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With the capability to control central door locking, retained accessory power, data communication, and power distribution, the body computer is also tied into alarm systems, immobilizers and remote keyless entry systems. The body electronics network is primarily a CAN bus today, frequently a low-speed, single-wire implementation to reduce cost. The local interconnect network (LIN) has been pioneered in Europe to reduce the cost in body electronics in high-end vehicles. In these systems, the manufacturers connect subsystems, such as mirrors and door motors using the LIN bus. Figure 1 shows how motor, heating and light loads in body electronics can be controlled by CAN and LIN and how these systems connect to the high-speed CAN bus in powertrain and a fiber-optic MOST network for audio systems.
As shown in Figure 2, the transition between high-speed CAN and low-speed CAN bus and between the low-speed CAN bus and other networks such as FlexRay or MOST requires a gateway. The LIN sub-bus connects directly to the CAN network. The recently approved (August 2004) SAE J2602 LIN sub-bus protocol should provide significant impetus for North American usage of LIN.
IT IS ALL ABOUT POWER
The motors, heaters and lighting loads in body electronics take a significant portion of the vehicle's power. (See “The Clout of Body Electronics,” on pg. 28.) As shown in Table 1, the power requirements can be only 10 W for some loads but is several hundred watts for others. In addition, the total number of these loads (many are doubled or quadrupled for right side and left side and front and rear) adds to the complexity of power control. For example, a high-end vehicle can easily have more than 60 motors and that does not include embedded motors in disk drives, DVDs or internal cooling fans for entertainment products.
Many of the lighting loads shown in Table 1 are industry standards based on headlamps and bulbs with SAE specifications. Incandescent lamps can have an inrush (peak) current up to 10 times the steady state value, which must be taken into account for protection especially if the loads are controlled by power semiconductors. As noted in Table 1, the maximum power in motors is several times the average value.
Most of the motors in body electronics are used infrequently making the permanent magnet brush dc motor the motor of choice. When the motor has a potential for continuous usage, such as the HVAC fan motor, a transition to brushless dc has started. In addition to improved reliability from eliminating brushes and their associated arcing, electronic control of the brushless dc motor reduces the power consumption, since the motor operates at the lowest speed required to perform its function with reduced current draw.
Many of the power motors in body electronics are still activated by electromechanical relays. To simplify the control of dc motors, semiconductor companies have designed a variety of power ICs. For example, Freescale Semiconductor introduced a relay replacement IC it calls the self-protected silicon switch (SPSS) at Convergence 2002. The 2 milliOhm on-resistance of the SPSS and its programmable functions were quickly identified as potential solutions for improved reliability. Since that time Freescale received considerable interest from automotive customers looking to improve reliability and reduce system complexity. They also developed a dual 4 milliOhm device with similar functionality. At Convergence 2004 the company introduced the MC33981, a 4 milliOhm high side power IC in a 12 mm × 12 mm power quad flat pack no-lead package for pulse width modulated (PWM) control at frequencies up to 60 kHz. As shown in Figure 3, one of the target applications is relay replacement in motor control.
RELAYS RULE
In spite of the number of power ICs that have been developed for automotive applications with the intent of replacing electromechanical relays, the electromechanical relay is still alive and well. David Goff, supervisor of new product development for Omron Automotive Electronics' estimates that the average North American vehicle has between 20 to 30 relays for a total annual number of about 400 million relays in North America alone. This includes plug-in and printed circuit board (PCB) mounted relays. Omron is the largest supplier of automotive relays in North America and one of the top suppliers globally offering both types of relays. Table 2 provides an indication of where some of these relays are used.
Vehicle relays are either replaceable, plug-in type or PCB-type. Body electronics typically use the circuit board type. According to Goff, the plug-in types are usually under the hood close to the fuse panel with common types available from many consumer stores. Typically, higher current relays or higher-temperature environment relays are plug-ins. However, the application frequently dictates the choice of relays, too. Systems such as the rear defroster, fuel pump and air conditioner clutch would typically be plug-in type. For printed circuit type relays there are several applications with dc motors common in body electronics, such as power windows, power door locks and power seat motors.
Goff observed that there hasn't been a major shift due to solid-state relays. The use of both is probably increasing based on the increased electronic applications but solid-state relays are not increasing at the expense of electromechanical relays. “We haven't seen a huge dent,” said Goff.
Smart junction boxes are one of the biggest applications for PCB relays according to Goff. PCB relays are integrated on the same board as the computing chips and reduce package size and reduce the cost associated with interconnecting plug-in relays. A relay driver IC controlled by the MCU would typically control the relay.
