Automotive Electronics Systems

Automotive electronic systems 1

An Electronic System is a physical interconnection of electronic components, or parts, that is able to gather information from a physical process, process the information, and act back on the physical process; it achieves this with the aid of input devices such as sensors, that gather information from the physical process and convert it into electrical signals, in the form of either voltages or currents within a circuit, and it exploits electronic components to manipulate and process the information.

Finally, the system uses electrical energy in the form of an output action to control the physical process, converts the electrical energy to a kind of energy which is compatible with the physical process we want to control by means of devices called actuators.

All the individual sub-systems are frequently interconnected with each other to leverage on the information produced by each subsystem: all modules can communicate with each other and with a diagnostic control unit.

Communications happen exploiting different buses: the control bus, carries signals that enable or disable the communication, the address bus, contains the address of each unit involved, and the data bus, contains the actual data.

Ex. automotive: controlled steering, collision avoidance, fuel injection control, ABS, ESP, ecc.

Automotive electronic systems must operate properly at low temperatures down to -40 °C, and also at very high temperatures, up to 165 °C. They must be very reliable and guarantee correct operation with no failures for more than 10 years, withstand severe vibrations and accelerations, and must be very insensitive to electrostatic discharge.

2. Nuts and bolts of electronics

A circuit comprises a number of interconnected parts, each of which imposes its own unique relationship, called constitutive relation, between two electrical quantities: voltage (V) and current (I), electrical current is similar to the flow of water, and voltage is similar to water pressure: given a pipe in which water can flow, the rate of flow will be governed by the difference in water pressure between its two ends.

Electric current is the ordered motion, in a piece of material, of those electrons within àthe material that are available for movement; the direction of the current is the one which is opposite to the one in which electrons move. Current is measured by amperometers, or ammeters, which are designed to have low impedance, to avoid disturb to the circuit.

Voltage is what encourages the free electrons to flow; it is actually a differential quantity, àand it is possible to define a voltage at a point in space only if a reference arbitrary voltage value is defined at another, well specified, point in space. Voltage is measured by voltmeter, which are designed to have high impedance, to avoid the connection disturbing the circuit.

Open circuit and short circuit

Open circuit: two-terminal component that will withstand any àarbitrary voltage with no current flow through its terminals (I = 0).

Short circuit: two-terminal component that will withstand any àarbitrary current with no voltage drop across its terminals (V = 0).

Ideal current source: two-terminal component àthat will withstand any arbitrary voltage while always displaying a given, fixed, cost. current flow through its terminals.

Ideal voltage source: two-terminal component àthat will withstand any arbitrary current while always displaying a given, fixed, cost. voltage across its terminals.

Resistor

Resistor: two-terminal device that establishes a linear àconstitutive relation between the current flowing through it and the voltage at its terminals. [Ω ℎ] =∙

Ohm’s Law: ! ! !" =∙ = [ ]# # ⁄

Power: We can classify: = ∙ = ∙ = [ ].à

• Passive devices: Always dissipates power to work, ex. resistors, capacitors, inductors and diodes.
• Active devices: Can generate power while working, energy conservation is verified: the generated power is always coming from an energy source, ex. voltage sources and current sources.

Capacitor

Capacitor: linear component that is able to store electric charge, and it establishes àa linear relation between the charge it holds and the voltage at its terminals =∙ → = = = [ ]

If a voltage is applied on the capacitor, then it will store some charge and, vice-versa, if some charge is stored in the capacitor, then the capacitor will exhibit a voltage at its terminals: for charge to be stored, some current must flow in, charge, or out of, discharge, the capacitor. Current in a capacitor only flows when the voltage across it changes over time.

Considering an RC circuit, by means of which it is possible to charge a capacitor: it is composed by a constant voltage source and an ideal switch, that behaves as an open circuit if it’s OFF and as a short circuit if it’s ON. =0$

For t < 0: Switch OFF, no current can flow, → = 0 → ! → = &'()*+ ,∆ = 0% !!( ) () ( )

For t ≥ 0: Switch ON → − ∙ − = 0 → = H1 − J"#, $ * * ,() () = ∙ * *() (0)→K − )* , * !( ) () = = = $%* $

Voltage delivered by the battery splits across the resistor and the capacitor, and their sum must be constant; as the high current flows, it quickly charges the capacitor, that now shows a higher voltage across it.

