1

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

3

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.

5

SERVO SYSTEMSERVO SYSTEM

Servo is mechanism based on feedback control.

The controlled quantity is mechanical.

6

7

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

8

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

10

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

11

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

18

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)

19

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 )

20

21

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

23

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

24

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

43

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

45

46

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

47

48

49

PNEUMATIC PNEUMATIC ACTUATORSACTUATORS

like hydraulic except power from compressed air

fast on/off type tasks big forces with elasticity no leak problems

50

piezoelectric magnetic ultra sound SMA inertial

EXAMPLESEXAMPLES