04/05/14 1Hareesha N G, Asst. Prof, DSCE, BLore-78
• Characteristics of actuating system: weight, power-
to-weight ratio, operating pressure, stiffness vs.
compliance, Use of reduction gears
• comparison of hydraulic, electric, pneumatic
• Hydraulic actuators-proportional feedback control
• Electric motors: DC motors, Reversible AC motors,
Brushless DC motors
• Stepper motors- structure and principle of
operation, speed-torque characteristics
• Actuators are the muscles of robots.
• If you imagine that the links and the joints are the skeleton of
the robot, the actuators act as muscles, which move or rotate
the links to change the configuration of robots.
• The actuator must have enough power to accelerate and
decelerate the links and to carry the loads, be light, economical,
accurate, responsive, reliable, and easy to maintain.
• There are many types of actuators available.
– Electric motors
– Stepper motors
– Direct-drive electric motors
– Hydraulic actuators
– Pneumatic actuators
– Shape memory metal actuators
– Magneto-strictive actuators
• Electric motors — especially servomotors — are the most commonly
used robotic actuators.
• Hydraulic systems were very popular for large robots in the past and
are still around in many places, but are not used in new robots as
often any more.
• Direct drive electric motors, the shape memory metal type-actuators,
and others like them are mostly in research and development stage
and may become more useful in the near future.
CHARACTERISTICS OF ACTUATING SYSTEMS
Weight, Power-to-Weight Ratio, Operating Pressure
• It is important to consider the weight of the actuating system, as well
as its power-to-weight ratio.
• For example, the power-to-weight ratio of electric systems is
• Stepper motors are generally heavier than servomotors for the same
power and thus have a lower power-to-weight ratio.
• The higher the voltage of an electric motor, the better power-to-
weight ratio it has.
• Pneumatic cylinders deliver the lowest power-to-weight ratio.
• Hydraulic systems have the highest power-to-weight ratio.
• However, it is important to realize that in these systems, the weight
is actually composed of two portions. One is the hydraulic actuator,
and the other is the hydraulic power unit.
• The system's power unit consists of a pump, which generates the
high pressure needed to operate the cylinders and rams, a reservoir,
filters, electric drive motors to drive the pump, cooling units, valves,
CHARACTERISTICS OF ACTUATING SYSTEMS
Weight, Power-to-Weight Ratio, Operating Pressure
• The actuators' role is only to move the joints.
• However, the power unit is normally stationary and located
somewhere away from the robot itself.
• The power is brought to the robot via an umbilical tether hose.
• Thus, the actual power-to-weight ratio of the cylinders is very high
for the moving parts.
• However, the power unit, which is very heavy, does not move and is
not counted in this ratio.
• If the power unit must also move with the robot, the total power-to-
weight ratio will be much less.
• The power that the hydraulic system delivers is also very high, due to
high operating pressures.
• This may range from 55 psi to 5,000 psi pressures. Pneumatic
cylinders normally operate around 100 to 120 psi.
• The higher pressures in hydraulic systems mean higher powers, but
they also require higher maintenance, and if a leak occurs, they can
become more dangerous.
Stiffness vs. Compliance
• Stiffness is the resistance of a material against deformation.
• It may be the stiffness of a beam against bending under the load, the
resistance of a gas against compression in a cylinder under load and
• The stiffer the system, the larger the load that is needed to deform it.
Conversely, the more compliant the system, the easier it deforms
under the load.
• Stiffness is directly related to the modulus of elasticity of the
• The modulus of elasticity of fluids can be around 1 x 106
psi, which is
• As a result, hydraulic systems are very stiff and noncompliant.
Conversely, pneumatic systems are easily compressed, and, thus, are
• Stiff systems have a more rapid response to changing loads and
pressures and are more accurate.
• Obviously, if a system is compliant, it can easily deform (or compress)
under changing load or changing driving force, and, thus, will be
Stiffness vs. Compliance
• Similarly, if a small driving force is applied to a hydraulic ram, due to
its stiffness, it will respond more rapidly and more accurately than a
pneumatic system, which can deform under the same load.
