Sensors, Digital Electronics, and Multiplexing

Copyright © 2014 Delmar, Cengage Learning

Sensors, Digital Electronics, and Multiplexing

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11CHAPTER


Learning Objectives

• List the various types of sensors used on modern trucks

• Explain the difference between a digital and analog signal

• List the main logic gates and develop the truth table for both

• Describe the memory commonly used in an electronic module


Learning Objectives (continued)

• List the four main types of electronic module inputs

• Explain the concept of a pull-up and pull-down resistor in an electronic module input circuit

• Discuss the various forms of multiplexing used on modern truck


Sensors

• Sensors convert physical properties to an electrical signal

• Sensors are also known as transducers or sending units

• Sensors are used by various electronic modules on modern trucks


Thermistors

• One of the most common sensors found on trucks

• Two terminal temperature measuring sensor typically made of semiconductors

• The resistance between the two terminals changes proportionally with temperature

• Typical temperature sensors have a negative temperature coefficient


RTD’s and Thermocouples

• Resistance Temperature Detectors (RTD’s)- Two terminal temperature measuring sensors

• Constructed of thin metal wire with a positive temperature coefficient

• Thermocouples are constructed of two dissimilar metals such as iron and constantan

• The junction of this combination of metals is know as a type J thermocouple


RTD’s and Thermocouples (continued)

• Both ends of the wires are welded to form a cold and hot junction

• The hot junction is where the temperature is being measured

• The cold junction is the reference with a known voltage

• Thermocouples require amplification of the voltage produced


Potentiometers and Rheostats

• Used to measure angle position of fuel gauges, accelerator pedal or other devices

• Some diesel engines use an idle validation switch (IVS) to prevent unwanted acceleration in the event of a wiring problem


Variable Reluctance Sensor

• Variable reluctance sensors are basically miniature AC generators

• Made up of a coil wrapped around a permanent magnet and a low reluctance rotor

• The rotor rotates past the tip of the sensor• The changing magnet field of the sensor

induces a voltage in the coil• The output is AC voltage


Variable Reluctance Sensor

Figure 11-7 Variable reluctance sensor.


Variable Reluctance Sensor (continued)

• As the rotor speeds up, the frequency increases

• As the rotor slows down, the frequency decreases

• The amplitude of the signal depends on the air gap and rotational speed

• Engine speed sensor and ABS wheel speed sensors use variable reluctance speed sensors


Tech Tip

Proper wheel bearing adjustment is very important on trucks with ABS because a loose wheel bearing can cause the ABS sensor to be pushed away from the tone wheel, resulting in an inconsistent air gap between the sensor and the tone wheel. Just a small increase in the air gap between the tip of the sensor and the tone wheel will result in a large decrease in the voltage produced by the sensor.


Hall Effect Sensors

• Current is passed through a thin layer of semiconductor

• When exposed to a magnetic field a voltage will be developed at the edges of the semiconductor

• The level of voltage is directly proportional to the strength of the magnetic field

• The voltage produced is referred to as a the Hall voltage


Hall Effect Principle

Figure 11-11 Hall effect principle.


Potentiometric Pressure Sensor

Figure 11-15 Potentiometric pressure sensor.


Strain Gauge Sensor

• Used to measure pressure in a chamber next to a known pressure

• A flexible material with conductive material in a zigzag pattern

• As the material stretches the conductive material stretches and changes its resistance

• The change is typically converted to a voltage by a Wheatstone bridge


Strain Gauge Sensor

Figure 11-16 Strain gauge resistance change due to deformation.


Wheatstone Bridge Used With a Strain Gauge

Figure 11-17 Wheatstone bridge used with the strain gauge (Rs).


Variable Capacitance MAP Sensor

Figure 11-18 Variable capacitance MAP sensor.


Piezoelectric Sensor

Figure 11-19 Piezoelectric pressure sensor.


Single Frequency Displayed as a Sine Waveform on Oscilloscope

Figure 11-20 Plucking a single guitar string results in a single frequency, displayed as a sine waveform on oscilloscope.


