Robot Technology And Application Computer Science Essay

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A robot is generally described as a mechanical device, which resembles human form that is programmed to carry out a number of different repetitive operations. A robot is a machine designed to execute one or more tasks with speed and precision and can be used for a variety of applications within industry, such as pick and place, welding operations, machine tool loading and unloading, fabrication, spray painting and assembly. They can be used to replace humans and perform continuous operations in hazardous or unsafe working environments.

1: Main components of an industrial robot

Types and their applications

There are various different types and forms of robots which are suited to certain applications and environments determined by their work envelope. For each type of robot there are benefits and limitations which would establish their ability and efficiency to perform a particular task.

The main types of robots are:

SCARA (Selective Compliance Assembly Robot Arms)

SCARA robots are also known as horizontal articulated arm robots and in some cases are able to rotate about all three axes, although some have sliding motion along one axis in combination with rotation about another. They are greatly used throughout the electronics industry and their design is ideal for use in peg board type assembly. They are capable of performing high speed operations very accurately and are also used for machine loading, assembly and palletisation. These robots tend to be fairly small as can be seen below:

Jointed arm Robot

A Jointed Arm robot has three rotational axes connecting three rigid links and a base. This type of robot is frequently called an anthropomorphic arm as it closely resembles a human arm and forms a polar co-ordinate system. It's structure is very flexible as it can generally achieve any orientation or position within it's work envelope and has the ability to reach over any obstructions. However this can cause complications when driving the robot within its joint space as it's tricky to visualise the motion of the robot as each joint moves the minimum angle required from one point to another. This prevents the motion of the tool being able to move in a straight line. The jointed-arm robot is the most popular form for a robot and is capable in arc and spot welding, fettling, machine tending and painting work. Examples of this type of robot can be seen below:

Cartesian Co-ordinate Robot

This robot's three principle axes of control are linear and are at right angles to each other which means it can only move in straight lines with no rotary motion. Due to its high rigidity these are most often used with machine tools and co-ordinate measuring machines. They are not very flexible but are very easy to program and operate and are ideal for pick and place operations. However they are not suitable for damp or dusty environments as their linear joints are difficult to seal. The structure of this type of robot can be seen below

Cylindrical Co-ordinate Robot

These robots operate in a very similar way to the cartesian co-ordinate robots. They have three degrees of freedom, but it moves linearly only along the Y and Z axes. Its third degree of freedom is the rotation at its base around the two axes. The work envelope is in the shape of a cylinder. They are suited to tasks requiring straight line movements and have good rigidity. They are easy to program as their motion can be visualised quite simply and they are ideally suited to jobs such as machine tending. They are good for reaching into cavities but are unable to reach around objects and require a large amount of clearance behind. As with the cartesian co-ordinate robots they are unsuitable for damp or dusty environments as the linear joint is difficult to seal.

Polar co-ordinate robots

The polar coordinate robot also known as the spherical arm robot, was the first robot to be used in industry. It has one sliding motion and two rotational around the vertical post and around a shoulder joint. It's work envelope is a partial sphere which has various length radii which produces a very fast and accurate robot. Since electric robots have been introduced these robots have been replaced by jointed arm robots. They are powered by hydraulics and are capable of welding, assembling and gluing operations.

Tricept and Hexapod robots

These robots also known as parallel robots control their tool position using linear motors. The tricept robot achieves its orientation by using three legs in conjunction with a central pillar to hold its standard wrist mounting rigidly in position. The Hexapod robot achieves both orientation and position using its six legs. Although both of these structures provide very good rigidity they have the disadvantage of limited ability of orientation and small work envelopes. Robots of this type are generally used in machining operations where flexibility is a requirement and machine tool levels are not necessary.

