MAS836 – Sensor Technologies for Interactive Environments
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MAS836 – Sensor Technologies for Interactive Environments. Lecture 10 – Digital Sensor Processing and Modules. The Next Steps. So far, we have concentrated on the use of a variety of sensors to measure human interaction However, a measurement alone is of little value Next steps:

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MAS836 – Sensor Technologies for Interactive Environments

Lecture 10 – Digital Sensor Processing and Modules

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The Next Steps

  • So far, we have concentrated on the use of a variety of sensors to measure human interaction

  • However, a measurement alone is of little value

  • Next steps:

    • Conversion of the data into a computer-readable format

    • Processing within the microcontroller

    • Communication with to other devices

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Analog Processing

Analog to Digital Conversion

Digital Processing

RF Transmission


One Approach

  • There are numerous possible data flow patterns

  • We will concentrate on one:

  • Each involves different technologies and issues

  • We will also discuss other networking protocols

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Analog Processing (1)

  • Most analog signal processing was covered in first three lectures

  • Some issues are of particular importance when preparing data for analog to digital conversion

  • Just as important for looking at data using other techniques (e.g. oscilloscope)

  • Three issues are quickly touched upon

    • Obviously there are many more

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Analog Processing (2)

  • Output Range

    • Voltage output should fill the input range of the analog to digital converter (ADC)

    • Unipolar/bipolar

  • Output Bandwidth

    • Amplifier should attenuate outside frequencies of interest (both high and low)

    • Take both sensor and data bandwidth into account

  • Output Impedance

    • Usually need to buffer signal to avoid loading of next portion of signal chain

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Analog to Digital Conversion:Basics

  • Converts analog data (continuous) into digital values (discrete)

  • Input range is divided into steps

    • Each is at a discrete voltage and represented by an integer value

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Analog to Digital Conversion:Figures of Merit

  • Number of bits: Size of the integer used to represent the analog value.

    • N bits provide 2N different values

  • Vref: Maximum voltage of the converter

    • Vref is represented by 2N-1

    • Minimum voltage is almost always 0V

  • Sampling rate: Number of individual measurements made in a fixed time

    • Limited by technique used

  • Impedance limit: Largest input impedance at which the ADC will still function properly

    • Based on size of internal hold capacitor and sampling rate

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Analog to Digital Conversion:Successive Approximation

  • Most common form of ADC

  • Bits are calculated one at a time and operation can be stopped at any point

  • Each bit is found by comparing the input value to the value represented by all the bits calculated so far

    • Requires a DAC

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Analog to Digital Conversion:Flash

  • Simplest, fastest form of ADC

    • Sometimes known as a parallel converter

  • Voltage ladder divides reference voltage into 2N steps, which are compared to the input voltage

    • Requires 2N resistors and comparators

    • Limited by resistor accuracy

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Analog to Digital Conversion:Misc

  • Highest accuracy ADCs are sigma-delta type

    • Gains arise from oversampling (alters noise spectra)

  • Chart shows range of ADCs discussed

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Analog to Digital Conversion:Sampling Rate

  • Aliasing is the folding over of higher frequencies when sampled at less than double their frequency

    • Leads to loss of data and increase in noise

  • To avoid:

    • Always sample at twice the highest frequency of interest (known as Nyquist sampling)

    • Always filter out higher frequencies before conversion

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Analog to Digital Conversion:Error/Noise

  • The error in an ADC can be treated as a white noise source

    • Only if number of bits is high enough and voltage is in range

  • Equivalent to half of the least significant bit

    • 6(N+1) dB

  • Comparison of this value and the noise in the system allows calculation of the maximum accuracy (number of bits) worth considering

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  • A microcontroller (uC) is a small, lightweight CPU which is usually combined with on-board memory and peripherals

    • Compact and low power (relatively)

  • Often used as a simple hardware to software interface as well as for in-situ processing

    • Analog to digital gateway

    • Allows for real-time feedback based on data

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Microcontrollers:Features (1)

  • Processor speed: Fundamental measure of processing rate of device

    • Value of interest is in MIPS, not MHz

  • Supply voltage/current: Measure of the amount of power required to run the device

