robotics - aast.edu

Robotics (EE 514)Lecture 2

Dr. Ahmed El-ShenawyElectrical and Control Engineering

Sensors and Actuators

Robotics (Lecture 2)

Dr Ahmed El-Shenawy 2

What is an actuator?

• A mechanical device for actively moving or driving something.

• Source of movement (drive), taxonomy:

Electric drive (motor).

Hydraulic drive.

Pneumatic drive.



Servomechanism

• Mechanism exploring feedback to deliver number of revolutions, position, etc.

• The controlled quantity is mechanical.



Properties of a servo

High maximum torque/force allows high (de)acceleration.

Can be source of torque.

High zero speed torque/force.

High bandwidth provides accurate and fast control.

Works in all four quadrants

Robustness.



What is Four Quadrant Control? If we consider both directions of operation (ClockWise and CounterClockWise) and both modes (acceleration and deceleration), we arrive at four distinct areas, or quadrants, of operation.



Quadrants 1 and 3 represent the motor applying torque in the direction ofmotion, while quadrants 2 and 4 represent applying torque opposite thedirection of the motion. In quadrants 1 and 3 the flow of energy is fromelectrical to mechanical. The servo motor is converting electrical power fromthe drive into motion in the system. In quadrants 2 and 4, the motor isactually acting as a generator. The motion of the system is being convertedinto electrical power, which is then absorbed by the drive.



Some motor drives are capable of operating in only quadrant 1, while others will work in quadrants 1 and 3, quadrants 1 and 4, etc. Since most servo drives are designed for accurate control in both directions, many will operate, at least transiently, in all for quadrants. To do this the drive must be able to both source and sink electrical power from the motor.



Robust Control

Robust control theory is a method to measure the performance changes of a control system with changing system parameters.



Classification of Electric Motors



How does a DC motor work ?



DC motor, a view inside

•Simple, cheap.•Easy to control. •1W -1kW •Can be overloaded.•Brushes wear.•Limited overloadingon high speeds.



DC motor control

Controller + H-bridge (allows motor to be driven in both directions).

Pulse Width Modulation (PWM)-control.

Speed control by controlling motor current=torque.

PID control.



DC motors

Field poleNorth pole and south poleReceive electricity to form magnetic

field

ArmatureCylinder between the polesElectromagnet when current goes

throughLinked to drive shaft to drive the load

CommutatorOverturns current direction in armature



DC motors, cont.

Speed control without impact power supply quality

Changing armature voltageChanging field current

Restricted use

Few low/medium speed applicationsClean, non-hazardous areas

Expensive compared to AC motors



Stepper Motors


Dr Ahmed El-Shenawy

Brushless DC Motors

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BLDC motors have many similarities to AC induction motors and brushed DC motors in terms of construction and working principles respectively. Like all other motors, BLDC motors also have a rotor and a stator.

BLDC motors have many similarities to AC induction motors and brushed DC motors in terms of construction and working principles respectively. Like all other motors, BLDC motors also have a rotor and a stator.


Dr Ahmed El-Shenawy

Brushless DC Motors

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StatorSimilar to an Induction AC motor, the BLDC motor stator is made out of laminated steel stacked up to carry the windings.


Dr Ahmed El-Shenawy

Brushless DC Motors

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RotorThe rotor of a typical BLDC motor is made out of permanent magnets. Depending upon the application requirements, the number of poles in the rotor may vary. Increasing the number of poles does give better torque but at the cost of reducing the maximum possible speed.


Dr Ahmed El-Shenawy

Brushless DC MotorsThe underlying principles for the working of a BLDC motor are the same as for a

brushed DC motor; i.e., internal shaft position feedback. In case of a brushed DC

motor, feedback is implemented using a mechanical commutator and brushes. With

a in BLDC motor, it is achieved using multiple feedback sensors. The most

commonly used sensors are hall sensors and optical encoders.


Dr Ahmed El-Shenawy

Brushless DC MotorsNote: Hall sensors work on the hall-effect principle that when a current-carrying

conductor is exposed to the magnetic field, charge carriers experience a force based

on the voltage developed across the two sides of the conductor.

If the direction of the magnetic field is reversed, the voltage developed will reverse as

well. For Hall-effect sensors used in BLDC motors, whenever rotor magnetic poles (N

or S) pass near the hall sensor, they generate a HIGH or LOW level signal, which can

be used to determine the position of the shaft.


Dr Ahmed El-Shenawy

Brushless DC Motors

Trapezoidal Commutation of BLDC Motor

One of the simplest methods of control for dc brushless motors uses what is

termed Trapezoidal commutation.

