The Laser Is Going Places
Laser beams can be used to precisely measure the vibrations of components. In their search to understand how noises are created, Bosch researchers are applying this measuring principle to moving objects. Sounds being scrutinized include the “hum” of a vehicle’s windshield wipers and the squeal of its brakes.
Acoustic researchers need to have the instincts of detectives. One of their primary tasks is to track down sources of noise.
At Bosch, this usually means that they hunt disturbing sounds and then suppress them. Finally, they work to understand the mechanisms that linger behind the production of sound. And the result of the work can lead to component designs that silence disturbing noises.
The scanning laser Doppler vibrometer (SLDV) has proved very useful to noise researchers. Every master spy knows what can be done with the SLDV: You can easily listen in on a conversation being held in a room if you shoot a laser beam at a glass window from outside and then catch the reflected beam.
That’s because the reflected beam shows the vibrations on the window pane that are set in motion by the sounds emitted during the conversation. The SLDV measures the most frequent causes of noise – the oscillation of component surfaces. The reflected laser beam experiences a slight shift in frequency as the surface structures vibrate. This Doppler shift is measured in the laser scanner by a complex optical system and converted into a speed component of the surface. The scanner automatically reads a previously defined measurement grid on the surface, and two mirrors arranged in the laser head move the laser beam from one measuring point to another.
Bosch researchers have applied this established form of measurement to moving objects – a process called “tracking.” As a result, components or systems with movable parts, such as radiator fans and windshield wipers, can be studied during actual operation. The laser beam tracks a fixed measuring point on the radiator-fan blade, or the wiper blade, and records the oscillation.
The vibrations and noises created by the wiping operation of the windshield wipers can be examined.
Researchers using the laser method and the acoustic camera (see cover page) promptly discovered two types of noises: When the blades reach the pinnacle of their cycle and move downward, the change of direction in the wipers’ edges produces a flip-flop noise. During the last third of the wiping cycle on some windshields, a light “hum” occurs at about 100 hertz.
To examine the types of noise more closely, the researchers marked the stiff plastic wiper arm and the flexible wiper edge with about a dozen equally spaced measuring points each.
The measurement process begins with a learning program: The laser is manually aimed at some measuring points from various angles of the wipers. Armed with the reference measurements, a computer determines the individual points’ paths of motion over the windshield. As a result, the laser scanner is able to track all measuring points throughout the entire wiping cycle.
These measurements, in turn, can be used to help in the design, construction and material selection of components. The researchers are also trying to filter out from the data those characteristic parameters for noise generation that are valid regardless of the wiper system used by vehicle makers.
3D Scanning
With the help of laser vibrometry, Bosch researchers have been able to find solutions to a technical question that could not be answered before: Why do brakes squeal?
Anybody who has ever spent time near a busy junction is only too familiar with how annoying the sound of squealing brakes can be. The noise actually begins just before the car comes to a stop, at walking speed.
The researchers are looking at the problem from a physical point of view: Energy is necessary to produce the plangent sound of squeal, and somehow the emitted sound must have picked up a small fraction of the rolling energy of the wheel and brake disk. But how?
Conventional measurement technology can do little to answer this question. Laser vibrometers can only perform measurements in the direction of the laser beam. If a measurement is made perpendicular to the brake disk surface, it’s impossible to see what happens in the plane of the disk. Only the so-called out-of-plane oscillations are visible. They certainly do radiate the sound, but they don’t reveal any information about the mechanism that actually creates the noise.
The key to finding this mechanism lies in observing the friction forces acting between the brake pads and brake disks, and the resulting oscillations. If these components are not observed, two-thirds of the truth remains invisible.
To get to the bottom of the problem, Bosch researchers and an industrial partner began development of a scanning laser vibrometer capable of performing three-dimensional measurements.
The measuring principle is based on a combination of three conventional SLDV systems that focus on the same point from three different directions. With trigonometrical conversions, researchers can use the information from the three beams to determine the oscillatory motion of the measuring point in all three directions in space (3D).
During measurements made on brakes on the test stand, the new system finally revealed the suspected interaction of in-plane and out-of-plane oscillations between brake pads and disks. At certain values of variables, such as brake pressure or disk temperature, the two oscillation components interact to mutually amplify each other. Frictional energy is converted into vibrational energy – the brakes squeal.
