Ch. 9: Sensors

In order to obtain amplitude and phase linearity, what about the system response of an accelerometer must be true?

- Amplitude linearity occurs at a damping ratio of 0.707 and a natural frequency that is as large as possible. - Phase linearity also occurs at a damping ratio of 0.707. To make the natural frequency large: choose a small seismic mass and a large spring constant - The above system response is why it is easy to produce commercial accelerometers in such small packages.

When a strain gage of known resistance R and gage factor F is bonded to the surface of a component and the component is then loaded, how do you determine the strain in the member?

- By simply measuring the change in the resistance of the gage

How do you debounce a switch? Draw a schematic of the circuit.

- Use a sequential logic circuit that contains 2 NAND gates

What are the 2 different types of rotary digital optical encoders?

1) Absolute encoder 2) Incremental encoder

1) How are accelerometers typically incorporated into a mechatronics system? 2) In a 3D world, how does the accelerometer detect acceleration? 3) What types of sensing elements are used to detect acceleration?

1) Attachment: Typically mechanically attached or bonded to structure 2) Detection: Detects acceleration in one direction, insensitive to motion in other directions 3) Sensing Elements: strain gages, piezoelectric elements, or capacitive elements

What are the 3 ways encoders are coded? Compare and contrast the 3 methods.

1) Binary (Absolute): - Multiple bits can change for each transition - Uncertainty can be multiple counts Pros: preferred method to directly interface with computer 2) Gray code (Absolute): designed so that only one bit changes state for each transition - Uncertainty during transition is only one count. Pros: coding method with the least amount of uncertainty 3) Quadrature Signals/Resolution (Incremental):

Why would you select a Wheatstone bridge configuration over a simple voltage divider when using strain gages?

1) It can be easily balanced to establish an accurate zero position. 2) It allows for temperature compensation. 3) It can provide better sensitivity and accuracy.

What are the common types of temperature measurement devices?

1) Liquid-in-glass thermometer (nonelectrical) 2) Bimetallic strip (nonelectrical) 3) Electrical resistance thermometer - Resistance temperature device (RTD) - Thermistor 4) Thermocouple

1) What is an accelerometer? 2) What does it do? 3) Applications?

1) What Is It: a sensor designed to measure 1) acceleration (rate change of speed) due to motion, 2) vibration 3) impact events, and 4) detect orientation. 2) What Does it Do: Converts acceleration into an electrical signal. Examples: - Acceleration: video game controller, smart phone shake detection - Vibration: rotating equipment - Impact Events: airbag deployment - Detect Orientation: determine inclination angle of robot or vehicle Switch between portrait and landscape on phone

What are common examples of applications of proximity sensors?

1. Detection the presence of an object. - Ex. Man in front of a urinal. 2. Counting moving objects. - Ex. Objects moving by on a conveyor belt. 3. Limiting travel of a machine. - Ex. detecting end of travel

What are the 5 Basic Laws that govern the function of thermocouples?

1. Law of leadwire temperatures. 2. Law of intermediate leadwire metals. 3. Law of intermediate junction metals. 4. Law of intermediate temperatures. 5. Law of intermediate metals.

What are the common types of position and speed measurement devices? What does each measure?

1. Proximity sensors and limit switches - Position sensors that detect whether or not something is close to reaching a limit of travel 2. Potentiometer - inexpensive, analog devices for measuring rotary or linear position 3. Linear variable differential transformer (LVDT) - Analog device capable of accurately measuring linear displacement 4. Digital encoder - useful for measuring position with a digital output that can be directly interfaced with a computer

How does a piezoelectric accelerometer work? What is worth of note?

1. When an object that the accelerometer is attached to begins to vibrate/accelerate, a relative displacement occurs btw the mass and the housing. (this occurs b/c of the mass's inertia) 2. The relative motion creates a strain in the piezoelectric crystal which in turns causes the crystal to output a charge as a result of the piezoelectric effect. Note: - Piezoelectric accelerometers do NOT require external power supplies. - Only measures acceleration in one direction.

How does an accelerometer work?

1. When object accelerates, there is relative motion btw the mass and the housing. 2. This relative motion is measured by the displacement transducer. 3. By performing a frequency response analysis of the 2nd-order system modeling the accelerometer, we can relate displacement transducer output to either: - absolute position or - acceleration of the object

What are quadrature signals? What is the purpose of quadrature signals on an incremental encoder?

