Cruise Control Devices

             A company named Vorad Technologies has developed a system which measured up to one hundred meters. A low powered, high frequency modulated laser diode was used to generate the light signal. 

 

        Most of the current acc systems are based on 77GHz RADAR sensors.  The RADAR systems have the great advantage that the relative velocity can be measured directly, and the performance is not affected by heavy rain and fog. LIDAR system is of low cost and provides good angular resolution although these weather conditions restrict its use within a 30 to 40 meters range.

 

3.2 RADAR (Radio Detection and Ranging):

         RADAR is an electromagnetic system for the detection and location of reflecting objects like air crafts, ships, space crafts or vehicles. It is operated by radiating energy into space and detecting the echo signal reflected from an object (target) the reflected energy is not only indicative of the presence  but on comparison with the transmitted signal, other information of the target can be obtained. The currently used ‘Pulse Doppler RADAR’ uses the principle of ‘Doppler effect’ in determining the velocity of the target [5].

 

3.2.1 PULSE DOPPLER RADAR:

 

The block diagram of pulse Doppler radar is as shown in figure.2.

             The continuous wave oscillator produces the signal to be transmitted and it is pulse modulated and power amplified. The ‘duplexer’ is a switching device which is fast-acting to switch the single antenna from transmitter to receiver and back. The duplexer is a gas-discharge device called TR-switch. The high power pulse from transmitter causes the device to breakdown and to protect the receiver. On reception, duplexer directs the echo signal to the receiver. The detector demodulates the received signal and the Doppler filter removes the noise and outputs the frequency shift ‘fd’.

Fig2. Block diagram of pulse Doppler radar

     

3.2.2 EFFECT OF DOPPLER SHIFT:

 

              The transmitter generates a continuous sinusoidal oscillation at frequency ‘ft’which is then radiated by the antenna. On reflection by a moving object, the transmitted signal is shifted by the Doppler Effect by ‘fd’.

 

       If the range to the target is ‘R’, total number of wavelength is ‘λ’ in the two way- path is given by,                                   

                                                                              n = 2R/ λ

The phase change corresponding to each λ =2π

So total phase change, p=2n П                                         

      =2(2R/ λ) π

So, if target moves, ‘R’ changes and hence ‘φ’   also changes.

 

Now, the rate of change of phase, or the ‘angular frequency’ is

                                   W=dφ/dt =4 π (df/dt)/ λ

 

Let Vr be the linear velocity, called as ‘radial velocity’

Wd = 4 πVr/ λ =2πfd.

 

Fd=2Vr / λ

But λ = ft, the transmitted velocity.

    Fd= (2c Vr)/ ft

So by measuring the shift, Vr is found. The ‘plus’ sign indicates that  the target and the transmitter are closing in. i.e. if the target is near, the echoed signal will have larger frequency.

 

3.2.3 RADAR ANTENNA SCHEMES:

 

 Radar systems employ a variety of sensing and processing methods to determine the position and speed of vehicles ahead.  Two such important schemes are:

 

1.      mechanically steered antenna

2.      electronically steered antenna

 

1.      Mechanically steered antenna:

                      

            A parabolic reflector is used as mechanically steered antenna. The parabolic surface is illuminated by the source of energy placed at the focus of the parabola. Rotating about      its axis, a circular parabola is formed. A symmetrical beam can be thus obtained. The rays originating from focus are reflected parallel to the axis of parabola. [fig (3).]

Fig 3.Parabolic reflector antenna

 

1.      Electronically steered phased array radar antenna

           A phased array is a directive antenna made up of a number of individual antennas, or radiating elements. The radiation pattern is determined by the amplitude and phase of current at each of its elements. It has the advantage of being able to have its beam electronically steered in angles by changing phase of current at each element. The beam of a large fixed phased array antenna is therefore can be rapidly steered from one direction to another without mechanical positioning [1, 5].

 

            Consider the following figure with ‘N elements placed (equally separated) with a distance‘d’ apart. Suppose they have uniform response to signals from all directions. Element ‘1’ is taken as reference with zero phase.

Fig 4. Phased array elements (example: reception of the beams)

 

           From simple geometry, we can get difference between path lengths of beam1 and that of beam2   is x = d sinθ, where ‘θ’ is the angle of incidence of the beams. This gives phase difference between adjacent elements as Φ= 2π (d sinθ)/ λ, where ‘λ’ is the wave length of the signal. But if the current through a ferro electric element is changed, the dielectric constant ‘ε’ is changed since electron density is changed, and for an electromagnetic radiation,                                                   Φ   = 2πx / λ

                                   =2πxf/v,

                       here the velocity  v = f λ                                                                                                                                    = 1/ (√μ ε)

                    Hence Φ=2πxf (√μ ε).

                   

           So if ‘ε’ is changed ‘Φ’ also changes and inserting ‘N’ phase shifting elements to steer the beam, we can obtain an electronically steered beam.

 

           Regardless of the scanning mechanism the radars typically operate in the millimeter wave region at 76-77 GHz.

 

           The system should be mounted inside the front grille of the car as shown in figure (5). So its size is to be small. A typical radar produced by Delphi-Delco Electronic systems is having the size of two stacked paper back books(14x7x10 cm)[1].

 

3.3 FUSION SENSOR

 

            The new sensor system introduced by Fujitsu Ten Ltd. and Honda through their PATH program includes millimeter wave radar linked to a 640x480 pixel stereo camera with a 40 degree viewing angle. These two parts work together to track the car from the non-moving objects. While RADAR target is the car’s rear bumper, the stereo camera is constantly captures all objects in its field of view.

Fig5. A prototype of a car with fusion sensor arrangement

Fig 6.Block diagram of sensing and controlling process

 

           The image processor measures the distances to the objects through triangulation method. This method includes an algorithm based on the detection of the vertical edges and distance. Incorporating both the 16-degree field of view of radar and 40-degree field of view of camera enhances the performance in tight curves [4].

 

4. SPACE OF MANEUVERABILITY AND STOPPING DISTANCE

 

           The space of maneuverability is the space required by the driver to maneuver a vehicle. An average driver uses larger sideways acceleration while vehicle speed is low. If the curve radius of a possible trajectory is ‘r’ for a given velocity ‘v’ and sideways acceleration ‘ay’ ,then  r= / ay [2].so to get the required ‘r’ ,when ‘v’ is low, ‘ay’ is also to be low correspondingly. The stopping distance is given by, Ds = .5 u/ax + td u, where ‘u’ is the initial speed ‘td’ is the time taken by the system to receive and process   the sensor data and ‘ax’ is the acceleration of the vehicle .the figure shows the detection of edges of the preceding vehicles.

Fig 7.Detection of vehicle edges by the fusion sensor

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