Designing for the auto-challenge

01 April 2007

Variable obstruction threshold

What consumers are demanding from their automobiles is changing, having a direct impact on the automotive electronics industry

Each year, automotive manufacturers are integrating more new or enhanced electronics and the current growth rate of body electronic systems is outpacing vehicle production by a factor of 4:1

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 fashionable, as is connecting an iPod to an in-dashboard entertainment system. Consumers now consider Bluetooth connectivity between handsets and integrated hands-free units to be standard.

These features are merely superficial, however. Highly complex features that are invisible and intangible further affect the passenger experience and are increasingly being incorporated. Adaptive forward lighting, multi-axis adjustment seating, intelligent climate control systems, collision avoidance and dynamic cruise control are becoming commonplace in the 21st-century automotive landscape.

Below the surface
Automotive electronics designers are expected to incorporate new components for passenger comfort, safety and enhancements. They have to shorten the overall design cycle and increase functionality of existing systems without failing to meet ever-tightening targets for quality, reliability and cost. Designers must look for more highly integrated solutions to increase the functionality of systems. Large-scale integration in mixed signal ICs is one attractive alternative.

Nearly every
embedded automotive system must perform three functions: capture, compute and communicate. ‘Capture’ means extracting information from the real world and translating it into the digital domain. ‘Compute’ means taking digitised information and manipulating it in the context of the application. ‘Communicate’ means distributing this result to other systems that may require that information. The degree to which the system can perform all three functions will ultimately determine the effectiveness of the solution.

Still waters run deep
Fuel-tank sensing is a good example of the challenges being placed on automotive designers. Only a few years ago, a fuel level sensor was a relatively straightforward design. It consisted of a simple float mechanism with a sweeping brush contactor across a resistive surface. The result was an analogue output proportional to the level of fuel in the tank. In today’s vehicles, fuel tanks are incorporated towards the end of the design process and are often required to use up any remaining space. This results in exotic tank geometries that have non-linear volume:displacement attributes thus complicating the float mechanism.

Furthermore, alternative fuels and fuel derivatives have different compositions, with the ratio of petroleum and ethanol affecting 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 a requirement for next-generation fuel-tank sensors.

Nearly every system within the automobile is being upgraded. Windscreen de-fogging functions are being replaced with active dewpoint controllers to eliminate the conditions necessary for condensation to form. Rainsensitive wipers combine both the motor control and rain-sensing functions in a unified system. Anti-pinch window and sunroof closures is another example of what is required from microelectronics.

Current anti-pinch technology
Anti-pinch technology 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 an obstruction. This would result in the reversal of the window direction from up to down (see figure 1).

There were limitations in this approach. The first was discriminating between the stall current of a motor at start-up and when the window encountered an obstruction. A fixed time delay was introduced to 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 window that was partially opened (see figure 2).

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 changing conditions of the driving environment. Temperature greatly affects the working load due to the effects of thermal expansion on the seals of the window. In a sun-roof, the force required for closure while the vehicle is stationary is very different to when the car is moving. The force required to raise a window while driving on a smooth surface is different to when driving over a cobblestone street. In both cases, the inability to compensate for these changing conditions results in unsafe or improper operation.

Different strokes
These three fundamental challenges were addressed differently by designers. In some cases, additional sensors were used or materials and components more tightly controlled. 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 microcontroller (MCU) that has a high-speed central processing unit (CPU) as well as an integrated highperformance analogue-to-digital converter (i.e. bandwidth greater than 180,000samples/sec and a resolution of 12 bits or greater) is an ideal solution for this problem (see figure 3).

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 in the motor supply circuit using the on-chip ADC. This method can determine more accurately and quickly 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.

Help is at hand
By implementing a variable motor current threshold based on both historical and calculated parameters, the system can respond to changes in motor loading and maintain the appropriate force limits in the system (see figure 4). Both long-term (e.g. motor wear, seal aging) and short-term (e.g. environmental, humidity, temperature, vibration) factors are included. In addition, by having a method for communicating the information between other ECUs, the system can use information as inputs of a weighted determination of the appropriate threshold (see figure 5). The overall system performance can be increased without being burdened by the cost of redundant sensors already deployed in other areas of the vehicle.

One in every three US dollars spent on an 8bit MCU goes into an automobile. This market is over US$3billion and is growing at close to 10 per cent each year. As automotive embedded designers are continually pushed to 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 analogue and digital performance are a cost-effective solution for this class of next-generation automotive applications.

KEITH ODLAND is product manager, MCU Products, Silicon Laboratories

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