|Home | Articles | Forum | Glossary | Books|
The elementary actuators and techniques described in Section 3 are used in concert with one another to accomplish different tasks. For instance, motors, gearboxes, bearings, and belts are combined within a frame to form a conveyor, or pneumatic actuators; vibratory thrusters and sensors are built into a vibratory part feeder.
Manufacturers often concentrate their expertise into combining these techniques into standard products, whereas custom machine builders use these systems to create unique combinations for each application.
Conveyors are used to move objects or substances from one point to another. They can take many forms and are usually driven by a motor, air, or gravity. Large conveyor systems often have a centralized control system controlled by a PLC. Because of the long distances associated with conveyor systems, sensors and actuators were historically often operated at 120VAC; however, with advances in technology using distributed I/O and modern safety regulations, 24VDC systems are now common. FIG. 1 shows a conveyor system in a cotton-testing facility.
Motors on these large systems are usually of the three-phase 480VAC variety. This requires I/O and motor power to be run separately if 24VDC I/O is used because of the potential of electrical interference. Distributed I/O using communication methods such as Profibus, Ethernet, or DeviceNet requires additional cabling that is also usually attached to the frame of the conveyors. A local disconnect is often provided near each motor and may be monitored by the control system. Safety devices such as E-Stop push buttons and cable pull actuated E-Stops are also generally mounted to the frames.
HMIs often depict the layout of the system, showing the status of the system components along with production or packaging machines integrated with the system. Conveyor control systems can be very elaborate and have hundreds or thousands of I/O points. They also often use multiple variations of the types of conveyors described in this section.
1.1 Belt Conveyors
A belt conveyor consists of two or more pulleys or rollers with a continuous loop of material with the conveyor belt rotating around them. One or both of the pulleys may be powered, moving the belt and the material on the belt forward. Powered pulleys or rollers are called drive or driven elements, while the undriven rollers or pulleys are called idlers. Idlers may also be located on the underside of the conveyor for support of the return strand of belt. Motors for belt conveyors are typically located at the head, or pulling end, of the conveyor. For reversing conveyors, the motor may be located in the middle.
Belts may be made of many different materials ranging from rubber or plastic compounds to metal mesh. Many belts are made of composites with an under layer for strength and a cover material to protect the product. Belt conveyors are usually used in applications requiring a solid surface, where materials cannot be easily passed across rollers. Belt materials are often chosen based on strength requirements or load, the amount of friction required, and the environment that they are exposed to. Cleats and sidewalls may be attached to the belt surface to help confine materials or reduce the need for high-friction surfaces that may damage products. Cleat spacing and durability are key factors in the choice of material and bonding methods for a cleated belt.
If belt conveyors are used on an incline or decline, the friction coefficient of the belt is typically high. A nose-over section is often placed on the top, bottom, or both sections of incline belt conveyors to allow for easy transitions of material from other conveyors or into hoppers. FIG. 2 shows a small cleated conveyor on wheels used for packaging.
Belt conveyors are one of the least expensive types of conveyor.
They typically have a metal framework with rollers at each end.
The belt may be pulled across a flat surface or bed. For heavier loads, it may also move across additional rollers. These are known as slider bed and roller bed conveyors, respectively. To ensure the belt is at the proper tightness and tracks well across the rollers, the end roller is often adjustable. Rollers may also be crowned to ensure belt centering.
1.2 Roller Conveyors
Roller conveyors can take several forms: they may be powered or unpowered, belt or chain driven, or even series of individually powered rollers.
Rollers are usually a metal shell with a shaft on each end.
Depending on the weight and material being conveyed, rollers may be thin-wall aluminum or heavier-gauge steel, rubber coated or individual gravity "skate wheels." Thin-wall rollers are easily bent, dented, or cut and are not suitable for all applications, but they are often used for package handling. Axles on these rollers are often spring-loaded for easy removal.
Roller conveyors are usually used for moving packages with flat bottoms, like boxes. Rollers should be spaced so that at least three rollers are underneath the package at any time. Rollers may be driven using various methods. A line shaft may be placed along the length of the conveyor with individual urethane belts attached to each roller from spools on the shaft. Another method of driving rollers is to place either a flat or V belt on the underside of the rollers.
