CHAPTER 2 INDUSTRIAL ROBOT TECHNOLOGY by NORHAFIZA BT SAMION PPD, UTHM DEK 3223 Automation System and Robotics
Sabda Nabi Muhammad SAW:
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Robot Reference Frames
Definition by Robotics Industries Association:
“ Industrial robot is an automatically controlled, reprogrammable, multipurpose manipulator programmable in three or more axes, which may be fixed in either place or mobile for use in industrial automation applications”
The first industrial robot was manufactured by Unimate and installed by General Motors in 1961.
Most applications of industrial robot are in welding, painting, and pick and place operations.
Welding and painting robots are very common used in automotive industry, which need accuracy, high-speed operation, and human safety.
Pick and place operations need a robot to pick up parts and place them elsewhere. This may include palletizing, placing, sorting and assembly parts or other similar routines.
Robot system is combination of multidiscipline area , which includes implementation of mechanics , electrical and electronic systems, software engineering, and numerous other fields of application interest.
Most of the industrial robots have six basic components : manipulator, end effector, actuators, sensors, controller, and teach pendant.
Manipulator is a main body for the robot and consists of the joints , links and other structural elements of the robot .
It is a collection of mechanical linkages (or link) connected by joints and included are gears, coupling devices , and so on.
Generally, joints of a manipulator fall into two classes:
Each of the joints of a robot defines a joint axis along which the particular link either rotates or slides (translates).
Every joint axis identifies a degree of freedom (DOF).
► No. of DOFs = No. of Joints.
Regardless of its mechanical configuration, the manipulator defined by the joint-link structure generally contains three main structural elements as human parts:
the end effector.
Most robots are mounted on stationary base on the floor and its connection to the first joint as called link 0. The output link of joint 1 is link 1, and so on.
Besides the mechanical components, most manipulators also contain the devices for producing the movement of the various mechanical members.
These devices are referred to as actuators and may be pneumatic, hydraulic, or electrical in nature.
They are either directly or indirectly, coupled to the various mechanical links or joints (axes) of the arm.
In the latter case, gears, belts, chains, harmonic drives , or lead screws can be used.
The interface between the last link and the end effector is called the tool mounting plate or tool flange.
End effector is the part that is connected to the last joint (hand) of a manipulator , which generally handles objects , makes connection to other machines, or performs the required tasks.
Robot manufacturers generally do not design or sell end effectors ; just supply a simple gripper.
This is the job of a company's engineers or outside consultants to design and install the end effector on the robot and to make it work for the given situation/task.
In most cases, either the action of the end effector is controlled by the robot's controller , or the controller communicates with the end effector's controlling device such as a PLC.
The end effectors can include a sensor to determine if a part is present.
The addition of a simple sensor can make a gripper a relatively intelligent device.
A simple gripper that has a sensor in it which tells if there is something between its jaws
This could be as simple as a light and phototransistor
If the robot is commanded to go and get a part, the manipulator will position the tool to the correct location and then check the gripper’s sensor before closing the gripper.
Actuators are used to move elements of the manipulators.
It must have enough power to accelerate and decelerate the links and to carry the loads , yet be light, economical, accurate, responsive, reliable, and easy to maintain.
Each actuator is driven by a controller.
Common types of actuators are electric motors (servomotors and stepper motors), pneumatic cylinders, and hydraulic cylinders.
Electric motor especially servomotors are the most commonly used.
Hydraulic systems were very popular for large robots in the past and still around in many places, but are not used in new robots as often any more.
Pneumatic cylinders are used in robots that have on-off type joints, as well as for insertion purposes.
Electric Motor Advantage Disadvantage
Good for all sizes of robots
Better control, good for high precision robots
Higher compliance than hydraulics
Reduction gears used to reduce inertia on the motor
Does not leak , good for clean room
Reliable , low maintenance
Can be spark free , good for explosive environments
Needs reduction gears , increased backlash, cost and weight
Motor needs braking device when not powered. Otherwise, the arm will fall.
Pneumatic Advantage Disadvantage
Many components are usually off-the-shelf.
No leaks or sparks.
Inexpensive and simple.
Low pressure compared to hydraulics.
Good for on-off applications and for pick and place.
Require air pressure, filter, etc.
Difficult to control their linear position.
Deform under load constantly.
Very low stiffness and inaccuracy response.
Lowest power to weight ratio.
Hydraulic Advantage Disadvantage
Good for large robots and heavy payload.
Highest power/weight ratio.
Stiff system, high accuracy, better response.
