The progress on flexible parts feeding
Progress usually moves along little by little, it is a continuous process driven by experimentation and awareness of the state-of-the-art technology to raise new ideas and requirements which are the driving force of development.
This brief article is meant to talk about the progress that has been done in the field of flexible parts feeding skimming through the most representative systems.
We define agile manufacturing as the ability to accomplish rapid changeover between the manufacture of different assemblies utilizing essentially the same workcell. Rapid changeover (measured in hours) is defined as the ability to move from the assembly of one product to the assembly of another product with a minimum of change in tooling and software. A central theme of our definition of “agile” manufacturing is the ability to rapidly introduce new assemblies and components into the system.
In this system, rapid changeover is accomplished through the use of reusable software, quick change grippers for the robotic manipulators, modular work tables, and parts feeders which are flexible enough to handle several types of parts without needing mechanical adjustment.
In order to determine the position and orientation of parts, these feeders use vision rather than hard fixturing.
Non-vision based feeders are usually based on a bowl top with a spiral track within. The component parts are delivered from the bottom of the bowl feeder up the track into the top of the feeder as the bowl vibrates.
A series of escapements, which are tailored to the specific part being fed, are located along the spiral track allowing only the properly oriented pieces, to proceed. The limit of these systems, however, is the need to redesign the mechanical track, whenever it’s necessary to switch to a new part.In vision based flexible feeders rapid changeovers are simplified by real-time programming, modular and object-oriented software environments and offline graphic simulations.
Adept is considered the pioneer of flexible parts feeding that it has been promoting since the 80’s.
The Adept FlexFeeder 250 is one of the first marketed solutions. The feeder utilizes three conveyors to singulate and present parts to a vision system. A backlight is located at the end of the second conveyor and presents a silhouette image to an overhead camera. The first conveyor is mounted horizontally and above the second. Parts drop from the first conveyor onto the second. Bulk parts are stored on the first conveyor. Parts which are in the incorrect orientation or are unreachable by the grippers drop onto a third horizontal conveyor. A bucket lifts parts from the end of the third conveyor and dumps them back onto the first conveyor.
Although the FlexFeeder 250 is quite compact, it has mechanical complexity, high prize and frequent maintenance as its main disadvantages. The system use to operate only with small-size and low-weight parts.
Other limitations concern the geometry of the parts (ex: cylindrical parts).
In recent years a new solution has been developed by Case Western Reserve University (CWSU) in collaboration with Adept.
The feeder consists of three conveyors working in concert to singulate and present parts for retrieval.
The first conveyor is inclined and is used to lift parts from a bulk hopper. By varying the angles of inclination and belt motion profile (acceleration, deceleration, and speed), a variety of parts are deliverable from the hopper.
Parts slide down a ramp at the end of the inclined conveyor onto a horizontal conveyor. The horizontal conveyor terminates within the work envelope of a robot, where a backlit vision window is located.
A third, fixed-speed conveyor is used to return un-retrieved parts to the hopper.
The first two conveyors (inclined and horizontal) are under servo control.
Part pose is determined at the end of the second conveyor. A backlit window is machined in the conveyor and provides a back-lit, silhouette image to an overhead camera.
A further parts feeder introduced by Adept is the iFeeder.
This feeder is similar to the FlexFeeder 250, but has one added feature. Incorporated into the second horizontal conveyor (the conveyor from which the robot retrieves parts) is a mechanism called a popper.
It is a small voice-coil that travels (servo’ed in x and y) under the belt. It has the ability to hit from below (or pop) any part within its field of motion. This action causes the part to tumble with the hope that it will come to rest in a new pose more advantageous to the subsequent assembly operation.
To enable this feature, two vision systems are used.
The first is used to determine parts which are not in the proper orientation and need to be re-oriented or popped.
The second determines the pose of parts which have passed the popper and are entering the pick-up area.
The system popper is very effective, yet it is very expensive and therefore it seldom was applied.
We intend to mention also the system AnyFeeder, which though not directly produced, is actually distributed
The experience gained by Ars srl in over 20 years upon integration and use of these systems, leaded the company to the design and development of a new concept of flexible parts feeding: the FlexiBowl®. The goal was to achieve a system of real flexibility, which might enhance the robotics’ effectiveness and lead vision based automation to be cost-competitive with non-vision based one. The FlexiBowl® is no longer based on the use of conveyor belts and vibrating drive.
It appears as a cylindrical cup of variable size composed of an upper rotating disc, able to accelerate and decelerate in both directions through a servomotor, and an impulse generator underneath.
The electronic control and the optional backlight, are placed within the stainless steel frame.
The FlexiBowl® is available in different sizes: from the smallest Ø350mm to the biggest Ø800mm.
The system is generally combined with a vibrating or elevating hopper which works as a buffer to allow a continuous operation.
The working principle is simple and straightforward: the pieces fall from the hopper on the rotating disc and are singulated by the bidirectional rotatory motion and the impulse generated below.
The mix of motion and impulse allows to switch the parts’ placement so that the silhouette might be seen by the vision system and the part can enter the picking area of the robotic arm.
The circular geometry and the plain rotating motion of the upper disc, allow software optimization through the communication between Flexibowl® and the hopper. Parts fall from the hopper into the rotating disc just after the vision system notices there’s a lack of components.
This avoids idle times and throughput reduction.
Moreover, it prevents the surface to be overloaded which would lead to troubles for the camera and for the vision system in locating parts’ silhouette.
The instruction set allows to tune speed, acceleration of the rotating disc and frequency of the impulse basing on the part’s geometry and weight, for better results.
The communication between robot and vision system is carried out by simple commands through the available ports: Ethernet (UDP), EtherCAT and digital I/O.
Field analysis show that FlexiBowl® allows a more uniform , leaner and waste-free parts feeding:
- OEE is never less than 99% for a single feeder
- 30% more productivity than its closest competitors
- Less than 2 hours to be put in operation.