Actuators

INDIAN INSTITUTE OF ROBOTICS

www.indianinstituteofrobotics.com , [email protected]

1. ACTUATORS

1.1 INTRODUCTION Actuators Drive motions in mechanical systems. Most often this is by converting electrical

energy into some form of mechanical motion.

1.2 SOLENOIDS Solenoids are the most common actuator components. The basic principle of operation is there is

a moving ferrous core (a piston) that will move inside wire coil as shown in Figure 1. Normally

the piston is held outside the coil by a spring. When a voltage is applied to the coil and current

flows, the coil builds up a magnetic field that attracts the piston and pulls it into the center of the

coil. The piston can be used to supply a linear force. Well known applications of these include

pneumatic values and car door openers.

Topics:

Objectives:

• Be aware of various actuators available.

• Solenoids, valves and cylinders • Hydraulics and pneumatics

FIGURE 1: A SOLENOID

As mentioned before, inductive devices can create voltage spikes and may need snubbers,

although most industrial applications have low enough voltage and current ratings they can be

connected directly to the PLC outputs. Most industrial solenoids will be powered by 24Vdc and

draw a few hundred mA.

1.3 VALVES

The flow of fluids and air can be controlled with solenoid controlled valves. An example of a

solenoid controlled valve is shown in Figure 2. The solenoid is mounted on the side. When

actuated it will drive the central spool left. The top of the valve body has two ports that will be

connected to a device such as a hydraulic cylinder. The bottom of the valve body has a single

pressure line in the center with two exhausts to the side. In the top drawing the power flows in

Current Off Current On



through the center to the right hand cylinder port. The left hand cylinder port is allowed to exit

through an exhaust port. In the bottom drawing the solenoid is in a new position and the pressure

is now applied to the left hand port on the top, and the right hand port can exhaust. The symbols

to the left of the figure show the schematic equivalent of the actual valve positions. Valves are

also available that allow the valves to be blocked when unused.

Figure 2: A Solenoid Controlled 5 Ported, 4 Way 2 Position Valve

Valve types are listed below. In the standard terminology, the ’n-way’ designates the number of

connections for inlets and outlets. In some cases there are redundant ports for exhausts. The

normally open/closed designation indicates the valve condition when power is off. All of the

valves listed are two position valve, but three position valves are also available.

2-way normally closed - these have one inlet, and one outlet. When unenergized, the valve is

closed. When energized, the valve will open, allowing flow. These are used to permit flows.

2-way normally open - these have one inlet, and one outlet. When unenergized, the valve is

open, allowing flow. When energized, the valve will close. These are used to stop flows. When

system power is off, flow will be allowed.

3-way normally closed - these have inlet, outlet, and exhaust ports. When unenergized, the

outlet port is connected to the exhaust port. When energized, the inlet is connected to the outlet

port. These are used for single acting cylinders.

3-way normally open - these have inlet, outlet and exhaust ports. When unenergized, the inlet is

connected to the outlet. Energizing the valve connects the outlet to the exhaust. These are used

for single acting cylinders

3-way universal - these have three ports. One of the ports acts as an inlet or outlet, and is

connected to one of the other two, when energized/unenergized. These can be used to divert

flows, or select alternating sources.

The solenoid has two positions and

when actuated will change the direction that fluid flows to the device.

The symbols shown here are

commonly used to represent this type

of valve.



4-way - These valves have four ports, two inlets and two outlets. Energizing the valve causes

connection between the inlets and outlets to be reversed. These are used for double acting

cylinders.

Some of the ISO symbols for valves are shown in Figure 3. When using the symbols in drawings

the connections are shown for the un-energized state. The arrows show the flow paths in

different positions. The small triangles indicate an exhaust port.

