Chapter 23: Repeatability |
1. Introduction and Chapter Summary |
Repeatability is one of the most important performance metrics for industrial robots. It describes how consistently a robot can return to the same programmed position over and over again. When a robot is commanded to move to a specific point in space, repeatability tells us how close the robot will actually come to that point on successive attempts. A typical mid-size articulated robot arm quotes repeatability between 0.02 and 0.1 millimeters. To put that in perspective, a human hair is roughly 0.07 millimeters wide. This means that a good industrial robot can return to a position with an error smaller than the width of a human hair. |
This chapter explains repeatability in plain language. It covers what repeatability means, why it matters, how it differs from accuracy, what factors influence it, and how it is measured. Above all, this chapter focuses on real-world applications across many industries. You will see how repeatability affects electronics assembly, automotive manufacturing, food processing, pharmaceuticals, aerospace, logistics, and more. By the end of this chapter, you should understand why repeatability is often the single most quoted performance number for an industrial robot, and why it is so critical to modern automated production. |
This chapter is organized into numbered sections for easy reading. Section 2 defines repeatability in simple terms. Section 3 explains the difference between repeatability and accuracy. Section 4 discusses how repeatability is measured. Section 5 covers the factors that influence repeatability. Section 6 looks at repeatability across different robot types. Section 7 through Section 18 present detailed application examples from many industries. Section 19 discusses the relationship between repeatability and other performance metrics. Section 20 covers common misunderstandings. Section 21 offers guidance on selecting a robot based on repeatability needs. Section 22 provides a detailed summary of the entire chapter. |
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2. What Is Repeatability |
Repeatability is the ability of a robot to return to the same position repeatedly. Imagine a robot arm that picks up a small part from a conveyor belt and places it into a fixture. The robot is taught a specific position for placing the part. Repeatability measures how much variation there is in the actual position where the robot places the part each time it runs the cycle. |
If a robot has a repeatability of 0.05 millimeters, it means that when the robot is commanded to go to the same point one hundred times, the actual positions will fall within a small sphere with a diameter of about 0.05 millimeters. In practice, repeatability is usually expressed as a plus or minus value. For example, plus or minus 0.05 millimeters means the robot will return to within 0.05 millimeters of the target position in any direction. |
Repeatability is not the same as precision in everyday language. In robotics, repeatability specifically refers to the consistency of returning to a taught position. It does not tell you whether the taught position itself is correct. That is a separate concept called accuracy, which we will discuss in the next section. |
Repeatability is often measured under specific conditions. These conditions include a constant payload, a constant speed, a constant temperature, and a constant approach direction. If any of these conditions change, the repeatability may change as well. Manufacturers usually quote repeatability based on ISO 9283, an international standard that defines how to measure robot performance. This standard ensures that different robots can be compared fairly. |
Repeatability is important because most industrial robots are used for tasks that require consistent positioning. Welding, dispensing, assembly, picking and placing, and many other tasks all depend on the robot going to the same spot every time. If the robot's repeatability is poor, the quality of the work will suffer. For example, if a robot is applying a bead of adhesive, poor repeatability could cause the bead to be placed slightly off the intended path, leading to a weak bond or a visible defect. |
It is also important to understand that repeatability is a statistical concept. It is usually expressed as a range or a standard deviation. A robot with a repeatability of plus or minus 0.1 millimeters will have most of its returns within that range, but occasionally a return might be slightly outside. The distribution of returns is typically bell-shaped, with most returns clustered near the center and fewer returns at the edges. This is why manufacturers often quote repeatability as a value that covers a certain percentage of returns, such as 99.7 percent or 99.99 percent. |
In summary, repeatability is the consistency with which a robot returns to a programmed position. It is a key performance metric that directly affects the quality and reliability of automated tasks. |
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3. Repeatability Versus Accuracy |
Many people use the words repeatability and accuracy interchangeably in everyday conversation. In robotics, however, they mean very different things. Understanding the difference is essential for anyone specifying or using an industrial robot. |
Accuracy is the ability of a robot to move to a commanded position in the workspace. If you tell the robot to go to a specific point in space, accuracy tells you how close the robot actually gets to that point. A robot with high accuracy will go very close to the commanded point. A robot with low accuracy might go several millimeters or even centimeters away from the commanded point. |
Repeatability, as we discussed, is the ability to return to the same position over and over. A robot with high repeatability will return to the same spot consistently, even if that spot is not exactly where it was commanded to go. |
To illustrate the difference, imagine a person throwing darts at a dartboard. If the person throws darts that all land close together in a tight cluster, but the cluster is in the wrong part of the board, that person has high repeatability but low accuracy. If the person throws darts that land all over the board but average out to the bullseye, that person has low repeatability but high accuracy. If the person throws darts that all land in a tight cluster on the bullseye, that person has both high repeatability and high accuracy. |
Most industrial robots have very high repeatability but relatively low accuracy. This might seem surprising, but it makes sense when you consider how robots are used. Most robots are taught a path by a human operator. The operator jogs the robot to the desired positions and records them. The robot then repeats those taught positions. The robot does not need to know where those positions are in absolute terms. It only needs to return to them consistently. This is why repeatability is more important than accuracy for most applications. |
