Chapter 20: Spherical Robots |
20.1 Chapter Overview |
This chapter examines spherical robots, one of the earliest and most influential families of industrial manipulators. A spherical robot, sometimes called a polar robot, positions its wrist by combining two rotational axes with one linear axis. The result is a working envelope shaped like a sphere or a thick spherical shell. That geometry gives the machine a very long reach relative to its footprint, which is why spherical robots dominated the first wave of automated welding and machining. |
The discussion begins with a short summary of the technology. It then moves through the mechanical layout, the nature of the spherical workspace, the control principles that make the geometry usable, and the historical role of these machines in factories. The heart of the chapter is a wide survey of real applications across many industries, including automotive welding, heavy equipment repair, shipbuilding, aerospace, foundry work, and general job-shop machining. The chapter closes with a detailed summary that ties the technical ideas back to practical decisions about when a spherical robot remains a sensible choice. |
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20.2 A Short Summary of Spherical Robots |
A spherical robot has three principal axes that position the wrist. Two of them are rotational, and one is linear. The rotational axes typically turn the arm around a vertical axis and tilt it up or down. The linear axis extends or retracts the arm along its length. Because the linear motion slides the wrist outward from a central pivot, the reach can be very large without making the machine itself enormous. The set of all points the wrist can touch forms a spherical shell. The robot can therefore work on parts that are far away, curved, or arranged around a central point. |
This design was the first to be used in industrial welding and machining. Its long reach and its ability to sweep around a workpiece made it ideal for tasks where the tool had to follow a curved seam or reach into a large structure. The trade-offs are equally clear. The spherical envelope is not a box, so some points in space are hard to reach and others are unreachable. The linear axis often uses a telescoping boom or a sliding carriage, which can be heavy and can introduce flexibility. The rotational joints must carry the full load of the extended arm, so payload capacity falls as the arm stretches out. Modern spherical robots are less common than articulated or Cartesian machines, but they remain in service in welding, coating, and heavy machining, and they still offer lessons about reach, stiffness, and workspace design. |
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20.3 The Mechanical Layout of a Spherical Robot |
20.3.1 The Three Principal Axes |
The classic spherical robot has a base that rotates about a vertical axis. This is usually called the theta axis, or the base rotation. On top of that base sits a shoulder joint that tilts the arm up and down. This is often called the phi axis, or the elevation axis. The arm itself contains a linear axis, often called the radial axis, that slides the wrist in and out. Together, theta and phi aim the arm in a direction, and the radial axis sets the distance. The wrist then adds orientation axes, usually two or three, to point the tool. |
The order of these axes matters. In most spherical robots, the base rotation comes first, then the elevation, then the extension. This arrangement keeps the heavy drive for the base near the floor and lets the elevation motor sit on the rotating base. The linear axis is usually the last positioning axis before the wrist, so it carries less of the structural load than it would if it were closer to the base. Even so, the extended arm acts like a cantilever, and the farther it reaches, the more it deflects under its own weight and under the weight of the tool. |
20.3.2 The Telescoping Arm |
Many spherical robots use a telescoping arm for the linear axis. The arm consists of nested tubes or a sliding carriage on a rail. A motor and a drive screw, belt, or rack-and-pinion push the inner section outward. Telescoping arms can achieve very long strokes in a compact retracted length. That is a major advantage in welding cells, where the robot must reach deep into a fixture but must also stay clear during loading. |
The alternative is a rigid boom that slides along a linear guide. This design is stiffer than a telescoping tube but takes up more space when retracted. Both designs share a common problem: the linear axis adds mass that the rotational axes must move. Engineers reduce this effect by placing the linear drive motor on the fixed part of the arm and using a light push-pull element to move the wrist. Cables, hoses, and welding wire must also be routed through or along the arm, and their stiffness changes as the arm extends. Good cable management is essential for reliable operation. |
20.3.3 The Wrist |