No reduction gear needed.
Can work in wide range of speeds without difficulty.
Can be left in position without any damage.
May leak and not fit for clean room applications.
Requires pump, reservoir, motor, hoses, etc.
Can be expensive, noisy and requires maintenance.
Viscosity of oil changes with temperature.
Very susceptible to dirt and other foreign material in oil.
High torque, high pressure, large inertia on the actuator.
Adding sensors to an industrial robot can increase the range of tasks the robot can perform.
It also decreases the mechanical tolerances required of both the robot and the robot’s environment.
Sensors can be divided into three categories:
internal sensors : tell a robot the position of its various joints and report other conditions such as fluid pressure and temperature.
external sensors : tell the robot what is happening outside.
interlocks : used to protect both humans and robots. They may possess features of both internal and external sensors.
If robots are to use sensors in the middle of the electrical noise of a factory, care must be used in transmitting information from them to the robot’s controller without interference.
If the sensor information is being shared with a production computer, the possibility of noise problems is increased.
Fiber optics or differential drivers may be needed to get the signals to the controller.
Many sensor devices give an analog signal, while the controller uses digital signals; therefore analog-to-digital and digital-to-analog converter circuits may be required.
The controller receives its data from the computer , controls the motions of the actuators, and coordinates the motions with the sensory feedback information.
Suppose that in order for the robot to pick up a part from a bin, it is necessary that its first joint be at 35°.
If the joint is not already at this magnitude, the controller will send a signal to the actuator (a current to an electric motor, air to a pneumatic cylinder, or a signal to a hydraulic servo valve), causing it to move.
It will then measure the change in the joint angle through the feedback sensor attached to the joint (a potentiometer, an encoder, etc.).
When the joint reaches the desired value, the signal is stopped.
The robot will move to various locations in performing its tasks.
These locations can be determined by a controller system whenever the robot’s working environment is defined.
However, these locations are usually taught to the robot controller and used by the operator to move the robot to desire locations.
Teach pendants sometimes can also be used to issue other commands to the robot or to teach a relatively simple program.
Quiz 1 (10 min)
Label Link & joint of figure below and determine how many DOF?
Each of these joints have a range over which it can be moved. The five joint types illustrated in the figure are:
Firman Allah SWT:
“ Barangsiapa yang bertakwa kepada Allah, akan diberi jalan keluar dari kesusahan, dan Allah akan memberi rezeki yang tidak disangka-sangka datangnya.”
Robot Reference Frames
Robots may be moved relative to different coordinate frames. There are 3 reference frame: world, joint, and tool.
Workspace or work envelope is an area which robot can reach .
The shape of the workspace for each robot is uniquely related to its characteristics of robot configuration, links and wrist joints.
The workspace may be found mathematically by writing equations that define the robot's links and joints and including their limitations, such as ranges of motions for each joint.
When a robot is being considered for a particular application, its workspace must be studied to ensure that the robot will be able to reach the desired points .
Typical Workspaces for Common Robot Configurations
Typical Workspaces for Cartesian Configuration
Typical Workspaces for Cylindrical Configuration
Typical Workspaces for Polar Configuration
Typical Workspaces for Jointed-Arm Configuration
Typical Workspaces for SCARA (Selective Compliance Assembly Robot Arm)
Robots may be programmed in a number of different modes, depending on the complexity of robot function.
4 common programming modes:
Physical Setup Mode
Continuous Walk Mode
Most industrial robots can be programmed in more than one mode.
Physical Setup Mode
In this mode, an operator sets up switches and hard stop that control the motion of the robot.
This mode is usually used along with other devices, such as Programmable Logic Controllers (PLC).
In this mode, the robot's joints are moved with a teach pendant.
When the desired location and orientation is achieved, the location is entered (taught) into the controller.
During playback, the controller will move the joints to the same locations and orientations.
This mode is usually point to point, where the motion between points is not specified or controlled.
Only the points that are taught are guaranteed to reach.
Continuous Walk Mode
In this mode, all robot joints are moved simultaneously, while the motion is continuously sampled and recorded by the controller .
During playback, the exact motion that was recorded is executed.
The motions are taught by an operator, through a model, either by physically moving the end effector, or by directing the robot arm and moving it through its workspace.
In this mode of programming the robot, a program is written off-line or on-line and is executed by the controller to control the motions .
The programming mode is the most sophisticated and versatile mode and can include sensory information, conditional statements (such as if...then statements), and branching.
However, it requires the knowledge of the operating system of the robot before any program is written.
