Cruise Control systems (Automobile)


Cruise Control systems

Cruise control systems were first introduced during the mid 1960′s as a means of reducing driver’s fatigue on long motorway journeys. The system is an ideal example of a closed loop control system. Figure 30.46 illustrates this control system in the form of a block diagram. Cruise control allows the driver to set the vehicle speed and when this control is activated the speed of the vehicle is maintained automatically without touching the accelerator pedal. The system is composed of three main assemblies. (0 A driver’s switch park.
(ii) A throttle actuator unit.
(Hi) Cruise control ECU.
The system adjusts the throttle according to the measured speed of the vehicle. Since the vehicle speed should not be felt to be surging up and down, the reaction time of the system is very important. Several other facilities are now being added. For example allowing the speed to be gradually increased or decreased at the touch of a button. Most systems also store the last set speed and resume this speed again at the touch of a button. The following is the list of functional requirements for a good cruise control system. It should
(a) hold the vehicle speed at the selected value,
(b) hold the speed with minimum surging,
(c) allow the vehicle to change speed,
(d) deactivate the control immediately after the brakes are applied,
(e) store the last set speed, and
(/) contain built in safety features.
A cruise control system in block diagram.
Fig. 30.46. A cruise control system in block diagram.
Since the main switch switches on the cruise control, it is therefore ignition controlled. When the main switch has been turned off, most systems do not retain the speed setting in memory. The memory is programmed through the set switch, which normally works only when conditions similar to the following are satisfied.
(a) The vehicle speed is in between 40 kmph and 200 kmph.
(b) The change of speed is less than 8 kmph/s.
(c) The automatic transmission, if installed, must be in drive.
(d) Brakes or clutch are not being operated.
(e) Engine speed is stable.
Once the system is set during the operation, it maintains the speed within about 3-4 kmph until cruise control is deactivated by pressing the brake or clutch pedal, pressing the resume switch or turning off the main control switch. The last speed value is retained in memory, except when the main switch is turned off.
To revive the cruise control system again, either the set bottom can hold the vehicle at the current speed or the resume button can accelerate the vehicle to the previous set speed. When cruising at a set speed, if the driver presses and holds the set button the vehicle accelerates until the desired speed is reached and the button is released. When the driver accelerates from the set speed to overtake for example, then if the throttle is released the vehicle slows down until it reaches the last set position.

System Components

The description of the main components of a typical cruise control system is as follows. Actuator.
Various methods are adopted to control the throttle position. Vehicles with wire drive systems use the same actuator to operate the cruise control. The actuator unit is connected to the throttle valve and controls the throttle butterfly position under the command of the cruise control ECU. Actuator mechanisms normally use either a permanent magnet DC motor assembly (Fig. 30.47) or a vacuum diaphragm powered by a motor-driven pneumatic pump and controlled by solenoid valves (Fig. 30.48) or, in many cases, a vacuum operated diaphragm controlled by three simple valves (Fig. 30.49).
To increase the speed, valve (X) is opened so that low pressure from the inlet manifold comes in contact with one side of the diaphragm. The atmospheric pressure on the other side forces thfe diaphragm and hence moves the throttle. For movement of the throttle in the reverse direction, valve (X) is closed and valve (Y) is opened, so that atmospheric pressure enters the chamber. The spring moves the diaphragm back. When both valves are closed the throttle position is held. Valve (X) is normally closed and valve (Y) normally open so that in the event
 Throttle position control through a motor.
Fig. 30.47. Throttle position control through a motor.
Vacuum type throttle actuator.
Fig. 30.48. Vacuum type throttle actuator.
of electrical failure, cruise does not remain engaged and the manifold vacuum is not disturbed. Valve (Z) ensures extra safety, which is controlled by the brake and clutch pedals.

Main Switch and Warning Lamp.

The main switch of the cruise control system is a simple on/off switch fitted on the dashboard within driver’s approachability. The warning lamp can be part of this switch or part of the main instrument display, but neither case it should be within driver’s visibility.

Set and Resume Switches

The set and resume switches are located either on the steering wheel or on a stalk from the steering column. When the switches are installed on the steer­ing wheel, slip rings are used to transfer the connec­tion. The set button is used to program the speed into memory as well as to increase the vehicle and memory
speed. The resume button permits the vehicle to attain its last set speed or deactivates the control temporarily.

Brake Switch.

The brake switch is extremely important, since it is dangerous to brake while the cruise control system is engaged to maintain the vehicle speed. This switch is fitted in place or as a supplement to the brake light switch activated by the brake pedal. Time to time adjustment of this switch is necessary and also important.

Clutch or Automatic Gearbox Switch.

