ee401-3!2015 embeddedsystems

EE401. Engineering Design by

Teams: Robotics 1

Lecture 2. Embedded Systems

for Robotics

Instructor: Huynh Viet Thang

Aug. 2015

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• Textbook: • T. Bräunl Embedded Robotics, Springer 2003

• The University of Western Australia, Electrical,

Electronic and Computer Engineering

Based on the materials of Prof. Marek A. Perkowski

Intelligent Robotics Laboratory

Portland State University

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What are in this lecture?

• Embedded systems

• Robots and Controllers

• Sensors

• Actuators

• Control techniques

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Embedded systems

Definition for Embedded System

• A combination of hardware and software which together form a component of a larger machine.

• An example of an embedded system is a microprocessor that controls an automobile engine.

• An embedded system is designed to run on its own without human intervention, and may be required to respond to events in real time.

• Source: www.computeruser.com/resources/dictionary

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Applications Areas

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Application Areas• TV• stereo• remote control• phone / mobile phone• refrigerator• microwave• washing machine• electric tooth brush• oven / rice or bread cooker• watch• alarm clock• electronic musical instruments• electronic toys (stuffed animals,handheld toys, pinballs, etc.)• medical home equipment (e.g. bloodpressure, thermometer)• …• [PDAs?? More like standard computer system]

Control Applications; Consumer Products

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Application Areas

• Medical Systems– pace maker, patient monitoring systems, injection systems,

intensive care units, …

• Office Equipment– printer, copier, fax, …

• Tools– multimeter, oscilloscope, line tester, GPS, …

• Banking– ATMs, statement printers, …

• Transportation – (Planes/Trains/[Automobiles] and Boats)

• Radar, Traffic lights, Signalling systems, …

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Application Areas• Automobiles

– engine management, trip computer, cruise control,

immobilizer, car alarm,

– airbag, ABS, ESP, …

• Building Systems

– elevator, heater, air conditioning, lighting, key card

entries, locks, alarm systems, …

• Agriculture

– feeding systems, milking systems, …

• Space

– satellite systems, …

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Application Areas

• Facts:– 1997: The average U.S. household has over 10

embedded computers (source: www.it.dtu.dk/~jan)

• 1998: 90% Embedded Systems vs. 10% Computers– (source: Frautschi, www.caliberlearning.com)

• 2001: The Volvo S80 has 18 embedded controllers and 2 busses (source: Volvo)

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Automobiles

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Microcontrollers

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Von-Neumann vs. Harvard

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Microcontrollers

• Microprocessor– CPU (on single chip)

• Microcontroller

• CPU + Timers + I/O (+RAM) (+ROM)• Reduced chip count for board design

• Embedded system

• Today’s Technology:

– Surface Mount Device (SMD)

– Ball Grid Array (BGA)

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Input and Output• Required to communicate with outside world

• PC System:– Keyboard

– Monitor

– Parallel port (printer port)

– Serial port + USB

• Embedded System:– Sensors (e.g. in automobile: acceleration sensor, seat

sensor)

– Actuators (e.g. in automobile: valves for airbags)

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Input and Output

• Input / output device implementation can be:

• • Memory-mapped

• • I/O mapped (ports)

• • DMA (direct memory access)

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Assignment

• You are required to design the controller (as an

embedded system) for a consumer product,

describe the system (give the specification) if the

product is

A) an air-conditioner

B) a washing machine

C) a smart TV

D) a smart phone

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Robots and Controllers

What are the advantages of using mobile robot

systems as opposed to traditional ways of

education, for example mathematical models

or computer simulation?

3 advantages

• Students can relate to a robot much better than to a

piece of software; tasks to be solved involving a robot

are of a practical nature and directly “make sense” to

students;

• A working robot program will be a robust system that

takes into account and overcomes inaccuracies and

imperfections: a valid engineering approach to a typical

(industrial) problem;

• Mobile robot programming is enjoyable and an

inspiration to students;

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Mobile robots

• A case study from the

Mobile Robot Lab at the

University of Western

Australia [Braunl 2006]

• “EyeBot family” using

“EyeCon”

– Wheeled robots

– Tracked robots

– Legged robots

– Flying robots

– Underwater robots

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An example: Wheeled Robot

• Require 2 motors for driving and steering

• Differential Drive is the most commonly used mobile

robot design

Driven and steered wheel Driven wheels Driven wheels

Steered wheels

Differential Drive “Ackermann Steering”Single Drive

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Braitenberg vehicles

• A conceptual abstraction of actuators, sensors, and

robot control by Braitenberg (1984)

– Simple interaction between motors and sensors

– If a light sensor is activated by a light source, it will proportionally increase the speed of the motor it is linked to.

• Braitenberg vehicles avoiding light

• How does it work?