The wrist of a spherical robot is usually a two-axis or three-axis unit. A two-axis wrist can tilt and rotate the tool, which is enough for many welding and cutting tasks. A three-axis wrist adds a second tilt, giving full orientation control. The wrist is mounted at the end of the linear axis, so its weight and the weight of the tool both contribute to the bending moment on the arm. Designers keep the wrist as light as possible and place the wrist motors close to the wrist to avoid long transmission paths. In some machines, the wrist is a compact unit with built-in motors and brakes, which simplifies the mechanical interface but adds cost. |
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20.4 The Spherical Working Envelope |
20.4.1 How the Envelope Is Formed |
The working envelope of a spherical robot is the set of points the wrist center can reach. The base rotation sweeps the arm around a vertical axis, creating a circular sweep. The elevation axis tilts the arm up and down, creating a range of angles from near the floor to near the ceiling. The linear axis extends the arm from a minimum radius to a maximum radius. The combination of these motions produces a partial sphere. The inner boundary is set by the minimum extension, and the outer boundary is set by the maximum extension. The top and bottom boundaries are set by the elevation limits, and the side boundaries are set by the base rotation limits. |
The result is not a full sphere. It is a thick shell with a wedge removed where the arm cannot go. The shape looks like a piece of an orange peel. The robot can reach points on the outer surface of that shell very easily, but it cannot reach points inside the inner boundary. It also cannot reach points outside the outer boundary, no matter how it turns. This shape is very different from the rectangular envelope of a Cartesian robot or the rounded envelope of an articulated robot. |
20.4.2 Reach and Dexterity |
The chief advantage of the spherical envelope is reach. A spherical robot with a one-meter linear stroke can reach parts that are two meters across, because the arm can extend outward in any direction. The same machine can often reach over obstacles and into recesses that a Cartesian robot could not enter without a very long travel. This makes spherical robots useful in welding large frames, in machining long shafts, and in servicing equipment that is already installed. |
The chief disadvantage is dexterity. Near the center of the sphere, the robot has very little room to move. The linear axis cannot retract below its minimum, so the robot cannot work close to its own base. Near the outer edge, the arm is fully extended, and small errors in the joints become large errors at the tool. The robot also cannot easily change its orientation while keeping the tool on a straight line, because the base rotation and the elevation axis change the direction of the linear axis. This makes path planning more complex than for a Cartesian machine. |
20.4.3 Singularities and Awkward Zones |
A spherical robot has singularities where two axes line up or where a small change in the tool position requires a very large change in joint angles. A common singularity occurs when the arm points straight up or straight down, because the base rotation no longer changes the tool position. Another occurs when the arm is fully extended, because the linear axis cannot extend farther. These singularities are not dangerous by themselves, but they can cause the robot to move suddenly if the controller is not careful. Good controllers limit joint speeds near singularities and plan paths that avoid them. |
Awkward zones are regions where the robot can reach a point but only with a poor posture. For example, the robot may have to fully extend its arm and tilt it to an extreme angle, leaving no room to adjust the tool orientation. In welding, this can mean the torch angle is wrong, which produces a bad weld. In machining, it can mean the tool deflects too much, which produces a poor surface finish. Experienced programmers learn to place the workpiece so that the critical tasks fall in the comfortable part of the envelope. |
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20.5 Control and Programming |
20.5.1 Coordinate Systems |
Spherical robots are usually programmed in Cartesian coordinates, even though their joints are rotational and linear. The controller converts the desired tool position and orientation into joint angles and extension lengths. This conversion is called inverse kinematics. For a spherical robot, the inverse kinematics are relatively simple because the geometry is clean. The base angle is found from the horizontal direction to the target. The elevation angle is found from the vertical direction and the distance. The extension is found from the distance to the target. The wrist angles are then found from the desired tool orientation. |
This simplicity is one reason spherical robots were popular in early automation. Computers of the 1960s and 1970s could solve the inverse kinematics quickly enough for real-time control. Modern controllers solve the same problem easily and add features such as collision avoidance, path blending, and sensor integration. |
20.5.2 Teach Pendant Programming |
Most spherical robots are programmed with a teach pendant. The operator jogs the robot to a desired point, records the point, and then moves to the next point. The controller stores the sequence and plays it back. This method is simple and works well for welding, where the path is often a series of straight segments or arcs. The operator can adjust the points by eye, which is useful when the workpiece is not perfectly positioned. |