Analog Signal

Figure 11-22 Analog signal produced by microphone.


Figure 11-23 Analog signal divided into equaltime increments

Figure 11-24 Amplitude measures at each time interval

Analog Signal (continued)


Figure 11-26 Re-created waveform fromnumerical information

Figure 11-27 Re-created waveform with increased sample time

Analog Signal (continued)


Digital Numbering

• Digital information is stored in a binary numbering system

• Only two numbers are used in the binary numbering system, 1 and 0

• Binary numbering works well with electronics because a switch has only two states, either off or on


The Number 9 Represented in Binary Format by Four Switches

Figure 11-30 Four switches used to represent the number 9 in binary format.


Digital Numbering (continued)

• Each digit in a binary number is a bit• A group of eight bits is one byte• A CD is capable of storing 682 megabytes

(682,000,000,) of information• 682 megabytes equals 682 million bytes

containing 8 individual bits • Computers store information on magnetic disk• A binary 1 is stored in one format and a 0 is

stored in another format


Logic Gates• Digital electronics is mostly concerned with

logic 0 and logic 1 values• Logic is the use of correct or valid reasoning

to come to a conclusion• Gates – Physical electronic components that

are composed of transistors and other hardware

• Inputs to gates will be a 0 or 1• Outputs form a gate will be a 0 or 1• Because logic gates are logical they will

always make the correct decision


AND Gate With Truth Table

Figure 11-31 AND gate constructed of two switches in series.

Figure 11-33 Status of Switch A and Switch B truth table.


AND Gate Symbol and Truth Table

Figure 11-34 AND gate symbol and truth table.


OR Gate and Truth Table

Figure 11-35 OR gate constructed of two parallel switches.

Figure 11-36 OR gate symbol and truth table.


Figure 11-38 NAND gate and truth table.

Figure 11-39 NOR gate and truth table.

Figure 11-37 NOT gate and truth table.

NOT, NAND and NOR Gates


Microprocessors

• Integrated Circuit – All components, such as transistors, resistors, and capacitors, are formed during the manufacturing process of the device

• Integrated circuits are commonly known as chips

• The components of a chip are formed in layers of P-type and N-type material

• Processors or microprocessors are the main component of computers


Integrated Circuit

Figure 11-40 Integrated circuit.


Microprocessors (continued)

• Processors process information based on programming information

• A microprocessor is an integrated circuit that may contain millions of transistors

• Most modern trucks contain a variety of electronic modules that are microprocessor based


Memory• Memory is a place where digital information is stored• Memory is binary, 1’s and 0’s• Volatile Memory – Lost or reset when power is lost or

reset• Non-Volatile Memory – Retained through power loss

or disconnect• RAM – Random access memory• EEPROM – Electronic, erasable, programmable,

read only memory• Flash Memory – Large sections must be must be

erased at the same time


Inputs and Outputs• All inputs to a modern microprocessor are

digital• All outputs to a modern microprocessor are

digital• Sensors provide all of the inputs • Sensor inputs must be converted from

analog to digital signals• A circuit called an analog to digital (A/D)

converter is used to convert from analog to digital signals


Connecting a Variable Resistor to the Input Terminal

Figure 11-47 Connecting a variable resistor to the input terminal.


Sensor Changes to 2kΩ and Voltage at A/D Converter Increase to 8V

Figure 11-49 Sensor changes to 2kΩ, and voltage measured by A/D converter increases to 8V as voltage divides between pull-up resistor and sensor resistance.


Pulled Down Input

• A pull-down resistor is a resistor between the input and ground

• Transfer Function – Microprocessor programming that converts input voltage to temperature, pressure, and other physical measurements


Pulled Down Input

Figure 11-50 Pulled-down input.


Digital and Analog Inputs

• Four basic types of inputs; pulled-up, pulled-down, analog, and digital

• Analog signals must be converted to digital• Electronic module digital signals are typically

switches with only two states: open or closed• Electronic module digital signals may also be

classified as pulled-up or pulled-down• A pulled-down digital input may also be

referred to as an active-high input


Tech Tip

Wiring harness problems with three wire sensor reference voltage and ground (return) circuits can result in sensor measurement errors. High levels of circuit resistance caused by terminal corrosion can result in misdiagnosis and unnecessary component replacement.