End Effectors and manipulators

An end effector is a device or tool connected to the end of a robot arm. They can be defined in technological requirement terms as a mechanism to be used for handling operation and environmental parameters. The structure of an end effector, and the nature of the programming and hardware that drives it, depends on the intended task. When designing a robotic system to carry out a certain process the end effector is one of the most important things to consider. There are various different types of tools and grippers available whether it be for specialist jobs such as arc welding, nut running or spray painting or basic functions such as a pick and place system. However a robot arm can not accommodate any end-effector without changes to the ancillary hardware and/or programming. For example it is not possible to directly replace a gripper with a screwdriver head and expect a favorable result. It is necessary to change the programming of the robot controller and use a different set of end-effector motors to facilitate torque rather than gripping force.

Gripper Types

A general feature of a gripper is grasping and releasing in sequence of various different components. Finger grippers are the most frequently used gripper type, they are usually made up of two or three fingers opposite to each other, which are driven together so that the component is centred between them. This provides some flexibility when locating components at the pickup point, but is not really suitable for fragile components. In these circumstances vacuum or magnetic grippers could be introduced so that the object, when in contact, is held by an applied vacuum or magnetic field. However these methods are not as accurate and would not be used for high accuracy applications as any error at the pickup point would reflect at the drop off point. Mandrel or plug grippers are the simplest form, they grip components on their inner surface, but again these are not suitable for fragile objects. Most grippers on the market are pneumatic because they are easy to apply and have no motors or gears; this means it is simple to translate the power of a piston/cylinder system into gripping force. Most manufacturing facilities already have compressed air, so little effort is required to bring it to a gripper in a cost efficient manner.

There are also a range of accessories available to help make a gripper more suitable to its application. These are used for mounting the gripper to achieve motion which can not be easily provided by the robot; linear and rotary units are produced in numerous different configurations. Linear units are used for applications where the robot is required to reach into confined spaces; whereas rotary units are more suited to operations which require more than one gripper mounted which can be switched from one to another.

Tool changers are also available for operations which require tools that can not be assembled together. A number of tool services, such as compressed air and electricity can be fed to the tools through these, so that individual supplies do not have to be connected to each tool. Unless entirely essential these would not be used as they had weight to the end effector and can increase cycle times.

Collision detection systems can also be attached between the robot flange and the end effector. They work in the same way as a probe on a co-ordinate measuring system, by using plates and electrical contacts which will cause a circuit to break with any motion. This circuit can then interrupt a robot's program or be wired to an emergency stop.

Surrounding environment

As end effectors often have to work in hostile surroundings, high temperatures, dust and solvents, thought has to go into what material will suit the application as well as the surrounding environment. For example rubber couldn't be used in certain conditions as it would become contaminated and start to crack and deteriate.

Safety Requirements

The increasing use of robot based automation within industry has led to a greater awareness of machine safety requirements and to the introduction of a range of related standards. When installing a robot cell there are a number of safety concerns that have to be considered regarding the safety of people working in the area as well as the plant. As modern robots can work at an extremely fast pace and make very little noise they can be incredibly dangerous to anyone working around them. They can also appear to be deactivated when they are still running through their program as pauses and decision making capabilities are often written into the program. Therefore the safest way to avoid accidents is to exclude people from the robot's work envelope. This can be implemented with the installation of an infer-red curtain, which will shut the robot down if the beams are broken; other methods would include safety fences and interlock switches on doors. The operator will not be exposed to danger as long as the fence and the interlocking system work correctly

However, all robots at some point require human intervention. Instances of human attended operation include robot set-up, teaching, process changeover and maintenance, and this often needs to be carried out by a worker within the fenced area. Although the operator has to be within the cell whilst the robot is running, during programming, the risk of injury can be greatly reduced by running the robot at a reduced rate and providing the operator with a deadman's handle on the teach pendant. All pendants and teaching controls must be provided with an enabling device equipped with an 'off-on-off' three-position enabling switch which can disable robot movement when either released or grasped tightly

To ensure the safety of the plant and equipment appropriate sensors can be included to ensure components are present and loaded correctly and that tooling is operating as it should. Collision sensors can also be installed to stop the robot arm if excessive pressure is applied to the tool.

