Introduction to Sensors

Introduction to Sensors

• Sensors are critical components in mechatronic systems
• Sensors provide data to monitor and control system operations
• Automated systems require sensory information to operate
• A sensor is a device that outputs a signal when it detects a physical phenomenon
• Transducers are devices that convert signals from one physical form to another
• Sensors are a type of transducer
• Measurement systems use transducers in different components of the system

Classifying Sensors

• Sensors are categorized by output signal- Analog or Digital
• Sensors are categorized by power supply- active or passive
• Sensors are categorized by operation mode- null type or deflection type
• Sensors are categorized by variables measured

Analog Sensors

• The output of analog sensors changes continuously and is proportional to the measured value
• Examples of analog sensors include potentiometers, strain gauges, and thermocouples

Digital Sensors

• Digital sensors provide a digital output, that consists of on/off signals representing a number related to the measured variable
• Encoders can be used in digital sensors

Active Sensors

• Active sensors need an external power source to function
• Strain gauges and resistance thermometers are examples of active sensors

Passive Sensors

• Passive sensors generate an electrical signal in response to an external stimulus
• Piezoelectric, thermoelectric, and radioactive sensors are examples of passive sensors

Deflection Sensors

• Deflection sensors produce an output proportional to the measured quantity in a physical setup

Null Sensors

• Null sensors balance any deflection caused by the measured quantity with an opposing force to detect imbalances

Static Characteristics

• Static characteristics are are time-independent sensor performance parameters and characterize sensor output after settling due to changes in a physical quantity

Range

• A sensor's range is the difference between the minimum and maximum input values that produce a valid output
• This is usually specified by the sensor's manufacturer
• A type-K thermocouple has a range of 800°C from -50°C to 750°C and A ten-turn potentiometer has a range of 3600 degrees

Error

• Error is the difference between the measurement result and the true value of the tested quantity
• error = Measured Value - True Value

Bias Errors

• Present in all measurements from a given sensor
• Cannot be statistically detected or removed
• Calibration errors: occur when there is a non-zero output with a zero input
• Loading errors: occur when the sensor affects the measured system

Precision Errors

• Precision errors are random in nature
• They are caused by noise, material quality, and sensor fabrication precision

Accuracy

• Accuracy is inversely proportional to error
• High accuracy means low errors
• Accuracy is expressed as a percentage of the full range output or full-scale deflection
• Accuracy % = (Measured value - True value) / Full scale *100
• For a temperature sensor ranging from 0 to 200°C with ±0.5% full-scale accuracy, the temperature reading will be within ±1°C of the true temperature

Precision

• Precision is the ability of an instrument to consistently reproduce readings within a specific accuracy range
• Precision depends on the instrument's reliability

Repeatability and Reproducibility

• Terms describe a transducer's capability to provide the same output for repeated applications of the same input value
• Smaller repeatability error means greater measurement precision
• Signal interference, vibration, and temperature fluctuation affect repeatability
• Repeatability error cannot be reduced by calibration
• Precision errors can be adjusted for by averaging measurements
• Repeatability is expressed as a percentage of the full range output

Non-Linearity Errors

• Most sensors target a linear output, but outputs are imperfectly linear
• Non-linearity error is the maximum difference between the sensor's actual output and a straight line fitted to its input-output data
• Non-linearity is specified as a full-scale output percentage
• There is no standard way to define a best fit straight line
• The line is either connecting min and max output values or derived from a least-squares fit

Sensitivity

• Sensitivity is the change in output for a change in input and is the slope of the output versus input line in analog sensors
• Sensors with linear behavior have constant sensitivity over their entire range
• Nonlinear behavior sensors have sensitivity that varies as input changes
• Sensitivity (S) = ΔOutput / ΔInput
• For an instrument where 0.001 mm movement causes a 0.02 V change, the sensitivity is 20 V/mm

Resolution

• Resolution is the minimum change in the input that results in an observable change in output
• Resolution for a wire-wound potentiometer is a specified value, often around 0.5°
• Resolution for sensors with digital outputs, the smallest change in the output signal is 1 bit
• A 10-bit absolute encoder has a resolution of 0.35 degrees/pulse which equals (1/1024) * 360°

Stability

• Transducer stability is its ability to provide identical output when measuring constant input over time
• Drift describes the change in output over time
• Drift can be expressed as a percentage of full range output
• Zero drift is the change in output when there is zero input

Zero Offset

• Zero offset is when there is a non-zero output for no input

Signal-to-Noise Ratio

• Signal-to-noise ratio (S/N ratio) is the ratio of signal strength to output noise magnitude

Nonlinearities

• Static and coulomb friction, backlash, hysteresis, saturation, deadband, etc are commonly found in mechatronic systems

Hysteresis

• Hysteresis is the maximum difference in sensor output for identical input quantity
• Averages one measurement while increasing input from zero, the other reduces input from the max value to zero
• Sensors with hysteresis have output values dependent on whether the input is increasing or decreasing

Backlash

• Backlash is the max distance or angle through which a mechanical system's part can move in one direction without affecting another part
• Clearance between gears can cause backlash
• Backlash is undesirable, but especially important in gear train precision design

Dead Band

• A transducer's dead band or dead space is the range of input values with no output
• Bearing friction in a flowmeter using a rotor may cause zero output until the input reaches a velocity threshold

Impedance

• Impedance is the Laplace transform of the ratio of voltage and current flow for a sensor
• Impedance Z is equivalent to resistance R in Ohms for resistive sensors like strain gauges and thermistors
• Two types of important apply to sensors: input impedance and output impedance

