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Designing high-performance automotive electronics

Posted: 14 May 2007 ?? ?Print Version ?Bookmark and Share

Keywords:electronics automotive? system body electronic? vehicle electronics?

By Keith Odland
Silicon Laboratories

Consumer buying habits for automobiles are changing, driving the growth in the automotive electronics industry. Each year, automotive manufacturers are integrating more new and enhanced electronics into passenger vehicles. The current growth rate of body electronic systems is outpacing vehicle production by a factor of four to one.

Some of the current trends in new or enhanced features are directly related to incorporating increasingly complex electronics to improve brand reputation, competitive differentiation, and consumer comfort and safety. Hybrid-electric vehicles are trendy, as is connecting an iPod to an in-dash entertainment system. Consumers now consider Bluetooth connectivity between handsets and integrated hands-free units a standard feature.

Complex features
However, such features are merely the surface. Other highly engineered, complex features, which passengers may not be able to see or touch but effect their experience, are also increasingly incorporated. These include adaptive forward lighting, multi-axis adjustment seating, intelligent climate control systems, collision avoidance and dynamic cruise control. And there is expectation of receiving a high-quality dashboard experiencebut implementation of these transcendental systems within the automotive framework comes at a price.

One challenge for automotive electronics designers is quickly introducing new electronic components for passenger comfort, safety, and enhancements. Engineers are required to shorten the overall design and qualification cycle and must increase functionality of existing systems without compromising ever tightening quality, reliability and cost targets. To address these challenges, automotive designers look for more highly integrated solutions and to increase systems functional densities. Large scale integration in mixed-signal ICs is one attractive alternative.

Capture, compute, communicate
Nearly every embedded automotive system must perform three functions: capture, compute, and communicate. Capture refers to extracting information from the real world and translating it into the digital domain. This could be an analog voltage from a pressure sensor used in a tire pressure monitoring system or the rising edge of a waveform as seen from an I/O pin in a collision detection sensor, which would be connected to an airbag firing mechanism.

Compute refers to the ability to take digitized information and manipulate it in the context of the application. An example would be an airbag controller making a split-second determination not to deploy because it has detected a child in the seat.

Communicate refers to taking this result and distributing it to other systems that may require that information. For instance, a simple function would be energizing an indicator lamp. A more complex function would be using a network bus to send CO levels from an exhaust system to the engine management computer in order to increase oxygen in the fuel/air mixture. The degree to which the system can perform all three capture, compute, and communicate functions will ultimately determine the effectiveness of the solution.

New design challenges
Fuel tank sensing is an excellent example of the challenges being placed on automotive design engineers. Only a few years ago, a fuel level sensor was a relatively straight-forward design problemconsisting of a simple float mechanism with a sweeping brush contactor across a resistive surface. The result was an analog output proportional to the level of fuel remaining in the fuel tank.

Fuel tank implementation in today's vehicles occurs at the tail-end of a platform design, and frequently the design is required to take advantage of any remaining unused space. This can result in exotic tank geometries that have non-linear volume to displacement attributes which complicate the implementation of float systems.

Even more significant, the introduction of alternative fuels and fuel derivatives make the composition of the fuel in the tank of interest. For example, the ratio of petroleum and ethanol based fuels can have effects on engine dynamics such as ignition, timing, and emissions. Determining fuel composition and communicating that information to other electronic control units (ECUs) in the automobile is now considered an application requirement for next-generation fuel tank sensors. So what was once an elementary-level sensing design is now a complex analytical control challenge.

It is important to note, such feature-set expansion is occurring in nearly every system within the automobile. Windscreen de-fogging functions are being replaced with active dew-point controllers to prevent or eliminate the conditions necessary for condensation to ever form. Rain sensitive wiper systems integrate both the motor control and rain sensing functions in a unified system. Next-generation anti-pinch window and sunroof closures are another application that is representative of the integration now required in the microelectronics of these safety systems.

The first generation anti-pinch implementations typically consisted of a mechanical drive system powered by an electric motor. The motor current was monitored by a controller and compared to a fixed threshold which represented a stall condition (i.e. presence of an obstruction). This would result in the reversal of the window direction from up to down (see below).

There were several limitations to this initial approach. The first was developing a method for discriminating between the stall current of a motor seen at start-up and when the window encountered an obstruction (see two figures below).

Current profile for window closure has peaks at beginning and end to start window movement and ensure closure, respectively.

Current profile super-imposed with fixed obstruction threshold will have a peak within the closure cycle.

A fixed time delay was introduced into the comparator circuit so the stall current threshold was compared only after the motor had started to move, but this prevented anti-pinch protection on a widow that was partially opened. For example, if the window was in a starting position of 10mm from the top of the sill, the window could close and engage the hard-stop prior to the expiration of the threshold timer.

The second limitation was that, over time, the parameters of the mechanical system would change and affect the working load of the motor resulting in a shift, either positive or negative, from the desired sensitivity of the anti-pinch threshold.

Finally, by using a fixed threshold, these systems were unable to adapt to the dynamically changing conditions of the driving environment. Changing temperatures greatly affect the working load due to the effects of thermal expansion on the seals of the window. In the application of a sun-roof, the force required for closure while the vehicle is stationary is significantly different than when the car is moving. The force required to raise a window on a smooth surface is different than when the vehicle is driving over a cobblestone street. In both cases, the inability to compensate for these changing conditions results in unsafe or improper operation.

These three fundamental challenges were addressed differently by designers. In some cases they were mitigated through the implementation of additional sensors or more tightly controlled materials and components. But either of these methods added cost and complexity to the design. There was an increasing need for a low-cost method for implementing anti-pinch functionality that would address the three limitations.

A mixed-signal MCU that has a high-speed CPU as well as an integrated high-performance ADC (i.e., bandwidth greater than 180,000 samples per second and a resolution of 12-bits or greater) is an ideal solution for this problem (see below).

This approach enables designers to have a single MCU responsible for both the commutation of the motor and monitoring the motor current. The commutation noise can be detected directly from a current sensor (e.g., shunt resister) in the motor supply circuit using the on-chip ADC. This method can more accurately and quickly determine if the motor is spinning or in a stall condition. This eliminates the need to use a fixed-time delay in the comparator circuitry and permits full anti-pinch functionality even when the window is slightly open.

By implementing a variable motor current threshold based on both historical and calculated parameters (shown below) the system can dynamically respond to changes in motor loading and maintain the appropriate force limits in the system. This response versatility also accommodates both long-term (e.g., motor wear, seal aging) and short-term (e.g., environmental, humidity, temperature, vibration) factors affecting window closure.

Current profile for window closure with variable obstruction threshold can accommodate various environmental conditions as well as system aging effects.

In addition, by having a method for communicating the information between other ECUs, the system can use information such as outside temperature and vehicular speed as inputs of a weighted determination of the appropriate current threshold (see below). Thus by leveraging other systems, the overall system performance can be increased without being burdened by the cost of redundant sensors already deployed in other areas of the vehicle.

A table in memory can consist of environmental and historical parameters used in the determination of current threshold.

A growing market
One in every three dollars spent on 8bit MCUs goes into automobile applications. This market is over $3 billion annually and is growing at close to 10% each year. As automotive embedded designers are continually pushed to quickly develop more highly integrated solutions with higher reliability and lower cost, they must have the most advanced microelectronic building blocks at their disposal. Mixed-signal MCUs that offer potent combinations of both analog and digital performance are a cost-effective solution for this class of next-generation automotive applications.

About the author
Keith Odland
is product manager for MCU products at Silicon Laboratories.

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