Figure 3: ISO Valve Symbols

When selecting valves there are a number of details that should be considered, as listed below.

pipe size - inlets and outlets are typically threaded to accept NPT (national pipe thread).

flow rate - the maximum flow rate is often provided to hydraulic valves.

operating pressure - a maximum operating pressure will be indicated. Some valves will also

require a minimum pressure to operate.

electrical - the solenoid coil will have a fixed supply voltage (AC or DC) and current.

response time - this is the time for the valve to fully open/close. Typical times for valves range

from 5ms to 150ms.

enclosure - the housing for the valve will be rated as,

type 1 or 2 - for indoor use, requires protection against splashes

type 3 - for outdoor use, will resists some dirt and weathering

type 3R or 3S or 4 - water and dirt tight

type 4X - water and dirt tight, corrosion resistant

Two Way Two Position

Three Way Two Position

Four Way Two Position



1.4 CYLINDERS A cylinder uses pressurized fluid or air to create a linear force/motion as shown in Figure 4. In

the figure a fluid is pumped into one side of the cylinder under pressure, causing that side of the

cylinder to expand, and advancing the piston. The fluid on the other side of the piston must be

allowed to escape freely - if the incompressible fluid was trapped the cylinder could not advance.

The force the cylinder can exert is proportional to the cross sectional area of the cylinder.

For Force:

where,

P = the pressure of the hydraulic fluid

A = the area of the piston

F = the force available from the piston rod

Figure 4: A Cross Section of a Hydraulic Cylinder

Single acting cylinders apply force when extending and typically use a spring to retract the

cylinder. Double acting cylinders apply force in both direction.



Figure 5: Schematic Symbols for Cylinders

Magnetic cylinders are often used that have a magnet on the piston head. When it moves to the

limits of motion, reed switches will detect it.

1.5 HYDRAULICS Hydraulics use incompressible fluids to supply very large forces at slower speeds and limited

ranges of motion. If the fluid flow rate is kept low enough, many of the effects predicted by

Bernoulli’s equation can be avoided. The system uses hydraulic fluid (normally an oil)

pressurized by a pump and passed through hoses and valves to drive cylinders. At the heart of the

system is a pump that will give pressures up to hundreds or thousands of psi. These are delivered

to a cylinder that converts it to a linear force and displacement.

Hydraulic systems normally contain the following components;

1. Hydraulic Fluid

2. An Oil Reservoir

3. A Pump to Move Oil, and Apply Pressure

4. Pressure Lines

5. Control Valves - to regulate fluid flow

6. Piston and Cylinder - to actuate external mechanisms

The hydraulic fluid is often a non corrosive oil chosen so that it lubricates the components. This s

normally stored in a reservoir as shown in Figure 6. Fluid is drawn from the reservoir to a pump

where it is pressurized. This is normally a geared pump so that it may deliver fluid at a high

pressure at a constant flow rate. A flow regulator is normally placed at the high pressure outlet

from the pump. If fluid is not flowing in other parts of the system this will allow fluid to

recirculate back to the reservoir to reduce wear on the pump. The high pressure fluid is delivered

single acting spring return

cylinder

double acting cylinder



to solenoid controlled vales that can switch fluid flow on or off. From the vales fluid will be

delivered to the hydraulics at high pressure, or exhausted back to the reservoir.

Figure 6: A Hydraulic Fluid Reservoir

Hydraulic systems can be very effective for high power applications, but the use of fluids, and

high pressures can make this method awkward, messy, and noisy for other applications.

1.6 PNEUMATICS

Pneumatic systems are very common, and have much in common with hydraulic systems with a

few key differences. The reservoir is eliminated as there is no need to collect and store the air

between uses in the system. Also because air is a gas it is compressible and regulators are not

needed to recirculate flow. But, the compressibility also means that the systems are not as stiff or

strong. Pneumatic systems respond very quickly, and are commonly used for low force

applications in many locations on the factory floor.

Some basic characteristics of pneumatic systems are,

- stroke from a few millimeters to meters in length (longer strokes have more springiness

- the actuators will give a bit - they are springy

- pressures are typically up to 85psi above normal atmosphere

- the weight of cylinders can be quite low



- additional equipment is required for a pressurized air supply- linear and rotatory

actuators are available.