Metal chain can also be used to drive the rollers. A single chain can be used to drive all of the rollers or rollers can be linked together with roller-to-roller links. A greater number of sprocket teeth in contact with the chain allows for heavier loading.
Roller conveyors present special challenges when used on a curved section. Rollers must be spaced farther apart at the outside edge of the curve. Using a double section of rollers with more rollers on the outside section can mitigate this. Some rollers are even made that are larger on one end than the other. One note on curved conveyors-product should never be accumulated on a curve.
FIG. 3 illustrates part of a roller conveyor system for cardboard containers. This section is known as a "merge." An interesting product sometimes used for roller conveyors is the individually powered roller. These are essentially cylindrical motors with fixed shafts. These are usually DC powered and can be used to drive products section by section.
Gravity roller conveyors may be of the roller, or "skate wheel," type. These unpowered conveyors are usually used in short horizontal runs where operators push products from one end to the other or when products drop from one level to another. Skate wheel conveyors are often placed on a wheeled frame so that it can be moved from one location to another. Another closely related nonpowered conveying device is the ball table, which allows products to be moved in any direction by pushing them across a table embedded with large ball bearings. These are often used when moving totes of parts in machine loading and unloading areas.
1.3 Chain and Mat Conveyors
Chain conveyors use a continuous chain that runs from one sprocket to another at each end of a frame. Pendants or containers may be attached to the chain for product containment and transport. The most common type of chain conveyor is the tabletop chain conveyor, which has flat plates connected to the chain. Cleats are sometimes added to these plates for product separation and indexing.
Chain conveyors often use parallel strands of chain mounted to dual sprockets or gears on each end of an axle or shaft. This allows for devices such as lifts, stops, or transfers to be mounted between the chains. Tabletop chains with slats or plates can then be used to move pallets or products between these devices.
Tabletop chains may be composed of a thermoplastic material or metal. The chain is usually contained in a channel between the sprockets that guide it. The plates can also be made in such a way that they overlap and can turn, allowing for curved conveyor runs.
Another term for this type of conveyor is multiflexing since the chain is flexible both sideways and on an incline. An example of a tabletop chain conveyor is shown in Fig. 4.4. Because the plates have space between them, they are also effective for drainage or airflow-an important consideration if working with metal machine parts.
Tabletop chain conveyors are not placed under tension like a belt conveyor since a sprocket is used to drive it. A catenary is typically used at the ends on the chain; this is a hanging loop that allows for an easy return of the chain on the underside of the conveyor frame. The catenary runs from the sprocket over a "shoe" and into containment by Teflon guides underneath.
Chain conveyors may also be used for suspending parts or pendants. A common application for this type of conveyor is for paint booths or ovens. In this case, the chain is almost always metal with hooks located at intervals along the chain.
The mat top conveyer is closely related to the single column of links used in a chain conveyor. This type of conveyor uses multiple columns of links chained together in a mat. Although not as flexible as tabletop chains on curves, mat top conveyors can generally support more weight.
Chain and mat conveyors are usually driven by AC motors, often with variable speed drives for speed control. Chain and mat conveyors with cleats may also be driven with a servo for indexing purposes.
Typically this is done using a sensor at the cleat for stopping the indexing motion and verifying position.
1.4 Vibrating Conveyors
Vibrating conveyors are used for moving bulk materials. Sometimes called shakers or shaker tables, they have a solid conveying surface with sides to contain the material being conveyed.
Vibrating conveyors operate on the natural frequency principle.
With only a small energy input, an object can be made to vibrate at some frequency by alternately storing and releasing energy using supporting springs. The drive mechanism is usually an electric motor with a fixed eccentric shaft or rotating weight. A flat pan vibrating conveyor will convey most materials at a 5° incline from horizontal.
Food grade applications use vibrating conveyors extensively.
Because vibrating conveyors are often made of stainless steel and can be easily coated with materials such as Teflon, they are suitable for wash-down and corrosive environments. They are low maintenance and excellent for sanitary applications. They are also used in applications for sorting, screening, classifying, and orienting parts.
