UNIT – 1 INTRODUCTION TO ROBOTICS CONTENT Section Topic Part I: Introduction to Robotics Objective 1.1 Introduction 1.2 Definitions 1.3 Industrial robots 1.4 The Robotic Motions 1.4.1 Robot Joints 1.4.2 Joint notations for major axes 1.4.3 Wrist Movements 1.5 Robot classification 1.5.1 Classification Based on Drive Technologies 1.5.2 Classification Based on work envelop geometries 1.5.3 Classification Based on Motion Control Systems 1.6 Robot Reach/ Work Volume 1.7 Robot Specifications 1.7.1 Number of Axes 1.7.2 Load Carrying Capacity 1.7.3 Maximum Speed of Motion 1.7.4 Reach and Stroke 1.7.5 Tool Orientation 1.7.6 Precision, Accuracy and Repeatability of movement 1.7.7 Robot Compliance 1.7.8 Operating environment 1.8 Speed of Response and Stability of Robot 1.9 End Effectors 1.10Architecture of Robotic Systems 1.11 Robot Selection 1.12 Robot Applications 1.12.1 Loading/unloading parts to/from the machines 1.12.2 Welding 1.12.3 Spray Painting 1.12.4 Assembly Operations 1.12.5 Inspection
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UNIT - 1 PART I: INTRODUCTION TO ROBOTICS Objective In the first part of this unit we would be studying the basics associated with the industrial robots, like basic types, classifications, the methods to specify the robots, types of drive technologies used in robotic applications, types of end effectors, the factors which must be taken care of while selecting the robots for a particular application, and finally the various applications of the robots. After studying this part of unit-1, we would be aware of most of basics of the industrial robots. In the second part of unit-1, we have developed the platform for understanding the coordinate transformation. The concept of fundamental rotations has been discussed along with the basic translations. Then the concept of homogeneous coordinates has been elaborated with examples. Then we devised the methodology to understand the homogeneous coordinate transformations, and the composite homogeneous transformations. 1.1 Introduction Having its origin from Science fiction way back in late 18th century, the field of robotics, as we see in modern world, has undergone tremendous developments. It just started with some fiction stories, that a robot looks and acts like human beings, and this concept was presented in various forms by number of authors in various languages. Issac Asimov, a famous science fiction writer, starting from 1939, has contributed, a number of fiction stories about robots, and has been credited with familiarizing with the term robotics. He has suggested that a robot must perform any task, according to the following three principles, famously known as three laws of robotics by Asimov, as written below: (i) A robot must not harm human being or through inaction, allow a human being to be harmed. (ii) A robot must obey orders given by human except the situation that violates the first law. (iii) A robot must protect itself unless that violates the first two above stated laws. Robots today, specially the industrial robots are the specialized, highly automated mechanical manipulators which are controlled by sophisticated electronic control systems and computer systems and are the most important part of flexible / programmable automation. The robots can be programmed to do a variety of operations by just changing the predetermined set of instructions called program, through some compatible software. This is also termed as soft automation. 1.2 Definitions An industrial robot can be defined as a general purpose reprogrammable manipulator, which has certain anthropomorphic or human like characteristics, which can be used to move its various linkages through a specified controlled sequence of motion in order to perform the desired task. Robot can also be defined as a software controllable mechanical device that uses actuators and sensors to guide one or more end effectors through programmed motions in a workspace in order to manipulate physical objects. 1.3 Industrial Robots The industrial robots are quite different from what was projected in the earlier day’s science fiction stories.
The Robotics Industries Association (RIA) defines an industrial robot as “programmable, multi-functional manipulator designed to move materials, parts, tools, or special devices through variable programmed motions for the performance of a variety of tasks” An industrial robot, as would be imagined by an unfamiliar person, does not look alike human or behave like human beings as generally these are like a one armed machine, which most of the time operate from a fixed location on the shop floor. But there are definitely some robots that have been developed which have got certain human or animal like appearance and working capabilities to certain extent, like ASIMO, a humanoid robot developed by Honda. The advances in the control technology, linkages actuation mechanisms, the enhancement in the computer technology and its miniaturization, and the development of advanced machine vision systems has added a number of capabilities to the industrial robots which one would have just imagined a century before. Thus, an industrial robot consists of a number of rigid links connected by joints of different types, controlled and monitored by a computer, to get the desired work from it. 1.4 The Robotic Motions The industrial robots are designed to perform some desirable work, and which can be performed by enabling the manipulator to move the body, arm and wrist through a series of motions and thereby helping the end effectors of the robot to achieve the desirable position and orientation in the three dimensional space surrounding the base of the robot. 