Robotic Manipulator Arm

8/4/2019 Robotic Manipulator Arm

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[MECHATRONICS MINI PROJECT]

[2010-11]

Robotic Manipulator ArmA Six Degree of Freedom Robotic Manipulator

Abhinav Tripathi

Kulbhushan

Kumar Akhilesh

Pankaj Singh

Navneet Verma

www.final-yearprojects.co.cc | www.troubleshoot4free.com/fyp/


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Introduction

Hydraulic Actuators employs hydraulic pressure to drive an output member. These are used wherehigh speed and large forces are required. The fluid used in hydraulic actuator is highly incompressible

so that pressure applied can be transmitted instantaneously to the member attached to it.

A robotic arm incorporates an articulate system, which together function in much same way as itsbiologica l counterpart. The skeleton is composed of rigid links that connect varying numbers of jointsthat are capable of sliding, twisting or rotating. The robot's muscles come in the form of actuators that

convert hydraulic, electrical or pneumatic energy into power for each joint. Next there is an electronicnervous system of wires and sensors that carries commands to the muscles of the robotic arm and

then back to an external computer.

The main difference between the arm of the robot and that of a human is found at the arm's extremity.

Rather than having a flexible, multi-fingered hand, typical robot arms end in special-purpose devicescalled end-effectors, which are installed directly into the wrist. To reduce the number of calculations,

needed to determine the robot's exact position, the base is generally kept stationary. In a few

applications, however, like the robots in spot welding, robots are programmed to follow an assemblyline. The motions of the robots can be programmed by means of direct teaching where the arm is

manually guided through its desired motion and the robot's computer remembers these specificmotions, sort of like a watch and learn method. Robots can also be taught by means of programming

by a computer specialist. Finally there is always the option of creating a learning computer that willgather data as it makes mistakes and on the following attempt, it won't make the same mistake again.

Principle Used in Hydraulic Actuator System

Pascals Law

Pressure applied to a confined fluid at any point is transmitted undiminished and equally throughoutthe fluid in all directions and acts upon every part of the confining vessel at right angles to its interior

surfaces.

Amplification of Force

Since pressure P applied on an area A gives rise to a force F, given as,F = PA

Thus, if a force is applied over a small area to cause a pressure P in a confined fluid, the forcegenerated on a larger area can be made many times larger than the applied force that crated the

pressure. This principle is used in various hydraulic devices to such hydraulic press to generate veryhigh forces.



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Conservation of Energy

Since energy or power is always conserved, amplification in force must result in reduction of the fluidvelocity. Indeed if the resultant force is applied over a larger area then a unit displacement of the area

would cause a larger volumetric displacement than a unit displacement of the small area through whichthe generating force is applied. Thus, what is gained in force must be sacrificed in distance or speed andpower would be conserved.

Pump

PA

Q

L

Travel/unit time

F

Major hydraulic and mechanical variables

Advantages of Hydraulic Actuation Systems

Hydraulics refers to the means and mechanisms of transmitting power through liquids. The original

power source for the hydraulic system is a prime mover such as an electric motor which drives the pump. However, the mechanical equipment cannot be coupled directly to the prime mover because

the required control over the motion, necessary for industrial operations cannot be achieved. In termsof these Hydraulic Actuation Systems offer unique advantages, as given below.

Variable Speed and Direction:Most large electric motors run at adjustable, but constant speeds.It is also the case for engines. The

actuator (linear here) of a hydraulic system, however, can be driven at speeds that vary by largeamounts and fast, by varying the pump delivery or using a flow control valve. In addition, a hydraulicactuator can be reversed instantly while in full motion without damage. This is not possible for most

other prime movers.



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Power-to-weight ratio:

Hydraulic components, because of their high speed and pressure capabilities, can provide high poweroutput with vary small weight and size, in comparison to electric system components. Note that in

electric components, the size of equipment is mostly limited by the magnetic saturation limit of theiron.

Stall Condition and Overload Protection:A hydraulic actuator can be stalled without damage when overloaded, and will start up immediately

when the load is reduced. The pressure relief valve in a hydraulic system protects it from overloaddamage. During stall, or when the load pressure exceeds the valve setting, pump delivery is directedto tank with definite limits to torque or force output. The only loss encountered is in terms of pump

energy. On the contrary, stalling an electric motor is likely to cause damage.

Components of Hydraulic Actuation Systems

Hydraulic Fluid

Hydraulic fluid is essentially non-compressible to be able to transmit power instantaneously from onepart of the system to another. At the same time, it lubricates the moving parts to reduce friction loss

and cool the components so that the heat generated does not lead to fire hazards. It also helps inremoving the contaminants to filter. The other desirable property of oil is its lubricating ability.

