Unit8_SensorsAndActuators

8/13/2019 Unit8_SensorsAndActuators

http://slidepdf.com/reader/full/unit8sensorsandactuators 1/33

Jeff Shelton – 26 February 2013

SENSORS & ACTUATORSSENSORS & ACTUATORSSENSORS & ACTUATORSSENSORS & ACTUATORS

UNIT 8:UNIT 8:UNIT 8:UNIT 8:

1




SENSORS AND ACTUATORSPower/energy transduction

Sensor – from physical property to electrical/optical Actuator – from electrical/optical to physical/mechanical

Power density Sensor – from potentially high power density to relative low power

density (signal level) Actuator – high power density conversion, using signal level (low

power density) to modulate/control the power conversion

2

Sensor

(Transducer)

Actuator

Physical Property

(may be high

power density)

Signal

Conditioning

Power

Amplification

Mechanical

Work




SENSOR

CHARACTERISTICSSensitivity

Transducer's ability to respond to changes in measured quantity (ratio of outputchange to input change).

Resolution Smallest increment of measured value that can be detected.

Dynamic range The maximum of number of the smallest increment changes that the sensor can

measure.Accuracy Difference between measured and actual values – depends on the inherent

instrument limitations.

Precision

Sensor's ability to reproduce readings within a given accuracy – a measure of itsreliability.

Repeatability An ability to exactly reproduce a particular output signal when the same

excitation is repeatedly applied.

3




SENSOR

CHARACTERISTICSLinearity

Percentage departure from a linear value, i.e. the maximum deviation of an

output curve from the best-fit straight calibration line.Output impedance

Impedance of the sensor output as it is connected either in series or parallelto an electrical circuit.

Response time Time needed for the sensor to reach a certain percentage (95%) of the final

value after an input step change.

Time constant

The 63.2% response time.

Bandwidth

Frequency range over which the magnitude variation is within 3 dB, or themagnitude is within 3dB, of the DC magnitude.

4




TYPES OF SENSORS Resistance

Potentiometer

Strain gage

Inductance Linear variable

differential transducer (LVDT)

Proximity sensor

Capacitance

Piezoelectric Accelerometer

Pressure/tactile

Hall effect

Rotational Eddy current

Optical Encoder

IR sensor

5

Temperature

Thermocouple

Resistive temperature detectors (RTD)

Thermistor

Infrared

Image or light

Photodiode

Phototransistor

Charge-coupled device (CCD)




OUT

POTENTIOMETER

Measures resistance variation.

Simple, low-cost Slider generates electrical noise,

wears out, and contamination.

Nonlinearity due to measurement input impedance

Maximum nonlinearity occurs at:

Max nonlinearity: fs!

6




LVDT

Linear Variable Differential Transformer

AC excited (Primary coil) Core changes relative coupling to secondary coils

AC output (AM) must be converted to DC to be useful.

Primary Coil

Secondary Coils

Moving

Core

,

,

"




HALL EFFECT

Hall effect sensors detect magnetic fields. Permanent magnets + Hall effect sensor = position sensor

Magnetic field changes the pattern of current flow in a

thin semiconductor sheet Voltage across the thin sheet changes, ~100 µV.

Either magnet or the Hall sensor can move.

Non-contacting positioning sensing. Requires magnets, which could generate unwanted

forces.

Hall element

(semiconductor)

Magnet

Vout

#




TYPES OF ACTUATORS

Electro-Mechanical Electromagnetic

Electric motors DC motors Stepper motors AC induction motors Servomotors

Solenoid and relays Piezoelectric

Hydraulic and Pneumatics Valves

Actuators – pumps, motors, and cylindersCombustion

IC engines Turbine engines

$




TYPES OF ACTUATORSHobby Servomotors

• Characteristics• Voltage: usually 5-6 VDC• Sizes: micro, standard, giant (1/4 scale)• Speed: time to rotate 60˚ (standard is 0.24 sec/60˚)• Torque: up to 200 oz-in (standard around 40 oz-in)

• Operation• Angle defined by duration of pulse (not duty cycle!)• 1.5 ms pulse goes to neutral (90 degrees)

• 1 ms is generally min duration, and 2 ms is max duration

• RC PWM is used differently than other forms of PWM 1.5 ms pulse every 6 ms goes to same position as 1.5 ms pulse every 16 ms.

• Servo will hold position up to its torque rating

• Analog (50 Hz)• Limited in speed and torque• Sluggish for small command signals (deadband)

• Digital (300 Hz)• Smaller deadband, quicker acceleration, less ripple in holding torque• Improved performance comes at the cost of greater power consumption

10




POSITION AND VELOCITY

MEASUREMENTPosition and velocity measurements are used in all

mechanical systems.They can be processed through many media. We willconcentrate on sensors with electrical output.

11




POSITION AND VELOCITY

MEASUREMENTAnalog or Digital?

Analog Information Coding (Processing) Difficult to achieve a high dynamic range (ratio between the

smallest resolvable increment and the maximum displacement).