1.4.1 Robot Joints A robot joint permits relative movement between parts of a robot arm. The joints of a robot are designed to enable the robot to move its end-effector along a path from one position to another as desired. The end effector is mounted on a flange or some plate secured to the wrist. It is most of the times, is the tool to perform some operation or some gripper for pick and place operations. The robot movements are broadly classified into two main categories, namely (i) arm and body motions (ii) wrist motions. The individual joint motions associated with these two categories are also referred to as the degrees of freedom. The first three axes of the robot are referred to as the major axes, most of the time the position of the end-effector of the robot is determined by the position of the major axes. Similarly three more axes associated with the wrist, are called minor axes and are used to establish the orientation of the tool or the gripper at wrist. Thus a minimum of six axes are required to achieve any desirable position and orientation in the robot’s work volume or work envelop or workspace. The locus of the points in the three dimensional space that can be reached by the wrist by the various combinations of the movements of the robot joints from base up to wrist, is called the gross work envelop of the robot. The robot motions are accomplished by means of powered joints. The rigid members connected at the joints of the robot are called links. In the link-joint-link chain the link closest to the base is referred to as the input link. The output link is the one which moves with respect to the input link. There are basically two types of joints commonly used in industrial robots, which are:
prismatic or linear joints, which have sliding or linear (translational) motion along an axis and revolute, which exhibits the rotary motion about an axis
Figure 1.1 shows some examples of the physical arrangements of the linkages to have prismatic or the revolute motions in robotics. The prismatic joints are called so because the cross section of the joint is considered as a generalized prism. They permit links to move in a linear relationship and are denoted by L. In a prismatic joint (L), the links are generally parallel to one another. In some cases, adjoining links are perpendicular but one link slides at the end of the other link. The joint motion is defined by sliding or translational movements of the links. The orientation of the links remains the same after the joint movement, but the lengths of the links are altered. The Revolute joints permit only angular motion between links. Their various types are: (i) Rotational joint (R) (ii) Twisting joint (T) (iii) Revolving joint (V) A rotational joint (R) is identified by its motion, rotation about an axis perpendicular to the adjoining links. Here, the lengths of adjoining links do not change but the relative position of the links with respect to one another changes as the rotation takes place. A twisting joint (T) is also a rotational joint, where the rotation takes place about an axis that is parallel to both adjoining links. A revolving joint (V) is another rotational joint, where the rotation takes place about an axis that is parallel to one of the adjoining links. Usually, the links are aligned perpendicular to one another at this kind of joint. The rotation involves revolution of one link about another.
Figure 1.1 Types of joints: (a) linear joint; (b) rotational joint; (c) twisting joint; (d) revolving joint
Figure 1.2 and 1.3 shows some examples of how we can use the various types of joint will give us a particular robotics configuration. The various robotic configurations will be explained later in detail. 1.4.2 Joint notations for major axes These motions are accomplished by movements of individual joints of the robot arm. The joint movements are basically the same as relative motion of adjoining links. The following figures shows the various possible combinations of the joints one may have in the robot. The basic movements required for a desired motion of most industrial robots are:
1. Rotational movement: This enables the robot to place its arm in any direction on a horizontal plane. 2. Radial movement: This enables the robot to move its end-effector radially to reach distant points. 3. Vertical movement: This enables the robot to take its end-effector to different heights.
Figure 1.2 Different Robot Configurations: (a) LL robot; (b) RRR robot; (c) TL robot
Figure 1.3 Different Robot Configurations: (a) LRL robot; (b) RRL robot; (c) TRL robot; (d) LVL robot
Figure 1.4: Wrist Movements
1.4.3 Wrist Movements The robots wrist movements are designed to orient the end effector or tool attached to the wrist of the robot in the desirable orientation. The robots wrist is generally having three degrees of freedom associated with it. These three degrees of freedom at the wrist are called wrist roll, wrist pitch and wrist yaw, as illustrated in the figure 1.4. 1. Wrist roll: it involves the rotation of the wrist mechanism about the arm axis. Wrist roll is sometimes also referred to as wrist swivel. 2. Wrist pitch: if the wrist roll is in its center position, the wrist pitch is the up or down rotation of the wrist. This is also called wrist bend. 3. Wrist yaw: if the wrist roll is in center position of its range, wrist yaw is the right or the left rotation of the wrist. The wrist yaw and pitch definitions are specified with respect to the central position of the wrist roll, as the rotation of the wrist about the arm axis will change the orientation of the pitch and yaw movements. The robot would have a spherical wrist if the axes used to orient the tool intersect at a common point. 1.5 Robot Classification Robots may be classified, based on: • Drive Technologies • Physical Configuration • Control Systems 1.5.1 Classification based on drive technologies An important element of a robot is the drive system that supplies the power for the actuation of various linkages and joints of a robot and thus enabling the robot to move. The dynamic performance of a robot mainly depends on the type of power source. There are basically three types of power sources for robots, which are mentioned below: Electric drive Most of the industrial robots use electric drive system, in the form of either DC stepper motor drive (open loop control), or, DC servo motor drive (closed loop control). The main
