Ultrasonic Sensors For Automotive Radar

[Sylvi]

Ultrasonic Sensors for Automotive Radar

March 1, 2010 By: Tom Adams, writing for the Fraunhofer Institute IZM Sensors

A novel assembly method for automotive long-range radar may enable wider adoption of the technology.

Onboard automotive sensing systems have increased in variety and usefulness in the past decade. They are most frequently offered as options on premium-priced automobiles, often from European manufacturers. There are ultrasonic sensors to measure distances in maneuvers such as parallel parking and also video systems for night vision. The various sensing systems can be classified as safety enhancers, convenience items, or a combination of the two.

Automotive radar has also been available, as with the other sensor types, chiefly on high-end vehicles. It can have high safety value because of its sensitivity in detecting and locating other vehicles, especially at highway speeds, and is referred to as long-range radar because it looks for obstacles that are 200–300 m ahead. Long-range radar has been combined with the vehicle's cruise control to create what is known as Adaptive Cruise Control.

Here's how it works: Suppose you are driving at 80 mph in the fast lane on an interstate highway and you're using cruise control. The car ahead of you, also in the fast lane, going at a speed of only 75 mph. The long-range radar will spot the car, note its location and speed, and communicate with your cruise control to lower your speed to 75 mph and to keep a safe distance between the two cars. If the car ahead speeds up to 80 mph or more, or if it moves to the right, Adaptive Cruise Control will boost your speed back up to 80 mph. At any speed, however, long-range radar can work to maintain a safe distance between your vehicle and the vehicle ahead and thus avoid rear-end collisions.

Because of long-range automotive radar's high safety value, it would be beneficial to extend its use beyond high-end cars to the less expensive models. Unfortunately, it is inherently costly to acquire and assemble the components needed to make an automotive radar system.

A few years ago the Fraunhofer Institute IZM, Germany's research center whose many divisions turn feasible concepts into manufacturable realities, saw a way to reduce the cost of these systems. Automotive radar systems typically consist of components attached to a printed circuit board (PCB). The researchers at Fraunhofer foresaw that costs could be reduced by using an assembly method that places the components inside the PCB. The German government then funded a consortium involving, among others: Bosch, the automotive parts manufacturer, Wuerth Elektronik, makers of PCBs, and the Fraunhofer Institute.

The benefits of moderate-cost, long-range automotive radar would extend far beyond the protection of the driver and passengers in a top-end BMW that is cruising the autobahn at high speeds. On a more mundane level, most of the truck accidents in Germany are rear-end collisions—caused when the truck driver couldn't quite stop in time—that take place at speeds from 10 to 30 km/h. The vast majority of these accidents could be prevented if Germany mandated the use of long-range radar on all trucks. Such legislation does not exist today, but substantially decreasing the cost of long-range radar systems would make the idea more attractive. Today, for example, BMW charges about 1800 € for Adaptive Cruise Control with stop-and-go capability. Therefore, a primary goal of the consortium was to reduce the cost of manufacturing for a long-range radar system by about 30% to make the technology feasible for integration in mid-priced autos.

Streamlined Assembly

Electronic systems installed in automobiles have generally undergone more rigorous development than systems destined for most other applications. The automotive environment subjects electronics to drastic temperature fluctuations, noxious fumes, and endless shock and vibration. Under normal circumstances, lopping a small percent off the cost of manufacturing a PCB populated with integrated circuits, resistors, capacitors, connectors, and all the rest would be a significant challenge.

We can understand part of the cost if we consider the steps involved in applying a plastic-encapsulated microcircuit (computer chip) to a PCB. The chip is removed from the plastic tube or tape that it was supplied on, is picked up by a vacuum or tweezers, and placed on the solder paste on the PCB. Later, the whole board is heated to around 260°C to melt the solder so that the metal leads sticking out of the microcircuit can make electrical connections with the board. After cooling, the board is cleaned and tested. Many handling steps are involved, presenting many opportunities for damage to the plastic-encapsulated microcircuit.