Finally, often, the fluid also acts as a seal against leakage inside a hydraulic component. The degreeof closeness of the mechanical fit and the oil viscosity determines leakage rate. F igure 2 below showsthe role played by hydraulic fluid films in lubrication and sealing. The fluid used is 40W oil reads as

40 weight oil. It has high viscosity whichmaintains a lubricating film between moving parts.

Film of hydraulicfluid lubricates

Film of hydraulic fluid seals passagefrom adjacent

Lubrication and Sealing by Hydraulic Fluid



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The Fluid Delivery Subsystem

It consists of the components that hold and carry the fluid from the pump to the actuator. It is made

up of the following components.

Reservoir

It holds the hydraulic fluid to be circulated and allows air entrapped in the fluid to escape. This is animportant feature as the bulk modulus of the oil, which determines the stiffness of hydraulic system,

deteriorates considerably in the presence of entrapped air bubbles. It also helps in dissipating heat.

From hydraulic system To hydraulic system

PUMP

Releases Bubbles

Dissipates Heat

The functions of the reservoir

Reservoir

Baffle

Filter

The hydraulic fluid is kept clean in the system with the help of filters and strainers. It removesminute particles from the fluid, which can cause blocking of the orifices of servo-valves or cause

amming of spools.An important part in designing hydraulic systems is to find an adequate filtration concept. Therefore

operating and ambient conditions are important. Equally important are requirements on oil purityand filter life time which affect maintenance intervals. Considering the increasing complexity o

hydraulic systems the design of adequate filtration concepts becomes more difficult. Thiscomplexity becomes a greater challenge for designers because until now the design of a filtration

system has mainly been based on experience.

Line

Pipe, tubes and hoses, along with the fittings or connectors, constitute the conducting lines that carryhydraulic fluid between components. Lines are one of the disadvantages of hydraulic system that we

need to pay in return of higher power to weight ratio. Lines convey the fluid and also dissipate heat.In contrast, for Pneumatic Systems, no return path for the fluid, which is air, is needed, since it can be

directly released into the atmosphere. There are various kinds of lines in a hydraulic system. Theworking lines carry the fluid that delivers the main pump power to the load. The pilot lines carry fluidthat transmits controlling pressures to various directional and relief valves for remote operation or

safety. Lastly there are drain lines that carry the fluid that inevitably leaks out, to the tank.



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Working line

Pilot line Drain line

Fig.4 The various kinds of lines in a hydraulic system

Fig 5 below shows a typical configuration of connecting the supply and the return lines as well as thefilter to the reservoir. The graphical symbol for a Reservoir and Filters is shown in Fig. 6.

Supply Line

Pump

Return Line

Filter

Reservoir

Connection Arrangement of Filter and Lines with a Reservoir

The graphical symbol for Reservoirs and Filters



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Fittings and Seals

Various additional components are needed to join pipe or tube sections, create bends and also to

prevent internal and external leakage in hydraulic systems. Although some amount of internal leakageis built-in, to provide lubrication, excessive internal leakage causes loss of pump power since high

pressure fluid returns to the tank, without doing useful work. External leakage, on the other hand,

causes loss of fluid and can create fire hazards, as well as fluid contamination. Various kinds osealing components are employed in hydraulic systems to prevent leakage. A typical such component,

known as the O-ring is shown below in Fig.7.

O-Ring

Sealing by O-rings

Hydraulic Pumps

The pump converts the mechanical energy of its prime-mover to hydraulic energy by delivering a

given quantity of hydraulic fluid at high pressure into the system.The pump used here is a hydrostaticor positive displacement. The symbol for a pump, is shown in Fig.8 below.

Pump

Reversible

The graphical symbol for Pumps

Hydrostatic or Positive Displacement Pumps

These pumps deliver a given amount of fluid for each cycle of motion, that is, stroke or revolution.Their output in terms of the volume flow rate is solely dependent on the speed of the prime-moverand is independent of outlet pressure notwithstanding leakage. These pumps are generally rated by

their volume flow rate output at a given drive speed and by their maximum operating pressurecapability which is specified based on factors of safety and operating life considerations. In theory, a

pump delivers an amount of fluid equal to its displacement each cycle or revolution. In reality, theactual output is reduced because of internal leakage or slippage which increases with operating

pressure. Moreover, note that the power requirement on the prime mover theoretically increases with

the pump delivery at a constant fluid pressure. If this power exceeds the power that the prime movercan handle the pump speed and the delivery rate would fall automatically. There are various types o

pumps used in hydraulic systems as described below.