Teach pendant programming has limits. It is slow for complex paths, and it depends on the skill of the operator. It also does not easily handle changes in the workpiece. If the part moves, the program must be taught again. For these reasons, many modern spherical robot cells use offline programming, where the path is created in a software model and then downloaded to the controller. Offline programming is especially useful in shipbuilding and aerospace, where the parts are large and the cost of stopping production to teach a path is high. |
20.5.3 Sensor Integration |
Spherical robots can be fitted with sensors to adapt to changes in the workpiece. A common example is through-arc sensing in welding. The welding current and voltage change as the torch moves across the joint, and the controller uses these changes to steer the torch along the seam. This lets the robot weld parts that are not perfectly aligned. Another example is laser vision, where a camera and a laser stripe measure the joint before the weld. The controller adjusts the path in real time. These sensors make spherical robots more flexible and reduce the need for precise fixtures. |
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20.6 Historical Role and Evolution |
20.6.1 The First Industrial Robots |
The first industrial robot, the Unimate, was installed in a die-casting plant in 1961. It was a spherical robot with a base rotation, an elevation axis, and a telescoping arm. It used hydraulic drives and a magnetic drum memory. It was not a welding robot, but its geometry proved that a machine could reach into a workspace and perform useful work. Other early robots, such as the Versatran, also used spherical or near-spherical layouts. These machines established the idea that a robot could be programmed by teaching points and then repeat a task without human intervention. |
20.6.2 The Rise of Welding Robots |
In the 1970s and 1980s, spherical robots became common in automotive welding. The long reach let them weld large body frames and truck cabs. The linear axis let them reach deep into fixtures. The base rotation let them swing from one side of a part to the other. Companies such as Unimation, Cincinnati Milacron, and ASEA built spherical or hybrid robots for these tasks. Many of these machines used hydraulic drives, which were powerful but messy. Later models used electric drives, which were cleaner and more precise. |
20.6.3 The Shift to Articulated Robots |
By the 1990s, articulated robots with three rotational axes and a wrist became the dominant design. Articulated robots are more flexible than spherical robots because they can reach around obstacles and can change orientation without moving the whole arm. They also have a more compact envelope and can be mounted on the floor, on a wall, or on a ceiling. Spherical robots lost market share, but they did not disappear. They remained in use in welding, coating, and heavy machining, especially where long reach was more important than dexterity. Some manufacturers still build spherical or hybrid machines for specific tasks. |
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20.7 Applications in Welding |
20.7.1 Automotive Body Welding |
The automotive industry was the first large market for spherical welding robots. A car body has hundreds of spot welds and many seam welds. A spherical robot can reach across a body frame and weld both sides without moving the car. Its long arm can reach into the engine compartment, the wheel wells, and the trunk. In a typical line, several spherical robots work together, each welding a different section. The robots are mounted on pedestals or on overhead rails so they can reach the top and bottom of the body. The program is taught by hand and then repeated thousands of times. |
Seam welding is more demanding than spot welding because the torch must follow a continuous path. A spherical robot can follow a seam by combining base rotation, elevation, and extension. The controller coordinates the three axes so the torch moves at a constant speed. The welder must set the voltage, current, wire feed, and travel speed for each joint. The robot repeats these settings exactly, which gives consistent welds. This consistency is the main reason automakers adopted robots. |
20.7.2 Truck and Bus Frame Welding |
Truck and bus frames are larger and heavier than car bodies. They are often welded in low-volume batches, so flexibility is important. A spherical robot with a long reach can weld several frame sizes without a major changeover. The operator loads the frame on a fixture, teaches the path once, and then runs the program for the batch. The robot can reach the top, bottom, and sides of the frame by rotating its base and extending its arm. The linear axis is especially useful for reaching the far corners of a long frame. |
Some truck frames are welded with a robot on a traveling carriage. The carriage moves along the frame while the robot welds a seam. This combination of a linear track and a spherical arm gives an enormous working volume. The track handles the long travel, and the arm handles the local reach and orientation. This arrangement is common in heavy equipment manufacturing. |