However, there are applications where accuracy matters. For example, if a robot is programmed offline using a computer model, the robot needs to go to absolute positions in the workspace. In this case, accuracy is important. Offline programming is becoming more common, especially in automotive and aerospace industries. To improve accuracy, manufacturers can use calibration techniques. Calibration involves measuring the robot's actual positions and adjusting the control software to compensate for errors. After calibration, a robot's accuracy can approach its repeatability. |
It is also possible for a robot to have good accuracy but poor repeatability. This can happen if the robot's joints have backlash or if the control system is unstable. In practice, most industrial robots are designed to have excellent repeatability first, and then accuracy is improved through calibration if needed. |
For most users, repeatability is the more important specification. If a robot has good repeatability, it can be taught to do a task well. If a robot has poor repeatability, no amount of teaching will make it consistent. This is why repeatability is often the first specification that engineers look at when selecting a robot. |
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4. How Repeatability Is Measured |
Measuring repeatability requires careful procedures and precise equipment. The most common standard for measuring industrial robot performance is ISO 9283. This standard defines how to measure repeatability, accuracy, path accuracy, and other performance metrics. Following the standard ensures that measurements are consistent and comparable across different robots and manufacturers. |
To measure repeatability, the robot is commanded to move to a specific position in its workspace. This position is usually called the test position. The robot moves to this position many times, typically thirty or more times. Each time the robot arrives at the position, its actual location is measured using a precise external measurement device. Common measurement devices include laser trackers, coordinate measuring machines, and camera-based systems. These devices can measure positions with sub-micron precision. |
The robot is usually allowed to settle at the position before measurement. The measurement device records the actual position. After many repetitions, the data is analyzed. The repeatability is typically expressed as the radius of a sphere that contains a certain percentage of the measured points. For example, a repeatability of plus or minus 0.05 millimeters might mean that 99.7 percent of the measured points fall within a sphere of radius 0.05 millimeters centered on the average position. |
The measurement conditions must be carefully controlled. The payload must be constant. The speed and acceleration must be constant. The temperature must be stable. The approach direction must be the same each time. If any of these conditions change, the repeatability measurement may not be valid. |
There are different types of repeatability. Position repeatability refers to the ability to return to the same point in space. Orientation repeatability refers to the ability to return to the same orientation. Path repeatability refers to the ability to follow the same path repeatedly. Most manufacturers quote position repeatability, as it is the most commonly needed metric. |
It is important to note that repeatability can vary depending on where the robot is in its workspace. A robot might have excellent repeatability near the center of its workspace but poorer repeatability near the edges. This is because the robot's joints are at different angles, and the mechanical stiffness varies with joint angle. Manufacturers usually quote a single repeatability value that represents the worst-case performance across the workspace. This is a conservative approach that ensures the robot will meet the specification everywhere. |
Repeatability can also vary with speed. A robot moving slowly might have better repeatability than a robot moving quickly. This is because higher speeds cause more vibration and dynamic deflection. Manufacturers usually quote repeatability at a specific speed, often the maximum rated speed. Users should be aware that repeatability might be better at lower speeds. |
Finally, repeatability can change over time. Wear in the joints, changes in lubrication, and thermal expansion can all affect repeatability. Regular maintenance and calibration can help maintain repeatability over the life of the robot. |
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5. Factors That Influence Repeatability |
Many factors influence a robot's repeatability. Understanding these factors helps users get the best performance from their robots and helps them diagnose problems when repeatability degrades. |
The first factor is mechanical design. The stiffness of the robot's structure, the quality of the bearings, and the precision of the gearboxes all affect repeatability. A robot with a rigid structure and high-quality components will have better repeatability than a robot with a flexible structure and low-quality components. Harmonic drive gears, which are common in robot joints, offer high precision and low backlash. This is one reason why modern industrial robots have such good repeatability. |
The second factor is backlash. Backlash is the small amount of play in a mechanical system. If you push on the end of a robot arm, it might move slightly without the motor moving. This play is backlash. Backlash can cause the robot to arrive at a position from different directions and end up at slightly different points. Manufacturers minimize backlash by using precision gears and preloading bearings. |
The third factor is thermal expansion. As a robot operates, its motors and bearings generate heat. This heat causes the metal parts to expand. Thermal expansion can change the geometry of the robot and affect repeatability. Manufacturers often warm up robots before precision work to allow the temperature to stabilize. Some high-precision robots have temperature sensors and compensation algorithms to correct for thermal expansion. |
The fourth factor is vibration. When a robot moves quickly, it can vibrate. Vibration can cause the robot to overshoot or oscillate around the target position. This can reduce repeatability. Manufacturers reduce vibration by designing stiff structures, using advanced control algorithms, and tuning the robot's motion profiles. |
The fifth factor is payload. The weight and inertia of the payload affect how the robot moves. A heavy payload can cause the robot to deflect more, reducing repeatability. Manufacturers usually specify repeatability at a specific payload, often the maximum rated payload. Users should be aware that repeatability might be better with a lighter payload. |
The sixth factor is speed and acceleration. Higher speeds and accelerations cause more dynamic forces on the robot. These forces can cause deflection and vibration, reducing repeatability. Users who need the best possible repeatability should operate the robot at moderate speeds. |
The seventh factor is the control system. The robot's controller determines how the motors move. Advanced controllers with high-resolution encoders and fast update rates can achieve better repeatability. Controllers also implement algorithms for backlash compensation, thermal compensation, and vibration suppression. |
