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Actuators
 

Actuators

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    Actuators Actuators Document Transcript

    • INDIAN INSTITUTE OF ROBOTICSwww.indianinstituteofrobotics.com , info@iir.co.in1. ACTUATORS1.1 INTRODUCTIONActuators Drive motions in mechanical systems. Most often this is by converting electricalenergy into some form of mechanical motion.1.2 SOLENOIDSSolenoids are the most common actuator components. The basic principle of operation is there isa moving ferrous core (a piston) that will move inside wire coil as shown in Figure 1. Normallythe piston is held outside the coil by a spring. When a voltage is applied to the coil and currentflows, the coil builds up a magnetic field that attracts the piston and pulls it into the center of thecoil. The piston can be used to supply a linear force. Well known applications of these includepneumatic values and car door openers.Topics:Objectives:• Be aware of various actuators available.• Solenoids, valves and cylinders• Hydraulics and pneumaticsFIGURE 1: A SOLENOIDAs 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 beconnected directly to the PLC outputs. Most industrial solenoids will be powered by 24Vdc anddraw a few hundred mA.1.3 VALVESThe flow of fluids and air can be controlled with solenoid controlled valves. An example of asolenoid controlled valve is shown in Figure 2. The solenoid is mounted on the side. Whenactuated it will drive the central spool left. The top of the valve body has two ports that will beconnected to a device such as a hydraulic cylinder. The bottom of the valve body has a singlepressure line in the center with two exhausts to the side. In the top drawing the power flows inCurrent OffCurrent On
    • INDIAN INSTITUTE OF ROBOTICSwww.indianinstituteofrobotics.com , info@iir.co.inthrough the center to the right hand cylinder port. The left hand cylinder port is allowed to exitthrough an exhaust port. In the bottom drawing the solenoid is in a new position and the pressureis now applied to the left hand port on the top, and the right hand port can exhaust. The symbolsto the left of the figure show the schematic equivalent of the actual valve positions. Valves arealso available that allow the valves to be blocked when unused.Figure 2: A Solenoid Controlled 5 Ported, 4 Way 2 Position ValveValve types are listed below. In the standard terminology, the ’n-way’ designates the number ofconnections for inlets and outlets. In some cases there are redundant ports for exhausts. Thenormally open/closed designation indicates the valve condition when power is off. All of thevalves 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 isclosed. 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 isopen, allowing flow. When energized, the valve will close. These are used to stop flows. Whensystem power is off, flow will be allowed.3-way normally closed - these have inlet, outlet, and exhaust ports. When unenergized, theoutlet port is connected to the exhaust port. When energized, the inlet is connected to the outletport. These are used for single acting cylinders.3-way normally open - these have inlet, outlet and exhaust ports. When unenergized, the inlet isconnected to the outlet. Energizing the valve connects the outlet to the exhaust. These are usedfor single acting cylinders3-way universal - these have three ports. One of the ports acts as an inlet or outlet, and isconnected to one of the other two, when energized/unenergized. These can be used to divertflows, or select alternating sources.The solenoid has two positions andwhen actuated will change thedirection that fluid flows to the device.The symbols shown here arecommonly used to represent this typeof valve.
    • INDIAN INSTITUTE OF ROBOTICSwww.indianinstituteofrobotics.com , info@iir.co.in4-way - These valves have four ports, two inlets and two outlets. Energizing the valve causesconnection between the inlets and outlets to be reversed. These are used for double actingcylinders.Some of the ISO symbols for valves are shown in Figure 3. When using the symbols in drawingsthe connections are shown for the un-energized state. The arrows show the flow paths indifferent positions. The small triangles indicate an exhaust port.Figure 3: ISO Valve SymbolsWhen 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 alsorequire 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 rangefrom 5ms to 150ms.enclosure - the housing for the valve will be rated as,type 1 or 2 - for indoor use, requires protection against splashestype 3 - for outdoor use, will resists some dirt and weatheringtype 3R or 3S or 4 - water and dirt tighttype 4X - water and dirt tight, corrosion resistantTwo Way Two PositionThree Way Two PositionFour Way Two Position
    • INDIAN INSTITUTE OF ROBOTICSwww.indianinstituteofrobotics.com , info@iir.co.in1.4 CYLINDERSA cylinder uses pressurized fluid or air to create a linear force/motion as shown in Figure 4. Inthe figure a fluid is pumped into one side of the cylinder under pressure, causing that side of thecylinder to expand, and advancing the piston. The fluid on the other side of the piston must beallowed 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 fluidA = the area of the pistonF = the force available from the piston rodFigure 4: A Cross Section of a Hydraulic CylinderSingle acting cylinders apply force when extending and typically use a spring to retract thecylinder. Double acting cylinders apply force in both direction.
