Sensors, Digital Electronics, and Multiplexing

CHAPTER Sensors, Digital Electronics, and Multiplexing 11 Instructor Name:(Your Name)

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.

Analog Signal (continued) Figure 11-23 Analog signal divided into equal time increments Figure 11-24 Amplitude measures at each time interval

Analog Signal (continued) Figure 11-26 Re-created waveform from numerical information Figure 11-27 Re-created waveform with increased sample time

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.

NOT, NAND and NOR Gates Figure 11-37 NOT gate and truth table. Figure 11-38 NAND gate and truth table. Figure 11-39 NOR gate and truth table.

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 – Lostor reset when power is lost or reset • Non-Volatile Memory – Retained through power loss or disconnect • RAM – Randomaccess 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

Sensors, Digital Electronics, and Multiplexing