Motors, Control, API

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Motors, Control, API

• 6.186, January 2003
• Edwin Olson

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Agenda for today

• This talk
• Motor Basics
• PWM, Encoders, Servos, etc.
• Robot Control Strategies
• Using the ORC API
• Open Lab
• Robot rolling
• Remote control
• Integrate a sensor
• Work towards Friday checkpoint

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Reminder Fridays Checkpoint

• Robot should be put together
• Temporary peg-board fine
• Dont need a target-collecting mechanism
• Robot should be controllable by remote, using
  color coded cards
• See Wiki for details
• Have a plan/strategy for contest
• Gripper mechanism?
• Waypoint demonstration

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Motors- Physics

• Motor windings have a fixed resistance (ignoring
  heating)
• Voltage differential on motor leads causes
  current to flow. (Ohms law)
• Magnetic field proportional to current
• Torque proportional to magnetic field strength
  (and thus current)
• Torque causes rotor to begin rotating.
• Which way? It tries to reduce the magnetic field
  strength.

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Motors- Physics (continued)

• (Magnetic) Rotor is now turning
• Faradays Law applies.
• A voltage is generated!
• Which way? To reduce the magnetic field strength
  gt to reduce the current gt cancelling out the
  externally-applied voltage
• Motor Voltage Applied voltage Back EMF
• This is the voltage we used when determining how
  much current flows

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Motors- Physics (continued)

• Scenario motor just turned on
• Back EMF 0, current is Vin/R
• Scenario motor rotor is stalled
• Back EMF 0, current is Vin/R
• Scenario motor rotating unimpeded
• Back EMF - Vin, current is 0
• (With friction, current is non-zero but small.)
• Scenario applied voltage changes polarity from
  Vin to -Vin
• Back EMF -Vin (inertia!), current is 2Vin/R

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Motors as Inductors

• Another complication motors windings are
  inductors
• Inductors oppose changes in current
• They store energy
• When current would otherwise change, inductors
  become voltage sources such that the current
  remains constant
• VLdI/dt

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Motors as Inductors (continued)

• VLdI/dt
• Scenario you unplug a running motor
• dt 0
• dI/dt infinity
• This makes V really big
• Makes a big spark so that dt isnt quite 0
• Scenario you rapidily change the external
  voltage
• dI/dt very large (finite switching times)
• V very big.

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Motor Physics- Take home lessons

• Dont unplug a running motor
• Dont rapidly change the voltage on a motor
  slowly increase/decrease it to avoid a current
  spike
• When accelerating, accelerate in a couple steps
  25 for 30 ms, 50 for 30ms, 75 for 30ms, then
  100.
• This goes for changing direction and decelerating
  too!
• Your wheels are less likely to slip too!

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Powering Motors

• CPU cant control directly
• uP pins usually can source/sink lt 10mA
• Our motors can use 200mA or more!
• uP pins are 1s and 0s
• There is no -1, so no backwards
• There is no 0.5, so no medium speed
• We want these things!
• How do we do it?

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Motors H-bridges

• H-bridge allows bidirectional control
• A D on forward
• C B on backwards
• A B on or
• C D on horrible!
• Our H-bridges have direction enable
• What would B D on do?

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Motors PWM

• How do we run at fractional power?
• Consider rapidly toggling H-bridge on off

85 power?
40 power?
40 power?
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Motors PWM continued

• Two degrees of freedom period and duty cycle

• Two degrees of freedom period and duty cycle
  (percentage on)

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PWM- Why does this work?

• Why does a 50 duty cycle look like half the
  voltage?
• It doesnt not always, at least
• Scenario period1 day, duty cycle0.5.
• Motor will run full blast for 12 hours, then stop
  for 12 hours

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PWM- how to chose period?

• Rotor/Magnets have a mechanical time constant
• Amount of time it takes for rotor to achieve
  63.2 of its final velocity after a step input.
• Motor windings (inductor) and resistance have
  electrical time constant
• Amount of time it takes for current to achieve
  63.2 of its final value after a step input with
  rotor locked in position.
• Motor drivers take time to switch. If period is
  too short, drivers will spend the whole time
  switching and not conducting gt low efficiency.

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PWM- how to choose period?

• Mechanical time constant 10 msec
• Electrical time constant 1 msec
• PWM must be faster than mech time constant, or
  motor will vibrate
• Faster than electrical time constant ensures PWM
  waveform is effectively an analog voltage
• Why do we care?
• Avoid frequencies lt 20KHz due to human hearing!
  (magnetostriction)
• Avoid frequencies gt driver switching frequency
  (100KHz?)