The eighth factor is calibration. Over time, a robot's calibration can drift. Regular calibration ensures that the robot's internal model matches its actual geometry. This helps maintain repeatability. Some robots have automatic calibration routines that can be run periodically. |
The ninth factor is maintenance. Worn bearings, degraded lubricant, and loose fasteners can all reduce repeatability. Regular maintenance, including lubrication and inspection, helps maintain repeatability over the life of the robot. |
The tenth factor is the environment. Temperature, humidity, and airborne contaminants can affect repeatability. For example, in a foundry, heat and dust can cause joints to wear faster. In a cleanroom, the robot must be designed to minimize particle generation. Users should choose a robot that is appropriate for their environment. |
By understanding these factors, users can take steps to maximize repeatability. This includes choosing the right robot, operating it within its specifications, maintaining it properly, and controlling the environment. |
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6. Repeatability Across Different Robot Types |
Different types of robots have different repeatability characteristics. The most common types of industrial robots are articulated robots, SCARA robots, delta robots, Cartesian robots, and collaborative robots. Each type has its own strengths and weaknesses when it comes to repeatability. |
Articulated robots are the most common type of industrial robot. They have a series of rotary joints that allow them to move in many directions. A typical six-axis articulated robot can reach almost any point in its workspace with almost any orientation. Articulated robots typically have repeatability between 0.02 and 0.1 millimeters for mid-size models. Larger articulated robots may have repeatability of 0.1 to 0.5 millimeters. Smaller articulated robots, sometimes called desktop robots, can have repeatability as good as 0.01 millimeters. Articulated robots are used in welding, assembly, material handling, and many other applications. |
SCARA robots have a horizontal arm that can move in the X-Y plane and a vertical axis for Z motion. SCARA stands for Selective Compliance Assembly Robot Arm. SCARA robots are very fast and are often used for assembly and pick-and-place tasks. They typically have repeatability between 0.01 and 0.05 millimeters. Because they have fewer joints than articulated robots, they can be very stiff and precise. SCARA robots are common in electronics assembly. |
Delta robots have a parallel structure with three arms connected to a common base. They are extremely fast and are often used for picking and placing small items. Delta robots typically have repeatability between 0.05 and 0.1 millimeters. Their speed makes them ideal for food packaging, pharmaceutical sorting, and electronics assembly. |
Cartesian robots, also called gantry robots, move in straight lines along linear axes. They are often used for tasks that require long linear motions, such as 3D printing, CNC machining, and large-scale assembly. Cartesian robots can have very good repeatability, often between 0.01 and 0.05 millimeters. Because they use linear bearings and rigid structures, they can be very precise. |
Collaborative robots, or cobots, are designed to work alongside humans. They are typically smaller and lighter than traditional industrial robots. Cobots often have repeatability between 0.02 and 0.1 millimeters. While they are not always as fast or as stiff as traditional robots, they are often precise enough for many assembly and inspection tasks. |
It is important to note that repeatability specifications are not always directly comparable between robot types. A delta robot with a repeatability of 0.1 millimeters might be perfectly adequate for picking and placing small parts, while an articulated robot with the same repeatability might not be suitable for a precision assembly task. The required repeatability depends on the application. |
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7. Application Example: Electronics Assembly |
The electronics industry is one of the largest users of industrial robots. Electronics assembly requires very high precision because the components are very small and the tolerances are very tight. Repeatability is critical in this industry. |
Consider the assembly of a smartphone. A smartphone contains hundreds of tiny components, including resistors, capacitors, integrated circuits, and connectors. Many of these components are placed on a printed circuit board by high-speed pick-and-place machines. These machines are essentially specialized robots. They must place components with repeatability of 0.01 to 0.05 millimeters. If a component is placed too far from its intended position, it might not make proper electrical contact, or it might interfere with a neighboring component. |
Surface mount technology, or SMT, is the most common method for assembling printed circuit boards. In SMT, components are placed on a board that has been coated with solder paste. The board is then heated in a reflow oven, which melts the solder and forms electrical connections. The placement of the components must be very precise. A typical SMT machine can place tens of thousands of components per hour with repeatability of 0.02 millimeters or better. |
Robots are also used for other electronics assembly tasks. For example, robots are used to apply underfill, a glue that is applied around the edges of a chip to strengthen the connection between the chip and the board. The underfill must be applied precisely, or it might flow into unwanted areas. Robots with repeatability of 0.05 millimeters are often used for this task. |
Another example is the assembly of connectors. Many electronic devices have connectors that must be inserted with precise force and alignment. A robot with good repeatability can insert the connector without damaging the pins. Force sensors are often used in combination with position control to ensure that the connector is inserted correctly. |
Robots are also used for testing electronic devices. A robot might pick up a device and place it into a test fixture. The robot must place the device precisely so that the test probes make good contact. Repeatability of 0.05 millimeters is often sufficient for this task. |
In the electronics industry, repeatability is not just about position. It is also about consistency. A robot that places components with high repeatability will produce boards with consistent quality. This reduces defects and improves yield. In a high-volume production line, even a small improvement in repeatability can lead to significant cost savings. |
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8. Application Example: Automotive Manufacturing |
The automotive industry is one of the oldest and largest users of industrial robots. Robots are used in automotive manufacturing for welding, painting, assembly, and material handling. Repeatability is critical in all of these applications. |
Welding is one of the most common applications of robots in the automotive industry. Spot welding is used to join sheet metal parts together. A typical car body contains thousands of spot welds. Each weld must be placed precisely to ensure the strength and integrity of the body. A robot with repeatability of 0.1 millimeters can place spot welds with high consistency. If the repeatability is poor, the welds might be placed too far from the intended location, leading to weak joints or visible defects. |