2 signals that are 1/4 cycle out of phase with each other. Purpose: Quadrature signals can be decoded to yield angular displacement and the direction of rotation.

What is a load cell? What does it consist of?

A sensor used to measure force. Consists of: 1. Internal flexible element that has several strain gages on it. 2. Strain gages are usually in a bridge configuration and output a voltage equal to the applied force.

What is 2) Law of intermediate leadwire metals?

A third wire being inserted into the thermocouple will have no effect on the resulting voltage as long as the two junctions at where the wire is attached to the thermocouple are both at the same temperature. Consequence: a voltage measurement device that creates two new junctions can be inserted into the thermocouple circuit without altering the resulting voltage.

What is an Absolute Encoder? What is an Incremental Encoder?

Absolute Encoder: - an encoder where a unique digital word corresponds to each rotational position of the shaft. Measures: angular position Incremental "Relative" Encoder: - an encoder that produces digital pulses as the shaft rotates, allowing measurement of relative displacement of the shaft. Measures: Distance, speed, linear position

Compare and contrast absolute and incremental encoders.

Absolute: - Definitive position is know at ALL times - More complex - Has N definitive positions - Do NOT require a reference point Incremental: "Relative Encoder" - Requires a reference point - Simpler

What are the advantages and disadvantages of an LVDT?

Advantages: 1) Accuracy over a linear range 2) Produces an analog output that doesn't necessarily require amplification 3) Less sensitive to temperature changes than other transducers Disadvantages: 1) Has limited range of motion 2) Has limited frequency response

Why are seismographs, used to measure motion due to earthquakes, so large?

B/c the largest frequency range that results in a uniform amplitude ratio for a vibrometer occurs at a damping ratio of 0.707 and a natural frequency that is as small as possible. In order to make the natural frequency as small as possible, a large seismic mass and a small spring constant are chosen for the vibrometer.

Why is stress in a mechanical component an important feature to measure? What other physical quantities can stress and strain measurements indirectly measure?

Because it determines whether or not a member will fail. Indirect Measurement of Physical Quantities: - Force (by measuring strain of a flexural element) - Pressure (by measuring strain of a flexural element) - Temperature (by measuring strain of a flexural element)

9.1: Introduction

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9.2: Position and Speed Measurement

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9.3: Stress and Strain Measurements

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9.6: Pressure and Flow Measurement

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9.7: Semiconductor Sensors and Microelectromechanical Devices

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What is a Wheatstone bridge? What purpose does it serve? 2 Modes of operation?

Common circuit that allows use to use strain gages by accurately measuring small changes in resistance. Consists Of: 4 resistors excited by a DC voltage Modes of Operation: 1) Static Balanced mode 2) Dynamic Unbalanced mode

What are the components of an accelerometer? Draw a figure. Also, include a FBD.

Components: - Seismic mass - Damper (b) - Spring (k) - Displacement transducer - Accelerometer housing - Vibrating object

What does a piezoelectric accelerometer consist of? Draw a schematic.

Components: 1. Preloaded spring - Why preloaded?: to keep the mass in contact w/ the crystal and keep the crystal in compression (prolongs life) 2. Damper 3. Mass 4. Piezo-crystal 5. Accelerometer housing

Sketch the standard configuration for a thermocouple configuration. What does it consist of?

Consists Of: 1) 2 wires of dissimilar metals (A and B) attached to a 2) Voltage measure device w/ terminals made of metal C. 3) Ice bath/ reference temp liquid. The Dissimilar metals form 2 Junctions: 1) A reference junction 2) Measuring junction

How does a 3-Way switch work? Draw the schematic. Why is it called a 3-way switch?

Consists of 2 SPDT switches, a light bulb, and a power source Note: 3-way refers to the number of terminals (3), not the number of switches (2)

Give an example of a cheap and basic accelerometer circuit. How does it work?

Example: a ball that lights up/flashes LEDs when it is bounce. How it Works: - Contains a spring with a metal rod that goes through the middle of the spring - Vibrations cause the spring to make contact w/ the rod. - This creates an electrical contact that triggers an oscillatory circuit that makes LEDs blink

What type of measurements are piezoelectric accelerometers good for? What type of measurements are they unable to measure?

