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...

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Feedback Signals

Principle of feedback signals
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.

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Switch Inputs

Pull-down switch circuit
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. FIGURE. Pull-down switch circuit. 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. The current limiting resistor used in the circuit is referred to as a pull-up resistor since it 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. FIGURE. Pull-up switch circuit. FIGURE. Pull-down circuit with a diagnostic resistor. The pull-up resistor is usually of a very high ohms value to keep amperage to a minimum. This resistor can have a value of 10K to 10M ohms. When the switch is open, there is no current flow through the resistor and no voltage drop over it. This results in the voltage sense circuit recording a value equal to the reference voltage. When the switch is closed, current flows through the resistor and results in a voltage drop. Since the switch should provide a clean contact to ground, the voltage sense circuit should read a value close to 0 volts. The pull-up circuit will have a reference voltage through the switch. Usually the reference voltage will be provided directly from the battery or by the ignition switch. The current limiting resistor performs the same function in this circuit as it does in the pull-down circuit. This resistor is referred to as a pull-down resistor since it assures a proper low voltage reading by preventing float when the switch is open. With the switch in the open position the voltage sense circuit will read 0 volts. With the switch closed, the sense circuit should read close to reference voltage. Both of these circuits...

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Position and Motion Detection Sensors

A potentiometer sensor circuit measures the amount of voltage drop to determine position
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. 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. FIGURE. A potentiometer sensor circuit measures the amount of voltage drop to determine position. The potentiometer usually consists of a wire wound resistor with a moveable center wiper . A constant voltage value (usually 5 volts) is applied to terminal A. If the wiper (which is connected to the shaft or moveable component of the unit that is being monitored) is located close to this terminal, there will be low voltage drop represented by high voltage signal back to the computer through terminal B. As the wiper is moved toward the С terminal, the sensor signal voltage to terminal В decreases. The computer interprets the different voltage value into different shaft positions. The potentiometer can measure linear or rotary movement. As the wiper is moved across the resistor, the position of the unit can be tracked by the computer. Since applied voltage must flow through the entire resistance, temperature and other factors do not create false or inaccurate sensor signals to the computer. A rheostat is not as accurate and its use is limited in computer systems. Magnetic Pulse Generator An example of the use of magnetic pulse generators is to determine vehicle and individual wheel speed. The signals from the speed sensors are used for computer-driven instrumentation, cruise control, antilock braking, speed sensitive steering, and automatic ride control systems. The magnetic pulse generator is also used to inform the computer of the position of a monitored component. This is common in...

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Pressure Sensors

Simple pressure switch uses contracts to complete electrical circuit
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 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. Current that is supplied to the switch now has a completed path to ground. In a very simple warning light circuit, the closed pressure switch completed the circuit for the bulb and alerts the driver to an unacceptable condition. For example a simple oil pressure warning lamp circuit will use a pressure switch. FIGURE. Computer monitors pressure switch circuit. Computer monitored pressure switch circuits use the change in voltage as an indication of pressure. When the pressure change (either from low to high or high to low) changes the state of the switch, the voltage change is interpreted by the computer. Pressure switches are used to monitor the presence of pressure that is above or below a setpoint; they do not indicate the exact amount of pressure being applied. Potentiometric Pressure Sensor FIGURE. Potentiometric pressure sensors use a Bourdon tube, capsule, or bellows to drive a wiper arm on a resistive element. One of the basic types of pressure sensor is the potentiometric pressure sensor. The potentiometric pressure sensors use a Bourdon tube, a capsule, or bellows to move a wiper arm on a resistive element. Using the principle of variable resistance, the movement of the wiper across the resistive element will record a different voltage reading to the computer. Although this type of sensor can be used as a computer input, a computer is not always involved. Some early analog instrument panels used this sensor unit with an air core...

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Thermistors

A thermistor is used to measure temperature. The sensing unit measures the resistance change and translates the data into temperature values
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. 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. NTC thermistors reduce their resistance as the temperature increases, while PTC thermistors increase their resistance as the temperature increases. The NTC is the most commonly used. FIGURE. Chart of temperature and voltage correlation. Using the circuit shown in Figure, if the value of the fixed resistor is 10K ohms and the value of the thermistor is also 10K ohms, the voltage sensing circuit will read a voltage value of 2.5 volts. If the thermistor is a NTC, as the ambient temperature increases its resistance decreases. If the resistance of the NTC is now 8K ohms, the voltage reading by the voltage sensing circuit will now be 2.22 volts. As ambient temperature increases and the NTC value continues to decrease, the voltage sensing circuit will measure a voltage decrease. If the thermistor was a PTC, the opposite would be true and the voltage sensing circuit would measure an increase in voltage as the ambient temperature increases. FIGURE. Dual ramp temperature sensor circuit. Some temperature sensing circuits are designed as dual range circuits. This circuit provides for a switch in the resistance values to allow the microprocessor to more accurately measure temperatures. When the voltage sensing circuit records a calibrated voltage value (1.25 volts for example), the microprocessor will turn on the transistor which places the IK resistor in parallel with the 10K resistor. The circuit will operate the same as described until the 1.25 volts is recorded. With the IK resistor now involved in the...

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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. 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...

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