Accessories for vibrating conveyors include counterbalance members for reducing reactions by generating an out-of-phase response to conveyor motion and weighted bases with isolation springs to reduce transmitted vibrations.
Air knife separators are an air-driven method of separating different weight materials. They are sometimes used with vibrating conveyors as a sanitary noncontact method of diverting material.
1.5 Pneumatic Conveyors
Pneumatic conveyors use pipes or ducts to transport materials using a stream of air. The most commonly transported materials using this method are dry pulverized or free-flowing powdery materials.
Carriers can also be transported using air. Items can simply be pushed from one location to another using a push or pull pressure system.
Following are three basic systems that are used to generate high velocity airstreams for conveying:
Suction or vacuum systems use a vacuum created in the pipeline to draw the material with the surrounding air. The system is operated at a low pressure, usually 0.4 to 0.5 atm of pressure. This method is used mainly in conveying light free flowing materials.
Pressure type systems use a positive pressure to push material from one point to the next. The system is ideal for conveying material from one loading point to a number of unloading points. It operates at a pressure of 6 atm and upward.
Combination systems use a suction system to convey material from a number of loading points and a pressure system to deliver it to a number of unloading points.
Air pressure may be generated using an industrial blower or fan.
Alternatively, compressed air is sometimes used for small-volume applications.
In addition to conveying components, there are various devices that are used to guide product in conveying systems.
Diverters are used to move product in a transverse direction to the direction of the conveyor. Sometimes called plows when used for bulk materials, diverters usually have a pivot point at one end.
Diverters may be used to move product off the side of a conveyor onto spurs or guide product into lanes for sorting.
When guiding objects, it is important to consider the angle at which the diverter will operate. To ensure a smooth transition for items such as packages, it is best to operate the diverter at 30° to the conveyor flow. Under no circumstance can they be at greater than 45°. Bulk materials can generally operate at greater angles.
Diverters can be used with belt, roller, and chain conveyors.
Pneumatic system diverters are also common. Diverters usually use air cylinders for actuation, but servo-operated diverters are also common where multiple positions are required.
Pushers are used to move objects at right angles to the conveyor.
They are usually pneumatically operated and are often used in roller conveyors; however, they are not appropriate for belt conveyors.
Gates and lifts can be used to allow passage of personnel and vehicles. These are essentially self-contained conveyors on hinges.
Elevators are also commonly used to move product from one conveyor level to another. These may be pneumatically or motor operated and usually include a short length of conveyor within the lifting platform. With safety devices and traffic control sensors, these are often self-contained machines.
2. Indexers and Synchronous Machines
Indexers move objects a fixed distance for repetitive positioning and to avoid cumulative errors. They are often used to move objects being worked between fixed location stations. Walking beams and pick-and-place mechanisms also move objects from one location to another.
2.1 Rotary Cam Indexers
Rotary indexers are used to move actuators to fixed points in a circular path. They are built to move to discrete points and are typically available in 2- to 12-point configurations. Because they are cam driven, they can be moved at a high rate of speed and handle heavy loads. They can be driven by constant speed motors and can drive auxiliary actuators to perform other repetitive tasks as part of their operation. A common name for a rotary indexer with a machined platform for stations on top is a dial table. An example of a four-station dial table is shown in Fig. 5. Well-known manufacturers of these indexing devices include Camco and Stelron.
Sensors are used to detect when the indexer is within the "dwell" part of the index so station devices can operate on the product. A common device that is often used on dial tables is an overload clutch.
If the motor tries to index the dial, but something is in the way, the dial will "break away" from the drive unit. A sensor is used to detect this condition, and the dial must be put back into position manually.
2.2 Synchronous Chassis Pallet Indexers
Synchronous chassis make use of a motor and line shaft to index pallets and synchronize devices performing operations around the chassis. Cams on the line shaft are used to operate devices in time with the movement of pallets, as well as control pallet movement and dwell times. Synchronous chassis are more robust than systems using sensors and independent control of stations but less flexible and more difficult to reconfigure. Indexing drives and cam motions are specific to the machine application. Machine timing is mechanically fixed, so there is no risk of losing timing or position on individual workstations.