- dampers can be used to cushion impact at ends of cylinder travel.

When designing pneumatic systems care must be taken to verify the operating location. In

particular the elevation above sea level will result in a dramatically different air pressure. For

example, at sea level the air pressure is about 14.7 psi, but at a height of 7,800 ft (Mexico City)

the air pressure is 11.1 psi. Other operating environments, such as in submersibles, the air

pressure might be higher than at sea level.

Some symbols for pneumatic systems are shown in Figure 7. The flow control valve is used to

restrict the flow, typically to slow motions. The shuttle valve allows flow in one direction, but

blocks it in the other. The receiver tank allows pressurized air to be accumulated. The dryer and

filter help remove dust and moisture from the air, prolonging the life of the valves and cylinders.

Figure 7: Pneumatics Components

Flow control valve

Shuttle Valve

Receiver Tank

Pump

Filter

Dryer

Pressure Regulator



1.7 MOTORS Motors are common actuators, but for logical control applications their properties are not that

important. Typically logical control of motors consists of switching low current motors directly

with a PLC, or for more powerful motors using a relay or motor starter.

1.8 COMPUTERS - More complex devices contain computers and digital logic.

- to interface to these we use TTL logic, 0V=false, 5V=true

- TTL outputs cards supply power and don’t need a separate power supply

- Sensitive to electrical noise.

1.9 OTHERS There are many other types of actuators including those on the brief list below.

Heaters - The are often controlled with a relay and turned on and off to maintain a temperature

within a range.

Lights - Lights are used on almost all machines to indicate the machine state and provide

feedback to the operator. most lights are low current and are connected directly to the PLC.

Sirens/Horns - Sirens or horns can be useful for unattended or dangerous machines to make

conditions well known. These can often be connected directly to the PLC.

1.10 SUMMARY • Solenoids can be used to convert an electric current to a limited linear motion.

• Hydraulics and pneumatics use cylinders to convert fluid and gas flows to limited

linear motions.

• Solenoid valves can be used to redirect fluid and gas flows.

• Pneumatics provides smaller forces at higher speeds, but is not stiff. Hydraulics

provides large forces and is rigid, but at lower speeds.

• Many other types of actuators can be used.

1.11 PRACTICE PROBLEMS 1. A piston is to be designed to exert an actuation force of 120 lbs on its extension stroke. The

inside diameter of the cylinder is 2.0” and the ram diameter is 0.375”. What shop air pressure

will be required to provide this actuation force? Use a safety factor of 1.3.

2. Draw a simple hydraulic system that will advance and retract a cylinder using PLC outputs.

Sketches should include details from the PLC output card to the hydraulic cylinder.

3. Develop an electrical ladder diagram and pneumatic diagram for a PLC controlled system. The

system includes the components listed below. The system should include all required safety and

wiring considerations.

a 3 phase 50 HP motor

1 NPN sensor

1 NO push button

1 NC limit switch

1 indicator light

a doubly acting pneumatic cylinder



1.12 PRACTICE PROBLEM SOLUTIONS

1. A = pi*r^2 = 3.14159in^2, P=FS*(F/A)=1.3(120/3.14159)=49.7psi. Note, if the cylinder were

retracting we would need to subtract the rod area from the piston area. Note: this air pressure is

much higher than normally found in a shop, so it would not be practical, and a redesign would be

needed.

2.

1.13 ASSIGNMENT PROBLEMS 1. Draw a schematic symbol for a solenoid controlled pneumatic valve and explain how the valve

operates.

2. A PLC based system has 3 proximity sensors, a start button, and an E-stop as inputs. The

system controls a pneumatic system with a solenoid controlled valve. It also controls a robot with

a TTL output. Develop a complete wiring diagram including all safety elements.

