Part 3 ― Advanced Solutions for Vehicle Control Applications

The Renesas Approach to Vehicle Control: Chassis and Airbag Control Solutions

Customers that are building vehicle chassis systems are making increasing use of mechanisms driven by electric motors. A prime example is the electric power steering system that replaces a hydraulic system to achieve multiple benefits, including increased fuel efficiency. Renesas supplies the SH7147 and SH7142 SuperH microcomputers for the advanced electric motor control functions that typify this application. To improve occupant safety, automotive customers are using an increasingly diverse range of airbag types and functions, adding complexity to the associated control systems. Our H8SX/1700 series achieves a 1.7 times increase in processing performance to handle the latest airbag control requirements.

Microcomputer control is also an essential part of the steering, brakes, and other chassis functions used to control vehicle movement such as cornering and stopping. Power steering has to transmit the driver's intentions accurately and smoothly to the components in the vehicle's front-end. In the past, steering assistance was achieved by utilizing the power of the engine to run a belt-driven hydraulic pump. Today, however, the trend is to use electric power steering (EPS), an approach that replaces the hydraulic pump with electric motors.

One advantage of changing the design of power steering systems to electric motor driven operation is an improvement in fuel consumption. This is because the pump in a hydraulic system runs continuously, driven by a belt at speeds proportional to engine rpm. By contrast, the electric motor in an ESP system only has to operate when power-assist is required. Also, an EPS system can provide power-assist even if the engine is turned off, as is often the case in hybrid vehicles or in models that switch off the engine when the vehicle is stopped in traffic. Another advantage is that the level of steering force can easily be modified in accordance with vehicle speed, so that assistance decreases at highway speeds and increases at slow speeds to ease parking maneuvers.

The electric motors traditionally used in EPS systems were brush-type DC motors. Now, though, more designs are using brushless DC motors, which have excellent durability but require complex control algorithms. The microcomputers used to control them must be high-speed types that provide a high-resolution A/D converter and a three-phase motor control function. For the main microcomputers in EPS control systems, Renesas supplies the SH7147 and SH7142 32-bit microcomputers, members of the SuperH family that use the SH-2 CPU core. For the subsystem microcomputers used in these applications, we supply the 16-bit devices in the R8C/Tiny series that operate at 20MHz and are available with 16, 32, or 48Kbytes of internal flash memory.

The SH7147 and SH7142 devices operate at frequencies that range from 64MHz to 80MHz. They incorporate fast internal flash memory that can be accessed in a single clock cycle even at full speed, and can readily handle the processing requirements of the main microcomputers in EPS control systems. Also, the devices have two 8-channel 12-bit AD converter modules, of which six channels support simultaneous sampling. Compared to the SH7047 microcomputer that was previously popular for EPS applications, the SH7147 achieves twice the speed of that older device and offers a four-fold improvement in A/D converter resolution.

Maximum flash memory capacity of the SH7147 and SH7142 is a large 512Kbytes, and both are also available in 256Kbyte versions. A 384Kbyte version of the SH7147 is available, as well. The internal RAM sizes are 12Kbytes and 16Kbytes.

The SH7147 has a single-channel CAN interface, while the SH7142 has two CAN interface channels. The CAN interfaces are complimented by an SSU (synchronous serial communication unit) and SCI (serial communication interface). Thus, one of the strengths of these microcomputers is an ability to support a wide range of different types of communication.

Renesas plans to meet future requirements for solutions for main microcomputers for EPS applications by developing devices that operate at higher frequencies (120 to 200MHz), have more flash memory (up to 1Mbyte), and provide support for FlexRay in-vehicle networks (see Figure 1).

Airbags, which were designed from the very beginning to use microcomputer control, obviously must satisfy extremely stringent operating standards. When the sensor detects a collision, the gas generation unit is triggered to inflate the airbag so it can catch the passenger's body and absorb the energy of the collision. The sequence of operations is nearly instantaneous, occurring in only about 0.2 seconds.

During vehicle operation, the airbag control system has to analyze all of the different sorts of shocks to which the vehicle is subjected and then determine, within a fixed amount of time, whether or not the shocks indicate a collision. The detection time is very short, only a few hundred microseconds and the detection routine executes several hundred times per second. When the control system detects a collision (shock event) sufficient to necessitate triggering the airbag, the microcomputer must switch instantly to the operations that activate the gas generation unit.

To improve occupant safety, there has been a steady increase in recent years in the number of different types of collision sensors and airbags. Examples include curtain airbags and the side airbags used to protect against side-impact collisions. These safety features require that the airbag control microcomputers have a high level of processing performance. Also, they must generate low levels of electromagnetic noise to prevent interference with radio reception in the car stereo and other systems.

In the past Renesas has supplied the H8SX/1527 for this application, a device with a 32-bit H8SX CPU core, 48MHz operating frequency and NOR-type internal flash memory (see Figure 2). To accommodate the greater needs of advanced airbag control systems, we have developed the higher-performance microcomputers in the H8SX/1700 series. Sample shipments of the first model in this product line, the H8SX/1725, will commence from October 2007. ( Click here to see related article. )

The new H8SX/1725 microcomputer has a maximum operating frequency of 80MHz, 1.7 times faster than the previous solution, and its 256Mbytes of internal MONOS flash memory can operate at up to 100MHz. (MONOS is a Renesas developed flash memory technology that uses a 150nm process.) Thus, when the CPU is running at its full 80MHz speed it can access that flash memory in a single clock cycle for maximum processing performance. This is an important advantage in airbag control applications.

System designs based on the H8SX/1725 can reuse software developed for the H8SX/1527, another major advantage of the new chip. Also, the device's noise emissions in the FM radio band (70MHz to 110MHz) and VHF TV band (150MHz to 400MHz) have been significantly reduced, making it much less likely that interference with car radio or TV systems will be a problem.

Our roadmap for the H8SX product line includes plans for extending the number of RAM-size, ROM-size, and pin-count variations in the H8SX/1700 series. Also, a CAN interface will be added to ensure that the series can cover the requirements of low-end to high-end airbag systems.

For more information on SuperH microcomputers, click here.

For more information on H8SX microcomputers, click here.