Thus, there is an increasing interest in PCB-style relays, which are getting smaller for a given current value. Since the mid-1990s, a PCB relay has been reduced to 25% less volume for the same current level and can address the same applications as the larger models. According to Goff, there has been quite a trend to miniaturize relays. So as solid-state relays get better, electromechanical relays are getting better, too. The other trend is surface-mounted relays. These are just starting to be available with circuit board and module makers providing the driving force to achieve easier process compatibility.
SOME OTHER TRENDS
In addition to transitions from brush to brushless motors and power ICs competing with electromechanical relays for control of body electronic loads, there are a number of other interesting trends that are occurring in body electronics.
To simplify the design and its installation, doors, seats and consoles are increasingly provided as completed assemblies to car manufacturers. A body electronics item that is affected in hybrid vehicles is the AC compressor. Since the engine is off at idle, one way of providing the AC is an electric compressor. However, an electric compressor requires 2800 W or more of power. Matsushita engineers developed an electrical compressor that operates from 200 V to 300 V for hybrid vehicles. The refrigeration capacity of the unit is 4.6 kW maximum with operation between 780 rpm to 9000 rpm. The design is said to be low cost. It weighs only 5.9 kg and is 115 mm in diameter and 199 mm in length.
In some cases, motor design can be improved without making the transition from brush to brushless dc motors. A flat armature motor developed by Brose for window lift applications takes less space than conventional drives promising new design approaches for door assemblies. The motor has a symmetric design, so it can be mounted in the left or the right door and it takes up far less space in the door structure. High efficiency gearing provides the same mechanical performance with 50% less power consumption making semiconductor-based control electronics more cost effective.
Electronically controlled mirrors are used externally and internally. For example, Gentex Corporation makes auto-dimming rearview mirrors that automatically darken to reduce glare from the headlamps of vehicles approaching from the rear. The brighter the glare, the darker the mirrors become, making nighttime driving safer.
The auto-dimming mirror can be upgraded to include Johnson Controls' HomeLink Wireless Control system, which adds three buttons to the face of the mirror that can be programmed to operate garage doors, estate gates, security systems, home lighting, and other radio frequency-controlled devices.
Gentex has identified that rearview mirrors are a natural, cost-effective location to locate various driver communication interfaces and displays. They project that future mirrors could integrate a number of features including:
- rain sensors;
- global positioning systems;
- cell phones;
- tire pressure indicators;
- carbon monoxide warning lights; and
- collision avoidance indicators.
If a rearview mirror has this kind of potential for customer differentiation, it is not surprising that carmakers will be adding more electronic functions to the other body electronics systems to attract customers.
REFERENCES:
Brose http://www.brose.net/en/pub/products/
Gentex http://www.gentex.com/wn_smartbeam.html
ABOUT THE AUTHOR
Randy Frank is president of Randy Frank & Associates Ltd., a technical marketing consulting firm based in Scottsdale, Ariz. He is an SAE and IEEE Fellow and has been involved in automotive electronics for more than 25 years. He can be reached at [email protected].
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THE CLOUT OF BODY ELECTRONICS
by Randy Frank, Contributing Editor
According to Chris Webber, vice president-Automotive Practice of Strategy Analytics, “The most significant electronic module and semiconductor device growth drivers we see in the body electronics area is lighting. This has already commenced with LED exterior and interior lighting and high-intensity discharge (HID) headlamps.” In addition, adaptive or dynamic beam-adjusting headlamp assemblies are emerging that use a significant amount of power semiconductors. Webber sees volume growth for body features accelerating, but he noted the dollar value increase is tempered by greater control module integration. Strategy Analytics expects significant changes, invisible to the customer, to body module designs and control partitioning in general with the introduction of the LIN/J2602 sub-bus protocol.
Figure 1 shows the contribution of the various subsystems for the main automotive vehicle-producing regions of Europe, NAFTA, Japan, South Korea and China. The largest portion is heating ventilation and air conditioning (HVAC) at 24%, followed closely by the body electronic module itself at 19%.
The projected sales growth of body electronics ECUs compared to all the other systems is shown in Figure 2. Body electronics was 28% of the total control ECU content in 2004 and will drop to about 26% by 2011 based on body electronics growth of 5.6% compared to the rest of the electronic total growth at 7.2%. The ECU split is based upon the penetration of the system function in car and light truck production, and its associated electronic value. Some of the functions are combined into integrated modules such as centralized body control modules (BCMs), door modules, column modules, etc.
Based on the extensive power control in body electronics, power and analog comprise 55% of semiconductor content as noted in Figure 3. MCUs with their control and communication responsibilities are next, but only half as large. Stand-alone sensors at 3% represent a rather small portion of body electronics with current sensing and temperature sensing for control and protection embedded in many of the analog/power semiconductors and not even counted in this number.