Excellent For: dynamic measurements - vibration and impact Unable to Measure: constant and slow changing accelerations

9.4: Temperature Measurements

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9.5: Vibration and Acceleration Measurements

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What is 3) Law of intermediate junction metals?

If a 3rd metal is inserted at a junction of a thermocouple (effectively creating 2 new junctions), the resulting voltage will not be impacted as long as the temperature values at these 2 junctions are the same. Consequence: Therefore, although soldered or brazed joints introduce thermojunctions, they have no resulting effect on the measured voltage.

What factors cause a conductor's resistance to increase? What factors cause a conductor's resistance to decrease?

Increases Conductor Resistance: 1) Resistivity (ρ) increases 2) Length (L) increases Decreases Conductor Resistance: 1) Cross-sectional area (A) increases

What is a strain gage rosette?

It is a combination of 3 different strain gages pointed in 3 different directions that are used to measure strain in complex loading (NOT uniaxial or biaxial loading) or complex geometry situations. - Various configurations are available

What is a proximity sensor?

It is a device that contains an object that changes in some form when it detects it is close to reaching a limit of travel. - Many types available.

What is the gage strain from a strain gage?

It is the strain on the surface of the component in the direction of the gage's longitudinal dimension.

Draw the schematic for a half-bridge circuit. What is it used for?

It is used to cancel out the effect temperature change has on the strain measurement. Specifically, temperature change that occurs beneath the gage.

What is a gage factor? What is the equation for the gage factor?

It's a factor that is reported for most commercially available strain gages that represents the material characteristics of the gage that relate the gage's change in resistance to strain

What is 4) Law of intermediate temperatures?

Junction pairs at T1 and T3 produce the same voltage as two sets of junction pairs spanning the same temperature range (T1 to T2 and T2 to T3) Consequence: result supports the use of a reference junction to allow accurate measurement of an unknown temperature based on a fixed reference temperature

What does MEMS stand for? What are they?

Micro-electro-mechanical Systems a complete microcontrollers system w/ sensors that comes packaged at the microscopic level on a single IC

How do most pressure and flow measurement devices operate? Examples of some pressure and flow measurement devices?

Most operate by measuring a deflection or displacement that is then relate to pressure via calibration or theory. Examples: - Pressure: Manometer, elastic diaphragm, piezoelectric pressure transducer, etc. - Flow: Pitot tube, etc.

What is a NO pushbutton switch? Draw the schematic of one. What is a NC pushbutton switch? Draw the schematic of one.

NO = normally open NC = normally closed

What are some naturally occurring piezoelectric materials? What are some crystalline materials that can be artificially polarized via heating and slow cooling in an electric field in order to take on piezoelectric characteristics? Which (naturally occurring or artificially enhanced) is more commonly used in accelerometers? Why?

Naturally Occurring: - Rochelle salt - Tourmaline - Quartz Artificially Produced: - barium titanate - lead zirconate (PZT) - lead titanate - Lead metaniobate Artificially are more commonly used in accelerometers b/c the sensitivity can be controlled in the manufacturing process.

What effect do lead wires have on a circuit with a strain gage that is located far away from the Wheatstone bridge? Draw a schematic of the set up. Why is this problematic?

Problem: - Leadwire resistances R′ add to the resistance of the strain gage branch of the bridge. - If the leadwire temperature changes, it causes changes in the resistance of the bridge branch. Substantial Problem If: 1) Lead wires are extremely long and 2) Extend through environments where the temperature changes.

Sketch a diagram of a dynamic unbalance Wheatstone bridge (dynamic deflection operation).

R_1 = strain gage R_4 = potentiometer Differential buffer amplifier

Sketch a diagram of a static balanced Wheatstone bridge. What is it used for?

R_2 and R_3 are precision resistors R_1 is a strain gage R_4 is a precision potentiometer (variable resistor) Hi Z VM = high-input impedance voltmeter R_4 is adjusted until the voltage between A and B is zero Static Balance W.S.B: - Can be used to measure a gage's resistance from a fixed load - Usually only used as a sort of calibration step for measuring changes in resistance.

Are linear sensors or rotary sensors more common? Why?

