The What, Why and How of Delta Robots
Isaac Maw posted on March 15, 2018 | | 10175 views

Biomimicry is the strategy of modeling designs and structures in technology after nature. Building a two-armed assembly robot to have wrist, elbow and shoulder joints, like a human, is a good example of biomimicry. In the design of robotic systems, engineers often look to the musculoskeletal structures of animals and humans to develop their designs. After all, if you are designing a robot to perform tasks usually done by human arms and hands, robotic arms and hands are a good starting point.

The design of this collaborative assembly robot is inspired by human physiology.(Image courtesy of ABB.)
The design of this collaborative assembly robot is inspired by human physiology.(Image courtesy of ABB.)

What makes the delta robot fascinating is that it represents a complete departure from the constraints of biomimicry. It’s an efficient, optimized machine, inspired not by nature, but by pure mathematics and geometry.

I spoke with experts from ABB, FANUC, Kawasaki Robotics, Omron and Yaskawa Motoman to learn the nuts and bolts of these fascinating robots, from best use cases to most common failure modes. I also spoke with their inventor, Dr. Reymond Clavel, for his unique perspective on the machines' early development.

Read on for a comprehensive guide to delta robots.

The Invention of the Delta Robot

Professor Reymond Clavel, inventor of the delta robot.

Professor Reymond Clavel, inventor of the delta robot.

Today, delta robots are well established in the automation industry. Unlike larger articulated arms, delta robots are often kept as a stock item at many manufacturers such as Yaskawa Motoman and ABB. But in 1985, few robots could perform light pick-and-place tasks quickly or repeatably.

Reymond Clavel and his team at the Robotics Systems Laboratory at Ecole Polytechnique Federale de Lausanne (EPFL) began the research that would produce the delta robot following a visit to a chocolate factory. Clavel’s team was looking for repetitive labor applications for robots, and they found that the packaging of chocolate pralines was a candidate for this type of high-speed, low-payload automation.

Clavel’s team began by setting constraints on their design. First, the robot must perform at a rate of 3 picks per second. In order to place the chocolates correctly, the mobility of the robot required 4 degrees of freedom: translations along 3 axes, as well as rotation about the vertical axis. In order to achieve a high rate of work, Clavel added two more constraints to the design: the actuators of the robot would be fixed on the frame, and the moving part of the robot would be kept as light as possible.

The first prototype of the delta robot (January 1986)

The first prototype of the delta robot (January 1986)

Six months after the visit to the chocolate factory, a prototype of the delta robot was complete; by December, a patent was filed. Two years later, the delta robot was industrialized by a small company called Demaurex Robotics and Microtechnology.

While engineers and industry players were impressed with the innovative design, first reactions to the new robot were tepid. Clavel recalled, “They would not take the first step and risk their reputation with a robot similar to an umbrella.” As with any new technology, potential customers demanded to know how the product would facilitate their operations. What’s the ROI?

Thirty-two years later, industrial professionals are no longer dubious of these umbrella-shaped robots, which have become unparalleled in pick-and-place, sorting, and other high-speed, low-mass applications. However, they’re still asking about how delta robots can benefit their production process, and for the hard numbers of the ROI.

Delta Robot Configurations

Delta robot kinematics (Image courtesy of Wikimedia commons)
Delta robot kinematics (Image courtesy of Wikimedia commons)

Here’s how a typical delta robot works: Three high-torque servomotors are mounted on a rigid frame. On each motor shaft, an arm is mounted perpendicular to the shaft’s rotational axis. Some robots use direct drive, in which the arm is mounted directly on the shaft, and some use a gearbox. These “Bicep” arms are connected to lightweight linking rods arranged in parallelograms to restrict twisting motion. These arms are connected to a central platform. The joints at both ends of each parallel rod move freely, typically in ball joints. At the lower platform, the end effector may be attached, as well as several other options including motors for additional axes of freedom. Most delta robots have, at least, a fourth axis, enabling objects to be rotated.

The main benefit of delta robots is that the heavy motors are fixed on the frame, allowing the moving parts of the robot to be very light. In contrast, each motor of an articulated arm robot carries the weight of all the successive motors. Dean Elkins, Senior General Manager at Yaskawa Motoman, agreed: “In the mechatronic design of delta robots, the motion is being translated down through, in most cases, carbon fiber arms, where there’s far less mass being moved. They’ve become a very, very efficient way of x, y, and limited z translation.”