A clutch may be used to disconnect the drive mechanism from the chassis. This must be done while the cam is in the dwell, or nondriving part, of the cam motion. Otherwise, a chassis must be slowed at a gradual rate to reduce stress on the cam-driven mechanisms.
2.3 Walking Beams
A walking beam uses an X-axis and Z-axis configuration to repetitively index parts a fixed distance in a single direction. The X-axis moves forward with the Z-axis raised, carrying or pushing the part in the desired direction. The Z-axis is then lowered and the X-axis returned to its origin to begin another index. Walking beams are common in packaging and assembly operations because of their relatively low cost and repetitive accuracy. Axes may be pneumatically or servo actuated. FIG. 6 illustrates the principle of a two-axis walking beam.
A variation of a walking beam for boxes, totes or flat products that does not require a Z-axis is a spring-finger walking beam. This is a horizontal axis with sloped fingers that are held in the raised position with springs. As the beam is moved backward underneath the product, the product pushes down the fingers; when the beam moves in the forward direction, the fingers pop back up and propel the product forward.
A pick-and-place is so named because it typically picks up an object and places it in another location. It consists of an X-axis, or horizontal axis; a Z-axis, or vertical axis; and a picking mechanism such as a mechanical gripper, vacuum cups, or even a magnet. If another horizontal axis, or Y-axis, is added, it is described as a gantry, which is further described in the robotic section 4.4 of this guide.
Pick-and-place mechanisms may be pneumatically or servo driven, depending on speed requirements and the number of locations that must be accessed. It is common to see mechanisms that are a combination of both. Servo axes provide flexibility since their positions and speeds can be reprogrammed for different products.
FIG. 7 is a three-axis robot from Adept Robotics. A gripper can be attached to the lower end of the vertical axis, or Z-axis, creating a pick-and-place.
Other variations of the pick-and-place can be fabricated using linkages, cams, and other basic mechanisms. An example is a literature or sheet feeder that strips sheets of paper from a hopper or rack using vacuum cups and places them onto a flat surface.
3. Part Feeders
Part feeders supply components to a variety of manufacturing processes. They often serve as a buffer and part orientation device.
3.1 Vibratory Bowls and Feeders
Vibratory bowls and feeders use a variable amplitude controller to control a drive unit with spring thrusters oriented in the direction of part movement. Similar to the method of driving vibratory conveyors, leaf springs are mounted on a base unit oriented in the direction of the desired travel of the part. A bowl with special tooling and tracks sized to the component is then mounted to the other end of the springs to guide the parts. The tooling is also used to orient components within the bowl, guiding improperly oriented parts back into the bowl and allowing parts with the correct orientation to proceed. FIG. 8 shows a vibratory bowl used to feed plastic caps.
Linear tracks also use vibratory drive units to move components in a straight line. Sensors and stops or gates may then be used to control the flow of parts along the tracks.
Drive units are available in electromagnetic and pneumatic drives. Parts are forced up a circular inclined track inside the bowl.
The track length, width, and depth are carefully chosen to suit the application and component shape and size. Special track coatings are applied according to shape, size, and material of the component, which aids traction, minimizes damage to the product, and lowers acoustic levels.
Different materials travel better with different vibration frequencies. The amplitude and frequency of the electronic drive unit is generally set based on the optimum movement rate for the part being moved. Weights can also be added or removed from motors to adjust coarse feed rates. Spring constant values and lengths are other important considerations that affect part movement.
Vibratory feeders are used by most industries, including pharmaceutical, automotive, electronic, food, packaging, and metalworking. They are most commonly used in the assembly process as they align components for access by other mechanisms.
Vibratory hoppers and trays are also used to move bulk materials and are not always associated with orienting or singulating parts.
These are more commonly used in the material handling and process industries for flow control.
3.2 Step and Rotary Feeders
Step feeders remove component parts from a hopper by either elevating parts with a single moving stepper plate onto stationary ledges, as illustrated in Fig. 4.9, or by counterrotating two stepper plates. Plates are moved on linear guides pushing product up out of the bin or hopper. Components are elevated until they reach the desired transfer height, generally feeding into a linear track or conveyor. Step feeders are often used on parts that are cylindrical and are not appropriate for vibrating feeders because of potential product abrasion.
