Computer Inputs: Summary

  • Inputs provide the computer with system operation information or driver requests.
  • Driver input signals are usually provided by momentarily applying a ground through a switch.
  • Switches can be used as an input for any operation that only requires a yes-no, or on-off, condition.
  • Sensors convert some measurement of vehicle operation into an electrical signal. There are many different designs of sensors: thermistors, Wheatstone bridge, potentiometers, magnetic pulse generator, and Hall-effect switches.
  • A thermistor is a solid-state variable resistor made from a semiconductor material that changes resistance in relation to temperature changes.
  • Negative temperature coefficient (NTC) thermistors reduce their resistance as the temperature increases.
  • Positive temperature coefficient (PTC) thermistors increase their resistance as the temperature increases.
  • Some temperature sensing circuits are designed as dual range circuits to provide more accurate temperature measurements.
  • Pressure switches will usually use a diaphragm that works against a calibrated spring or other form of tension. When pressure is applied to the diaphragm that is of a sufficient value to overcome the spring tension a switch is closed.
  • A strain gauge sensor determines the amount of applied pressure by measuring the strain a material experiences when subjected to the pressure.
  • Piezoresistive devices are similar to variable resistors whose resistance values change as the pressure applied to the crystal changes.
  • The Wheatstone bridge is a series-parallel arrangement of resistors between an input terminal and ground.
  • Piezoelectric devices are voltage generators with a resistor connected in series that is used to measure fluid and air pressures.
  • The capacitance discharge sensor uses a variable capacitor constructed of two flexible alumina plates that are separated by an insulating washer. As the distance between the electrodes changes, so does the capacity of the capacitor. A measure of capacitance constitutes a measurement of pressure that is detected by a bridge circuit.
  • A potentiometer is a variable resistor that usually consists of a wire wound resistor with a moveable center wiper.
  • Magnetic pulse generators use the principle of magnetic induction to produce a voltage signal and are commonly used to send data concerning the speed of the monitored component to the computer.
  • Magnetoresistive (MR) sensors consist of the magnetoresistive sensor element, a permanent magnet, and an integrated signal conditioning circuit to change resistance due to the relationship of the tone wheel and magnetic field surrounding the sensor.
  • Magnetically coupled linear sensors use a moveable magnet that is attached to the measured element, a resistor card, and a magnetically sensitive comb. When the magnetic field passes the comb, the fingers are pulled against the resistor card contacting resistors that represent the various positions of the measured element.
  • Hall-effect switches operate 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 Hall-effect can also be designed as an analog (or linear) sensor that produces an output voltage that is proportional to the applied magnetic field.
  • Accelerometers are sensors designed to measure the rate of acceleration or deceleration.
  • Accelerometers react to the amount of G force associated with the rate of acceleration or deceleration.
  • The piezoelectric accelerometer generates an analog voltage proportional to a G force.
  • Accelerometers can also be designed as piezoresistive sensors.
  • The pull-down circuit will close the switch to ground.
  • The pull-up circuit will close the switch to voltage.
  • The pull-up resistor assures the proper high voltage reading by connecting the voltage sense circuit to an electrical potential that can be removed when the switch is closed.
  • The pull-down resistor assures a proper low voltage reading by prevent float when the switch is open.
  • The resistive multiplex switch is used to provide multiple inputs from a single switch using one circuit.
  • Feedback signals are used to confirm position and operation of an actuator.