The Yaskawa MPP3H has a horizontal reach of 1300mm. (Image courtesy of Yaskawa Motoman.)

The Yaskawa MPP3H has a horizontal reach of 1300mm. (Image courtesy of Yaskawa Motoman.)

Because of this lever-based design, the motors must deliver high torque in order to counteract the increase leverage applied by the payload on the shaft. This contributes to the low payloads of most deltas. Elkins elaborated on that point: “There’s a lot of torque on the motors that gets translated all the way down to the wrist of the robot. So, because of that offset, because of that distance away from the motor itself, you’re moving a wand that can be 1,300 millimeters or even longer in length. So, the idea here is while the motors are high-torque, you have this offset that you have to deal with.  To keep the speed, you limit the payload.”

While most delta robots have three arms and four axes of freedom, there are other configurations available. Omron, for example, offers the Quattro parallel robot. According to the company, this design allows the robot to carry an increased payload, or to perform a faster cycle time.

I asked Atef Massoud, automation engineering manager of servo, robotics and automation at Omron Automation Americas, about the configuration options. “We have a unique design which uses 4 motors with 4 links. Instead of 3 parallel links, it has 4 parallel links. This allows it to carry up to 15 kilograms if you don’t do rotation at the bottom, or 6 kilograms if you do rotation. But, our traditional robots for example can do 150 parts per minute, while with this unique design you can do  300 parts per minute,” he said.

The Omron Sysmac Delta. (Image courtesy of Omron.)

The Omron Sysmac Delta. (Image courtesy of Omron.)

This brings up an interesting point about theta motion around the z axis, which is the fourth axis of freedom for the delta robot. There are two main design approaches to this. As Massoud explained, “The traditional solution would be to have three motors mounted above, and then a separate motor to do rotation,” he said. “Omron has that. You can mount the motor on top or at the platform. But if you mount the motor on the top, then you must have a drive shaft. You can mount the motor at the platform, but it will be a smaller motor and the added weight will increase the inertia of the platform.”

According to Roy Fraser, global product manager, packaging robots at ABB, the shafted configuration allows certain benefits, including higher payload. “We have a number of patented designs for the theta shaft and the theta fully rotational shaft, which can move in continuous 360-degree rotation. The reason we perform theta motion by a shaft, and not by adding a motor at the central plate, is that such a motor adds weight and reduces payload and reliability. The shafted design avoids these issues.” The reliability issue Fraser mentioned is with the cable that must run to the motor. The cable must not interfere with the high-speed arms and must stand up to repetitive bending and twisting strain.

On the other hand, the shafted approach adds another element which requires maintenance. The shaft can wear out over time, and needs regular replacement, and in some cases, regular maintenance such as greasing.

What are the Axes a Delta can Manipulate in?

The range of motion of delta robots is simpler than that of a serial manipulator robot. Delta robots can move freely within their workspace in the x and y directions. Z-axis motion is more limited. However, depending on the application, rotations about the x, y and z axes are sometimes required. Elkins of Yaskawa Motoman explained: “It’s not uncommon anymore to see rotational axes or pitching axes to upright parts, for example. If you were to look at a blow molding application where you perhaps wanted to descramble horizontally oriented bottles and upright them into a soldiering or singulation type of arrangement, you can pitch those parts by adding external axes and then you could actually add in many cases, another roll axis.”

The FANUC M-1iA/0.5A can manipulate objects in six axes. (Image courtesy of FANUC.)

The FANUC M-1iA/0.5A can manipulate objects in six axes. (Image courtesy of FANUC.)

Phil Zanotti, material handling engineer at FANUC America, agreed. “The majority of operations taking place with delta robots are going to require a four axis.  There are certainly applications that require that six-axis because a six axis will let you, for example, flip a small bottle up or something similar. You could pick something laying down and place it standing straight up. That’s where you see that kind of robot.”

