1、By Keith OdlandSilicon 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 electroni
2、c 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-electr
3、ic 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 featuresHowever, such features are merely the surface. Other highly engineered, complex feat
4、ures, 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 expectatio
5、n 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.
6、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
7、 to increase systems functional densities. Large scale integration in mixed-signal ICs is one attractive alternative. Capture, compute, communicateNearly every embedded automotive system must perform three functions: capture, compute, and communicate. Capture refers to extracting information from th
8、e 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.
9、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 distri
10、buting 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
11、/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 challengesFuel tank sensing is an excellent example of the challenges being placed on automotive design engineers. On
12、ly 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 implement
13、ation in todays 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 s
14、ystems. 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 fue
15、l 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 imp
16、ortant 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 inte
17、grate 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 implemen
18、tations 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 fr
19、om 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 ha
20、s 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 on
21、ly 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 time
22、r. 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
23、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 stat
24、ionary 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 oper
25、ation. 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
26、 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 o
27、r 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 mo
28、tor 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
29、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
30、 (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
31、, 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 perfo
32、rmance 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 marketOne in every three dollars spent on
33、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 a
34、dvanced 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 authorKeith Odland is product manager for MCU products
35、 at Silicon Laboratories. 高效能汽车电子设计挑战作者：Keith 源自：Silicon Laboratories 微控制器产品 访问：1311 日期：2008-3-24 【 字号： 大 中 小 】 消费者的买车习惯正在转变，这也带动了汽车电子产业的增长。汽车制造商每年都为载客车辆增加更多新型或加强型电子元器件，使得车身电子系统目前的增长率比汽车产量还高出四倍。目前新功能或加强功能的趋势是增加更复杂的电子元器件，以便提高品牌声誉和竞争差异性，同时让消费者更安全舒适。例如复合动力电动车就像把 iPod连接到汽车娱乐系统一样，现已成为一种流行时尚。消费者还把手机与整合型免持
36、听筒装置之间的蓝牙连结视为标准配备。复杂功能这些特色仅是冰山一角，其它精心设计的复杂功能虽不会被乘客看到或摸到，却会影响他们的行车经验，这些功能也逐渐导入汽车设计。感应照明系统、多轴 调整座椅、智能型天候控制系统、防撞系统和动力巡航控制在 21世纪汽车市场变得格外重要。消费者甚至期望车商提供高质量的仪表板功能。要将这些先进功能导 入汽车系统往往需要付出代价。汽车电子设计人员的一项挑战是迅速推出新的电子元器件，提高乘客的舒适性、安全保护和其它加强功能。设计人员必须缩短整体的设计与认证时间和增强现 有系统功能，并且不能影响日益严格的质量与可靠性要求和成本目标。为了克服这些挑战，汽车电子设计人员需要
37、集成度更高的解决方案以便提高系统的功能密度。 混合信号元器件的高功能集成就是很有吸引力的一项替代方案。捕捉、运算和通讯几乎所有的嵌入式汽车电子系统都必须执行捕捉、运算和通讯等三种功能。“捕捉”是从实际世界取得信息，再将它转为数字形式。这可能是车胎监控系统的 压力传感器所传来的模拟电压，或是碰撞侦测感应器 I/O接脚的上升沿波形，这个感应器可能会连接到安全气囊的触发系统。“运算”是指在应用环境下处理数字 信息的能力，例如安全气囊控制器可能在极短时间内就决定不启动安全气囊，因为它发现座位上有小孩。“通讯”是指将处理结果传送给其它需要该信息的系统，譬 如启动指示灯就是很简单的例子。其它复杂功能可能会
38、通过网络总线把排气系统的一氧化碳含量告知引擎管理计算机，以便提高燃油的氧气混合比例。解决方案的有 效性最终将由系统执行这三种功能的程度决定。新设计挑战油箱感测是一个很好例子，说明汽车电子设计人员所须面对的挑战。仅在几年前，油量传感器还是一个相当直接的设计问题。它包含一个简单的浮筒装置，上 面有扫描式碳刷接触着电阻性表面，它会使得模拟输出电压正比于油箱的剩余油量。但对今日汽车而言，通常必须等到平台设计快结束时才会开始油箱设计，而且多 半要利用任何尚未使用的空间。这可能使得油箱的形状怪异，容量也不再与液面高低成正比，这会让浮筒系统的设计变得很复杂。更重要的是，替代燃料的出现和燃 料衍生物让油箱的燃
39、料成份变得很重要。举例来说，汽油与乙醇燃料的比例会影响点火、燃烧时间和废气排放等引擎动力特性。厂商现已认为新一代油箱传感器必须 能决定燃油成份，同时将这项信息提供给汽车的其它电子控制系统。这使得过去被认为很简单的感测设计现已变为一种复杂的分析控制挑战。值得注意的是，几乎车内的所有系统都在扩充功能。主动式露点 (dew-point) 控制器正在取代挡风玻璃除雾功能，它可以避免或排除水滴凝结所需的条件。雨水感应雨刷系统则会把马达控制和雨水感应功能整合为一套系统。下一代防夹车窗与 天窗的关闭则是这些安全系统的微电子元器件所需整合的另一代表性应用。第一代防夹技术第一代防夹设计通常包含一套由电动马达驱动
40、的机械驱动系统。马达电流由一颗控制器监测，然后与代表失速状态 (stall condition，亦即马达转动受阻) 的固定临界值比较；只要达到该临界值，车窗方向就会从上升反转为下降。这套系统如图 1所示。图 1：第一代防夹车窗升降系统的控制图第一代设计有几项缺点。首先是要开发一套方法分辨马达启动和车窗受阻时的马达失速电流 (图 2和 3)。为了达到这项要求，比较电路中增加一段固定延迟时间，确保它只在马达转动后才开始比较失速电流临界值，只不过这种做法有时无法为半开的车窗 提供防夹保护。举例来说，如果车窗的起始位置仅距顶端10毫米，那么在临界定时器的计时结束前，车窗很可能早已撞到顶端的挡板 (ha
41、rd-stop)。图 2：关闭车窗时的电流变化图 3：关闭车窗遇到阻碍时的电流变化第二个缺点是机械系统的参数会随着时间改变，这会影响马达的工作负载，使得防夹临界值变大或变小。最后，这些系统由于使用固定临界值，所以无法适应行车环境的改变。车窗密封条的热膨胀效应会让温度变化对工作负载产生很大影响。汽车静止时关闭天窗 所需的力量与行驶中车辆有很大不同，在平滑路面升起车窗所需的力量也不同于车辆在石头路上的行驶。在这两种情形中，无法补偿这些变动的状况都会影响安全或 造成车窗无法正常操作。设计人员过去是以不同方式应付这三项重要挑战。在有些情形下，他们会增加更多的传感器或使用更精确的控制材料与元器件来减轻这
42、些问题，但这些方法都会增加设计的成本与复杂性。这使他们日益需要一套低成本的防夹功能设计来克服这些缺点。新的设计解决方案如图 4所示，一颗包含高速中央处理单元 (CPU) 和高效能模拟数字转换器 (亦即带宽大于 180MSPS和分辨率超过 12位) 的混合信号微控制器是此问题的最佳解决方案。图 4：采用混合信号微控制器的防夹系统这种做法让设计人员利用一颗微控制器同时执行马达的通讯功能和监控马达电流。通讯噪声可由芯片内建的模拟数字转换器直接在马达电源电路的电流传感器 (亦即分流电阻) 上侦测。这种方法能更精确分辨马达处于转动或失速状态，不仅比较器电路不需再增加一段固定延迟时间，就算车窗已经半开也可
43、提供完整的防夹功能。如图 5所示，系统会根据历史数据和参数计算结果设定可变的马达电流临界值，以便动态响应马达负载变动和将系统扭力限制在适当范围，同时将长期因素 (例如马达磨损和密封材料老化) 和短期因素 (例如环境、湿度、温度和振动) 都列入考虑。另外，系统还能与其它的电子控制装置 (ECU) 交换信息，把外界温度和车速等信息当成加权输入来决定适当的临界值 (参考图 6)。利用其它系统不仅会提高整体系统效能，还能避免在车上重复安装传感器的额外成本。图 5：使用可变临界值后的车窗关闭过程电流变化图 6：存储在内存表格的环境参数与历史数据增长市场汽车应用占 8位微控制器销售额的三分之一，不仅市场规模已超过 30亿美元，每年还以将近 10%的速率增长。汽车嵌入式系统设计人员必须发展更可靠、 更低成本和更高集成性的解决方案，因此需要最先进的微电子建构方块供他们使用。对这类下一代汽车电子应用来说，兼具强大模拟与数字效能的混合信号微控制器 正是最符合成本效益的解决方案。