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Gear Pumps

Outlet

DriveGear Free

Gear

Inlet

The construction of a Gear Pump

A gear pump develops flow by carrying fluid between the teeth of two meshed gears. One gear is driven by the drive shaft and turns the other, which is free. The pumping chambers formed betweenthe gear teeth are enclosed by the pump housing and the side plates. A low pressure region is createdat the inlet as the gear teeth separate. As a result, fluid flows in and is carried around by the gears. As

the teeth mesh again at the outlet, high pressure is created and the fluidis forced out. Figure .9 showsthe construction of a a typical internal gears pump; Most gear type pumps are fixed displacement.They range in output from very low to high volume. They usually operate at comparatively low

pressure.

As the gears rotate they separate on the intake side of the pump, creating a void and suction which isfilled by the fluid. The fluid is carried to the discharge side of the pump, where meshing of gearsdisplaces the fluid. The mechanical clearances are small-in the order of 10microns. The tight

clearances, along with the speed o f rotation, effectively prevent the fluid from leaking backwards.



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ValvesA valve is a device that regulates the flow of a fluid by opening, closing, or partially

obstructing various passageways.

Hydraulic solenoid valves are in general similar to pneumatic solenoid valves exceptthat they control the flow of hydraulic fluid (oil), often at around 3000 psi which is

210 bar. Solenoid-controlled valves are used, where a relatively weak solenoid opensand closes a small pilot valve, which in turn activates the main valve by applying fluid

pressure to a piston or diaphragm that is mechanically coupled to the main valve.

3/2 Valve:

In a three way, two position valve, there are three inlet/outlet ports in the valve andthe spool can be in one of two positions. A 3/2 valve would be used to allow fluid

flow into or out of actuator or motor. There are 18- 3/2 valves connected in series.

AA A

.

Actuator

A

B

C

To understand the working of 3/2 valve let us consider the figure above.

The figure shows pump connected to A, which in turn is connected to the actuator. Also there is a

way from the portion A to the sink. The truth table can be obtained, in which the state 1 represent

open and state 0 means close, and is given below.

From Pump

To Sump



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A B C

Forward 1 1 0

Backward 1 0 1

Hold 0 0 0

4/2 Valve:

In a four way, two position valves there are four inlet/outlet ports in the valve andthe spool can be located in one of two positions. For 4/2 valve fluid is always flowing

through the valve with system pressure supplied to one of the two outlet ports at all times.

The other port would then be ported to return. 4/2 valves would normally be used inhydraulic systems in conjunction with an upstream shut valve .

E2I2E1I1

Actuator Actuator



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The above is the truth table for 4/2 valve. For the forward stoke to take place the pump I1 and the sump

E2 must be open while the rest are closed. For the backward stroke pump I2 and sump E1 are open, while

I1 and E2 remain closed. The hold state is obtained by closing all the valves.

Modern 3/2 valve (Three way, twoposition)

Modern 4/2 valve (Four way, two position)

I1 E1 I2 E2

orward Stroke 1 0 0 1

ackward Stroke 0 1 1 0

Hold state 0 0 0 0



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DC Motor

Motors

Motors work exactly on the reverse principle of pumps. In motors fluid is forced into the motor from pump outlets at high pressure. This fluid pressure creates the motion of the motor shaft and finally

goes out through the motor outlet port and return to tank. The motor used in this case in a DC motor.The direct current (DC) motor uses Permanent magnet (PM) to convert electrical energy into

mechanical energy through the interaction of two magnetic fields. One field is produced by apermanent magnet assembly; the other field is produced by an electrical current flowing in the motorwindings. These two fields result in a torque which tends to rotate the rotor. As the rotor turns, the

current in the windings is commutated to produce a continuous torque output. The stationaryelectromagnetic field of the motor can also be wire-wound like the armature (called a wound-field

motor) or can be made up of permanent magnets (called a permanent magnet motor).In either style (wound-field or permanent magnet) the commutator. acts as half of a mechanical

switch and rotates with the armature as it turns. The commutator is composed of conductive segments(called bars), usually made of copper, which represent the termination of individual coils of wire

distributed around the armature. The second half of the mechanical switch is completed by thebrushes. These brushes typically remain stationary with the motor's housing but ride (or brush) on therotating commutator. As electrical energy is passed through the brushes and consequently through the

armature a torsional force is generated as a reaction between the motor's fie ld and the armaturecausing the motor's armature to turn. As the armature turns, the brushes switch to adjacent bars on the

commutator. This switching action transfers the electrical energy to an adjacent winding on thearmature which in turn perpetuates the torsional motion of the armature.



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Gas underpressure

HydraulicFluid

Fig.16 A gas-charged accumulator

Cylinders

Cylinders are linear actuators, that is, they produce straight- line motion and/or force. Cylinders are

classified as single-or double-acting as illustrated in Figures.17 and 18 with the graphical symbol foreach type.