advantages of using electric drive system are that this drive system gives better positioning accuracy and repeatability, and is suitable to keep cleaner environment around. But it gives slower movement compare to the hydraulic robots and the electric drive system is good for small and medium size robots only. Hydraulic drive In case when industrial robot is required to work at higher speeds and at substantial loads hydraulic drive robot are preferred. The main disadvantage of using hydraulic drive system is that it occupies large space area and there is a danger of oil leak to the shop floor. Pneumatic drive For both electrical and hydraulic drive robots most of the time make use of the pneumatic tools or end effectors. Pneumatic drives are used especially when the gripping action of the end effectors is simple open and close operation to pick light objects. But the pneumatic drive system is preferred for smaller robots as these are less expensive than electric or hydraulic robots and suitable for relatively less degrees of freedom design for simple pick and place application. 1.5.2 Classification based on work envelop geometries Based on the physical configuration or the combination of the revolute or prismatic joints for the three major axes, a particular geometry of the work envelop is achieved. The table below shows the some of the most common robot work envelops based on the major axes. Robot work envelop Cartesian Cylindrical
Spherical SCARA Articulated
Table 1.1: Robot work envelops based o major axes Axis 1 Axis 2 L T L L T R R
L L T V R R R
Axis 3 L L L L L L R
Thus based upon the combination of the various joints types for the major axes, the industrial robots can be classified into the following four types: 1. Cartesian configuration 2. Cylindrical configuration 3. Polar configuration 4. Joint-arm configuration Cartesian Configuration As shown in figure 1.4, the robots with Cartesian configuration consist of links connected by linear joints (L). Thus the resulting configuration is (LLL). Here the three joints corresponds to the notation for the moving the wrist up and down, in and out, and back and forth. Thus the work envelop/ work volume generated by this robot is a rectangular box. The example of Cartesian robots is the gantry robot and they are commonly used for pick and place work for heavy loads, assembly operations, handling machine tools, and arc welding operations.
Figure 1.4: Cartesian coordinate robot
The major advantages of Cartesian coordinate robot are: 1. Ability to do straight line insertions into furnaces. 2. Easy computation and programming. 3. Most rigid structure for given length. But Cartesian coordinate robots do have certain disadvantages also as listed below: 1. Requires large operating volume. 2. Exposed guiding surfaces require covering in corrosive or dusty environments 3. Can only manipulate the objects in front of it. 4. Axes of robot are hard to seal Cylindrical Configuration Changing the first prismatic joint of the Cartesian coordinate robot by revolute joint, to have TLL configuration we bet the cylindrical coordinate robot. The space in which this robot operates is cylindrical in shape, hence the name cylindrical configuration. As shown in figure 1.5, the first axis twists the robot wrist about a vertical base axis and second and third joints which are prismatic moves the wrist up and down along the vertical axis, and in and out in horizontal direction respectively. As there is always some minimum radial position up to which the third joint would be moving, the work envelop we get from this configuration is actually the volume between two concentric cylinders. In short a robot with cylindrical configuration has one revolute (T) joint at the base and two linear (L) joints succeeded to connect the other links. But in addition to this TLL configuration, we can also get the cylindrical robot configuration by LTL and LVL configurations as listed in table 1.1.
Figure 1.5: Cylindrical coordinate robot
The cylindrical coordinate robots are commonly used for handling at die-casting machines, assembly operations, handling machine tools, and spot welding operations. The cylindrical coordinate robots have the following major advantages over other robot configurations: 1. can reach all around itself 2. rotational axis easy to seal 3. relatively easy programming 4. rigid enough to handle heavy loads through large working space 5. good access into cavities and machine openings The main disadvantages of the cylindrical coordinate robots are: 1. can't reach above itself 2. linear axes is hard to seal 3. won’t reach around obstacles 4. exposed drives are difficult to cover from dust and liquids Polar (spherical) configuration Polar robots have a work envelop of spherical shape. Generally, the arm is connected to the base with a twisting (T) joint, and rotary (R) and linear (L) joints follow to give us a configuration TRL. Here the first joint swings the arm back and forth about a vertical base axis, the second revolute joint moves the arm up and down about the horizontal shoulder, as shown in figure 1.6.
Figure 1.6: Polar coordinate robot
Robots with the designation TRL are also called spherical robots. Those with the designation TRR are called articulated robots. An articulated robot more closely resembles the human arm. The Spherical/Polar Robots are commonly used for material handling at die casting or fettling machines, handling machine tools and for arc/spot welding etc. The following are the advantages of spherical coordinate robots: 1. Large working envelope. 2. Two rotary drives are easily sealed against liquids/dust. The disadvantages are: 1. Complex coordinates more difficult to visualize, control, and program. 2. Exposed linear drive. 3. Low accuracy.
Figure 1.7: SCARA (Selective Compliance Assembly Robot Arm)
Like a spherical coordinate robot, a SCARA robot (Selective Compliance Assembly Robot Arm) is a robot with at least two parallel rotary joints (R) and having one linear joint for the positioning of the wrist. But for a SCARA robot all three joint axes are vertical as shown in figure 1.7. The first rotary axis swings the arm back and forth and can be thought of as the vertical shoulder axis. The second revolute joint swings the forearm back and forth about the vertical elbow axis. The third joint which is linear joint, slides up and down, to give vertical motion to the robot wrist. Thus the two rotary joints are in the same horizontal plane. The SCARA robot is commonly used for pick and place work, and assembly operation with high working speeds.