20.7.3 Shipbuilding and Offshore Structures |
Shipbuilding involves large steel structures with long seams. A spherical robot can be mounted on a gantry or on a portable platform and moved to the seam. The long reach lets it weld stiffeners, bulkheads, and hull plates. The robot can also be used for cutting and gouging. In shipyards, the robot is often programmed offline because the parts are too large to teach by hand. The offline model includes the ship structure, the robot, and the fixture. The programmer creates the path in the model and downloads it to the controller. |
Offshore structures, such as oil platforms and wind turbine foundations, have similar requirements. They are large, heavy, and often welded in difficult positions. A spherical robot on a track or a gantry can reach the joints and weld them with consistent quality. The robot can also be used for inspection and repair, where it carries a camera or a sensor instead of a torch. |
20.7.4 Pipe and Pressure Vessel Welding |
Pipe welding is a classic application for spherical robots. A pipe joint is a circle, and a spherical robot can follow the circle by rotating its base and adjusting its elevation and extension. The robot can weld the joint in one pass or in multiple passes, depending on the wall thickness. It can also weld nozzles and flanges, which are harder to reach. In pressure vessel fabrication, the robot welds shells, heads, and nozzles. The long reach lets it weld both inside and outside the vessel. The robot is often mounted on a positioner that turns the vessel while the robot welds. This combination is called a coordinated motion system, and it requires the controller to synchronize the robot with the positioner. |
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20.8 Applications in Machining |
20.8.1 Drilling and Tapping |
Spherical robots were used for drilling and tapping before they were used for welding. A drilling robot needs a stiff arm and a precise feed. The linear axis of a spherical robot can provide the feed, and the base and elevation axes position the drill. The robot can drill holes in large parts, such as engine blocks, transmission cases, and structural beams. It can also tap threads, countersink holes, and chamfer edges. The main challenge is stiffness. A drilling tool pushes back against the robot, and the arm must resist that force. Spherical robots with rigid booms and strong bearings can handle moderate drilling loads. For heavy drilling, a special-purpose machine is usually better. |
20.8.2 Milling and Grinding |
Milling and grinding require even more stiffness than drilling. The tool must follow a path with small tolerances, and the cutting forces can be large. A spherical robot can perform light milling and grinding if the arm is stiff and the path is planned to keep the tool close to the base. This is called working in the stiff zone. The robot can grind weld seams, deburr castings, and polish molds. It can also mill pockets and slots in soft materials, such as aluminum and plastic. For hard materials and tight tolerances, a dedicated machining center is usually the better choice. |
20.8.3 Deburring and Chamfering |
Deburring is a common task for spherical robots. After a part is machined, it often has sharp edges and burrs. A robot with a grinding wheel or a deburring tool can remove these features. The long reach lets the robot work on large parts, such as castings and forgings. The robot can also chamfer edges, which prepares them for welding or assembly. Deburring is a good application for spherical robots because the forces are moderate and the path is often simple. The robot can be taught by hand or programmed offline. |
20.8.4 Cutting and Gouging |
Spherical robots can carry plasma torches, oxy-fuel torches, or waterjet nozzles for cutting. The long reach lets them cut large plates and shapes. In shipbuilding, the robot can cut openings and contours in hull plates. In construction, it can cut steel beams and pipes. Gouging is similar to cutting, but the goal is to remove metal rather than to separate parts. A gouging robot can prepare a weld joint or remove a defective weld. The robot must be able to control the torch angle and the travel speed to get a clean gouge. |
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20.9 Applications in Coating and Surface Treatment |
20.9.1 Spray Painting |
Spray painting is a natural application for spherical robots. The robot can reach around a part and apply paint from many angles. The long reach lets it paint large parts, such as car bodies, truck cabs, and appliance panels. The robot can also paint the inside of a part, such as a cabinet or a container. The base rotation sweeps the spray gun around the part, and the elevation axis moves the gun up and down. The linear axis sets the distance from the gun to the surface, which controls the paint thickness. The robot repeats the path exactly, which gives a uniform finish and reduces paint waste. |
20.9.2 Powder Coating |
