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This type of robot is very versatile and it closely resembles that of a human arm, it has a shoulder motion, an elbow motion as well as wrist and hand movements as shown in the image to the right. It has the ability to generally achieve any position and orientation within its working area in 6 or more different ways, although this is beneficial it can lead to causing control problems. When programming this type of robot it is hard to visualise its motion as it moves all its joints using the easiest route (straightest line possible) due to this it is a difficult robot to programme.
This robot is ideal for applications which require a larger level of manoeuvrability or an application which needs a high level of movement with regards to axis movement.
Selective Compliance Assembly Robot Arms (SCARA):
This type of robot was initially designed to be used for the specific application of peg board assembly, its more rigid working envelope and its fewer axis movements allow it to work with less chance of error, it is used mainly within the electronics industry. This robot can operate at very accurate and high speeds, they are usually small in size and are also used for machine loading, the image to the left shows the SCARA robot and the axis movements in which it can participate.
Taken from Wikipedia.com:
"SCARA's are generally faster and cleaner than comparable Cartesian systems. Their single pedestal mount requires a small footprint and provides an easy, unhindered form of mounting. On the other hand, SCARA's can be more expensive than comparable Cartesian systems and the controlling software requires inverse kinematics for linear interpolated moves. This software typically comes with the SCARA though and is usually transparent to the end-user."
Tricept and Hexapod Robots:
This style of robot uses linear motors to control the desirable condition, it has very precise and accurate movements but works within a very small working area and the orientation ability is limited. The head of the tricept robot is held rigidly by 3 legs in conjunction with a central pillar, a wrist is then mounted onto the head to achieve the required orientation.
Taken From Autokinematics.com:
"Tricept and hexapod robots use linear motors to control the position of the tool. The tricept uses three of these legs in conjunction with a central pillar to hold the head rigidly in position and then has a standard wrist mounted on it to achieve the orientation. A hexapod uses six legs and achieves both position and orientation using them. Both of these structures give very rigid robots but both have the disadvantage of small working envelopes and limited orientation ability. These structures tend to be used for machining operations where machine tool level tolerances are not required but greater flexibility is."
An image of a tricept robot is shown to the right:
Cylindrical Co-ordinate Robot:
The cylindrical co-ordinate robot is generally compared to the Cartesian robot, this is probably due to the fact that they both have good rigidity and are good for jobs which involve straight line movements. The image below shows an interpretation of movements in which this robot can do and upon which axis:
Some of the common applications for the cylindrical co-ordinate robot are; handling at machine tools, spot welding, and handling at diecasting machines. They are also used for assembly operations.
"as their motion is easy to visualise and they are good for reaching into cavities which makes them ideal for machine tending applications. The disadvantage of these robots is their inability to reach around objects and the amount of clearance required behind the robot. The linear joint makes them unsuitable for working in dusty or damp environments as it is difficult to seal just as it is with the Cartesian robot"
This style of robot is commonly used within the PAG head/block line I am currently working on at Bridgend Ford, it is known for its very high rigidity and so it is often seen in machine tools and co-ordinate measuring machines. Within the line I am working on these Cartesian robots (also known as gantry robots) are widely used for pick and place activities, for example; taking a head or block from one operation to another. The reasoning for these robots being used so widely in Ford is because of its high accuracy and repeatability, the programming aspect of this robot is also generally pretty basic. These robots have difficulties when working in damp and dusty conditions, this is because there linear joints are hard to seal to prevent any grit or dirt from getting in. An image of a Cartesian robot is shown below with an illustration of which axis it can move in.
Polar Co-ordinate Robot:
This type of robot isn't commonly found within industry in our modern day, this is because it has been replaced by the jointed arm robot which is better in many ways. Although not so common anymore, the polar co-ordinate robot was one of the first to be used in any type of industrial activity, the main reason for this is because it has hydraulic drives, which at the time were the main type of power drive. Electrically driven drives are found more often these days, but the polar co-ordinate robot is still used for simple tasks such as spot welding.