Following are the main advantages of SCARA: 1. High speed. 2. Height axis is rigid. 3. Large work area for floor space. 4. Moderately easy to program. The main disadvantages of SCARA are: 1. Limited applications. 2. Two ways to reach a point. 3. Difficult to program off-line. 4. Highly complex arm. Jointed Arm Configuration The jointed-arm is a combination of cylindrical and articulated configurations. The arm of the robot is connected to the base with a twisting joint, as shown in figure 1.8. The links in the arm are connected by rotary joints. Many commercially available robots have this configuration.
Figure 1.8: Jointed Arm Robot
A jointed arm robotic configuration that is a robot with at least 3 rotary joints, are commonly used for assembly operations, welding, weld sealing, spray painting, and handling at die casting or fettling machines. The main advantages of a jointed arm robotic configuration are: 1. All rotary joints allows for maximum flexibility 2. All joints can be sealed from the environment. The main disadvantages are: 1. Extremely difficult to visualize, control, and program these robots. 2. Restricted volume coverage. 3. Low accuracy. 1.5.3 Classification based on motion control systems To have the proper motion of the various joints of the robot it must have some appropriate control system to regulate the drive system which provides the motion to the various robot joints. The control system used in the industrial robots can be classified into the following categories: 1. Limited sequence robots
2. Point-to-point (PTP) control robot 3. Continuous-path (CP) control robot 4. Controlled-path robot 5. Intelligent robots. Limited Sequence Robots The limited sequence robots do not make use of the feedback control to indicate the relative positions of the joints, and hence we get no feedback that the desired position has been achieved or not. In these robots the joint motions are controlled by the limit switches to detect the end point of travel of the respective joints. Thus in this method of robot control the individual joints can only be moved to their respective extreme limits of travel. This limits the number of distinct positions which can be specified for the robot in a program. The sequence in which the robot’s various joints will move to get the desired motion of the end effector is controlled by a sequencing device, which is called robot controller. The robot controller gives the input signals to each actuator to operate in a particular sequence. These types of robots generally make use of pneumatic drives and are used for rather simpler applications like pick and place operations. Point to Point Control Robot (PTP) The PTP robot is capable of moving from one point to another point. These point locations are taught to the robot and recorded in the memory of robot control unit. During operation cycles the robot controller commands the actuation of the various robot joints in a proper sequence so that the robot’s end effector moves from one point to another in a desired sequence. Point to point robots do not control the path followed while moving from one point to the next point. Thus if the programmer want the robot to follow a particular path while moving from one position to another, then he must specify a series of intermediate points along the desired path between the given points. Some common applications of the point to point robots include: component insertion, spot welding, hole drilling, machine loading and unloading, and assembly operations. Continuous-Path Control Robot (CP) The continuous-path control robot is capable of performing movements in which the path followed by the robot is controlled. For continuous-path control, the points along the path must be stored explicitly in the robot's control memory. Thus actually continuous-path control is accomplished by making the robotic manipulator move through the sequence of closely spaced points which describe the path to be followed by the robot. These individual points along the path are defined by the robot controller unit. Straight-line motion is the simplest example for this type of robot, where the task of the programmer is to just specify the start and the end point of the path, and the controller calculate the intermediate points along the path and allow the robot to move along the straight line path. Some continuous-path controlled robots also have the capability to follow a smooth curve path that has been defined by the programmer. In such cases the programmer manually moves the robot arm through the desired path and the controller unit stores a large number of individual point locations along the path in memory. Some typical applications of continuous-path control robot include: spray painting, gluing, and arc welding operations Controlled-Path Robot In controlled-path robots, the control equipment can generate paths of different geometry such as straight lines, circles, and interpolated curves with a high degree of accuracy. Good accuracy can be obtained at any point along the specified path. Only the start and finish
points and the path definition function must be stored in the robot's control memory. This controlled path control is accomplished by making the robot move through a series of closely spaced points which fall along the desired path to be followed by the robot. These intermediate points along the path are defined by the controller of the robot, not by the programmer. Straight line motion is the common type of controlled path robots, where in the task of the robot programmer is to specify the start and the end points of the path, and the robot’s control unit determines the sequence of the intermediate points along the straight line path. Similarly robots for certain industrial applications possess the capability to follow a smooth curved path that is defined by the robot programmer by manually moving the robotic arm through the desired complicated curved path. Here in such case the controller memory should be sufficiently large to store the large number of individual point locations that define the curved path with sufficient accuracy. It is important to mention that all controlled-path robots have a servo capability to correct their path. Intelligent robots Intelligent robots are the type of industrial robots that posses the capability to interact with the environment they are working in addition to play back the programmed motion cycles. These robots possess the capability to alter the programmed instructions by perceiving the change in the working environment through the data received from the various robotic sensors. These robots also possess the capacity to communicate with the humans or the computer based systems. These robots are programmed using the higher level (English like and symbolic) language so that the complex activities could be accomplished, like complex assembly operations etc.