This decreases the voltage across the resistor, and also the current in the circuit; the now smaller current causes a less rapid charge of the capacitor: the process ends, at t = ∞, when the voltage across the capacitor equals that delivered by the voltage source, with no voltage drop across the resistor and no current in the circuit, after a time = RC from the moment the switch is closed, the charging process is completed at around 68%. After a time = 3RC, almost the 95% of the charging process is complete.

Inductor

Inductor: dual of the capacitor and is a linear component that is able to store magnetic àenergy; it establishes a linear relation between the voltage at its terminals and the change over time in current through it. If a voltage is applied on the inductor, then the current through it will vary over time, storing or extracting some magnetic energy: inductor lets current flow with zero voltage drop at its terminals only if I = constant. =∙

Kirchhoff’s law

Kirchhoff’s law:

An electrical circuit can be seen in terms of branches and nodes:

Node: junction where two or more elements connect, characterized by a unique V value; à

Branch: path connecting two nodes, characterized by a unique value of the current. à

KCL: Kirchhoff's first law states that, for any node in an electrical circuit, the sum of currents flowing into that node = to the sum of currents flowing out of that node.

KVL: Kirchhoff's second law states that the directed sum of the voltages around any closed loop is zero.

Parallel and series configuration

Parallel Configuration: two devices are in this àconfiguration if they share the same nodes, so they experience the same voltage. ∙ = = " # " # = →! → = ( + )-. " # = + + " #" #

Series Configuration: two devices are in this àconfiguration if they lie along the same branch, so they experience the same current. = + " # = + → ! → = ∙ -. " # -. = = " #

Switch: three-terminal device which allows controlling if current can or cannot circulate àin a branch. Two terminals are connected in series with the circuit branch in which current control wants to be obtained, while the third terminal is used as a control terminal. The behavior that can be forced is either akin to a short circuit or close to an open circuit.

Superposition principle

Superposition principle:

It consists of eliminating all but one source of power within a network at a time, using circuit analysis to determine voltage drops, and/or currents, within the modified network for each power source separately. To eliminate a voltage source, we set it to zero volts, so we replace it with a short circuit; to eliminate a current source, we set it to zero amperes, therefore replacing it with an open circuit. Then, once voltage drops and/or currents have been determined for each power source working separately, the values are all “superimposed” on top of each other, added algebraically, to find the actual voltage drops/currents with all sources.

Superposition principle can be extended to circuits that include reactive components by introducing the generalized concept of impedance.

N.B. This principle works only for circuits that are reducible to series/parallel combinations for each of the power sources at a time and only when the underlying equations are linear! Principle is only applicable for determining voltage and current, not power, and cannot be applied to circuits where the resistance of a component changes with voltage or current!

Equivalent circuits

Equivalent circuits:

Suppose that the circuit is enclosed in a black box with two wires coming out of it; we test the circuit by the two wires, and we can replace the real circuit with one that behaves like the real one, based on the measurements we got from the wires.

Thevenin: a generic non-dynamic linear circuit observed by two nodes can be represented àby a voltage source (V ) in series with a resistor (R ).th th

Norton: a generic black box non-dynamic linear circuit observed by two nodes can be àrepresented by a current source (I ) in parallel with a resistor (R ).no no

D.C. vs. A.C.

D.C. vs. A.C. (Direct Current vs. Alternating Current):

• DC signal is defined as a perfectly constant function of time.
• AC signal not only changes over time, but periodically reverses its sign, ex. sine wave.

Pulsating Signal: has a DC component superposed to an AC one. à

According to the Fourier analysis, all signals can be seen as a superposition of sine waves with different amplitudes and phases: FOURIER TRANSFORM, decomposition process of an arbitrary signal, which is also called the SPECTRUM of a signal.

Signals can be exactly represented as the sum of an infinite number of components that are sine, or cosine, waves at different frequencies, each with well-specified amplitude and phase. The first component, wave 0, has zero frequency, and represents the mean value of the signal in the range [0, T], while the other components are defined at specific frequencies, that are integer multiples of (DISCRETE SPECTRUM for periodic signals); if we ∆ω = 2π/T consider a non-periodic signal, it can be mathematically seen as a signal that is periodic but has an infinitely extended period, T → ∞, then Δ→0, and this results in a CONTINUOUS SPECTRUM, so the signal is defined at all possible frequency values.

As a result of the Fourier analysis, if the response of a system to a sine input at any frequency àis known, then it is possible to know the response of the system to any kind of input, simply because an arbitrary input can be seen as a sum of sine components!

