04/28/23 1Hareesha N G, Asst. Prof, DSCE, BLore-78

UNIT-7: ACTUATORS: Syllabus• Types• Characteristics of actuating system: weight, power-

to-weight ratio, operating pressure, stiffness vs. compliance, Use of reduction gears

• comparison of hydraulic, electric, pneumatic actuators

• 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

Introduction-Types • 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– Servomotors– Stepper motors– Direct-drive electric motors– Hydraulic actuators– Pneumatic actuators– Shape memory metal actuators– Magneto-strictive actuators

Introduction• 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 SYSTEMSWeight, 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 average. • 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, etc.

CHARACTERISTICS OF ACTUATING SYSTEMSWeight, 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 so on.

• 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 material.

• The modulus of elasticity of fluids can be around 1 x 106 psi, which is very high.

• As a result, hydraulic systems are very stiff and noncompliant. Conversely, pneumatic systems are easily compressed, and, thus, are compliant.

• 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 inaccurate.

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 resistive force.

• 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 characteristics.

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 noise.

• 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 Actuators• 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 hazardous environments.

• 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 any more.

• 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 hydraulic pump.

4. Cooling system, which rids the system of the heat generated. In some systems, in addition to cooling fans, radiators and cooled air are used.

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 hydraulic servomotor.

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

LINEAR ACTUATOR

ROTARY ACTUATOR

LOADPUMP

PUMP

PISTON & ROD

TO RESERVOIR

HYDRO MOTOR

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Limited-RotationHydraulic Actuators

• Rack-and-pinion limited rotation actuator

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only. 25

Limited-RotationHydraulic Actuators

• Vane limited-rotation actuator

THE LIMITED ANGLE ROTARY ACTUATOR:• The limited angle rotary actuator is applied when the shaft has to rotate over a

limited angle.• 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

and excavators.

• Figure is a schematic drawing of a typical hydraulic system.

• 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 controlled.

• 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.

• The sensors provide feedback to the controller for accurate and continued control.

• Figure shows the flow of the fluid as the spool valve moves up or down.

• As you can see, a simple motion of the spool controls the motion of the cylinder.

• 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.

ELECTRIC MOTORS• 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 electric motors.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 constant.

• 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 well.

• 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 damage occurs.

• 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• DC motors are very common in industry and have been used for a

long time. As a result, they are reliable, sturdy, and relatively powerful.

• 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 windings:

• 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 [3]); 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.