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Cruise Control: How It Works One option that has become popular on cars in recent years is the cruise control. Here’s a quick rundown on how they work. By JULIAN EDGAR Cruise control systems are now widely used in cars. A cruise control allows the driver to select a speed, with the system then maintaining that speed irrespective of gradient or aerodynamic loadings. Cruise controls have benefits in reducing fuel consumption, decreasing driver fatigue, and – sometimes – avoiding speeding tickets! All cruise control systems compare the actual vehicle speed with the speed set in the system’s memory. A signal is then transmitted according to the difference between the two. This signal is used to control an actuator linked to the throttle butterfly, with the throttle being opened or closed as appro­priate. Fig.1 shows the layout of a typical Aftermarket cruise control systems generally use a magnetic pickup sensor to determine vehicle speed. The magnets are typi­cally attached to the tailshaft or to the transaxle, 4  Silicon Chip cruise control sys­tem, in this case from a Subaru. The major input signal is derived from the speed sensor. Depending on the car, this sensor can be located on the tail­shaft, within the transmission or within the speedometer. The location of the sensor will depend on its design and whether the system is an aftermarket unit or one designed and fitted by the vehicle’s manufacturer. Aftermarket cruise controls generally use an inductive pulse sensor, whereby bar magnets are attached to the tailshaft and a pick-up coil is positioned close to the rotating shaft. Fig.2 shows an example of this type of sensor. It generates a waveform whose frequency is proportional to the car’s speed. By contrast, original equipment Hall Effect sensors (Fig.3) are usually mounted on the transmission and generate a square-wave output. Yet another scheme uses optical sensors mounted within the speedometer assembly, or a reed switch excited by the speedometer drum can be used to make and break the cir­cuit. Other input signals to the ECU are also used. An engine rpm signal is derived from the engine management system in some cars, while brake and clutch position indicators (usually simple switches) and automatic transmission status inputs are also utilised. The latter are used to disable the cruise control function if the brake or clutch pedals are depressed, or if the transmission is shifted into neutral. Electronic control unit Fig.4 shows the layout of a typical Bosch cruise control ECU. During op- Fig.1: the cruise control system used in the Subaru Liberty is typical of current designs. The electronic control unit receives inputs from a number of sensor and activates sole­noid-operated pressure control valves to permit the engine vacuum to control the actuator. The actuator in turn controls the throt­tle valve via a cable. Note that the cruise control throttle cable operates in parallel with the cable from the accelerator pedal. eration, the speed sensor (1) provides an AC vol­tage signal to the evaluation circuit (7), which is a frequency to voltage converter. The actual speed signal is then compared with the set speed stored in the memory (12). Once the Activate/Set button (2) is switched, the speed at which the vehicle is travelling when the button is pressed is stored digitally in the set-speed memory (12). Older systems used capacitor storage of the set-speed December 1994  5 Fig.2 (above): aftermarket cruise controls often use an inductive speed input sensor. This comprises magnets attached to the driveshaft which then spin past a pick-up coil. In systems employing a Hall Effect speed input sensor (Fig.3, right), the device is usually installed on the gearbox. but the more-modern digital approach has advantages in terms of ease and precision, particu­larly when it comes to long-term storage. A control circuit (8 & 9) acts on the comparison between the actual and set speeds. The acceleration controller (8) acti­vates when the car is travelling more slowly than the set speed. The speed controller (9), operates within the control range. If the speed is within the control range, the position controller (10) receives a signal which is proportional to the deviation between the set and actual speeds. This deviation is the refer­ence input signal for the electromagnetic actuator used in this system. The potentiometer (18) registers the position of this actuator, giving closed-loop feedback. The actuator (17) is driven by the output stages (11). Should the brake (5), clutch (6) or Off switch (4) be acti­vated, then the cruise control is disabled. It is also disabled if the car’s speed drops below the Vmin (minimum velocity) threshold (14), or if the rate of speed change (ie, acceleration) exceeds a preset value. In the Subaru Liberty, this preset accel­eration value is 25 km/h per second. Fail-safe functions Most cruise controls use a vacuum operated servo to open and close the throttle butterfly. These photos show two examples. 6  Silicon Chip The Bosch unit discussed above does not have extensive fail-safe functions. However, current units are designed so that a breakdown (eg, of a component) will not cause a dangerous situation to develop; eg, if an erroneous circuit or switch operation is sensed, then the cruise control will be switched off or the memory speed cancelled. One of the conditions which would cause this to occur is if the actuator’s output signal was on for at least a second – something which would not normally happen. (1) SPEED SENSOR (7) (8) EVALUATION CIRCUIT V1st/Vact FINAL CONTROLLING ELEMENTS ACCELERATION CONTROLLER (10) (17) (11) POSITION CONTROLLER M OUTPUT STAGES (9) SPEED CONTROLLER (18) CLOCK SIGNAL (13) STEERING COLUMN SWITCH (2) ACTIVATE SET Vmin THRESHOLD (15) DISCONNECT LOGIC AND RELAY (14) DIGITAL SET-SPEED MEMORY Vset v THRESHOLD (19) (3) COUPLING RE-ACTIVATE (4) OFF (20) (5) DRAG SWITCH BRAKE (6) (21) SAFETY CIRCUIT CLUTCH Fig.4: this diagram shows the basic circuit elements of the electronic control unit in the Bosch cruise control. The cruise control is deactivated immediately if the brake or clutch are operated. RELAY Self-diagnostics are incorporated into some ECUs. In one system, a handheld “Select Monitor” (a proprietary service tool) is used. Diagnostics can be conducted in either real time or by using the service tool’s memory. During real-time fault diagnos­es, the Select Monitor is used to enter dummy data to simulate operating conditions. Output actuators The electronic control module accepts inputs from the speed sensor & the various control switches & outputs a signal that controls the throttle opening via an actuator. While an electromagnetic actuator is used to change the throttle butterfly opening in the Bosch control system, most systems use an actuator that’s operated by the engine vacuum. The vacuum servo output device uses the low pressure ex­ perienced in the manifold of a throttled engine (and hence at­mospheric air pressure) to do the hard work. Engine vacuum and atmospheric pressure are admitted to one side of a diaphragm. Depending on the opening and closing of the solenoid pressure control valves, the diaphragm will be deflected by December 1994  7 Different control stalks & panels are available for use with cruise control systems, with two stalks & a control switch plate shown here. varying amounts. This diaphragm is attached to a throttle cable which operates in parallel with the usual cable connected to the accel­ erator and so the throttle valve is opened and closed appro­priately. The amount of manifold vacuum available at large throttle openings is small (and a positive pressure will, of course, exist in the manifold at large throttle openings in a turbocharged car). In some cars, a vacuum accumulator (Fig.5) is used in conjunction with a one-way valve, to provide a reservoir of low pressure. Stepper motors and electric DC motors used in conjunction with an epicyclic gear train have also been employed by some manufacturers as the actuator. However, by far the most common servo is the vacuum-assisted SC design. Fig.5: a vacuum reservoir is used in some systems so that actua­tor operation can still occur at the low vacuum levels experi­enced at large throttle openings. 8  Silicon Chip