Figure 1.9: Work Volume for Various Robot Configurations (a) Cartesian coordinate (b) Cylindrical coordinate and (c) Polar coordinate.
1.6 Robot Reach/ Work Volume Robot reach, also known as the work envelope or work volume, is the space of all points in the surrounding space that can be reached by the robot arm. Reach is one of the most important characteristics to be considered in selecting a suitable robot because the application space should not fall out of the selected robot's reach. The definition of the robots work volume is conventionally done based on the positions in the space surrounding the robot which can be reached by the robots wrist. The positions of the robots wrist are used for the purpose of defining the robots work volume mainly to avoid the complication of different sizes of the end effectors that may be attached to the robots wrist. The end effector is in addition to the basic robot and it should not be accounted as the part of the robot’s work volume.
The work volume of an industrial robot is determined by the following physical characteristics of the robot: (i) The robot’s physical configuration (ii) The sizes of the links for the body, arm and wrist of robot (iii) The limits of the robot’s joint movements. As shown in figure 1.9, the work volume for a Cartesian coordinate robot is a rectangulartype space, where as for a cylindrical configuration, and polar configuration the reach/ work volume of the manipulator is a hollow cylindrical space, and a part of a hollow spherical shape. Robot reach for a jointed-arm configuration does not have a specific shape. 1.7 Robot Specifications We broadly categorize the robots based upon the drive technologies, work volume geometries, and the motion control methodologies. But in addition to these three things, there are several additional characteristics that help us categories the industrial robots which are mentioned below: (i) Number of axes (ii) Load carrying capacity (kg) (iii) Maximum speed (mm/sec) (iv) Reach and stroke (mm) (v) Tool orientation (deg) (vi) Precision, accuracy and Repeatability of movement (mm) (vii) Robot compliance (viii) Operating environment 1.7.1 Number of Axes The industrial robots have got a number of axes about which its various links rotate or translate. As explained earlier the first three axes of the robot called major axes are used to establish the position of the wrist. The remaining axes of the robot are used to establish the orientation of the robots wrist, called minor axes. Thus a six axes robot is a general manipulator which can move its end effector to both an arbitrary location and an arbitrary orientation with in its work volume. Some industrial robots have more than six axes, where the additional axes after the six axes in a robot are termed as the redundant axes, which are generally used to avoid certain obstacle in the robots work volume. The mechanism to activate the robot tool (end effector), or the opening and closing of the robots gripper, is not considered as the independent robot axis, as this mechanism (axis) do not contribute to acquire either the position or the orientation of the end effector in robots working space. 1.7.2 Load Carrying Capacity The load carrying capacity of industrial robots is mainly determined by various factors such as robot’s size, configuration, type of drive system and the type of application for which the robot is designed. The load carrying capacity of modern robots has a very wide range, from few grams to several thousand of kilograms. The maximum load carrying capacity of the industrial robots arm should be specified for the condition that it is in its weakest position. The weakest position most of the time is the position when the robots arm is at maximum horizontal extension. This can be understood in a way that if compared with the human arm, it is most difficult to lift a heavy load when the arm is fully extended rather than a position when some load is to lifted which is placed close to the body. The specification of load carrying capacity provided by the most of the robotic manipulator manufacturers is actually the gross weight capacity that can be put at the robotic wrist. Thus
to make use of this specification the user must be aware of the weight of the end effector. For an example, if the gross load carrying capacity of a robot is 10.0 kg and it’s end effector weigh 3.0 kg, then the net load carrying capacity of the robot would be only 7.0 kg. 1.7.3 Maximum Speed of Motion The maximum tool tip speed of the robots also vary widely, from a few mm per second to a magnitude of several meters per second. The speed of the robot is measured at robot’s wrist. Thus the highest speeds can be achieved by the larger industrial manipulators with maximum horizontal extension of the arm away from the base of the robot. Also the type of the drive system of the robot affects the joint speeds, for example, the hydraulic robots are having faster joint motions than the electrical drive robots. A more meaningful measure of the robot speed is the cycle time, which is the time required to accomplish a given work cycle, consisting of several periodic motions of robot. As it is desirable for any production operation to minimize the cycle time of the given task, most of the robots have the provision to regulate or adjust the speed. The selection of the desirable speed of the robot depends upon the number of other factors as well, like: (i) The accuracy of positioning of the end effector or robot’s wrist (ii) The distance to be moved by the robot’s wrist (iii) The load at the wrist. There is an inverse relation between the accuracy of wrist positioning and the speed of the robot motions. As the desired accuracy is increased, the robot need more time to reduce the location errors in its various joints to achieve the final desirable position of end effector or the robot wrist. The load at the wrist also affects the operational speed of the robot as the heavier loads would generate greater inertia and momentum, which would restrict us to lower robot speeds for safer manipulator operations. The length of distance to be traveled by the robot wrist will also affect the operation speeds, as the acceleration and deceleration of the various joints take some time. Thus a longer distance will be traveled in relatively shorter time than a series of short distances. Thus the shorter distances will prevent the robot to achieve its maximum operating speeds. The maximum load carrying capacity and maximum operating speed of industrial robot have got a wide range, thus we must consider the aforesaid factors while selecting the robot for a particular application. In some cases the load carrying capacity required may be more, but accuracy may not be a constraint, thus we should select the appropriate configuration, as there is no use of paying for the characteristics which are not at all necessary for a particular required application where the robot is to be used. 1.7.4 Reach and Stroke Reach and stroke of the robot are the measure of the work volume of the robot. The horizontal reach is the maximum radial distance at which the robotic wrist can be positioned away from the vertical axis about which the robot rotates, or the base of the robot. The horizontal stroke is the total radial distance the wrist can move. There is always a certain minimum distance the robot’s wrist will remain away from the base axis. Thus, the horizontal stroke is always less than equal to the horizontal reach. For a cylindrical coordinate robot the horizontal reach is the outer cylinder of the workspace, while the horizontal stroke is the difference between the radii of the concentric outer cylinder and the inner cylinder, as shown in figure 1.10.