    • Multiple modes (sleep, idle, etc)

  • It is possible to adjust the voltage and frequency of some devices in real time, thereby trading off speed and power usage

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Microcontrollers:Features (2)

  • Internal memory: Sometimes divided between program and data memory, the amount of information that can be stored on board

    • Can sometimes be supplemented by external memory

  • I/O Pins: Individual points for communication between the uC and the rest of the world

    • Can be digital or analog, general or special purpose

  • Interrupts: Non-linear program flow based on event triggers from peripheral or pins

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Microcontrollers:Peripherals (1)

  • Timers: Internal registers (any size) in the uC that increment at the clock rate

    • May have prescaler

    • May be combined with range testing for interrupt

    • Watchdog timers reset processor if it hangs.

  • Comparators: Input that effectively functions as a 1-bit ADC with an adjustable threshold

    • Used for real-time data monitoring

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Microcontrollers:Peripherals (2)

  • ADC: Most ADCs used in sensor data collection are integrated with uC

    • Watch for number of channels vs number of inputs

    • Sampling speed does not include input switch time

    • Very fast ADCs often combined with DMA

  • DAC: Digital to analog converters are also include in some data collection driven uC

    • Mostly used for feedback and control

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Microcontrollers:Communication (1)

  • UART: Basic hardware module which mediates serial communication (RS232)

    • Simplest form of communication but limited by speed

    • Most modules are full duplex

  • USB: High bandwidth serial communication between uC and a computer or an embedded host

    • Usually requires chips with specialized hardware and firmware

    • Host side issues

  • SPI: Full duplex master-slave 4-wire protocol for data transfer between uCs

    • Mbit transfer rates, but somewhat quirky protocol

    • Unlimted (almost) nodes, can change master

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Microcontrollers:Communication (2)

  • I2C: Half duplex master-slave 2-wire protocol for data transfer between uCs

    • kbit transfer rates

    • Tx/Rx based on slave addressing

    • Can invert protocol with sensors as masters

  • RF: Radio frequency (>100 MHz) EM transmission of data

    • Built in to some newer special-purpose uC

    • Wireless spherical transmission

    • More later

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Microcontrollers:Atmel AVR

  • 8-bit RISC series of microcontroller chips

    • Large range of available devices covering many interfaces, speeds, memory sizes, and package sizes

    • Large hobbyist development community with many available free toolchains and sample applications

  • General specs

    • One MIPS per MHz

    • Models available up to 20MHz

    • Max 128K program space / 8K RAM

    • ADC/LCD Driver/Motor Control



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Microcontrollers:TI MSP430

  • Proprietary TI low-power low-cost RISC chips

    • Well supported by TI with good program chain

    • Designed for intermittent sampling and fast startup

  • General specs

    • Very low power (flexible)

    • Max 32KHz / 8 MIPS

    • Max 50K program space / 10K RAM

    • Max 16 bit ADC

    • UART/SPI/DAC/LCD/PWM/Comparators


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Microcontrollers:Silicon Labs (FKA Cygnal)

  • 8051 derivate uC with high reconfigurability

    • Many programming environments available

    • Vary from 3mm2 to 100 pin packages

  • General specs

    • Medium power

    • Max 100 MHz / 100 MIPS

    • Max 128K program space / 8K RAM

    • Max 16 bit ADC

    • UART/USB/SPI/CAN/PWM/Comparators


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Microcontrollers:Atmel ARM7 (AT91SAM7S series)

  • 32-bit ARM microcontroller

    • Low power (for 32-bit machines)

    • Can run in 16-bit mode if needed

  • General specs

    • Lots of memory (8-64KB RAM, 32-256KB flash)

    • Variable speed up to 55MHz

    • Packed with peripherals (USB, ADC, SPI, etc.)