2: Power units used in modern robotic operations

There are three forms of power supply available to robots, which are individually suited to different applications and environments.

Hydraulics

Hydraulics were used for the majority of the early robots. They could provide greater power than the electric robots of the time and were easier to control and far more powerful than the pneumatic robots. They are very rigid robots but tended to be fairly slow and often sprung leaks due to the high pressures involved in their operation. The basic components of a hydraulic system have been included beneath:

This type of system uses high pressure, driven by an electric pump, to push fluid (oil) though pipes to create movements in cylinders or actuators. Control is achieved using directional control valves which are usually powered by solenoids.

Pneumatics

Pneumatics are still used to power a number of modern robots, but more commonly their end effectors. They tend to be used more for simple pick and place type robots as they are fairly cheep, not that powerful and are difficult to control. They are suitable for tasks that don't require a great deal of accuracy as speed and positioning is difficult to control. These would also not be suitable for an application which required strength as hydraulics is far more powerful. However there benefits of using pneumatics, especially in certain industries like medical or food products. Benefits would include:

Air is clean:- As mentioned it is ideal for the food and medical industries as it is a clean method of powering robots.

Air is readily available:- This means that it is cheap.

No environmental restrictions:- Air can be vented straight into the atmosphere, so there is no cost of disposing of it.

Air can travel long distances:- It can be adopted into any part of industry without large alterations.

Easily combined with control systems:- It works simply with solenoid control valves and PLC programming.

The physical elements are inexpensive.

A basic example of the necessary components for a pneumatic robot system can be seen below:

Electric

Electric is the most commonly used power source, especially in the newer robots. There are three different types of drive used for robots:

Stepper motors

A stepper motor is a brushless, synchronous electric motor that can divide a full rotation into a large number of steps. When commutated electronically, the motor's position can be controlled precisely, without any feedback mechanism. Stepper motors are constant-power devices, as the motor speed increases, torque decreases. The torque curve may be extended by using current limiting drivers and increasing the driving voltage. Steppers display more vibration than other motor types, as the discrete step tends to snap the rotor from one position to another. This vibration can become very bad at some speeds and can cause the motor to lose torque. The effect can be mitigated by accelerating quickly through the problem speed range, physically dampening the system, or using a micro-stepping driver. Motors with greater number of phases also exhibit smoother operation than those with fewer

DC servo

In recent years the AC servo has taken over from the DC. As in the DC motor, a current is passed through the coil, generating a torque on the coil. Since the current is alternating, the motor will run smoothly only at the frequency of the sine wave. It is called a synchronous motor. More common is the induction motor where electric current is induced in the rotating coils rather than supplied to them directly. One of the drawbacks of this kind of AC motor is the high current which must flow through the rotating contacts. Sparking and heating at those contacts can waste energy and shorten the lifetime of the motor. In common AC motors the magnetic field is produced by an electromagnet powered by the same AC voltage as the motor coil. The coils which produce the magnetic field are sometimes referred to as the stator, while the coils and the solid core which rotates is called the armature. In an AC motor the magnetic field is sinusoidally varying, just as the current in the coil varies. It's workings can be seen below:

AC servo

In recent years the AC servo has taken over from the DC. As in the DC motor, a current is passed through the coil, generating a torque on the coil. Since the current is alternating, the motor will run smoothly only at the frequency of the sine wave. It is called a synchronous motor. More common is the induction motor where electric current is induced in the rotating coils rather than supplied to them directly. One of the drawbacks of this kind of AC motor is the high current which must flow through the rotating contacts. Sparking and heating at those contacts can waste energy and shorten the lifetime of the motor. In common AC motors the magnetic field is produced by an electromagnet powered by the same AC voltage as the motor coil. The coils which produce the magnetic field are sometimes referred to as the stator, while the coils and the solid core which rotates is called the armature. In an AC motor the magnetic field is sinusoidally varying, just as the current in the coil varies. It's workings can be seen below:

These provide high power output and are almost silent in operation. As they don not use brushes they are also more reliable than the DC servo.