The Fraunhofer Institute had a big advantage in modifying this scenario. Its Chip in Polymer technology has the same basic purpose as the solder-reflow process described above—to connect the silicon chip with display units, controls, and other parts of the system of which the chip is a part. The approach, however, is very different.

Chip in Polymer

Chip in Polymer begins with a very thin substrate—much thinner than the typical PCB. The substrate may be FR4 (a glass-reinforced epoxy laminate) or copper. The item to be embedded is typically a silicon chip. The silicon chip, previously thinned to a thickness of approximately 50 µm, is adhesively bonded face up onto this substrate. Then a layer of resin-coated copper is placed on top. The resin conforms to the height of the chip so that the copper layer on top remains flat.

When the resin has cured, a laser drills holes down through the copper-resin to the contact pads on the chip. The holes are plated with copper, and the copper layer on top is etched in a pattern that leaves only the copper traces that will connect the chip to the rest of the system (Figure 1)

The Fraunhofer team used the Chip in Polymer approach along with a second method called Duromer Embedding. In this method, the chip is first attached face down onto a carrier tape. The tape and chip are placed in a transfer-molding tool, where the chip is overmolded by transfer molding. The thickness of the mold compound over the chip backside can be controlled so that all chip-mold compound combinations have the same thickness, no matter what the thickness of the silicon. To assemble the 77 GHz radar system, all of the individual radar ICs were embedded by the Duromer process into one molded module (Figure 2), and Chip in Polymer was then used to attach this 1 mm thick module to the core substrate.

The radar chip itself is a 1.8 by 1.7 mm silicon germanium (SiGe) 77 GHz voltage control oscillator (VCO) that runs off a 5.5 V supply voltage and has an operating temperature range of –40°C to 125°C. The VCO replaces the Gunn oscillator; both devices can be tuned to oscillate between 70 and 80 GHz, but the Gunn oscillator is hundreds of times larger than the chip. When a diode, such as those on the VCO, is biased, the usual result is an upward curve into a positive resistance area. The SiGe chip, in contrast, goes through an S curve, alternating between negative resistance (the downward part of the curve) and the upward positive resistance. This alternation into the negative region is what causes the oscillation and launches the radar signals.

By removing several process steps and by doing without wires, wire-bonding, and solder, Fraunhofer researchers lowered the cost of the whole radar assembly by the desired 30%. Because the radar ICs are all embedded at the same level, it was possible to use the antenna arrays within the IC to employ a narrower radar beam. A wide radar beam has a lower spatial resolution; it can tell how far away an obstacle is, but it cannot reliably determine the obstacle's left-to-right position. To make a narrower beam requires the development of new algorithms, which were written by researchers at the university in Stuttgart; the corresponding antenna array layout was designed by university scholars at Erlangen. The resulting narrow-beam radar sweeps back and forth and does a better job of locating an object and of judging the object's dimensions to distinguish, for example, between a car and a motorcycle.

Installation on the Horizon

The completed module with its embedded radar ICs (Figure 3) passed the electrical and longevity tests that are standard at the conclusion of development. One test, though, was unnecessary—the shock and vibration tests that are often carried out on electronic assemblies destined for harsh applications. Earlier work during the Chip in Polymer development program had already shown that the chips were so well protected that further testing was unnecessary. The embedding and material-matching necessary to create this type of assembly ensures minimal movement of parts relative to one another during vibration or mechanical shock and ensures minimal differential stress under large temperature excursions.

The embedded-chip radar system is not being installed in new vehicles just yet, primarily because the incorporation of new safety equipment in general proceeds slowly. However, expect to see the first versions in two to three years.

ABOUT THE AUTHOR

Tom Adams, BA, MA, can be reached at [email protected].

Edited June 16, 2010 by Sylvi

[Sylvi]

From the web

Advanced high-strength steels help automotive designers cut weight and increase crash worthiness

Advanced High-Strength Steels Add Strength and Ductility to Vehicle Design

By Jessica Shapiro

Authored by:

Paul Geck

Edited by Jessica Shapiro

Key points:

• AHSS steels have both high strength and good formability.