The clutch switch is installed in a similar manner to the brake switch. When clutch is pressed, it deactivates the cruise system to prevent increase of the engine speed. The automatic
Vacuum operated diaphragm controlled by two simple valves.
Fig. 30.49. Vacuum operated diaphragm controlled by two simple valves.
gearbox switch permits engagement of the cruise only when it is in the drive position. This prevents over speeding of the engine when the cruise tries to acceterate the speed with the gear selector in the 1 or 2 positions. The gearbox still changes gear while accelerating back up to a set speed as long as it recognizes the availability of top gear.

Speed Sensor

The speed sensor is often the same sensor used for the speedometer. The most common sensor produces a pulsed signal frequency, which is proportional to the vehicle speed.

System Operation

The electrical circuit diagram of a cruise control system using a vacuum diaphragm throttle actuator is shown in Fig. 30.50. With the vehicle running above a minimum cruising speed (typically 40 kmph) the cruise control ON switch is pressed, and the system becomes operational. When the vehicle attains the desired cruising speed, the SET button is pressed. The ECU’s speed control microprocessor then operates the vacuum pump, which in turn moves the throttle valve actuator diaphragm until the SET speed is maintained without the use of the accelerator pedal so that the driver can remove his foot from the pedal. The microprocessor continuously monitors the vehicle speed signal and constantly changes the throttle position taking account of variations in road gradients, wind resistance etc. and thereby maintains the memorized cruising speed. To increase speed, the pump is operated for a short time to increase the vacuum, and to reduce speed the control valve is pulsed open to reduce the vacuum slightly.
In order to increase the cruising speed of the vehicle, the SET button is held down so that the ECU commands the vehicle to smoothly accelerate until the switch is released or maximum
Cruise control system electrical diagram.
Fig. 30.50. Cruise control system electrical diagram.
cruising speed is attained. At the instant the set switch is released the new cruising speed is stored in the microprocessor memory. To overtake another vehicle, the accelerator pedal is pressed in the normal way to increase speed. When the pedal is released the cruise control automatically takes over, and the system returns to the memorized speed.
Once the brake pedal is pressed,-the microprocessor detects the closure of the stop lamp switch and immediately opens the pressure release valve (dump valve) to rapidly decrease the vacuum to disengage the system. As a safety measure, the brake pedal uses a additional pair of switch contacts, which open to disconnect the positive supply from the solenoid valves, allowing them to open. This action is also carried out on manual transmission vehicles by using a switch on the clutch pedal. On automatic vehicles a relay contact closes the circuit when the selector lever is in DRIVE. To reengage the system the RESUME (RES) switch is pressed so that the ECU operates the vacuum pump to restore the previously memorized SET speed.


Electronic Control Unit (ECU)

The cruise control ECU microprocessor takes input from the switch park and various sensors to find out the vehicle operating parameters. It then sends out signals to the throttle actuator to regulate the throttle valve position so as to maintain the memorized cruising speed.
A block diagram of a cruise control ECU is shown in Fig. 30.51. Normally cruise control systems work on the proportional-integral control technique. In this control technique an error signal is generated through the feedback loop. This error signal is proportional to the difference between the required and actual outputs. The final output of a cruise control system is the vehicle speed, but this depends on the throttle position, which is controlled by the actuator. The system electronics account for the lag between throttle movement and the required change in vehicle speed.
Early cruise control ECUs incorporated analogue circuitry, and most modern system are totally digital, using at least an 8-bit microprocessor. The microprocessor constantly samples the road speed signal and compares it with the SET speed stored in memory. When a difference is found, a proportional integral (PI) control algorithm generates drive pulses for the pump and valve such that the SET speed is quickly restored without overshoot or oscillation.
When the system overreacts in either way, the vehicle speed becomes too high or too low. In other words the system is not damped correctly (under-damped) and oscillates, much like a suspension spring without a damper. Proportional control alone cannot handle this problem,
Block diagram of a cruise control ECU.
Fig. 30.51. Block diagram of a cruise control ECU.
because of steady state errors in the system.a nd to solve the problem integral control is included. There­fore, the final signal is the sum of proportional and integral control signals. An integral controller produces a signal which is a ramp, increasing or decreasing, proportional to the original error signal. The integral control causes the final error signal to approach towards zero. Therefore the combination of these two forms determines the damping factor of the control electronics. Figure 30.52 shows the effect on vehicle speed of different damping factors. These four responses can be modelled mathematically to calcu­late the response of a system.
The above technique can be based on either analogue or digital electronics and the principle is much the same. The theoretical valves can be evaluated prior to circuit design as follows;
Effect on vehicle speed control of different damping factors.
Fig. 30.52. Effect on vehicle speed control of different damping factors.

