Adaptive Cruise Control (ACC)

It's a mechatronic system that controls the velocity of the car and distance of the car from the certain object or car in-front of it. The goal of ACC is a partial automation of the longitudinal vehicle control and the reduction of the workload of the driver with the aim of supporting and relieving the driver in a convenient manner. ACC cars has the radar sensor fitted on the front bumper that emits waves when these waves are reflected back it senses the car or object in-front,its distance and its velocity

In such type of cars the drivers can set the velocity of the car and distance from the front car or object (input to control unit) than accordingly the car holds the set velocity (ACC working in cruise control or velocity control mode) as long as the any object or car may not be detected by the radar sensor. When the object or car is detected, the ACC automatically shifts to distance control mode immediately.

Working of  Adaptive cruise control (ACC)

The ACC is connected to following systems

• Engine's ECU(Electronic control unit):- If ACC want car to go faster it sends command to engine ECU and the angle of the throttle changes.
• Electronic stability programme (ESP):-If the ACC detect that the distance between your car and front car is smaller than the distance set by driver than ACC sends command to ESP and ESP increases the fluid pressure which leads to braking.
• Gear ECU:-ACC can change the gear whenever needed or If the ACC operates in a broad velocity range, a connection to an automated manual or to an automatic transmission is useful.

Calculation of throttle angle

The ACC has acceleration limits for braking and acceleration which are dependent on velocity and are standardized by ISO (the International Organization for Standardization). The standard acceleration is entered into the fundamental equation of longitudinal dynamics then ACC has to estimate rolling resistance, aerodynamic resistance, grade resistance & inertial resistance to enter them into fundamental equation of longitudinal dynamics. The moment of engine is thus calculated which than enters to the characteristic map of the engine.

Why estimation??

·         Grade resistance estimation-The accelerometer can be used to measure the acceleration on slope but it cannot distinguish between the acceleration of car and gravitational acceleration of the car so need an estimation.

·         Aerodynamic resistance estimation-The ACC do not know the speed of the wind so estimation is needed.

·         Rolling resistance-The ACC do not know the pressure in the tyres and type of tyre if tyre pressure sensor is used ACC could know the tyre pressure but tyre type is still unknown so the exact rolling resistance is unknown.
Acceleration resistance-ACC does not know the exact payloads on the vehicle.

Mathematically, Fundamental equation

Me*ig*id/R=Fr+Fa+Fg+mλa

Me=R/ig*id(Fr+Fa+Fg+mλa)

Me-Moment of engine

R-static radius of tyre

ig-transmission gear ratio

id-differential gear ratio

Fr-rolling resistance

Fa-aerodynamic resistance

Fg-Grade resistance

Fi-inertial or acceleration resistance(its known from ISO) 

Characteristic map of the engine

Me is the moment of engine needed to overcome all the resistance at acceleration limit ‘a’ defined by ISO. This engine moment at some defined rpm (angular velocity) of engine enters the characteristic map to give the throttle angle as shown in graph above

Example-ISO 15622:2010 contains the basic control strategy, minimum functionality requirements, basic driver interface elements, minimum requirements for diagnostics and reaction to failure, and performance test procedures for Adaptive Cruise Control (ACC) systems

Acceleration limits,e.g ISO 15622 or ISO 22179

In the graph(standardized by ISO),the limit of acceleration and braking are shown which are dependent on the velocities. The acceleration is higher at lower velocities and retardation is higher at lower velocities.

Working algorithm of ACC (Distronics)

Measurement of distance and relative velocity

Frequency modulated continuous wave-FMCW is used

The measurement of distance to front car and relative velocity between front car and your car is done by the use of radar system of frequency of 76.5 GHz.The radar system emits the wave which propagates to front car and reflects back to radar.

Two effects are measured

1. The propagation time is calculated for measuring distance(D=c*t/2).
2. Frequency change due to Doppler effect for measuring relative velocity(Vr=Δf*c/2fs)

D-distance between front car and your car

c-speed of light=3*10^8 m/s

t-propagation time from sending till receiving the wave back

Vr-relative velocity between front car and your car

Δf-change in frequency of reflected wave due to Doppler effect

fs-sending frequency of wave

Calculation of frequency shift

When the reflected signal of different frequency due to doppler effect interferes with the the transmitted signal forming the resultant wave this is known as beat. As the signal is always sweeping through a frequency band, at any moment during the sweep, the frequency difference,  , is a constant between the transmitted signal and the received signal.  is usually called the beat frequency (resultant wave frequency)which is small frequency.FMCW radars estimate the target range using the beat frequency.

Low pass filter

As the beat frequency is small so low pass filter is used to reduce the amplitude of the signals with frequency higher than cutoff frequency.

Fast fourier transform FFT

It converts frequency into time after passing beat frequency through low pass filter.

hence finally the Δf frequency shift can be calculated.

Don't forget to comment