    • Comes in LQFP 48 and 64 packages

    • Not suitable for beginners


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  • Analog Devices ADUC8xx:

    • More of an ADC with a uC attached

    • Some models include 24 bit sigma-delta converter

    • Useful with IEEE 1451 (see later)


  • Chipcon CCxxxx:

    • More of an RF transceiver with a uC attached

    • Variety of frequency ranges and modulation schemes


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Digital Filter h[n]


Digital Signal Processing:Basics

  • A discrete time stream x[n] is convolved with a discrete time impulse response h[n] to produce an output y[n]

    • x[n] is usually acquired from a continuous time signal x(t) using an ADC

    • h[n] is the response to a single input of 1 at time 0

      • Can be finite (FIR) or infinite (IIR)

      • Can be described as a difference equation

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Digital Signal Processing:Considerations

  • Why would I want to do this?

    • Reconfigurable in real-time

    • Allows off-line processing

    • Power savings

  • What are the drawbacks?

    • Non-parallel

    • Memory intensive

    • Stability issues can be complex

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Digital Signal Processing:Algorithms (1)

  • Simplest and most common DSP algorithm is the N-point running average:

  • Acts as a smoothing filter

    • Pseudo-lowpass

    • Recursive implementation in two operations per cycle

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Digital Signal Processing:Algorithms (2)

  • A wide variety of frequency filters (HP/LP/BP) can be constructed as both FIR and IIR filters

    • FIR more stable

    • IIR shorter impulse response

      • See MATLAB for more details

  • Fast Fourier Transform allows quick ( ) conversion from time domain to frequency domain (for N = power of two)


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Digital Signal Processing:Algorithms (3)

  • Outlier detection

    • Not strictly DSP (no impulse response)

    • No analog equivalent

  • Difference with adjoining points must meet certain criteria:

    • Greater than some factor of a baseline value

      • ie More than three greater than the standard deviation

  • Sign of values are opposites

    • ie Curve changes direction (spikes)

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Radio Frequency Transmission:Basics (1)

  • Radio frequency (RF) electromagnetic transmission is the use of high frequency radiation to transmit data wirelessly between devices

    • Can be very high speed (50 Mbits+)

    • Can have enormous range (10 km+)

    • Does not require line-of-sight

  • HCI systems tend to be <1Mbit and have a range of <10m, but the same principles apply in all cases

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Radio Frequency Transmission:Basics (2)

  • Data is transmitted on a carrier wave of a fixed frequency

    • The centre frequency is known as the carrier frequency

    • Data is introduced into the carrier through means of a modulation scheme

  • Often, multiple transmitters want (need) to share the channel, requiring channel access schemes

  • Transmissions are made more robust to (non-flat) interference through spread spectrum techniques

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Radio Frequency Transmission:Figures of Merit

  • Transmitter power: Measured in dBm (dB referenced to 1 mW), the fundamental measurement of the power in a signal

    • Most unlicensed transmitters are ~0dBm

  • Receiver sensitivity: The smallest signal which can be adequately detected

    • Usually around –90dBm

  • BER: Bit error rate, the frequency with which data is received incorrectly

    • Can be in 10-9 range for simple short transmissions

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Radio Frequency Transmission:Modulation – OOK/ASK

  • On-Off Keying (OOK) is the most trivial of all modulation schemes.

    • The transmitter is turned on to full power to send 1 and off to send 0

      • Can be driven directly with a UART

      • Most power efficient since transmitter is only on 50% of the time on average

  • Amplitude Shift Keying (ASK) is similar, except the TX power is merely lowered for a 0 bit

    • Allows for faster transmission

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Radio Frequency Transmission:Modulation - BPSK

  • Binary phase shift keying (BPSK) uses a carrier continuously broadcast at the same amplitude

  • A 1 is indicated if there is a 180 phase shift in the bit window, otherwise the bit is 0

  • Allows for better carrier lock at receiver

  • We see at right that OOK is equivalent to multiplying the carrier by the data, while BPSK is XOR

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RF Channel Sharing Protocols

  • Time Division Multiple Access (TDMA)

  • Frequency Division Multiple Access (FDMA)

  • Carrier Sense Multiple Access (CSMA)

  • Direct Sequence Spread Spectrum (DSSS)

  • Frequency Hopping Spread Spectrum (FHSS)

Laibowitz and Paradiso, “Embedded Wireless Transceivers and Applications in Lightweight Wearable Platforms,” Circuit Cellar, February 2004