The image below shows a polar robot and the axis in which it can move in:
Taken from robots.com:
"Polar - Also called spherical robots, in this configuration the arm is connected to the base with a twisting joint and a combination of two rotary joints and one linear joint. The axes form a polar coordinate system and create a spherical-shaped work envelope."
Principles of Operation:
A robot working envelope is its range of movement, this range is determined by the size of each of the robots arms and how the axis are constructed. An image of a jointed arm robot's working envelope is shown below:
A robot can only perform its operations within the space of this working envelope, some robots are designed to have larger working envelopes for differing applications and certain types also have the ability to reach behind themselves.
Gantry or Cartesian robots, don't tend to follow the regular pattern of working envelopes due to the track systems in which they work on to create a bigger working area.
A statement taken from robots.com giving a description of a working envelope:
"It is the shape created when a manipulator reaches forward, backward, up and down. These distances are determined by the length of a robot's arm and the design of its axes. Each axis contributes its own range of motion."
Basic Mechanical Principles:
A robot is basically made up of the following structure:
A moveable base with an arm or other type of end effectors which will be capable of interacting with its environment.
Various type of sensor is then used to sense the environment and provide feedback to the device allowing a movement or output/input to be processed.
Systems to process the input from the environment and to instruct the device to perform actions in response to the situation.
Robots are built up with mainly mechanical components and its function is commonly compared to that of the human body.
The robot structure consists basically of the robot body that includes arms and wheels. A force such as an electricity supply is required to be used in conjunction with this to make the arms and wheels turn under command.
The robots structure will affect the size of the working envelope in which the robot can move in, if the robot has a larger working envelope with many axis movements it generally means there will be a decrease in accuracy, wasted power and longer cycle times.
On the other hand, robots with a smaller working envelope and a smaller mechanical structure with less degrees of movement will generally be more accurate and with cycle times being completed a lot quicker.
The diagram below shows the working axis' in which the Cartesian robots at Bridgend Ford work in:
As you can see it works on numerous different axis' from looking at the machine from the systems control panel the axis do as follows:
Y axis - This is the travel which is along the vertical plane (up and down motion)
X axis - This is the travel which is along the horizontal plane (going in and out to and away from you)
Z axis - This axis is the spindle moving in and out
W axis - This is the travel of the part which is going to be cut, it runs along the same plane as the z axis
S axis - This is the rotation of the spindle
Axis C2 - This is the rotation of the tooling magazine
B axis - This is the rotation of the part which is going to be cut, in our case either a engine head or engine block
End Effectors, Grippers & Manipulators:
An end effector is a tool which is on the end of a robot arm performing an operation, some commonly used end effectors are mentioned below:
By changing these end effectors it enables us to completely change the operation without changing the robot. Below is a quote taken from ask.com giving a brief description of what an end effector is:
"In robotics, an end effector is the device at the end of a robotic arm, designed to interact with the environment. The exact nature of this device depends on the application of the robot."
Within Bridgend Ford, grippers are commonly used to transport engine heads and engine blocks from one operation to another in conjunction with our gantry robots. When choosing these grippers there are a few factors which have to be considered such as:
Weight of the object to be lifted.
Size or shape of the object.
Speed of movement.
Taken from festco.com about selecting a desired gripper:
"Once these factors have been considered we can then decide what grippers to use, the most commonly used grippers are the finger grippers these will generally have two opposing fingers or three fingers like a three finger lathe chuck. The fingers are driven together such that once gripped any part is centred in the gripper. This gives some flexibility to the location of components at the pick-up point"
An image of a servo motor gripper is shown below:
Surrounding environment and safety requirements:
When working with anything within industry safety is also a massive consideration, it is the main priority to ensure that all the tasks are done as safely and efficiently as possible. Within Bridgend Ford before starting a job a risk assessment will take place, this risk assessment will analyse the safety of the job and to find a suitable and safe way of completing the task in hand. A risk assessment will look similar to the one created below:
1 -Extremely Low Chance
2 - Low Chance
3 - Satisfactory
4 - Slightly Dangerous
5 - Could Cause Fatality
No Emergency Stop
No Light Beam
No Safe Isolation/Lock off point
Working at Height
When working with robotics, it can be very dangerous as they can be very silent and can move at a fast pace with a larger amount of force, this is why we employ safety rules such as risk assessments, to do our best to avoid any accidents from occurring.