• A mixture of microstructures gives AHSS their properties.

• Designers should aim for structural efficiency, thin plates, and strong joints to take advantage of AHSS.

Resources:

American Iron and Steel Institute, www.steel.org [2]

Society of Automotive Engineers, “Advanced high-strength steels for vehicle weight reduction” seminar, taught by the author, www.sae.org [3]

“Basics of Design Engineering: Steels for strength,” machinedesign.com/article/steels-for-strength-1115 [4]

What material is seeing the most rapid growth in automobiles? If you guessed aluminum or composites, you’d be wrong. It’s advanced high-strength steel (AHSS). The material comprised just a small fraction of cars and light trucks a few years ago, but it could grow to over 30% of vehicle weight within 10 years.

[5]Although higher-priced, lower-volume vehicles like the Audi A8 have converted many of their parts to aluminum in response to fuel-economy pressures, the trend has been for moderately priced vehicles to stick with steel. These mainstream vehicles are using better manufacturing techniques — like laser-welded blanks, hydroformed components, and better joining techniques — in addition to containing up to 30% AHSS.

When the Honda Insight was first launched, it had among the highest percentages of alternate materials of any vehicle on the road. However, the latest version of the Insight is one of the most AHSS-intensive vehicles. Likewise, BMW came out with an aluminum front end on its 5-Series a few years ago but recently switched back to steel.

So what is AHSS, and what makes it so attractive automakers? Grades of AHSS have strengths to 1,500 MPa but retain the formability of lower-strength steels. In general, elongation, the property that equates to formability, degrades as strength increases. AHSS is formulated for more elongation at equivalent strengths.

[6]Microstructure magic

The high-strength, high-ductility characteristics of AHSS come from the metals’ unique microstructures. Where most steels have primarily one microstructural phase, like ferrite, AHSS typically has a combination of martensite, bainite, and ferrite phases.

Each microstructural phase has a different crystalline or molecular structure. For instance, ferrite has a body-centered-cubic (BCC) structure, and martensite has a body-centered-tetragonal structure. Each structure has its own set of physical properties due to the forces within the crystals and the densities with which atoms are packed in a crystal cell.

Phases form during annealing, a process in which steel is heated to 850°C so that it becomes pure austenite. From there, the steel is cooled in a controlled manner that determines the final phase mixture (see “Transformative cooling”).

[7]For instance, if the steel is cooled at a very slow rate, the austenitic phase will transform into ferrite without transitioning into any other phase. Conversely, very rapid cooling or quenching produces 100% martensitic steel (MART). A slow cool followed by a rapid quench produces dual-phase (DP), complex-phase (CP), or transformation-induced plasticity (TRIP) steels, all of which have AHSS properties.

Alloying elements like silicon, manganese, chromium, and molybdenum shift the phase lobes on the transformation diagram, making it possible to form the desired phase mix with feasible cooling cycles.

Forming and springback

The combination of phases keeps AHSS formable while they retain the high strength of martensite. Formability is especially important in automotive manufacturing where large presses and multipiece dies form parts.

AHSS parts are formable by traditional methods, but they typically need stronger die materials or coatings that boost die life. Manufacturers may also need higher tonnage presses to form these steels.

[8]A key difference designers need to take into account when using AHSS is a greater degree of springback, the tendency of a metal to partially return to a previous shape. While all steel parts have some degree of springback when removed from the press, steels that undergo more work hardening or strengthening during forming have more springback.

There are several strategies for reducing springback. One is overforming the metal so it springs back to the desired dimensions. For example, to stamp a 90° angle into a sheet of steel, overstamp it to about 87° and it will springback to the desired angle.

Designers can also optimize radii in the edges and corners of the stamped piece. Other strategies involve changing the design of the finished part to minimize springback.

Structural strategies

Despite AHSS’ better strength, steel still has to compete with aluminum’s lower density. But aluminum may not be a panacea for automakers. For one thing, 30 times as much steel is produced than aluminum, so it can be hard to sustain production or keep prices down on an all-aluminum car.