3. A system contains a pneumatic cylinder with two inductive proximity sensors that will detect

when the cylinder is fully advanced or retracted. The cylinder is controlled by a solenoid

controlled valve. Draw electrical and pneumatic schematics for a system.

4. Draw an electrical ladder wiring diagram for a PLC controlled system that contains 2 PNP

sensors, a NO pushbutton, a NC limit switch, a contactor controlled AC motor and an indicator

light. Include all safety circuitry.

5. We are to connect a PLC to detect boxes moving down an assembly line and divert larger

boxes. The line is 12 inches wide and slanted so the boxes fall to one side as they travel by.

One sensor will be mounted on the lower side of the conveyor to detect when a box is present. A

second sensor will be mounted on the upper side of the conveyor to determine when a larger box

is present. If the box is present, an output to a pneumatic solenoid will be actuated to divert the

box. Your job is to select a specific PLC, sensors, and solenoid valve. Details (the absolute

minimum being model numbers) are expected with a ladder wiring diagram. (Note: take

advantage of manufacturers web sites.)



MOTORS

DC GEARED MOTORS

SPECIFICATIONS

• DC SUPPLY 4 to 12V

• RPM: 10 TO 600 OR MORE

• TOTAL LENGTH : 46 mm

• Motor Diameter : 36 mm

• Motor Length: 25 mm

• Brush Type : Plastic / Metal

• Gear Head Diameter : 37 mm

• Gear Head Length : 21mm

• Output Shaft : Centred

• Shaft diameter: 6mm

• Shaft Length : 22mm

• Gear Assembly : Spur

DC ENCODER MOTORS

1. These DC motors uses 2 phase (quadrature) incremental encoders to detect the speed of

the motor and the distance it has travelled.

2. These motors are designed to run at 6 to 24 volts DC and they draw less than 3 amps

w/locked rotor. (This is normally referred to as the stall current and is usually the

maximum current draw for the motor). Motors are rated at 12 VDC but can be kicked up

to 24 VDC for short periods to get a higher power level if the amplifier used to run the

motor can provide the needed power.

3. Controller and Amplifiers for these motors should be able to provide a continuous current

of 3 amps and extremely short bursts of 6 amps. Popular, amplifiers and controllers

based on the National Semiconductor LMD18200 series chips will do this.





HIGH TORQUE DC GEARED MOTOR

Features:

100RPM 12V DC motors with Metal Gearbox

25000 RPM base motor

6mm shaft diameter

Gearbox diameter 37 mm.

Motor Diameter 28.5 mm

Length 63 mm without shaft

Shaft length 15mm

300gm weight

35kgcm torque

No-load current = 800 mA(Max), Load current = upto 9.5 A(Max)

12V DC geared motors for robotics applications. It gives a massive torque of 35Kgcm.

The motor comes with metal gearbox and off-centered shaft. Shaft has a metal bushing

for wear resisinance.



GEAR MOTOR – 90 DEGREE SHAFT

Features: Working voltage : 3V to 9V

Compatible wheel available as optional item

40gm weight

Same size motor available in various rpm

3 Kgf.cm torque

No-load current = 60 mA, Stall current = 700 mA

DC geared motor which gives good torque and rpm at lower voltages. This motor can run

at approximately 150 rpm when driven by a single Li-Ion cell.

DUAL SHAFT GEAR MOTOR

This is a low cost low voltage durable dual

shaft DC geared motor. It is most suitable

for light weight robot running on small

voltage. Out of its two shafts one shaft can

be connected to wheel, other can be

connected to the position encoder. Drive

shaft has clutch for non continuous

protection from overload. Motor runs

smoothly from 2V to 9V and gives wide

range of RPM, and torque.

