FAQ Automotive Electrical Signals Glossary

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Глоссарий автомобильных электрических сигналов​

	Amplitude: The voltage level of a signal above or below zero volts. The signal in the example at left has an amplitude of 2V.
	Analog Signal: An electrical signal whose amplitude can be measured at an infinite number of positions along the waveform.
	Digital Signal: An electrical signal with an instantaneous change in amplitude (called a pulse) from low to high and high to low. Since the change in state is instantaneous, the amplitude can only be measured in two positions, high or low. The pulse shown at left is a positive pulse, because the normal state of the waveform is low and the pulse goes high. However, with a negative pulse, the normal state of the waveform is high and the pulse goes low.
	Sine Wave: An analog signal where the current reverses direction at regular intervals, also called alternating current (AC). In automotive applications, sine waves are produced by either the alternator (unrectified) or inductive sensors (such as the RPM sensor).
	Square Wave: A digital signal that continuously alternates between on and off. A true square wave is on and off for an equal length of time. A variation of the square wave is the rectangular wave, which is on and off for an unequal length of time, but is usually still called a square wave.
	Period: The time required for a signal to complete one cycle. It can be measured in seconds (s), milliseconds (ms) or microseconds (μs).
	Frequency: The number of times a signal repeats in one second (cycles per second), measured in Hertz (Hz). The example at left has a frequency of 3Hz. The frequency of a signal can be fixed or variable. Any sensor that measures a rotating component (such as the camshaft position sensor) generates a variable frequency signal.
	Pulse Width: The time that a signal remains on during one period. It can be measured in seconds (s), milliseconds (ms) or microseconds (μs). Pulse width is similar to duty cycle, except duty cycle is measured in percent (%) instead of time, see duty cycle.
	Duty Cycle: The percentage (%) of time a signal remains on during one period. Duty cycle is similar to pulse width, except pulse width is measured in time instead of percent, see pulse width. Duty cycle is calculated by dividing the pulse width (s, ms or μs) by the period (s, ms or μs), and then multiplying the result by 100. For example, a signal with a 50 ms pulse width and a 100 ms period has a 50% duty cycle.
	Pulse Width Modulation (PWM): A signal that varies the pulse width of a signal. It is also called variable duty cycle.
	Waveform: The graphic representation of an electrical signal as displayed on an oscilloscope screen. While waveform is the preferred name, it is also called a trace or a pattern.
	Leading Edge: When viewing a waveform, the change in vertical height at the beginning of the signal. It is also called the rising edge or positive edge.
	Trailing Edge: When viewing a waveform, the decrease in vertical height at the end of a signal. It is also called the falling edge or negative edge.
	Networked Signals: A signal that consists of a sequence of coded pulses (sequence of event signals) used to broadcast data between a network of control modules. The CAN and LIN busses use networked signals.
	Sawtooth Wave: A signal in which the amplitude instantaneously rises and then ramps down, giving the appearance of a sawtooth. Sawtooth signals are used in the hood switch circuit in late model Volkswagens.

Note: The following lists provide general information on sensors, actuators and their signals. They are not
intended to account for every sensor and actuator in the vehicle, and applications include, but are not limited
to, those listed.


Analyzing Automotive Electrical Signals

Three factors affect automotive signals:
Amplitude
Frequency
Sequence of Events

Amplitude: On/Off, analog, pulse width modulated and duty cycle signals are characterized by the rate
of change in amplitude or the time the signal remains in the high or low state. When used in sensor
applications, the amplitude or pulse width (duty cycle) of a signal is varied to supply data to a control
module. Thermistors, potentiometers, Hall switches and pressure sensors are commonly used in this
way.

Frequency: Square and sine wave signals are examples of signals that are characterized by changes in
frequency (the number of times they repeat themselves per second). In sensor applications, Hall and
inductive sensors are used to provide rotational data such as RPM, CMP and wheel speed sensors.

Sequence of Events: Sequence of event signals are characterized by a series of pulses that can be
compared to messages sent by Morse code. By altering the sequence of the pulses, an almost infinite
number of coded messages can be quickly and accurately transmitted between different control modules.
Networked signals that are used by the CAN and LIN buses are examples of sequence of event signals.