Microprocessor Controlling Output

Figure 11-57 Microprocessor controlling one of its outputs based on its input values and programming instructions.


Multiplexing• Multiplexing is methods used to combine

more than one channel of information into a single signal path

• Analog multiplexing would be a group of switches wired in parallel connected to a single input

• Time-Division Multiplexing uses a single wire several sensors and microprocessor inputs

• Serial and USB ports found on a PC are forms of serial data communication using time division multiplexing


Multiplexing (continued)

• Information sent out on the serial data link can be used by any device on the link

• SAE defines the standard for digital information transmitted on the data link


SAE J1587/J1708 Standard• SAE J1587 protocol have a bit transmission

rate of 9600 bps• SAE J1708, standard that defines the

hardware including the physical data wiring used with J1587, often referred to as ATA data link

• MID Message Identifier – Transmitted first to identify major system or device sending information

• PID Parameter Identifier – Indicates the specific information


SAE J1939 Standard• J1939 replaces most J1578/1707 and J1922

serial communication standard• J1939 transmission rate is either 250k or 500k• J1939 requires specific twisted copper wire

with special plastic insulation• The conductors are like two plates of a

capacitor and the insulation acts as a dielectric

• The impendence of J1939 cable is 120Ω• Cable that meets J1939/11 standards has a

metal foil shield


J1939/11 Shielded Cable

Figure 11-60 J1939/11 shielded cable.


Network Topology

• Topology refers to the manner the computer network is laid out

• J1939 uses bus topology, a single communication line shared by several devices

• The single cable is called the backbone• Each module is connected to the backbone by

a cable less than 1meter called a stub• Terminating Resistor – a 120Ω resistor across

each end of the backbone


Oscilloscope Patterns

Figure 11-62 J1939 oscilloscope patterns with terminating resistors missing (upper) and present (lower).


J1939 Message Structure

Figure 11-63 J1939 message format.


Dual-Channel Oscilloscope Trace

Figure 11-65 Dual-channel oscilloscope trace of J1939 messages.


Figure 11-67 6-pin J1578/J1708 diagnosticconnector pin assignment

Figure 11-68 9-Pin diagnostic connector pin assignments

6-Pin and 9-Pin Diagnostic Connectors


Summary• A modern truck may several different types of

sensors used to measure temperature, pressure, speed, and position.

• A variable reluctance sensor is a small version of an A/C generator. These types of sensors are often used to measure a rotational speed such as an ABS wheel speed sensor.

• The Hall effect describes the generation of voltage in the presents of a magnetic field. The voltage that is generated is proportional to the magnetic field strength. Unlike electromagnetic induction, no magnetic lines of force have to be cut to generate this voltage.


Summary (continued)• An analog signal is a signal that can be

any level within a range. A digital signal can only be one of two different values.

• Logic gates are hardware devices used to make a decision. There are several types of logic gates. A truth table describes the decision made by the logic gate to all possible inputs.


Summary (continued)• A microprocessor is an integrated circuit that

contains thousands of logic gates. The microprocessor only accepts digital information and provides results in digital format. An analog to digital converter is used to convert real-world analog signals into the digital equivalent for use by the microprocessor.

• The main two categories of memory are volatile and non-volatile. Volatile is lost when power is disconnected from the device while non-volatile is retained through power disconnection.


Summary (continued)• The internal circuitry in an electronic module

typically has pull-up or pull-down resistors.• Electronic module inputs can be digital or

analog types of inputs. These digital or analog inputs can be pulled-up or pulled-down.

• Multiplexing refers to methods used to combine more than one channel of information into a common signal path. Common forms of multiplexing used on modern trucks include analog and time-division multiplexing.


Summary (continued)• Serial data communication is a form of

time-division multiplexing. An example of serial data communication is the J1939 data link.

• A controller area network (CAN) is a communication network without a master control device.

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Sensors, Digital Electronics, and Multiplexing