A new intelligent device - the cobot

Citation

(1998), "A new intelligent device - the cobot", Industrial Robot, Vol. 25 No. 3. https://doi.org/10.1108/ir.1998.04925caf.006

A new intelligent device - the cobot

A new intelligent device ­ the cobot

Keywords Cybernetics, Intelligence, Robots

Unlike robots that perform specialized tasks only in restricted areas, cobots ­ a new class of intelligent devices ­ have been designed to work with human operators in a shared workspace.

Cobots is a conflation of the words "collaborative" and "robot". Although the first robot designs are only now being tested, one of the devices' primary advantages has already been demonstrated: helping human workers use their muscle power more safely and effectively. According to one of the engineers who designed them, the most novel feature of this new breed of intelligent devices is that they provide guidance rather than raw power. "Having humans in the same workspace with robotic devices is a hot-button issue because of safety concerns," said Michael Pashkin, a professor of mechanical engineering at Northwestern University in Evanston, Illinois. "The robot approach is based around the recognition that in the industrial environment,intelligent tools are needed more to supply guidance than to supply power. It's the power aspect that is potentially dangerous." While the worker still must supply some force when working with a cobot, the benefit to his or her safety and long-term health can be significant. For example, it's relatively easy for workers to push objects directly in front of them. By contrast, moving heavy objects in a sideways direction typically is more difficult because the worker must stretch and turn, which can place severe stress on arm and back muscles.

Industrial applications

The industrial environment has a wide army of potential applications for cobots. In a car manufacturing plant, there are applications where it makes sense for workers to do a task manually, while for other applications, the best option is complete automation. Cobots are useful for many of the tasks that fall somewhere in between ­ tasks in which workers' abilities to see, feel, and react are needed, but where it is also desirable to spare the operator from having to perform certain physically taxing motions. The cobot concept and early designs were developed at Northwestern's Laboratory for Intelligent Mechanical Systems, with funding from General Motors. A team comprising engineers from Northwestern and GM has honed the final designs, and is overseeing testing and implementation. The technology has now been licensed to Collaborative Motion Control Inc. in Evanston, a new company that will focus on bringing cobots to a variety of industrial applications.

GM refers to these technologies collectively as "intelligent assist devices." The cobots developed as part of the GM-Northwestern collaboration are the first such devices to appear. The cobot has two modes: free and constrained. In free mode, the operator could push the cobot anywhere and provided all the motive force. In constrained mode, the operator still provided the motive force. However, if the computer sensed that the cobot was coming up against a preprogrammed virtual boundary, a motor turned the wheel about its vertical axis and prevented the cobot from crossing that boundary. When in contact with a virtual wall, the wheel simply steered tangent to the wall. If something went wrong with the motor or controls, the unicycle might begin to steer improperly, but it still would not go anywhere unless the operator pushed it. The device also could not harm theoperator because the user supplied the only kinetic energy in the system. Once the development team proved the effectiveness of the concept, it built several more prototypes, eventually leading to a tricycle cobot in which all three wheels are independently steered. This cobot can control translation like the unicycle could, but it can also simultaneously control angular orientation. The prototype served as the basis for the larger version that will eventfully be used in GM's manufacturing facilities.

The first cobot will be used to handle car doors on a GM's vehicle assembly line. In the factory, car doors are connected to the chassis relatively early in the manufacturing process so that their colour and finish match exactly during painting. To make it easier for workers further down the line to install the instrument panel and other components, the doors are removed as soon as the car comes out of the paint shop. The doors are put back on near the end of the assembly process. Currently, workers handle the doors with one of two tools ­ a pneumatic air balancer or an overhead-tail-based arm with a pneumatic actuator. Both tools do the actual lifting of the door, but the human worker still needs to push or pull the door as well as guide it. The pushing or pulling motion is relatively easy; guiding the door can be difficult, however, because of the stretching and turning. With the cobot, all the worker will need to do is push or pull the door, and the cobot will guide it.

