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Robots are typically comprised of the following major subsystems
- Mechanical
- Electrical Power and Signals
- Sensing
- Software
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Program Control Levels
- Teleoperator: responds to user-supplied commands
- Blind mobility: executes a program of instructions
- Teach/playback: copies historical behavior of itself
- Convoy: copies behavior of another vehicle
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Supervised control
Operator specifies broad goals at various frequencies
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Analog connections are used to transmit a
voltage to the device
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Parallel communication
transmits several streams of data simultaneously along multiple channels
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Serial communication
is the process of sending data one bit at a time, sequentially, over a communication channel or computer bus
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Benefits of Serial communication
Less mass, don't have to worry about interference
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Serial sends information
one bit at a time
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Serial has specific timing requirements, specified by
baud rate, similar to a bar code
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A bus is used to transfer
digital information between components
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A bus uses a specific
protocol for reading digital signals
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A bus would be like a human
spine
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Information transfer over a bus typically has a
host and at least one device
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The host offers
information, resources, services and applications to a device
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A device is connected externally or internally to the host and is
directed and used by the host
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Host and Device relationship is in many ways a
slave and master relationship
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Controller Area Network (CAN)
- Primarily in automobile industry
- Automatic retransmission of corrupted messages as soon as the bus is idle again
- Priorities are given to messages based on importance
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Inter-Integrated Circuit (I2C)
- Master/Slave Serial Bus
- Used when low cost is more important than speed
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TCP
- Synchronous
- Built in error detection and correction
- High Reliability, low efficiency
- (Ex: Webpage)
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UDP
- Asynchronous
- Error detection, no correction
- Low reliability, high efficiency
- (Ex: streaming)
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ASCII Encoding
- Human readable
- Easy to debug
- Easy to program
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Binary Encoding
- Pack more data into fewer number of bytes
- Computationally more efficient
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Asynchronous
- Occurs at any time without warning
- Variable bit rate
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Synchronous
- Steady stream of data
- When stream ends, communication is considered terminated
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Commonly used data fields
- Start Byte/Bit
- Message ID
- Packet Size
- Addressing for point to multipoint
- Out of Order Checking – timestamp, sequence number
- Error Checking – Checksum, CRC
- Error Correction
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Start/End Bit/Byte
Needed in synchronous, stream-based transport–To tell where stream ends/starts
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Out of Order
Time Stamp
–Sender includes a timestamp in every message and receiver compares timestamp to previous messages
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Out of Order
Sequence Number
–Sender includes a unique sequence number for every message. Increment by one for every message sent
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Error Detection Types
- Parity
- Checksum
- Cyclic Redundancy Check
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Error Correction
ACK/NAK
- Simplest form of error correction
- Ignore data with errors, ask for data to be sent again
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Error Correction
Hamming Codes
- –Redundant data
- Inefficient, lots of extra data
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Pulse-Width Modulation (PWM) is another form of
Digital communication
signals are pulsed and pulse width indicates value
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PWM has relatively low power losses.Makes it an efficient way of encoding _____ signals
analog
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Information bandwidth is ______ proportional to frequency
directly
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Fundamental trade-off:
propagation vs. bandwidth
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Signal degradation is ______ proportional to frequency
inversely
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High frequency =
Low frequency =
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Radio Transmitters
- Encode information and generate electric output signal at acceptable power level
- Signal must be modulated around carrier frequency
- Most modern radios are digital
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Spread Spectrum
Greater data rates can be achieved via using distributed carrier waves (spread spectrum)
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Frequency Hopping Spread Spectrum (FHSS)/ Time Domain Multiplexing (TDM)
- Pseudorandomly jumps between carrier channels
- Robust to Doppler shift = good for aerial
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Orthogonal Frequency Division Multiplexing (OFDM)
- Transmits on multiple frequencies simultaneously
- Robust to interference and multipath = good for ground
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Ideal "isotropic" antenna tranmits
Equal power in all directions
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Antennas
- Convert electrical power and RF waves
- Must be matched to transmitter
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Gain is the increase in radiated power relative to the
ideal isotropic model
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Total power
does not change
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Omnidirectional Antennas
Radiate power in “all” directions equally
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Directional Antennas
- Focus power in one direction to achieve gain
- More gain = narrower band
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Radio Receivers
Filters and processes electrical signals from antenna to decode signal
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Radio Receivers
Two important factors
- Signal-to-Noise Ratio (SNR)
- Receive sensitivity
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Cabling and Connectors
- Significant signaling losses occur in cables and connectors
- Losses increase with frequency
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Radio Units
Power is typically expressed in dB
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RF power often uses
Antenna gains expressed in
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Pulse-Width Modulation (PWM) is another form of ________ communication
digital
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Pulse-Width Modulation (PWM) signals are
pulsed and pulse width indicates value
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Pulse-Width Modulation (PWM) signals have relative
low power losses and makes it an efficient way of encoding analog signals
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Given a robot’s geometry and the velocity of its wheels, how does the robot move?