• Additionally, the stiffer the system, the less it gives or deforms under
load, and thus the more accurately it holds its position.
• Now consider a robot that is used to insert an integrated circuit chip
into a circuit board.
• If the system is not stiff enough, the robot will not be able to push
the chip into the board, since the actuator may deform under the
• On the other hand, if the part and the holes are not perfectly aligned,
a stiff system cannot give enough to prevent damage to the robot or
the part, whereas a compliant system will give to prevent damage.
• So, although stiffness causes a more responsive and more accurate
system, it also creates a danger if all things are not always perfect.
• Thus, a working balance is needed between these two competing
Use of Reduction Gears
• Some systems, such as hydraulic devices, produce very large forces
with short strokes.
• This means that the hydraulic ram may be moved very slightly while
delivering its full force.
• As a result, there is no need to use reduction gear trains to increase
the torque it produces and to slow it down to manageable speeds.
• For this reason, hydraulic actuators can be directly attached to the
links, which simplifies the design, reduces the weight and cost and
rotating inertia of joints, reduces backlash, increases the reliability of
the system, due to simpler design and fewer parts, and also reduces
• On the other hand, electric motors rotate at high speeds (up to many
thousands of revolutions per minute) and must be used in
conjunction with reduction gears to increase their torque and to
decrease their speed, as no one would want a robot arm to be
rotating at such speeds.
• This, of course, increases the cost, number of parts, backlash, inertia
of the rotating body, etc., as was mentioned earlier, but also
increases the resolution of the system, as it is possible to rotate the
link a very small angle.
• So, the motor will only "feel" a fraction of the actual inertia of the
load (which in the case of a robot, constitutes both the manipulator
and the load it carries with it)
• Hydraulic systems and actuators offer a high power-to-weight ratio, large
forces at low speeds (both linear and rotary actuation) compatibility with
microprocessor and electronic controls, and tolerance of extreme
• However, due to leakage problems, which is almost inevitable in hydraulic
systems, and due to their power unit weight and cost, they are not used
• Nowadays, most robots are electric. However, there are still many robots in
industry that have hydraulic actuators.
• Additionally, for special applications such as very large robots and civil
service robots, hydraulic actuators may be the appropriate choice.
• The total force that a linear cylinder can deliver can be tremendously large
for its size.
• A hydraulic cylinder can deliver a force of F = p x A lb, where A is the
effective area of the piston or ram and p is the working pressure.
• For example, for a pressure of 1.000 psi, every square inch of the cylinder
develops 1.000-lb of force.
• In rotary cylinders, the same principle is true, except that the output
is a torque where:
• where dx is the desired displacement and dx/dt is the desired velocity
of the piston.
• As you can see, by controlling the volume of the fluid going into the
cylinder the total displacement can be controlled.
• By controlling the rate in which the fluid is sent to the cylinder, the
velocity can be controlled.
• This is done through a 'servo-valve’ that controls the volume of the
fluid, as well its rate.
A hydraulic system generally consists of the following parts:
1. Hydraulic linear or rotary cylinders and rams. These provide the force
or torque needed to move the joints and are controlled by the servo
valves or manual valves.
2. A hydraulic pump which is a high-pressure pump that provides high-
pressure fluid to the system.
3. Electric (or others such as diesel engine) motor, which operates the
4. Cooling system, which rids the system of the heat generated. In some
systems, in addition to cooling fans, radiators and cooled air are
5. Reservoir, which keeps the fluid supply available to the system. Since
the pump is constantly supplying pressure to the system, whether or
not the system is using it, all the extra pressurized fluid, as well as all
the returned fluid from the cylinders, flow back into the reservoir.
A hydraulic system generally consists of the following parts:
6. Servo-valve is a very sensitive valve that controls the amount and the
rate of fluid to the cylinders. The servo-valve is generally driven by a
7. Safety check valves, holding valves, and other safety valves
throughout the system.
8. Connecting hoses, which are used to transport the pressurized fluid
to the cylinders and back to the reservoir.
9. Sensors, which are used to control the motion of the cylinders. They
include position, velocity, magnetic, touch, and other sensors.