Efficient coding – single wire carries all the information.

Intuitive

Need careful design for grounding and shielding – this increasescost per wire.

Digital Information Coding Decoupled design between minimum count (physical constraint)

and maximum range (word size).

Computation speed is the major limitation.

12




ANALOG VELOCITY

MEASUREMENTTachometer – Same operating principle as DC motor/generator

Runs as an unloaded generator.

Produces open-loop voltage proportional to rotational speed: Back-emf constant is the primary functional parameter, which is also called the

voltage constant:

Output is usually fed into a device with high input impedance –

negligible current drain. Since the current is zero, the other side of the motor electrical equation is

irrelevant.

Brush and commutation switching are the two main noise source for

analog tachometer. Noise reduction is the main design consideration – heat is not an issue.

High velocity limitation is the voltage limit of the attached equipment.

Low velocity performance limited by noise level.

⋅ ⋅

13




ANALOG VELOCITY

MEASUREMENTLow Velocity Behavior

Stop – How to determine?

Obvious and intuitive definition Difficult to determine under noise and resolution limitation.

Wait for a specific period of time after reaching and maintaining belowthe lowest measurable velocity.

Apply brake when reaching a suitably low (but measurable) velocity.

Stiction Unique to mechanical systems. Two surfaces will lose relative motion when the relative velocity drops

below a sticking velocity – the system will stop!

Good when system needs to stop and bad when trying to startmotion. Especially bad for precision positioning.

Low velocity scanning Requires constant (low) speed to perform operation such as

inspection, welding, cutting, and laser texturing.

14




PULSE MEASUREMENT

OF VELOCITYPulses generated at uniform intervals, creating discrete points ofmotion.

Pulse Frequency Modulation – Analog velocity information is carried inthe pulse frequency.

Good for unidirectional motion or when the direction can be inferredfrom other information.

Pulse Generating Device Optical reflective, optical occluding, magnetic, pressure, reluctance, and

mechanical switches all can be used to generate pulses.

Pulse should be reasonably clean to avoid false triggering.Frequency-to-Voltage Converter

Generate an analog voltage that is proportional to the frequency. Additional ADC needed for digital systems.

Noise is always a limiting factor.

15




LOGIC OF FREQUENCY

MEASUREMENTA timer, a counter and a few control circuits are needed tomeasure frequency.Measure (count) the number of pulses that occur within a fixed

period of time – result is frequency, which is proportional tovelocity (if the time period does not change).Timer control the timing and the counter counts the number ofpulses:

Q: What are the potential errors that can occur in this system?

%lear

&ut'ut(atch

)eset %ounter

Set *imer

Start *imer)unnin+

)ea! %ounter

,(oa! &ut'ut (atch-

*imer not

!one

*imer !one

S*.)*

16




LOGIC OF FREQUENCY

MEASUREMENTAdd check for counter overflow to avoid erroneous readings:

Further Safety Measure – Use handshake to make sure outputlatch information (data) is only read when it is valid.Can be implemented in software:

Timer is normally still done with hardware timer. Timer interrupt willbe initiated to trigger the software to count. A hardware counter canalso be used.

Processor speed is the major limitation.

*imer not

!one an!no o/erflo

*imer !one

an! no

o/erflo

%lear

&ut'ut

(atch

)eset %ounter

Set *imer

Start *imer

)ea! %ounter

,(oa! &ut'ut (atch-

)unnin+

S*.)*

Set &ut'ut

(atch to

Max%ounter

o/erflo

1"






DIGITAL POSITION

MEASUREMENTIncremental Encoders

Related to pulse generators.

Extends the dynamic range of analog position instruments. Uses quadrature + index pulse.

Quadrature consists of two channel signalsthat are 90˚out-of-phase.

Quadrature Decoding Decoders usually convert quadrature signals to two pulse channels:

One for forward counts and one for backward Can be send directly to up/down counters.

Channel A

Channel B

Index

1$




QUADRATURE DECODING

Quadrature decoding can be 1X, 2X, or 4X.

1X Decoding – decodes the positive edges in one channelonly

The other channel is used to detect direction changes.

2X Decoding – decodes only the positive edges or all theedges in one channel.

4X Decoding – decodes all transition edges.

%hannel .

%hannel

4

2

1

20




QUADRATURE DECODINGDecoders must be able to send the result of the counter to thecomputer.

Decoder cannot lose count.

If the counter is binary (as it usually is), a data-valid handshakeis required.

Avago HCTL-2001-A00

21




INCREMENTAL ENCODERPrimary disadvantage – incremental position information only.Index pules can be used for homing purposes.

No way to recover from error – errors are cumulative. Decoder must not miss any transition – must not lose count.

Quadrature rate can often be up in the MHz range Limited by the speed of the decoder circuit (clock rate).

No theoretical limit to dynamic range.

Quadrature is grey code – if two bits change that is an error. Can use to develop error checking capability.