    • INDIAN INSTITUTE OF ROBOTICSwww.indianinstituteofrobotics.com , info@iir.co.inFigure 5: Schematic Symbols for CylindersMagnetic cylinders are often used that have a magnet on the piston head. When it moves to thelimits of motion, reed switches will detect it.1.5 HYDRAULICSHydraulics use incompressible fluids to supply very large forces at slower speeds and limitedranges of motion. If the fluid flow rate is kept low enough, many of the effects predicted byBernoulli’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 thesystem is a pump that will give pressures up to hundreds or thousands of psi. These are deliveredto a cylinder that converts it to a linear force and displacement.Hydraulic systems normally contain the following components;1. Hydraulic Fluid2. An Oil Reservoir3. A Pump to Move Oil, and Apply Pressure4. Pressure Lines5. Control Valves - to regulate fluid flow6. Piston and Cylinder - to actuate external mechanismsThe hydraulic fluid is often a non corrosive oil chosen so that it lubricates the components. This snormally stored in a reservoir as shown in Figure 6. Fluid is drawn from the reservoir to a pumpwhere it is pressurized. This is normally a geared pump so that it may deliver fluid at a highpressure at a constant flow rate. A flow regulator is normally placed at the high pressure outletfrom the pump. If fluid is not flowing in other parts of the system this will allow fluid torecirculate back to the reservoir to reduce wear on the pump. The high pressure fluid is deliveredsingle acting spring returncylinderdouble acting cylinder
    • INDIAN INSTITUTE OF ROBOTICSwww.indianinstituteofrobotics.com , info@iir.co.into solenoid controlled vales that can switch fluid flow on or off. From the vales fluid will bedelivered to the hydraulics at high pressure, or exhausted back to the reservoir.Figure 6: A Hydraulic Fluid ReservoirHydraulic systems can be very effective for high power applications, but the use of fluids, andhigh pressures can make this method awkward, messy, and noisy for other applications.1.6 PNEUMATICSPneumatic systems are very common, and have much in common with hydraulic systems with afew key differences. The reservoir is eliminated as there is no need to collect and store the airbetween uses in the system. Also because air is a gas it is compressible and regulators are notneeded to recirculate flow. But, the compressibility also means that the systems are not as stiff orstrong. Pneumatic systems respond very quickly, and are commonly used for low forceapplications 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
    • INDIAN INSTITUTE OF ROBOTICSwww.indianinstituteofrobotics.com , info@iir.co.in- additional equipment is required for a pressurized air supply- linear and rotatoryactuators 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. Inparticular the elevation above sea level will result in a dramatically different air pressure. Forexample, 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 airpressure might be higher than at sea level.Some symbols for pneumatic systems are shown in Figure 7. The flow control valve is used torestrict the flow, typically to slow motions. The shuttle valve allows flow in one direction, butblocks it in the other. The receiver tank allows pressurized air to be accumulated. The dryer andfilter help remove dust and moisture from the air, prolonging the life of the valves and cylinders.Figure 7: Pneumatics ComponentsFlow control valveShuttle ValveReceiver TankPumpFilterDryerPressure Regulator