Methods of Control

A control system will determine how efficient or how flexible an industrial robot will be. The most common kind of robot failure is not mechanical or electronic failure but failure of the software that controls the robot. A control system should provide the robot with a logical sequence to follow. So the system will give the robot the exact position values to which it wants the robot to follow and will continuously monitor the robots positions by receiving information from the encoders. Any difference between the actual and theoretical position values is amended by actuating variables which drive the robot. Almost all modern robots are controlled using either microprocessors or auxiliary computers which interface control associated elements and carry out all the computational functions. This means the sequencing, memory functions for online sensing, branching and integration of any other equipment is performed by the control system. Downtime can be significantly reduced with its self-diagnosis ability to troubleshoot. There are two main types of control system:

Continues Path

This is where an irregular path is programmed for the robot to follow. The desired path is represented, within the control system, as a number of positions in a very close proximity. This is then stored in the robot's memory so that this desired route can be followed during the robot's working cycle. This method of programming is suited to activities such as welding, applying adhesive or painting, where the robot can't just transfer from one place directly to the next.

Point to point

Unlike continuous path this method of programming does move from one point to another. Point to point programming, as the title suggests, records positions at which the robot has to perform a function. So co-ordinates are recorded at the position where the robot has to pick a component up, and the position where the robot has to release it. Commonly used in pick and place procedures, this type of programming is suited to systems which require large amounts of repeatability.

Maintenance requirements

Even before selecting robots for performing operations in an industrial environment maintenance issues must be considered. If a company supplies a robot they will usually supply their own drive system and control system. More often than not these systems are very different to each other, which means that specialist training has to be given to maintenance personnel to deal with different robots. Different manufacturers may supply different electrical connections, such as power supplies and safety circuits. These are not to much of a problem, however they will most likely supply different programming languages and operating software. So a number of different robots could use completely different software which can cause great difficulty to maintenance personnel who have to deal with them. To relieve some signalling problems a PLC can be included into a robot cell rather than within the controller. As long as PLCs were kept standard throughout the plant problems in machine communication could be easily dealt with by anyone of the maintenance team

Even though there have been attempts at producing a universal programming language, they have never been adopted as manufacturers continue to produce their own programs trying to get a technology lead over their competition

If a robot arm has had maintenance carried out or the arm has been replaced it is a requirement that that robot is reprogrammed. This is because of the difficulty involved in setting up the datums or zero points on the robots axes. Methods and tools are available for setting these zero points, but are complex and expensive and cannot be used on all robots. In this respect some robots are better than others and more recent robots are less likely to suffer with these problems.

3: Methods used to gain speed and accuracy

Forms of feedback

To send positional information back to the control system robots use encoders. An encoder is a feedback device which translates mechanical motion into electronic signals. There are two different types of encoders, rotary and linear. A rotary encoder will output digital pulses which correspond to incremental angular motion. For example, a 1000 line encoder will output 1000 digital pulses for every one mechanical revolution. They are basically made up of a metal sheet cut into a complex pattern which is affixed to an insulating disc, which is rigidly fixed to the shaft. A row of sliding clear and opaque contacts is fixed to a stationary object so that each contact wipes against the metal sheet at a different distance from the shaft. As the disc rotates with the shaft, some of the contacts touch metal, while others fall in the gaps where the metal has been cut out. The metal sheet is connected to a source of an electric current, and each contact is connected to a separate electrical sensor. The metal pattern is designed so that each possible position of the axle creates a unique binary code in which some of the contacts are connected to the current source, switched on, and others are not , switched off. The code can then be read by a controlling device, such as a microprocessor to determine the angular position of the shaft. A diagram of how a rotary encoder is built up can be seen below

Acceleration / Deceleration ramps

Although a robot is required to perform a task as fast as it possibly can, it also has to stay accurate. To achieve a balance between speed and accuracy acceleration and deceleration ramps are included into its cycle. An acceleration ramp allows a robot to reach its operating speed at a more gradual rate, rather than trying to reach full speed immediately. This allows greater control and prevents errors, such as dropping components, and puts less stress and strain on the robot. A deceleration ramp behaves the same way but is activated as the robot is coming to its destination. So before the robot stops the deceleration ramp slows it down to come to a gradual stop. It is particularly useful when there is a need to place components down gently, if they are fragile or positioning is crucial, i.e. no sudden stops.

