MME 3374A Unit 3: Signal Conditioning

Introduction to Signal Conditioning

• What it is and Why we use it: Signal conditioning is about using electrical circuits to change physical measurements (like temperature or pressure) into electrical signals that computers or other devices can understand and use.

  • We'll focus on the main ideas, not every single type of sensor.

  • Some parts might remind you of our previous course, MME 2285b.

Transducers

• What they do: A transducer is a device that changes one kind of energy into another.

  • They are really important for watching and controlling things like force and temperature. They take these physical things, turn them into electrical energy, and make sure the electrical signal is the right size.

Voltage Divider

• Why it's important: The voltage divider is a very basic circuit in electrical engineering. It's like a building block for many circuits that measure and adjust electrical signals.

• How it's set up:

  • It uses resistors:

  • $V_{I}$: The voltage going into the circuit.

  • $V_{O}$: The voltage coming out of the circuit.

  • $R1$, $R2$: The values of the resistors.

  • $I$: The electrical current flowing through the circuit.

Choosing Resistor Values

• Example: Imagine you need to measure a voltage between 0 and 30 V, but your tiny computer (microcontroller) can only handle up to 5 V.

  • Let's pick $R1 = 10 \ \text{k}\Omega$.

  • Now, we calculate $R2$:
    $R2 = R1 \cdot \frac{(V<em>{I} - V</em>{O})}{V_{O}}$

  • If $V<em>{I} = 30 \ \text{V}$ and $V</em>{O} = 5 \ \text{V}$, then $R2$ turns out to be $2 \ \text{k}\Omega$.

What "Tolerance" Means

• Resistor Accuracy Explained: Each resistor isn't perfect; it has a small error range, usually +/- 5%. This is called tolerance.

  • So, an $R1$ of $10 \ \text{k}\Omega$ could actually be anywhere from $9.5 \ \text{k}\Omega$ to $10.5 \ \text{k}\Omega$.

  • And an $R2$ of $2 \ \text{k}\Omega$ could be from $1.9 \ \text{k}\Omega$ to $2.1 \ \text{k}\Omega$.

Calculating Output Voltage ($V_{O}$) with Tolerance

• Highest Output Voltage ($V<em>{O</em>{max}}$): This happens when:

  • $R2$ is at its largest ($2.1 \ \text{k}\Omega$).

  • $R1$ is at its smallest ($9.5 \ \text{k}\Omega$).

  • Calculation: $V<em>{O</em>{max}} = \frac{R2}{R1 + R2} \cdot V_{I} = \frac{2.1 \ \text{k}\Omega}{9.5 \ \text{k}\Omega + 2.1 \ \text{k}\Omega} \cdot 30 \ \text{V} = 5.43 \ \text{V}$

• Lowest Output Voltage ($V<em>{O</em>{min}}$): This happens when:

  • $R2$ is at its smallest ($1.9 \ \text{k}\Omega$).

  • $R1$ is at its largest ($10.5 \ \text{k}\Omega$).

  • Calculation: $V<em>{O</em>{min}} = \frac{1.9 \ \text{k}\Omega}{10.5 \ \text{k}\Omega + 1.9 \ \text{k}\Omega} \cdot 30 \ \text{V} = 4.60 \ \text{V}$

• How much the Voltage Changes due to Tolerances:
  $\Delta V<em>{O} = V</em>{O<em>{max}} - V</em>{O_{min}} = 5.43 \ \text{V} - 4.60 \ \text{V} = 0.83 \ \text{V}$

  • This change is 16.6% of the ideal 5 V output.

• Problems in Making Products: Such a large voltage difference might not be acceptable. In mass production, big errors can mean products get rejected or need expensive fixes.

Thinking About Better Tolerance

• Using More Accurate Resistors: If we use resistors with a tighter tolerance (+/- 1%), the range for $R1$ becomes $9.9 \ \text{k}\Omega$ to $10.1 \ \text{k}\Omega$, and for $R2$ it's $1.98 \ \text{k}\Omega$ to $2.02 \ \text{k}\Omega$.

  • With these new, more accurate parts, our output voltage calculations are:
    $V<em>{O</em>{min}} = 4.92 \ \text{V}$
    $V<em>{O</em>{max}} = 5.08 \ \text{V}$
    $\Delta V_{O} = 5.08 \ \text{V} - 4.92 \ \text{V} = 0.16 \ \text{V}$

  • This change is now only 3.2% of the ideal 5 V output, which is much better.