Various research studies (see “High-strength history” sidebar) have shown that proper application of AHSS can cut a vehicle’s weight between 10 and 25%. When fuel economy is paramount, the 5% fuel-economy boost a 10% reduction in weight provides is a nice carrot. But designers should be aware of certain approaches that can take full advantage of AHSS’ capabilities.

Automotive bodies can be considered to be a combination of plates, beams, and joints — the intersections of two or more beams. If all the steel in a car was replaced with aluminum and no design changes were made other than making the aluminum parts thick enough to achieve the same overall stiffness and strength of the steel vehicle, the distribution of strength and stiffness would still be somewhat different on the aluminum car versus the steel one.

For mechanical responses that are proportional to the specific modulus — the modulus of elasticity divided by the density — there’s no advantage of switching from one metal to another from a weight perspective. For instance, the torsional stiffness of circular thin-walled tubes made from aluminum and steel of the same weight is the same. Looked at another way, two thin-walled tubes having the same torsional stiffness, one made out of steel and the other made out of aluminum, weigh the same.

However, the torsional stiffness of a plate is not directly proportional to specific modulus. A steel plate weighs twice as much as an aluminum plate of the same area (but not thickness) and the same torsional stiffness.

So, designers can improve structural efficiency and minimize the benefit of converting to aluminum in two ways. First, use higher strength steels to minimize the weight of plates in a vehicle. Second, get the vehicle’s internal beams to behave more like closed-section, circular tubes.

Advanced architectural elements like laser-welded blanks or hydroformed beams can help a steel body or frame compete with aluminum in terms of weight. Laser welding two sheets of dissimilar-thickness or dissimilar-grade steel together puts strength where it is needed in a single steel stamping.

Hydroforming a continuous hollow tube to the desired contour in a die using water pressure eliminates spot-welded sheet-metal beams. Because the tubes are continuously fastened and of a thicker gage than the sheet-metal beams, they have higher strength and stiffness.

Joints are a third area designers should consider. The joints of an all-steel vehicle will usually have lower strength and stiffness than those of its all-aluminum counterpart because the joints have typically higher stresses and steel is thinner gage than in the aluminum vehicle. So, strengthening and stiffening the steel vehicle’s joints will reduce the weight benefit of aluminum.

Designers can add internal stiffeners to boost joint strength. More manufacturers are also converting spot welds into continuous welds or adding continuous adhesives in the seams of the joint.

These strategies let automotive designers maintain or reduce vehicle weight in the face of increasing safety and crashworthiness requirements. One example is an upcoming change to the National Highway Transportation Safety Administration’s roof-crush requirements. Since 1994, vehicle roofs have had to withstand a load of 1.5 times the gross vehicle weight (GVW). In 2011, this will grow to 2.5 to 3 times GVW, necessitating thicker or stronger roofs.

High-strength history

1960s and 1970s:

— Vehicles used predominantly mild steel because structural performance was dominated primarily by stiffness. Higher-strength steels have the same stiffness as mild steel, so automakers saw no reason to switch.

1980s:

— Fuel economy and safety requirements pushed manufacturers and steel suppliers to introduce conventional high-strength steels.

— High-strength steel content topped out at 30% in the middle of the next decade.

1990s:

— Designers turned to aluminum, magnesium, and fibrous composites to cut vehicle weight. More-stringent fuel-economy requirements looked to outweigh increased material cost.

— AHSS being studied in laboratories were costly and hard to weld. Despite better formability than conventional high-strength steels, springback was a barrier to designers using AHSS.

2000s:

— The International Iron and Steel Institute began the Ultralight Steel Auto Body Advanced Vehicle Concepts (ULSAB-AVC) study.

— Improved Materials and Powertrain Architecture for 21st Century Trucks (IMPACT), jointly funded by the U.S. Army, Ford Motor Co., University of Louisville, and American Iron & Steel Institute, sought to trim pickup-truck weight 25% with steel construction