    • INDIAN INSTITUTE OF ROBOTICSwww.indianinstituteofrobotics.com , info@iir.co.in1.7 MOTORSMotors are common actuators, but for logical control applications their properties are not thatimportant. Typically logical control of motors consists of switching low current motors directlywith 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 OTHERSThere 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 temperaturewithin a range.Lights - Lights are used on almost all machines to indicate the machine state and providefeedback 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 makeconditions 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 limitedlinear motions.• Solenoid valves can be used to redirect fluid and gas flows.• Pneumatics provides smaller forces at higher speeds, but is not stiff. Hydraulicsprovides large forces and is rigid, but at lower speeds.• Many other types of actuators can be used.1.11 PRACTICE PROBLEMS1. A piston is to be designed to exert an actuation force of 120 lbs on its extension stroke. Theinside diameter of the cylinder is 2.0” and the ram diameter is 0.375”. What shop air pressurewill 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. Thesystem includes the components listed below. The system should include all required safety andwiring considerations.a 3 phase 50 HP motor1 NPN sensor1 NO push button1 NC limit switch1 indicator lighta doubly acting pneumatic cylinder
    • INDIAN INSTITUTE OF ROBOTICSwww.indianinstituteofrobotics.com , info@iir.co.in1.12 PRACTICE PROBLEM SOLUTIONS1. A = pi*r^2 = 3.14159in^2, P=FS*(F/A)=1.3(120/3.14159)=49.7psi. Note, if the cylinder wereretracting we would need to subtract the rod area from the piston area. Note: this air pressure ismuch higher than normally found in a shop, so it would not be practical, and a redesign would beneeded.2.1.13 ASSIGNMENT PROBLEMS1. Draw a schematic symbol for a solenoid controlled pneumatic valve and explain how the valveoperates.2. A PLC based system has 3 proximity sensors, a start button, and an E-stop as inputs. Thesystem controls a pneumatic system with a solenoid controlled valve. It also controls a robot witha 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 detectwhen the cylinder is fully advanced or retracted. The cylinder is controlled by a solenoidcontrolled valve. Draw electrical and pneumatic schematics for a system.4. Draw an electrical ladder wiring diagram for a PLC controlled system that contains 2 PNPsensors, a NO pushbutton, a NC limit switch, a contactor controlled AC motor and an indicatorlight. Include all safety circuitry.5. We are to connect a PLC to detect boxes moving down an assembly line and divert largerboxes. 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. Asecond sensor will be mounted on the upper side of the conveyor to determine when a larger boxis present. If the box is present, an output to a pneumatic solenoid will be actuated to divert thebox. Your job is to select a specific PLC, sensors, and solenoid valve. Details (the absoluteminimum being model numbers) are expected with a ladder wiring diagram. (Note: takeadvantage of manufacturers web sites.)
    • INDIAN INSTITUTE OF ROBOTICSwww.indianinstituteofrobotics.com , info@iir.co.inMOTORSDC GEARED MOTORSSPECIFICATIONS• 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 : SpurDC ENCODER MOTORS1. These DC motors uses 2 phase (quadrature) incremental encoders to detect the speed ofthe 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 ampsw/locked rotor. (This is normally referred to as the stall current and is usually themaximum current draw for the motor). Motors are rated at 12 VDC but can be kicked upto 24 VDC for short periods to get a higher power level if the amplifier used to run themotor can provide the needed power.3. Controller and Amplifiers for these motors should be able to provide a continuous currentof 3 amps and extremely short bursts of 6 amps. Popular, amplifiers and controllersbased on the National Semiconductor LMD18200 series chips will do this.
    • INDIAN INSTITUTE OF ROBOTICSwww.indianinstituteofrobotics.com , info@iir.co.in
    • INDIAN INSTITUTE OF ROBOTICSwww.indianinstituteofrobotics.com , info@iir.co.inHIGH TORQUE DC GEARED MOTORFeatures:100RPM 12V DC motors with Metal Gearbox25000 RPM base motor6mm shaft diameterGearbox diameter 37 mm.Motor Diameter 28.5 mmLength 63 mm without shaftShaft length 15mm300gm weight35kgcm torqueNo-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 bushingfor wear resisinance.
