jussi suomelahut/automation1 actuators for robots actuators are used in order to produce mechanical...
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Jussi Suomela HUT/Automation 1
Actuators for Actuators for RobotsRobots
Actuators are used in order to produce mechanical movement in robots.
Slides from Braunl and Jussi Suomela
ActuatorsActuatorsIn this lecture we will present:
Motor and Encoder H-Bridge Pulse-Width-Modulation (PWM) Servos Other robotic actuators
ActuatorsActuators 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.
Jussi Suomela HUT/Automation 5
Servo SystemServo System
Servo is mechanism based on feedback control.
The controlled quantity is mechanical.
Jussi Suomela HUT/Automation 6
Servo Control of an Electrical Servo Control of an Electrical MotorMotor
Jussi Suomela HUT/Automation 7
Properties of Servo 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
Jussi Suomela HUT/Automation 8
Electrical ActuatorsElectrical 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
Electric actuatorsElectric actuators•Mainly rotating but also linear ones are available
•linear movement with gear or with real linear motor
Jussi Suomela HUT/Automation 10
Electrical Actuator TypesElectrical Actuator Types
DC-motors brushless DC-motors asynchronous motors synchronous motors reluctance motors (stepper motors)
Jussi Suomela HUT/Automation 11
DC-Motors simple, cheap easy to control 1W - 1kW can be overloaded brushes wear limited overloading
on high speeds
DC-motor controlDC-motor control
Controller + H-bridge PWM-control Speed control by
controlling motor current=torque
Efficient small components
PID control
H-BridgeH-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
Jussi Suomela HUT/Automation 18
Brushless DC-Motors Brushless DC-Motors (pm synchronous motor) (pm synchronous motor)
no brushes no wearing parts high speeds coils on cover => better cooling excellent power/weight ratio simple needs both speed and angle feedback more complicated controller From small to medium power (10W – 50kW)
Jussi Suomela HUT/Automation 19
Asynchronous MotorsAsynchronous 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 )
Jussi Suomela HUT/Automation 20
Structure of an Asynchronous motorStructure of an Asynchronous motor
Jussi Suomela HUT/Automation 21
SyncSynchhronous Motorsronous 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
Jussi Suomela HUT/Automation 23
Reluctance (Stepper) MotorsReluctance (Stepper) Motors
angle control slow usually no feedback used accurate positioning with out feedback not servos easy to control
Stepper MotorsStepper 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
Stepper MotorsStepper 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)
Motor and EncoderMotor 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)
Motor and EncoderMotor 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
Pulse-Width ModulationPulse-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
Pulse-Width ModulationPulse-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
ServosServos 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.
ServosServos 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.”
Jussi Suomela HUT/Automation 43
Hydraulic ActuatorsHydraulic 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
Jussi Suomela HUT/Automation 46
Hydraulic ValvesHydraulic 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
Jussi Suomela HUT/Automation 49
Pneumatic ActuatorsPneumatic Actuators like hydraulic except power from
compressed airfast on/off type tasks big forces with elasticityno leak problems
Jussi Suomela HUT/Automation 50
Other ActuatorsOther Actuators
piezoelectric magnetic ultra sound SMA inertial
PracticallyPractically
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.