1 Actuators are used in order to produce mechanical movement in robots.
Jan 03, 2016
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Actuators are used in order to produce mechanical movement in robots.
In this lecture we will present:
Motor and Encoder H-Bridge Pulse-Width-Modulation (PWM) Servos Other robotic actuators
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Electrical Hydraulic Pneumatic Others
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 SYSTEMSERVO SYSTEM
Servo is mechanism based on feedback control.
The controlled quantity is mechanical.
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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 ELECTRICAL ACTUATORSACTUATORS
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
•Mainly rotating but also linear ones are available
•linear movement with gear or with real linear motor
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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
Controller + H-bridge
PWM-control Speed control by
controlling motor current=torque
Efficient small components
PID control
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
POWER POWER AMPLIFIERAMPLIFIER
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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)
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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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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
Stepper Stepper MotorsMotors
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angle control slow usually no feedback used accurate positioning with out feedback not servos easy to control
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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
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 MOTOR AND ENCODERENCODER
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)
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 PULSE-WIDTH MODULATIONMODULATION
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
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.
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 SuomelaHUT/Automation
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HYDRAULIC HYDRAULIC ACTUATORSACTUATORS
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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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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PNEUMATIC PNEUMATIC ACTUATORSACTUATORS
like hydraulic except power from compressed air
fast on/off type tasks big forces with elasticity no leak problems
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piezoelectric magnetic ultra sound SMA inertial
EXAMPLESEXAMPLES