Powder coating uses a dry powder instead of a liquid paint. The powder is sprayed onto the part and then cured in an oven. A spherical robot can apply powder with the same motions as a paint robot. The long reach is useful for large parts, such as fence panels, shelving, and automotive wheels. The robot can also coat the inside of a part, which is hard to do by hand. The main difference from painting is that powder coating requires a clean environment and a grounded part. The robot must be designed to avoid sparks and to handle the powder recovery system. |
20.9.3 Sealant and Adhesive Dispensing |
Sealant and adhesive dispensing require a precise bead of material along a seam or a joint. A spherical robot can follow the seam with its long arm and apply the bead at a constant speed. The robot can dispense sealant on car bodies, appliance cabinets, and window frames. It can also dispense adhesive for bonding panels, glass, and trim. The key requirement is smooth motion, because a stop or a jerk can cause a lump or a gap in the bead. The controller must blend the motions of the base, elevation, and extension axes to keep the nozzle at the right speed and angle. |
20.9.4 Thermal Spraying |
Thermal spraying uses a hot gas or a plasma to melt a material and spray it onto a surface. The coating can protect against wear, corrosion, or heat. A spherical robot can carry a thermal spray gun and move it across the part. The long reach lets it coat large parts, such as turbine blades, pump shafts, and paper mill rolls. The robot must be able to maintain the correct spray distance and angle, because these affect the coating quality. The environment is harsh, with high temperatures and dust, so the robot must be protected with covers and cooling. |
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20.10 Applications in Material Handling |
20.10.1 Machine Loading and Unloading |
A spherical robot can load and unload machines, such as lathes, milling machines, and presses. The long reach lets it reach into the machine and place the part on the chuck or the fixture. The base rotation lets it swing from the conveyor to the machine and back. The linear axis lets it insert the part into a deep machine. This application is common in job shops, where the robot handles a variety of parts. The robot can also change tools, such as drills and taps, and can load parts into a pallet or a bin. |
20.10.2 Palletizing and Depalletizing |
Palletizing is the task of stacking parts on a pallet. A spherical robot can pick parts from a conveyor and place them on a pallet in a pattern. The long reach lets it build a tall stack without moving the base. The base rotation lets it cover the whole pallet. The linear axis lets it reach the far corners of the pallet. Depalletizing is the reverse task, and it is common in warehouses and distribution centers. The robot can handle boxes, bags, and drums. The main challenge is the weight of the parts, because the payload capacity falls as the arm extends. |
20.10.3 Foundry and Forging Handling |
Foundries and forges are harsh environments with high heat, dust, and vibration. A spherical robot can handle hot parts, such as castings and forgings, and can move them from one station to another. The long reach lets it reach into a furnace or a press. The robot must be protected from heat and sparks, often with a heat shield and a cooling system. The linear axis can be fitted with a special gripper that can hold a hot part without damaging it. This application is dangerous for humans, so robots are often used to improve safety. |
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20.11 Applications in Assembly and Inspection |
20.11.1 Large Part Assembly |
Spherical robots are not usually used for small-part assembly, because articulated robots are more dexterous. But they can be used for large-part assembly, such as joining sections of a fuselage or a ship. The long reach lets the robot hold a part in place while a second robot or a human fastens it. The robot can also install bolts, rivets, and pins. The key requirement is stiffness, because the robot must resist the forces of the assembly process. A spherical robot with a rigid arm and a strong wrist can handle these tasks. |
20.11.2 Inspection and Measurement |
A spherical robot can carry a camera, a laser scanner, or a probe to inspect a part. The long reach lets it inspect large parts, such as aircraft wings, ship hulls, and wind turbine blades. The robot can follow a path and record the data. The data can be used to find defects, measure dimensions, or create a digital model of the part. This application is common in aerospace and energy, where the parts are large and the cost of inspection is high. The robot must be very accurate, because the inspection results depend on the position of the sensor. The controller must compensate for the deflection of the arm and the errors of the joints. |
20.11.3 Non-Destructive Testing |
Non-destructive testing uses sound, radiation, or magnetic fields to find defects inside a part. A spherical robot can carry an ultrasonic probe, an X-ray source, or an eddy current sensor. The long reach lets it test large parts, such as pipes, pressure vessels, and welds. The robot can follow a weld seam and test it point by point. The data is recorded and analyzed. This application is common in oil and gas, power generation, and aerospace. The robot must be able to move smoothly and to maintain the correct coupling between the sensor and the surface. |