Samir Patel, director of product and advanced engineering at Kawasaki Robotics added that payload can be another indicator of your best robot option. “In theory, the delta model could be designed to handle a higher payload, but the heavier design is going to start reducing the cycles per minute. At that point it may not be worth all the complex mechanics of a delta robot. From a reliability or price standpoint, a SCARA robot could be more effective at a higher payload. For example, some SCARA robots can handle 20kg,” said Patel.

How Much Robot Do You Need?

Image courtesy of Cama Group.
Image courtesy of Cama Group.

This can be a tricky question for fledgling automation customers to answer. Consider the following:

  • Form factor and work envelope
  • Vision system requirements
  • Programming and controls

Beyond these secondary factors, the robot you need is essentially a function of your needed cycle rate and the mass of your products. This information is available on robot datasheets and from integrators.

Delta robots excel at providing high-speed pick-and-place, but they are sadly still bound by the laws of physics, especially Newton’s first law. Some delta robots can produce accelerations of up to 12-15 g. “Because delta robots start and stop so fast, when you decelerate with the product in hand, if you don’t have enough grasping power, the product can be released and go flying. In some applications you need very high power vacuum etc. There are certain products you cannot grip using suction. You have to use mechanical grippers. Of course, mechanical grippers add weight to the end effector, so the capacity is reduced,” said Patel of Kawasaki Robotics.

Fraser, of ABB, agreed that end effectors are a key part of selecting the best solution for the job: “When you calculate the cycle time, you have to look at the payload that you want to move, and the cycle that you need to achieve. You don’t want to move any faster than you absolutely need to. The key point is designing the gripper effectively to hold onto your parts under your given acceleration. The design of the gripper is always the key to that.”

While the most common end effectors for pick-and-place robots are suction cup grippers due to their fast actuation and high repeatability, not all objects can be manipulated by suction. For example, objects with porous or rough surfaces are typically manipulated by a mechanical gripper. However, mechanical grippers reduce cycle time, as the fingers must actuate open and closed to pick and release each object.

Why Use Delta Robots?


The ABB IRB 360 FlexPicker. (Image courtesy of ABB.)

“They’re certainly becoming a mainstream alternative for high speed picking and placing applications. It was that for the longest time we were limited to slower articulated arm technology or slightly faster SCARA technology, and now with Delta robots we’re able to get the heavy components up in the air, onto a super structure, and really increase speed because in many cases we’re moving much less mass,” said Elkins of Yaskawa Motoman. “So, they’re great for when you’ve got to go fast,” he emphasized. Fraser of ABB echoed his statement. “This is really about looking at the architecture of the robot. Due to global megatrends, production lines are getting faster. Because of the parallel kinematic architecture, the delta robot has a very high mechanical advantage in performance and flexibility when compared to traditional articulated robots, and even cartesian robots,” said Fraser.

Delta robots are purpose-built for pick-and-place, going back all the way to their inception with Dr. Clavel. Zanotti of FANUC agreed. He summed it up: “Typically you’d need a Delta robot if you have rate that’s going to be sustained—a lot of parts flowing down the conveyor belt, hundreds of parts per minute. There are other robots that can do fast rates, but they can’t keep up with the rate of a delta without overheating or working over their motor duty.”

Massoud of Omron says that delta robots are on the rise in the industrial pick-and-place market. “I see a growing use of Delta robots, especially in packaging and in high speed pick-and-place.  I think in the future, we will see them moving into different industries for lightweight objects that require high speed pick-and-place. So, I see them spreading more,” Massoud predicted.

Delta Robots in Industrial Food Production and Packaging

Of course, there are a few applications beyond pick-and-place for delta robots. Many of these applications are in the food industry, an industry characterized by high volume and strict health and safety regulations. For example, food manufacturers must meet standards for equipment washdown, and prevent exposure of food product to potential contaminants such as lubricants, fragments of metal or plastic, or dust. Because the architecture of the delta robot, the motors can be well isolated in enclosures, and many models are available up to IP69K ingress protection rating, allowing the robot to be blasted by high-pressure, high-temperature wash down.

Samir Patel of Kawasaki gave a fascinating example of a unique application: “One customer has  robots in big bakeries to do bread scoring, scoring the tops of loaves of bread as they go by on a conveyor. That’s one example of non-pick-and-place application,” Patel said. “Traditionally, one of our integrators has used six-axis robots for this. Now, the same integrator is looking at using our delta robot.”