Single Acting Cylinder: It has only one fluid chamber and exerts force in only one direction.When mounted vertically, they often retract by the force of gravity on the load.

Load

Symbol

Load

From Pump To Tank

Extend Retract

Fig.17 Cross Sectional View of Single-acting Cylinder



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LoadLoad

ExhaustTo TankFrom Pump

From Pump

ExhaustTo Tank

Extend Cylinder Retract Cylinder

Fig.18 Cross Sectional View of Single-acting Cylinder

Double-Acting Cylinder:The double-acting cylinder is operated by hydraulic fluid in both directions and is capable of a power

stroke either way. In single rod double-acting cylinder there are unequal areas exposed to pressureduring the forward and return movements due to the cross-sectional area of the rod. The extending

stroke is slower, but capable of exerting a greater force than when the piston and rod are being

retracted.

Double-rod double-acting cylinders are used where it is advantageous to couple a load to each end, orwhere equal displacement is needed on each end. With identical areas on either side of the piston,

they can provide equal speeds and/or equal forces in either d irection. Any double-acting cylinder maybe used as a single-acting unit by draining the inactive end to tank.

In a single acting cylinder, oil only acts on one side of the piston so it can only be mechanica lly moved

in one direction. An external force (gravity, or sometimes a spring or another hydraulic cylinder)provides force in the opposite direction.

Single acting cylinders can also be of the "displacement" type where the oil pressure acts directly on

the end of the rod, and there is no piston. In this cylinder design the force is limited by the surface areaof the rod, whereas in a cylinder with a piston, the rod can be of any size and the force can be

calculated or controlled by the piston design.Typically one end of the tube is fixed and on the end ofthe rod is attached the object to be moved, although it is possible to fix the end of the rod, and attach

the object being moved to the end of the tube



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Troubleshooting the parts:

1. Electrical connections: The existing control panel consisted of a circuit disfigured beyondfixation. Several ICs were missing; several components were burnt (including buses, ICs, wires

etc), while several others have become obsolete. So we decided to make the electrical &electronic circuits afresh.

2. Locate the motor; find its type & specifications: The oil reservoir & the other parts of thehydraulic system were concealed inside a sealed compartment. The seal was broken, the pump

was taken out & the outer covering was removed. The control board gave an impression that the

motor driving unit was not connected to the rectifier unit & thus it was suspected that the pump is

run by an AC motor. We fabricated an ad-hoc protection circuit using fuses for testing.

Successive testing on AC power repeatedly caused the fuses to blow up. The testing was done till

6A fuse rating. It was then concluded to be a DC motor operated pump. It was then tested with

increasing levels of DC power & the minimum power requirement was found to be 12V, 1A. This

was supplied, again, by an ad-hoc DC source created by using a 900mhA DC adapter used for power supply to PC Speaker Systems, coupled with variable resistors for current control.

Ultimately, due to excessive current being drawn when trying to lift a 500gm load, which means

that the power is insufficient. Our current estimates stand at 15V, 5A.

3. Figure out the hydraulic circuit: Since several of the pipes connecting the hydraulic actuatorswere disconnected, the connections had to be totally removed & redone. The filter & the pressure

regulator unit were not working & were thus removed from the circuit. The actuators were sorted

in order of their estimated torque requirements based on the kinematics of the structure &

subsequently the decision whether to use a 3/2 or a 4/2 valve to operate it was taken.

4. Determine the operations envelop: Based upon the degrees of freedom of the structure & theranges of motion for each degree of freedom had been measured. This data was used to do the

operational envelop calculations. The operations envelop of a robotic manipulator is the region in

space it can traverse corresponding to all its degrees of freedom.

5. Determine the value configurations: The configurations of the valves driving the actuators werefigured out, essentially on trail & error basis. The unit, instead of using distinct 4/2 & 3/2 valves

as available in the market so easily today, was built using 4 & 3 units of direction control valves.

These are 2-state valves; that operate due to magnetic action, allowing the flow through them

when input to them is high & block the flow in absence of input.

6. Draw the operation table for valves for each actuator: The operation tables for each set, i.e.,4/2 & 3/2 were figured out using algebraic deduction. The most likely scheme was then tested

practically tested & verified for operation.

Future Scope:

1. Incorporate driver board circuits for giving power for driving the system.2. Introduce a suitable filter into the hydraulic circuit.3. Incorporate safety circuits to save future damage to the circuit4. Repair, lubricate & replace mechanical components at the joint level to reduce friction5. Devise a sensor-actuator configuration to implement a feedback mechanism6. Develop a program for microprocessor based control of the robotic arm to implement full

autonomous action.


Robotic Manipulator Arm

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