Robotics (EE 514)Lecture 2
Dr. Ahmed El-ShenawyElectrical and Control Engineering
Sensors and Actuators
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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.
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Servomechanism
• Mechanism exploring feedback to deliver number of revolutions, position, etc.
• The controlled quantity is mechanical.
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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.
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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.
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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.
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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.
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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.
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Robust Control
Robust control theory is a method to measure the performance changes of a control system with changing system parameters.
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Classification of Electric Motors
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How does a DC motor work ?
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DC motor, a view inside
•Simple, cheap.•Easy to control. •1W -1kW •Can be overloaded.•Brushes wear.•Limited overloadingon high speeds.
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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.
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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
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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
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Stepper Motors
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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.
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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.
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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.
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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.
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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.
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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.
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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
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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
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HUMAN SENSES[Vision, hearing, smell, taste, touch]
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SENSORS IN EVERYDAY LIFE
Automobiles Cell Phones Remote Controls Traffic Lights Appliances (stove, refrigerator, furnace,
thermometer) Motion Sensors Smoke Detectors Gas Detectors
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Savannah, River Site NuclearSurveillance Robot
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Evolution Robotics Scorpion
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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
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– Random/Non-Deterministic Error• No prediction possible– E.g. Hue instability of camera, black level noise of camera– Precision• Reproducibility of sensor results
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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
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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
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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
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Quadrature Wheel/Motor Encoders
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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.
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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.
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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.
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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
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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.
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Inertia Motion UnitIMU
ACCELEROMETER AND GYROSCOPES SENSORS
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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
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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.