Light source

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Braitenberg vehicles (cont.)

• Braitenberg vehicles searching light

Light source

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Embedded controllers

• The EyeCon: 32-bit CPU

• “RoBIOS” (Robot Basic Input Output System) operating

system

• What are the advantages of using 32-bit CPUs vs. Using

8-bit CPUs? 29

RoBIOS

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Sensors

What is important is to find the right sensor for a particular application.

Overview

• Data transfer from the sensor to CPU

– either CPU-initiated (polling)

– or sensor-initiated (interrupt)

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Sensor categories

• Based on sensor output

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Other sensor classification

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Binary sensor

• Simplest

• Easy to design

• Active low in

this example

• What if “active high”?

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Analog vs. Digital sensors

• A number of sensors produce analog signals,

A/D converter is required

– Microphone

– Analog infrared distance sensor

– Analog compass

• Digital sensors are usually more complex and

more accurate than analog ones

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Synchronous serial interface

• “synchronous serial” means that the converted

data value is read bit by bit from the sensor

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Shaft encoder

• Encoder is fundamental feedback sensor for

motor control

• Magnetic encoders and Optical encoders

• Incremental encoders:

– Count number of

segments passed from

a certain starting point

– Not sufficient to locate

a certain absolute position

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Gray code disk

• help locate a certain absolute position

• How to determine orientation of the motor?

– use 2 sensors (magnetic or optical) positioned with a

small phase shift to each other (see previous slide).

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Other sensors

• Sonar (ultra-sound)

• Infrared sensors

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Other sensors (cont.)

• Compass,

• Orientation sensors– Gyroscope: rotational change of orientation about one axis

– Accelerometer: acceleraion along one axis

– Inclinometer: absolute orientation angle about one axis

• Digital Camera

• Microphone

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Actuators for Robots

Actuators are used in order to produce mechanical movement in robots.

Slides from Braunl and Jussi Suomela

Actuators• Motor and Encoder

• H-Bridge

• Pulse-Width-Modulation (PWM)

• Servos

• Other robotic actuators

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Actuator Types• Electrical

• Hydraulic

• Pneumatic

• Others

• Actuators can be built in may different ways, most prominently:– electrical motors– pneumatics and valves.

• we will only deal with electrical motors

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DC-Motors• simple, cheap

• easy to control

• 1W - 1kW

• can be overloaded

• brushes wear

• limited overloading

on high speeds

DC-motor control

• Controller + H-bridge

• PWM-control

• Speed control by controlling motor current=torque

• Efficient small components

• PID control

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H-Bridge

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H-Bridge• Hardware Implementation with

Microcontroller:

• 2 Digital output pins from microcontroller,

[one at Gnd, one at Vcc] feed into a power amplifier

• Alternative: use only 1 digital output pin plus one inverter, then feed into a power amplifier

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Stepper Motors

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Reluctance (Stepper) Motors

• angle control

• slow

• usually no feedback used

• accurate positioning

• easy to control

Stepper Motors• Stepper motors are another type of motors that do not require feedback

• A stepper motor can be incrementally driven, one step at a time, forward or backward

• Stepper motor characteristics are:– Number of steps per revolution (e.g. 200 steps per revolution = 1.8°

per step)– Max. number of steps per second (“stepping rate” = max speed)

• Driving a stepper motor requires a 4 step switching sequence for full-step mode

• Stepper motors can also be driven in 8 step switching sequence for half-step mode (higher resolution)

• Step sequence can be very fast, the resulting motion appears to be very smooth

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Stepper Motors

• Advantages– No feedback hardware required

• Disadvantages– No feedback (!)

Often feedback is still required,

e.g. for precision reasons, since a stepper motor can “lose” a step signal.

• Requires 2 H-Bridges plus amplifiers instead of 1

• Other– Driving software is different but not much more complicated

– Some controllers (e.g. M68332) support stepper motors in firmware (TPU)

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Motor and Encoder

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Motor and Encoder

• Motor speed determined by:

supplied voltage

• Motor direction determined by:

polarity of supplied voltage

• Difficult to generate analog power signal

(1A ..10A) directly from microcontroller

→ external amplifier (pulse-width modulation)

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Motor and Encoder

• Encoder disk is turned once for each rotor revolution

• Encoder disk can be optical or magnetic

• Single detector can determine speed

• Dual detector can determine speed and direction

• Using gears on motor shaft increases encoder accuracy

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Pulse-Width Modulation (PWM)

• A/D converters are used for reading analog sensor signals

• Why not use D/A converter for motor control?– Too expensive (needs power circuitry)

– Better do it by software, switching power on/off in intervals

– This is called “Pulse-Width Modulation” or PWM

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Pulse-Width Modulation

• How does this work?