Arc welding is another common application. Arc welding is used to join thicker metal parts. The robot must move the welding torch along a precise path while maintaining the correct angle and speed. Repeatability of 0.1 to 0.2 millimeters is often sufficient for arc welding. However, the robot must also be able to maintain a consistent path. This requires good path repeatability, not just position repeatability. |
Painting is another important application. Robots are used to paint car bodies with a uniform coat of paint. The robot must move the paint sprayer along a precise path while maintaining the correct distance and angle. Repeatability of 0.5 millimeters is often sufficient for painting, because the paint spray pattern is relatively wide. However, the robot must be able to repeat the path consistently to ensure a uniform finish. |
Assembly is another major application. Robots are used to install windshields, seats, engines, and many other components. For example, a robot might pick up a windshield and place it into the car body. The windshield must be placed precisely to ensure a proper seal. Repeatability of 0.1 to 0.2 millimeters is often required for this task. |
Material handling is also common in automotive manufacturing. Robots move parts from one conveyor to another, load and unload machines, and stack finished products. Repeatability of 0.5 millimeters is often sufficient for these tasks. |
In the automotive industry, repeatability is important for quality and consistency. A car is made of thousands of parts, and each part must fit together properly. If a robot places a part slightly off, it can cause problems downstream. For example, if a robot places a door slightly off, the door might not close properly, or it might leak water. This is why automotive manufacturers demand high repeatability from their robots. |
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9. Application Example: Food and Beverage Processing |
The food and beverage industry uses robots for picking, packing, palletizing, and processing. Repeatability is important in these applications, but the requirements are often different from those in electronics or automotive manufacturing. |
In food picking and packing, robots are used to pick up items such as cookies, candies, fruits, and vegetables and place them into packages. The items are often irregular in shape and size, so the robot must be able to adapt. Vision systems are often used to locate the items, and the robot must move to the correct position to pick them up. Repeatability of 0.1 to 0.5 millimeters is often sufficient for these tasks. However, the robot must be fast, because food production lines often run at high speeds. |
In palletizing, robots are used to stack boxes or bags onto pallets. The robot must place each item precisely so that the stack is stable. Repeatability of 0.5 to 1 millimeter is often sufficient for palletizing. However, the robot must be able to handle heavy payloads and work in harsh environments. |
In food processing, robots are used for tasks such as cutting, slicing, and decorating. For example, a robot might be used to cut a cake into precise slices. The robot must follow a precise path to ensure that the slices are uniform. Repeatability of 0.1 to 0.2 millimeters is often required for these tasks. |
One of the challenges in food processing is that the environment is often wet and cold. Robots used in food processing must be designed to withstand washdown and sanitization. They must also use food-grade lubricants. Repeatability can be affected by temperature changes and by the presence of water or steam. |
Another challenge is that food items are often delicate. The robot must handle them gently. Force sensors and soft grippers are often used. The robot's repeatability must be good enough to place the item without crushing it. |
In the beverage industry, robots are used for packing bottles and cans into cases and stacking cases onto pallets. Repeatability of 0.5 millimeters is often sufficient for these tasks. However, the robot must be able to keep up with high-speed production lines. |
In summary, food and beverage processing requires robots with good repeatability, but the requirements are often less stringent than in electronics or automotive manufacturing. The focus is often on speed, hygiene, and reliability rather than sub-millimeter precision. |
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10. Application Example: Pharmaceutical and Medical Device Manufacturing |
The pharmaceutical and medical device industries require very high levels of precision and cleanliness. Robots are used in these industries for tasks such as dispensing, assembly, inspection, and packaging. Repeatability is critical because the products are often expensive and the consequences of errors can be severe. |
In pharmaceutical manufacturing, robots are used to dispense precise amounts of liquid into vials or wells. For example, a robot might be used to fill a 96-well plate with a reagent. The robot must move the dispensing needle to each well and dispense the correct volume. Repeatability of 0.05 to 0.1 millimeters is often required to ensure that the needle is positioned correctly over each well. If the needle is misaligned, it might hit the side of the well or dispense the liquid outside the well. |
Robots are also used in pharmaceutical packaging. They might pick up vials, syringes, or blister packs and place them into boxes. Repeatability of 0.1 to 0.2 millimeters is often sufficient for these tasks. However, the robot must operate in a cleanroom environment, where particle generation must be minimized. This requires special robots with sealed joints and low-particle lubricants. |
In medical device manufacturing, robots are used to assemble devices such as catheters, syringes, and implants. These devices often have very tight tolerances. For example, a robot might be used to assemble a syringe by inserting the plunger into the barrel. The plunger must be inserted straight and to the correct depth. Repeatability of 0.02 to 0.05 millimeters is often required for these tasks. |
Robots are also used for inspection in the medical device industry. A robot might pick up a device and place it under a microscope or camera for inspection. The robot must place the device precisely so that the inspection system can see the features clearly. Repeatability of 0.05 millimeters is often sufficient for this task. |
One of the challenges in pharmaceutical and medical device manufacturing is validation. Regulatory agencies such as the FDA require that processes be validated to ensure that they consistently produce products that meet specifications. Repeatability is a key part of validation. Manufacturers must demonstrate that their robots can consistently perform the required tasks. This often involves extensive testing and documentation. |
In summary, the pharmaceutical and medical device industries require robots with excellent repeatability, often in the range of 0.02 to 0.1 millimeters. The robots must also meet strict cleanliness and validation requirements. |
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11. Application Example: Aerospace and Defense |