HYDRAULIC POWER TRANSMISSION
PISTON & ROD
THE LIMITED ANGLE ROTARY ACTUATOR:
• The limited angle rotary actuator is applied when the shaft has to rotate over a
• The animation shows how this simple actuator works: in this case the shaft can
rotate over an angle of about 270 degrees.
• This type of actuator is, among others, used as a rotator actuator on (small) cranes
• Figure is a schematic drawing of a typical hydraulic
• Figure is a schematic drawing of a position control pilot valve for a
hydraulic cylinder, also called a spool valve.
• This is a balanced valve, which means that the pressures on the two
sides of the spool are equal. Thus, it takes very little force to move
the spool even though it may be under very high pressures.
• When a servomotor is attached to the spool valve to operate it, a
servo-valve is created. The servo valve and the cylinder together
form a hydraulic servomotor.
As the spool moves up or
down, it opens the supply
and return ports through
which the fluid travels to
the cylinder or is returned
to the reservoir.
• Depending on the size of the opening of the port, the supply fluid
flow rate is controlled, and so is the velocity of the cylinder.
• Depending on the length of time that the port is kept open, the total
amount of the fluid to the cylinder, and thus its total travel, is
• The command to the servomotor controlling the spool valve comes
from the controller.
• The controller sets the current to the servomotor, as well as the
duration the current is applied, which, in turn, controls the position
of the spool.
• Thus, for a robot, when the controller has calculated how much and
how fast a joint must move, it sets the current and its duration to the
servomotor, which, in turn, controls the position and rate of
movement of the spool valve, which, in turn, controls the flow of the
fluid and its rate to the cylinder, which moves the joint.
• Thesensors provide feedback to the controller for accurate and
• Figure shows the flow of the fluid as the spool valve moves up or
• As you can see, a simple motion of the spool controls the motion of
• To provide feedback to the servo-valve, either electronic or
mechanical feedback can be added to the valve. (Otherwise, it will
not be a servo-valve, but a manual spool valve).
• Figure shows a simple mechanical feedback loop.
• A similar design is used in a two-stage spool valve to provide
feedback to the valve.
• Figure shows the schematic of the, block diagram
for the feedback loop.
• When a wire carrying a current is placed within a magnetic field, it experiences a
force normal to the plane formed by the magnetic field.
• If the wire is attached to a center of rotation, the resulting torque will cause it to
rotate about the center of rotation.
• Changing the direction of the magnetic field or the current causes the wire to
continuously rotate about the center of rotation, as shown in Figure .
• In practice, to accomplish this change in the current, either a set of commutators
and brushes are used for DC motors, the current is electronically switched for DC
brushless motors, or AC current is used for AC motors.
This is the basic principle behind all
Similarly, if a conductor is moved within a
magnetic field crossing the flux, a current
develops through the conductor. This is
called a generator.
Figure: When a wire carrying a current is
placed within a magnetic field it will
experience a force in a direction normal to a
plane formed by the current and the field.
• There are many types of electric motors that are used
in robotics. They include the following:
– DC motors
– reversible AC motors
– brushless DC motors
– stepper motors
• Except for stepper motors, all other types of motors
can be used as a servomotor.
• In each case, the torque or power output of the motor
is a function of the strength of the magnetic fields and
the current in the windings.
• Some motors have permanent magnets (PMs). These
motors generate less heat, since the field is always
present and no current is needed to build them.
• Others have a soft iron core and windings, where an
electric current creates the magnetic field.
• In this case, more heat is generated, but when needed, the
magnetic field can be varied by changing the current,
whereas in permanent magnet motors, the field is
• Additionally, under certain conditions, it is possible that the
permanent magnet may get damaged and lose its field
strength, in which case the motor becomes useless.
• For example, you should never take a motor apart, as the
permanent magnet will become significantly weaker.
• This is because the iron mass around the magnet holds the
field intact until they are separated.
• To increase the strength of the permanent magnets in
motors, most manufacturers magnetize the magnets
after assembling the motor. Motors without
permanent magnets do not have this problem.
• One important issue in the design and operation of all
motors is the dissipation of heat.