Rotary sensors because : 1) Most applications require measuring and controlling shaft rotations. - Ex. lathes, motors, generators 2) Linear motion can easily be converted to rotary motion which allows for rotary motion sensors to be used in linear motion applications. - Ex. W/ a belt, gear, or wheel mechanism

What is a SPST switch? Draw the schematic of one. What is a SPDT switch? Draw the schematic of one.

SPST: single-pole (SP), double-throw (DP) switch SPDT: single-pole (SP), double-throw (DP) switch; switches pole between 2 different positions

How do solve the problems associated with lead wires attached to a strain gage that is located far away from the Wheatstone bridge? Draw the schematic.

Solution: Add a third wire in the configuration shown How It Works: Equal leadwire resistances are added to adjacent branches in the bridge so the effects of changes in the leadwire resistances offset each other.

What is a strain gage? Draw schematic. What does it consist of?

The most common transducer for measuring strain in a mechanical component. Consists Of: 1) Thin foil of metal (usually constantan) in a pattern on 2) a thin plastic backing (usually polyimide).

What is 5) Law of intermediate metals?

The voltage produced between 2 metals A and B is the same as the sum of the voltage produced between each metal (A and B) relative to a third metal C. Consequence: This result supports the use of a standard reference metal (e.g., platinum) to be used as a basis to calibrate all other metals.

What is the Seebeck effect?

Two dissimilar metals in contact form a thermoelectric junction that produces a voltage that is proportional to the temperature of the junction.

How do you make the rectified signal of an LVDT into a smooth signal that tracks the core displacement?

Use a low pass filter.

What is the 1) Law of Leadwire Temperatures? What does this mean in relation to out thermocouple?

Voltage from 2 junctions of dissimilar metals in a thermocouple is only dependent on temperatures at junctions 1 and 2. - the temperature environment of the leads away from the junctions (T3,T4,T5) does not influence the measured voltage. Consequence: we need not be concerned about shielding the leadwires from environmental conditions

How do you determine the direction of the core displacement in an LVDT? Draw the circuit.

You use a demodulation circuit. - Contains a full bridge rectifier

When measuring strain on an object with a strain gage, how do you compensate for temperature changes that occur beneath the gage in such a way that it does impact the resistance measurement?

You use a half-bridge configuration that contains one active strain gage and one strain dummy gage. 1) The dummy gage is a strain gage that is placed on another identical, unloaded sample that is close to the original sample, and therefore at the same temperature. 2) The strain caused from temperature cancels out.

What is a digital optical encoder? Components? How does it work? Types? Draw the schematic for one.

a device that converts motion into a sequence of digital pulses. Digital bits can then be converted into absolute or relative position measurements Components: 1) Glass or plastic code disk w/ radial patterns on the tracks 2) 1 or more photoemitters/LEDs 3) Photodetectors/phototransistors How It Works: Interruption of the photoemitter and the photodetectors by the radial lines produces digital pulses Types: 1) Linear 2) Rotary (most common)

What is a charge amplifier?

a device that converts the displacement charge from the crystal in a piezoelectric accelerometer into a measurable voltage.

What is a piezoelectric crystal?

a material whose deformation results in charge polarization across the crystal. - In a reciprocal manner, application of an electric field to the piezoelectric material results in deformation.

What is a linear variable differential transformer (LVDT)? What is it made of? How does it work? Draw a schematic of one.

a transducer for measuring linear displacement. Consists Of: 1) Primary winding 2) 2 Secondary windings connected in series (series-opposing) 3) Movable iron core How it Works: https://www.rdpe.com/us/hiw-lvdt.htm Functions like a transformer: 1) Voltages are created in the secondary coils as a result an AC voltage being applied to the primary coil. 2) To find the core position, you simply measure the sum of voltage response from 2 secondary coils: - Amplitude = magnitude of core displacement - Direction (+/-) of wave = direction of core displacement

What is a rotary potentiometer (pot)? How does it work? Types? Imagery?

a variable resistance device used to measure angular position. The resistance between the wiper and the resistive element changes proportionally to the angular displacement. All it does is make the resistive wire in the potentiometer longer as the wiper moves. Consists of: 1) Wiper that contacts a 2) Resistive element Types: 1) trim pot: "trim" resistance with a screw 2) rotary pot: uses voltage division, adjusts resistance so voltage is equivalent to displacement Imagery: sounds like "potential meter" - Image: windshield wiper with debris on window that slows it down