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20.12 Applications in Specialized Industries |
20.12.1 Aerospace |
Aerospace manufacturing uses large parts, such as fuselage sections, wings, and engine components. A spherical robot can drill, rivet, and inspect these parts. The long reach lets it work on a fuselage without moving the whole structure. The robot can be mounted on a gantry or a track to extend its reach further. In engine manufacturing, the robot can grind and polish turbine blades and disks. The key requirements are accuracy and repeatability, because the tolerances are tight. The robot must also be able to work in a clean environment, because contamination can ruin a part. |
20.12.2 Automotive |
The automotive industry uses spherical robots for welding, painting, and handling. In body assembly, the robots weld the frame and the panels. In the paint shop, they apply primer, base coat, and clear coat. In the powertrain plant, they load and unload machines and handle parts. The long reach is useful in all these tasks, because the parts are large and the cycle times are short. The robots are often mounted on overhead rails or on pedestals to keep the floor clear. The controllers are integrated with the plant network, so the robots can be monitored and updated from a central computer. |
20.12.3 Construction and Heavy Equipment |
Construction and heavy equipment manufacturing involves large steel structures, such as excavator booms, crane sections, and bulldozer blades. A spherical robot can weld, cut, and grind these structures. The long reach lets it work on a part without moving it. The robot can be mounted on a positioner that turns the part, so the robot can work in the flat position, which is best for welding. This combination is common in job shops that build one-of-a-kind parts. The robot can also be used for painting and coating, which protects the structure from corrosion. |
20.12.4 Energy and Power Generation |
The energy industry uses spherical robots for welding and inspecting pipes, pressure vessels, and turbines. In a power plant, the robot can weld and inspect the boiler tubes, the steam lines, and the turbine casing. In a nuclear plant, the robot can work in radioactive areas, where humans cannot go. The long reach lets it reach into a confined space and perform the task. The robot must be reliable and easy to decontaminate. In the wind energy industry, the robot can weld and inspect the towers and the blades. The blades are large and curved, so the long reach of a spherical robot is a good match. |
20.12.5 Rail and Transportation |
Rail manufacturing uses spherical robots for welding and cutting. A rail car has a large steel frame, and the robot can weld the frame and the panels. The long reach lets it work on both sides of the car without moving it. The robot can also cut openings for doors and windows. In maintenance depots, the robot can repair and inspect the cars. The key requirement is flexibility, because the cars are not identical. The robot can be programmed to handle a variety of car types, and the teach pendant lets the operator adjust the path for each car. |
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20.13 Advantages and Limitations |
20.13.1 Advantages |
The main advantage of a spherical robot is its long reach. A single machine can cover a large workspace, which reduces the number of robots needed for a task. The linear axis gives a long stroke in a compact package, so the robot can reach into deep spaces. The base rotation gives a wide sweep, so the robot can work on both sides of a part. The geometry is simple, so the inverse kinematics are easy to solve, which makes the controller fast and reliable. The design is also robust, because the heavy parts are near the base and the arm is supported by strong bearings. Finally, spherical robots are often less expensive than articulated robots of similar reach, because they have fewer joints and a simpler structure. |
20.13.2 Limitations |
The main limitation is the shape of the working envelope. The spherical shell is not a box, so some points are hard to reach and others are unreachable. The robot cannot work close to its base, because the linear axis has a minimum extension. The robot cannot easily change orientation while keeping the tool on a straight line, because the base rotation and the elevation axis change the direction of the arm. The arm is a cantilever, so it deflects under load, which limits accuracy and payload. The payload capacity falls as the arm extends, so the robot cannot lift heavy parts at full reach. The linear axis adds mass and complexity, and the cables and hoses must be managed carefully. Finally, the spherical robot is less dexterous than an articulated robot, so it is not the best choice for complex assembly or for tasks that require many orientations. |
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20.14 Selection and Integration Guidelines |
20.14.1 When to Choose a Spherical Robot |
