Part 1 discussed "surround sensing" and frequency modulated continuous wave (FMCW) radar as the basis of an Adaptive Cruise Control (ACC).
How ACC workssystem hardware
A Gunn oscillator (Gunn VCO) is often used to generate the very high frequency transmit signal. If combined transmit and receive antennas are used, the transmit signal is multiplexed with the receive signal by means of a circulator (below). The receive signal is combined with the current transmit signal. The differential signal thereby arising is designated the intermediate frequency. The intermediate frequency is a much lower frequency than the transmit and the receive signals. Sample values of the intermediate frequency are therefore highly suitable for further processing with digital processors.
While the ACC radar sensor works in the high frequency range (radio frequency, RF), the signal processing for calculating the distance and the relative speed takes place in the low frequency (LF) range. The figure below shows the system block diagram for an ACC system. The RF part (left) consists of the Gunn control, the Gunn oscillator, a mixer, and a preamplifier. The LF part comprises an analog-digital converter, the device for signal processing and system control, as well as the power supply and the network interface with the car.
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The microcontroller (TMS470R1VF76B from Texas Instruments) has two CPUs, an ARM7 RISC (microcontroller, MCU) and a C54x 16-bit fixed-point digital signal processor (DSP). It is therefore ideally suited to applications that must perform both control tasks and high-performance digital signal calculations. Communication between the two CPUs, the numerous peripheral interfaces, and the memory can also be accelerated using direct memory access, DMA. The TMS470R1VF76B meets automotive requirements and is well suited for an ACC system. The figure below shows the block diagram of the microcontroller with a typical division of the ACC application tasks.
ACC system software
In addition to the usual diagnostic tasks, the following system tasks belong to the ACC system. The sequence of these system tasks is annotated in the ACC system block diagram below this list.
1. Read in the control presets entered via the HMI (speed, time interval) and the current driving-specific parameters detected by sensors (steering angle, wheel speed, yaw rate, etc.)
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2. a) Set the frequency ramps to be transmitted (start frequency, end frequency, ramp time)
b) Set the A/D converter (conversion rate, number of samples)
3. Set the transmit frequency and start the Gunn VCO
4. Generate the transmit signal
5. a) Simultaneous transmission of the transmit signal via all antennae and mixing of the intermediate frequency
b) Control loop for Gunn control
6. Filtering and amplification of the intermediate frequency
7. Sampling of the intermediate frequency
8. DMA transmission of the samples to the DSP
9. Digital Signal Processing (part 1 of the frequency modulated continuous wave (FMCW) radar tasks)
10. Exchange of the data calculated in the DSP
11. Digital Signal Processing (part 2 of the FMCW radar tasks)
12. Communication via the car network (CAN bus) with the electronic control units (ECUs) to adjust the speed or distance