Sinusoidal Commutation for

BDLC Motors

Trapezoidal commutation is

inadequate to provide smooth and

precise motor control of brushless

dc motors, particularly at low

speeds. Sinusoidal commutation

solves this problem.



Hydraulic actuators

linear movementbig forces without gearsactuators are simple in mobile machinesBad efficiencymotor, pump, actuator combination is lighter than motor, generator, battery, motor & gear combination



SENSORSA sensor (also called detector) is a converter that measures a physical quantity and converts it into a signal which can be read by an observer or by an (today mostly electronic) instrument. (wikipedia)

Are devices capable of detecting change:TemperaturePressureHumiditySpeed

http://en.wikipedia.org/wiki/Energy_conversion

http://en.wikipedia.org/wiki/Physical_quantity

http://en.wikipedia.org/wiki/Electronics



HUMAN SENSES[Vision, hearing, smell, taste, touch]



SENSORS IN EVERYDAY LIFE

Automobiles Cell Phones Remote Controls Traffic Lights Appliances (stove, refrigerator, furnace,

thermometer) Motion Sensors Smoke Detectors Gas Detectors



Savannah, River Site NuclearSurveillance Robot



Evolution Robotics Scorpion



Sensor Performance

– Sensitivity• Ratio of output change to input change– Cross-Sensitivity• Sensitivity to environmental parameters that are orthogonalto the target parameters

– Error/Accuracy• Difference between the sensor’s output and the true value

– Systematic/Deterministic Error• Caused by factors that can (in theory) be modeled

– E.g. calibration of a laser sensor



– Random/Non-Deterministic Error• No prediction possible– E.g. Hue instability of camera, black level noise of camera– Precision• Reproducibility of sensor results



Classification Of Sensors

Proprioceptive sensors– Measure values internally to the system (robot)– E.g. motor speed, wheel load, heading of the robot,battery status

Exteroceptive sensors– Information from the robots environment– Distances to objects, intensity of the ambient light,unique features



Classification Of Sensors (cont…)

Passive sensors– Energy coming for the environment

Active sensors– Emit their proper energy and measure the reaction– Better performance, but some influence onenvironment



Wheel/Motor EncodersWheel/motor encoders measure the position or speed of wheels.Wheel movements can be integrated to get an estimate of the robots position – odometry.

Optical encoders are proprioceptive sensors

– Thus the position estimation inrelation to a fixed referenceframe is only valuable for shortmovements

Typical resolutions of 2000increments per revolution



Quadrature Wheel/Motor Encoders


Dr Ahmed El-Shenawy

Absolute Encoder

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Every position of an absoluteencoder is unique. Unlike anincremental encoder, whereposition is determined bycounting pulses from a zeromark or home base, theabsolute encoder reads asystem of coded tracks toestablish position information.No two positions are alike.


Dr Ahmed El-Shenawy

Absolute Encoder

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Absolute encoders do not loseposition when power is removed. Since each position is unique, true position verification is available assoon as power is up. It is notnecessary to initialize the system by returning to home base.


Dr Ahmed El-Shenawy

Absolute Encoder

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Absolute encoders do not lose position when power is removed. Since each position is unique, true position verification is available as soon as power is up. It is not necessary to initialize the system by returning to home base.

The coded format is a variation of Binary code called Gray code. Gray code is used because only one "bit" changes between adjacent words on the disk.

This limits any transition errors of the word tracksensors to plus or minus one count. In all other codes, such as Binary or Binary Coded Decimal (BCD), several bits are required to change state between adjacent words.



Range Sensors

Large range distance measurement -> called range sensors

Ultrasonic sensors as well as laser range sensors make use of propagation speed of sound or electromagnetic waves respectively



The traveled distance of a sound orelectromagnetic wave is given by

d = c . T

Where

– d = distance traveled (usually round-trip)– c = speed of wave propagation– t = time of flight.


Dr Ahmed El-Shenawy

Inertia Motion UnitIMU

ACCELEROMETER AND GYROSCOPES SENSORS

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Dr Ahmed El-Shenawy

The Accelerometer's Sensing Mechanism

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The displacement of the movable mass (micrometer) is caused by acceleration,

and it creates an extremely small change in capacitance for proper detection


Dr Ahmed El-Shenawy

The Accelerometer's Sensing Mechanism

This mandates using multiple movable and

fixed electrodes, all connected in a parallel

configuration. The configuration enables a

greater change in capacitance, which can

both be detected more accurately, and

ultimately makes capacitance sensing a

more feasible technique.

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