Another interesting example is this video from ABB Robotics, showing a case study in which delta robots are used to spread sauce on pizza dough rounds.

Delta Robot Programming

In general, any technician experienced with programming industrial robots will be fully equipped to handle delta robot programming. The experts agreed that delta robots are in some ways simpler to program than serial manipulators, and in some ways, more complex.

Working at high speeds, delta robots are carefully programmed to work in synchronization with the conveyors parts move along. The conveyor may be equipped with an encoder to pass information to the PLC. In order to organize efficient picking of these parts, a vision system is used to give the correct parts to the robots, which is also controlled by the PLC.

Where it gets really interesting is in cases where multiple robots work together to pick from the same line. Software uses the vision data to schedule picks among the robots to optimize the duty of each robot.

Besides these complexities, the robots are, in the end, programmed according to their spatial coordinates, in x,y, and z. Most manufacturers use the same language and even the same environment to program and control delta robots as they use for all their industrial robots. FANUC, for example, uses the same interface for both four axis delta robots and their six axis arm robots. Zanotti noted that if you try to enter a value that requires movement in the fifth or sixth axes, the software will simply return a fault.

Fraser of ABB gave an excellent description of the role of vision in the function of these robots.

 “The purpose of the vision system is to identify good from bad products. We use a variety of algorithms to filter out the good products and give those to the robots. We have a range of cameras on offer, including 3D cameras. 3D vision is exciting because it can open up particular benefits for data analysis of products. There was a time where we were only using greyscale cameras, now we’re predominantly color, and in the future, we will be moving even further into 3D cameras.”

Today’s vision systems can identify objects of different shapes, sizes and even colors, enabling sorting and quality operations.

Common Failure Modes and Maintenance

One of the advantages of these robots is that the design is open and easy to access for repair and maintenance. I asked each expert about the most common wear and tear issues to watch out for.

Most of the experts agreed that due to the repetitive motion, the ball and socket joints connecting the parallel arms were subject to wear and eventual replacement. Luckily, it’s an easy fix.

“Most of these robots have a ball and socket construction, where the carbon fiber arms either meet the main drive at the top of the robot, or the wrist at the bottom of the robot. Inside of those drives there’s typically an insulator ring that’s made of a nylon or Delrin, and that provides a friction bearing type of arrangement inside that joint. Those usually will get changed out about every six months,” Elkins of Yaskawa Motoman said. “Other than that, it’s your traditional greasing of some Zerk fittings on the motors,” he added. Zanotti, of FANUC, highlighted the same issue. “Assuming that a cell’s running normally, the wear is going to be in the linkages in the parallel links coming down from the base of the robot. Follow a recommended maintenance plan to check the certain wear points. Washers, springs, that sort of thing,” described Zanotti. “If they are worn, they’re usually cheap, non-invasive fixes that don’t take too long. They don’t bring your line down for too long.”

According to Massoud of Omron, those rings typically last up to one million cycles before needing replacement.

Kawasaki YF003N Delta. (Image courtesy of Kawasaki Robotics.)

Kawasaki YF003N Delta. (Image courtesy of Kawasaki Robotics.)

Patel of Kawasaki filled me in on what to expect for motor wear: “In general, the main motors wear at the same rate. It depends on the motion pattern that repeats, but in general the three motors work within a reasonable range. AC servomotors driving the arms are one of the most reliable motor products for very fast start stop motion. So, there are no brushes, you have a magnetic rotor rotating inside of your stator windings. There are hardly any components inside to wear,” he stated.

Safety

Safety for delta robots does not differ from requirements for other industrial robots. Patel of Kawasaki has experience with the guarding required for delta robot pick-and-place applications. “You definitely need guarding around the delta robot. It could be Plexiglas, it could be wire mesh structure. At the same time, on long large lines, where you can’t get anywhere close to the delta robot because of the conveyor infeed/exits, I have seen open layouts with nothing around the delta, because you can’t reach into the envelope.”

One thing to consider is that with product being accelerated to high speeds, it’s possible that a suction or mechanical gripper could fail during motion, and send the payload flying. Therefore, it’s important to ensure guarding protects against this risk.

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