Drive ratios

Using just gears alone is never accurate anough for robotics as there is far too much play in the gears and bearings. This is known as backlash and for this application it needs to be kept to a bare minimum. To reduce this robots can use 300-1 ratio gear boxes, so that the motor shaft rotates 300 times fot the output shaft of the gearbox to rotate once. This provides more accurate movements of the robot.

Another example of a drive ratio would be a robot joint which has a support carrying a rotary robot arm. An electric drive motor is mounted on the support and its drive shaft is coupled to the robot arm by harmonic drive-type reduction gears. The reduction gears include an elastically deformable, externally toothed ring whose elliptical shape is rotated by the drive shaft. The externally toothed ring itself is stationary and is firmly connected to the support by a flexible sleeve provided at its free end with a rigid mounting flange. The annular mounting face of the flange and the opposite mounting face on the support is each provided with radial teeth, preferably in the form of Hirth-type gears, which guarantee an accurate centering of the sleeve and safeguard the same against turning. Preferably, the flange is fastened to the end face of a steel bushing which is pressed into a bore in the support and secured against turning by knurled serrations. The robot arm is mounted for rotation in a pretensioned cross-roller bearing which provides a compact and bend resistant connection to the support.

Types of sensors and feedback

In order to perform their operations correctly robots use external control or feedback systems. They provide the robot with signals telling it what operation to perform and when to stop and perform a different operation. The most commonly used type of feedback systems are PLCs. Sensors are used to inform the PLC that a task has been achieved, so that the PLC can send another signal for the robot to perform a new task. Sensors are able to provide the PLC with information about the robots surrounding environment. They allow the robot to recognise when and where to pick up a component, or where to start welding etc… There are many different types of sensors available for various purposes; they can be divided into two categories contact and non contact, examples have been included below.

Presence or proximity sensors

There are various different types, inductive: These detect ferromagnetic metallic materials, they can sense up to 50mm. They comprise of an oscillator whose capacitors constitute the sensing face; an alternating magnetic field is generated in front of these windings. An example can be seen below:

Capacitive: These detect solids and liquids (any material) to a distance of 40mm. They comprise of an oscillator whose capacitors constitute the sensing face; when a conducting or insulating material is placed within its field, it modifies the coupling capacitance and starts the oscillator. Both of these sensors are suitable for use in wet or dirty areas and are immune to dust. A capacitive proximity sensor can be seen below:

Ultrasonic: These are non contact and detect both solid and liquid up to a range of 8m, but must be perpendicular. It is basically a large capacitor with an ultrasonic resonance which can detect and transmit.

Laser: These detect solids and are more precise for smaller objects of a distance of up to 8m and are immune to dust. Fibre optic: Detects solid objects, photoelectric principle but scaled down for reduced machinery size, with separate amplifiers, ranges up to 150mm.