Surrounding environments and safety requirements:
Safety is always a massive concern when working in industry and when using robotics this is no exception, robots can be extremely dangerous due to the fact that they can move with virtually no noise and in a very fast and powerful manner. Accidents do occur with robots such as operators turning off isolation switches when maintenance is being carried out or robots even acting unexpectedly due to sensors being triggered or when it is out of sequence. The quote below is taken from the website manufacturingtalk.com:
``When going within reach of a robot whilst it is in operation is not a good idea, but at times it is required when servicing, tool changing or faultfinding. Robot Operators may also work in close proximity to a robot at times, for example while loading or unloading also some feed devices and work pieces. The measures taken to safeguard a robot will entirely depend on the circumstances of its operation and surrounding environment. However any safeguarding put in place will be done so with the intention of keeping people at a safe distance from the robot while it is operating and ensuring the robot and equipment is in a safe state in the instances where access is required ``
Some of the main safety features in which we have employed at Bridgend Ford and are used widely within industry world wide are mentioned below:
Safety Surrounding fencing - This is the fencing in which goes around the machinery so access can not be obtained easily, a request form to the foreman has to be signed off for removal of the fencing when a job needs to take place, only then can the fencing panels be removed. If regular access is required then a gate can be on there which will work along side an interlocking system.
Interlocking systems - these are the locking systems in which are placed on the gates or on some guards which required regular access to, operators cant gain access through these gates but fitters can and authorisation is not required in the form of a form because usually the gates with the interlocking systems are put up where common problems occur.
Light Curtains - These are sensors which are put up usually by where access is required frequently (usually between a cycle) but moving parts are still in play, the light curtain will activate when movement is taking place and if interrupted will stop the cycle. It will then de-activate at the point in cycle when safe access can be obtained.
Lock off system - This system is introduced to prevent yourself and others from harm when working in a machine, the idea is that when you isolate your machine you can lock it off yourself to prevent anyone turning it back on before its meant to be done or in a worse case scenario; whilst someone is in there.
Pressure sensitive mats - These mats are put in place to avoid people stepping in a place which could cause them harm whilst a machine is in cycle. When pressure is applied to the surface an output signal will be triggered causing the required action to take place (e.g. machine stopping in cycle)
The statement that follows is taken from http://www.osh.net/articles/archive/osh_basics_2002_may24.htm
"Studies in Sweden and Japan indicate that many robot accidents do not occur under normal operating conditions but, instead during programming, program touch-up or refinement, maintenance, repair, testing, setup, or adjustment. During many of these operations the operator, programmer, or corrective maintenance worker may temporarily be within the robot's working envelope where unintended operations could result in injuries.
Typical accidents have included the following:
* A robot's arm functioned erratically during a programming sequence and struck the operator.
* A materials handling robot operator entered a robot's work envelope during operations and was pinned between the back end of the robot and a safety pole.
* A fellow employee accidentally tripped the power switch while a maintenance worker was servicing an assembly robot. The robot's arm struck the maintenance worker's hand.
Robotic safeguarding systems protect not only the operators but also engineers, programmers, maintenance personnel, and any others who work on or with robot systems. A combination of safeguarding methods may be used. Redundancy and backup systems are especially recommended, particularly if a robot or robot system is operating in hazardous conditions or handling hazardous materials.
The safeguarding devices employed should not themselves constitute or act as a hazard or curtail necessary vision or viewing by attending human operators."
Power units which are normally used in robot applications:
There are 3 main power units which are used within robotic applications, they are:
The electrically driven power units can be broken down into the further subheadings as follows:
Each of the above is described below, some in more detail than others:
Pneumatics are used widely within robotics for many applications, a brief overview of how pneumatics are employed follows:
There are many types of actuators used within robotics and industry, a few are mentioned below:
Used in pick and place activities
Used for repetitive actions
Not accurate, tend to creep along
Mechanical coupling type - this type of coupling offers a larger force capability, but is not totally leak free because it is the more simpler type of coupling and used for activities which aren't critical.