Forward Kinematics
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Given where a robot needs to move and robot geometry, what wheel velocities are required for motion
Inverse Kinematics
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Wheel Kinematic Constraints Assumptions
- Movement on horizontal plane
- Point contact of the wheels
- Wheels not deformable
- Pure rolling ( = 0 at contact point)
- No slipping, skidding or sliding
- No friction for rotation around contact point
- Steering axes orthogonal to the surface
- Wheels connected by rigid frame (chassis)
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Given a robot with M wheels. What wheels impose constraints ?
Only fixed and steerable standard wheels impose constraints
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In a plane you only have three degrees of freedom which can be controlled via:
- Wheel Steering
- Driven Wheels
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Why do we prefer to control the degrees of freedom via driven wheels?
Must have one and are easier to control
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A holonomic constraint can be expressed explicitly in terms of
position
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A nonholonomic constraint requires expression in terms of
erivatives
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An omnidirectional robot can move at any time in any direction along the ground plane (x,y) regardless of the orientation of the robot around its ____ axis.
vertical
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Is the fixed wheel sliding constraint holonomic?
No
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Note that robots with only holonomic constraints are considered
holonomic robots
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Differential Steering
- Two Drive Wheels
- Caster for Stability
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SKID Steering
- Wheeled and tracked
- Wheels slide and slip
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Ackerman Steering
- Front Wheels Rotate
- Not Parallel Steering!
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High Maneuverability
Skid, Differential
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Energy efficient
Ackerman, Differential
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Good for Off-road Use
Skid, Ackerman, 4-Wheel
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Low Mechanical Complexity
Differential, Skid
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Easy to Model
Differential, Ackerman, 4-Wheel
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In quasi-static stability any snapshot of the robot shows that the robot is
statically stable
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A robot must have at least _ legs for quasi-static walking stability
4
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Why must a legged robot maintain ground contact for it to have a kinematic model?
gravitational force
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Aerial vehicles do not have kinematic equations because of
gravitational forces
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Proprioceptive sensors measure
values internal to the system, for example, engine temperature, motor speed, wheel speed, vehicle acceleration,
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Exteroceptive sensors
acquire information from the environment, for example distance measurements,
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Passive sensors measure
ambient environmental energy entering the sensor
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Active sensors
emit energy into the environment, then measure the environmental reaction
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Ideally, the analog output or digital signal value will be proportional to the
measurement
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Systematic error also known as a
deterministic errors
caused by factors that can (in theory) be modeled -> prediction
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Random error also known as a
non-deterministic
no prediction possible
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describes the closeness of output readings when the same sensor input is applied repetitively over a short period of time and the same experimental conditions
Repeatability
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describes the closeness of a sensor's readings when there are changes in the experimental conditions.
Reproducibility
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It is normally desirable that the output reading of a sensor is _____ proportional to the quantity being measured.
linearly
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Typical types of nonlinearities
hysteresis and dead space
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describes the effect in which the zero reading of an instrument is modified by a change in ambient conditions.
Zero drift or bias
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defines the amount by which an instrument's sensitivity of measurement varies as ambient conditions change
Sensitivity drift (also known as scale factor drift)
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ratio of output change to input change
Sensitivity
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sensitivity to environmental parameters that are orthogonal to the target parameters
Cross-sensitivity
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Eliminating Noise
Hardware and transmission lines
- Shielding
- Using twisted pair wires
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Eliminating Noise
Signal processing
- Kalman Filtering
- Particle Filtering
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measuring instrument describe the behavior between the time a measured quantity changes value (from one constant to another) and the time when the sensor attains a steady value in response
dynamic characteristics
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An example of a zero order instrument
potentiometer
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Example of a first order instrument
liquid-in-glass thermometer
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Example of a second order instrument
accelerometer
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used to measure the speed with which a sensor can provide a stream of readings.
Bandwidth
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Formally, the number of measurements per second is defined as the sensor's frequency in
hertz.
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IP ratings are expressed with two (or three) numbers.
First number =
Second number =
Third number =
- solid objects
- liquid
- mechanical impacts
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