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.
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Feedback Signals

Principle of feedback signals

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FIGURE. Principle of feedback signals.

If the computer sends a command signal to open a blend door in an automatic climate control system, a feedback signal may be sent back from the actuator to inform the computer the task was performed. The feedback signal will confirm both the door position and actuator operation (Figure 10-32). Another form of feedback is for the computer to monitor voltage as a switch, relay, or other actuator is activated. Changing states of the actuator will result in a predictable change in the computer’s voltage sensing circuit. The computer may set a diagnostic code if it does not receive the correct feedback signal.… READ THE REST

Switch Inputs

Switches are simplest of all input devices. The computer monitors the two states of the switch by measuring the voltage on the sense circuit. There are two types of voltage sensing circuits used with switches; the pull-down circuit and the pull-up circuit. Basically, the pull-down circuit will close the switch to ground and the pull-up circuit will close the switch to voltage.

Pull-down switch circuit

FIGURE. Pull-down switch circuit.

Pull-down switch circuit with external voltage source

FIGURE. Pull-down switch circuit with external voltage source.

A pull-down voltage sense circuit usually uses an internal voltage source within the computer. It is also possible to use an external voltage source. The current limiting resistor is used to protect the computer and the circuit. It also prevents input values from floating. Floating occurs when the switch is open resulting in the input to the voltage sense circuit of the control module being susceptible to electrical noise that may cause the control module to misread the switch state.… READ THE REST

Position and Motion Detection Sensors

Many electronic systems require input data concerning position, motion, and speed. Most motion and speed sensors use a magnet as the sensing element or sensed target to detect rotational or linear speed. The types of magnetic speed sensors include magnetoresistive (MR), inductive, variable reluctance (VR), and Hall-effect. In addition, the potentiometer and commutator pulse counting can be used to detect position.

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Some systems require the use of photoelectric sensors that use light sensitive elements to detect the movement of an object. In addition, solid state accelerometers, axis rotation sensors, yaw sensors, and roll sensors are becoming common components on many systems. This chapter will explore the operation of common position and motion detection sensors.

Potentiometer

A common position sensor used to monitor linear or rotary motion is the potentiometer. A potentiometer is a voltage divider that provides a variable DC voltage reading to the computer. These sensors are typically used to determine the position of a valve, air conditioning unit door, seat track, and so on.… READ THE REST

Pressure Sensors

This section discusses the various types of pressure sensors that are used in automotive applications. In some instances, a simple pressure switch is used. In systems that require monitoring of the exact pressure electromechanical pressure sensors, piezoresistive, or piezoelectric sensor are used. These sensors convert the applied pressure to an electrical signal. A wide variety of materials and technologies has been used in these devices. These sensors are used to measure the atmospheric air pressure, manifold pressure, pressure of a gas (such as R134a), exhaust pressures, fluid pressures, and so forth. The types of sensors that can be used include; potentiometric, strain gauges using Wheatstone bridges or capacitance discharge, piezoelectric transducers, and pressure differential sensors.

Pressure Switches

Simple pressure switch uses contracts to complete electrical circuit

FIGURE. Simple pressure switch uses contracts to complete electrical circuit.

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.… READ THE REST

Thermistors

Thermistors are commonly used to measure the temperature of liquids and ambient air. A thermistor is a solid-state variable resistor made from a semiconductor material, such as metal oxides, that have very reproducible resistance verses temperature properties.

A thermistor is used to measure temperature. The sensing unit measures the resistance change and translates the data into temperature values

FIGURE. A thermistor is used to measure temperature. The sensing unit measures the resistance change and translates the data into temperature values.

By monitoring the thermistor’s resistance value, the computer is capable of observing very small changes in temperature. The computer sends a reference voltage to the thermistor (usually 5 volts) through a fixed resistor. As the current flows through the thermistor resistance to ground, a voltage sensing circuit measures the voltage after the fixed resistor. The voltage dropped over the fixed resistor will change as the resistance of the thermistor changes. Using its programmed values, the computer is able to translate the voltage drop into a temperature value.

There are two types of thermistors: negative temperature coefficient (NTC) thermistors and positive temperature coefficient (PTC) thermistors.… READ THE REST

Computer Inputs

The input signals are processed in the microprocessor. The microprocessor directs the output drivers to activate actuators as instructed by the program

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.… READ THE REST