Autonomous Intelligent Cruise Control (AICC) or Adaptive Cruise Control

Conventional cruise control has now been improved substantially, which works well in steady traffic conditions and maintains constant speed. But it is not very practical on many occasions as the speed of the general traffic is constantly varying. In such cases the driver has to take over from the cruise control system to speed up or slow down the vehicle. AICC is a concept that eliminates this drawback by allowing the vehicle speed to continually change to the traffic flow and hence maintains a safe following distance. The system has three main functions :
(a) To maintain a speed as set by the driver.
(6) To adapt this speed and maintain a safe distance from the vehicle in front, (c) To provide a warning if there is a risk of collision.
AICC needs information on the range and velocity of vehicle immediately in front of it. The major components of basic as well as more complex adaptive cruise systems are shown in Fig. 30.53. The main extra components are the head-way sensor and the steering angle sensor, the former is obviously the most important. Information on steering angle is used to further support the data received from the head-way sensor by permitting greater discrimination between hazards and spurious signals. Two types of the headway sensor considered are radar and lidar. Both incorporate transmitter and receiver unit. In the radar system microwave signals at about 35 GHz, is transmitted from the front of the car and then reflected back to the transmitter by obstruction in is path. Velocity and range information is obtained by measuring the radar signal’s Doppler frequency shift and reflection delay respectively. The reflection time of these determines the distance to the object in front. In the lidar system a laser diode produces infra red light signals, the reflections of which are detected by a photo diode.
Both these sensors have advantages and dis­advantages. The radar system is not affected by rain and fog. The lidar is more selective by recog­nizing the standard reflectors on the rear of the front vehicle. Radar produces strong reflections from bridges, trees, posts and other normal road­side items. It can also suffer loss of signal return, due to multi-path reflections. Under ideal weather conditions the lidar system seems to be the best, but it becomes very unreliable when the weather changes. A beam divergence of about 2.5 degrees vertically and horizontally has been found to be the most suitable for both the head-way sensors. It is important that the signals from other vehicles fitted with this system must not produce erroneous results. There are two basic types of head-way radar; pulse and frequency modulated continuous wave (FMCW).

Pulse Radar.

These systems send out a microwaves pulse at a fixed frequency. The time taken for the pulse to reach an obstacle and bounce back to the transmitter is proportional to the range of the obstacle. If the echo returns after a time (t) then the range (R) of the obstacle is given by R – ct/2, where ‘c’ is the speed of the radar waves (speed of light). If there is a difference in speed between the car-mounted transmitter and the obstacle, then the returning radar signal has a slightly altered frequency (i.e. a Doppler frequency shift). This frequency change can be converted into speed information by using signal processing techniques.

FMCW Radar.

In this system a continuous radar signal is sent out, the frequency of which is repeatedly swept either side of a centre frequency. During the time taken for the transmitted waves to reach an object and bounce back, the transmitter frequency changes slightlj Mixing the two signals provides a beat frequency, which is proportional to the distance of the target. Together with Doppler shift detection the strategy provides both range and the speed informa­tion for the AICC ECU.
The principle of operation of an adaptive cruise system is the sair : as that of a conventional system, but when a signal from the head-way sensor detects an obstruction the vehicle speed is decreased. If the optimum stopping distance cannot be achieved just by backing off the throttle, a warning is supplied to the driver. The more complex system also controls the transmission and brakes. It is to be noted that the adaptive cruise control is designed to relieve the burden on the driver, not take full control of the vehicle.
In 1960, the Lucas company fitted an experimental 24 GHz radar system to a Ford Zodiac to give information on the range and speed of traffic in front of the vehicle. The problem of false alarms on bends was encounted, where fixed objects like trees and road signs gave unwanted radar signals.
The problem has been overcome in latter designs by adopting sophisticated digital signal producing technology with a higher frequency (77 GHz), which provides better resolution of the target. A contemporary system typically uses three radar antennae, mounted behind the plastic
Main components of adaptive cruise system.
Fig. 30.53. Main components of adaptive cruise system.
number plate at the front of the car, to give a 75 mm x 225 mm microwave beam (Fig. 30.54). These radio signals are reflected by other cars and fixed obstacles and then fed to the ECU (Fig. 20.55), which analyses the road situation approximately twenty times each second, taking into account the vehicle’s own speed and steering angle. Since only the events in the vehicle’s path are considered the obstacles such as trees and signposts do not provide error messages.
Typically, such a system has a detection range of up to 150 m and can resolve an object’s position and speed to within 1 m and 1 kmph respectively. This speed and distance information is supplied to the AICC ECU, which operates a conventional throttle actuator to maintain a present safe following distance behind the object in front of the vehicle. In the event of a likely collision the ECU is able to provide audible and visual warnings, enabling the driver to take evasive action.
Microwave beam geometry.
Fig. 30.54. Microwave beam geometry.
Obstacle detection i collision avoidance system used with AICC.
Fig. 30.55. Obstacle detection i collision avoidance system used with AICC.

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