• As with the heat generated in many other devices, the
generated heat in motors eventually becomes the
deciding factor about its size and power.
• The heat is generated primarily from the resistance of
the wiring to electric current (load related), but
includes heat due to iron losses, including eddy current
losses and hysteresis losses, friction losses, brush
losses, and short-out circuit losses (speed related) as
• The higher the current, the more heat is generated, as W = RI2
• Thicker wires generate less heat, but are more expensive, are heavier
(more inertia), and require more space.
• All motors generate some heat. However, what is important is the
path that the heat must take to leave the motor since if the heat is
dissipated faster, more generated heat can be dissipated before
• Figure shows the heat leakage path to the environment for an AC-
type motor and a DC-type motor.
• In DC-type motors, the rotor contains the winding and
carries the current, and thus, heat is generated in the rotor.
• This heat must go from the rotor, through the air gap,
through the permanent magnets, through the motor's
body, and be dissipated into the environment.
• As you know, air is a very good isolator. Thus, the total heat
transfer coefficient for the DC motor is relatively low.
• On the other hand, in an AC-type motor, the rotor is a
permanent magnet, and the winding is in the stator.
• The generated heat in the stator is dissipated to the air by
conduction through the motor's body.
• As a result, the total heat transfer coefficient is relatively
high, especially because no air gap exists.
• As a result, AC-type motors can be exposed to relatively
higher currents without damage, and thus they are
generally more powerful for the same size.
• Stepper motors, although not AC motors, have a similar
construction; the rotor is a permanent magnet, and the
stator contains the windings. Thus, stepper motors have
good heat dissipation capability.
• Of course, another major factor in the difference between
brushed and brush-less motors is the life of the brushes
and commutators, as well as the physical limitation of
mechanical switching by brushes.
• Brushless DC motors, AC motors, and stepper motors are
all brushless, and thus they and are sturdy and generally
have long life (only limited by the life of rotor bearings).
• DC motors are very common in industry and have been used for a
long time. As a result, they are reliable, sturdy, and relatively
• In DC motors, the stator is a set of fixed permanent magnets,
creating a fixed magnetic field, while the rotor carries a current.
• Through brushes and commutators, the direction of current is
changed continuously, causing the rotor to rotate continuously.
• Conversely, if the rotor is rotated within the magnetic field, a DC
current will develop, and the motor will act as a generator.
• If permanent magnets are used to generate the magnetic field, the
output torque TM is proportional to the magnetic flux φ and the
current in the rotor windings Irotor , Then,
• where kt is a constant. Since in permanent magnets, the flux is constant, the
output torque becomes a function of /rolor, and to control the output toque, /
(or corresponding voltage) must be changed. If instead of permanent
magnets, soft iron cores with windings are used for the stator as well, then
the output torque is a function of currents in both the rotor and the stator
• Here both k, and kfarz constants.
• Through the use of powerful magnets made of rare earth materials
and.alloys, the performance of motors has been improved significantly. As a
result, the power-to-weight ratio of motors is much better than before, and
they have replaced almost all other types of actuators.
• To overcome the problem of high inertia and large size of many electric
motors, a disk or shell motor can be used. In disk and shell motors, the iron
core of the rotor winding is removed to reduce its weight and inertia, and
as a result, these motors are capable of producing very large accelerations
(zero to 2,000 rpm in one ms ); they respond very favorably to changing
currents for control purposes. A shell motor's rotor looks similar to a
regular DC rotor without the massive iron core. However, in a disk motor,
the rotor is a flat, thin, plate, with windings pressed (etched) into it, as if
one would flatten a rotor into a disk. The wires are generally cut out of a
copper plate and embedded into a disk. The permanent magnets are
• generally small, short cylindrical magnets that are
placed on the two sides of the disk. As a result, disk
motors are very thin and are used in many
applications where both space and acceleration
requirements are important. Figure 6.15 is a
schematic of a disk motor.
Figure 6.15 Schematic of a disk motor. The
rotor has no iron core and thus has very little
inertia. As a result, it can accelerate and
decelerate very quickly.