Riassunti Automotive Electronics Systems, prof. Puglisi

AUTOMOTIVE ELECTRONIC SYSTEMS

NUTS AND BOLTS OF ELECTRONICS

• OPEN CIRCUIT ➜ AN IDEAL OPEN CIRCUIT IS SOMETHING WHERE THE CURRENT IS ALWAYS GOING TO BE ZERO, REGARDLESS THE VOLTAGE ACROSS IT.
• SHORT CIRCUIT ➜ AN IDEAL SHORT CIRCUIT IS AN ELEMENT THAT HAS A ZERO DROP VOLTAGE BETWEEN ITS NODES, REGARDLESS THE CURRENT FLOW ACROSS IT.
• IDEAL VOLTAGE SOURCE ➜ IT IS AN ELEMENT THAT HAS A CONSTANT VOLTAGE, REGARDLESS THE CURRENT FLOW THROUGH IT.
• IDEAL CURRENT SOURCE ➜ IT IS AN ELEMENT THAT PRODUCES A CONSTANT CURRENT, REGARDLESS THE VOLTAGE AT ITS ENDS.
• RESISTOR ➜ IT IS AN ELEMENT THAT ESTABLISHES A LINEAR RELATIONSHIP BETWEEN VOLTAGE AND CURRENT (FIRST OHM'S LAW): ΔV = R·I WHERE [R] = OHM = Ω IS THE RESISTANCE! A RESISTOR CAN ONLY ABSORB ENERGY AND CANNOT PRODUCE IT:

  [P] = WATT = W ➔ P = ΔV·I = R·I² = ^ΔV2/_R

SERIES CONNECTION:

• I_a = I_z ➜ R_eq = R₁ + R₂

PARALLEL CONNECTION:

• ΔV_a = ΔV_z ➜ R_eq = (¹/_R1 + ¹/_R2)^-1

KIRCHHOFF LAWS

• A NODE IS A POINT WHERE AT LEAST THREE BRANCHES MEET.
• A BRANCH IS WHAT STAYS BETWEEN TWO NODES.

FIRST KIRCHHOFF LAW - THE ALGEBRAIC SUM OF THE CURRENTS IN A NODE IS ZERO!

SECOND KIRCHHOFF LAW - STARTING FROM A POINT OF OUR CIRCUIT AND COMPLETING A CLOSED PATH, THE ALGEBRAIC SUM OF THE VOLTAGE DROPS IS ZERO!

• CAPACITOR - IT IS AN ELEMENT THAT ESTABLISHES A LINEAR RELATIONSHIP BETWEEN THE STORED CHARGE AND THE VOLTAGE; Q = C ∆V WHERE [C] = FARAD = F IS THE CAPACITANCE!

dQ/dt = I = C dV/dt - IF THERE IS A CONSTANT VOLTAGE DROP BETWEEN THE TWO ENDS OF THE CAPACITOR, THERE WON'T BE CURRENT FLOW THROUGH IT; INSTEAD, IF THERE IS A TIME-DEPENDENT VOLTAGE DROP, THERE WILL BE CURRENT FLOW!

SERIES CONNECTION - Vₐ C₁ C₂ Vᵦ Q₁ = Q₂ => C_eq = (1/C₁ + 1/C₂)^-2

PARALLEL CONNECTION - Vₐ C Vᵦ ∆V₁ = ∆V₂ => C_eq = C₁ + C₂

• INDUCTOR - IT IS A NON-LINEAR ELEMENT; INDUCTORS STORE MAGNETIC ENERGY WHEN THEY ARE FED WITH A NON-CONSTANT CURRENT OVER TIME:

∆V = L dI/dt WHERE [L] = HENRY = H IS THE INDUCTANCE

SERIES CONNECTION - Vₐ L₁ L₂ Vᵦ L_eq = L₁ + L₂

PARALLEL CONNECTION - Vₐ L₁ L₁ Vᵦ L_eq = (1/L₁ + 1/L₂)^-2

• EQUIVALENT CIRCUITS:

1. THEVENIN'S THEOREM - A GENERIC BLACK BOX NON-DYNAMIC CIRCUIT OBSERVED BY TWO NODES CAN BE REPRESENTED BY A SERIES OF AN IDEAL VOLTAGE GENERATOR AND A RESISTOR

ANALOG vs DIGITAL

Analog signals change continuously over time so its plot is usually smooth and continuous. Digital signals instead have a finite set of possible values allowed; usually the digital technology uses the binary numbering, so timing graphs look like square waves. Many and many actions can be done only in the digital domain so analogic signals are converted to digital, manipulated and then re-converted in analog.

The most important thing is that analogic systems are more likely to get corrupted by noise, while digital ones are well understandable even with noise.

SENSORS

A sensor is the primary element in a measurement chain that converts the variation of a physical quantity to a related variation of electrical quantity so it is also a transducer!