The aerospace and defense industries require extremely high precision and reliability. Robots are used in these industries for tasks such as drilling, riveting, composite layup, and inspection. Repeatability is critical because the parts are often large, expensive, and safety-critical. |
In aircraft assembly, robots are used to drill holes in the fuselage and wings. These holes must be drilled with very high precision to ensure that the rivets fit properly. A typical aircraft might have hundreds of thousands of holes. Each hole must be positioned within a tight tolerance. Repeatability of 0.1 to 0.2 millimeters is often required for drilling. However, the robot must also be accurate, because the holes are located based on a CAD model. This requires calibration and often external measurement systems to correct for positioning errors. |
Robots are also used for riveting. After a hole is drilled, a rivet is inserted and formed. The robot must place the rivet precisely and apply the correct force. Repeatability of 0.1 millimeters is often required for riveting. |
Composite layup is another important application. Composite materials, such as carbon fiber, are used in modern aircraft. These materials are laid up in layers and then cured. Robots are used to place the layers precisely. Repeatability of 0.5 to 1 millimeter is often sufficient for composite layup, because the layers are relatively large. However, the robot must be able to handle large sheets of material and follow complex contours. |
Inspection is another key application. Robots are used to move inspection sensors, such as ultrasonic probes or cameras, over the surface of a part. The robot must follow a precise path to ensure that the entire surface is inspected. Repeatability of 0.2 to 0.5 millimeters is often sufficient for inspection. |
In the defense industry, robots are used for tasks such as assembling missiles, loading ammunition, and handling hazardous materials. These tasks often require high repeatability and reliability. Robots used in defense applications must also be rugged and able to withstand harsh environments. |
One of the challenges in aerospace and defense is that the parts are often very large. A robot might need to reach several meters. Large robots typically have lower repeatability than small robots. For example, a large gantry robot might have repeatability of 0.5 millimeters or more. To achieve better precision, manufacturers often use external measurement systems, such as laser trackers, to correct the robot's position in real time. This is called closed-loop control. |
In summary, the aerospace and defense industries require robots with good repeatability, often in the range of 0.1 to 0.5 millimeters. For critical tasks, external measurement systems are used to improve accuracy and repeatability. |
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12. Application Example: Logistics and Warehousing |
The logistics and warehousing industry has seen rapid growth in the use of robots. Robots are used for picking, sorting, palletizing, and transporting goods. Repeatability is important in these applications, but the requirements are often less stringent than in manufacturing. |
In e-commerce fulfillment centers, robots are used to pick items from shelves and place them into bins or boxes. The items are often stored in random locations, so the robot must use vision systems to locate them. The robot must then move to the correct position to pick the item. Repeatability of 0.5 to 1 millimeter is often sufficient for picking. However, the robot must be fast, because fulfillment centers often handle thousands of orders per hour. |
Robots are also used for sorting packages. A robot might pick up a package from a conveyor and place it onto the correct chute based on its destination. Repeatability of 1 millimeter is often sufficient for sorting. The robot must be able to handle a wide range of package sizes and weights. |
Palletizing is another common application in logistics. Robots are used to stack boxes onto pallets. The boxes must be placed precisely so that the stack is stable. Repeatability of 0.5 to 1 millimeter is often sufficient for palletizing. However, the robot must be able to handle heavy payloads and work at high speeds. |
Autonomous mobile robots, or AMRs, are also used in logistics. These robots move around the warehouse to transport goods. AMRs do not have arms, so repeatability in the traditional sense does not apply. However, the navigation system must be repeatable to ensure that the robot follows the same path and stops at the same locations. Repeatability of a few centimeters is often sufficient for AMR navigation. |
In logistics and warehousing, repeatability is important for efficiency and reliability. A robot with good repeatability will be able to pick and place items consistently, reducing errors and improving throughput. However, the requirements are often less stringent than in manufacturing, because the items being handled are often larger and the tolerances are looser. |
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13. Application Example: Metal Fabrication and Machining |
The metal fabrication and machining industries use robots for tasks such as welding, cutting, grinding, and machine tending. Repeatability is important in these applications because the robot must follow precise paths and position parts accurately. |
In welding, as discussed earlier, repeatability of 0.1 to 0.2 millimeters is often required. The robot must follow the weld seam precisely to ensure a strong joint. If the repeatability is poor, the weld might be offset, leading to a weak joint or a visible defect. |
In cutting, robots are used to cut metal sheets, pipes, and profiles. The robot must follow a precise path to ensure that the cut is accurate. Repeatability of 0.1 to 0.5 millimeters is often required for cutting. The robot must also be able to maintain the correct cutting speed and standoff distance. |
In grinding, robots are used to remove weld spatter, smooth surfaces, and deburr edges. The robot must apply the correct force and follow the surface contour. Repeatability of 0.1 to 0.5 millimeters is often required for grinding. Force sensors are often used to maintain consistent contact with the workpiece. |
In machine tending, robots are used to load and unload parts from machines such as lathes, milling machines, and presses. The robot must place the part precisely into the chuck or fixture. Repeatability of 0.05 to 0.1 millimeters is often required for machine tending. If the part is not placed correctly, it might be damaged or the machine might be damaged. |
One of the challenges in metal fabrication is that the environment is often harsh. There may be metal chips, coolant mist, and high temperatures. Robots used in these environments must be protected. Repeatability can be affected by thermal expansion and by the presence of contaminants. |
In summary, metal fabrication and machining require robots with good repeatability, often in the range of 0.05 to 0.5 millimeters. The robots must also be rugged and able to withstand harsh environments. |
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14. Application Example: Plastics and Rubber Industry |