Figure 1.10: reach and stroke for a cylindrical robot
The vertical reach is the maximum vertical distance above the working surface that can be reached by the robot’s wrist. The vertical stroke is the total vertical distance that the wrist can move. Similar to the horizontal stroke, the vertical stroke is also always less than equal to the vertical reach. Figure 1.10 shows the concept of horizontal and vertical stroke, and maximum and minimum reach along horizontal and vertical direction. 1.7.5 Tool Orientation The three major axes of the robot determine the work volume, while remaining additional axes of the robot determine the orientation of the robot’s end effector. If three independent minor axes are present then the end effector of the robot will be able to achieve any arbitrary orientation in the three dimensional work volume of the robot. As defined earlier the position of the three axes associated with the wrist of the robot, namely yaw-pitch and roll, are used to define the orientation of end effector of robot. 1.7.6 Precision, Accuracy and Repeatability of movement The precision of movement for the robotic manipulator is basically a function of three features namely, special resolution, accuracy, and repeatability. These terms are defined for the robot's wrist end without any tool attached and for the conditions under which the robot's precision will be at its worst. Generally the robot has least precision of movement with the robot's arm is fully extended. For any robotics configuration, it is easier to define the various precision features in a static context rather than a dynamic context. (i) Spatial Resolution The spatial resolution of a robot can be defined as the smallest increment of movement into which the robot can divide its work volume. It depends on the system’s control resolution and the robot's mechanical inaccuracies. The control resolution for a robot is determined by the position control system and the feedback measurement system. It is the controller's ability to divide the total range of movement for the particular joint into individual increments that can be addressed in the
controller. The ability to divide the joint range into small special increments depends on the bit storage capacity in the memory of the control units. The number of separate, identifiable increments of movements for a particular axis is equal to 2n. Here n is the number of bits in the control memory. For example, a robot with 8 bits of storage can divide the range of a joint movement into 256 discrete positions. The control resolution would be defined as the total motion range divided by the number of increments. A robot with several degrees of freedom would have a control resolution for each joint of motion. Example 1.1 If we have a robot’s one particular joint’s motion range from Rmin to Rmax and the position control signal is generated by controller of the robot through a n bit digital to analog converter. Determine the precision of movement (control resolution) of the robot joint. Solution: The number of control increments obtainable in the specified range of joint movement = 2n. The total range of movement = (Rmax - Rmin). Thus, the control resolution or the special resolution for robot joint will be = (Rmax - Rmin)/ 2 n To determine the control resolution for the entire robot, component resolutions for each joint would have to be summed vectorially. Since some of the joints are likely to be rotary while others are sliding, the robot's control resolution can be a complicated quantity to determine. Mechanical inaccuracies which arises from imperfections in the mechanical system in the robot, is another factor that degrades the spatial resolution. The inaccuracies would also be influenced by such factors as the load being handled, the speed with which the arm is moving.