Temperature sensors

Again there are a few different types. Thermostats: These are basic on and off temperature regulation in equipment and alarming for temperature limits. An example can be seen below:

Infra-red pyrometer: These are non contact, non-invasive temperature measurement and control for hygienic applications, one can be seen below:

Thermocouples: These are fairly inexpensive and available in a wide range for temperatures up to 1300 degrees, again an example can seen below:

Micro switches

Micro switch-piezo-electric sensors: They consist of thin stripes of quartz or lithium crystals which form wafers. They are used to record the amount of force in which the gripper is applying to an object. Micro switch: These consist of a lever roller, or probe, which will either break a circuit or energise one. They can be used for a number of operations with robots but they can be quite fragile. A micro switch has been included below:

Potential sources of error

One of the main sources of error, amongst other factors, within a robot cell is the age or wear and tear subjected to the robots sensory devices. Although some sensors are more suitable for the more hostile environments they still become damaged over time. There is no way of preventing this, however we can minimise errors occurring with regular maintenance. As well as sensors becoming dirty or damaged gearboxes can become worn and bearings can start to seize, which obviously effects the movements of the robot. If the robot is knocked out of position it will carry on performing its operation without knowing any different, which could lead to equipment being damaged. So regular maintenance is a way of monitoring and correcting faults to prevent errors from happening.

4: Devices used to increase the perceived intelligence of a robot

Advanced devices

The more advanced the equipment being used is determines how intelligent the robot is seen to be. So the quality and technology of the components used improves a robot's perceived intelligence. The more sensory devices a robot has the more feedback it's receiving and the more decisions the robot can make. The advances in technology are allowing robots to become more like humans all the time, without being able to think for themselves without programmed scenarios.

Difficulties with advanced devices

By increasing the capabilities and intelligence of a robot the memory also has to be enlarged to be able to store the programs which will operate it. The more hardware which is added to a robot the more expense. Although these advanced devices can create greater efficiency and ability there is more which can go wrong and fault finding is difficult and solutions expensive. Hopefully with new technologies problems shouldn't occur very often, but replacement parts are extremely expensive due to their high accuracy.

Cost analysis

Although this technology could have massive benefits for a company there are numerous factors which have to be considered to determine if it will actually benefit them financially. As these systems are so expensive cost analysis has to be undertaken between an advanced system and a human employee. An employee would include annual wage, tax, PPE, pension, and any health considerations, such as heating or extraction fans. A system would include actual cost, installation cost, training fees, maintenance and spares. Other considerations would include product quantities and any differences in productivity between the two. For example it would be pointless installing a system if the products weren't being mass produced as it would never pay for itself. Taking all this into consideration, the cost of employing staff for a year would have to be weighed up against how many years it would take the system to pay for itself. Anything less than five years would pretty much guarantee a profit increase as long as the company stayed successful in the market place.

5: Common methods of programming robots and peripherals

Task programming

There are three basic types of programming methods for industrial robots:

Teach method

Over 90% of industrial robots are programmed using this method. The robot is taught its positional data using a menu based system or a text editor. The robot is driven to its desired position using a teach pendant, the individual locations are then recorded, with names, into the robots program.

Robots can also be taught via a teach pendant; a handheld control and programming unit. The common features of such units are the ability to manually send the robot to a desired position, or "inch" or "jog" to adjust a position. They also have a means to change the speed since a low speed is usually required for careful positioning, or while test-running through a new or modified routine. A large emergency stop button is usually included as well. Typically once the robot has been programmed there is no more use for the teach pendant.

Lead through method

This was originally the main method for programming robots but it is only used for certain applications these days. The robot is programmed by physically moving the robot arm through the process in which you want it to perform. This method requires great skill and can be exceptionally difficult on larger robots. It is also difficult as any inaccuracies introduced to the program can not be edited out, which means the whole task has to be reprogrammed. Each joint position is recorded at fixed time intervals by the robot controller so that it can be played back. This method is used for applications such as paint spraying and welding and any applications which require smooth robot movement as appose to point to point movements.

Offline programming

This method works in a similar way to CNC where CAD/CAM programs are used to generate numerical code in which the machine will read and follow. The program structure is built up in a similar way to the teach method, but it also uses intelligent tools that allow the CAD data to generate sequences of location and process information. This method is not used by a great deal of companies at the moment, due to infancy, but its use is still increasing each year. However this method does have great benefits which include:

Reduced down time

Reduces product lead time

Programming is made easier by programming tools

The method assists the cell design and optimises the process

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