A spherical robot is a good choice when the task requires long reach and the part is large. It is also a good choice when the task is welding, cutting, coating, or simple handling, because these tasks do not require a high degree of dexterity. It is a good choice when the workspace is arranged around a central point, such as a turntable or a positioner. It is a good choice when the budget is limited, because spherical robots are often less expensive than articulated robots of similar reach. It is a good choice when the environment is harsh, because the simple design is easy to protect and to maintain. |
20.14.2 When to Choose Another Type |
An articulated robot is a better choice when the task requires dexterity, such as assembly, machine tending, or complex welding. A Cartesian robot is a better choice when the task requires a rectangular workspace, such as palletizing or large-part handling. A SCARA robot is a better choice when the task requires fast, horizontal motion, such as small-part assembly. A delta robot is a better choice when the task requires very fast pick-and-place. A specialized machine is a better choice when the task requires high stiffness, such as heavy machining. |
20.14.3 Integration Checklist |
When integrating a spherical robot, consider the following items. First, define the task and the required reach, payload, and accuracy. Second, lay out the workspace so that the critical tasks fall in the comfortable part of the envelope. Third, choose a mounting method that gives the robot a stable base and good access to the part. Fourth, plan the cable and hose routing so that they do not restrict the motion or wear out. Fifth, select a controller that can handle the path and the sensors. Sixth, train the operators and the programmers so they understand the envelope and the limitations. Seventh, plan for maintenance, including lubrication, belt tension, and calibration. Eighth, test the cell with a representative part before going into production. |
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20.15 Safety |
20.15.1 Hazards |
A spherical robot can be dangerous because it moves quickly and has a long reach. The main hazards are impact, crushing, and trapping. The arm can swing around the base and strike a person who is standing too close. The linear axis can extend suddenly and hit a person or a fixture. The wrist can rotate a sharp tool or a hot part. The robot can also throw sparks, hot metal, or paint mist. The controller can also fail, causing the robot to move unexpectedly. |
20.15.2 Safeguards |
The first safeguard is a fence or a light curtain that keeps people out of the workspace. The fence must be strong enough to stop the robot if it moves unexpectedly. The gate must have an interlock that stops the robot when the gate is open. The second safeguard is a safety-rated controller that monitors the robot speed and position. The controller can limit the speed in certain zones and can stop the robot if it detects a fault. The third safeguard is a teach pendant with an enabling device, so the robot only moves when the operator holds the device. The fourth safeguard is a clear floor marking and a sign that warns of the robot. The fifth safeguard is training, so the operators and the programmers know the hazards and the rules. The sixth safeguard is a risk assessment, which identifies the hazards and the required safeguards for the specific cell. |
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20.16 Maintenance and Reliability |
20.16.1 Routine Maintenance |
A spherical robot needs routine maintenance to stay reliable. The linear axis needs lubrication and inspection for wear. The rotational axes need lubrication and inspection for backlash. The belts and the drive screws need tension checks and replacement when worn. The cables and the hoses need inspection for cracks and wear. The wrist needs inspection for play and for seal damage. The controller needs a backup of the program and the parameters. The robot needs calibration to maintain accuracy. The maintenance schedule should be based on the manufacturer's recommendations and on the operating conditions. |
20.16.2 Common Failures |
Common failures include worn bearings, broken belts, loose connections, and damaged cables. The linear axis is often the first part to wear, because it moves the most and carries the most load. The wrist is often the second, because it is small and works hard. The controller can fail because of heat, dust, or voltage spikes. The motors can fail because of overload or because of a bad encoder. The gearboxes can fail because of lack of lubrication or because of a shock load. A good maintenance program can prevent many of these failures and can extend the life of the robot. |
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20.17 The Future of Spherical Robots |
20.17.1 Hybrid Designs |
Some manufacturers build hybrid robots that combine a spherical arm with an articulated wrist or a linear track. These designs keep the long reach of the spherical arm and add the dexterity of an articulated wrist. They can also add a track to extend the reach further. Hybrid designs are common in welding and in large-part handling, where the extra reach and the extra dexterity are both useful. The controller must handle the extra axes and must coordinate them with the arm. |
20.17.2 Advanced Sensors and Control |