Specifications

· Voltage: 2V to 9V

· Current: No load and stall currents are

function of voltage. Fore more data refer

below tables

· RPM: 20 to 200. RPM is a function of

voltage. For more data refer below tables

· Clutch for non continuous protection from

overload conditions

· Motor weight: 30gms



SERVO MOTORS

• A "servo" is a generic term used for an automatic control system. It comes from

the Latin word "servus" - slave. In practical terms, that means a mechanism that

you can set and forget, and which adjusts itself during continued operation through

feedback. Disk drives, for example, contain a servo system insuring that they spin

at a desired constant speed by measuring their current rotation, and speeding up or

slowing down as necessary to keep that speed.

• The position-sensing mechanism tells the servo what position the shaft currently

has. The control circuitry notes the difference between the desired position and the

current position, and uses the motor to "make it so". If the difference in position is

large, the motor moves rapidly to the correct position; if the difference is small,

the adjustment is more subtle. As for the operator, all he knows is that he moved a

slider half-way up, and the rudder on his model plane moved to the center

position, and will stay there until he moves the slider again.

• The most commonly available servo motors on the market are the range from

Futaba. These include high tech digital servo motors. The regular servo motors

used in radio controlled cars is small and inexpensive. These servo motors are

usually made from plastic and have plastic gears.

• Usually rc servo motors come in standard sizes and have similar control schemes.

Unlike other servo motors rc servo motors are constrained from full rotation. RC

servo motors are restricted to a limited rotation of 180 degrees or less. Standard

servo motors such as the Futaba S148 can undergo mods which are designed to

make it rotate a full 360 degrees.



CONNECTIONS OF A SERVO MOTOR

• a high torque plastic gear servo motor measuring with dual ball bearings. It gives

6.4Kg/cm cm torque. This servo motor is ideal for making your own light weight

hexapod, walking insects, sensor / camera pod etc. Servo motors are used in radio control

models. They are very useful in robotics applications because of there small size and low

cost. Servomotor has built in motor, gearbox, position feedback mechanism and motor

controller. The servo motor can be controlled to move any position just by using simple

pulse controlling. This motor has three wire interfaces for control and power supply

• Specifications • Dimension: 40.7mm x 20.5mm x39.5mm

• Torque: 6.4kg/cm

• Motor weight: 41gms

• Operating speed: 0.17sec/60 degree

• Operating voltage: 4.8V to 6V

• Temperature range: 0-55C .

HIGH TORQUE RC SERVO MOTOR WITH METAL

GEARS

Specifications :

· Dimension: 40.7mm x 20.5mm x39.5mm

· Torque: 15.5kg/cm at 4.8V, 17kg/cm at 6V

· Dual bearing with metal gear

· Motor weight: 60gms

· Operating speed: 0.15sec/60 degree

· Operating voltage: 4.8V to 6V

· Temperature range: 0-55C



It gives whooping 15.5Kg to 17kg cm torque. This servo motor is ideal for making your own

hexapod, walking insects, heavy duty sensor / camera pod. Servo motors are used in radio

control models. They are very useful in robotics applications because of there small size and low

cost.

Servomotor has built in motor, gearbox, position feedback mechanism and motor controller. The

servo motor can be controlled to move any position just by using simple pulse controlling.

HS 485B DELUX HEAVY DUTY SERVO WITH

KARBONITE GEARS

Specifications :

• Dimension: 39.8mm x 19.8mm x37.8mm

• Torque: 5.2kg/cm at 4.8V, 6.4kg/cm at 6V

• Motor weight: 45gms

• Operating speed: 0.20sec/60 degree

• Operating voltage: 4.8V to 6V

• Temperature range: 0-55C

This is high torque Karbonite gear servo motor measuring 39.8mm x 19.8mm x37.8mm. It gives

5.2Kg to 6.4kg cm torque. This servo motor is suitable for making your own light weight

hexapod, walking insects, heavy duty sensor / camera pod etc. Servo motors are used in radio

control models.

They are very useful in robotics applications because of there small size and low cost.

Servomotor has built in motor, gearbox, position feedback mechanism and motor controller.

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Actuators