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Radio Frequency Transmission:Channel Access – TDMA/FDMA

  • Time division multiple access (TDMA) divides the channel up into chunks of time, with a different transmitter for each chunk

    • Requires master receiver to allocate chunks and keep synchronization

  • Frequency division multiple access (FDMA) divides the channel up into chunks of frequency, with a different transmitter for each chunk

    • Master not necessarily required

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Tx1 Range

Tx2 Range

Radio Frequency Transmission:Channel Access – CSMA

  • For low duty cycle transmitters, we can avoid masters and complicated schemes by using carrier sense multiple access (CSMA)

    • Listen before you talk

    • Hidden node problem:

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Radio Frequency Transmission:Spread Spectrum - DSSS

  • Direct sequence spread spectrum (DSSS) expands the frequency range of a signal by modulating it (xor) with a much faster sequence

known as a chip

  • Chips must be orthogonal to each other

  • Receiver must have same noise (chip) generator

  • Provides immunity to localized (in frequency) noise

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Radio Frequency Transmission: Walsh Codes (Orthogonal)

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Radio Frequency Transmission: Spread Spectrum - FHSS

  • While DSSS spreads at greater than the data rate, frequency hopping spread spectrum (FHSS) spreads at a much lower rate

    • Shifts happen ~ 10 bits (or so)

    • Again, receiver must know sequence

  • Originally invented during WWII by Hedy Lamarr and George Antheil to avoid jamming of radio-controlled torpedoes

    • Player piano rolls used to synchronize

  • In both techniques, all other sequences appear to be noise at the receiver

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    Radio Frequency Transmission:Bands

    • Low frequency unlicensed bands at 433 and 915 MHz are often used by low power ASK devices

      • Can only transmit for 36 seconds each hour (1%)

    • The high frequency unlicensed bands at 2.4 GHZ and 5.8 GHz are used by high speed spread spectrum devices

      • Eg. Wireless LAN

    • Both parts of the ISM band

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    Radio Frequency Transmission:Usage Notes

    • RF components are enormously sensitive to:

      • Placement

        • ground plane and traces

      • Power supply

    • Antenna can also have huge effect:

      • Quarter-wave whip best, but large for lower frequencies (c=f)

      • Helical antennae attenuate by 5dB, printed whip by 10dB

      • Need to match impendence to transmitter output (50)

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    The RFRAIN Card

    • Made by Mat Laibowitz for the UbER-Badge

    • GP RF card based on the Chipcon CC1010

      • Circa 70 kbps

      • CSMA Scheme

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    The CC2500 Daughtercard

    • Made by Mat Laibowitz for the Plug and other projects

    • GP RF card based on the Chipcon CC2500

      • Circa 500 kbps

      • CSMA Scheme, but can be used with other methods

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    Sensor Networking Protocols:Overview

    • Our previous discussion considered sensor modules which did not talk to each other or have any response more complicated than continuous collection, processing and transmission of data

    • Sensor networking protocols allow:

      • To establish clusters on the fly

      • To be remotely interrogated and controlled by master devices over a variety of communication systems

      • Modules to describe their own data

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    Sensor Networking Protocols:Importance

    • Allow for easier creation of sensor networks, including central node which understands and processes the data

    • Increase user acceptance of modules by trivializing their setup

    • Decentralize the data processing to reduce communication costs

    • Allow for remote collaboration through searching protocols and shared open sensors

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    Local Processor


    Star Topology

    Intelligence at the Extremities

    • Local processor detects, processes or compresses local features

    • High data rates possible with limited node densities

    • Wearable, medical applications


    • Feature extraction via local communication

    • Results routed out node-node

    • Potentially scalable to very high density

    • Electronic skins, sensate media

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    Some Sensor Net Architectures

    • Berkeley Motes

      • Current version favors size and integration over modularity

      • Commercial Version from Crossbow (also moteiv)

      • Concentrates on ad-hoc networking application


    • Philips SAND

      • Modular system for wireless sensing

      • Multiple panes with different functionality (IMU/ECG/DSP)