Magnetic coupling type - this is coupled magnetically which leads to less leaks being able to occur this also helps the length of the stroke to be longer, generally a better version but more expensive due to the design.
Air Chuck Cylinder:
This type of cylinder is used within gripping activities, more commonly used with positional control valves, this is a safety feature as if it was used with a spring return then when the machine is isolated the grippers would release and drop any part which it could be holding.
These special cylinders are commonly found on the ends of SCARA robots (selective compliance assembly robot arms) and are used for pick and place activities.
Types of control valves which operate these devices:
Positional control valve - this control valve is commonly used because if the electrics are shut off this valve will keep the part in the same place as opposed to returning to its home position or dropping a component., this as mentioned above is generally used with application such as the air chuck cylinder which would be used for picking and placing objects which would want to be dropped when the system was isolated.
Although this type of valve can still cause creeping, depending on the cylinder it is used with, there is a lot more control over it.
Spring return valve - this valve is used in smaller simpler activities as the position can not be control in the same way as a positional control valve can control it.
5/2 valves are used commonly with the rodless cylinders for pick and place activities, it gives them more control over which the action is going, the valves will be on either end of the cylinders track and both be permanently on, the way it will be moved is by dropping the pressure off one of the valves so that the other over powers it and pushes it in its required direction.
The 5/2 control valves is better than the 5/3 for controlling the position of the actuator as there is always a constant pressure being applied to either port of the cylinder, where as the 5/3 wont have a constant pressure either side it is just shut off. When using simple double acting cylinders with 5/2 valves they can generally creep along this is due to the surface area from the back of the rod is larger than the front due to the rod being there.
Some of the main manufacturers of specialised pneumatic equipment are mentioned below:
Provide an over view of the characteristics for pneumatically operated robots:
Simple enough to set up
Easy to work on
Can be strong when used as hydro pneumatics
Advantages and disadvantages:
Not used for heavy lifting, hydraulics is much better for this
Can't just use any air, needs to be compressed
It is adaptable and safe
Ability to control isn't as easy as hydraulics as air compresses.
The quote below is taken from http://en.wikipedia.org/wiki/Hydraulic_drive_system
"A hydraulic drive system is a drive or transmission system that uses pressurized hydraulic fluid to drive hydraulic machinery. The term hydrostatic refers to the transfer of energy from flow and pressure, not from the kinetic energy of the flow."
This power source is used widely when using robotics, it is used so widely as it has many advantages over pneumatics and electrical drives. Hydraulically driven devices are powerful, this is because the oil which runs through the system is a lot harder to compress than air, and when the oil is compressed it almost becomes a flexible solid and due to this a lot more control is obtained over the system.
Advantages of Hydraulics:
Disadvantages of Hydraulics:
Parts can be expensive
Oil leaks, can be messy
Expensive to install and maintain
An AC motor, is a motor which is driven by an alternating current. They are made up of two simple parts, an outside stationary stator or housing and an inside rotor. The outside stationary stator has coils supplied with alternating current which produces a rotating magnetic field whilst the inside rotor is attached to the output shaft and is given torque by the rotating magnetic field.
This type of motor is the most commonly used within industry, it has pretty much replaced the DC motor. These motors are so common because they give a higher output than a DC motor with the same size system, it also has the advantage that it doesn't have any brushes within itself, this helps it to run silently.
These are pictures of an AC industrial motor:
This type of motor is commonly found with Bridgend Ford Motor Company and are used in many applications such as driving gantry cranes or in many other robot applications, they have the advantage that as they are so cheap it doesn't cost much just to replace them when they break down.
A disadvantage of this type of motor is that it requires a lot of power for there initial start up, this generally doesn't affect us within our work place as the majority of these motors are running constantly.
These are hardwearing motors which generally run until the bearings fail.
A DC motor is a motor which runs off of a direct current,
There are a few different types of DC motor; Brushed, synchronous and brushless are more common ones.
Brushed DC motor:
This type of motor generates torque by using a commutator, stationary permanent magnets and rotating electrical magnets.
It works on the principle of Lorentz force; Lorentz force states that any current carrying conductor placed within an external magnetic field experiences a torque. This force is known as Lorentz force.
