Once the computer’s programming instructs that a correction or adjustment must be made in the controlled system, an output signal is sent to an actuator. This involves translating the electronic signals into mechanical motion.
An output driver is used within the computer to control the actuators. The circuit driver usually applies the ground circuit of the actuator. The ground can be applied steadily if the actuator must be activated for a selected amount of time. For example, if the BCM inputs indicate that the automatic door locks are to be activated, the actuator is energized steadily until the locks are latched. Then the ground is removed.
Other systems require the actuator to be turned either on and off very rapidly or for a set amount of cycles per second. It is duty cycled if it is turned on and off a set amount of cycles per second. Most duty cycled actuators cycle ten times per second. To complete a cycle it must go from off to on to off again. If the cycle rate is ten times per second, one actuator cycle is completed in one tenth of a second. If the actuator is turned on for 30% of each tenth of a second and off for 70%, it is referred to as a 30% duty cycle.
FIGURE. Duty cycle is the percentage of on time per cycle. Duty cycle can be changed; however total cycle times remains constant.
If the actuator is cycled on and off very rapidly, the pulse width can be varied to provide the programmed results. For example, the computer program will select an illumination level of the digital instrument panel based on the intensity of the ambient light in the vehicle. The illumination level is achieved through pulse width modulation of the lights. If the lights need to be bright, the pulse width is increased, which increases the length of on-time. As the light intensity needs to be reduced, the pulse width is decreased.
FIGURE. Pulse width is the duration of on time. (A) Pulse width modulation to achieve dimmer dash lights. (B) Pulse width modulation to achieve brighter dash illumination.
Most computer-controlled actuators are electromechanical devices that convert the output commands from the computer into mechanical action. These actuators are used to open and close switches, control vacuum flow to other components, and operate doors or valves, depending on the requirements of the system.
Although they do not fall into the strict definition of an actuator, the BCM can also control lights, gauges, and driver circuits.
FIGURE. The computer’s output driver applies the ground for the relay coil.
Relays. A relay allows control of a high-current draw circuit by a very low current draw circuit. The computer usually controls the relay by providing the ground for the relay coil. The use of relays protects the computer by keeping the high current from passing through it. For example, the motors used for power door locks require a high current draw to operate them. Instead of having the computer operate the motor directly, it will energize the relay. With the relay energized, a direct circuit from the battery to the motor is completed.
FIGURE. The computer controls the operation of the door lock motors by controlling the relays.
Solenoids. Computer control of the solenoid is usually provided by applying the ground through the output driver. A solenoid is commonly used as an actuator because it operates well under duty-cycling conditions.
One of the most common uses of the solenoid is to control vacuum to other components. Many automatic climate control systems use vacuum motors to move the blend doors. The computer can control the operation of the doors by controlling the solenoid.
Motors. Many computer-controlled systems use a stepper motor to move the controlled device to whatever location is desired. A stepper motor contains a permanent magnet armature with two, four, or more field coils. By applying voltage pulses to selected coils of the motor, the armature will turn a specific number of degrees. When the same voltage pulses are applied to the opposite coils, the armature will rotate the same number of degrees in the opposite direction.
FIGURE. Typical stepper motor.
Some applications require the use of a permanent magnet field servomotor. A servomotor produces rotation of less than a full turn. A feedback mechanism is used to position itself to the exact degree of rotation required. The polarity of the voltage applied to the armature windings determines the direction the motor rotates. Hie computer can apply a continuous voltage to the armature until the desired result is obtained.
FIGURE. Reversible permanent magnet motor.