High-speed measurements for electric, hybrid vehicles
Alfred Kless, Vector Informatik
3/11/2013 3:40 PM EDT
In the development of electric and hybrid vehicles, in particular, the requirements for instrumentation used to measure internal ECU signals are very high. Nonetheless, measurement data rates of up to 30 Mbyte/s and the necessary sampling rates of 100 kHz can be achieved with the latest generations of microcontrollers and an intelligent measuring instrumentation solution. The ECU’s CPU is not loaded here.
The drives of electric or hybrid vehicles are generally controlled by pulse-width modulation (PWM) signals. The advantage of PWM technology is that it incurs very low power losses at power switches, because they only need to be operated in two operating states: fully conducting or fully blocking. The frequency of the PMW signals typically lies in the 10 – 20 kHz range, and in exceptional cases up to 100 kHz. Maximum sampling rates of only 1 kHz are achievable for internal ECU signals when XCP – a widely used standardized measurement and calibration protocol for vehicle development – is used together with communication over the CAN or FlexRay bus system. PMW signals cannot be acquired in this method.
That is why the debug and data trace interfaces are used for fast access to ECU variables. These interfaces can vary significantly depending on the type of microcontroller that is implemented. The measurement hardware is interfaced to the ECU over a “Plug-On Device” (POD). The maximum allowable distance between the microcontroller's debug pins and the POD is 10 cm. Communication between the measuring instrumentation module and the test PC is over XCP on Ethernet in accordance with the MCD-1 XCP standard from ASAM. The physical connection is made by a standard CAT-5 Ethernet cable. Essentially, two different measurement methods are distinguished: the “RAM copy method” and the “data trace method.” They are presented in this article, together with their advantages and disadvantages, based on current microcontrollers and new microcontrollers that will be available soon. The different data trace methods refer to two types of 32-bit microcontrollers that are primarily used in powertrain ECUs and their successors: Freescale PowerPC (primary market: USA) and Infineon TriCore (primary market: Europe).
RAM copy method
The RAM copy method is a generic method, and can be used for current and future generations of 32-bit microcontrollers from various manufacturers. For the Infineon TriCore or XC2000, access is via the Device Access Port (DAP) interface; for the PowerPC devices from Freescale or V850 E2 processors from Renesas, access is via the Nexus Class 2+ interface. In this method, the ECU software initiates a RAM copy function according to the cycle time of the various ECU tasks. The measurement signals must be preconfigured over XCP on Ethernet. The mapping of signal names and RAM addresses is described in an A2L file (ASAM standardized ECU description file for signal-oriented RAM accesses). Once all measurement signals have been copied, the signals are transmitted to the base module for measurement data according to the existing debug interfaces (Figure 1). This concept is referred to as “Online Data Acquisition” (OLDA).
Figure 1: Data flow concept for measurement signals by the RAM copy method and Nexus Class 2+ interface
Compared to CAN, the measurement data rate and sampling rate are improved by a factor of 20, i.e. 0.5 to 1 Mbyte/s of measurement data can be acquired with a sampling rate of 10 – 20 kHz. The copying operation loads the CPU approx. 4 percent at 1 Mbyte/s.