Theory of Operation
The throttle is simultaneously linked by cable to the cruise control
actuator and by a second cable to the accelerator pedal. The core of
the cruise control actuator is a small motorized drum which winds or
unwinds a belt of rubberized fabric. This control belt is affixed to the
cable connected to the throttle. Thus, by winding the belt the throttle
cable can be retracted, or by unwinding it can be extended.
When the cruise control is not in use, the motor drive is disengaged
from the drum. This allows the drum to turn easily and the throttle
cable to move freely in response to accelerator pedal action. A
clockwork style spring supplies mild tension to keep the belt wrapped
around the drum and also takes up slack from the throttle cable.
A solenoid is used to engage the motor drive with the drum. When the
solenoid is energized a small armature is moved which pushes a
sprocket into contact with the integrated gear on the drum. At this
point, if current is supplied to the drive motor, the drum will wind. If
the polarity of the current is reversed, the drum will unwind.
The control unit (“brain”) is located in the electronics bay. The control
unit can thus command the actuator to engage or disengage, to reel in
the throttle cable or extend it. This provides the necessary control
capabilities.
To maintain a set vehicle speed, the control unit needs to know the
rotational position of the drum. A potentiometer (variable resistor)
integrated into the drum supplies this information to the control unit.
A resistance proportional to the position of the drum is constantly
presented to the control unit. This feedback allows the control unit to
determine how the actuator has responded to its commands.
By using the current vehicle speed (supplied by another sensor) along
with the feedback from the actuator, the control unit can adjust the
actuator as necessary to maintain vehicle speed.
BMW Cruise Control Actuator 1985 to 1996 – Technical Data page 5 of 25
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