It is very important how the sensor is connected to what comes later because the connections have to adapt the signal to the A/D converter requirements!

There are 3 types of connections:

1. SINGLE ENDED CONNECTION

This kind of connection has a single return line and only the upper branch brings relevant information. Theoretically the ground is always at same level; but if the 2 ground connections are in different places there might be a different voltage level, so some current can flow through it producing noise! On the other hand, if many channels have to be acquired, a single multiplexer with one switch per channel is enough!

2. DIFFERENTIAL CONNECTION

Also this kind of connection has a single return line but both lines carry important information. There is no noise since the differential connection has only 1 ground connection! The downside is that we need 2 switches per channel in a single multiplexer.

Torque Sensors

1. Using strain gauges: These can measure the deformation so we are able to trace the applied torque if we place them in a shaft. The problem is that the shaft rotates so it is difficult to power the gauge! Obviously, we can't use wires so the solution could be using batteries but this option depends on how fast the battery drains! It also could be an expensive solution!

2. Using infrared signaling: We use three discs → two of them are static and the third is integral with the shaft; one of the static has some LED; the other one instead has light receiving units on it! The trick is that when a torque is applied to the rotating shaft, the light won't completely pass through the holes! The upside is that the two static discs can be easily supplied with power and they are also used as reference.

Position Sensors

1. Potentiometers →

   It is an analog position transducer that can be linear or rotational! In the linear potentiometer a cursor is free to move along a track and it is also able to measure the voltage of the fraction of the resistor in relation to the full voltage on the resistor thanks to a voltmeter. In this way we can know the position of the pin, since the resistance depends on the length of the resistor.

   Which are the problems? Sliding contacts made by brushes, cheap but less reliable!

   Furthermore the voltmeter has a finite resistance and even if I chose it very high, when the pin is close to the full scale, I may make mistakes because I'm going to have a parallel of two resistances! These are called loading effects.

2. Encoders → Encoders can be incremental or absolute, linear or rotational and also optical or magnetic!

   Encoders convert analog position to a digital electronic signal. Encoders can work up to 80°/s and the magnetic ones have larger durability and reliability so they are also more expensive.

KELVIN BRIDGE

IN THIS CIRCUIT WE HAVE 6 ELEMENTS BUT TWO OF THEM ARE 4-TERMINAL RESISTORS: THUS WE CAN EXCLUDE THE DEPENDENCE OF R_X FROM THE RESISTANCE OF THE BRANCH BETWEEN R₄ AND R_A! UNFORTUNATELY WE CAN'T EXCLUDE THE EXTERNAL BRANCHES. FURTHERMORE IF THE RESISTORS ARE MADE OF THE SAME MATERIAL AND ARE SUFFICIENTLY CLOSE TO EACH OTHER, RATIOS ON WHICH IS BASED THE UNKNOWN RESISTANCE DON'T DEPEND ON TEMPERATURE!

MAXWELL BRIDGE

THIS CIRCUIT IS USED TO MEASURE A REAL UNKNOWN INDUCTANCE BY USING AN IDEAL INDUCTOR AND AN IDEAL RESISTOR! THIS CIRCUIT WORKS WITH AC CURRENT ONLY AND THE ONLY THING THAT CAN BE ADJUSTED IS THE FREQUENCY. ALSO IN THIS CIRCUIT, AS IN THE PREVIOUS ONES, THE VOLTAGE DROP IN THE HORIZONTAL BRANCH MUST BE ZERO. IN THIS BALANCE CONDITION V_ABC = V_BC, SO WITH JUST ONE COMPLEX EQUATION WE CAN FIND 3 UNKNOWNS, THE RESISTANCE R₃ AND THE IMPEDANCE L_X!

AMPLIFYING AND FILTERING

OPERATIONAL AMPLIFIER

AN OPERATIONAL AMPLIFIER IS A VERY GOOD DIFFERENTIAL AMPLIFIER: IT BRINGS TO THE OUTPUT THE AMPLIFIED DIFFERENCE OF THE TWO INPUTS! THE UPSIDE IS THAT IF THERE IS THE SAME NOISE BETWEEN THE 2 INPUTS, IT WILL BE NEGLECTED! THE NEGATIVE ASPECT INSTEAD IS THAT IT NEEDS POWER SUPPLY: UNFORTUNATELY THE OUTPUT VOLTAGE IS LIMITED BY THIS POWER SUPPLY, SO IT REMAINS BETWEEN -V_SS AND +V_SS. THE GAIN IS VERY HIGH, TYPICALLY 10⁶ OR 10⁷, AND IN REAL OP-AMP I WANT TO LOWER IT AS MUCH AS I CAN SIMPLY BECAUSE I DON'T NEED IT TO BE SO HIGH! FURTHERMORE THE GAIN IS A FUNCTION OF THE FREQUENCY BUT IT'S BAD FOR ME! I WANT IT TO BE CONSTANT.