Potentiometers

• How they are built and how they work: Potentiometers (or 'pots') come in different forms (like thin film) and their resistance changes as you turn a knob or slide a part (called the wiper).

  • You find them in cars, for example, as gas pedal sensors or in air conditioning systems.

• How they are used: The resistance, and thus the output voltage ($V_{O}$), changes as you move the wiper.

Measuring Temperature using a Voltage Divider

• How it's done: We can put a thermistor (a special type of resistor) where $R2$ would normally be in the voltage divider.

  • There are two main kinds: NTC (Negative Temperature Coefficient, resistance goes down as temperature goes up) and PTC (Positive Temperature Coefficient, resistance goes up as temperature goes up).

Thermistors

• How good they are at sensing change: NTC thermistors are very sensitive; their resistance changes a lot with even small temperature changes.

Semiconductor Temperature Measurement

• Using Diodes: The way a diode's voltage changes when current flows through it (about -2.1 mV for every degree Celsius or Kelvin) can also be used to measure temperature.

Measuring Light with Photocells

• Cadmium Sulfoxide Photocell: By replacing $R2$ with a light-dependent resistor (often called a photocell), we can measure how much light there is. The resistance of these cells goes down as more light hits them.

The Bridge Circuit

• Why we use it: The Wheatstone bridge circuit is an advanced version of the voltage divider. It's used for very precise measurements, especially to remove any unwanted starting voltage (offset) that might come from the sensor itself.

• How it's set up:

  • Resistors $R1$, $R2$, $R3$ are picked to make the bridge balanced (output voltage is zero when balanced).

  • When the sensor resistor ($R<em>{sensor}$) changes, it causes current to flow between different parts of the bridge, sending out a signal ($V</em>{O}$) that tells us the change.

Strain Gauges and Bridge Circuit

• What a Strain Gauge is: A strain gauge is a device whose electrical resistance changes when it's stretched or squeezed (strained). This change in resistance is how it works.

  • Gauge Factor (GF): This number tells us how much the resistance of the strain gauge changes in relation to how much it's stretched or squeezed.

Everyday Problems with Strain Gauges

• Affected by Temperature: The resistance of a strain gauge can also change with temperature, which can mess with readings.

• Damaged by too much strain: If you stretch or squeeze it too much, the gauge can break or come off.

• How well it's attached: The quality of how the gauge is stuck to a surface affects how consistent the measurements are.

Hot Wire Anemometer

• What it is and How it Works: This device measures how fast air is moving. It has a heated wire (like tungsten) whose resistance changes as air flows past it, cooling it down. This change in resistance tells us the airflow speed.

Transducer Frequency Response

• What it's for: This is about making sure a sensor only responds to the important signals (frequencies) and ignores unwanted electrical background noise.

• Types of Filters Used: We use different kinds of filters to do this, such as:

  • Low-pass filters

  • High-pass filters

  • Band-pass filters

  • Band-stop filters

Low-Pass Filter

• How it works: It lets signals with lower frequencies pass through but blocks higher frequencies.

• How it's built: You can make one by replacing one of the resistors in a voltage divider with a capacitor (an energy-storing component).

• Mathematical formula and Resistance:
  This formula comes from the voltage divider idea but is for electrical signals that change over time (AC voltages).

High-Pass Filter

• How it works: It does the opposite of a low-pass filter; it lets signals with higher frequencies pass but blocks lower ones.

• How it's built: The parts in the circuit are set up differently compared to low-pass filters.

Band-Pass and Band-Stop Filters

• Band-Pass Filter: This filter lets frequencies within a specific range go through.

  • It's like combining a high-pass filter followed by a low-pass filter.

• Band-Stop Filter: This filter blocks frequencies within a specific range.

  • It can be made in various ways using different circuit designs.

Bode Plots

• What they are: These are graphs that show how a circuit's output strength and timing (phase) change when the frequency of the input signal changes.

  • Why they are important: They help us understand and predict how a circuit will behave with different frequencies.

  • They also help us tell the difference between various filter designs (like Butterworth vs. Chebyshev filters).

Conclusion and Further Studies

• Homework Reading: Read Chapter 6 – Sections 6.1, 6.2, 6.3, 6.4, 6.5, 6.8.

• Practice Questions: Work on problems from Chapter 6: Questions 6, 21, 22, 23, 25, 26, 27, 33, 34, 39, 42, 44, 48, 51, 52, 54, 62, 63, 67, 69.