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High-speed measurements for electric and hybrid vehicles

High-speed measurements for electric and hybrid vehicles

Technology News |
By eeNews Europe



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% at 1 Mbyte/s.

Data trace concept for Nexus Class 3 – current Freescale PowerPC

Most devices of the current Freescale PowerPC series support the data trace method of Nexus Class 3. In this case, the developer configures one or two monitoring windows with a maximum total size of 512 kByte in the ECU RAM. Any changes within these monitoring windows are transmitted to the POD via Nexus Class 3 without any additional CPU load. Transmission rates for raw data of up to 100 MByte/s are possible over the High Speed Serial Link cable. The advantage of this concept is that the base module for measurement data always contains a consistent mirrored RAM of the ECU’s RAM. An ECU software trigger interrupts the data flow within the measurement data base module, where new changes are saved in a First In, First Out (FIFO) buffer in RAM. The measurement is initiated by one of up to 256 different software triggers, and the contents of the mirrored RAM are “frozen”. Based on the measurement configuration, the signals are read out from the mirrored RAM in the base module for measurement data and are sent to the measurement and calibration tool over XCP on Ethernet (Figure 2).

Figure 2: Data flow concept of measurement signals by the data trace concept and Nexus Class 3 interface

Advantages of the Nexus Class 3 solution:

  • The maximum measurement data rate of 30 Mbyte/s is a factor of 30 times larger than with Nexus Class 2+ and 600 times larger than with XCP on CAN.
  • The CPU is typically not loaded by the measurement.

All PWM drive signals can be measured at the 100 kHz sampling rate without any problems.

The disadvantage of this solution lies in the fact that significant effort is involved in connecting the POD with its 25 pins to the microcontroller, and it must process a very large raw data stream of 100 Mbyte/s.

Data trace concept for next generation microcontrollers

The main disadvantage of the Nexus Class 3 solution will be eliminated in next generation microcontrollers, because the pin count has been reduced from 25 to 5. However, the measurement data rate and sampling rate will remain at the same unchanged high level. This data trace solution will also be supported by future processors from the Infineon and Freescale companies. The raw data stream von 100 Mbyte/s must still be processed.

Data trace concept for the current Infineon TriCore

A concept comparable to Nexus Class 3 may also be used for DAP. This involves reserving a 256 kByte memory range of the ED-RAM (Emulation Device RAM) for measurement data acquisition. In contrast to the 100 MByte/s of the Nexus Class 3 concept, the trace transmission rate for raw data must be limited to 5 MByte/s; just 4 pins suffice instead of 25 pins. A maximum of four RAM monitoring windows may be configured. They must be configured so that there is no overrun of the trace data. Generally, this permits monitoring of just 10–20 kByte of memory instead of 512 kByte and measurement of signals in this memory without processor loading. Signals outside of these trace monitored memory areas can be measured by the RAM copy method.

Advantages of the Infineon DAP data trace solution:

  • The maximum measurement data rate of 3 Mbyte/s is a factor of 3 larger than in the RAM copy method.
  • The microcontroller is not loaded by the measurement.
  • All known PWM drive signals can be measured at a 100 kHz sampling rate without any problems.

Data trace concept for future Infineon controllers

In the next generation of microcontrollers, Infineon is also offering the latest generation Device Access Port (DAP). One advantage lies in its higher raw data transmission rate, which is now 20 MByte/s in contrast to the previous 5 MByte/s. This is attained by the higher frequency of 160 MHz at the DAP interface instead of the previous 80 MHz and by a new type of three-line concept, which permits parallel transmission on two lines.

The greatest improvement to the DAP2 interface is that it now lets users set up hardware-based data trace filters with extremely fine granularity. This significantly reduces the transmission of unnecessary data trace information from the microcontroller to the POD. Despite the maximum measurement data rate of 10 Mbyte/s, it is only necessary to process 15 instead of 100 Mbyte/s of raw data (Figure 3). Due to the considerably reduced requirements for processing the measurement data, cost-optimized measuring instrumentation can be used for DAP2.

Figure 3: In the data trace concept, fine grain filters reduce the raw data stream to 15 Mbyte/s over the DAP2 interface.

Summary

Many aspects of modern drive concepts for vehicles with pure or hybrid electric motors make it necessary to develop new strategies for measurement data acquisition. Existing measuring instrumentation concepts for internal ECU signals often reach their limits in terms of data rate or sampling rate. The sampling rates of up to 100 kHz that are necessary for electric drive systems can be implemented for existing and future microcontrollers using the VX1000 measurement and calibration hardware from Vector. Over the course of this year, new controller generations will be available from Freescale and Infineon, which can perform their tasks with a data trace that requires significantly fewer connection pins. In combination with the high-speed VX1131 measurement module from Vector – which will be available in the second half of 2012 – they will enable measurement data rates of 30 Mbyte/s without CPU loading.

In the case of Infineon, DAP2 with finely granulated signal filters in the microcontroller make it possible to reduce the raw data stream from 100 to 15 Mbyte/s, which permits the use of very cost-efficient measurement hardware to achieve high data rates. When used with the ASAM-standardized XCP on Ethernet as the PC interface, the measurement and calibration hardware is also ideal as a flexible and powerful bypass solution with short latency times.

About the author:

After graduating from the University of Applied Science Esslingen with a degree in Electrical Engineering, Alfred Kless initially worked for Alcatel where his roles included team leader for software development and business development of test systems.

Since May 2004, he has been employed at Vector Informatik in Stuttgart as Business Development Manager for the product lines “Measurement & Calibration” and “Network Interfaces.”

All figures: Vector Informatik GmbH

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