Actuators for Robots

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Actuators for Robots

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

Slides from Braunl and Jussi Suomela
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Actuators

• In this lecture we will present
• Motor and Encoder
• H-Bridge
• Pulse-Width-Modulation (PWM)
• Servos
• Other robotic actuators

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Actuator Types

• Electrical
• Hydraulic
• Pneumatic
• Others

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Actuators

• Actuators can be built in may different ways,
  most prominently
• electrical motors
• pneumatics and valves.
• In this course we will only deal with electrical
  motors
• In past we built pneumatic robots which you can
  still find in the lab.
• We will build them again after purchasing air
  compressor
• My first robot was very strong and it was
  hydraulic. It pissed hot oil at students in
  Warsaw.

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Servo System

• Servo is mechanism based on feedback control.
• The controlled quantity is mechanical.

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Servo Control of an Electrical Motor
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Properties of Servo

• high maximum torque/force allows high
  (de)acceleration
• high zero speed torque/force
• high bandwidth provides accurate and fast control
• works in all four quadrants
• robustness

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Electrical Actuators

• easy to control
• from mW to MW
• normally high velocities 1000 - 10000 rpm
• several types
• accurate servo control
• ideal torque for driving
• excellent efficiency
• autonomous power system difficult

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Electric actuators

• Mainly rotating but also linear ones are
  available
• linear movement with gear or with real linear
  motor

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Electrical Actuator Types

• DC-motors
• brushless DC-motors
• asynchronous motors
• synchronous motors
• reluctance motors (stepper motors)

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DC-Motors

• 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
• PWM-control
• Speed control by controlling motor currenttorque
• 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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Power Amplifier
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Brushless DC-Motors (pm synchronous motor)

• no brushes ? no wearing parts ? high speeds
• coils on cover gt better cooling
• excellent power/weight ratio
• simple
• needs both speed and angle feedback
• more complicated controller
• From small to medium power (10W 50kW)

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Asynchronous Motors

• very simple, very popular in industry
• 0,5kW - 500kW
• More difficult to control (frequency)
• nowadays as accurate control as DC-motors
• In mobile machines also (5kW ?)

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Structure of an Asynchronous motor
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Synchronous Motors

• usually big 100 kW - XXMW
• also small ones brushless DC-motors from 50W to
  100 kW
• controlled like as-motors (frequency)
• ships
• industry
• Mobile machines

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• Stepper Motors

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Reluctance (Stepper) Motors

• angle control
• slow
• usually no feedback used
• accurate positioning
• with out feedback not servos
• easy to control

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Principle of Stepper Motor
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Stepper Motors

• Stepper motors are another kind 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 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

• 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.

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Servos

• As you may know, torque is related to current in
  the brushed servo motor.
• So the designers need to keep in mind the ability
  of the motor to handle short bursts of very high
  current, which can be many times greater than the
  continuous current requirements.
• Another key characteristic of the brushed servo
  motor is a high torque to inertia ratio.
• This ratio is an important factor in determining
  motor responsiveness.
• Further, servo motors need to respond to small
  changes in the control signal.
• So the design requires reaction to small voltage
  variations.

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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

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Hydraulic actuators
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Hydraulic motor
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Hydraulic Valves

• servo valves
• complicated structure, expensive
• good control
• proportional valves
• simple, cheap
• robust
• more difficult to control
• Digital hydraulics, new!
• several fast on/off valves (2n)
• digital control of the flow

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Servo Valve
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Proportional Valve
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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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Other Actuators

• piezoelectric
• magnetic
• ultra sound
• SMA
• inertial

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Examples
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Arska
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Workpartner
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Shape Memory Alloy Robot
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Practically

• In this class we will use only servos
• In past we used DC motors with H-bridge,
  pneumatic actuators, nintinol wires and hydraulic
  actuators.
• So far, if you want to build rather small robots
  and you want to concentrate on intelligence and
  sensing, RC servos are the best choice. Many new
  types arrive every year, from very small to big
  powerful ones. Look to internet.
• We will learn about some new actuators if time
  will allow at the end of the class.