FIGURE. The input signals are processed in the microprocessor. The microprocessor directs the output drivers to activate actuators as instructed by the program.
The microprocessor receives inputs that it checks with programmed values. Several types of input devices are used to gather information for the computer to use in determining the desired output. Many input devices are also used as a feedback signal to confirm proper positioning of the actuator. Depending on the input, the computer will control the actuator(s) until the programmed results are obtained. Hie inputs can come from other computers, the vehicle operator, the technician, or through a variety of sensors.
Driver input signals are usually provided by momentarily applying a ground through a switch. The computer receives this signal and performs the desired function. For example, if the driver wishes to reset the trip odometer on a digital instrument panel, he would push the reset switch. This switch will provide a momentary ground that the computer receives as an input and sets the trip odometer to zero.
Switches can be used as an input for any operation that only requires a yes-no, or on-off, condition. Other inputs include those supplied by means of a sensor and those signals returned to the computer in the form of feedback.
This chapter discussed the many different designs of sensors and inputs. Sensors convert some measurement of vehicle operation into an electrical signal. Some sensors are nothing more than a switch that completes the circuit. Others are complex chemical reaction devices that generate their own voltage under different conditions. Repeatability, accuracy, operating range, and linearity are all requirements of a sensor.
Sensors discussed in this chapter include common forms of electrical and electronic devices. These include the following:
- Thermistor — a solid-state variable resistor made from a semiconductor material that changes resistance in relation to temperature changes.
- Wheatstone Bridge — A series-parallel arrangement of resistors between an input terminal and ground. The sensing circuit will receive a voltage reading that is proportional to the amount of resistance change.
- Piezoelectric device — A voltage generator with a resistor connected in series that is used to measure fluid and air pressures.
- Piezoresistive device — Similar to a piezoelectric except they operate like a variable resistor. Its resistance value changes as the pressure applied to the crystal changes.
- Potentiometer — A voltage divider that provides a variable DC voltage reading to the computer. The potentiometer usually consists of a wire wound resistor with a moveable center wiper.
- Magnetic pulse generators — Commonly used to send data to the computer about the speed of the monitored component. They use the principle of magnetic induction to produce a voltage signal.
- Hall-effect switch — A switch that operates on the principle that if a current is allowed to flow through thin conducting material that is exposed to a magnetic field, another voltage is produced. The switch contains a permanent magnet, a thin semiconductor layer made of gallium arsenate crystal (Hall layer), and a shutter wheel.