The following characteristics are typically used to describe commercial industrial robots:
Payload is the weight a robot can carry and remain within its specifications .
If load capacity larger than its specified payload, it may become less accurate, may not follow its intended path accurately, or may have excessive deflections.
The payload of robots compared with their own weight is usually very small.
>> Fanuc Robotics LR Mate™ robot has a mechanical weight of 86 lbs
and a payload of 6.6 lbs.
Reach is the maximum distance a robot can reach within its work envelope .
Many points within the work envelope of the robot may be reached with any desired orientation (called dexterous).
However, for other points, close to the limit of robot's reach capability, orientation cannot be specified as desired (called non-dexterous point).
Reach is a function of the robot's joint lengths and its configuration.
Precision is defined as how accurately a specified point can be reached.
This is a function of the resolution of the actuators, as well as its feedback devices.
Most industrial robots can have precision of 0.001 inch or better.
Repeatability is how accurately the same position can be reached if the motion is repeated many times.
Suppose that a robot is driven to the same point 100 times . Since many factors may affect the accuracy of the position, the robot may not reach the same point every time, but will be within a certain radius from the desired point.
The radius of a circle that is formed by this repeated motion is called repeatability. Most industrial robots have repeatability in the 0.001 inch range.
Repeatability is more important than precision. If a robot is not precise, it will generally show a consistent error, which can be predicted and thus corrected through programming.
As an example, suppose that a robot is consistently of 0.05 inch to the right. In that case, all desired points can be specified at 0.05 inch to the left, and thus the error can be eliminated. However, if the error is random, it cannot be predicted and thus cannot be eliminated.
Safety is the method to avoid accidents .
Industrial robot must follow the Isaac Asimov’s First Law of Robotics : “a robot shall not harm a human being.”
The cause of human injury in a robotic environment vary and include the following:
Parts of the body being caught
Being struck by a part or robot gripper.
Falling from equipment or structure
Slipping or tripping on walking or working surfaces
Exposure to dangerous levels of heat or electricity
Excessive physical strain
Safety monitoring involves the use of sensors to indicate conditions or events that are unsafe or potentially unsafe.
The objectives of safety monitoring include not only the protection of humans who happen to be in the cell, but also the protection of the equipment in the cell.
The sensors used in safety monitoring range from simple limit switches to sophisticated vision systems that are able to scan the workplace for intruders and other deviations from normal operating conditions.
Great care must be taken in workcell design to predict all possible accidents that might occur during the operation of the cell , and to design safeguards to prevent or limit the damage.
The National Bureau of Standards defines three levels of safety sensor systems in robots:
to detect that an intruder has crossed the workcell perimeter without regard to the location of the robot.
to detect the presence of an intruder in the region between the workcell perimeter and the limit of the robot workspace.
provide intruder detection inside the workspace of the robot.
Robot Safety Three Levels of Robot Safety Sensor System
There are two common sensors for robot safety system are pressure sensitive floor mats and light curtains.
Pressure sensitive mats are area pads placed on the floor around the workcell that sense the weight of someone standing on the mat. It can be used for Level 1 or 2 system.
Light curtains consist of light beams and photosensitive devices placed around the workcell that sense the presence of an intruder by an interruption of the light beam. It can be used for Level 1 system.
For Level 3 safety system, the Proximity sensors could be used and located on the robot arm.
The following guidelines are for safe use of robots in a production environment which given by K. M. Blache in Industrial Practices for Robotic Safety (1991) :
If the robot is not moving , DO NOT assumes it is not going to move.
If the robot is repeating a pattern , DO NOT assumes it will continue.
Always be aware of where you are in relationship to the possible positions that the robot may reach.
Be aware if there is power to the actuators. Indicator lights will be on when there is power to the actuators.
Limit switches or software programming will not be used as the primary safeguard.
Teaching, programming, servicing, and maintenance are the only authorized reasons for entry into the work envelope.
Safeguards with at least one level of redundancy will be present when employees are required to enter the work envelope.
Never climb on, over, or under a barrier for any reason .
Before activating power to the robot , employees should be aware of what it is programmed to do , that all safeguards are in place, and that no foreign materials are present within the work envelope.
Eliminate any source of stored energy prior to entry into the work envelope.
Notify supervision immediately when any unexpected interruption to the normal robot work cycle occurs.
Servicing should never occur within the work envelope with power on to the robot.
Report any missing or defective safeguard to supervision immediately. Check all safeguards at the beginning of each shift .