The plastics and rubber industry uses robots for tasks such as injection molding, blow molding, and assembly. Repeatability is important in these applications because the robot must handle hot, soft, or sticky materials. |
In injection molding, robots are used to remove parts from the mold. The robot must reach into the mold, grab the part, and pull it out. The robot must move precisely to avoid hitting the mold. Repeatability of 0.1 to 0.5 millimeters is often required for this task. The robot must also be fast, because the mold cycle time is often short. |
Robots are also used for post-processing operations, such as trimming, degating, and assembly. For example, a robot might pick up a molded part and place it into a trimming fixture. The robot must place the part precisely so that the trim is accurate. Repeatability of 0.1 to 0.2 millimeters is often required for these tasks. |
In blow molding, robots are used to remove the finished part from the mold and place it onto a conveyor. The part is often large and hot. The robot must handle it carefully. Repeatability of 0.5 to 1 millimeter is often sufficient for this task. |
In rubber processing, robots are used to handle rubber compounds, which are often sticky and difficult to handle. The robot must place the compound precisely into the mold. Repeatability of 0.5 to 1 millimeter is often sufficient for this task. |
One of the challenges in the plastics and rubber industry is that the materials are often hot. The robot must be able to withstand high temperatures. Repeatability can be affected by thermal expansion. Robots used in these applications often have special heat-resistant components. |
In summary, the plastics and rubber industry requires robots with good repeatability, often in the range of 0.1 to 1 millimeter. The robots must also be fast and able to withstand high temperatures. |
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15. Application Example: Foundry and Die Casting |
The foundry and die casting industries use robots for tasks such as pouring, ladling, and extracting parts from dies. Repeatability is important in these applications because the robot must handle molten metal and hot parts. |
In die casting, robots are used to extract the cast part from the die. The robot must reach into the die, grab the part, and pull it out. The robot must move precisely to avoid hitting the die. Repeatability of 0.5 to 1 millimeter is often required for this task. The robot must also be able to withstand high temperatures and lubricant spray. |
Robots are also used for ladling molten metal. The robot must dip a ladle into the furnace and pour the metal into the die. The robot must move precisely to ensure that the correct amount of metal is poured. Repeatability of 1 to 2 millimeters is often sufficient for ladling. However, the robot must be able to withstand the extreme heat of the molten metal. |
In foundries, robots are used for tasks such as sand core placement, mold handling, and shot blasting. Repeatability of 1 to 2 millimeters is often sufficient for these tasks. The environment is very harsh, with dust, heat, and vibration. Robots used in foundries must be specially protected. |
One of the challenges in foundry and die casting is that the environment is extremely harsh. The robots must be able to withstand high temperatures, dust, and molten metal splashes. Repeatability can be affected by thermal expansion and by the presence of contaminants. Regular maintenance is essential to maintain repeatability. |
In summary, the foundry and die casting industries require robots with moderate repeatability, often in the range of 0.5 to 2 millimeters. The robots must be rugged and able to withstand extreme environments. |
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16. Application Example: Additive Manufacturing and 3D Printing |
Additive manufacturing, also known as 3D printing, is a rapidly growing industry. Robots are used in additive manufacturing for tasks such as material deposition, post-processing, and inspection. Repeatability is important in these applications because the robot must follow precise paths to build up layers of material. |
In large-scale additive manufacturing, robots are used to deposit material such as plastic, concrete, or metal. The robot must follow a precise path to build up the part layer by layer. Repeatability of 0.5 to 1 millimeter is often sufficient for large-scale additive manufacturing, because the layers are relatively thick. However, the robot must be able to move smoothly and maintain a consistent speed. |
In metal additive manufacturing, robots are used to deposit metal powder or wire. The robot must follow a precise path to build up the part. Repeatability of 0.1 to 0.5 millimeters is often required for metal additive manufacturing, because the layers are thinner and the tolerances are tighter. |
Robots are also used for post-processing of additively manufactured parts. For example, a robot might be used to remove support structures or to machine the surface of the part. Repeatability of 0.1 to 0.5 millimeters is often required for these tasks. |
Inspection is another important application. Robots are used to move inspection sensors, such as cameras or laser scanners, over the surface of the part. The robot must follow a precise path to ensure that the entire surface is inspected. Repeatability of 0.5 to 1 millimeter is often sufficient for inspection. |
One of the challenges in additive manufacturing is that the process is often slow. The robot must be able to move smoothly and consistently for long periods of time. Repeatability can be affected by thermal expansion and by wear in the robot's joints. |
In summary, additive manufacturing requires robots with good repeatability, often in the range of 0.1 to 1 millimeter. The robots must be able to move smoothly and consistently for long periods of time. |
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17. Application Example: Agricultural Robotics |
Agricultural robotics is an emerging field. Robots are used in agriculture for tasks such as planting, harvesting, spraying, and sorting. Repeatability is important in these applications because the robot must handle delicate plants and fruits. |
In harvesting, robots are used to pick fruits and vegetables. The robot must locate the fruit, move to the correct position, and pick it without damaging it. Repeatability of 1 to 5 millimeters is often sufficient for harvesting, because the fruits are relatively large and the robot can use vision systems to adjust its position. However, the robot must be gentle and fast. |
In planting, robots are used to place seeds or seedlings into the soil. The robot must place them at the correct depth and spacing. Repeatability of 5 to 10 millimeters is often sufficient for planting. However, the robot must be able to work in outdoor environments, where the terrain may be uneven. |
In spraying, robots are used to apply pesticides or fertilizers to crops. The robot must follow a precise path to ensure that the entire crop is covered. Repeatability of 10 to 50 millimeters is often sufficient for spraying. However, the robot must be able to work in windy conditions and avoid obstacles. |
