66 Industrial Roboticsota 2wsne

3.7 ACTUATORS don oiltt dna oe (9 u0

Actuators are the devices which provide the actual motive force for the robot
joints. They commonly get their power from one of three sources: compressed air
pressurized fuid, or electricity. They are called pneumatic, hydraulic, or electric
actuators respectively. We will discuss all three types in this section. o o e

3-7.1 Pneumatic and Hydraulic Actuators

Pneumatic and hydraulic actuators are both powered by moving fluids. In the
first case, the fiuid is compressed air and in the second case, the fiuid is usually
pressurized oil. The operation of these actuators is generally similar except in their
ability to contain the pressure of the fluid. Pneumatic systems typically operate at
about 100 1b/in and hydraulic systems at 1000 to 3000 1b/in We discussed the
relative advantages and disadvantages of these types of drive systems in Chap. Two.
The simplest fluid power device is'the cylinder as illustrated in Fig. 3.17, which
could be used to actuate a linear joint by means of a moving piston:. This example is
called a single-ended cylinder as the piston rod only comes outofthe cylinder at one
end. Other types of cylinders include double-ended cylindersandrodless cylinders
There are two relationships of particular interest when discussing actuators: the
velocity of the actuator with respect to the input power and the force of the actuator
with respect to the input power. For the cylinder type actuator these relationships are
given by
(3.35)
A
FO P)A (3.36)

Cylinder

iiovol
Fluid port

Oruor Pstongsoy ucuo nt et () 919/W

10h6 nolsretotPiston rod hivut ot b9eiitnog se2otomoiloni

01 29019 n 20 021 Fig. 3,.17 Cylinder and pistonog ot be o 20
o d

where V() is the velocity of the piston, f{t) is the fluid flow rate
(volumetric),
F) is the force, P(1) is the pressure of the fluid, and A is the area of the piston. Since
the requirements of a robot are to carry a payload at a
given speed, we canuse the
relations described for choosing the appropriate
actuator.voqrtoirw rolomordout on
3e0 S19W oteTO0on3 10at1o fit s8 vofT giiooloV 1O
 Control Systems and Components 67

3.7.2 Electric Motors ebhoee to robouk

As their capabilities improve, electric motors are becoming more and more tne
actuator of choice in the design of robots. They provide excellent controllability with
a minimum
of maintenance required. There are a variety of types of motors in use in
robots; the most common are dc servomotors, stepper motors, and ac servomotors,
DC motors that are used in closed loop position control are called servo motors.
As servo motors are required to have very large acceleration or deceleration in order
to have faster response, the armatures of these motors are specially designed to have
low inertia. DC servo motors can be either brushed motors or brushless motors. The
basic construction difference between a brushed and brushless motor is that in a

brushless motor, the armature winding are in the stator and the permanent magnets
are in the rotor. While brushed motor, the armature windings are in the rotor and
in a

the permanent magnets are in the stator. adt otsluoloo ot oino

stator and
Figure 3.18 shows a typical dc servomotors with a permanent magnet
and the
winding on rotor. The main components of the dc servomotor are the rotor
and
stator. Usually, the rotor includes the armature and the commutator assembly
When current flows
the stator includes the permanent magnet and brush assemblies.
the field
through the windings of the armature it sets up a magnetic field opposing
rotor. As the rotor rotates, the
set up by the magnets. This produces a torque on the
armature so that the field
brush and commutator assemblies switch the current to the
the torque produced
remains opposed to the one set up by the magnets. In this way,
the rotation. Since the field strength of
by the rotor is constant throughout
it can be shown that for a dc
the rotor is a fünction of the current through it,
servomotor
g1torw 91TES ard lo sor
Tm)=Kmlat) (3.37)
artubsrtiA
9gallov

vifnoloy

Permanent
3-7-3 magnet (stator)
Stepper o0

ro
Amature
winding (rotor)

Is eopio

esotuako?
showing the armature (roto) andpermanent magnet (stator).
Fig. 3.18 Brushed DC motor
of the motor, Ia is
the current flowing through the
where Tm is the torque constant.
armature, and K is
the motor's torque
 Industrial Robotics eylotnuo
68 is
tor is the
back-emf. A dc motor
similar
Another effect associated with a de
the presence
of a nmagnetic
a r m a t u r e in
the
ning
to a de generator or tachometer. Spinnin terminals, This
voltage is propo portiona
a voltage across the
armature
produces
field
bopo14008.13ln of
totheangular velocity of therotor h i atoniuos
teoir a (338
e o t et)
=
K^o(1)
2 voltage constant
or the motor
back-emf (voltage), K, is called the act as
VISCOus
damping
is the back-emf is to
where
e of the I
velocity. The effect increases proportionately.
and o is the angularthe velocity increases the damping and the resistance
1Or the motor: as terminals of is
the motor would be Vn/R
were to supply a voltage
across armature
through the
we then the current motor to spin. As the
armature were Ra, causes the
ne and
This voltage must be
rotor
n i s current produces a
torque on the or e,(?).
to K,@),
a back-emf equal currént. The
actual armature
armature spins it generates
the armature
Subtracted from Vin in
order to calculate

current is therefore n
(3.39)
ens roOimu Vnt)-ep0 ord o/01 nt rilcap
Ra buio0pi0
increases accordingly
back-emf voltage
and the
motor velocity increases, reduces the
As the
the armature
decreases. The decreasing current of the
the current available to decreases the acceleration rotor
by the rotor. As the torque
torque generated the motor maintains steady-state
a
well. At the point at which e, Vin The block
decreases as
no external disturbances
on the motor).
there are
velocity (assuming constant and back-emfon the
3.19 illustrates the effects of the torque
diagram Fig.
in discounts such effects as friction or
model of a motor. Note that
this simplified model LEY
inductance of the armature windings.