Figure 1.11: Accuracy and control resolution of a robot
(ii) Accuracy Accuracy can be defined as the ability of a robot to position its wrist end at a desired target point within its reach. In terms of control resolution, the accuracy can be defined as one-half of the control resolution. This definition of accuracy applies in the worst case when the target point is between two control points. The reason is that displacements smaller than one basic
control resolution unit (BCRU) can be neither programmed nor measured and, on average, they account for one-half BCRU. The accuracy of a robot is affected by many factors. The accuracy of robot varies within the work volume. It is worse when the arm is in the outer range of its work volume and better when the arm is closer to its base. The reason for this is that when the arm is fully stretched out, the mechanical inaccuracies tend to be larger. Another factor influencing accuracy is the load being carried by the robot. Heavier workloads cause greater deflection of the mechanical links of the robot, resulting in lower accuracy. This relationship is il1ustrated in figure 1.11. In fact, the mechanical inaccuracies would affect the ability to reach the target position. (iii) Repeatability Repeatability is the measure of the ability of the robot to position the tool tip at same position repeatedly. There is always some repeatability error associated because of backlash in gears, flexibility of the mechanical linkages and drive systems. The repeatability errors are generally very small in magnitude for well designed robotic manipulators. Repeatability and accuracy refer to two different aspects. Accuracy is an absolute concept, repeatability is relative. Accuracy relates to the robot's capacity to be programmed to achieve a given target point. The actual programmed point will probably be different from the target point due to limitations of control resolution. Repeatability refers to the robot's ability to return to the programmed point when commanded to do so. A robot that is repeatable may not be very accurate, and visa versa. The repeatability could be better understood from figure 1.12. Let T be the desired target point to where the robot is commanded to move, but because of the limitations on its accuracy, the programmed position becomes point P. The distance between points T and P is robot's accuracy. When, the robot wrist is commanded to the programmed point P, however, it does not return to the exact same position. Instead, it returns to position R. The difference between P and R is a result of limitations on the robot's repeatability. The robot will not always return to the same position R on subsequent repetitions of the motion cycle. Instead, it will form a cluster of points on both sides of the position P in figure 1.12.
Figure 1.12: Repeatability and Accuracy
Repeatability errors form a random variable and constitute a statistical distribution as shown in the figure 1.12. It would be convenient if the repeatability errors formed a nice bell-shaped curve, suggesting a normally distributed random variable. Mechanical inaccuracies of the various joints of the robot which are basically responsible for the repeatability errors do not have the nice symmetric bell-shaped distribution as shown in the figure 1.12. However, when the errors from several axes of motion are combined together, the resulting aggregate error is influenced by the central limit theorem in probability. This theorem states that the sums of random variables tend to form a normally distributed variable, even though the individuals come from a distribution other than the normal. Thus we can say that the repeatability error of a multi-jointed robot will follow the normal distribution. In three-dimensional space, the repeatability errors will surround the programmed point P, forming a distribution whose outer boundary can be conceptualized as a sphere. A robot manufacturer typically quotes the repeatability of its manipulator as the radius of an idealized sphere, usually expressing the specification as plus or minus a particular value. The size of the sphere will tend to be larger in the regions of the work volume that are further away from the center of the robot. It is likely that the shape of the sphere is not perfectly round, but instead is oblong in certain directions due to compliance of the robot arm. 1.7.7 Robot Compliance The compliance of the robotic arm refers to the displacement of the wrist end in response to a force or torque exerted against it. Higher compliance will tend to displace the robotic wrist by a large amount with a relatively small force, where as a low compliance robotic arm would be relatively stiff and will not be displaced by a significant amount. Robot manipulator compliance is a directional feature, which means that the compliance of the robot arm will be greater in certain directions than in other directions because of the mechanical construction of the arm. Compliance is important aspect to be considered for robot design because it reduces the robot's precision of movement under load. If the robot is handling a heavy load, the weight of the load will cause the robot arm to deflect and the robot's performance will be degraded because of compliance when it operates under loaded conditions. 1.7.8 Operating Environment The uses of the robots are justified for the jobs which are repeated in nature, or to get the work in some harsh or dangerous environment by the robot. For example for transportation of radioactive materials, spray painting, welding, and the loading and unloading of furnaces. For each of these applications the robot must be specifically designed to operate in extreme working conditions. The various robot joints must be protected from the exposure to dust and foreign contaminating materials, so that the robot may be able to perform its stipulated task with desired accuracy and precision. 1.8 Speed of Response and Stability of Robot The characteristic which determine the dynamic performance of the robot are speed of response and stability. These characteristics are the inherent features of the control systems of robot. The speed of response corresponds to the ability of robotic manipulator to move its wrist to the next desirable position in its work volume in a short span of time and it is directly related to the motion speeds of various joint actuators of robot. The stability of robot manipulation is a parameter to determine the amount of oscillations occurring in the robotic arm when it moves from one position to another. Robot with higher
stability will have little or no oscillations, either during start of operation, during intermediate joint motions or at the end of the arm movement. Where as poor stability would be indicated by a large amount of oscillation. The most desirable aspect about the design of the robot control system is to design the controller which can provide good stability and a fast response time. But these two characteristics are just opposite to each other, as if we increase the stability the response time increases, and vice versa. The stability of a robot can be improved by using damping elements into the robot's design and this will reduce its tendency toward oscillation. But this will not be desirable as use of high damping reduces the speed of response. Thus one must find an optimum balance between the two conflicting parameters.