Encoder interface Single ended vs differential (IEEE-422) (A,B,GND) vs (A,A’,B,B’)

Alignment is very important!

22




INCREMENTAL ENCODERS

Optical Shutter

Moving grate with light source on one side and detector on theother.

Allows grid size to be smaller than light source. Two receivers are appropriately aligned (critical) to give quadrature.

Light Source

Moving Grate

Light Receiver

Collimated

Beam

23





Micro-Decoding Analog signal from each channel is (ideally) triangular –

real signals are more sinusoidal.

As long as the shape is reproducible and stationary, it

can be decoded (ADC) to get finer position. Limitation is the stability of the signal waveform and

analog noise.

Channel A

Channel B

24





Velocity from Encoders Same approaches as pulse frequency measurement.

Logic is complicated by the potential change in direction.

Simplest method is to approximate velocity by compute the differences in position(counts) between fixed samples times.

Precision is not so good at low velocity (why?).

Period measurement can be used, but must take direction change into account.

Linear vs. Rotary Encoders Rotary Motion – measured directly with rotary encoders.

Linear Motion

Linear Encoder – must be of the length of the motion

Gives greatest accuracy, but expensive.

Rotary Encoder – use with either rack/pinion or lead screw type transmission

Can be less expensive

Accuracy depends on quality of the transmission mechanism.

25




INCREMENTAL ENCODERSLaser Interferometry

Precise but expensive!

Very high resolution – down to about 2 nanometer (10-9).

Output depends on phase difference between signal reflected frommoving object and a reference signal.

Laser is necessary to get phase-coherent mono-frequency light.

Major errors are from alignment problems, temperature changes inthe air path, motion of air cause density changes, ...

Target

Retro-reflector

Beam Splitter

Interferometer Receiver

& Signal Processing

Collimated

Laser Beam

Electrical Signal

Quadrature Output

26




ABSOLUTE ENCODER

Multi-channel extension of incremental encoders.One track for each digital channel.

Rotary only – difficult to implement in linear applications. It is a digitalalternative to a resolver.Optical or brush contact.

Common coding: Natural binary. Grey code. Binary-coded decimal.

Q: How would one implement a Grey coded absolute encoder?

2"

SYNCHROS AND




SYNCHROS AND

RESOLVERSAnalog devices that measure absolute angular position.

Resolvers and synchros are, in principle, rotating transformers.

Excite winding B with sinusoid: sin

Voltage will be induced acrosswinding A: ⋅ sin

Place winding C at 90° w.r.t.winding A: ⋅ !s

Resolver has a 4-wire output whilea synchro has a 3-wire output

2#




RESOLVER OUTPUTS

Outputs from the resolver (synchro) are amplitude modulated ACsignals:

Angular position information is encoded in the amplitude of themodulated signal.

Processing the modulated signal is often thought as phase detection.It is NOT!

Phase and carrier frequency of the outputs are nominally (ideally) thesame (fixed).

Two important deviations are:

Phase shift between the input excitation (AC carrier signal) and the outputsignals.

Rotor motion induced voltage fluctuation (back EMF).

sin ⋅ sin

" !s ⋅ sin

#$s!%&$'

( sin ⋅ sin

) !s * +-. ⋅ sin

/ !s +-. ⋅ sin

01n2'!

2$

RESOLVER SIGNAL




RESOLVER SIGNAL

CONVERSION

Estimate the shaft angle to be φ – DC analog signal.

Output of the multipliers are:

When subtracted, the signal is proportional to:

Which has the error in position estimation – can be used as the input to afeedback loop to eliminate the position error.

!s 3 ⋅ sin ⋅ sin

" sin 3 ⋅ !s ⋅ sin

sin 3 ⋅ sin

30

sin 3

sin

RESOLVER SIGNAL




RESOLVER SIGNAL

CONVERSIONOne way of demodulation is to divide the previous signal by the reference ACexcitation signal:

Will not work when sine is zero – apply only when the absolute ofthe reference AC excitation is above a specified limit.

Two stage PI and I controller is used to eliminate steady-state

position error under constant velocity operation. Input to the second integrator is the estimated velocity.

4$5!4 sin 3 ⋅ sin

sin sin 3

31

RESOLVER TO DIGITAL




RESOLVER-TO-DIGITAL

CONVERTER

Same basic scheme:

Replace second stage integrator with an up/down counter.

Drive counter with VCO (voltage controlled oscillator). Input to the VCO is the output of the PI compensator.

Overall precision is 12 to 16 bits.

The output represents the absolute angular position within one resolver rotation.

32




BRUSHLESS RESOLVERContact resolvers have the same drawback as DC motors; brushcontact is needed to commutate the rotor coils.

Brush or slip-ring contact introduces additional friction andregular wear-and-tear to the system

Rotor excitation can be inductively coupled (induced) throughadditional windings to eliminate the need for slip-ring contacts.

33

Unit8_SensorsAndActuators

Documents