Controlling rotation

Before developing the cobot, team members considered a number of existing technologies to accomplish the same goals. For example, haptic devices ­ the name of which comes from the Greek term meaning "to touch" ­ allow an operator to interact with a computer through hand and arm motions. In such an application, a high-performance servo typically would be connected to a computer controller linked to the haptic device. "At first glance, it would seem natural to adapt existing haptic technology to industrial uses," Northwestern's Peshkin said. "However, the project team considered this approach unsuitable for safety reasons. The forces involved with existing haptic devices are relatively small, but the motors would have to be much more powerful to lift and guide large objects. If control of these motors was lost, or if one or more of the motors malfunctioned, there is the chance that they could harm the operators." For this reason, the development team had to come up with an entirely new approach to providing workers with guidance so that they, in turn, can manipulate objects safely on the factory floor. Controlling the translational motion of an object is relatively easy through the use of a wheel steered by a motor. The heading of the wheel determines how fast it moves in the X-axis relative to the Y-axis. The challenge for Northwestern was that most applications require control of not only translation but rotation as well, and restricting angular motion is not nearly as straightforward as connecting a wheel to a motor. To address this problem, the development team created a new type of continuously variable transmission (CVT) that imposes a fixed ratio on a pair of angular velocities. The first rotational CVT model that the team developed consists of a sphere caged by six rollers, with the rollers arranged as if on the faces of a cube surrounding the sphere. Each roller is pressed in toward the centre of the sphere by an externally applied force, which serves to keep the rollers in rolling contact with the sphere. Two rollers are considered drive rollers and interface to other parts of a machine that incorporate the CVT. Two other rollers, diametrically opposite the drive rollers, are followers; they serve only to confine the sphere and apply the externally applied force. The drive rollers and followers have axes of rotation that lie in a single plane, which passes through the centre of the sphere. The remaining two rollers are steering rollers, located at the top and bottom of the sphere, that can turn freely on their axes. Unlike the drive rollers and followers, the axis of the steering rollers is adjustable. The angle that the axis of the steering roller forms with the horizontal is the steering angle.

Rolling ­ as opposed to sliding ­ occurs between two rotating rigid bodies in contact when the axes of the two bodies are coplanar. Their axes do not need to be parallel: the axes of two bevel gears, which are in rolling contact, are coplanar but not parallel. If, on the other hand, the axes of the two bodies are skewed, sliding occurs. Similarly, if the coefficient of friction is adequate, motion is prevented. The rotational CVT requires a sufficient force and coefficient of friction high enough to prevent sliding. Considering all possible axes of rotation of the sphere, the sphere must be in rolling contact with all six rollers if it is to move at all. Since the centre of the sphere is stationary, the sphere's axis of rotation must pass through its centre. Rolling contact with a given roller requires that the axis of the sphere lie in the place containing the axis of the roller and passing through the centre of the sphere. Each roller forms such a plane. The planes for the followers and the bottom steering roller can be ignored because of symmetry. Accordingly the two follower rollers were eliminated in the version that the developers actually built. The rollers contact the sphere at four points that form the corners of a tetrahedron. (The tetrahedron is not regular but is stretched such that the angle subtended by the points of contact of either pair of rollers with the centre of the sphere is 90°; this makes the device easier to machine.) Moreover, in the CVT used in production cobots, the rollers do not need to be independently preloaded. Instead, a rigid frame holds the two drive rollers, and another rigid frame holds the two steering rollers. The frames can be simply drawn together by a spring, which applies the same force to all four contacts.

Once the cobot for unloading doors had been designed and proven using the new CVT, the team began testing the device at GM's General Assembly Centre in Warren. Meanwhile, GM, along with other partners in industry and academia, is developing other intelligent assist devices. With the University of California, Berkeley, for example, GM engineers are developing a device that will amplify the power a human worker supplies in the course of doing his or her job. One of the first applications of this device will be to lift batteries from a skillet and install them in vehicles. At GM's assembly plants, several different vehicles ­ each of which may require a different battery ­ are manufactured on the same line. Workers use pneumatic tools to lift each battery off the skillet and put it into the car.