In sorting, robots are used to sort fruits and vegetables by size, color, and quality. The robot must pick up each item and place it into the correct bin. Repeatability of 1 to 5 millimeters is often sufficient for sorting. |
One of the challenges in agricultural robotics is that the environment is unstructured and variable. The robot must be able to adapt to different conditions. Repeatability can be affected by uneven terrain, weather, and lighting. Agricultural robots often use advanced sensors and artificial intelligence to cope with these challenges. |
In summary, agricultural robotics requires robots with moderate repeatability, often in the range of 1 to 50 millimeters. The robots must be rugged and able to work in outdoor environments. |
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18. Application Example: Research and Laboratory Automation |
Research laboratories and laboratories in the pharmaceutical and biotechnology industries use robots for tasks such as sample handling, liquid dispensing, and analysis. Repeatability is critical in these applications because the samples are often small and the results must be reliable. |
In sample handling, robots are used to move vials, tubes, and plates from one location to another. The robot must place the sample precisely so that it can be picked up by another robot or analyzed by an instrument. Repeatability of 0.1 to 0.5 millimeters is often required for sample handling. |
In liquid dispensing, robots are used to dispense precise amounts of liquid into wells or vials. The robot must move the dispensing needle to each well and dispense the correct volume. Repeatability of 0.05 to 0.1 millimeters is often required for liquid dispensing. If the needle is misaligned, it might hit the side of the well or dispense the liquid outside the well. |
Robots are also used for analysis. For example, a robot might pick up a sample and place it into a spectrometer or a microscope. The robot must place the sample precisely so that the instrument can analyze it correctly. Repeatability of 0.05 to 0.1 millimeters is often required for analysis. |
One of the challenges in laboratory automation is that the samples are often very small and delicate. The robot must handle them gently. Force sensors and soft grippers are often used. The robot must also be able to work in a clean environment, because contamination can affect the results. |
In summary, research and laboratory automation requires robots with excellent repeatability, often in the range of 0.05 to 0.5 millimeters. The robots must be precise, gentle, and clean. |
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19. Repeatability and Other Performance Metrics |
Repeatability is not the only performance metric for industrial robots. Other important metrics include accuracy, path accuracy, speed, payload, and reach. Understanding how repeatability relates to these other metrics helps users make informed decisions. |
Accuracy, as discussed earlier, is the ability to go to a commanded position. Repeatability is the ability to return to the same position. A robot can have high repeatability but low accuracy, or high accuracy but low repeatability. For most applications, repeatability is more important, because robots are usually taught rather than programmed offline. |
Path accuracy is the ability to follow a commanded path. This is important for tasks such as welding, cutting, and dispensing. A robot with good path accuracy will follow the path closely. A robot with good repeatability but poor path accuracy might return to the same points but deviate between them. Path accuracy is often specified separately from position repeatability. |
Speed is the ability to move quickly. Speed and repeatability are often in tension. Higher speeds can cause vibration and deflection, reducing repeatability. Users who need both speed and repeatability must choose a robot with a rigid structure and advanced control algorithms. |
Payload is the weight the robot can carry. Payload and repeatability are also in tension. A heavier payload can cause deflection, reducing repeatability. Manufacturers usually specify repeatability at a specific payload. Users should be aware that repeatability might be better with a lighter payload. |
Reach is the size of the robot's workspace. Reach and repeatability are often in tension. A larger robot has longer arms, which are more flexible and more prone to deflection. This is why large robots typically have lower repeatability than small robots. Users who need both long reach and high repeatability must choose a robot with a very rigid structure or use external measurement systems. |
In summary, repeatability is one of several important performance metrics. Users should consider all of these metrics when selecting a robot. The relative importance of each metric depends on the application. |
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20. Common Misunderstandings About Repeatability |
There are several common misunderstandings about repeatability. Clearing up these misunderstandings helps users make better decisions. |
The first misunderstanding is that repeatability and accuracy are the same thing. As discussed earlier, they are different. Repeatability is about consistency, while accuracy is about correctness. A robot can have one without the other. |
The second misunderstanding is that repeatability is a fixed number. In reality, repeatability can vary depending on the position in the workspace, the speed, the payload, and the environment. Manufacturers usually quote a worst-case value, but users should be aware that the actual repeatability might be better or worse depending on the conditions. |
The third misunderstanding is that repeatability is the only thing that matters. While repeatability is important, other factors such as speed, payload, and reliability also matter. A robot with excellent repeatability but poor reliability is not useful in production. |
The fourth misunderstanding is that a robot with good repeatability will always produce good parts. Repeatability is necessary but not sufficient. The robot must also be programmed correctly, the fixtures must be accurate, and the process must be stable. A robot with good repeatability can still produce bad parts if the other elements of the process are not correct. |
The fifth misunderstanding is that repeatability is permanent. Repeatability can degrade over time due to wear, thermal effects, and other factors. Regular maintenance and calibration are needed to maintain repeatability. |
The sixth misunderstanding is that repeatability is the same for all directions. In reality, repeatability can be different in different directions. For example, a robot might have better repeatability in the vertical direction than in the horizontal direction. This is because the robot's structure and joints respond differently to forces in different directions. |
The seventh misunderstanding is that repeatability can be improved simply by slowing down the robot. While slower speeds can reduce vibration and improve repeatability, there are limits. If the robot has mechanical backlash or thermal drift, slowing down will not help. The root cause of the repeatability problem must be addressed. |
By understanding these misunderstandings, users can avoid common pitfalls and get the best performance from their robots. |