A.C. currents to passive devices

A.C. currents to passive devices:

• Resistors: I, V are ISO-FREQUENTIAL and IN PHASE! = = ∙ + à$ /())( ) sin( )
• Capacitors: I, V are ISO-FREQ. and OUT OF PHASE BY 90°, QUADRATURE, I anticip. V. 2/()) 3 {\ 45)() } = = ∙ sin Y + + Z = ∙ c\ = ∙ ∙ ∙X !2) # c 46{c = ∙ 46 45) 45) }() = ∙ cos( + ) = a ∙ ∙ b = ∙ c 1 1 | |→ = = − = ∠ = −% % %\ 2
• Inductors: I, V are ISO-FREQ. and OUT OF PHASE BY 90°, QUADRATURE, V anticip. I. c( ) {\ \45)( ) cos( ) } 46 = = ∙ + = ∙ = = ∙ K K {c c \45) }() = ∙ cos( + ) = ∙ = ∙ ∙ ∙c | |→ = = − = ∠ = 07 7 7\ = + (: , : )

In general, impedance can be defined as: à ! !" = = + (: , . )7

Diodes

Diodes: two-terminal, non-linear, non-symmetric àelectronic device that conducts current primarily in one direction; it consists of a crystalline piece of semiconductor, mostly Si, material with a p–n junction: it has low, ideally zero, resistance in one direction, and high, ideally infinite, resistance in the other, so that the diode will conduct a significant current for positive voltages, while behaving almost as an open-circuit for negative voltages, I doesn’t depend on V, it’s constant at -I ).0

The positive direction of the current is taken to be from the anode to the cathode.

Considering the THRESHOLD V , the constitutive relation of the diode is: d 89 .9 = ∙ i − 1j = ∙ i − 1j X9 :;$8 8

• Forward-bias region: When the applied voltage is < V , then the current is small. If the d voltage changes significantly, the current doesn’t show significant variations. When the applied voltage is > V , the current is significant and increases very rapidly with d the applied voltage.
• Reverse-bias region: If a positive current flows in the device, then the voltage across it is V , regardless of the magnitude of the current. If no current flows in the device, d then the voltage must be < V ; d
• Breakdown region: For large negative voltage an unexpected breakdown happens. This is a region to avoid, since in those conditions the device cannot tolerate the very strong electric field that develops in the semiconductor. The device cannot block the current and starts conducting strong heat dissipation permanent damage. à à

It’s possible to identify different types of diodes:

• Zener Diode: Able to work in the breakdown regime without experiencing damage.
• Light Emitting Diode (LED): Forward-biased, it shines light at a specific frequency that depends on the employed material. Intensity controlled by the magnitude of current.
• Photodiode: The dual of LED, it responds with current flow when light is shone on it.

Analog vs. digital

Typically the information that is available in terms of analog signals is converted in digital form and then processed by means of digital circuitry. Then, when needed, it is transformed back to the analog domain to allow further interaction with analog physical systems.

Analog signals: describe physical quantities, position, pressure, temperature, and are àcontinuous function of time, defined at every point in time, and are real-valued.

Digital signals: describe quantities like answers to yes/no questions and are not time-continuous à, so they are defined at well-specified instants of time; Moreover, at any instant of time at which they are defined, digital signals can take one out of just two values. They are useful when dealing with logic and evaluation of logical propositions, other then arithmetic operations between different signals.

3. Sensors

A sensor is a device that transforms the variations of a physical quantity in a related variation of an electrical quantity, like voltage, current, resistance…: it translates information from a physical domain to another, getting as an input a physical variable and converting it into a variable according to a well-defined relation.

A sensor is placed in a specific position, and the information that is present at its output, that is an electric signal, must travel through wires or by means of a wireless link toward to electronic circuit that will handle the data; voltage is always defined with respect to a point in space at which the voltage is arbitrarily taken to be 0 V, ground, but considering two different points with different potentials, there might be problems in communications.

To avoid problems due to reference differences, we set different configurations:

• Single-ended: Transducer enclosed in its metal case delivers its output in between two terminals, high-end and low-end. The information is encoded in terms of voltage difference; the low-end terminal is physically connected to ground, so if the two local grounds are not indeed at exactly the same potential, some parasitic current will flow along the cable connecting the low-end terminals, causing a voltage drop: the voltage seen by the receiving circuit is therefore the sum of this parasitic voltage and of the useful voltage that is given as output by the transducer. The advantage of this connection typology is that many transducers that have to be connected to the same receiving circuit may share the low-end li