What is a thermocouple? Draw a thermocouple circuit schematic.

an electrical temperature measurement device that functions by taking advantage of the Seebeck effect. It consists of 2 dissimilar metals connect by 2 thermoelectric junctions

What is a resolver? What does it consist of? How does it work?

analog rotary position sensor that operates like an LVDT Components: 1) Rotor with a primary winding 2) Stator with 2 secondary windings, offset by 90 degrees How it Works: 1) A primary AC voltage is sent over the primary coil, which causes AC voltages to appear in the secondary coils. 2) These voltages in the secondary coils are proportional to the sine and cosine of the shaft angle.

What is switch bounce?

brief current oscillations due to mechanical bouncing or electrical arcing that occur when a mechanical switch is open or closed. -a single closing of a switch can result in multiple voltage transitions that usually occur within a few milliseconds.

How does a piezoelectric microphone work? How about a piezoelectric buzzer?

crystal vibrations produce pressure waves (sound) and visa-versa

What are photoemitter detector pairs? What do the pairs consist of? What types are there? Sketch each type.

device that uses interruption or reflection of a beam of light to detect an object in a non-contact manner. - Contains an emitter (laser/focused LED) and a detector (phototransistor or photodiode) Types: 1. Opposed Mode Alignment (Obj. interrupts beam) 2. Retroreflective Mode Alignment (Obj. interrupts beam) 3. Proximity (Diffuse) Mode Alignment (Obj. reflects beam)

What are limit switches? Examples? How are they characterized?

devices that open or close connections within circuits Ex. pushbutton, levered microswitches Characterization: 1. The number of poles (P) and the number of throws (T) 2. Whether the connections are normally open (NO) or normally closed (NC)

What is inertia? What is the law of inertia? Imagery?

inertia = the natural tendency of an object to resist change in its state of motion - If an object is at rest, it wants to remain at rest. - If an object is in motion, it wants to stay in motion. - Connection: Alana not wanting to get off the couch to do something. Me not wanting to stop running on the treadmill until I finish my run. law of inertia = Newton's 1st Law: an object at rest tends to stay at rest, and an object in motion will stay in motion at the same velocity and direction unless it is acted on by an unbalanced force.

How are mass and inertia related? What is mass (in terms of inertia)?

mass and inertia are directly related. - Increasing mass also increases inertia. mass: the measure of an object's inertia

What type of accelerometer are the highest quality ones?

piezoelectric accelerometers

What is the equivalent circuit for the a piezoelectric crystal? Draw a schematic of it. Also, draw a schematic of the Thevenin equivalent piezoelectric cystal.

piezoelectric crystal = a capacitor and a charge source

What is a pole? Image? What is a throw? Image? What does a NO switch mean? What does a NC switch mean?

pole: the moving element in a switch; makes/breaks contact - Connection: pole vaulting think of the pole throw: the contact point for a pole in a switch - Connection: the point of contact btw the pole and the ground. NO = normally open NC = normally closed

What other types of devices does switch terminology apply to?

relays

What is a sensor? Imagery? What is a transducer? Imagery?

sensor: (combo of transducer and signal processor) a device that 1. senses magnitude of some physical parameter 2. converts it into an output signal, and 3. preforms processing on the signal Imagery - Image: Mom feeling my head to see if a I have fever. - Transducer: mom's hand that converts temp to electrical signals to it sends to brain - Processor: mom's brain / mouth transducer: the active element of a sensor that converts the physical input into an output Imagery: sounds like "transfer" - Image: heat being transfer from hand to electrical signals that go to brain

What is the piezoelectric effect?

the ability of certain materials to generate an electric charge in response to applied mechanical stress

What is a vibrometer?

the exact same spring-mass-damper configuration as an accelerometer, except the vibrometer measures displacement instead of acceleration.

What is the design of an accelerometer based on?

the inertial effects of a mass that is connected to a spring and a damper and is measured by a displacement transducer.

What is the equation for the resistance of foil wire in a strain gage/ resistance of a wire?

ρ is the foil metal resistivity L is the total length of the grid lines A is the grid-line cross-sectional area