Modern sensors, such as laser scanners, vision systems, and force sensors, make spherical robots more flexible. A vision system can find the part and correct the path. A force sensor can detect a collision and stop the robot. A laser scanner can measure the part and adjust the path in real time. These sensors reduce the need for precise fixtures and make the robot easier to program. Advanced control algorithms can plan paths that avoid singularities and that keep the tool in the stiff zone. They can also optimize the motion for speed and for energy use. |
20.17.3 New Applications |
New applications for spherical robots include additive manufacturing, where the robot deposits material to build a part. The long reach lets the robot build large parts, such as molds and dies. Another new application is composite layup, where the robot places fibers or tapes on a mold. The long reach lets it work on large parts, such as aircraft wings and wind turbine blades. Another new application is inspection and maintenance of large structures, such as bridges and buildings. The robot can carry a camera or a sensor and can reach areas that are hard to access. These applications show that the spherical robot is not obsolete. It is a specialized tool that remains useful when reach and simplicity matter more than dexterity. |
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20.18 Detailed Summary |
This chapter has examined spherical robots from their mechanical layout to their real-world applications. A spherical robot positions its wrist with two rotational axes and one linear axis. The base rotation sweeps the arm around a vertical axis. The elevation axis tilts the arm up and down. The linear axis extends the arm in and out. The result is a spherical working envelope with a long reach. The envelope is not a full sphere but a thick shell with limits at the top, bottom, inside, and outside. The shape gives the robot a long reach but also creates awkward zones and singularities. |
The mechanical layout is simple and robust. The heavy parts are near the base, and the arm is supported by strong bearings. The linear axis is often a telescoping boom or a sliding carriage. The wrist is usually a two-axis or three-axis unit. The design is easy to protect and to maintain, which makes it suitable for harsh environments. |
The control of a spherical robot is based on inverse kinematics. The controller converts the desired tool position into joint angles and extension length. The geometry is simple, so the conversion is fast and reliable. Most robots are programmed with a teach pendant, but offline programming and sensor integration are becoming more common. |
Historically, spherical robots were the first industrial robots. The Unimate and the Versatran used this layout. In the 1970s and 1980s, spherical robots dominated automotive welding. Later, articulated robots became more popular because they are more dexterous. Spherical robots remain in use in welding, coating, machining, and handling, especially where long reach is important. |
The applications of spherical robots are wide. In welding, they are used for automotive bodies, truck frames, ships, pipes, and pressure vessels. In machining, they are used for drilling, milling, grinding, deburring, and cutting. In coating, they are used for painting, powder coating, sealant dispensing, and thermal spraying. In handling, they are used for machine loading, palletizing, and foundry work. In assembly and inspection, they are used for large-part assembly, measurement, and non-destructive testing. In specialized industries, they are used in aerospace, automotive, construction, energy, and rail. |
The advantages of spherical robots are long reach, simple geometry, robust design, and low cost. The limitations are the shape of the envelope, the lack of dexterity, the deflection of the arm, and the falling payload at full reach. A spherical robot is a good choice when the task requires long reach and the part is large. It is a good choice for welding, cutting, coating, and simple handling. It is a good choice when the budget is limited and the environment is harsh. It is not a good choice for complex assembly or for tasks that require many orientations. |
Safety is a critical part of any spherical robot installation. The hazards are impact, crushing, trapping, and projectiles. The safeguards are fences, interlocks, safety-rated controllers, enabling devices, floor markings, training, and risk assessment. Maintenance is also critical. The linear axis, the wrist, the cables, and the controller need regular inspection and care. A good maintenance program can prevent failures and extend the life of the robot. |
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The future of spherical robots lies in hybrid designs, advanced sensors, and new applications. Hybrid designs add dexterity and reach. Advanced sensors make the robot more flexible and easier to program. New applications, such as additive manufacturing and composite layup, show that the spherical robot is still a useful tool. It is not the most common robot type, but it is a proven design that solves specific problems. For tasks that need long reach and simple motion, the spherical robot remains a practical and cost-effective choice. |
cs@easiersoft.com