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Motor performance measurement

• How do we find out how a motor is performing?
• Current Sensing
• Back EMF (tricky!)
• Tachometer
• Encoders

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Current Sensing

• Tells you how much torque the motor is producing.
• Not directly related to angular velocity
• Dependent on terrain!

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Back EMF

• Use Back EMF to measure speed
• Current must be zero
• Must disconnect power and wait! (a couple
  electrical time constants)
• ORC board can measure, but requires great care
  and special cable.

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Tachometer

• Add an additional little motor to the output
  shaft with very low inertia
• This motor generates a back EMF
• This voltage proportional to rotational velocity.

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Simple Encoders

• Attach a disk to the motor shaft and attach a
  break-beam sensor across the teeth.

Output of Encoder

• Or, use a reflectivity sensor and a disk with
  black white colored wedges.
• Are we going forwards or backwards?

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Quadrature Phase Encoders

• Use TWO single encoders, 90 degrees out of phase.

Close-up of teeth
Forward
Backward

• Forward and backward are now distinguishable!

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Servo Motors

• Servo motors seek and hold a specific angle
  automatically!
• Have integrated gearhead control electronics
• Use PWM as interface!
• Huh?
• About 180 degrees of rotation, to within 1
  degree
• Pretty good torque, but take it easy on them!
• Power dissipation can be bad
• Plastic gears are wimpy
• Intermittent duty only

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Servo Motors, continued

• PWM is not used in the same way as normal motors.
• Servo uses PWM to build an analog voltage that is
  compared to a potentiometer on the output
  (feedback).
• Just send a PWM with a duty cycle proportional to
  the desired angle!
• Pulse widths that are too short/long cause servo
  to shake violently!
• Not very good for them.
• Consumes a lot of power

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Other Actuators

• Other actuators you might consider
• Solenoids
• Muscle Wire
• Cheap simple DC motors (5 or 12V)
• Electromagnetic ltsomethinggt
• An actuator a spring is a different actuator
• Dont forget 8.01
• Use mechanical advantages when you can levers,
  pulleys, ramps, winches
• Servos have a fair amount of torque for a motor
  their size, but can easily be overworked!
• Design so that actuators expend power only
  intermittently!

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Robot Control Strategies

• How do you make your robot do what you want?
• Common design build behaviors, then coordinate
  behaviors from a planner

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Control Strategies

• Fixed Plan (Scripted)
• Preprogram a number of operations or phases which
  will run in a fixed sequence
• Cant cope well with the unexpected!
• Finite State Machine
• Preprogram a number of contingencies
• E.g., If Im looking for a target and I hit a
  wall, turn right.
• Presents some ability to cope with the unusual
• But still cant cope with things you havent
  considered
• AI Planner
• Consider all possible plans (sequence of
  actions)
• Give each one a score
• Pick the best one
• Might be able to figure a way out of a bind that
  you hadnt considered.
• Complicated, sometimes slow. Assigning scores to
  agree with how you would decide can be really
  hard.

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Control Strategies

• Hybrid
• Combine the above
• Example Fixed plan Finite State Machines
• For the first minute, run the find a target FSM
• For the second minute, run the search for and go
  to a scoring area
• Release the target
• For the third minute, try to go home

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Philosophy of Control

• Know Everything
• My current position is (2.6,3.7), Im facing
  exactly east. Theres a target behind me at
  (1.5,3.6) and a wall between us. The following
  series of precise moves will get me there.
• Robot maintains a lot of state (information about
  the world)
• Very easy to be wrong about some of it
• Very powerful. Very hard.
• Know Nothing
• Gee, George. That sure is a pretty target over
  there. Mmmm, pretty red color. I should go
  there. (later) Ah, I have a red thing. I feel
  happy. But Id feel happier if I saw some yellow
  right about now so Ill spin!
• Not much state to maintain.
• Hard to be wrong!
• Hard to make good decisions.

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Philosophy of Control (continued)

• You want to be somewhere in the middle. For
  example
• Maintain as much state as you can, but understand
  the margin of errors in your state and take this
  into account. Expect to be wrong.
• Maintain only a little state. Do lots of
  wandering, but try to remember where youve
  explored so you maximize the ground youve
  covered.
• Gather information about particular places. When
  you get to one, know how to recognize it. Then
  figure out what the next few things you should
  try to do are. As you wander, youll get lost
  adapt and simplify your plan based on what goes
  wrong.