    • INDIAN INSTITUTE OF ROBOTICSwww.indianinstituteofrobotics.com , info@iir.co.inGEAR MOTOR – 90 DEGREE SHAFTFeatures:Working voltage : 3V to 9VCompatible wheel available as optional item40gm weightSame size motor available in various rpm3 Kgf.cm torqueNo-load current = 60 mA, Stall current = 700 mADC geared motor which gives good torque and rpm at lower voltages. This motor can runat approximately 150 rpm when driven by a single Li-Ion cell.DUAL SHAFT GEAR MOTORThis is a low cost low voltage durable dualshaft DC geared motor. It is most suitablefor light weight robot running on smallvoltage. Out of its two shafts one shaft canbe connected to wheel, other can beconnected to the position encoder. Driveshaft has clutch for non continuousprotection from overload. Motor runssmoothly from 2V to 9V and gives widerange of RPM, and torque.Specifications· Voltage: 2V to 9V· Current: No load and stall currents arefunction of voltage. Fore more data referbelow tables· RPM: 20 to 200. RPM is a function ofvoltage. For more data refer below tables· Clutch for non continuous protection fromoverload conditions· Motor weight: 30gms
    • INDIAN INSTITUTE OF ROBOTICSwww.indianinstituteofrobotics.com , info@iir.co.inSERVO MOTORS• A "servo" is a generic term used for an automatic control system. It comes fromthe Latin word "servus" - slave. In practical terms, that means a mechanism thatyou can set and forget, and which adjusts itself during continued operation throughfeedback. Disk drives, for example, contain a servo system insuring that they spinat a desired constant speed by measuring their current rotation, and speeding up orslowing down as necessary to keep that speed.• The position-sensing mechanism tells the servo what position the shaft currentlyhas. The control circuitry notes the difference between the desired position and thecurrent position, and uses the motor to "make it so". If the difference in position islarge, 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 aslider half-way up, and the rudder on his model plane moved to the centerposition, and will stay there until he moves the slider again.• The most commonly available servo motors on the market are the range fromFutaba. These include high tech digital servo motors. The regular servo motorsused in radio controlled cars is small and inexpensive. These servo motors areusually 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. RCservo motors are restricted to a limited rotation of 180 degrees or less. Standardservo motors such as the Futaba S148 can undergo mods which are designed tomake it rotate a full 360 degrees.
    • INDIAN INSTITUTE OF ROBOTICSwww.indianinstituteofrobotics.com , info@iir.co.inCONNECTIONS OF A SERVO MOTOR• a high torque plastic gear servo motor measuring with dual ball bearings. It gives6.4Kg/cm cm torque. This servo motor is ideal for making your own light weighthexapod, walking insects, sensor / camera pod etc. Servo motors are used in radio controlmodels. They are very useful in robotics applications because of there small size and lowcost. Servomotor has built in motor, gearbox, position feedback mechanism and motorcontroller. The servo motor can be controlled to move any position just by using simplepulse 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 METALGEARSSpecifications :· 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
    • INDIAN INSTITUTE OF ROBOTICSwww.indianinstituteofrobotics.com , info@iir.co.inIt gives whooping 15.5Kg to 17kg cm torque. This servo motor is ideal for making your ownhexapod, walking insects, heavy duty sensor / camera pod. Servo motors are used in radiocontrol models. They are very useful in robotics applications because of there small size and lowcost.Servomotor has built in motor, gearbox, position feedback mechanism and motor controller. Theservo motor can be controlled to move any position just by using simple pulse controlling.HS 485B DELUX HEAVY DUTY SERVO WITHKARBONITE GEARSSpecifications :• 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: 0-55CThis is high torque Karbonite gear servo motor measuring 39.8mm x 19.8mm x37.8mm. It gives5.2Kg to 6.4kg cm torque. This servo motor is suitable for making your own light weighthexapod, walking insects, heavy duty sensor / camera pod etc. Servo motors are used in radiocontrol 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.