Advantages of DC drives are mentioned below:
Low initial cost
Simple control of motor speed
Produce larger torques
Disadvantages of DC drives:
Low life span for high intensity uses
Problems which commonly occur when using DC brushed motors are replacing brushes and springs which carry the DC or cleaning/ replacing the commutator.
The main difference between stepper motors and AC motors is that AC motors repel windings to cause a movement where as a stepper motor attracts teeth to cause its movement.
This type of motor is also brushless, it has multiple toothed electromagnets in an arrangement around the motor which surround a central, iron gear. These magnets are arranged so that each one will activate in turn and cause the central gear to move due to the attraction of the teeth, this is best shown in the diagram below:
Electromagnet 1 is activated causing the teeth of the central gear to a line with it.
Electromagnet 2 is then activated and 1 is de activated causing the central gear to move slightly clockwise to align the teeth with the electromagnet.
Electromagnet 3 is then activated and number 2 is deactivated causing the same clockwise action to occur
Electromagnet 4 is then activated number 3 will deactivate and the process continues.
Characteristics of a stepper motor:
Taken from http://en.wikipedia.org/wiki/Stepper_motor
"Stepper motors are constant power devices.
As motor speed increases, torque decreases. (most motors exhibit maximum torque when stationary, however the torque of a motor when stationary 'holding torque' defines the ability of the motor to maintain a desired position while under external load).
The torque curve may be extended by using current limiting drivers and increasing the driving voltage (sometimes referred to as a 'chopper' circuit, there are several off the shelf driver chips capable of doing this in a simple manner).
Steppers exhibit more vibration than other motor types, as the discrete step tends to snap the rotor from one position to another (called a detent). The vibration makes stepper motors noisier than DC motors.
This vibration can become very bad at some speeds and can cause the motor to lose torque or lose direction. This is because the rotor is being held in a magnetic field which behaves like a spring. On each step the rotor overshoots and bounces back and forth, "ringing" at its resonant frequency. If the stepping frequency matches the resonant frequency then the ringing increases and the motor comes out of synchronism, resulting in positional error or a change in direction. At worst there is a total loss of control and holding torque so the motor is easily overcome by the load and spins almost freely.
The effect can be mitigated by accelerating quickly through the problem speeds range, physically damping (frictional damping) the system, or using a micro-stepping driver.
Motors with a greater number of phases also exhibit smoother operation than those with fewer phases (this can also be achieved through the use of a micro stepping drive)"
Communication and methods to gain accuracy:
In robotics there are various different ways in which you can gain more accuracy, speed and control within a system. One of which is used is by having a feedback loop, a feedback loop is where something such as a sensor in a system will obtain information from the surrounds or cycle and this in turn will change the output. The best example of this would be a central heating system with a thermostat, the heating system will turn on and once the temperature meets the set requirements it will turn back off and keep the room(s) at that constant temperature, this way of feeding back information is known as a closed loop system.
Taken from http://en.wikipedia.org/wiki/Control_theory#Closed-loop_transfer_function:
"The concept of the feedback loop to control the dynamic behaviour of the system: this is negative feedback, because the sensed value is subtracted from the desired value to create the error signal which is amplified by the controller."
On the other hand is an open loop system, also known as a non-feedback controller, this type of system does not have anything within it feeding back information to change the outcome, it is just a set system.
Taken from http://en.wikipedia.org/wiki/Open-loop_controller:
"An open-loop controller, also called a non-feedback controller, is a type of controller which computes its input into a system using only the current state and its model of the system.
A characteristic of the open-loop controller is that it does not use feedback to determine if its output has achieved the desired goal of the input. This means that the system does not observe the output of the processes that it is controlling. Consequently, a true open-loop system can not engage in machine learning and also cannot correct any errors that it could make. It also may not compensate for disturbances in the system.
For example, an irrigation sprinkler system, programmed to turn on at set times could be an example of an open-loop system if it does not measure soil moisture as a form of feedback. Even if rain is pouring down on the lawn, the sprinkler system would activate on schedule, wasting water."