In their experiments, the researchers “play” with material parameters and component geometry in an effort to discover their effect on the squeal noise. Their aim is to eliminate squeal by selecting the right design and material parameters during the brakes’ development phase.
At Bosch, this usually means that they hunt disturbing sounds and then suppress them. Finally, they work to understand the mechanisms that linger behind the production of sound. And the result of the work can lead to component designs that silence disturbing noises.
The scanning laser Doppler vibrometer (SLDV) has proved very useful to noise researchers. Every master spy knows what can be done with the SLDV: You can easily listen in on a conversation being held in a room if you shoot a laser beam at a glass window from outside and then catch the reflected beam.
That’s because the reflected beam shows the vibrations on the window pane that are set in motion by the sounds emitted during the conversation. The SLDV measures the most frequent causes of noise – the oscillation of component surfaces. The reflected laser beam experiences a slight shift in frequency as the surface structures vibrate. This Doppler shift is measured in the laser scanner by a complex optical system and converted into a speed component of the surface. The scanner automatically reads a previously defined measurement grid on the surface, and two mirrors arranged in the laser head move the laser beam from one measuring point to another.
Bosch researchers have applied this established form of measurement to moving objects – a process called “tracking.” As a result, components or systems with movable parts, such as radiator fans and windshield wipers, can be studied during actual operation. The laser beam tracks a fixed measuring point on the radiator-fan blade, or the wiper blade, and records the oscillation.
The vibrations and noises created by the wiping operation of the windshield wipers can be examined.
Researchers using the laser method and the acoustic camera (see cover page) promptly discovered two types of noises: When the blades reach the pinnacle of their cycle and move downward, the change of direction in the wipers’ edges produces a flip-flop noise. During the last third of the wiping cycle on some windshields, a light “hum” occurs at about 100 hertz.
To examine the types of noise more closely, the researchers marked the stiff plastic wiper arm and the flexible wiper edge with about a dozen equally spaced measuring points each.
The measurement process begins with a learning program: The laser is manually aimed at some measuring points from various angles of the wipers. Armed with the reference measurements, a computer determines the individual points’ paths of motion over the windshield. As a result, the laser scanner is able to track all measuring points throughout the entire wiping cycle.
These measurements, in turn, can be used to help in the design, construction and material selection of components. The researchers are also trying to filter out from the data those characteristic parameters for noise generation that are valid regardless of the wiper system used by vehicle makers.
3D Scanning
With the help of laser vibrometry, Bosch researchers have been able to find solutions to a technical question that could not be answered before: Why do brakes squeal?
Anybody who has ever spent time near a busy junction is only too familiar with how annoying the sound of squealing brakes can be. The noise actually begins just before the car comes to a stop, at walking speed.
The researchers are looking at the problem from a physical point of view: Energy is necessary to produce the plangent sound of squeal, and somehow the emitted sound must have picked up a small fraction of the rolling energy of the wheel and brake disk. But how?
Conventional measurement technology can do little to answer this question. Laser vibrometers can only perform measurements in the direction of the laser beam. If a measurement is made perpendicular to the brake disk surface, it’s impossible to see what happens in the plane of the disk. Only the so-called out-of-plane oscillations are visible. They certainly do radiate the sound, but they don’t reveal any information about the mechanism that actually creates the noise.
The key to finding this mechanism lies in observing the friction forces acting between the brake pads and brake disks, and the resulting oscillations. If these components are not observed, two-thirds of the truth remains invisible.
To get to the bottom of the problem, Bosch researchers and an industrial partner began development of a scanning laser vibrometer capable of performing three-dimensional measurements.
The measuring principle is based on a combination of three conventional SLDV systems that focus on the same point from three different directions. With trigonometrical conversions, researchers can use the information from the three beams to determine the oscillatory motion of the measuring point in all three directions in space (3D).
During measurements made on brakes on the test stand, the new system finally revealed the suspected interaction of in-plane and out-of-plane oscillations between brake pads and disks. At certain values of variables, such as brake pressure or disk temperature, the two oscillation components interact to mutually amplify each other. Frictional energy is converted into vibrational energy – the brakes squeal.
In their experiments, the researchers “play” with material parameters and component geometry in an effort to discover their effect on the squeal noise. Their aim is to eliminate squeal by selecting the right design and material parameters during the brakes’ development phase.