 

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Automotive Electrical Sensors and Actuators

Analog Sensors

Thermistor: A two wire sensor that utilizes a resistor whose resistance varies with temperature. The
thermistors used in most automotive applications have a Negative Temperature Coefficient (NTC), where
the resistance of the thermistor decreases as the temperature increases. In a Positive Temperature
Coefficient (PTC) thermistor, the resistance of the thermistor increases as the temperature increases.
NTC thermistors are commonly used as temperature sensors. The temperature value is not obtained by
reading the sensor resistance directly, but instead by placing a reference voltage (usually 5 volts) and
ground across the sensor and then reading the resulting voltage drop.

Potentiometer: A three wire variable resistor that is used as a voltage divider. A reference voltage
(usually 5 volts or battery voltage) and ground are placed across a resistance element. A wiper is moved
across the element to produce an infinitely variable voltage signal from zero up to the reference voltage,
which is measured on the third wire. In automotive applications, potentiometers are commonly used as
position sensors for motors or measuring throttle plate position.

Inductive Sensor: A two wire sensor that measures the rotation of a shaft. Unlike other sensors, this
sensor does not have an external power supply. Instead, it contains a permanent magnet that creates a
magnetic field which collapses and expands when a sensor wheel is rotated through it, generating an AC
sine wave signal. The frequency of the signal varies with changes in the RPM of the sensor wheel. Many
crankshaft position (RPM) sensors and older ABS wheel speed sensors are inductive sensors.

Knock Sensor: A two wire sensor that is used to measure spark knock in an engine. This sensor uses a
crystal material that generates an AC voltage when mechanical stress is applied to it (piezoelectric effect)
when spark knock occurs. During installation, a knock sensor must be properly torqued to read spark
knock correctly.

Digital Sensors

Hall Sensors and Switches: A two or three wire electronic sensor that produces a variable frequency
square wave signal. Power and ground are supplied to a Hall Effect Transistor which is located in a
magnetic field generated by a permanent magnet. As the magnet field is altered by moving the magnet in
relation to the transistor or by moving a shutter wheel through the magnetic field, the reference voltage is
alternately pulled high or low resulting in a square wave signal. Hall Sensors are often used to measure
the position of rotating components such as camshaft position sensors.

Pressure Sensor: A three wire electronic sensor that converts pressure measurements into an electrical
signal. Power and ground are supplied to a pressure sensing device, which then produces a PWM or
analog signal relative to the measured pressure. The third wire transmits the PWM signal to the control
module. While majority of automotive pressure sensors fall into this category, there are a small number of
pressure sensors that use potentiometers to read pressure (such as in the Routan HVAC system).

Actuators

Solenoid: A two wire electromechanical device used to control the flow of liquids, gasses or the
operation of mechanical components. To operate the solenoid, an on/off, PWM or variable frequency
signal (commonly a switched ground) is supplied to a winding inside the solenoid, which in turn generates
a magnetic field that moves a plunger. Depending on the design of the solenoid, the plunger may be
normally open or normally closed in its rest state. A fuel injector is an example of a solenoid.
When the signal to the solenoid is switched off and the magnetic field around the winding collapses, the
winding produces a phenomenon called “inductive kick”. Inductive kick is a high voltage pulse that is
injected back into the control circuit and is similar in principal to the pulse produced by an ignition coil,
although the voltage is much lower (generally around 30 to 60 volts).

Relay: An electromechanical switch that uses a low current input signal to control a high current output
signal. It contains a winding that is used to magnetically move a set of points (switch), similar to the
operation of a solenoid. When an on/off signal is supplied to the winding, a magnetic field is generated
which changes the position of the switch. Depending on the design of the relay, the switch may be
normally open or normally closed in its rest state. The most common type of relay is a four wire relay,
which uses two wires for the control circuit and two wires for the switched circuit. Relays that use more
than four wires are usually variations of this design, usually containing multiple control and switched
circuits. If a relay contains logic circuits, it is generally considered a control unit although it may still be
called a relay.
Like solenoids, relays also produce an inductive kick. Volkswagen relays have a built in suppression
circuit consisting of a resistor placed parallel to the winding.

Motor: A device that converts electrical energy into rotational motion. On late model vehicles, the speed
of most motors is controlled using PWM circuits. If a motor has low output, checking the motor amperage
can determine if the problem is electrical or mechanical. Increasing the electrical resistance in a motor
circuit will decrease the amperage in the circuit, while increasing the mechanical load on the motor shaft
will increase the amperage in the circuit.
The direction of motor rotation can be changed by reversing the polarity of the signals to the motor.