Input Armature
voltage voltage o Acceleration 1VelocityOutput
velocity
Inertia Time

Back EMF

Voltage constant

Fig. 3.19 DC motor block diagram.

Amotor has a torque constant, K =10 0z-in./A and a voltage

I
Example 3.10 constant of 12 V/Kr/min (1 Kr/min= 1000 r/min). The
armature resistance is 2 SQ. If 24 V were applied to the terminals what would be: (a)
the
torque at stall (0 r/min), 6) speed at 0 load (torque 0), and (c) the
the torqueat
100 r/min? Plot the results on a speed versus torque graph.

Solutions volunetach
(a) At O r/min the value of e, =0. Therefore, the armature current is
24V/2 Q= 12A
and the torque is s030 9f1 to 9upro 9d i or91
=
(12 A) (10 oz-in./A) =120 oz-in.
 Control Systems and Components 69

(b) At no load the output voltage is equal to the input voltage so that
24V - (12 VKr/min) w)

w-2 Kr/min =2000 r/min

the
(c) At 1000 r/min the output voltage is 12 V. Therefore the current through
armature is
otol 24V -12V6A olot
22
and the torque is her
We can see that the relationship between speed and torque in Fig. 3.20 is a straight
line. The feature is one of the desirable feature of de servomotors.

150

120|
DC motor
100 Speed vs. torque

60

1000 1500 2000

500
Motor speed (RPM)

Plot for Example 3.11. Be

o s 9 Fig. 3.20

3.7-3 Stepper Motors iiaore

of actuator and have
(also called stepping motors) are a unique type the
motor provides output in
motors
Stepper
peripherals, A stepper
been used mostly in computer increments. It is actuated by a series of discrete
motion
form of discrete angular is a single-step rotation of
the
For electrical impulse there
electrical pulses. every
are used for relatively light
motors
duty applications.
In robotics, stepper the closed-
motor shaft. systems rather than
are typically used in open-loop
Also, stepper motors chapter
in this
which we have been concentrating motor. The
type of stepper
on
loop systems schematic representation of one
Figure 3.21 provides a and the rotor is a two-pole
permanent
electromagnetic poles that pole 3
stator is made up of four are activated in such a way
stator poles the
magnet. If the
electromagnetic as illustrated. If
I is S then the rotor is aligned
and pole rotor makes a 90°
turn in the
is N (magnetic North)
4 is N and pole 2 is S, the
stator is excited so that pole to the stator electronically,
it is
switching the current
clockwise direction. By rapidly conunuous wopooA
rotor aPpear
motion of the
possible to make the booubet
 (70 Industrial Robotics

Pole 1

wwMN

Pole 4 Pole 2

Pole 3

Fig. 3.21 Stepper motor schematic. 021

eblog

Fig. 3.22 Toothed stepper motor (Courtesy: Litton System, Incorporated Clifon Precision
Division)
The resolution (number of steps per revolution) of a stepper is determined
by the
number of poles in the stator and rotor. Figure 3.22 show a commercially available
stepper motor which has a notched stator and rotor. This effectively increses the
number of poles and hence the resolution of the device. The relation between a
stepper motor's resolution and its step angle 1s given Dy
oY lug soitosk
noitnoigea aub iigil vlevitalo n
A/36090ioot ego.0itodovgl fis (3.42)
where n is the resolution and A is the step
angle.
Unlike the de servomotor, the relation between a
is not
stepper motor's speed and torque
necessarily a straight line. Because of the discrete nature of the stepper
construction the torque is also a function of the motor's
angle between the stator and rotor
poles. The torque is greatest when the poles are aligned. This maximum
known as the holding torque torque is
of the motor. It is possible to increase the resolution of a
stepper by using a technique known as half-stepping or
current to more than one set of field microstepping. By applying
windings, it is possible to make the rotor
out an
'average' position. Of course, when using this technique the holding seek
is reduced. torque
 Control Systems and Components

The control of a stepper motor is dependent on the ability of the switching

electronics to switch the
windings at precisely the right moment. If the windings
are switched too quickly, for example, it is possible that the motor will not be able
to keep up with the command signals and will perform erratically, in some cases
oscillating. With some steppers, the speed-torque relation degrades badly at certain
frequencies of operation, and operation of the motors at these frequencies must be
avoided. JE O1 19WOG edt asups nie T86 Sd o o 15wog sii haigumbsp

3-7-4 AC Servomotors and Other Types

There are numerous
orea
other aspects of electric motors which may be investigated.
Recent advances in control electronics are producing ac servomotors. These motors
have the advantages of being cheaper to manufacture than de motors; they have no
brushes, and they possess a high power output. With proper electronics package,
however, their performance can be made to look very much like the performance of
a dc motor.
Another type of electric motor is the brushless de motor. It is constructed like an
inside-out' de motor. It has a permanent magnet rotor and an electromagnetic stator.
Instead of using brushes, however, commutation is performed electronically using an
encoder to inform the electronics of the relative positions of the stator and rotor. Also
available are linear electric motors. Their construction is similar to a dc servomotor
that has been cut open and flattened out.
In almost all cases of electric motors the limiting factor on power output is heat
dissipation. Some of the current used in the motor must be dissipated as heat. Two
ways to increase the performance of a motor is to remove heat more quickly or to
reduce the current requirements. The latter may be done by increasing the magnetic
flux of the permanent magnets. Recent advances in magnetic materials are allowing
for performance improvements of almost 10 times with the samne power requirements.