      • Networking via ZigBee


    • Millenial Net

      • MIT ME Spinoff (lower-end)


    • Ember

      • Media Lab Spinoff (higher-end)


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    Sensor Networking Protocols:802.15

    • IEEE 802.15 standard defines specifications for wireless personal-area network (PAN)

      • Defined as a number of devices communicating solely amongst themselves in a 10m radius

      • Transmission is in the 2.4 GHz band

      • Master-slave protocol with automatic discovery

      • TDMA or CSMA channel sharing

    • Sub-standards based on transmission speed, number of nodes, and packet size

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    Sensor Networking Protocols:802.15.1 – Bluetooth

    • Designed for cable replacement for peripherals communicating with a single master

      • 7 nodes with 720 kbps total bandwidth

      • Packet overhead of 250kB

      • Power can be carefully managed (battery life on week scale)

        • Max data rate of 1Mb/s and power consumption at 0.3mA in standby, 30mA maximum while transferring at full speed

      • Some issues

        • 7 Slaves, 1 Master (although can nest subnets with shared node)

        • Takes 100’s of msec(!) to shift between nodes in Bluetooth 1

      • Single-Chip manufacturers

        • Cambridge Silicon Radio ( )

        • SiliconWaves ( )

        • Zeevo ( )

      • Modules

        • BlueRadios (

        • Infineon BlueMoon (

        • National Semiconductor SimplyBlue (

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    Sensor Networking Protocols:802.15.4 – Zigbee

    • Designed for monitoring of large network of very low duty cycle sensors

      • 255 nodes with 200 kbps total bandwidth

      • Packet overhead of 32kB

      • Battery life on year scale

      • Zigbee System-On-Chip

        • Chipcon CC2431

        • Ember EM250

    "Using Low-Cost Low-Power Wireless Sensor Devices to Monitor the Health of Structures", Anthony Allen, Ralph D'Souze, Oleg Andric, Minh Pham, Wayne Chiou, and Lance Hester (Motorola), 4th International Workshop on Structural Health Monitoring, Stanford CA, September 2003.

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    Sensor Networking Protocols:CAN

    • Controller Area Network (CAN) was designed for automotive use to enable robust serial communications while simplifying wiring

    • Nodes share a common bus (using CSMA) and can send messages:

      • Only on sensor/device failure

      • Continuously to update parameters

      • When instructed by master or another node

    • Message format is fairly complex, but system could be useful if integrated with sensors and processor of interest

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    Sensor Networking Protocols:IEEE 1451 - Goals

    • Develop network independent and vendor independent transducer interfaces.

    • Achieved through Transducer Electronic Data Sheets (TEDS) that remain together with the transducer during normal operation.

      • Support a general transducer data, control, timing, configuration and calibration model.

      • Allow transducers to be replaced/moved with minimum effort.

      • Eliminate error prone, manual system configuration steps.

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    Sensor Networking Protocols:IEEE 1451 - Standard

    • IEEE 1451 is actually a family of standards which define various portions of the interface between sensor modules and a network

    • X.1: Define processor block to isolate sensing and communication

    • X.2: Define TEDS to store data about sensor with sensor

    • Communication:

      • Wired (X.3), wireless (X.5) and raw signal (X.4)

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    Sensor Networking Protocols:IEEE 1451 - TEDS

    • Transducer Electronic Data Sheets (TEDS) allow for sensors to contain their own calibration data and other fundamental characteristics

      • Number of different templates

      • Standardizes interface

      • Simplifies integration of sensor into larger system and replacement if necessary

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    Sensor Networking Protocols:SensorML

    • Sensor Model Language (SensorML) is an XML based description language for web-resident devices

    • These devices are self-describing and provide information necessary for discovery, processing, and location of sensor observations

    • System allows for:

      • Search and discovery of available public sensors

      • Dynamic request and fusion of data from disparate sources

      • Control and handling of remote data sources

      • Direct transmission of data and processing information to remote sites

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    Sensor Networking Protocols:SensorML - Format

    • The SensorML format allow sensor to identify themselves, give their security limitations, describe their measurements and specify their location