– We do not change the supplied voltage

– Power is switched on/off at a certain pulse ratio

matching the desired output power

• Signal has very high frequency (e.g. 20kHz)

• Motors are relatively slow to respond

– The only thing that counts is the supplied power

– ⇒ Integral (Summation)

• Pulse-Width Ratio = ton / tperiod

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Servos

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Servos

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Servos

• Terminology:

• Do not confuse “servos” with “servo motors”

• DC motors (brushed or brushless) are also sometimes also referred to as “servo motors”

• See: http://www.theproductfinder.com/motors/bruser.htm

• “So when does a motor become a servo motor? There are certain design criteria that are desired when building a servo motor, which enable the motor to more adequately handle the demands placed on a closed loop system.

• First of all, servo systems need to rapidly respond to changes in speed and position, which require high acceleration and deceleration rates.

• This calls for extremely high intermittent torque. 66

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Hydraulic Actuators• linear movement

• big forces without gears

• actuators are simple

• in mobile machines

• Bad efficiency

• motor, pump, actuator combination is lighter than motor, generator, battery, motor & gear combination

Hydraulic actuators

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Pneumatic Actuators�like hydraulic except power from compressed air

�fast on/off type tasks

�big forces with elasticity

�no leak problems

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Control techniques

Closed loop control is an essential topic for

embedded systems, bringing together actuators

and sensors with the control algorithm in software

Control Techniques

Problem: supplying the same analog voltage (or the same

PWM signal) to a motor does not guarantee that the motor

will run at the same speed under all circumstances!

• On-off control

• PID control

• Others: • Adaptive control (LMS, NLMS, RLS)

• Fuzzy control

• Neural Networks

Solution: Feedback is everything!

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On-off control

• The power to motor is either switched on or

switched off

72R(t): control signal (control voltage) over time

• Behavior over time

• Advantage: simplest control method; used in

refridgerators, heater, thermostat, etc.

• Disadvantage: the motor control signal is only updated at fixed time intervals (e.g., 10ms) � hysteresis (trễ)

On-off control (cont.)

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• Use a hyteresis band with 2 desired signals to prevent a

high switching frequency

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• Software implementation

• See: Chapter 4, T. Bräunl Embedded Robotics, Springer 2003

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Step-response of an on-off

controller

• Not smooth

• Can we improve this?76

PID Control

• PID = P + I + D

• P = Proportional

• I = Integral

• D = Derivative

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Proportional Controller (P)

• The control voltage is directly proportional to the error

signal (error function)

• P controller is only slightly different from on-off controller

• Varying the “controller gain” Kp will change the behavior

of the P controller

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Proportional Controller (cont.)

• Step response for P controller

• Higher Kp � Faster response

• Important: Too high Kp

leads to undesirable

oscillating system!

• Require fast response

and stable system

(e.g., Kp = 0.45)

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Proportional Controller (cont.)

• P controller’s equilibrium state is not at the desired

velocity due to control formula

• Steady-state error is the difference between desired

velocity and equilibrium-state’s velocity

• Can we reduce the

steady-state error?

� Integral controller

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Integral Controller (I)

• The idea of the I controller is to reduce the steady-state

error of P controller

• The I controller is commonly used with the P or PD

controller

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Integral Controller (cont.)

• Define the error function : e(t) =

• The formula for PI controller is

• Rewrite for 2 independent additive terms for P and I

• How can we compute the integral?

– naive way (see textbook)

– proper implementation: replace the integral with a sum and use the trapezoidal rule

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Integral Controller (cont.)

• Use the term Rn-1 to remove the sum

• Substitute KI for

• We only need to store

– the previous control value Rn-1

– the previous error value en-1

to calculate the PI output

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• The idea of using the D controller is to speed up the P

controller’s response to a change of input

• The D controller is commonly used with the P or PI

controller

• Recall:

– P provides a better step response than on-off controller

– I for reducing steady-state error of P

– D for speeding up step response of P

• How can a PID controller help us?

Derivative Controller (D)

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P, PD and PID

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• A complete PID formula

PID controller

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PID parameter tuning

Find parameters experimentally

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Other control techniques

• Adaptive control (LMS, NLMS, RLS)

• Fuzzy control

• Control using Neural Network

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Summary

• Embedded systems– MCU vs. Computer

• Robots and Controllers– Braitenberg vehicles, Operating System (RoBIOS)

• Sensors– Binary, Encoder, Sonar, Infrared sensors, Orientation sensors,

Cameras, Microphones

• Actuators– DC motor, Servo, Stepper motors, H-Bridge and PWM

• Control techniques– On-off, PID

• Next: “artificial intelligence of robots”89

Projects

1. Auto-driving Car

2. Smart 2-DOF Robot Arm

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