Another commonly used way of gaining more accuracy, speed and control over a robotic system is by utilising motor ratios along with their encoders.
A motor has 360 degrees of motion, when coupled with a robotic arm or joint it will then in turn give us 360 degrees of motion providing there is nothing obstructing it.
As a method of gaining more control encoders are attached to the back of the motor, this enables us to see where exactly, for example; the robotic arm will be positioned.
The way the encoder works is that it is a plastic disc which is generally on the back of a motor, notches also known as tracks, will be cut out of it and a light source is used to gain its position.
An image of an encoder is shown below:
Sensors are commonly used within robotics, these help us feedback information into the system and trigger next actions within cycles.
There are many different types of sensors available such as; temperature sensors and proximity sensors. Proximity sensors are generally split up into 3 main categories:
Inductive sensor - This type of sensor is activated by metal objects and only metal objects, this is so that if something or someone was to get in the way of the sensor then it won't activate it unless it is made of metal.
Capacitive sensor - This sensor is activated by either metal or non-metal objects, depending on the application decides which sensor is preferable for you.
Photoelectric sensor - This type of sensor is also triggered by any object, the way it works is by the object reflecting an infra-red beam back on the sensor to activate it. This is shown below.
BeforePhotoelectric sensors can work in different ways, the first way is shown above where the sensor sends out a beam then the object will reflect the beam back into the sensor, this is known as the optical reflective type. Images of before and after of this sensor are shown below.
Another type of sensor is the retro-reflective type. This is similar to the optical reflective type except it has a separate transmitter and receiver as opposed to it being on the same component. This is shown below:
AfterThe last type of Photo-electric sensor is the optical one (separate type) this is where as opposed to figure 2 and 4 where the receiver is on the same sensor, the receiver is a separate unit all together. This is shown below:
To the inexperienced eye a robot can be perceived as having some sort of intelligence, this is due to the highly advanced plc equipment which is available along with using sensors and switches to trigger actions.
The problem with robots is getting them to think for themselves, this requires a lot of programming, equipment and the cost can mount up to be quite high.
Feedback loops are used to give the robot some sort of self alteration abilities, although these are installed it still is hard to get a robot to see and think the way a human does.
Taken from http://www.livescience.com/technology/090318-robot-madness-future-robots.html
"As humans, we can detect where there's shadows, colors and objects," said Chad Jenkins, a robotics expert at Brown University. "That has proven extremely difficult for robots."
It is hard for a robot to fully connect to the outside world, as it doesn't have any feelings or senses, taken from the same site as above:
"A robot fetching a beer has to realize that it should go to the fridge, figure out where the handle is and how to open the fridge door, and distinguish between beer cans and soda cans. It should know not to crush the beer can in its grasp. Finally, it should know that handing a beer over isn't the same as dropping the can in someone's lap."
The idea of pressure sensitive fingers on a robot has been tried, this gives a robot some sort of feeling ability to judge how tightly or loosely it is holding something. But in my opinion I believe that a robot will never be as clever as the person who programmes it.
Programming is a vital part of robotics and there are many ways to programme robots along with there peripheral equipment. These vary from:
Having manual data be input
Using a teach pendant
The task programming method was one of the first types of programming around, it is also called the lead through method and its not as commonly found used today. This type of programming requires an operator to lead the robot through its movements and actions. This method of programming is commonly found used with the paint spraying of cars, this is because as it has an operator it makes it a lot easier to paint in those harder to reach areas. One problem with task programming is that it can be unreliable in the sense that it needs to be re-programmed for every type of car and then re-programmed again if you need to change it back.
A teach pendant is commonly used in conjunction with the Cartesian robots or gantry robots in which we have in Bridgend Ford. They can be used to control a robot and move it into a position where it is safe to work on. A good advantage of this method is that you are able to easily visualise where the robot needs to go as you can see it go through its motions. A disadvantage is that if there are obstacles in its way it then becomes difficult to visualise its route. Another advantage of this programming method is that changes can be easily and quickly made, however although this can be done you cannot test to see if they correct. You have to trust the changes which are made are accurate and correct hence why this type of programming requires a fair bit of training.
A teach pendant is shown in the image below: