible panel to which the master cylinder mounts. The master cylinder connects to the two extra ABS valves through hydraulic tubing. Those valves, which open and close rapidly whenever ABS is called for, pass their vibration along to the master cylinder. The master cylinder acts surprisingly like a speaker coil, driving the firewall, which acts surprisingly like a speaker cone.
To verifY this, the Lucas Varity engineers removed the master cylinder from the firewall, and placed it in another room. Running the tests again, they found the ABS measurably quieter.
For all the benefits of testing in the lab, though, there are some things that simulations simply cannot provide. "In a true ABS stop," Johnson said, "the surface is never exactly the same from stop to stop." Mter extensive testing in the lab, Johnson and his associates take their binaural head out to the test track and put it in a real car.
There they see how much noise their brakes add to the real world.