Key features of a step feeder are that it operates quietly and without vibration. Width and thickness of the stepper plates are important variables to consider in feeder design. When considering using a step feeder over a vibratory system, it is important to remember that the part will be elevated considerably above the level of the hopper.
Centrifugal feeders, also referred as rotary feeders, have a conical central-driven rotor surrounded by a circular bowl wall. The feeder separates component parts using rotary force; then parts revolve with high speed and are pulled to the outside of the bowl. As parts accumulate at the outside edge of the bowl, they tend to line up allowing for orientation and singulation of the parts.
Centrifugal feeders can be operated at higher feed rates than vibratory bowls. They are also better at handling oil-covered parts but do not work as well if parts tangle easily. Specialized tooling for parts orientation is used in much the same way as a vibratory bowl, with machined protrusions on the outside track of the feeder.
3.3 Escapements and Parts Handling
An escapement is a pair of actuators that allows for singulation of parts. If components are separated by a space, gates or pop-up stops can be used between parts as they move through the system. As a part is detected at the forward stop, another stop is raised behind the part. After the forward stop allows the part to exit and is raised again, the rear stop is lowered, allowing the part to move forward, after which the cycle is repeated.
Escapements may take the form of stops operating from above, below, or the sides. They may also use pressure against the side of the part itself; this is useful if parts do not have space between them.
Escapements are used in conveying systems, feeders, pallet indexing systems, and assembly. They are one of the basic elements of parts handling.
In general, parts handling involves controlling the movement of components within a system. A good rule of thumb is once you have control of randomized parts through orientation and singulation, you should never relinquish it. This means that if parts have been individualized, do not let them recombine. If they have been oriented, do not allow them to return to a mixed state.
Most part-handling techniques have been discussed in other sections of this section. Pushers and diverters, pick-and-places, and other actuator-based mechanisms are all examples of parts handling.
Additional methods include using parallel urethane cord conveyors at an angle on the sides of parts to lift them out of moving conveyor pockets, using rubber wheels to accelerate low-friction parts for singulation, and using air to blow rejected or disoriented parts off a conveyor or out of a track. Actuators, air, and sensors can be used in many creative ways to create the desired part movement.
4. Robots and Robotics
A robot is an electromechanical machine that can perform tasks on its own or with guidance. Industrial robots are used widely throughout the manufacturing sector, and the various categories of these robots come in different configurations and sizes. Robots are most often driven by coordinated servo gear motors moving directly on axes; however, hydraulic robots are also used in some applications.
An industrial robot is defined by ISO 8373 as an "automatically controlled, reprogrammable, multipurpose manipulator programmable in three or more axes." In industry, the term robotics can be defined as the design and use of robot systems for manufacturing.
The most commonly used robot configurations are articulated robots, SCARA robots, and Cartesian coordinate robots (also known as gantry robots or x-y-z robots). Speed requirements, the positions that must be attained, and the cost of the system are factors that determine which type of configuration is generally used for a particular function.
4.1 Articulated Robots
An articulated robot is one that uses rotary joints to access its work space. Usually the joints are arranged in a "chain," so that one joint supports another farther in the chain. Another term for an articulated robot is a "robotic arm." Articulated robots usually have anywhere from three to six joints.
More than six joints are possible, but these robots generally fall into the custom category. Another term for this is "degrees of freedom," defined as the number of independent motions that make up the robot's area of operation. Joints are usually defined as J1-Jx, where x is the number of joints in the robot. J1 is the joint nearest the base of the robot, and other joints increment from there. Typically, J1 rotates horizontally around the robot base. Because of the cables that need to make their way through the various joints for servo power and position, joint rotation for J1 is usually less than 360°. FIG. 10 shows a six-axis Denso robotic arm mounted on a base.
J2 and J3 usually operate in the vertical plane. Along with the rotation of J1, this allows the other joints to be placed close to nearly every point within the robot's operating envelope. J4, J5, and J6 typically act as manipulators, with the last joint, J6, usually being a rotary to which grippers or other devices are attached.