Input Impedance

• Input impedance measures current draw needed to power a sensor or circuit
• Modelled as resistor in parallel with inputs
• High impedance is desirable, as device draws less current to minimize loading effect
• Oscilloscopes and DAQ equipment have at least 1 MΩ input impedance to minimize current and loading

Output Impedance

• Output impedance gauges the ability of sensors/circuits to provide current for subsequent system stages
• Output impedance is frequently modeled as a series resistor
• Low output impedance is desirable
• Piezoelectric sensors have high output impedance
• Op-amp circuits are used to buffer outputs and lower output impedance

Dynamic Characteristics

• Dynamic characteristics are time-dependent sensor performance parameters

Rise Time

• A certain percentage of output change defines its rise time
• Often output change is measured from 10-90% final steady state

Time Constant

• Time constant: Time needed for the output to reach 63.2% of final output
• Large time constant = sluggish sensor
• Small time constant = rapidly responding sensor
• Thermocouples have a time constant from 40-100s
• The time constant measures a sensor's responsiveness to input changes

Settling Time

• The period for the sensor output to settle within a certain percentage from steady-state value(usually 2-5%)

Bandwidth

• The bandwidth represents the frequency range for a sensor's operation
• Sensor outputs are 70.7% of DC level(3db reduction) at bandwidth limit frequency
• A sensor's bandwidth should be wider than the bandwidth of the feedback information in closed loop systems

Resonance

• Resonance is the frequency at which the output's magnitude peaks

Sensor Data Sheets

• Each sensor's performance characteristics are in the data sheet from the manufacturer
• The data sheet's categories are dynamic/performance, electrical, mechanical, environmental and physical

Sensor Selection Factors

• What needs to be measured and the accuracy required
• The expected value range
• Speed requirements
• Environmental circumstances
• What the Static and dynamic properties are
• Sensor life expectancy (reliability, maintenance, lifetime, resilience, availability, and cost)
• Signal conditioning needed for a suitable output

Cost-Effective Sensor

• When a cost-effective sensor doesn't exist, redesigning the mechatronic system may be needed
• It's better to use a systems level approach

Measuring Corrosive Acid Level

• Example: sensor choice for measuring corrosive acid in a vessel, which varies between 0-2 meters
• The diameter of circular vessel is 1 m
• Empty vessel weight is 100 kg
• Minimum level variation detectable is 10 cm
• Acid density is 1050 kg/m3, need electrical output
• Determining acid level indirectly is better, using load cells for outputting electrical for vessel weight
• Calculate weight change of liquid as it goes between empty and the full volume
• With the weight of the empty vessel, calculate what the weight change will turn out to be
• Calculate the resolution required for a 10 cm level change
• With three load cells, you will need a certain range and resolution so consult manufacturers catalogues

Analog Output Sensors

• Have continual signals that vary by voltage/current, relative to the variable that must be measured
• Common signal ranges are 0-10V, 0-5V, and 4-20mA
• Simple to use and interface with DAQ or microcontrollers

0-10 V Output

• A common and straightforward voltage output in analog sensors
• The start and end for measuring is distributed linearly between 0 and 10 V

Sensor Output that Work with Voltage

• More prone to electrical noise and ill-suited for transfers across long distances
• Harder to identify a malfunction since signal starts at 0V

Sensor Output that Works with Current

• When producing is cheaper, explains expensive analog sensors with a certain signal range
• Voltage out is sensitive to EL noise and voltage losses. Starting at 0V creates fail-safe design
• An electrical connection feeds a constant voltage to the sensor, for something like 24 DC
• There Is a converter that turns into a certain Direct Current
• Industrial standards dictate 4mA at 0% usage and a certain mA at 100% throughout range
• Starting at the 4 mA leads to distinction between signal and malfunction
• Signal becomes unaffected to voltage losses and unsusceptible to electrical noise

Digital Output Sensors

• Measure physicals, turn into discrete digitals described by Binary code
• An AD turns analog as digital, through Protocols with delivery

Switches

• Best to highlight the being of a subject with certain distance
• Distinguish which values have been reached by switching outputs with transistors' relays

Digital vs Analog Characteristics

• Analog has wide compatibility, high resolution, instantaneous analysis and real time analysis
• Analog also experiences signal noise and has limited signal processing
• Digital has great precision, signal processing with easy communication and immunity with noise
• Some shortcomings are high expenses and delayed reactions

Position Sensor Details: Potentiometer

• Potentiometers measure the angular position in the analog
• Sliding contact can be shifted across an element converting displacement to potential

Position Sensors: Encoders

• Encoders group between incremental for identifying transition/absolute position, and where the actual angle occurs
• Higher slots allows the degree with which encoder determined, the bit determine what tracks are

Switches: Proximity Sensors

• Turns switch through the existing of an object, classifying it based on how it reacts
• Classifying can use magnetic, ultrasound, or the optical that has the transistor, and transistors can offer speed

Potentiometers

• Common sensors for linear variable differential transformers and ultrasonic sensors

Temperature

• Temp is usually measured through Thermistors RTD and IC chips

Resistance Temperature Detector

• The principle relates the resistance of metal where the temp increases or decreases
• RTDs create common metals, but can still use copper and nickel

Integrated chips

• Depends on transistor tech where the different is the tension and temp correlates
• Integrated ones can still show precise results, can use an integrated AD where volts converts like pulse, I2c
• The sensor represents the manufacturer where they’re shown.

Strain Gauge Applications

• Measure strain with a gauge whose varying resistance measures it
• Force/load cells can either use pneumatic, strains where they measure the pressure where they use circuits
• The range from the voltage represent required by the sensor