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21. Selecting a Robot Based on Repeatability Needs |
When selecting a robot, repeatability is one of the first specifications to consider. The required repeatability depends on the application. Here are some general guidelines. |
For high-precision applications such as electronics assembly, medical device assembly, and laboratory automation, repeatability of 0.02 to 0.1 millimeters is often required. Robots in this range include small articulated robots, SCARA robots, and delta robots. |
For medium-precision applications such as automotive assembly, metal fabrication, and machine tending, repeatability of 0.1 to 0.5 millimeters is often sufficient. Robots in this range include mid-size articulated robots and Cartesian robots. |
For lower-precision applications such as palletizing, packaging, and material handling, repeatability of 0.5 to 2 millimeters is often sufficient. Robots in this range include large articulated robots and Cartesian robots. |
For outdoor and unstructured applications such as agriculture and construction, repeatability of 1 to 50 millimeters may be sufficient. Robots in this range include mobile robots and ruggedized articulated robots. |
In addition to the repeatability specification, users should consider the conditions under which the repeatability is quoted. Is it quoted at full payload or at a lighter payloadIs it quoted at full speed or at a moderate speedIs it quoted at room temperature or at an elevated temperatureUsers should choose a robot whose repeatability specification matches their actual operating conditions. |
Users should also consider the robot's accuracy if they plan to use offline programming. If offline programming is used, the robot will need to be calibrated, and the accuracy should be specified. Some manufacturers offer calibrated robots with accuracy approaching their repeatability. |
Finally, users should consider the robot's reliability and maintenance requirements. A robot with excellent repeatability but high maintenance requirements may not be the best choice for a production environment. Users should choose a robot from a reputable manufacturer with a proven track record. |
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22. Detailed Summary of Chapter 23 |
Repeatability is a measure of how consistently a robot returns to a programmed position. It is typically quoted between 0.02 and 0.1 millimeters for mid-size articulated arms. This means that the robot can return to a position with an error smaller than the width of a human hair. Repeatability is one of the most important performance metrics for industrial robots because most robot applications depend on the robot going to the same spot every time. |
Repeatability is different from accuracy. Accuracy is the ability to go to a commanded position, while repeatability is the ability to return to the same position. Most industrial robots have high repeatability but lower accuracy. This is because robots are usually taught by a human operator rather than programmed offline. Repeatability is more important than accuracy for most applications. |
Repeatability is measured using standards such as ISO 9283. The robot is commanded to move to a test position many times, and the actual positions are measured with a precise external device. The repeatability is expressed as the radius of a sphere that contains a certain percentage of the measured points. Measurement conditions such as payload, speed, and temperature must be carefully controlled. |
Many factors influence repeatability. These include mechanical design, backlash, thermal expansion, vibration, payload, speed, control system, calibration, maintenance, and environment. Understanding these factors helps users maximize repeatability and diagnose problems when it degrades. |
Different types of robots have different repeatability characteristics. Articulated robots typically have repeatability between 0.02 and 0.1 millimeters for mid-size models. SCARA robots can have repeatability as good as 0.01 millimeters. Delta robots typically have repeatability between 0.05 and 0.1 millimeters. Cartesian robots can have repeatability between 0.01 and 0.05 millimeters. Collaborative robots often have repeatability between 0.02 and 0.1 millimeters. |
Repeatability is critical in many industries. In electronics assembly, repeatability of 0.01 to 0.05 millimeters is often required to place tiny components on printed circuit boards. In automotive manufacturing, repeatability of 0.1 to 0.2 millimeters is often required for welding and assembly. In food and beverage processing, repeatability of 0.1 to 1 millimeter is often sufficient for picking, packing, and palletizing. In pharmaceutical and medical device manufacturing, repeatability of 0.02 to 0.1 millimeters is often required for dispensing and assembly. In aerospace and defense, repeatability of 0.1 to 0.5 millimeters is often required for drilling and riveting. In logistics and warehousing, repeatability of 0.5 to 1 millimeter is often sufficient for picking and sorting. In metal fabrication and machining, repeatability of 0.05 to 0.5 millimeters is often required for welding, cutting, and machine tending. In plastics and rubber processing, repeatability of 0.1 to 1 millimeter is often sufficient for injection molding and post-processing. In foundry and die casting, repeatability of 0.5 to 2 millimeters is often sufficient for extracting parts and ladling. In additive manufacturing, repeatability of 0.1 to 1 millimeter is often sufficient for material deposition and inspection. In agricultural robotics, repeatability of 1 to 50 millimeters is often sufficient for harvesting and spraying. In research and laboratory automation, repeatability of 0.05 to 0.5 millimeters is often required for sample handling and liquid dispensing. |
Repeatability is related to other performance metrics such as accuracy, path accuracy, speed, payload, and reach. Users should consider all of these metrics when selecting a robot. The relative importance of each metric depends on the application. |
There are several common misunderstandings about repeatability. It is not the same as accuracy. It is not a fixed number. It is not the only thing that matters. It does not guarantee good parts. It is not permanent. It can vary with direction. It cannot always be improved by slowing down. |
When selecting a robot based on repeatability needs, users should match the robot's repeatability specification to the requirements of the application. They should also consider the conditions under which the repeatability is quoted and the robot's accuracy, reliability, and maintenance requirements. |
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In conclusion, repeatability is a fundamental performance metric for industrial robots. It defines how consistently the robot returns to a programmed position. It is critical for quality, reliability, and efficiency in automated production. By understanding repeatability and its influencing factors, users can choose the right robot for their application and get the best performance from it. As robots become more capable and more widely used, repeatability will remain a key specification that engineers and managers rely on to ensure successful automation. |
cs@easiersoft.com