4.2 SCARA Robots
The SCARA acronym stands for Selective Compliant Assembly Robot Arm or Selective Compliant Articulated Robot Arm. These are usually of the four-axis variety, with J1 and J2 being horizontal rotary joints to access X-Y points, J3 being a Z-axis, and J4 being a rotary or T-axis mounted at the end of J3.
Because of the parallel axes of J1 and J2, the end of the vertical axis J3 is rigidly controlled in the X-Y position, hence the term "selective compliant." SCARA robots are widely used for assembly operations that require this rigidity in the X-Y plane, such as placing a round pin into a vertical hole without binding. An example of a SCARA-type Adept robot is shown in Fig. 11.
SCARA robots are less expensive than similar-size, fully articulated robots by virtue of the lower number of joints they possess. They are also faster and more compact than Cartesian gantry systems because the pedestal mount has a smaller footprint than the multiple-point mounting of a gantry.
4.3 Cartesian Robots
A Cartesian robot, also called a linear or gantry robot, has three linear axes of control in the X, Y, and Z directions. Rather than having rotary joints, the X-axis is usually mounted at both ends with the Y-axis mounted to it. Some gantries suspend the Y-axis between two X-axes using a four post arrangement. This creates a box-shaped working envelope. The Z-axis is mounted to the Y-axis and may have an additional rotary axis mounted to the end. A gripper or other end effector is then attached to this for part handling. The Adept "Python" three-axis robot shown in Fig. 7 is an example of a Cartesian robot.
Gantry arrangements are the simplest control scheme for robots since coordinates are in the familiar X-Y-Z or Cartesian system and do not have to be converted or interpolated as with other systems. This allows for separate controllers or servo drives to be used for movements if coordinated moves are not required.
A popular application for Cartesian robots is the Computer Numerical Control (CNC) machine. These machines are used widely in industry for the automated machining of metal parts.
4.4 Parallel Robots
A parallel robot makes use of four or more linkages or kinematic chains from a central actuation point to an end effector. They are considered closed-loop systems since each of the links are constrained by the others. When compared with serial manipulators, such as robotic arms and SCARAs, the structural members are very light and therefore provide a much greater linear speed. The disadvantage of a parallel robot is that its work space is limited when compared to the space it occupies. FIG. 12 shows an Adept "Quattro" parallel robot.
Parallel robots are usually suspended above the objects being manipulated. A common use for parallel robots is the insertion of components into printed circuit boards.
4.5 Robot Basics and Terminology
Robotic systems are made up of several components. The robot itself, with its motors, joints, and structures, makes up the moving part of the system. Motor and feedback cables are usually routed through the structural members of the robot for protection. Motors used in smaller robots are usually high-speed, low torque DC motors with high gear ratios. Larger robots use various different types of servomotors, depending on speed and payload requirements, but all use gearing or gearboxes of some kind.
For stability, the robot is mounted to a base, which is usually bolted to a solid foundation or steel frame. The base also usually has cable connections from the controller.
The robot controller contains drives for each of the axes along with the "brains" for running robot programs and coordinating axis movements. Communication ports for interfacing with programming computers and other controllers are also present. Safety interface connections for E-Stop and guard circuits are generally mounted here as terminal connections. Cables to the robot base connect to the controller as well as a port for the robot's programming pendant. This is also where the power connection for the robot is made.
The robot pendant is used to write or edit robot programs as well as to manually move the robot and "teach" positions. Pendants resemble an HMI with a touch screen and usually a membrane keypad. They also have an E-Stop button for integration into a machine safety circuit. A "dead-man" switch is built into the pendant, which must be held down during any manual robot movements.
The end effector is tooling placed on the working end of the robot.
It is typically used to manipulate parts, but it may also be used to carry a welding tip or sprayer head. End effectors may carry pneumatic or hydraulic grippers, magnets, suction cups, or various other types of tooling. Some end effectors can be quite elaborate, having several actuators and sensors. Cameras and measurement devices are also often mounted on the end effector.
Terminology for robots and robotics can vary widely, depending on the manufacturer. Following are some of the more general terms that apply to a variety of platforms. Other terms for specific platforms can be found in manufacturers' documentation.