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Building Behaviors

• How would you build a right-wall follower?
• Finite State Machine
• Example
• If Im getting farther away from the wall, turn a
  unit to the right
• If Im too close to the wall, turn a unit to the
  left
• Go forward a bit, then back to step 1.
• Easy to write, easy to understand
• Not terribly flexible
• What if were getting farther away from the wall
  (a wheel is slipping, for example) faster than we
  are turning to the right?

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Building Behaviors- Wall Follower

• Use a feedback-based control system
• Example
• Compute errordistance from wall desired
  distance from wall
• Errorlt0 too close
• Errorgt0 too far away
• Compute some function of error, and use this to
  determine how sharply (and in what direction) to
  turn.
• Rich mathematics available to help design
• Robust in theory, a bit tricky to perfect in
  practice
• Potential for instability gt amusement for
  audience
• F(error) often has derivative and integral
  components (PID control)

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PID control

• If youre really far away from the wall, turn
  sharply. If youre only slightly veering, turn
  just a bit.
• Thats Proportional
• If youre rapidly getting farther away from the
  wall, start turning harder.
• Thats Derivative
• If, despite your efforts, you keep getting
  farther away for a long time, start turning
  harder.
• Thats Integral
• TurnrateCpe Ci ?e Cdde/dt
• Discrete-time calculus
• This is actually what a servo motor does to hold
  a particular angle

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Using the ORC API

• Im almost done talking
• Where is the ORC library?
• Look on the cs00s NFS server
• Patches come out periodically. Each version is
  time stamped. E.g., orc010803.tgz.
• What do I do with it?
• Copy it somewhere useful
• Untar it (tar xzvf orc010803.tgz)
• Edit/copy the helloworld example to get started
  quickly
• API available from the Wiki
• Largely self explanatory
• Its worth sitting down and reading through

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ORC API Documentation

• Getting started
• You must call orc_initialize() before anything
  else.
• You must link with liborc.a after your .o files
• E.g. g o myprogram main.o liborc.a
• Use g, not gcc

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ORC Communication

• Bandwidth very limited
• Only a couple hundred transactions per second
• Starvation!
• Consider caching results
• Latency very high
• Single transaction can take gt10ms
• Lost opportunity for computation
• Consider doing communication asynchronously or
  heavy computation in separate thread.

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LCD commands

• API
• orc_lcdHome
• orc_lcdPrint, orc_lcdPrintf
• orc_lcdGoto
• LCD screen is only 16x2 characters!

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Analog Command

• API
• orc_analog
• orc_analogSetResolution
• orc_analogSetMaxPort
• orc_currentSense
• Analog values are 0,65535 corresponding to
  0,5 volts.

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Digital Commands

• API
• orc_buttons, orc_buttonsWait
• orc_digitalRead
• orc_digitalWrite
• orc_digitalSetDirection
• ORC supports digital out, but this is dangerous
• If you plug in sensor that outputs a value into a
  digital out port, you can damage your ORC.
• Ask a TA if you need this, and always be careful!

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Motor Commands

• API
• orc_motorPWM
• orc_motorDir
• Motors are indexed 1,4 different than other
  ports which are 0,x!
• Use symbolic directions MOTOR_FORWARD,
  MOTOR_REVERSE, MOTOR_BRAKE
• These may not correspond to forward/backward
  for your robot
• Motors have different strengths forward and
  backward

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Servo Commands

• API
• orc_servoPWM
• orc_servoSetProfile
• orc_servoSeek
• hitec300profile
• Servos should not be seeked out of range for
  extended periods of time (gt2 seconds?)
• Servos support profiles, allowing you to specify
  an angle, rather than a pulse width.
• Purely a convenience function just calls
  servoPWM.

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Ultrasound Commands

• API
• orc_ultrasoundPing
• orc_ultrasoundRead
• Can only use one ultrasound at a time
• Must wait for the sonar to echo before reading!

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Batteries Care and Feeding

• 12V Lead Acid Battery, 5 Ah
• Battery recharges whenever ORC is plugged into AC
  battery is connected
• Recharging battery can use lots of current
• Dont deeply discharge battery. Its bad for it
  anyway!
• Disconnect geode if regulator gets hot
• That frees up 0.7A for battery charging!
• If regulator still too hot, disconnect battery.
  Recharge with a bench supply.

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Soldering Iron Care

• Immediately tin (cover with solder) new tips.
• When soldering, if tip isnt shiny, it needs to
  be cleaned
• Sponge
• Re-tin with solder, or retinning goop
• Keep solder on tip when storing to prevent
  oxidation of tip
• Never, ever, sharpen the tip!
• Tip has special alloy on outer coating. Dont
  remove it!

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Im done talking!

• Now get to work!
• Not sure where to start?
• Talk to your advisor!