Cruise control relieves the driver from the task of maintaining a constant speed over an extended period of time, for example when driving on a motorway. The system can maintain a particular speed irrespective of gradient, headwinds etc. thus reducing the possibility of driver fatigue.
It is becoming a common accessory in modern vehicles. This is because most vehicles have electronic controls already capable of performing this feature. No additional parts are required, unlike in the past where additional actuators were needed to be fitted to the
throttle assembly and additional cruise control units had to be installed in the vehicle. Modern cruise control systems perform the function with existing components and the cruise controller is merely a software extension in the engine management system.
The system works as a closed-loop controller for vehicle speed (see Chapter 2). The desired speed is set by the driver and the actual speed is monitored by the system via vehicle speed sensing. Any deviation is fed back to the controller as an error. The controller then adjusts the throttle position to compensate and try to maintain the desired speed. A diagram of the process is shown in Figure 7.26.
Figure 7.25 Radio transmitter system
7.4
DRIVER COMFORT AND ASSISTANCE
Figure 7.26 Control loop – cruise control system
164 Comfort and control systems Fundamentals of Motor Vehicle Technology: Book 3
The main system components are:
● Controls: mounted near or on the steering wheel, this selects on/off and operation mode. Typically: – activate/set: this sets the current speed as the
desired target speed. It can also sometimes have up/down to increase/decrease the desired target speed;
– off: this switches off cruise control but retains the last set speed in memory;
– reactivate: the vehicle returns to the last-set speed.
● Vehicle speed sensor: most vehicles have an
electronic speedometer with a sensor that can be utilised. Alternatively, ABS wheel-speed sensor signals could be used.
● Cruise control unit: commonly an engine/powertrain
ECU function in modern vehicles.
● Throttle actuator: this is not necessary for drive- by-wire (electronic throttle or EGAS (electronic gas pedal)) systems; older systems used pneumatic or electrical actuators.
● Brake and clutch switches: if either pedals are operated, the cruise control operation is suspended until the driver operates the reactivate command via the control lever.
Figure 7.27 shows how the components integrate within the cruise control system.
The speed signal is processed and compared with the desired value. This gives the error signal for the control loop. The acceleration and speed controller process this signal to generate the throttle position demand which is applied to the throttle actuation control system.
The system is deactivated by the switch-off logic circuit if the driver lever ‘off’ switch, clutch or brake are activated. Also, this happens automatically under the following conditions:
● minimum speed threshold is crossed (approximately
20 mph)
● the speed difference threshold becomes too great between actual and desired speed.
In addition, operation of other vehicle dynamic control systems will deactivate cruise control operation, e.g. stability control activation. When the reactivate control is given, the vehicle accelerates to the last speed-set point and then the speed controller takes over to maintain constant vehicle speed. If the driver accelerates beyond the set speed and then releases the throttle, the vehicle will gently decelerate to the set speed at which point the cruise control resumes speed control. The system is not deactivated under these conditions.
New developments – adaptive cruise control This technology allows an addition to the basic cruise control system by introducing the ability to automatically adjust the vehicle speed to maintain a set distance to the vehicle in front according to that speed. That is, the vehicle is capable of safely following a vehicle in front that is travelling at a slower speed than the set speed of the cruise control system.
If the vehicle in front is travelling at a constant speed, a car with ACC (adaptive cruise control) can follow at the same speed and constant distance because the distance is proportional to speed. The constant time gap is equal to the time required for the front of the car with ACC to reach the position of the rear of the vehicle in front and can be expressed by:
Time gap = Clearance ACC vehicle speed
Switching between normal cruise control condition and ACC modes is done automatically by the system which can adapt immediately to changing traffic conditions, e.g. a car pulling into the gap in front and reducing the distance between vehicles.
The ACC system uses a microwave radar transceiver to measure the distance to the vehicle in front. This sensor is mounted at the front of the vehicle (behind the bumper) and includes all of the necessary control electronics and processing power. It is therefore known as the ACC sensor and control unit. Apart from this sensor, the other inputs and outputs in addition to the normal cruise control system are:
● engine management system with electronic throttle/ fuel injection control
● electronic brake control system with pressure-
increase capability (ESP).
A system overview is shown in Figure 7.29.
The ACC sensor detects the distance to the vehicle in front using radar. The fundamental principle is that the time between transmission and reception of the radar beam is measured, and from this the distance can be calculated (known as ‘echo timing’). If the target object is moving, the echo signal undergoes a frequency shift relative to the transmitted signal and this gives rate of change of distance information (Doppler effect). The ACC sensor/controller uses this information to calculate distance to the vehicle in front. The sensor is shown in cut-away view in Figure 7.30.
The ACC system is integrated with the other sensors and actuators fitted to the vehicle via the CAN bus. It uses existing hardware and this reduces installation and implementation costs.
ACC vehicle Target vehicle Clearance
(time gap = clearance/vehicle speed)
Forward vehicle
Figure 7.28 Adaptive cruise control – time gap
Figure 7.29 ACC – structure and components of system (Bosch)
166 Comfort and control systems Fundamentals of Motor Vehicle Technology: Book 3
The system interfaces to the drive train control via the engine control/throttle (and can thus control engine/ vehicle speed) and also with the brake control system (ABS, ESP) in order to be able to reduce the vehicle speed if necessary. In addition, yaw sensors, wheel- speed sensors and steering angle sensors give the ACC system information about the vehicle positional dynamics (see Figure 7.31).
The main operational features of the ACC system are:
● Cruise control: operation as a standard cruise control system.
● Constant gap: determination of gap and maintenance of this calculated by the ACC sensor. Intelligence built into the sensor selects the target vehicle. The required distance is calculated and then implemented by the system. The driver can also adjust the time gap according to the driving conditions and this is done via selection buttons in the steering wheel.
● Bend detection: the ACC can take into account bends in the road (to a limited degree). Adjustments are made to the speed and acceleration characteristics to compensate for this.
ACC is a clear step forward in the development of systems to improve the safety of the driver by taking over tasks that require high levels of concentration for extended periods of time. It is likely that as the technology improves, the system will evolve further and progress towards providing an autopilot mode for the driver that can react more quickly and drive more safely than a human being!