Figure 1.13 Speed of response and damping in robotics: (a) low damping and fast response, (b) high damping and slow response
A good robot is one that is fast enough but at the same time has good stability. The figure 1.13 below shows the concept of stability and its relation to damping. Figure 1.13(a) shows the position of the robot's wrist as a function of time for small damping; where as figure 1.13(b) shows the position of the robot's wrist as a function of time for large damping. With low damping, the robot arm moves to the target position rapidly, but experiences large amount of oscillation about the final position. And in the second case, with a large amount of damping, the arm movement becomes is very slow but will not have oscillatory motion about the final position. 1.9 End Effectors End effectors are the devices which are attached to the robot wrist to enable the robotic manipulator to perform the intended task for which it has been designed. They include the tooling which attaches to the robot's wrist and the sensor systems which allow the robot to interact with its environment. The robotic end effectors are generally custom engineered to fit them for the particular purpose/job. End effectors are broadly divided into two main categories namely grippers and tools. Grippers are used for pick and place operations, and are utilized to grasp the objects/ workpart, and hold it during the robot work cycle. The most common methods of grasping used in grippers in industrial robot are: (i) Mechanical Grippers: this method consists of mechanical means of grasping the part between two or more fingers as shown in figure 1.14. (ii) Suction cups: this method uses the vacuumed cups to hold the flat objects. (iii) Magnetized grippers: can be used for ferrous parts (iv) Hooks: used to lift the parts from the conveyors (v) Scoops or ladles: these are used for the fluids, powdered, or granular substances.
Tools are mostly directly mounted on the wrist and acts as the end effector. Only in some applications it may happen that a gripper is used to hold the tool. A tool would be used as an end effector in applications where the robot is required to perform some operation on the work part. These applications include spot welding, arc welding, spray painting, and drilling.
Figure 1.14: Mechanical Grippers
1.10 Architecture of Robotic Systems The general robot architecture comprises of the following components, as listed below: (a) Mechanical Structure • Kinematics model • Dynamics model (b) Actuators: Electrical, Hydraulic, Pneumatic, Artificial Muscle (c) Computation and controllers (d) Sensors (e) Communications (f) User interface (g) Power conversion unit
Configuration Sensor Figure 1.15: General architecture of the robotic system
These components are connected to each other in a way shown in figure 1.15, for the proper execution of the robot motions. We shall be discussing about these components in details in the next units. 1.11 Robot Selection After the selection of the application which needs to be automated using robot, a suitable robot should be chosen from the many commercial robots available in the market. The characteristics of robots generally considered in a selection process include:
(i) (ii) (iii) (iv) (v) (vi) (vii) (viii) (ix) (x) (xi) (xii) (xiii) (xiv)
Size of class Degrees of freedom Velocity Drive type Control mode Repeatability Lift capacity Right-left traverse Up-down traverse In-out traverse Yaw Pitch Roll Weight of the robot
The size of the robot is given by the maximum dimension (x) of the robot work envelope. Based upon the size of work volume the robots are classified as: (i) Micro (x < 1 m) (ii) Small (1 m < x < 2 m) (iii) Medium (2 < x < 5 m) (iv) Large (x > 5 m) The cost of the robot increases with the number of degrees of freedom. Six degrees of freedom is suitable for most works. Robots with higher degrees of freedom are called redundant robot, and are used most of time for complex applications and generally to avoid obstacles in their work volume. Velocity consideration is effected by the robot’s arm structure, which may be rectangular, cylindrical, spherical, and articulated. It also depends upon the drive technology used in the robots. 1.12 Robot Applications Industrial robots have been successfully used for almost all industrial applications. Major applications of industrial robots are as follows: 1.12.1 Loading/unloading parts to/from the machines (i) Unloading parts from die-casting machines (ii) Loading a raw hot billet into a die, holding it during forging and unloading it from the forging die (iii) Loading sheet blanks into automatic presses (iv) Unloading molded parts formed in injection molding machines (v) Loading raw blanks into NC machine tools and unloading the finished parts from the machines Single machine robotic cell applications include: (i) The incoming conveyor delivers the parts to the fixed position (ii) The robot picks up a part from the conveyor and moves to the machine (iii) The robot loads the part onto the machine (iv) The part is processed on the machine (v) The robot unloads the part from the machine
(vi) The robot puts the part on the outgoing conveyor (vii) The robot moves from the output conveyor to the input conveyor Multi-machine robotic cell application: Two or three CNC machines are served by a robot. The cell layout is normally circular. 1.12.2 Welding (i) Spot welding: Widest use is in the automotive industry (ii) Arc welding: Ship building, aerospace, construction industries are among the many areas of application. 1.12.3 Spray Painting Robots provide a consistency in paint quality and widely used in automobile industry for medium batch production. 1.12.4 Assembly Operations Electronic component assemblies and machine assemblies are two areas of application. 1.12.5 Inspection Industrial robots are used for inspection applications, in which the robot end effector is special inspection probe.
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K. S. Fu, R. C. Gonzalez, and C. S. Lee “Robotics: Control, Sensing Vision and intelligence”, McGraw Hill Book Company Pvt. Ltd., 1987, Chap 2
Robert J. Schilling, “Fundamentals of Robotics- analysis and control”, Prentice-Hall India, 2003, Chap 1 and 2.
John J. Craig, “Robotics”, Pearson Education, New Delhi.
A. Ghosal, “Robotics”, Oxford, New Delhi, 2006.
S. K. Saha, “Introduction to Robotics”, Tata McGraw-Hill, New Delhi, 2008.