Specifications for a robot involve an analysis of the product and movements required. The payload, or carrying capacity, is the amount of weight that the robot must lift, including the weight of end effector tooling. This can be affected by the speed, acceleration, and force that are required. Kinematics is the arrangement of the rigid members and joints as described previously. Choosing between articulated, SCARA, Cartesian, or other configurations involves determining the envelope of the points that need to be accessed and at what angle. Two axes are required to access any point in a plane, while three are required to reach any point in a X-Y-Z space. In order to completely control orientation, an additional three rotational axes are required-pitch, yaw, and roll. Additional axes are sometimes added to reach around obstructions or into cavities.
Accuracy and repeatability are measurements of the precision at which the robot operates. Accuracy is a measurement of how closely a robot will move to a programmed or commanded position, while repeatability is a measure of how well the robot will return to the same position each time. Compliance is a measure of how much a robot will move when a force is exerted against it. When a load is being carried, positions will be slightly different than when there is no load at all. Joint positions may be detected accurately, but even solid members or joints will bend slightly under load. Acceleration can affect compliance even further and must be considered because of potential overshoot in position.
A frame or tool frame is a coordinate reference that is used to allow an entire set of points to be translated to a new location. As an example, if you have taught a large group of points around a station that the robot will access, then either the robot or the station needs to be moved. By using a frame recorded at X-Y-Z on the station, all that needs to be done is to teach the new frame, rather than re-teaching all of the points.
A singularity is a condition at which robots can reach a point through more than one joint configuration, making the axes redundant. The American National Standard for Industrial Robots and Robot Systems-Safety Requirements (ANSI/RIA R15.06-1999) defines a singularity as "a condition caused by the collinear alignment of two or more robot axes resulting in unpredictable robot motion and velocities." It is most common in robot arms that utilize a "triple roll wrist." This is a wrist about which the three axes of the wrist- controlling yaw, pitch, and roll-all pass through a common point.
The ANSI/RIA has mandated that robot manufacturers make users aware of singularities if they can occur while robots are being manually manipulated. Some industrial robot manufacturers have attempted to sidestep the situation by slightly altering the robot's path to prevent singularity conditions.
Often, SCARA robot arm positions are defined as being right- or left-handed to avoid singularities arising when the robot could access a point from either configuration. Articulated robots may have even more joint combinations that can theoretically reach a particular point through various arm configurations. Some singularity problems can be avoided by switching from Cartesian moves to individual joint movements within a program. Another method is to slow the robot's travel speed, thus reducing the speed required for the wrist to make the transition, avoiding the condition sometimes referred to as "wrist flip." An area definition or vector can be used to define an area for safety or movement control. By defining a point along X, Y, and Z dimensions, a three-dimensional space can be used to control operations within an area. This is often used to set or reset an output when the robot enters or leaves a space. Terminology for this varies widely for different platforms.
An approach or depart move generally describes moving to or from a point at a certain angle. Rather than having to define a specific position for a move, the approach or depart move can make use of a defined position by telling the robot to move toward or away from a point using the current orientation of the end effector tooling.
4.6 Robot Coordinate Systems
Robot movements and positions can be defined in a number of different coordinate systems. "World" coordinate systems apply to any coordinate system using the robot's base as the origin. "Tool" coordinates use the end of the robot's arm where the tooling is mounted as the origin. "Workpiece" coordinates use a point on the work area tooling rather than a point on the robot itself as the origin.
The most familiar X-Y-Z, or Cartesian, coordinates are usually easiest for a human to visualize and so are often used for position definitions. Additional coordinates are sometimes added to the X (primary horizontal), Y (secondary horizontal), and Z (vertical). These are sometimes referred to as A (rotation around X), B (rotation around Y), and C (rotation around Z). SCARA and gantry-type robots require little interpolation or conversion to use this system. This coordinate system is sometimes referred to as a "space" coordinate system.
Joint coordinates describe the angular position of each of the robot's joints. Controllers use joint coordinates and perform mathematical calculations or interpolations to arrive at Cartesian points. These may be addressed in a program in variables such as J1-Jx or A1-Ax, depending on the software platform.
When operating a robot from a pendant, it may be advantageous to switch between different coordinate systems and work spaces, depending on the ease of visualization.
Further information on robot programming and software is addressed in Section 6.